Racing rim options - standard 1991 SE PCD, offset etc

[wiseraven]

Yes, SE hubs = 60mm bore F & R

I wouldn't get too hung up on getting the widest possible tyres. What is often overlooked is that the size of the contact patch is entirely determined by the tyre pressure. A fat tyre and a skinny tyre at the same pressure will have exactly the same contact size e.g. with 600lb on a corner, and tyres at 30psi, you'll have a contact patch of 20 sq in. Drop the pressure to 20psi and you'll have a 30 sq in contact patch.

The two main advantages of wider tyres are that you need less deformation in the tyre to achieve the same contact size, and the stresses (and heat) are spread over a wider area. This is particularly important when cornering, as that 600lb easily becomes 1200lb. A skinny tyre at 30psi may not be able to deform enough to generate a 40 sq in contact point, so you go sliding off.

Since in your case, you're talking about the difference between fat tyres and slightly fatter tyres, the effect is of course much less. In addition, wider tyres are also more sensitive to incorrect camber settings, so it may be better for your first year to stick with the narrower tyres until you're more familiar with the car's set-up.

Just my .02.

Phil

[M Blur]

Cheers Phil,

and the new aero will of course increases the vertical load onto the contact patch.. I can live with the drag as my wee engine will still push a hole through the air regardless.

so up front: 7.5 (+15et) or 8J (+20et) if I wanted to run a 225 profile.. both okay? both okay for arch clearance?

As you say in time I'd hope to add more rims of varying sizes to suit different set-ups.

and skinnier can be better if it turns slippy by dropping the pressures down or pressures up and less aquaplaning. However I want to get as much width into the front as the rear aero will prb be stronger than the front initially. But generally - the bigger the contact patch the better and I'm running seperate wets in the original 15/16" OZ split-rims.

Cheers

Jon

Edited February 12, 2007 by M Blur

[M Blur]

The two main advantages of wider tyres are that you need less deformation in the tyre to achieve the same contact size, and the stresses (and heat) are spread over a wider area. This is particularly important when cornering, as that 600lb easily becomes 1200lb. A skinny tyre at 30psi may not be able to deform enough to generate a 40 sq in contact point, so you go sliding off.

and conversely then as the weight shifts ot the outside wheels in a corner then the vertical load on the inside wheels reduces and reduces the overall grip. If you have less contact patch to begin with (assuming decent -ve camber) then a skinnier wheel will likely let go first tho may prove more progressive when it slides.. but my pref would always be to have thw wider wheel to give max contact patch initially to the outside wheel giving the majority of grip but also the inside wheel to work betetr with the slipper and hang on for longer when it starts to slide. We know this form days of old classic rwd cars that would slide very quickly under load in a corner.

stil to talk to John @ Leda - hope to do that week and get new springs ordered up.. phew!!

so if i can get away with the 8J (=20et inset) wheels including 20mm spacers then that would seem the best choice. If there are any potential clearance problems then the 7.5J (+15et) would seem the better choice for the fronts.

Edited February 12, 2007 by M Blur

[M Blur]

so if I run the 8J with +20et instead of 7.5J +15et the net result is approx half an inch or 1.252cm or 12.52mm difference.. +5et difference for the offset then the net increase would only be 7.52mm or jst under a cm.. is that right?

Sounds as if I could get away with that = ideas on profile - 225, 235.?

I'm thinking run a modest profile (225) so that it's nicely fitting to the rim with little slop.. only risk being that it will overheat quicker if I do get understeer but I'd rather have some front slip and balance early into the turn than that wooly turn in feel and then you're never quite sure how much frotn bite there is so you never push the tire, get heat up and will likely feel skittish. Plus you can't then find the set and start working the rear if the front is all over the place..

Nature of the event is short sessions so I can afford to scrub the tire quicker as it's not as if I have to try and run 30 laps on them in one go so easier to keep the temps/wear down.

[M Blur]

Order in 30 mins ago.. 17x8J +20et and 18x9J +15et, 60mm bore, 108pcd 5 stud

Have gone for soft spigots initially in case there's anything funky with the hubs Hazzard was using. Not doing any long events so hopefully wont melt them to quickly and can get alloy spigots made up later and dedicated bores when I order the next set.

will post up pics once they arrive, boots-on and on the car.

Q - what size bolts - lenght/thread are usually used for the SE? I'll check the existing bolts while I'm at it. New bolts supplied with wheels. 3 wk lead time.

Jon