Ride Height, Suspension Geometry and Tire "Squish"?

[BBShark]

I would like to know how 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, big time.

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! The suspension geometry would be COMPLETELY different from what had been calculated!

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?

[GetMore]

I'm not sure how that "squish" would have an effect on geometry. The suspension components are all going to be at the same angles regardless. The only thing that can change is the ground clearance and possibly the rake of the vehicle (from the front squishing more than the rear.)
One other thing to keep in mind is tire pressure. Most OEMs use lower pressure for better ride, while sacrificing some handling. Add more pressure and you will decrease squish.

[BBShark]

I'm not sure how that "squish" would have an effect on geometry. The suspension components are all going to be at the same angles regardless. The only thing that can change is the ground clearance and possibly the rake of the vehicle (from the front squishing more than the rear.)
One other thing to keep in mind is tire pressure. Most OEMs use lower pressure for better ride, while sacrificing some handling. Add more pressure and you will decrease squish.

If you maintain the same ground clearance that you planned on when you designed the suspension, and now the centerline of the wheels is 1 inch lower, there is going to be a major effect on geometry.

If you reduce ground clearace to keep your geometry, now your calculations are wrong (because you used the wrong ride height). Also, if you could get by with 1 inch less ground clearance, you probably should have done that when you laid out the suspension. After all, you want to minimize ground clearance to reduce CG.

[sik68]

I too, don't see how tire deflection affects your suspension geometry, other than ride height. Your springs are supporting the same load regardless of the pressure in the tires.

As to how you got 1.5" of tire "squish...no way was that a properly inflated tire. 1.5" would produce a huge rolling resistance.

[BBShark]

OK how about this. You are building a car an a frame jig and you have designed the lowest part of the car to have 4 inches clearance to the ground. You have calculated the pickup points of the suspension based on the roll center and other parameters based on this 4 inch clearance.

You finish the car and it is now at 3 inches of clearance because of tire squish. So what do you do? Raising the car 1 inch changes your geometry. Leaving the car as is is OK if you are comfortable with occasionally dragging the oil pan (for instance) however points that you have calculated like IC are now wrong.

As far as a properly inflated tire, I measured my Dodge Magnum with 225/60-18 tires inflated to 32psi (factory recommendation). Once again, I measured 12.5 inches to the center of the tire with a 28 inch diameter tire. 14-12.5 = 1.5 inches.

So, can someone tell me if this number is published by the tire manufactures?

[Derek69SS]

Well, I'd assume most of the guys serious enough to build a full custom chassis car on a jig aren't going to be running 225/60s. ;) That's a tall side-wall, and probably soft too.

I haven't taken any measurements, but I doubt the "squish" on my 255/45/17 Kumho Ecsta MXs is anywhere close to 1.5".

[BBShark]

I used the Magnum as an example. However, that does not make my question irrelevant. It doesn't really matter. Even if is 1/2 inch of squish, it is important (take a look at IC calculations with a 1/2 inch difference).

If you don't believe your tires have 1/2 inch of contact patch, you probably should measure.

So, can someone tell me if this number is published by the tire manufactures?

[LateNight72]

When a tire "squishes" it does not change control arm instant centers. The only thing it will change is rollcenter (in relation to the ground).

Well, I'd assume most of the guys serious enough to build a full custom chassis car on a jig aren't going to be running 225/60s. ;) That's a tall side-wall, and probably soft too.

I haven't taken any measurements, but I doubt the "squish" on my 255/45/17 Kumho Ecsta MXs is anywhere close to 1.5".

A 30 series tire (what I am going to run) will not really squish at all.

[GetMore]

I see what you are saying, but as you can tell you misdescribed your concern.
The squish doesn't change the geometry, but as you mention, raising the car to compensate for the loss of ride height does change the geometry.

I'm sure the experts have learned to account for the tire squish when they decide on ride height and where the suspension will be sitting.

I'm also pretty sure that no tire manufacturer publishes numbers for how much the tire will compress when installed, since that depends on too many factors, such as: tire pressure, axle weight, rim width, section width, overall diameter, and sidewall height.

[The WidowMaker]

i accounted for about 1/2", but im running a shorter sidewall. i only have a little over 3" of sidewall, so 1.5" would be pretty flat.

i dont think anyone is going to have those numbers. its going to vary by sidewall height and strength, rim width, weight supported....... and you'd have to do that for every size for every tire model.

[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 0.05"

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

[MonzaRacer]

As an alignment tech and chassis tuner. Your points have little merit as most low profile tires will lose very little height upon sitting on ground.
I have set up cars with anywhere from 35s to 70s and have yet to see many cars have any real ride height issues. Namely because you will place a fresh car on the ground, allow for new springs and such and most cars sit high in the beginning.
AS such I have yet to find many cars that have such majors issues.

[79-TA]

Right now, I run 245 60R15 BF Goodrich Radial TA's at only 28 psi on my 3550 pound Trans Am. I just measured the diameter of one front wheel/tire both horizontally and vertically and there was only about a thee quarter inch difference between the two diameters. Remember, this is at a low 28 psi, so I don't see it being a huge issue.

Also, I don't see how it would affect the actual suspension geometry (assuming equal squish on all corners.) The suspension only makes its physical reactions with what is essentially the center of the wheel. All of the instant centers will remain the same relative to the wheel, but will be lower relative to the ground. Basically the only effect (past sidewall flex and other large profile tire issues) would be lowering the car's center of mass. I have only theoretical experience here, so I should really leave this issue to the more experienced members.

Lastly, if a measurement as small as tire squish is a large concern, it seems that one would want to consider the amount of tire growth at speed as well.

[The WidowMaker]

you guys are missing his pt;

If you maintain the same ground clearance that you planned on when you designed the suspension, and now the centerline of the wheels is 1 inch lower, there is going to be a major effect on geometry.

if he built the car for 4" of ground clearance and couldnt run any less, then he set the car on the ground and the 1.5" of squish gave him only 2.5" of clearance, he would have to raise the car back up 1.5" by using spring adjustments. this would alter the angles of the suspension and give him significantly different numbers.

Tim

[LateNight72]

He could also design some new "raised" spindles and be done with it.

But to that point, yes you would have to compensate. Just like I will have to design the coil-over mounts so they preload and settle at the ride height I want.

[BBShark]

you guys are missing his pt;





if he built the car for 4" of ground clearance and couldnt run any less, then he set the car on the ground and the 1.5" of squish gave him only 2.5" of clearance, he would have to raise the car back up 1.5" by using spring adjustments. this would alter the angles of the suspension and give him significantly different numbers.

Tim

Thank you. This is exactly what I was asking. I would like to start with the most accurate numbers.

I have an aquaintance who works for Goodyear. I talked to him about this and he said that this number is known and is how they control the contact patch and some other information about the tires contribution to wheel rate, etc. Apparently there is more to tire design that I had thought.

[greencactus3]

you need to account for the effective tire radius when designing the geometry

[Norm Peterson]

I'd guess that the vertical stiffnesses of most tires that you'd use on most average-weight cars fall in the 1000 - 2000 lb/in range, depending on the actual size and inflation, and that about 1/2" to 3/4" deflection with the car at rest and the tires at proper inflation pressure is more reasonable. Perhaps tires for more luxury-oriented rides or SUVs will deflect slightly more - I just don't have any "feel" at all for those situations (though I might be able to check my daughter's Honda Pilot later). Even so, 1.5" sounds a bit high - that much corresponds to about 2° of the total visual roll in a fairly hard but entirely do-able (if perhaps noisy) cornering situation.

As I understand it, tire load ratings vs inflation pressure are associated with some sort of deflection criteria (I think the term used in a discussion elsewhere was "over-deflected" rather than "overloaded"). Makes sense from a structural fatigue point of view.

The effect of tire stiffness may affect the handling balance as much by its effect on the overall corner stiffnesses than by its effect on roll center heights and instant center locations.


Norm

[astroracer]

The "squish" is called "rolling radius" by tire and suspension designers. This is a known quantity and is calculated through load ratings and tire size/aspect ratios. Static rolling radius is generally a bit shorter than dynamic rolling radius and all of this has to be taken into account when using wheel speed sensors to input vehicle dynamics into the ECM in late model vehicles.
Rolling radius is taken into account when establishing ride height and alignment settings but is not a definitive factor during geometry design. As was stated earlier the geometry takes it's input from the wheel center and has no real clue as to where "ground" is during operation. Hope this helps.
Mark
Edit:
Just to clarify one other concern that was mentioned about altering ride hgt for grd clearance... Raising the suspension for additional grd clearance does not alter the geometry of the suspension. The geometry doesn't change unless you start moving hard points. What the raised suspension does do is alter the "position" of the suspension as compared to jounce and rebound. A raised suspension will increase jounce travel as it reduces rebound travel. A re-alignment will be required to correct toe and camber but the suspension will not be changed.
Mark

[79-TA]

This seems mostly to be an issue of maintaining ground clearance without having to change any suspension settings. Clearance is the only thing I can see squish negatively affecting. Most cars with larger profile tires aren't going to have serious clearance issues. I'd say if the half inch or so of squish is compromising the car's clearance, perhaps the car is just too low to begin with. That and avoid speed bumps.

If it really is only a matter of clearance, just estimate the squish and then give yourself a bit of a safety net in the initial design.

I suppose that's why some folks like the options an air ride system provides.




Another factor to consider is tire width. If two tires are identical in all dimensions except width, the wider tire should "squish" less. Assuming both tires are bearing the same load, they both must apply an equal force at the contact patch to remain static. That is, force = pressure x area. Both contact patches will be the same size, but will have different shapes. Given equal tire pressures, the wider tire needn't deform as much to get more rubber to the road.

So, it seems the more performance-oriented a car is, the more likely it is to have lower profile and wider tires which both make the tire's rolling radius a much smaller factor.


ps: my car often scrapes its mufflers on speed bumps . . .