Track change with suspension movement

[formula94lt1]

I realize that if one wheel hits a bump and the track of that side changes it results in unnecessary tire wear. However, what other effects does it have? Also what if say you are entering a right hand turn with a perfectly smooth surface, so no jounces, and in roll the outside, left, track is extended outward away from the vehicle centerline and the inside wheel, right, extends inward the same amount? My assumption is there would be a combination of moving the mass of the sprung body toward the inside of the curve and whatever resistance that gave would also deflect the tires path outward depending on lateral stiffness and load etc.. Im really trying to ask what would be the harm of this hypothetical situation? Seems like in a situation like this if the effects of "scrubing" the tires a little to the outside are not very noticable or detrimental in any way, the benefits of transfering the sprung mass toward the inside of the turn in relation to the track centerline may slightly load the tires more evenly... Just a thought.

 

[GregLocock]

"I realize that if one wheel hits a bump and the track of that side changes it results in unnecessary tire wear. However, what other effects does it have?"

It also creates a lateral force tending to steer the car.

In your smooth road example I'd find it hard to get excited about it as a problem.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[formula94lt1]

Thank you for the info, greg. What I'm getting at with the smooth track was a suspension that has no track change in bump but only in roll, if there was/is such a thing. I thought maybe roll would be slow enough with dampening that the lateral forces and even track changes on either side of each axle wouldnt have much effect on the wear or steering of the car. The steering of the car especially if the lateral forces generated were even such that it did not steer the car, but slightly laterally displace it. This theory make much sense?

 

[BrianPetersen]

This is a tiny aspect of an extremely complicated situation.

Take two simple suspension geometries that are almost polar opposites, pure trailing arm independent, and the old "swing axle" independent (original VW Beetle, original Corvair, many others).

Pure trailing arms let each wheel independently go straight up and down. If the bodyshell is held steady (or assumed to be thus) then there is absolutely (in theory) no side scrubbing whatsoever. The spindle swings in an arc when viewed from the side, but this doesn't matter. Pure trailing arms have no resistance to body roll on account of the geometry (in old terminology, the "roll centre" is at ground level) and when the body leans in a corner, the wheels lean with it.

Swing axles swing the wheels in a massive arc (when viewed from the end of the vehicle) as they go up and down. Unlike pure trailing arms, there is massive camber change with suspension movement and plenty of what you call "scrubbing" action. Since the forces that lead to body roll are transferred at the pivot points of the swing axles which are necessarily well above the ground, the geometry provides a resistance to body roll, BUT, independent suspensions like this transmit a vertical component of the side loads to the suspension attachment point and this leads to a "jacking" effect.

If used on the front suspension, this type of sideways motion of the contact patch can cause reactions in the steering wheel. For decades, Ford trucks and vans had Twin-I-Beam front suspension which is sort of a variation of swing-axle suspension but with the effects moderated by having the swing-axle point on the opposite side of the vehicle. Those didn't exactly have outstanding handling (nor front tire wear) characteristics - particularly in the vans after the bushings started wearing. If conditions weren't perfect, old Ford vans could be a chore to keep in one lane on the highway.

VW original Beetles had a form of pure trailing arm front suspension and swing axle rear suspension ... If you poke around on the internet you can find pictures of those cars either jacked up off the ground, or on slammed suspension - note the huge difference in camber between the front and rear wheels, in either case. Let's just say that achieving outstanding handling qualities was not in the design objectives of that car.

Modern good suspension geometries use multiple links to more carefully control how the steering axis and the contact patch move as the suspension goes up and down, and some front or rear multi-link designs have complex relationships in how all the links work together to make this happen. Designed-in bushing deflection complicates this even further.

 

[BrianPetersen]

By the way, I don't think you can have a geometry that has no track change in bump but only in roll. You either have track change with suspension movement (multilink, swing axle, MacPherson, semi-trailing arm) or you don't (pure trailing arm and rigid-axle geometries, and the halfway-house of twist-beam trailing axles).

 

[formula94lt1]

Understood, I was just speaking in what if's. It is more of a numbers crunch to see what the exact effects are of the situation I described, I wanted to be sure I wasn't leaving anything out of consideration.