Re: [T3] Wheel offset

[Jim Adney <jadney@vwtype3.org>]

On 22 Oct 2003 at 11:28, KenSimons wrote:

> 	I am curious as to what kind of offset some of you are using on
> non-stock rims? Who has had success with a 6" rim, and what kind of
> offset are you using? Has anyone found out how much is too much? Which
> way is better/worse? Too much? Or too little. Thanks for all input, Ken.

For fronts:

In general, for neutral effects on the steering, you want to put the center of 
the tire contact patch in the same place as the OE wheels did it. Here's why:

Consider a sort of X-ray view of your front suspension. There are 2 ball joints 
associated with each wheel and all the steering angle motion takes place around 
these 2 centers. Since there are exactly 2 centers, we can draw a straight line 
thru them which runs up and down at a funny angle. This is your steering axis.

If you look at the diagrams, you may be able to see that this straignt line 
passes thru the approximate center of the tire contact patch with the road. The 
effect of this is to null out any torque about the steering axis that might 
occur when braking.

I think it's safe to assume that VW designed the car this way on purpose.

If we add wider wheels, but with the same offset, then the additional width 
will have been added equally on both sides, and the steering torque remains 
minimized. But if we change to a different offset, wider or not, then there is 
a torque about the ball joint axis any time we brake.

This torque won't be noticed if the tie rods and tie rod ends are in good shape 
and if the traction on both wheels is the same, since they will still balance 
out (assuming we did the same thing to the wheels on both sides,) but now we 
are asking the tie rod ends to take additional loads every time we brake, and 
this is bound to reduce their lifetime.

In addition, the torque will become evident any time we try to stop on pavement 
that is not the consistent from left to right, as in a patch of sand on one 
side. This has an impact on directional stability while braking and wear on the 
steering gearbox as we try to correct the tendency to pull to one side.

For rears:

The steering effects don't come into play here, but if you move the tire 
centers outward on IRS cars the stress on the wheel bearings increases. At some 
point this will reduce their lifetime. Shad has also pointed out that the IRS 
diagonal arms are a weak point in the design and the stress on them also goes 
up as the tire moves outwards.

I would think that for swing axle cars there would be very little negative 
effect.

-- 
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Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA
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