Aero Placement And Mounting
Wings have been a common feature on FSAE cars for many years, but the placement and mounting of wings is often overlooked or underthought. In this article I’ll cover some of the important aspects of placing wings on your car. For articles about if you should put wings on your car, check out this article from Skitter Yaeger.
Where the wings can (and can’t go):
If you look at the bottom of most FSAE cars you’ll see one thing in common: damage from hitting the ground. It may surprise most of your team just how much ground clearance you will need. To easily visualize this for all members of your team, create a surface in the full assembly of your car that represents the ground during every movement: full pitch forwards and backwards, full roll left and right, every combination of those, as well as full suspension compression. Do not forget, the tires are not rigid bodies and will compress, and as much as we all wish it would be the ground is not flat! Add a safety factor to cover those pesky bumps!
In the image below I’ve constructed this surface by drawing a sketch on the centerline plane that defines the maximum pitch in each direction and the maximum heave, then drawing a sketch on the front plane that represents the maximum roll angle in each direction and intersects the previous sketch, and then sweeping the second sketch along the first.
You can limit how much ground clearance you need by limiting your suspension travel. FSAE 2023 V3.1.1 requires 50mm of usable wheel travel but you can engineer how much of that is compression and how much is extension through a well designed helper / tender spring setup. Alternatively, you can adjust wheel rates, roll rates, and anti-geometries to develop a car that will limit body movement during weight transfer (ignoring tire compression) but this can come at a serious cost to your mechanical grip.
The front wing has become part of the suspension.
You can also design the shape of your front wing to ensure ground clearance through roll by raising the outboard ends of the wing. This has the added benefit of maintaining downforce, or even increasing it, during roll.
A front wing assembly which slopes up as it moves outwards allows for ground clearance during roll.
“But why is ground clearance important? Last year we ran our wing really low and just repaired it every time it broke.” FSAE 2023 V1.4: Ground Clearance says:
“Ground clearance must be sufficient to prevent any portion of the vehicle except the tires from touching the ground during dynamic events.“
When your front wing hits the ground it stops making downforce and instead lifts the tire off the ground. This causes three problems:
It makes the car very unpredictable because the outside tire suddenly loses grip.
It massively increases the risk of breaking the front wing.
During any dynamic event at competition, if any part of your car breaks off, or even could break off and damage the track surface or pose a danger to anyone, you can be disqualified.
FSAE 2023 D3.9: Vehicle integrity says:
“Officials may revoke the Inspection Approval for any vehicle condition that could compromise vehicle integrity, compromise the track surface, or pose a potential hazard. This could result in DNF or DQ of any Dynamic event.”
At FSAE Michigan 2018 the endurance event tested cars’ ground clearance by forcing the drivers to go up onto the banking as part of a minimum radius turn. This caused countless DNF’s through the loss of front endplates, not to mention many penalties for cutting the course to avoid the risk. The teams who had their front wings mounted to allow for approach angle (and the teams who made their wings strong enough to repeatedly scrape the ground) were fine.
Living with wings:
The positioning of your aerodynamic devices can also make working on and transporting the car much more difficult. When we [Rensselaer Motorsport] designed our 2017 car, we constrained the placement of the diffuser to ensure that we could leave it installed from the time our car left the shop to when we returned; so the diffuser had to clear the ramp of our trailer and allow room for a safe jacking point (rules required anyways). We may have given up a small amount of downforce, but instead of having to transport the diffuser separately and then spend time installing it, we could leave the diffuser installed for everything but oil changes; and the extra test time was worth whatever downforce we lost. We also fastened the diffuser to the car with hardware that could be accessed entirely from above, so we could lay the diffuser on the floor, roll the car over it, lift it 2” and install nuts on the studs. The whole process took about 5 minutes and 4 people.
The same issues go for your front and rear wings, if they can be left on while you service the car and transport it to and from your test track you can save lots of time. If the wings have to be removed for transport, ensuring that the wing is installed on the car in the same place every time will save you setup time at the track; designs with 6 degrees of freedom which all need to be adjusted every time the car comes off the trailer waste valuable test time.
If you can make the height of your wings easily adjustable you can tune their height to match any changes you make to ride height, spring rates, bump stops, roll bars, etc. This kind of aero validation data is very impressive during design judging (not to mention it’ll make your car faster!), and running skidpad a few times with different wing heights does not take much time. Lastly, do not forget that the height of your wings is measured without a driver in the car (FSAE 2023 T7.3.1.b).
Unexpected loadings:
While it’s important to design your wings and mounting to withstand the usual loads, don't forget about the unusual load cases:
What happens to the wing and mounting when the car spins out and goes backwards at 60mph?
What about the drag caused by the endplate as the car spins?
What about when that freshman leans on the wing while trying to look cool?
What about when a cone flies up and hits the rear wing?
What about when a cone gets lodged under the front wing at 70mph?
What about when a volunteer has to push your car by the rear wing to move if off track?
It’s very likely that some of these forces are higher than the downforce or drag produced by the wings.
Good vibes only:
When my team built our 2017 rear wing mounts we designed them to withstand every load case we could come up with, but we neglected to analyze the natural frequency of the wing assembly. Unfortunately the natural frequency of the rear wing assembly was about 2 Hz, which was close enough to the suspension frequency that after the car would go over a bump the rear wing would begin to oscillate about its fore/aft roll axis. We solved this by pre-stressing our wing mounts to raise their natural frequency and supporting them with two rods mounted below the wing, but it's something that shouldn’t be overlooked.
Before the two rods under the wing were added, the rear wing would oscillate whenever the car hit a bump.
Build A Better Wing Cable:
GFR 2022
Whether they are a last minute addition or part of your aero package design from day 1, tension cables can be a critical part of your aero assembly. There are many ways to make these cables, and while teams have had success using parts bought from the hardware store they can be heavy and cumbersome for a lightweight racecar. Thankfully, small aircraft use similar tension cables and there are a variety of crimp-on cable ends available.
For example, if you use two MS21259 stud wire ends in different thread directions you can mount a rod end on each end of the cable and use it as a turnbuckle. Or you could bolt a tab onto your wing or diffuser and use an MS20667 fork end, or an MS20668 eyelet end to attach the cable.
An MS21259 stud end with an aluminum turnbuckle to connect to a rod end allows the cable tension to be adjusted in place.
The options are plentiful, and suppliers like Aircraft Spruce will make custom cable assemblies for you at incredibly reasonable prices. Alternatively, you can use a hydraulic wire crimper to crimp the ends yourself, but always remember to safely test your cables to a load higher than they’ll see in use before installing them on the car.
While the hardware store may have something that will work, there is almost always a better solution for an FSAE car. Don’t be afraid to go looking for what you want in another industry’s parts bin.