adventures with alignment and tyres...

[105gta]

The wheel with the 'most' positive caster is the dominant wheel. So if you have Toe out it will pull to that side. If you have toe in then it will pull to the opposite side.
By load bar I mean a spring loaded telescopic bar you place between the wheels on the forward side to simulate road drag on the tyres. Pretensioning the suspension in an 'as driven' condition. Rarely seen or used these days as its accepted that generally fwd cars counter act this by the pull of the driven wheels. Hence fwd cars being set up with toe out.

[johnl]

The wheel with the 'most' positive caster is the dominant wheel. So if you have Toe out it will pull to that side. If you have toe in then it will pull to the opposite side.

I'm not sure I agree that this is as simple as that, or universally true. It doesn't fit with my experience with toe (in or out) having no discernible affect on a given steering pull, with a pull remaining much the same whether or not I've set the toe to 'in' / 'out' or zero (at least with the FWD cars I've played with). Having said that, I'm fairly sure some cars are likely to respond differently to different changes to alignment.

I suspect that scrub radius (lateral offset of the contact patch from the steering axis ground intersection point) may be one of the geometries that influences the manner in which other changes affect steering behaviour. Most RWD cars have quite significant positive scrub radius, but most FWD cars have zero SR (or very near to it). I have a suspicion (and I could be wrong) that quite a lot of 'accepted wisdom' related steering geometry (as often encountered on the interweb) tends to assume a more or less typical RWD geometry, with significant positive SR...

If a car has substantial SR then this creates an effective lever arm that causes the vertical load (gravity) to make the wheel 'want' to rotate around the rearward inclination of the steering axis, which with substantial SR will cause the wheel to rotate inward (the strength of which will be affected by the angle of the caster). If SR is zero then this lever arm doesn't exist and gravity won't cause the wheel to 'want' to rotate around the steering axis. Gravity, in this case, won't create a wheel self rotating affect (which it will when SR is significant), and thus differences in caster won't cause a pull. With zero SR the only self centering effect will come from 'trail' (i.e. the longitudinal offset of the contact patch from the centre steering axis ground intersection point).

With trail, the centre of the contact patch will 'want' to follow directly behind the point at which the steering axis intersects the ground, and greater or lesser trail on one side will not cause a pull because both wheels 'want' to follow behind the steering axis on that side, and the force this creates on one side is transferred to the other side through the steering linkage.

That's all fine in theory, but in reality it's not quite that simple. Both SR and trail are 'nominal' and somewhat arbitrary numbers (i.e. the length of the SR and trail), but in the real world are affected by camber (effective SR) and tyre casing stiffness (effective SR and effective trail). You can have a nominal SR of X (length, as seen in the drawing board), but if the wheel is cambered then the effective centre of the contact patch will move in the direction of the camber lean. An example would be if we had a nominal SR of zero, but if the wheel has significant neg camber then the inside of the tyre will be more loaded than the outside, and this will cause the effective SR to become somewhat negative. Effective SR changes all the time with camber changes, with steering input (which causes camber changes), body roll and changes in road surface flatness. These changes are quite normally felt as the steering momentarily pulling one way or the other, and will tend to be felt more or less strongly if the tyre casing is stiffer or softer.

Of course if all is equal side to side then the steering shouldn't more or less consistently pull in any direction on a smooth and level road. It isn't always the case though, and not helped by all radial tyres having some (varying) degree of inbuilt directional bias due to the manner in which the radial belts are oriented inside the tyre...

Having written all this, it occurs to me that there may be a theoretical basis for your contention that front toe might affect a steering pull. For example if we start with zero SR and zero toe, then the distance between the nominal centres of both contact patches will equal the distance between both steering axis ground intersection points, and the contact patches will follow directly behind the steering axis ground intersection points. If we then add (say) toe-in then the centres of the contact patches will be marginally wider apart than the two steering axis intersection points. If caster is exactly the same side to side, then still no pull should exist. However, if caster were greater on say the left side then the force created by trail (which is associated with caster angle, i.e. greater caster creates greater trail) will be stronger on the left side than the right, and the steering may pull slightly to the left because the left side contact patch will 'try harder' to follow directly behind it's steering axis intersection point than the right side contact patch will. If you can follow my clumsy explanation...

But, I suspect any such effect would be vanishingly small considering the difference in lateral distances between the two steering axis intersection points and the two contact patch centres would be truly tiny.

By load bar I mean a spring loaded telescopic bar you place between the wheels on the forward side to simulate road drag on the tyres. Pretensioning the suspension in an 'as driven' condition. Rarely seen or used these days as its accepted that generally fwd cars counter act this by the pull of the driven wheels. Hence fwd cars being set up with toe out.

I suspected that might be your meaning, but wasn't sure.

OK, but how much pre-load should be used? Different suspension designs will suffer from different degrees of longitudinal force induced toe change (for X force), and the hardness of the bushes will have an affect. The direction of longitudinal force varies dependant on whether the car is accelerating, braking, or cruising.

I suspect that the 147 suspension may suffer relatively badly (with unstable toe angle) due to the geometry of the lower wishbone where longitudinal force is resisted inwardly and outwardly by the two bushes, which are not spread all that far apart (longitudinally). A good reason to install much harder bushes in the lower wishbone, i.e. more stable toe angles.

IMO lower 'wishbones' that consist of a lateral control arm and a long radius rod are theoretically a better design because the distance between the two bushes is typically much greater, meaning loads are less leveraged into the bushes, so less bush deformation for X longitudinal load, so more stable toe angles, especially under heavy braking (less toe-out gain).

I think it's heavy braking that will induce the most toe instability. Heavy acceleration will be next (toe-in gain), but I doubt cruising causes much of a problem because the force involved is relatively minor (it doesn't take all that much power to keep a car at a constant cruising speed).

Apologies for the long stream of consciousness post...

Regards,
John.

[Olgun]

Although this is an old thread, I have a similar problem in my 916 spider twinnie.

The alignment is totally ok, every suspension part and bushes including shocks and spring are new, steering rack and pump are also totally ok but my car still drifts through right. Since I have tried everything, now I think that ply steer of tires may cause this issue.

So, did you managed to solve the pulling issue?