Donald

My first time to use AO's calculator, so would like some assistance. It appears that front can take a 225/40, but no more, and rear will take more rubber than I will put on- 295/35.
Is that correct, and any other comments?
TIA

Donald

Really, no comments?
bump.

GlenL

OK. I'll comment.

Is the a 49mm offset in front? That may give handling problems.

DougM

295 tires on a 9.5" rim is stretching it a bit.

pcar928fan

Would not put 295's on a 9.5" rim and seems like an 8.5" ET49 would be an iffy fit... Though we generally talk about 8" rims on the front and look for an offset larger than 50... I know there have been several explanations about the ET #, but I STILL don't understand that... sure would be easier if they just measured from the back of the rim to the mounting face of the wheel! That would be WAY to easy for a German to do though!

DougM

James
You are half way there with offsets
I = center of wheel where it meets the hub
- = front and rear backspace

----I---- center is right in middle of rim barrel so "0" offset or equal frontspace and backspace
---I----- rim barrel pushed outward(more lip) is negative offset or less backspace and more frontspace
-----I--- rim barrel pushed inward(less lip) is positive offset or more backspace and less frontspace

When looking at offsets and backspace it gets confusing when mm is mixed with " 25.4mm = 1"

imagine a 2 piece rim with the barrel and the center spokes. If the center spokes mount to the hub at the center of the barrel you have 0 offset.
slide the barrel away from the center to get more lip and you are getting more negative offset(deeper dish)
slide the barrel inward to get less lip and you get positive offset.

Donald

Gents- thanks for the comments and yes, 49 is for front and rear.

Glen: In what way do you mean the 49 in front will give handling problems?
Doug & James: I do not intend to put on rear 295s, that is the max based on Andrew's chart. Issue I see is what happens on fronts.

Hilton

On the "S4" style suspension (86.5 onwards) there is only 10mm of negative scrub radius in the suspension. This provides stability and a natural tendency for the car to hold a straight line if you release the wheel.

Using front wheels with a smaller negative offset than the stock ET65 ones reduces the effective negative scrub radius, reducing stability, and increasing the tendency of the front wheels to follow road camber, grooves, etc. without driver input. More importantly, negative scrub radius also has a large impact on braking stability, so under heavy braking the car doesn't pull left or right as badly.

Once you get to ET55, you've eliminated the negative scrub radius, and you'll find that the car has a tendency to want to follow the road's shape, rather than the direction you're steering, so you end up spending some time pushing against the wheel to make the car go where you want.

Below ET55 for late cars, you're actively fighting the wheel more of the time, to hold the car on the line you want, and the further below, the less stable the car becomes.

For earlier cars (pre -86.5) the suspension actually has slightly more negative scrub radius (15mm rings a bell), so early cars can get by on lower offsets (i.e. down to ET50).

Having driven S4's with ET50, ET55, ET57 and ET65 fronts, the lowest I'll put on my cars is ET57 (Carrera III's), and even those I'm tempted to ditch if I can find a local set of 18" Panamera wheels at a decent price.

finally!

KEEP IN mind that spreadsheet is a GUIDELINE. For my wheels it said the rears would rub on the inside unless put between 10 and 20 mm spacer. Got a 17 mm and guess what? rubbed on the outside! Removed spacer and fit perfectly.

mickster

That might just be the most succinct explanation of the effects of differing offsets on steering and braking I've read here. Thanks.

Jerry Feather

Looks great to me too, except I don't have a concept of "scrub radius" in my head to use to think through the explaination. Can anyone help?

Jerry Feather

Jerry Feather

Back to what I thought was the OP's original question about tire width, I would suggest that he become his own calculator. Start with the rim width. Here that is 9.5 inches. Add to that a little bit to incorporate the "balooness" of the tire profile, perhaps an inch, and then convert to milimeters. Here that works out as 9.5 plus 1 equals 10.5, times 25.4 equals 266.7 mm. That suggests that the width of the tire should be about 265 or not greater than 275.

Then take the wheel diameter of 18 inches and decide what the overall height/diameter of the tire should be, say 25.5 inches for the rear, and do a little more math. Subtract 18 from 25.5 and come up with 7.5 inches. divide that in two and get 3.75 inches. Now, convert that to milimeters by multiplyuing by 25.4 and get 95.25. Finally, divide 95.25 by 275, if that is your choice on width, and come up with 34.6. That figure is a percentage that the height of the tire profile is in comparison the the width. That tells you you need a 275 by 35 profile tire for the rears.

Jerry Feather

Jerry Feather

If you do the math backwards to check the 295 35 tires, you come up with a 26 inch tall tire, which is the very max I would put on the rear of a 928, and the need for a 10.5 inch wide rim. To put a 295 tire on an 18 inch rim on a 928, I would suggest a 295 30. That would give you a 25 inch tall tire. That is just about exactly what my S4 has now with 255 40 17's on 9 inch rims. The offset is 47, I think, and there is just about 3/8 to 1/2 inch clearance to the wheel arch, and that is without spacers.

Jerrry Feather

mickster

http://en.wikipedia.org/wiki/Scrub_radius

Hilton made perfect sense. Then I read this to add visualization.

Jerry Feather

Thanks Michael, Now I understand it. What I don't know is what the offset is for a 928 originally in front. This analysis suggests that it is ET65 for both the early and the later cars. Is that correct?

Jerry Feather