Would a front tire diameter increase compensate for a front offset decrease?

If you were to fit front alloy wheels to a 928 with say an et50 offset, this would reduce the 15mm negative scrub radius to zero (from et65) right?
(or is it then a 5mm positive from a 10mm negative? Not too sure but I think the lower 10mm scrub radius is on the S4s & up? )

Anyway, if you increased the diameter of the front tire by say 15mm, wouldn't this compensate the loss of negative scrub radius to some extent?
( I know larger front diameters would interfere with the front guards a bit more but please ignore this for the moment...)

I'm not sure of the angle of the steering axis so don't know how to work it out exactly, but surely this diameter increase would ad a few mm back into the negative scrub radius?

Thanks in advance for any help.

Tom

 

A wider tire centre line is still the same so, no and I think you mean width not diameter.

 

Hi Pagnobito,

No, I mean diameter.

I larger diameter would push the tire's road contact point away from the center of the wheel & I would think, increase the distance between the steering axis center line & wheel axis center line, increasing the negative scrub radius.
Whether it would be by a significant enough amount or not is the bit I really don't know without the angle of the steering axis.

 

To put it another way, if you reduced the wheel/tire diameter, at a certain point the road contact point would meet the intersection of the steering axis & wheels axis & you would have zero negative scrub radius on an et65 wheel.

I'm not 100% sure about all this but think that's right?

 

I am assuming you know what offset is, the measurement between the wheel centre line and hub.

I don't see how increasing the diameter of wheel has any effect on offset at all in a straight line and when turning the difference in angle has to be so small to be negligible.

 

Yes, the effective increased ride height would increase the negative scrub radius r. By how much, would involve some trigonometry. Begs the question of how much it changes with ride height adjustment...

 

Thanks for the image Rob.

Going off that it looks roughly like a 15mm diameter increase would give you about 4mm of increased offset (15mm is the legal diameter increase here in Australia)

So I guess a diameter increase could be some help!

 

I think the steering axis inclination is 15 deg (looks about right from that pic).

So tan(15 deg) is 0.27, so for every inch (25mm) that the tire radius were increased, the steering offset (scrub radius) increase (become more negative) by 7mm.

The stock setup has a scrub radius of 10mm ("r" in Rob's picture), a bit smaller is fine (the factory used ET60 for the club-sport wheels, for an offset of 5mm). But for ET50 you need another 10mm, so the tire radius would need to be 1.5" larger, or a 3" increase in diameter. That is not going to end well.

That said, we have ET52 18" wheels on the front of our GT and it works fine, in my opinion. There is a bit more tramlining but part of that is wider rubber, and it is also very sensitive to toe adjustment. I've also used ET52 17" wheels on our S4, the tramlining in that case was better not worse... go figure (same toe spec).

So I am not sure that I would worry about it, I think alignment (esp. toe) is a larger effect.

 

Yes in theory you can negate some of the loss of negative offset by increasing the tire diameter. Its not very practical on a sports car though. Assuming a roughly 20 degree king pin angle you will need to increase the radius by about 27.5 mms for a 10 mm offset change. So to compensate for the 65 mm to 50 mm change you'd need to go from a 25" tire to a 28.25" tire. This method can be some what successful on a lifted 4x4, but doubt it will help with a 928.

 

Nonsense... Remember this bad boy??

928-4x4.jpg

 

Now that was funny!

Regards

Fred

 

No.

 

Great, now I'm gonna have to build another hybrid...

 

Hi Tom.

You need 1/2"+ increase in rolling radius to gain 2-3mm in scrub radius.

S4's+ have a 10mm scrub radius with a 65mm offset. ET.

If you fit an 50mm ET wheel the scrub will go from -10mm to +15mm.

So if you fit a larger overall diameter wheel as well you'll end up with +12mm scrub. Still not good.

Then there's the issue of speedo error, but that can be corrected with a black box.


HTH.

.

 

You mean -10mm s.r. to +5mm scrub radius going from ET65 to ET50.

But still, you're right, a 1" rolling diameter change won't make a helluva lot of difference.

Should be enough to make an ET55 boxster wheel nicer to drive on a 928 tho' - the caveat being actually fitting under the front arches without tearing the aluminium as you turn the steering rack. Brief experiment with a 997 wheel (and 997 rubber) says an inch bigger diameter is a lot to fit under there.