Ideal Geometry

[parsonsj]

I occasionally dabble in design (consulting only) for folks who want to build their own suspensions. I've been working on one that uses C5 components. Those numbers look something like this:

Roll Center location Static: +2.0"
Roll Center Migration In Roll: 0.62" / Degree of Roll
Roll Center Height in Dive and Lift: just about 1 to 1
Caster: ~8 Deg.
Caster gain per inch of travel: 0.5+ Deg Pos. / Inch of Travel
Camber: -1 Deg (adjustable to whatever)
Camber gain per inch of travel: 0.9+ Deg Neg. / Inch of Travel
Toe: whatever
Bump steer: less than .005 for up 1.5" dive, .010 for up to 1" lift, .020 for 2"
King Pin angle: 9.9 Deg.
Anti-dive: 23%

As Matt mentioned, the hardest part of the whole design is the packaging: finding workable racks, getting the engine in the car with the rack, figuring out how to get a workable sway bar, and, in general, making sure none of the parts hit each other as the front suspension moves.

jp

[parsonsj]

My rack in II Much is solid mounted. I don't notice it being any different than my wife's Mazda or my big-ass Duramax in terms of vibration or other annoying behaviors.

jp

[ccracin]

Thanks John. Since you do some design work, what are your thoughts on the process of designing suspension system? I threw some things out in an earlier post as to logical steps in mapping out a suspension system. Where do you jump into the water? I think your opinion will be valuable to alot of people. 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? Anyway, I hope it did.

Thanks Again,

[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

[chevyIIpost]

Originally Posted by ccracin

They talk about improving the handling of their cars but have no idea what it takes to do that. They don't even know what questions to ask. They only know what they see in ads and from marketing wizards. The "average Joe" has never heard the terms Matt, You, and myself are throwing around here. If someone is going to spend thousands of dollars to upgrade their car or truck (we can't forget the trucks ) they should actually get an upgrade rather than some pretty tubular control arms that don't do anything but be tubular. Don't anybody get upset that sells tubular control arms! They have a place, but I just thought an open discussion about these principals would help alot of people.

Chad, this is the exact road I have travel over the last 5 years. My original thoughts where for a S-10 pickup suspension but I didn't want a salvage yard car. I went to the school of "Hard knox$" with a Jim Meyer set up because I had no clue what questions to ask let alone what the correct answers might be. I read and learned some after this purchase and found that what I had welded in my car had bad bump steer, only a couple of degrees of caster, a bad camber curve, no anti dive to speak of, and poor ackerman, etc. Basicly a street rod suspension for 40 Ford. I am now finishing up another learning process on a c5/c6 set up that has great specs and should do the job very well. Now I know enough to get in trouble when on my own but I can at least ask good tough questions and sort through the posers and the real doers. It just cost me $$ and time to learn this. I would hope this thread could help others avoid this long painful process.

[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.

[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

[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.