See Design Fall 2019/Winter 2020 for steering geometry used in MSXIV.
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Rack and Pinion
Utilizes a rack and pinion gear system to turn rotational (steering wheel) motion into linear motion, which in turn moves the other linkages to make the wheels turn.
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Meeting Minutes - Conversation with the FSAE team
The following notes are from a conversation with Theo Camilleri from the FSAE team, who was heavily involved with the design and manufacturing of the steering system.
- Rules of thumb
- Think of how you (or someone with little manufacturing experience) would manufacture a part. This helps with choosing good tolerances
- Document your entire design process
- Tolerances are incredibly important
- Problematic things
- Getting sponsor parts made - takes a long time. Manufacture on a reasonable time scale
- Making sure gears are easily manufacturable; DFM is critical
- Rack housing needs to have concentric holes
- Critical tolerances on gear meshing
- Ackermann Steering
- Figure out way to vary Ackermann settings; i.e moving it forward and backwards. Make position customizable because testing actual physical components is a great way to validate the system
- Look at skid pad and accelerometer to determine lateral scrub
- Bump steer
- They use Adam’s Car, a software for vehicle dynamics simulation. The hardest part is setting the car up, but past that it’s fairly straightforward
- Their car was already set up a while ago in Adam’s Car
- Goal; just minimize bump steer as much as possible; the FSAE team doesn’t have an exact number target
- Google bump steer testing and perform tests in real life; try locking out steering wheel, put a plate on the wheel, and figure out how much the plate turns when the tire is moved up and down
- They currently have 0.3° under max travel
- Bump steer is based on tie rods, i.e. it’s independent of rack position
- The FSAE team is almost never at full lock
- The FSAE team’s process for rack design
- Start with load cases
- Get driver input loads
- Validate loads with strain gauges on an existing system
- Look at other steering systems and their solutions
- Look at FSAE-specific racks
- The FSAE team’s rack and pinion design/manufacturing
- Spur gear is used for pinion; they’ve been doing this since 2013
- Designing for manufacturability is extremely important
- They use mostly 4140 and 4140 steel (steel that hardens well) for gears
- Gears get treated to 40-44 Rockwell C (Rockwell is a hardness scale for metals, and C specifically pertains to hard steels)
- They use 6061-Aluminum for the rack
- Aluminum parts are anodized for aesthetics
- Manufacturing company probably just hobs the gear
- Difficult to get sponsors to do helical and double herringbone gears
- Both rack and pinion are equally critical to get tolerances right for
- pinion was designed to last for 2 or 3 years; figure out how many comps it should be able to go through
- Other steering component design and manufacturing
- Determine car’s life and design less critical parts/ parts that can be maintained easily based on that period of time
- Rack Housing
- Concentricity of holes is critical for having a good rack and pinion
- They made it from separate pieces and used adhesives to bond parts together
- Steering column
- top of column is held together with a crossbolt
- no team really does belt drives
- Comments on manufacturing in general
- Tolerances are incredibly important; FSAE has standards for some areas
- The rack and pinion require tight tolerances
- Rack housing requires tolerancing for concentricity of holes
- Specific to the FSAE team - low tolerance: +/- 0.25mm, high tolerance: +/- 0.1mm. Anything higher is unattainable for manual machinists
- High tolerance places: concentric holes for housing, diameter of steering rack
- Set a target for how much compliance you want (they have 7º of backlash)
- Mechanical issues
- linkage failure is large problem at comp
- Quick release on steering wheel had biggest source of backlash
- Maintenance
- Buy new u joints every year
- Don’t bring u joints past max angle
- Grease rack every month
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- When cornering, there will be weight transfers between the two tires.
- The tire slip data will show us the lateral force that can be achieved with the tire at different slip angles, there will usually be a range of slip angles that will allow for a peak force before dropping off.
- After finding the normal loads on each tire by using the first set of equations in the paper above and the inclination angle of each tire (variables in the inclination angle computation equations were not explained), the raw tire slip data can be put into a software (Optimum T, has free trial). Using the inclination angle and normal load, optimal slip angle to reach peak lateral force can be found.
- If the outer wheel prefers a larger slip angle, then anti-Ackermann should be chosen, vice versa. Due to a larger slip angle arising from a larger turning angle. However, there is another factor affecting the type of steering chosen; tire drag.
- Due to the drag on the outer tire of an anti-Ackermann setup, it will produce a moment that will want to turn the car out of the corner. So usually an FSAE car will choose Ackerman due to the factor of tire drag.
- The example in the paper chose to do an imperfect Ackerman geometry to produce some effects of anti-Ackermann and minimize tire drag. Analysis should then be performed to check if the geometry can turn the smallest bends in the track.
tl;dr: we need tire slip data before we do anything, then we need to calculate tire drag. to see what type of geometry is suitable for the tires.
Running Anti-ackermann:
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Due to the lack of pre-existing experimental data, it was decided that pro-ackermann and parallel steering components will be designed and manufactured. The parallel steering geometry will be used to collect experimental data of the new ties in various toe conditions while running on a skid-pad. There is a correlation between toe and ackermann, so by running our own experiment, we can determine the amount of ackerman desired by the new tires. The only difference between the pro-ackermann and parallel steering components should be an adjustable tie rod (to adjust toe). Pro-ackermann will be run on MSXIV due to the majority of solar car teams running 100% ackermann.
References:
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