Determine Ride Height on a Frame Up Build?

[BBShark]

I would like to know how you determine the finished ride height before the car is finished for a car with a one of a kind suspension? I know that you have some flexibility with coilovers but I'm not sure how you account for tire "squish" when all of the weight (of the finished car) is on the tires. I thought this dimension might be negligible but I measured my daily driver and came up with 1 1/2 inches of difference on a 28 inch diameter tire (distance from ground to center of wheel = 12 1/2 inches). That is significant.

So, is there a fixed number you use or do the tire manufacturers give you this info or................?

[JRouche]

Originally Posted by BBShark

I would like to know how you determine the finished ride height before the car is finished for a car with a one of a kind suspension? I know that you have some flexibility with coilovers but I'm not sure how you account for tire "squish" when all of the weight (of the finished car) is on the tires. I thought this dimension might be negligible but I measured my daily driver and came up with 1 1/2 inches of difference on a 28 inch diameter tire (distance from ground to center of wheel = 12 1/2 inches). That is significant.

So, is there a fixed number you use or do the tire manufacturers give you this info or................?

You have some good questions. Tire size will vary from manufacture, so the shouldnt be too much of an issue. I have GY F-1s and they dont compress much (I could measure) when loaded. A 28" dia tire sounds like it has a tall sidewall. 26" is pretty much the standard, give or take a lil, for preformance tires. No matter the wheel size. But if you have a 20" rim with a 26" tire its gonna deflect less than a 15" rim with a 26" tire. So with that you should know what size wheel you are gonna be thinking about. But tires are gonna be pretty close.

Now... Overall ride height with all the suspention parts is more difficult to figure. You have pinion angles to think about, trailing arm angles, A arm angles, total vehicle weight as finished, shock or coil over mounting placements, additional linkage arm placements, say like a pan hard bar or watts link, all affected by ride height.

My car is a one of a kind as far as suspension and determining ride height. I had to put it all down on paper first. And if you really want to do it right you make a scale drawing and "line" the entirety of it all up. You can get roll centers, front and rear, roll axis, center of gravity with some scales, roll couple, anti squat, and a whole lot of good figures. It starts on paper before you even make the first weld. Ooops, I kinda went off the road huh from your question LOL But yeah, the tires are an easy step. The rest is just fun. JR

[BBShark]

OK, maybe I'll rephrase the question.

I see a lot of people on here and other forums laying out suspension geometry. I have always seen this done as though the tire has no "squish". This makes the assumption that the centerline of the spindle is centered on the tire.

In my example above, that would be completely wrong. The centerline would actually be 1 1/2 inches lower at ride height! That's significant, big time.

So, can someone tell me if this number is published by the tire manufactures or if it is ignored by builders or if anyone has any information about this or?

[LateNight72]

It really depends on the tire, pressures, etc. For instance, I will be running 30 series tires on the front suspension I am building (the one with a thread a couple down), and 35 series on the back, whenever I get around to designing an IRS.

The tire "squish" is negligible in a 30/5 series that's properly inflated.

[BBShark]

Originally Posted by LateNight72

It really depends on the tire, pressures, etc. For instance, I will be running 30 series tires on the front suspension I am building (the one with a thread a couple down), and 35 series on the back, whenever I get around to designing an IRS.

The tire "squish" is negligible in a 30/5 series that's properly inflated.

Even if it's a 1/2 inch, it's not "negligable". Take a look at what it does to your IC calculation in your thread. Seems like if you are doing these layouts for geometry, you want to start with the most accurate information.

So, do the tire manufacturers give you this info or................?

[LateNight72]

To add to your question,

I redimensioned, taking 1" away from the bottom half of the tires. Mind you 1" of "squish" is very generous for track tires.

Anyhow, the only thing it changed was my rollcenter. It increased my rollcenter by a total of 0.054"

So yes, my notion of it being negligible is sufficient.

[mmliljoker]

if you build everything to function properly at ride hight that you want i don't think that the tire squish is going to be a problem. Think of it like this the tire is squishing, the suspension is not moving. As long as your front lower control arms a as close to parallel with the ground at ride hight during fabrication you will be fine. you will have enough adjustment in the suspension to fix any tire squish woes. There is no possible way on a project like that to know the exact total finished weight of the car and that would make a lot bigger diff on ride hight geometry then tire squish. Don't over think it. set every thing up at ride hight and adjust out. One more thing put the calculator away and have fun its only metal.

[B_Alley]

if you wanted to get really fancy...

in a race setup, you want the vehicle as low to the ground as possible, and you can find that once you know the vehicle's wheel rates (a function of the tire spring rate and the spring rate of the suspension, effected by motion ratio) and know how much lateral acceleration and longitudinal acceleration the car will see...

Also, the tire will compress under static loading because it is a spring itself. If at a certain inflation, the tire spring rate is 2,500 lbs/in and you have a 3300 lb vehicle w/ 50/50 weight distribution, thats almost 3/8" of compression, just in the tire alone...