The drivetrain on the buggy is completely separate for each axle. The front rider drives the front axle and the rear rider powers the rear axle. This method keeps the center of the buggy open for folding and separation. The rearward facing rear rider allows for the segments of the drivetrain to be shorter, more compact, and lighter.

A standard pedal method of capturing rider power is used because it’s a well developed method that most people are accustomed to and efficient at using. In addition bicycle parts are readily available for purchase and are relatively inexpensive. A combination of mountain bike and BMX parts were used where they’re benefits were determined to be best.

To allow the buggy to fold in its basic design, the crank assemblies would have to fold in. To fold the cranks without changing the chain length the cranks would have to fold about an axle that the chain would rotate about. By fixing the chain length the chain could always remain in the same tensioned state while folded or extended. Because it wasn’t ideal to pivot the cranks about the drive axle, an intermediate axle is added to the drivetrain. The fact that the cranks need to fold also makes it unfavorable to standard derailleur shifting, as the derailleurs would be likely to get in the way and the potential for the chain to fall off during assembly would be higher.

The intermediate axle is a convenient place to locate the shifting. It is a well-constrained axle, and does not have to house a differential unit. Internal hub shifters had many advantages in this area. They didn’t involve derailleurs, so the chain line and length would never have to change. Because the chain line would always be the same, wider, stronger, and thus less likely to derail single speed chain could be used. The internal hub shifter is comparable to a rear cassette on a bicycle.

From the internal hub shifter the chain goes to the roller clutch based differential unit. The differential unit provides power to the slowest wheel so that the buggy remains powered even if one wheel loses traction. The torque from the roller clutches is transferred from the outer casing to the universal joints, which is connected to the half shafts.

The half shafts connect the two universal joints that allow the axle to move with the wheel. Through the suspension travel the half shafts vary in angle and length. To allow the variation in length the outer universal joints are keyed along with the half shaft. At the wheel the universal joint is attached to the stub axle, which supports the wheel in the upright.

The stub axles are made of large hollow steel tube, and attach to the disk brake mounting holes. The disk brake holes make for convenient mounting points to the axle, as the hubs are readily available for purchase. The disk mounting holes are strong enough because the braking loads are the greatest torque loads, so they are plenty strong for the pedaling torque.


Introduction / Design Goals

We used bicycle cranks for their high efficiency in transferring human energy into usable power. We purchased the cranks to eliminate needless machining and cost. We aimed to get the cheapest cranks that would work.

Design Notes

The cranks are chosen for they’re compatibility with gearing options, and common bottom bracket bearing assemblies. There are three types of bolt patterns available today. The newest option is a 4-bolt pattern, however gears aren’t made in as many sizes as other options, so these cranks are avoided. The older styles of cranks use a five bolt pattern. There are two varieties of the 5-bolt pattern, one is a 110mm/74mm pattern, and the other is a 94mm/58mm pattern.

The 110mm pattern is known as Standard. The 74mm rings are typically 24 or 26 teeth, The middle 110mm rings are typically 34 or 36 teeth, and the large 110mm ring is typically a 46 or higher. To achieve the optimal chain line with the shifter hub the 110mm mounts should be used on the crank. This limits the gearing options to 34 teeth and larger. This limitation is avoided by going to the 94mm cranks.

The 94mm/74mm cranks are known as microdrive or compact cranks. With recent attempts to race with two chainrings instead of three many different chainrings are available in numerous gearings. The 58mm pattern will fit 20 and 22 teeth rings, and the 94mm will fit 29 to 32, and 42 and 44 tooth chainrings. Although 32 tooth rings are the standard, 29 and 30 tooth gears can be ordered. QBP (Quality bicycle Products, a wholesale distributor) sells 30 tooth rings. 29 and even 28 (must have special bolts to use 28t) tooth gears are made by Ritchey and some other companies.

Fabrication Notes

We eliminated the need for fabrication by purchasing the cranks.

Performance / Lessons Learned

The cranks worked flawlessly. They were not the most expensive or flashy. Possibly next year we could get cranks that are the same color on both sides.

Eccentric Bottom Bracket

Introduction / Design Goals

On of the biggest problem with the old buggy were chain derailments. These occurred for two reasons improper chain alignment or improper chain tension. To solve the tension and alignment problem in the crank to internal hub shifter chain line an eccentric bottom bracket was used.

Design Notes

The need for a reliable and easy way to tension and align the chain between the cranks and internal hub shifter was needed.  This is very similar to the rear end and drive train of a bike, however we only have one gear so we do not need a derailleur shift and therefore do not have a chain tensioning device.  If the gears are not properly aligned then when the buggy is pedaled the chain will derail. If the chain is tensioned too low then the buggy will work till a bump or turn causes the chain to swing out of alignment and result in a derailment. If the chain is tensioned too high then the chain, gears, and bearings will not function properly and will wear very quickly.

On last year’s buggy putting slots in the crank support members tensioned the crank to internal hub shifter chain line. This design did not perform well.  The first issue was that the bolts on the internal hub shifter could not be tightened enough to keep the crank supports from sliding resulting in a non-fixed distance chain line.  Another issue was that when the cranks were rotated for folding the chain line became shorter.  In the end the slots were difficult to tension and would not remain tensioned.

This year we examined a couple of options.  The same pulleys used to tension the internal hub shifter to drive axle could have been used.  These however would have been hard to mount since the crank supports were small and at weird angles.  In addition they could have interfered with other parts when the cranks were in the folded position.

The other option examined was the use of an eccentric bottom shell.  This is a part commonly used on single speed bikes.  It is composed of an enlarged bottom bracket shell that has been cut and bolts mounted to clamp down on a cylindrical aluminum insert.  The insert has a hole that is tapped and accepts a normal bottom bracket placed off center in it.  A bottom bracket is what holds the cranks to the bike and allows them to spin.  By adding or removing links of chain and spinning the insert the chain can be tensioned to account for different chain length requirements such as is required for different gear combinations.

This design has been tested and is a tried and true method for tensioning chains in similar situations to the one we encountered.  In addition to being able to rotate, the insert can slide laterally to help with chain alignment.  Another benefit is the commercial availability of all involved components.  The only down side is a moderately heavy design.  Since the buggy would not be ridden much the bottom bracket used was the most inexpensive one we could find at a local bike shop.

Fabrication Notes

Since eccentric bottom brackets can be purchased at any bike store fabrication was minor.  The shell should be purchased uncut.  Then weld on the bolt clamps, tubes, and anything else that is going to be attached to it.  Then cut the shell so it can clamp down on the insert.  This will ensure that the shell does not warp during welding and that insert will fit properly.

In the purchasing of the bottom bracket make sure it is compatible with the cranks.  Either splined or square taper joints do not make any difference for our purposes because the low usage.  We recommend using the cheapest design that has different length bottom brackets, this year we used square taper.  Also when placing the bottom bracket into the shell, make sure it goes in the correct direction, with the fixed cups on the drive side.


We experienced no chain derailments during testing or racing.  This in itself is a testament of the perfect performance of a properly set up eccentric bottom bracket.  The one problem we did experience was that the eccentric is not properly designed and does not complete adjust to any size chain.  If the chain happens to result in the shell being at its limits there is a small window where proper tensioning cannot be achieved.  This was frustrating and was fixed by changing to a slightly different size gear that was needed anyway.

Lessons Learned

The biggest thing we learned was to make sure that all members that were going to be welded to the shell were attached before it was cut.  We did not do this and as a result the shell warped making it hard to adjust.  In addition, really examining the specifications of the eccentric bottom bracket to make sure that it will be able to be fully adjustable.

Internal Hub Shifters

Introduction / Design Goals

With the complications of the buggy needing to fold, a robust shifting system was desired. Derailleurs are vulnerable to damage, move the chain line, and there is a high chance that the chain may come off during assembly. The use of an internal hub shifter has many advantages over derailleurs. The chain line is fixed, the chain length is fixed, and there are no derailleurs that must move around during folding.

Design Notes

The internal hub shifter allows for shifting to take place without moving the chains. The 7-speed Sachs shifter gets 303% gearing range, comparable to a mountain bike cog set. Although the hub is heavy, no derailleur or cog set is required.

The hub conserves the power transferred through it. Since it changes the speeds the net torques on the hub are not zero. The torque difference comes from the axle attachment to the dropouts. The axle must be secured and not permitted to rotate, otherwise it will spin and damage the dropouts or the bearings of the hub will tighten or loosen. The Sachs hub comes with two flat edges on the axle. Either an included washer can grab the flats and the dropout, or the dropout can be made narrower to grab the flats on the hub. The narrower dropouts are preferred as it is a stronger interface and it reduces items on the axle of the hub.

The gearing is idealized so that the middle gears would be used most frequently. Within the net gearing ratio, torques are minimized prior to the shifter hub to reduce loads on the shifter. The Shifter hub is only able to withstand a limited amount of torque. For this reason the smallest possible gear is placed on the body of the shifter hub to build as much torque after the hub.

Fabrication Notes

The shifters are purchased from Glenn Swan. The cog may be changed to change gearing. The shifter is adjusted while in fourth gear, and the white arrows are aligned in the shifting window.

Performance / Lessons Learned

It is hard to get different sized cogs for the hub. The gear from Shimano’s Nexus hub fit the SRAM hub. The outer body for the SRAM hub is smaller than the Nexus hub, allowing for smaller gears to be placed around it. The side mounted shifting on the SRAM hub worked better than the Nexus hub for folding the cranks down.

Chain Guides / Tensioners

Introduction / Design Goals

One of the primary problems with many buggies, including the 2001 buggy is keeping the chain from derailing.  A derailed chain means the buggy has to stop until the chain is fixed, or in our case finish the course under half power (unless the other chain derails as well).  In some cases, a derailed chain might require the rider to get off the buggy to fix it, and incur a ground penalty.  Keeping the chains properly tensioned and guided is therefore of paramount importance to finishing the race cleanly and quickly.

Design Notes

Last year, chain tension on the front (the model for both halves of this year’s buggy) left much to be desired.  Between the cranks and the hub shifter, the adjustment was achieved by slotting the tabs attached to the crank support tubes.  This worked horribly.  Because the cranks have to rotate, they cannot be bolted securely down.  In addition, There is sufficient force on the crank supports to move them even if they were bolted firmly.  The result was that the cranks shifted back and forth, and the chain was still not properly tensioned.

The problem of tensioning the chain from the cranks to the hub-shifter was solved on this year’s buggy by the eccentric bottom bracket.  We needed only to make sure we had proper gear alignment, and we would not have to worry about derailments.

The other chain, located between the hub shifter and the roller clutch assembly, also had tensioning problems last year.  Tension was achieved by sliding the entire roller clutch support assembly forward and back.  Aside from being very difficult to do, this also failed to keep the chain properly tensioned.  The force on the chain was often enough to move the whole assembly, and if the chain did derail (which it often did), it was exceedingly difficult to get back on.  Based on these failures, we identified several qualities that would be necessary for a proper chain tensioning system.

1. The tensioning should be achieved on the slack half of the chain line.  Thus it would only have to withstand forces due to the actual chain tension and not due to the motive force.
2. Chain tension should ideally be an independent system.  That is, other important buggy systems, including the roller clutch assembly and the hub shifter should be placed where they belong, and not moved for the purpose of chain tensioning.
3. Tensioning the chain should be a quick, simple, one-tool job.

We also realized that chain guides placed before the chain meets gear would help reduce derailments (this function is performed by the derailleurs on a bicycle).  It would be ideal if we could incorporate our chain tensioners with chain guides of this type

Based on all these considerations, we decided our chain guide/tensioners would be pulley-style spinners that would be mounted in a slot that allowed their position to be adjusted.  On the front, there would be only one, placed just before the chain reaches the roller-clutch assembly chain ring.  It would be mounted vertically off of the bottom frame rail, and would slide in a vertical slot.  On the back, three guides would be necessary to guide the chain around the roller clutch assembly for the reversal mechanism.  Two of these would be mounted below the bottom frame rail on either side of the gear, and the third vertically above the one closer to the center of the buggy.  This last guide would slide in a vertical slot to adjust chain tension.  All of these guides would be on the slack half of the chain line so as to encounter minimal loads.  The guides would be mounted on bearings to minimize frictional losses (this was of arguable necessity, and could be skipped if finances were tight).

The final tensioning goal was single tool adjustment.  This was achieved by using a T-slot nut in the slot.  These are normally used to bolt things to machine tables.  This setup allowed the loosened, slid along the slot and tightened effectively with one tool.  We decided to use allen-head bolts so the tension could be adjusted with an allen-wrench.
Fabrication Notes

We decided to use ultra-high molecular-weight polyethylene to make our chain guides out of because of its resistance to wear.  The bearings were the smallest, cheapest ones we could get that would fit a ¼-20 bolt.  We got two bearings for each guide with OD’s of 0.625”.  In order to be able to tighten down on the bearings, we had to make a spacer to go between them, and small OD washers between bearing and the slot.  The slots were made about an inch and a half long in 0.040 chro-moly sheet.  This much adjustability was only really necessary in the front where the tensioner simply deflected the chain line from the side.  On the rear, where the chain wrapped 180 degrees around the tensioner, only ½” of adjustability is required to account for a 1” link.  The slots were supported with more 0.040” sheet and welded to the lower frame bar.   On the rear, the non-adjustable guides were bolted through holes in the 0.040” plate instead of through slots.

When the chain guide mounts were first constructed, they were equipped with clamping mechanisms that allowed them to be bolted to the frame, and adjusted as necessary.  We found that they did not clamp well enough to resist being moved by the chain tension.  Therefore, once we decided where they worked best we welded them onto the frame.

The guides themselves were turned on a lathe out of a piece of 2” diameter UHMW PE we purchased from McMaster.  Putting the bearing-reliefs in was a little tricky because they had to be put on both sides.  Therefore, one side had to be done after the guide was cot off the stock.  It was pretty difficult to hold the guide properly in the lathe, therefore one of the bearing-reliefs was a little larger than 0.625”.  This made some of the chain guides wobbly, and therefore less effective than they might have been.  In the end, the bearings probably weren’t necessary, and we felt like the guides might have been better off if we had just drilled a bolthole in them and attached them with shoulder bolts.

Performance / Lessons Learned

The chain guides worked flawlessly.  Since they were welded on we have not experienced a single chain derailment.  Aside from the shoulder bolt idea mentioned above, we recommend using an identical technique on future buggies.

There was some argument about whether welding the chain guides onto the frame was necessary or appropriate.  The argument is that the more things you weld to the frame, the less replaceable your frame is.  There was also the worry that many welds on the lower frame bar would weaken the already highly loaded member.  Though the clamps that were made for our chain guides where ineffective, a better design could allow them to be clamped rather than welded.  In the end, none of our fears were justified, and the trussing probably strengthened the frame beyond any chance of failure.  Still, a clamping mechanism could be considered for future buggies.

Reverse Gear

Introduction / Design Goals

With the decision made to have the rear rider reversed the drive direction on the rear drivetrain had to be reversed so the two riders would be powering the buggy in the same direction.  This could be done using a couple different methods.  We chose the one that worked best for our design and was easy to manufacture.

Design Notes

One of the biggest cons of switching the rear rider around was the weight and complexity of reversing the drive direction of the drivetrain.  The method used to reverse the drive would be the only difference between the front and rear drivetrain.  To get ideas of what is the best way to accomplish this task we looked at what methods are currently employed.  Three main ideas were found and examined.
The first idea was the use of mesh gears.  Two gears next to each other spin in opposite directions.  This idea would have worked well and was close to becoming the design used.  The down sides to this design are mainly in its complexity.  In order to make sure the gears are properly aligned and stay there under load would have required extremely closely tolerance parts that are very strong.  This would have resulted in a heavy-complex design and one that is slightly less efficient than a bike chain and gears.

The next other two ideas considered were using conventional bike chain in a different way than usual.  The simplest is to simply run the chain in a figure eight pattern around two gears.  This design is used on many recumbent however they do not ride over rough terrain and tend to have longer chain lines so a slight misalignment in the chain is less noticeable.  This design was rejected for two reasons.  First the chain would have to be tensioned in some way.  Second the chain would have a tendency to derail because a perfectly straight chain line would be impossible.

The final idea examined was the use of pulleys to wrap the chain in the reverse direction around one of the gears.  This idea would be a little more complex than the figure eight design however the pulleys used to reverse the chain could also be used to tension and align the chain.  For these reasons this design was chosen.  A schematic of this type of design is below (file “reverse idea” on black CD in “all pictures” folder).

Fabrication Notes

Fabrication of the pulley reverse drive system was relatively simple.  The main factors that had to be accounted for were space considerations, tension, and pulley placement.  After the pulleys and pulley supports (see pulley section of report) were made they were placed and test for fit then welded in the correct position.
When considering pulley placement it is important to know that it is best if pulleys are not placed on the drive side on the chain loop.  The drive side of the chain loop is the one that transfers the torque from one gear to the other.  This side of the chain loop has the chain in very high tension and if pulleys were placed in this area would have to be very strong.  The non-drive side on the other side of the chain loop can have almost no tension in it. This side is where the pulleys should be used.

The pulley locations were placed on the non-drive side of the chain loop and what was required to get the chain too loop around the gear.  The pulley arrangement could have been arranged in any configurations to allow clearance around other parts of the buggy.  One of the pulleys was attached so that in addition to its role as a guide pulley it could be adjusted to allow for chain tension adjustment.  Using a slotted bolthole allowed for this adjustment


The reverse drive system worked perfectly.  Once everything was aligned and welded in place its performance was indistinguishable from the front drive system.

Lessons Learned

The use of pulleys to wrap the chain in the reverse direction around a gear to get a reverse drive proved to be effective and reliable.  To date the buggy has reported zero problems with the reverse drive system.  It is important note that the pulleys must be strongly secured to ensure they do not more or flex under the heavy pedal loads.  For next year the only suggestion would be to try and accomplish the same effect with only two pulleys versus the three used this year.

Roller Clutch Assembly

Introduction / Design Goals

When turning the outside wheel must turn faster than the inside wheel, as the outside wheel must follow a longer path. To allow the wheels to travel at different speeds and maintain all wheel drive, some form of a differential unit must be used. Roller Clutches allow for a wheel to roller faster than the powered drive axle, but not slower. Thus, the slower wheel is always powered by the drive axle.

Roller clutches were used for the effective differential because they offered many advantages over standard differential units in performance, cost, complexity, and weight. A roller clutch will always power the slowest wheel, where as a differential will power the wheel with least resistance. If a wheel powered by a standard differential were to lift off the ground, it would spin freely and the wheel in contact with the ground would recieve no driving force. This was not desirable in the event that a wheel lift off the ground in the race, as it would make a rider power less. A roller clutch based differential would prevent this effect.

Design Notes

The 2002 roller clutch assembly is an improved version of the 2001 assembly. The 2001 assemblies were very wide, very heavy, and required many precisely made parts. The 2001 assembly used 3 rollerclutches per side to handle the torques, and it was felt that only two would be needed to transmit the torques as needed. The 2001 assembly placed its many components along the axis, while they could have been laid out in a radial series, reducing the assemblies length considerably, and thus it's weight as well.

A main tube was used to house the roller clutches. The main tube was bored precisely to the diameter required for the roller clutches. The roller clutches were press fit inside the tube. On the outside of the tube a shaft collar is used to clamp on the drive gear. The shaft collars also serve the purpose of constraining the roller clutch assmbly in it's frame mounts.

The inner shafts that spin inside the roller clutches are made of hardened steel shafts. The insides were bored out to allow the universal joint to fit inside the end of the shaft. The universal joint and the hardened steel shaft were then keywayed to transmit the torque.

The frame mounts for the roller clutches use two needle bearings and 2 thrust bearings. Sealed cartridge ball bearings were considered for use in this area, however their use was deemed unnecessary. The ball bearings would have been considerably larger, heavier, and had much more friction. The needle bearings were much lighter, and rolled very easily. The disadvantage of the needle bearings were that they didn't support axial loads, so thrust bearings were used, they weren't sealed, and they didn't come with an inner race. because the loads placed on the bearings were low compared to the bearings' rating, and the axial loads were almost neglegible, needle bearings could be justified. The 2001 buggy used needle bearings and it they worked very well, so there was little reason to use ball bearings in this location.

Fabrication Notes

The Spcifications for the Torrington roller clutches are available online at
under drawn cup and clutch needle bearings. The clutches used housed support needle bearings within them. The Clutch used was for a 30mm shaft, with the highest torque rating. They were purchased from Kaman.

The bore of the main tube was machined to the tolerances specified by torrington. The tubes were cut to 5" long, slightly longer than the width of four roller clutches. The inner shafts were cut to 2.5" each, so they would be fully in contact with the roller clutches. The shafts were purchased as presicion hardened shafts, exactly to the specs and tolerances of the roller clutches.


The rollerclutches performed flawlessly, as expected. The clutches never seemed to be slipping under load. However, the shaft collar would slip around the main tube of the assembly unless tightened very tight.

Lessons Learned

Two roller clutches work very well, and three are not necessary for each side of the assembly. The hardened shafts are very brittle if cut too thing. When broaching the keyways one cracked because it was cut too deep. To cut the hardened shafts they must be ground, the outside is too hard to cut with normal cutting tools.

It is very difficult to bore the main tube to the specs required by the roller clutches. it may take a few attempts, but it is possible. The surfaces were honed after machining to make them smooth and to bring them to the final tolerance.

Half Shafts

Introduction / Design Goals

The purpose of the half shafts is to transfer torque from the roller clutches to the wheels. They were made of cro-moly steel and each has a U-Joint at both ends. The U-joints transfer torque through variable angles, allowing the torque to follow the wheels through the suspension motion. Because the wheel's position changes relative to the roller clutch assembly, the length of the half shafts must change, while still transfering torque to the wheels.

On the 2001 buggy the half shafts were overbuilt in many areas, and as a result were much heavier than they could have been. A primary goal was to minimize the weight of the half shafts, while ensuring that they would be strong enough for the torques they would experience. The 2001 buggy also had the half shafts welded to different components, preventing them from the potential to be rebuilt or easily replaced. Ease of manufacture and replacability was another goal.

Design Notes

The half shafts were made hollow, as hollow tubes are better able to withstand torque with less material, resulting in the ability to make lighter half shafts. As the buggy's suspension goes through its travel and as the front wheels turn, the effective length of the half shafts must change. To allow for this, the outboard U-Joints were allowed to slide along the inside of the hollow half shafts. To allow the change in length while transmitting torque, the outside of the universal joints were keyed, and a complimentary key was attached to the inside of the halfshaft. The key assembly allows for continued torque transfer while allowing the half shaft to slide on the universal joint. The inside ends of the halfshaft can remain fixed with the inner universal joint. Being convenient to take apart and easy to manufacture, the connection between the half shaft and the inner universal joint was pinned.

The halfshaft experiences greater sress at the ends where the key or pinned joint creates a stress concentration on the half shaft. To minimize the weight while maintaining sufficient strength throughout the half shaft, the ends of the half shafts are made much thicker than the middle section.

The universals were chosen based on their torque rating, resulting in universals with 1" outer diameters. They are manufactured by Curtis, and purchased through Kaman. The outer front universals were custom ordered with 45 degree misalignment clearance, increased from the standard 35 degrees. The extra clearance would allow for sharper turning and large suspension induced misaligment.

The stresses were approximated in the shaft by assuming that the maximum torque that could be transfered to the the shafts would be limited by the roller clutches. This effectively limits the torque to around 1500 in-lbs. The stress in torsional shear is given by T*r/J, where T is the torque, r is the outer radius, and J is the polar moment of inertia. The maximum predicted shear stress is then calculated to be around 29ksi, providing a reasonable factor of saftey for surface scratches and a reasonable fatigue life.

Fabrication Notes

The half shafts were made from 1.25" OD with .94" ID 4130 steel tube. To fit the universals they were bored their entire length to a 1" ID. The ends were left with a 1.25" OD, while the center section was turned down to 1.125" OD.

The half shaft keys were constructed by first cutting a slot through the full wall thickness of the shafts using the horizontal mill. The key-piece, made from 1/8" steel sheet, was then fit into this slot with a portion of it sticking out past the inner diameter acting as the "key". The "key" was aligned and held in place by an aluminum cylinder with a keyway cut 1/8" deep. The key was then welded to the shaft, and then the surface was ground to the contour of the half shaft. This permitted the key to be structurally attached to the half shaft, and it permitted the key to be placed only partially down the length of the half shaft. A typical keyway could not have been broached the full length of the half shaft, they are too long.

The key-way of the U-Joints was also fabricated using the horizontal mill. The cut was made 1/8" deep, with enough clearance as to not catch on the key in the half shaft.

The torque is transferred from the inboard U-Joints to the half shafts with a simple pin. The hole for this pin was drilled though the shaft and the U-Joint at the same time to make sure the proper alignment was achieved. This was done on a mill with the U-Joint clamped to the shaft with a vise grip.

Performance / Lessons Learned

The half shafts performed perfectly as expected. The only problem we have encountered so far occurred at the competition. Since it rained much of the time we were in Alabama the entire buggy got wet. The problem with the half shafts is that they were not painted and we found that chrome-moly rusts. As a temporary fix we coated them with oil to prevent further rust. In the future they should be coated with grease or possibly painted to prevent rust.

It is difficult to bore the entire length of the half shafts, as they are very long. It is important to use a sharp cutting tool mounted in a very stiff boring bar. At the length the boring bar must be extended it is important to try to minimize vibration to ensure a good surface finish inside the halfshaft.

Stub axles

Introduction / Design Goals

Once of the continuing issues with the ’01 buggy was the stub axles. They were made too small in diameter for the loading conditions and subsequently bent or sheared off under heavy loading. This year we set out to make purposely overbuilt stub axles. If something were to fail in the drivetrain we did not want it to be the stub axle. With that in mind we built large outside diameter axles with hollow insides to create the largest moment of inertia while minimizing weight.

Design Notes

The first step in designing the stub axle was to find what hub they would be attached to. We used oversized “Dimension” hubs from QBP, a bicycle distribution company. This gave us one of the largest outside diameter stub axles possible with commercial bicycle hubs.  The axle itself was 1 ¼” diameter chrome-moly steel with a 1” inner diameter. One half of the axle slid inside the wheel hub. The other half rested on the upright bearings. A universal joint slid inside the stub axle from the frame side of the upright. It was pinned in place with 3/8” pin and a 1/8” keyway helped to transmit the torque. The pin was chosen to be larger than the pin inside the universal joint carrying nearly identical loading. Simple calculations showed that if we needed to put in better pins, high carbon steel pins would transmit the torque with a factor of safety of over 5 for the 3/8 size. We used the standard surface ground pins and decided that we would replace them with the high strength ones if they broke.
On the wheel side transmitting the torque was a little more difficult. Last year the team used the disk brake mount to transmit the torque. This worked well and we saw no reason to change it.

A problem with last year’s design was that the stub axle had a pin that transferred torque to a disk right at the highest bending stress point.

This year we wanted to eliminate that problem. To do this we welded a flange to the disk and put a keyway along the inside length. This way we could transfer the torque inside the hub and away from the high bending stress area.

To keep the flange from moving laterally it was welded to the stub axle. With the flange disk bolted into the disk brake holes on the hub this constrained everything outbound of the upright.

To keep the stub axle positioned laterally inside the upright, we made a round spacer for bolt clearance and then held the whole apparatus in place with an external grooved snap ring. The flange and spacer was pulled up flush against the bearing race and a snap ring was snapped on the far side to prevent lateral movement.

Fabrication Notes

We bought the axle stock of nearly the correct dimensions and bored out the inside slightly to fit the universal joints. To keyway the inside we used a 1/8” broach tool on an arbor press. To do this we first needed to make an aluminum guide too. This was a cylinder with an outside diameter equal to the inside diameter of the tube. Along the length of that tube we cut a 3/16” slot such that with the broach inserted into the slot the teeth would just touch the surface of the tube to be keyed. To cut the keyway the guide tool and broach were pressed through the tube in successive passes until the desired depth was reached. After each pass shims were placed under the broach tool to let it cut deeper into the tube. To keyway the outside of shafts we used the horizontal mill in Emerson and ran the machine in sequential passes to the desired depth. We used standard keystock material from McMaster-Carr.
Fabrication of the torque transmitting flange consisted of a spider which was milled on the rotary table and a simple tube which was bored to the 1 ¼” inside diameter. We put a keyway in the tube using the above technique. These two parts were then welded together.

To cut the snap ring groove we first assembled all of the pieces into place and then carefully scribed a line at the edge of the bearing race on the axle where we wanted the groove to sit. It was essential that we placed this groove correctly to eliminate slop. The actual cutting of the groove was done on a lathe and eyeballed to the correct position. To cut the groove we narrowed a cutoff tool to the specified width and cut the specified depth (given by McMaster).

Performance / Lessons Learned

The stub axles worked well and have shown no signs of failure. They are relatively heavy, but our aim was too make them very strong. In the future the overlap of metal inside the hub could be reduced to reduce weight, while maintaining nearly the same strength.

A few minor improvements could be made on the fabrication of the current design. First, the flange should not be welded, it could be pinned instead. Pinning would allow the flange to be reused if the axle did somehow fail. It would also eliminate the warping due to welding. In the manufacturing of the flange itself, warping naturally occured when joining the disk to the tube section. Welding techniques exist to minimize this, consult George Petry or Rob Shydo for help. Another method would be to leave the disk extra thick before welding and then once it is welded face the disk again on the lathe to ensure perpendicularity.

As for the overall design, it was not very user friendly. Perhaps some thought could be put into ways to make it easier to take apart and replace wheels etc.


Introduction / Design Goals

Last year the choice of wheels was a heated debate. Major debates were held over whether to use mountain bike wheels, ATV wheels, wheel chair wheels, make our own hubs, make our own rims, and make our own entire wheel. It was decided upon to use all mountain bike parts to make as strong a wheel as possible. Testing was done to prove that they would be strong enough. In the end last year's wheels performed well and were lightweight, strong, inexpensive, and readily available. After hours of riding the wheels only required a slight truing, as a result of cornering loads. With this knowledge under our cap we decided to use similar wheels this year.

Design Notes

In designing the wheels we chose to use all mountain bike technology, built to our specifications. It was decided that all four wheels would be built identically. This would reduce the design and engineering time and complexity as well as cutting cost of repair. First we decided on some requirements that this year's wheels needed to meet. In choosing each component it was important to consider cost since four would multiply it. With this in mind stronger or lighter weight wheels could have been built but cost would have been much larger.

Most of these decisions stem from last year's experience. Because last year's stub axles were prone to bending a hub that could accept a 20 mm bolt through axle was required. This would allow for a larger (and stronger) stub axle to be used. We decided to use a similar disc brake setup as last year's buggy (see brakes section) so we required disc brake hubs. 26-inch wheels were used because they were the current standard for mountain bike wheels providing the widest range of available parts.

The first thing chosen on the wheel was the hub. The hub would serve as the connection between the buggy and the rest of the wheel. Front hubs were used because we did not have any use for the cassette body that comes on a rear wheel. In order to help counter the lateral loads on a wheel in a turning situation the angle between the left and right spokes should be increased and the length of spoke should be shortened. To accomplish this the hubs chosen had the widest spoke flange (increasing spoke angle) width with the largest diameter spoke flange (reduce spoke length). In addition the hub had 36 spoke holes, the largest number available for a standard mountain bike wheel. The hub chosen was the Dimension front disc hub.

The rim was the same used as last year, Sun Mammoth. In addition to having a large box cross-section for rigidity, they are very heavy, meaning they consist of a lot of metal, making them stiff. A lighter stronger rim could have been chosen however the cost would have been much greater. The Sun Mammoth is a very inexpensive and strong rim. The rim was laced to the hub using large 14- straight gauge spokes.

Tires were chosen based on availability and cost. Since our buggy wheels do not need to lean into corners like a bike tire, our only goal was to get a fast rolling tire. An ideal tire for this is a semi-slick mountain tire. The center section is solid so the tire will roll without the usual small bumps because of text that mountain bike tires have. These were out of stock at time of our order so we chose to use the Ritchey Z-Max 2.35 tires. These have a relatively solid center tread and were low in cost.

Fabrication Notes

The wheels were purchased and assembled to our specifications by a local bike shop. In building the wheels we had them laced offset with high tension. The high tension makes the wheel stronger, while giving a slightly rougher ride (not noticed by our riders due to 8 inches of suspension travel). The offset was towards the inside of the buggy. This makes the wheel able to handle slightly larger loads when it is the outside wheel of the turn. The outside front wheel in a turn takes slightly larger loads because of the weight shift forward and to the outside that occurs in a turn.


These wheels preformed flawlessly. They held up to all the bumps and hard turns we could throw at them. In certain conditions the buggy went up on two wheels and the wheels did not even need to be trued, this proves their strength.

Lessons Learned

The wheels held up to everything that was thrown at them without even the slightest sign of failure. In the future maybe try and make them lighter in weight using different components.


Introduction / Design Goals

Traditionally stopping the buggy has not been a major concern. The reports from last year's competition show brake power was not a major concern, even though the buggy only had a single brake on the left rear wheel. With this in mind we decided to you a similar set up as last year with only some minor changes. This year we wanted the front driver to control the braking and to have one on each rear wheel allowing more control.

Design Notes

Brakes were chosen to be at the rear wheels. The rear wheels were braked because they encounter less loadings than the front. The front wheels have to handle larger suspension and steering loads than the rear wheels. It would be possible to brake the drive axle closer to the frame but because of the large torques required of braking (multiples time larger then pedal torques). These larger torques would then have to be transmitted through the entire drivetrain requiring a much bulkier and heavier design.

We chose to use Avid mechanical disc brakes again this year. Disc brakes were chosen because our design does not have any structural members that could support a rim mounted braking system and the use of disc brake hubs allowed direct attachment of the disc to wheel. Mechanical discs were chosen over the more powerful hydraulic brakes because mechanical disc brakes are lighter, less expensive, easier to fit, and easier to adjust than hydraulic disc brakes.

Because the buggy folded at the center, as opposed to completely separating, the brake cables could be run to allow the front driver to control the rear brake. Tandem bike cables were used to get the extra length we needed.

Fabrication Notes

The brakes were a purchased item so no designing was needed. The mounting for them had to be custom done however. The use of steel uprights allowed threaded inserts to be welded to them and the disc brakes to screw right into them.

To eliminate some of the problem with the cables getting in the way they were run internal on the handlebars. This created a cleaner look and kept the cables from getting caught on everything. Where the cables exit the handlebars a gusset was used to reinforce the bar and a slot was drilled and ground to allow the cable to pass through the bar. The cable was then zip tied to the frame down the length of the buggy and out the A-arms to the brakes. It is important to note that the brake cables should be routed and cut only when the buggy is completely done. Try not to create sharp bends in the cable as this causes more friction and less control of the brake.


The brakes performed very well at the competition and during testing. The two brakes allowed for a slight increase in control and resulted in less skidding than one brake. Having the cables running all over the buggy was a slight hassle but well worth the control gained from having the driver control braking to both wheels.

Lessons Learned

The mechanical disc brakes are the perfect brakes for the buggy. Braking the rear wheels gives adequate stopping power and allows better breaking in turns. In the future more attention could be given to the cable routing. At the handlebar end the use of an angled cable guide (similar to those used on rear derailleurs) my help. Also, instead of running the cable housing the length of the buggy use the cable stops on the long straight sections (similar to those used on bikes). This would cut weight result in better performance (cable housing compresses and results in less braking power and feel) and make for a very professional look.


Introduction / Design Goals

The gearing was optimized for reliable function and gearing for above and below the most common speeds the buggy would be ridden at. Within the optimal gearing the hope was for easy adjustability gearing through simple changing of the gears. It was also hoped to reduce the stresses on critical components, such as the hub shifter, as much as reasonably possible.

Design Notes

The gear on the cranks should be mounted in the position that provides the best chain line with the shifter hub. Although single speed gears would be ideal because they are wider and less likely to allow a derailment, they aren’t available in all sizes, so standard shifting gears are used. Fancy chainrings come with features to aid shifting, such as pins and ramps. These shifting aids are not wanted for a single speed moonbuggy. The cheapest rings without ramps and pins will keep the chain on the best.

The 94mm bolt pattern chainrings have been found to be the smallest that will fit over the body of the shifter hubs. The smallest gear is placed on the shifter hub to reduce the gearing to the optimal range. The cog on the shifter hub is limited to what is available by the manufacturer. At the cranks the largest feasible gear is used, to minimize the torque at the internal hub shifter. The gear on the drive axle is then made reasonably large to make the gearing low as desired.

Spiders were designed to mount the gear to the internal hub shifters. The spider has a 94mm diameter bolt pattern with five 10mm holes. The spider mounts to the holes for the spokes on the hub. The hubs were 36 spoke, with 18 holes per flange. The spider was designed with holes to line up with the holes on the hub flange. The spider on the drive axle was similarly made with a 94mm bolt pattern.

Fabrication Notes

The gears for the SRAM shifter are compatible with Shimano Nexus gears, so the larger nexus gears were used. The nexus gear was mounted in the opposite orientation from what was intended because its offset improved the chain line. The use of larger gears at the cranks and the internal hub shifter reduced the chain tension between the hub and the crank. This will extend the life of the chain and minimize the chain tension induced stresses on the crank assembly.

The Final gearing for the front:

The final gearing for the rear

The spiders were made on the rotary table on the mill. The bolt pattern and cuts were specified by radial and angular dimensions. Extra narrow track chainring bolts were used to hold the chainrings to the spider. The bolts must be narrow since they are only holding one chainring, instead of two, to a thin sheet of steel.

Lessons Learned

The gearing worked very well, and functioned reliably. The change to the 94mm bolt pattern from the 2001 buggy’s larger standard pattern allowed for greater adjustability in the gearing. The use of narrower shifting gears instead of single speed gears did not cause unwanted derailments.