Ideal Geometry

[parsonsj]

Originally Posted by Chad

Do you agree that once you get the best geometry on the front end that can be packaged properly for the vehicle that you can tailor the rest around this starting point? Did that make sense?

Sorry, but not really. What do you mean by "the rest"?

jp

[ccracin]

Originally Posted by parsonsj

Sorry, but not really. What do you mean by "the rest"?

jp

Sorry John,

What I mean is what process would you follow to design a complete chassis? I personally start with what wheels and tires will be on the car. Then I do the front end. Then the rear, weight & balance etc. I guess I'm just looking for some rules of thumb to help people evaluate what to look at. Hopefully that helps. I started this thread on front ends, because it is an area that people spend alot of money on and can sometimes take a bath doing it.

Thanks,

[chevyIIpost]

All suspensions have some bump steer the attempt is to minimized it as much as possible. But with the bumpsteer that is left, if you have a choice, do you want it toe out or toe in during bump steer?

[Silver69Camaro]

Originally Posted by chevyIIpost

All suspensions have some bump steer the attempt is to minimized it as much as possible. But with the bumpsteer that is left, if you have a choice, do you want it toe out or toe in during bump steer?

Doesn't matter (for the most part). What you want is for the correct steering in a full cornering situation (body roll), corner exit (body roll + rear squat/front lift) and corner entry (body roll + front dive/rear lift). You also want to consider what it does on acceleration and braking.

[Silver69Camaro]

Chad,
I know you didn't ask me but I pretty much set the wheels and tires last.

For some reason, people always want to build a car around an existing set of wheels, a back seat, and a fuel tank. None of those should matter. You should never limit yourself to a specific backspace, but obviously you need to keep it reasonable so a manufacturer can actually produce the product (this is what JP is referring to). But here's what I do:

1. Build front suspension first; have ride height, approx. tire diameter and ride height, and CG location.
2. Build rear suspension to accomodate front suspension for solid axles; if IRS is used, build front and rear suspensions together.
3. Repeat, because the first design never works the way you thought.

But first and foremost, you need to be realistic about what suspension properties you want. 99% of the people here want the appeal of a race car, but don't actually want to drive one. If it is a true street car, even with some moderate autocross or lapping, don't get overboard with caster and camber gain; and choosing a suspension frequency "race car" high will make it terrible to drive on real roads. Be realistic.

[parsonsj]

Originally Posted by Matt

If it is a true street car, even with some moderate autocross or lapping, don't get overboard with caster and camber gain; and choosing a suspension frequency "race car" high will make it terrible to drive on real roads. Be realistic.

Amen. Double Amen. Sing it to the heavens, my brotha!!!

Race car suspensions aren't really that enjoyable on the street. No doubt it's cool to take freeway off-ramps at triple their rated speed, but the rest of the time a twitchy, tire-wearing, bone-rattling, NVH on steriods ride will just annoy and tire you out.

Make your shocks adjustable, keep your caster and camber at reasonable levels, don't go for too-quick steering, and use bushings with some compliance.

jp

[parsonsj]

Originally Posted by Matt

For some reason, people always want to build a car around an existing set of wheels

You're right: it's all about making sure the suspension will fit inside the car body. Reasonable choices here make the rest go much better.

jp

ps. And Matt, it was your boss who taught me to choose the wheels and tires first. Art wouldn't make me a chassis until I spec'ed the wheels.

[parsonsj]

Oh, ok, I think I understand now.

Sure: wheels and tires first. Without that, you don't know anything.

Then begin iteration:
1. ride height
2. track width
3. front suspension
4. rear suspension
5. Go to 1.

After that, you've got (in loose order) steering linkage, drivetrain, pedal linkage, seat, a/c, exhaust, cage, floors, trunk, wheel wells, firewall, radiator, etc.

jp