Why is it?....

[BDA]

I know when you are aligning your car on the floor of your garage, you need to bounce the car and roll it around a bit between adjustments.

But my question is "Why?" Let me provide some context. In the case where there is no compliance (eg. rod-ends, roller bearings, etc., no rubber bushings), since you are measuring the angle off the rim, not the tire, the rim should also move without compliance. What ever angle you measure should be the angle. Certainly, any of those joints or bearings would force the tires to deflect, but the angle of wheel should be determined by geometry of the suspension regardless of whether it is bounced or rolled.

Now, I understand that in most suspensions, the bushings are rubber or poly and those would deflect based on changes in the geometry and the traction of the tire on the floor. I get that! But in cases were there is no rubber, why is there any need for turn plates, rolling, bouncing, etc.?

[4129R]

To let the suspension settle in all 4 corners.

You have probably jacked up one corner to adjust something
prior to measuring, so the car will not be evenly balanced on all 4 corners until it has settled.

[BDA]

Actually, I'm thinking of setting my rear camber which doesn't require jacking up the car at all. But more broadly, back when I was racing, the Formula Atlantic racers I knew (and so I did too on my Lola T492 sports racer) did the same thing and there is no need to jack up anything to adjust a formula car. Maybe they were just doing it out of "habit?"

[TCS4605R]

I used to crew for a Formula Atlantic - Swift DB4 - probably about 15-20 years ago.  Jim, the driver, is still racing in Vintage races - Jim is also Vintage at age of about 77 or 78.  Before that is was Formula Ford and before that Formula V.  We would adjust the suspension on the ground and roll it back and then forward to a stop to 'settle the suspension and also to let the tires seek their normal 'straight ahead' posture.  The driver was always in the car when we made adjustments.

Tom
74 TCS - 4605R

[jbcollier]

When you make an adjustment, the “system” of tires, bushings, joints, chassis, etc is put under tension.  This tension may cause some elements to “distort”.  Moving the vehicle back and forth allows this tension to dissipate and each part in the system to assume its new “normal” position.

[BDA]

I used to crew for a Formula Atlantic - Swift DB4 - probably about 15-20 years ago.  Jim, the driver, is still racing in Vintage races - Jim is also Vintage at age of about 77 or 78.  Before that is was Formula Ford and before that Formula V.  We would adjust the suspension on the ground and roll it back and then forward to a stop to 'settle the suspension and also to let the tires seek their normal 'straight ahead' posture.  The driver was always in the car when we made adjustments.

Tom
74 TCS - 4605R

That's what I remember we did. One friend had sacks of sand that added up to his weight so he could adjust the alignment by himself. And that's what we said back then, we were settling the suspension, but with a suspension that is completely solid ball joints and rod ends, I don't see how there is anything to settle. The tires might get to a more relaxed posture but you don't measure the alignment on your tires, you measure it on your wheels.

[BDA]

When you make an adjustment, the “system” of tires, bushings, joints, chassis, etc is put under tension.  This tension may cause some elements to “distort”.  Moving the vehicle back and forth allows this tension to dissipate and each part in the system to assume its new “normal” position.

I agree that for most cars because which have rubber or even poly bushes there can be distortion but if your suspension is steel tubing with rod ends, you won't have any distortion (if you do, you're putting WAY too much pressure on it). With a full rod-end suspension, any distortion would have to be taken up by the tire, right? And since you measure your alignment on the wheel, you don't really care what the tire is doing, right?

I suppose it's more of a theoretical question since very few of our cars have only rod ends or ball joints in their suspensions but it got me to thinking since six of the eight rod ends on my twin-link rear suspension have rod ends and the fourth end has a very thin rubber bush (which I expect has very little deflection).

[TCS4605R]

If you look really close even though the suspension points are solid, taken collectively they are somewhat compliant including the wheel bearings - nothing is perfect.  My TCS has all spherical joints in the front suspension but I find I get different results when I don’t roll the car back and then forward.

Tom

[BDA]

 Interesting, Tom! If you didn't tell me I would never believe it! Thanks!

[ACW270]

Isn't the change caused by the lateral friction on the tyres as you let down the hanging (unloaded) suspension? My Midget is a couple of inches higher at the front when I lower her.  Rolling her back and forward a few feet allows the front tyres to 'walk' their way back to the correct track.

[BDA]

Jacking up the car and then lowering it on the floor will definitely require "resettling" as per 4129R's post. The suspension will droop the tire will contact the floor on its corner and will have to be set right again before measuring anything.

I should have taken that out of the equation when I set up the question. I was really asking about a suspension jointed entirely by rod ends or ball joints that did not need to be jacked up. Maybe a more practical example is setting the toe on the front end. I suppose there is a bit of stress that's built up in the system when adjusting the toe but I would think that the vast majority would be taken up in the tires and a tiny amount in the bushings so resettling seems unnecessary. Tom said that even though suspension links might be solid (rod end or ball joint) that there is still enough compliance that resettling is necessary. I'm surprised but I believe him. In any case, it might be easier to consider it good practice if you don't use turn plates.