Scrub Radius and wheel offset .

discopotato03 · Post by **discopotato03** » Thu Apr 01, 2010 7:32 pm

This is the simplest definition I can find that explains why changing wheel offsets from std is less than ideal .

Its in relation to Performance wheels and the humble L Series with its Grrrr 4 x 140mm PCD wheels and front wheel drive type steering geometry .
Often AWD cars come from a FrWD basis inc all Subarus . Many of them have wheels with a lot of positive offset (center towards the outer face of the wheel) to get zero scrub radius geometry with McPherson struts .

The trouble is that many aftermarket wheels are intended for RWD cars with different steering geometry to FrWD/AWD cars . Often it means their offset is less positive than front/all WD cars so even if they bolt up the steering characteristics change because the scrub radius has changed .

Soo , if the 15" Superlight is not close to the std L offset then maybe the 14" is closer .

Anyone seen or used 14" Superlights on an L Series ?

Cheers A .

2coupedup · Post by **2coupedup** » Thu Apr 01, 2010 8:16 pm

as far as i know you can order them in whatever offset you need???
i got measured up for them once and the brilliant tyre man measured offset and everything for the order... maybe they were something similar to superlights... also i have 2 sets of momo vega's, each set have different offset...

you should chat with a good tyre shop... maybe they know what wheels they where...

also i once got told by an engineer to get the offset ground down on a set of alloys as they had a pretty thick base and could handle it...

cheers

Venom · Post by **Venom** » Fri Apr 02, 2010 8:22 am

Is the negative scrub radius geometry really as much of an issue once you are running AWD? Considering an Outback and Forester have 7mm extra offset per wheel over a Lib/Impressa. Granted both are at different heights to the original vehicle but isn't the relative steering geometry maintained? Pretty sure you explained to me that it is the same regardless of spring lift, body lift etc.

That been said i have really noticed a difference to how my L series handles with my 14x6 rally rims repaired and replacing the desert rats with their huge offset. Rally rims are using a +48 offset BTW.

Cheers
Rhys

discopotato03 · Post by **discopotato03** » Fri Apr 02, 2010 1:09 pm

Don't know the relative differences between those models .
Aren't there wheelbase and or track width difference between some of these cars ?
Also what changes with rim diameter and tyre profiles ?

I finally looked up the wheel specs in my 87 L Series factory workshop manual and it states 55mm/2.17 inch positive offset , right across the range from the 2WD 4 1/2J steelies through to the 4WD Turbo 5J alloy wheels .

I suspect the aftermarket wheels are more like 26 - 38P (positive offset in mm) and this is why for their 6 odd inch width they sit so far out on an L Series . They are a bit more RWD oriented offset wise .

AWD doesn't alter the fact that generally the front wheels do most if not all of the steering so its up to the front wheels to make the vehicle change direction . AWD really means that the drive loads are shared by four contact patches instead of two so basically you are less likely to overcome the front tyres limit of adhesion for a given surface .
The other thing is that most Subarus are front heavy so most of the weight is supported by the front wheels which are doing the steering .

My other computer has the Vortex Factory WSM on it and isn't working ATM . I wanted to see if XT6s also have 55P offset 14" wheels .

Cheers A .

A .