Rear outboard suspension design questions

[Mike Huang]

Hi guys, I am new to FSAE, and I am now just started to do research and design the rear outboard suspension. There are very few book/website that explain these components in detail, and I have searched all the related post I can find in thie forum, but just got myself confused.

Can anyone kindly explain to me that how many forms/combinations of upright/hub/spindle can be used for rear? what are the pros/cons? can a fixed spindle used for rear?

Thank you

[Mike Huang]

Hi guys, I am new to FSAE, and I am now just started to do research and design the rear outboard suspension. There are very few book/website that explain these components in detail, and I have searched all the related post I can find in thie forum, but just got myself confused.

Can anyone kindly explain to me that how many forms/combinations of upright/hub/spindle can be used for rear? what are the pros/cons? can a fixed spindle used for rear?

Thank you

[Drew Price]

Assuming that you will have rear wheel drive, it would be very difficult to drive the wheels with them rotating around a dead spindle!

You will have some sort of halfshaft or rear axle assembly powered by the engine, and you have to couple that to the wheel flanges. Usually there is some sort of drive hub that passes through the rear uprights to drive the wheels.

Look through photos in the old 'Competitions' threads.

Do some google image searches, like:

'rear suspension uprights' (http://images.google.com/images?gbv=2&ndsp=20&hl=en&q=rear+suspension+uprights&start=0&sa=N)


Best,
Drew

[Mike Huang]

Thanks for reply

Am I right that torque from dirve shaft goes to spindle (spin indside upright on the wheel bearing) through CV joint, and then to the wheel hub by spinle, and the wheel rim is bolted onto the hub?

if I am right, the question is where does the brake rotor sits? is it between the wheel rim and hub or between hub and upright/bearing? Thank you

[Drew Price]

Yes, you're right.

Usually the brake rotors are between the hub (wheel drive flange) and the upright, however, on our car last year we have the outboard rear brake rotors mounted to the back of the CV flanges on the inboard side of the uprights.

Just remember you have to hang the brake calipers off the upright somewhere, and you want to try to remove any bending in the caliper mounts -- the force going through them can be fierce.

Best,
Drew

[Mike Huang]

Wondering what are the pros and cons for each design? the only thing I can think of are making the scrub smaller and can change brake disc without taking the wheel off?

[Drew Price]

Right, but why does scrub matter on the rear wheels? Will you be steering them?

Also, if they are inboard you still can't change them with the wheel on, unless you split them in half to get them off the halfshaft.

What about cooling?

What about unsprung mass / inertia?

What about heating the wheel bearings?

What about heating the CV joints?



It's mostly a packaging exercise, we did it because it packaged nicer with the bearing / hub / CV package we used for a given track width and a-arm length.

Best,
Drew

[Mike Huang]

Thanks Drew, Seems I came out some pretty stupid answers...

I understand the cooling bit, but how can it make difference on the unsprung mass and inertia? only thing i can think of is it makes the weight distribution closer to the centre of the car...

Cheers

[Drew Price]

Definitely not stupid questions. Don't stop asking questions, it's the only way to learn!


<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Huang:
.... how can it make difference on the unsprung mass and inertia? only thing i can think of is it makes the weight distribution closer to the centre of the car...

Cheers </div></BLOCKQUOTE>



Exactly. Depending on how much the rotor weighs, how much the caliper weighs, and how far inboard you move it, it has to move through a smaller arc when it's closer to the center of the car. How much smaller is dependent on a lot of things, and usually that isn't a good enough reason to justify it on it's own, but if you're moving a good bit inboard it could make a difference.

But only if your driver is sensitive enough to tell.

Best,
Drew

[PatClarke]

Quote Drew
"Right, but why does scrub matter on the rear wheels? Will you be steering them?"

Oh Drew, think about what you just wrote =]
Compliance steer at the rear of the car is a very undesirable trait.
It is certainly something I look at when assessing a car

Cheers
Pat

[Drew Price]

You're right Pat, what I meant in my head was the difference in the inch or so decrease in scrub just from packaging the rotors inboard of the uprights, not if you are going to add 6 inches and run the thing on actual kingpins or something like that.

Hopefully most of the rear steer compliance if affected by a good toe control base anyways, and the rear bearing and drive assemblies have to be pretty large diameter to accomodate the drive stubs anyways, so at least on our 2008 car we had a pretty good hope of having decent toe stiffness.

I was not thinking large scrub radius implied large moment arm for the Fx reactions. You could achieve the large moment with a small scrub radius with a lot of KPI, but that would be just as silly as running 6" of scrub!

Though, a little bit of thought and you could work in some stabilizing toe-in under throttle, and toe-out under the brakes like street cars do with their floppy-handed rubber bushings, but I don't really have the time to investigate such niceties!

Best,
Drew