If an 8" wheel with the correct offset hits the strut, then it doesn't fit. I've never run wheel with the wrong offset on track. Anyone tried it and noticed a difference?
Yup I have. It's something to do with torque reaction of the contact patch of negative vs positive scrub radius and the effect is more noticeable on corner exit when deploying power. - Lower offset wheels which could fall into positive scrub. Turn in is very good but when putting the power down the front end tends to push wide. can be corrected with more King pin angle but this itself has negative side effects. - Higher offset wheels which could fall into negative scrub. Turn in seems a little less sharp but when putting the power down the front end holds line better. The offsets I have tried are between et24 through to et37. Gurds
All that said, I've compared logs of my corner speeds between lower and higher offset wheels and the min apex speeds have always been better on the low offset wheels. Gurds
The Red Rocket has some high corner speeds and that has a final offset of around 0 at the front on 9" wheels
Red rocket is Robs car I take it? Yea kinda throws the scrub radius argument in the air a little. Gurds
No Robs is the Green Ibiza. The Red Rocket is the Mk2 that was at Bedford next to me. It looked awesome. HR Engineering's IIRC.
That's not as extreme as it sounds. Using this handy chart: It looks like you're only generating scrub of about 6mm to 12 mm, which would be easily overcome with some other changes. I'd love to see a build thread on that car, with all the changes used to make that work.
My track car. Suspension development There's one on here but a lot of changes got it to where it was at Bedford.
A 7 x 15 ET33 with a 205/50 Dunlop DO1J rubs on the inner wing on full lock but no real issue on track. Changed to a 195/55 same tyre and OK. Toe set parallel with 1.5` -ve camber.
What I've said above contradicts my previous statement that the only way to keep the scrub at 0 is to use the same offset wheels. I think I've misread that chart, and you do still have a large scrub radius. My head hurts. lol
This is the way I see it Alex. The KPI, caster and scrub go hand in hand. You really want to minimise the KPI as much as possible as this is what causes positive camber on the outside wheel during a turn. We can practically only get this down a little due to design constraints and trying to keep scrub to a minimum. Caster is your friend here and can combat the effects of KPI and a general rule is caster should be half that of your KPI. i.e 12 degrees of KPI and 6 degrees of caster. This is enough to keep your camber stable. After than you only need enough static camber to keep the contact patch flat with the floor due to body roll. However, having to much caster will create a strong self centering feeling as the axle stub travels in an upside down U shape through turning left and right. So turning the steering wheel causes the wheels to lift the front of the car and makes the steering heavier. On the subject of scrub, I don't think we need to be exactly at 0. The whole negative scrub on FWD comes from the want to control stability and torque steer. Being a little one way or the other won't suddenly ruin your car. Easy test for you, wack on some spacers on the front axle and go drive it round some of your favorite roads and feedback what you find. Gurds
Interesting reads below. Me? Obsessive? lol http://www.clubprotege.com/forum/showthread.php?47204-Effect-of-Wheel-Offset-Change-on-Scrub-Radius http://www.m3forum.net/m3forum/showthread.php?t=433015 (Not read all of this yet) And finally, an interesting image: I've widetracked mine... What does this all mean? It means, if I really want to know what's going on with my car, I need to spend some time measuring everything, and then modelling it properly.
Really good article: http://www.motoiq.com/MagazineArtic...ur-Caster-King-Pin-Inclination-and-Scrub.aspx It suggests maximum 2" scrub for FWD cars, which explains why Rob and Rob are not feeling any ill effects of running no half an inch to an inch of scrub. It also says zero scrub is bad, as it deadens steering feel.
Good article. so many options to think about.. I suppose you need to know what settings you have already.
