In the initial phase of designing the steering system, many ideas including the design from the previous were discussed and evaluated. We did a little research and found a great page, http://www.ihpva.org/people/tstrike/steering.htm, which gave us other options to look at. Many of the new ideas were unfeasible for our design, involved too many moving parts with numerous pivots or were too time consuming to manufacture. Returning to the team’s initial goal of designing a better Moonbuggy from the lessons and mistakes learned from the previous year, the team decided to simply create a more robust steering system with better handling characteristics based on last year’s model. This model was simple, easy to manufacture, proven to work, and gave the team the incentive to replicate it in this year’s buggy. However, this design was not without its flaws. First, we needed to recalculate the Ackerman angle to accommodate the proper turning radius. Ackerman angle should dictate the steering geometry so our wheels have the proper angle in a turn- the inside wheel must turn a sharper angle than the outside wheel due to the fact that the inside wheel moves through a smaller arc. The Ackerman angle on last year’s buggy was too large and we wanted to correct this on the new buggy. Secondly, mounts were not secure enough, causing too much slop in the connections and pivots. Tighter connections ensure accuracy in the steering and also decrease stress throughout the steering assembly. Therefore, mounts and connections were all redesigned to provide a more robust steering system. The design was then drawn in a 2D modeling program to simulate the steering system in motion and confirm our calculated dimensions.
Before any dimensions could be determined, the Ackerman angle had to be calculated to provide the necessary turning angles of the inner and outer wheels. There are two methods to perform this calculation. The simplified method states that the toe link should point at the center of the rear differential. A more comprehensive method involves the use of formulas taking into account the turning radius, wheelbase and the width of the buggy. Both methods were performed and compared. The simplified method yields an angle of 18.94 degrees. The comprehensive method can be calculated as follows:
Wheelbase length =
Width of buggy (inboard side of wheel) = L = 40”
Turning radius = 20’
Length of toe links (measured from pivot to pivot) = l = 7”
Known values: Neglecting the N/l terms and solve for
Use q to find M and O:
Use M and O to find N:
(These equations were taken from The Automotive Chassis by P.M. Heldt)
Substituting N and l in equation (1) yields = 19.31 degrees
Last year’s buggy had a total Ackerman angle of 38 degrees with the same turning radius. This year we reduced it to 24±2 degrees, 10 degrees from the toe links and 14 degrees from the rotator (angle between the line from the pivot on the aluminum mount to the tie rod pivot with the vertical).
In designing the handlebars, many factors were taken into account. The position of the handlebars along the sides of the frame was placed as far forward as possible while leaving enough clearance (about 1”) between the edge of the rotator and the front gears. We found that this was still not enough for an ergonomic steering position. The upper portion of each of the bars was angled forward. The handlebars are positioned 5” away from either side of the frame for a suitable width, both from a rider’s viewpoint and a design consideration. Initially the handlebars were to be mounted on tabs that extended out from the side of the frame. However, the handlebars 5” away from the frame applied considerable stress to the .035” steel frame. This problem was solved by welding a 1” square cross beam, 22” long made, from .035” steel and attaching tabs on either end of the beam mount the handlebars. Finally, the handlebars had to mounted onto the tabs without any play. Steel rings were welded onto both sides of the handlebars with press fitted ABEC1 ball bearings. A ¼” 20 bolt was inserted through the bearing with two spacers and fastened with a lock nut. This arrangement allowed for double shear, which minimizes the stress on the bolt and the bearing. Similar ball bearing arrangements were used on the rotator’s pivot.
We decided that the rotator would be placed inside the frame instead of underneath it since this renders the system more compact and increases stability because the weight of the rotator assembly will rest on the frame instead of hanging from it. This arrangement also provides better clearance from the A-arms because it only needs to clear 3” of A-arm travel upwards instead of 5”of A-arm travel downwards. A decision also had to be made about whether the assembly should be welded or clamped to the frame. It was proven to be advantageous to have the assembly clamped to the frame due to adjustability without affecting the structural integrity of the frame. A set of aluminum mounts was designed to hold the rotator in place while clamped onto the frame.
In addition to the toe link angular offset, its location along the upright
is also crucial. If the geometric relationships are not correct, bumps
can produce steering inputs. The toe links are placed along the upright
according to a proportional relationship in relation to where the rotator
is located between the pivots of the upper and lower A-arms. The pivots
of the upper and lower A-arms on the frame are 6” apart and the rotator
is located 1.5” from the bottom A-arm, which is 25% of the total distance.
Relating that same percentage to the uprights with A-arm pivots separated
at a distance of 9”, the toe link should be positioned 2.25” from the bottom
A-arms on the front uprights.
All connecting rods of the steering assembly are made from .5” x .035” 4130 chromoly steel. Threaded steel inserts were manufactured so rod ends could be threaded into each end of the connecting rods. The rod ends are ¼”-20 thread obtained from National Rod Ends ( http://www.Nationalrodends.com ). Each connecting rod is made with one right and one left-handed threaded insert. This design allows the overall length of the connecting rod to be adjusted.
The rotator is made from two ¾” square tubes of .035” thickness, welded together to form a T shape. We used square tubing because it was easy to integrate into the design and lightweight. Square tubing also allows for the rod ends to sit inside a slot we cut into the sides. These were fastened by a bolt, which caused the bolt to be in double shear. Two .035” thick plates were welded on both side of the rotator to hold one end of the tie rods. Material was cut from the sides to reduce the weight.
To eliminate the play in the handles and rotator, 5/32” OD, ABEC 1 double shielded ball bearing were used, a pair on each handle and another pair on the rotator. Steel rings were machined precisely within a few thousandths of an inch and then welded concentrically onto the piece. The bearings were then press fitted into the steel rings.
The mount clamp, machined from aluminum stock, holds the rotator in place as it rotates about an axis. The mount is clamped instead of welded onto the frame to allow for adjustability.
The toe links are made from 1”square .035” thick chromoly steel stock. One end of each of the toe links is angled 10 degrees inboard and one welded onto the side of the each front upright.