315mm tires on 17" x 10.5"

[Incan]

So, I need wheels & tires wide enough to stick out 4 inches on each side for a dsm experiment: (the objective is skidpad, not slalom) http://cellcluster.net/turbo/renderings/

Now i'm willing to have the front be 1 or 2 inches less if that won't damage the center diff. If the tires are the same exact height, then is it ok for the AWD if they are of different widths?

What I can afford right now are mustang wheels and spacers (the kind that have holes for the stock bolts, and then use thier own bolts to attach the wheel).
So I think this will be sufficient 315 millimeters = 12.4 inches
This is the tire size recommended to fit these 10.5 wide mustang wheels. They claim a certain backspacing, but that didn't quite match my calculations. Perhaps it has something to do with the outboard flange?
http://www.mustangtuning.com/silver-deep-dish-fr500-wheel-17x10-5.html

I can do that cheaper using tire rack tires and discounttire wheels, but maybe mounting & balancing will cancel it out. I don't care about looks, because what I really want costs $1200 each for 17x11. Hence the mustang wheels.

Anyway, I can afford spacers, but 1 inch wide seems to be the limit. I'm afraid that won't give me clearance for the suspension, knuckle, etc. I'm going to try and get that ebay wheelspacer machinist to make me 2 inchers. I started doing extensive calculations and research and created a diagram. I'm not worried about fender clearance.

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The only offset guide that mentions the flange: http://www.motegiracing.com/techzone/wheeltechdetail.asp?filename=wheeltech4#

 

[ACM]

A 10.5" wide rim with a 10mm offset will fit with 285/30.18 Kumho V710s. The same rim with same size Hoosier AS05 requires a 1/4" or 3/8" spacer (10mm I think). The specific tyre you choose will determine the offset/backspacing you require, it is related to the curve of the upright, the OD of the tyre, the sidewall width, and the profile. Since each tyre varies slightly from the nominal spec, it's incredibly hard to know.

In short, the only way to determine it is best guess or suck it and see. When you are operating this close to the physical limits, production tolerances have a large effect on the end result, what works on one car doesn't work on another.

Also, be aware that if you start generating any significant lateral G on a skidpad from this setup you better have reworked the oil system otherwise you'll be loosing #3 or #4 big ends every few minutes. I have the row of failed engines to prove it ! At a minimum you will need the Moroso pan and a 3 qt AccuSump to survive the high G duration you will see on a skidpad, but you'd be better served just going straight to a dry sump system. At 1.4 G you will be suffering regular oil starvation - on a skid pad the motor will be knocking before you get the car stabilised if you're on a stock oiling system.

Charles

 

[jtmcinder]

Incan -

The reason your calculation of back-spacing disagrees with theirs is you did not take the rim's lip into account. Rim lips are usually around 1/2" thick.

Charles -

Is the starvation worse in one direction?

- Jtoby

 

[ACM]

Incan -

The reason your calculation of back-spacing disagrees with theirs is you did not take the rim's lip into account. Rim lips are usually around 1/2" thick.

Charles -

Is the starvation worse in one direction?

- Jtoby

I don't know - I turn left regularly as well, I just know that after several lefts and rights it goes rappety-rappety-rappety

Charles

 

[Incan]

Thanks for the info, I'll look into that and do it before trying high lat g's.

So that lip is the flange. well I bought one wheel with the tire mounted, and a set of 2" spacers. so I'm going to test fit it front and rear. Once I get it monday, I'll measure the actual height so I can contemplate getting narrower tires for the front. If I can even figure out an identical height for a different width.

- No one seems to use 300+ tires on the front, probably because when you turn the tires tilt slighly? (I'm not worried about the stock wheel well)
- If I keep the front tire wide as well, then I should have less understeer right? That's not really a big concern, actually.

 

[mavisky]

I don't know anyone running a dsm with over 285's on the front. My buddies old 91 mustang lx had 315's all around but I think he was using 12" wheels.

 

[ACM]

On a 2G ride height is critical to maintaining the best geometry possible. The inner fender comes out over the top of the tyre, limiting how much the suspension can compress - the taller the tyre the less compression travel is available, or the higher the car must ride to maintain acceptable compression travel. Solo ESP does not allow the inner fender to be modified here, SM may not either, so tyre OD becomes critical. 285/30.18 has the same OD as 245/45.16 - that's one of the primary reasons for the choice. An important consideration is that any extra tyre width on a DSM is added to the outside of the rim, thus making the car wider - this makes slaloms harder, offsets tighter, clam shells and Chicago Boxes more difficult to navigate - ie all of these elements become slower. The extra width must supply sufficient grip to offset the lost time through the above elements, and the only place the time is going to be regained is in sweepers of one form or another - places where being tight on the cones doesn't matter. There are few of those on an autocross course.

This applies to a lesser degree on a road course - the wider the car the further it has to travel around a turn, which costs time. The added grip must allow a sufficiently higher corner speed to make up for the added distance - diminishing returns.

There's also the added mass which slows accelleration, the added contact patch which increases rolling resistance, and the additional protrusion beyond the bodywork which really hurts aerodynamics (particularly a 2G, because they're so clean to start with).

It all adds up. Pony cars carry a lot of weight and a lot of torque - but they have really big wheel cavities. It makes perfect sense for them to go up as far as 335, it's a much tougher justification for a DSM.

Charles