Upright Camber Stiffness

[Dave K]

Design question:

When you guys design your uprights, what is usually your target camber stiffness? Mine so far are looking to be in the neighborhood of +.25 degrees of camber gain under about 1.5g of cornering and .5g of braking. This is all due to FEA results of course...

[J.R.]

This is my first year, but I was under the assumption that "we want no compliance within the suspension system or its attachment to the sprung mass" CS, Tune to Win, but I haven't done anything with the uprights yet.

Is this "no compliance" actually practical (down to negligible camber changes that won't influence tyre F&M)?

[PSUAlum06]

on production vehicles, compliance is actually designed into the suspension and used as a tuning knob to adjust ride and handling characteristics. while it may be tempting to do the same with a FSAE suspension, my suggestion would be to keep it simple and design everything to be as rigid as possible.

my other recommendation is that if you have a sponsor with a K&C rig you should see if they're willing to put your car on it.

[Dave K]

Ah yeah i might have worded that question a bit wrong now that i look at it.. of course it makes sense for them to be as stiff as possible, but i want to keep them lightweight as well.

I should ask what you guys see during your FEA/testing...

[RacingManiac]

0.25 degree sounds like a lot...for us who usually run less than 4 deg of neg camber thats like 6% camber change on upright deflection alone. On top of everything else that deflects by whatever amount....I am sure you can do better than that without having to add too much weight to it. How much does it weigh now in the model?

[flavorPacket]

.25 deg/1.5g is a good target for total stiffness (aside from wheels), not upright stiffness. What is your upright weight target?

[BillCobb]

You might want to head into the lab real quick. Your wheel bearings will have a large effect on your camber (degrees) and camber GAIN) deg/degree roll, deg/N Fy, deg/deg steer, and deg/Nm Mz.

Don't fool yourself into thinking that your frame plus chassis plus wheel and bearing compliances can be ignored.

This is best managed by identifying what vertical and lateral forces will be expected. Get this right and you will rewarded with a winning design.

[exFSAE]

You and your "science" and "engineering" terms, Bill...

Just make your uprights and whatever outta some chro-moly.. harden 'er up.. pretty much can't even bend that.

In all seriousness though.. with hindsight I'd design for 2-10x more stiffness than my target, because you'll get compliance here and there and everywhere. Bearings.. joint stiffness (killed me!).. compliance in mounts.. in bolts.. WHEELS(!!)..

[Dave K]

Yeah target weight is around 2 pounds for an aluminum (7000 series) upright...

I still have a bit more to go with FEA and design iterations, just wanted to get a feel of what to aim for.

[murpia]

Originally posted by BillCobb:
You might want to head into the lab real quick. Your wheel bearings will have a large effect on your camber (degrees) and camber GAIN) deg/degree roll, deg/N Fy, deg/deg steer, and deg/Nm Mz.
Good advice. Bearing stiffnesses are something you just have to measure for yourself, with rigourous attention paid to repeatable preloading procedures and measurements.

Don't be surprised if the wheel rims + wheel bearings contribute more that 50% of total deflection (chassis + suspension + uprights + wheelbearings + axles + rims).

Regards, Ian

[Bill_Murray]

Dave, I was shooting for less than 0.05 degrees per g lateral camber loss and less than 0.08 degrees toe per g longitudinal from just the upright alone in FEA when I did our upright last year. The numbers were based off that stiffness of our previous uprights in FEA. The weight of the final upright was under one pound. Material was 6061 to facilitate welding (make sure to post heat treat). Box structures work miracles for stiffness per weight, but they are more difficult to manufacture.

As has been suggested though, the bearings, wheels, spindles, A-arms, and chassis all play a part and cannot be forgotten. Upright stiffness alone is hard to measure on the car. If you run FEA on the whole system and then look at where most of the deflection is coming from it will give you insight into where to cut weight. This total system deflection predicted by FEA is then much more easily measured on the car to verify your FEA work.