SCCA Stock Class Tire Thoughts

As mentioned in another thread, I think I'm going to build my 1984 CRX for H-Stock SCCA Autocross.

According to stock rules you can adjust the suspension within the limits of the OEM parts and you can run any size tire you can squeeze on stock size rims.

The CRX has torsion bars in the front and therefore has a good bit of height adjustment that can be done to the front in stock form. By lowering the front you also gain some chamber.

The rear has springs which cannot be cut etc.

So, my thought is, what if I run a smaller diameter tire in the rear than in the front. For example, what if I run a 20" diameter tire in the rear and a 22" diameter tire in the front.

This would allow me to drop the front 1" and still have a level car.

Good idea? Bad idea?

Someone on another forum also mentioned running wider tires in the front than on the back for FWD car but didn't give a reason why. Any thoughts on that?

Well the benefit of the narrower tires on the rear is that front-heavy, front wheel drive cars typically run out of traction in the front way before they run out of traction in the rear, which causes understeer.

This is because compared to the rear, the front tires have to handle more braking, accelerating, turning and weight. The weight would generally increase the tire's traction but on most soft-sidewall tires it really just means they deflect more under lateral load.

The back tires basically only have to hold some weight up and handle the inertia of the back end when steering, but the back of the car is NEVER turning as hard as the front, and always has less weight on it. And because the camber angle doesnt change due to steering, and there is less sidewall deflection ,etc, the back contact patch is always pretty good.

Most people tune out the understeer by keeping the same size tires and modifying the suspension. There are lots of benefits to that, but one downside is that by the time you've made a FWD rotate hard at autox, you've also made it rotate hard when you dont want it to. Things that seem manageable in an empty parking lot at 40 mph become deadly in 75 mph avoidance maneuvers on minorly wet and traffic-packed freeways. If you simply run different tires only at autox, you keep the safety margin of the stock handling for normal driving, if you intend to do any.

When Pontiac added weight to the front of their Grand Prix by adding a V8 for the GXP, they actually adopted a staggered setup like you're talking about to maintain a similar front/rear traction balance, and that's oem with a warranty.

jwdmotorsports wrote: The rear has springs which cannot be cut etc.... So, my thought is, what if I run a smaller diameter tire in the rear than in the front. For example, what if I run a 20" diameter tire in the rear and a 22" diameter tire in the front. This would allow me to drop the front 1" and still have a level car. Good idea? Bad idea?

In stock class you can not drop the suspension height so this would not allow you to drop the front 1" and/or no spring cutting.

Was your car from the factory with 13" or 14" rims?
Not "dealer option" but from the factory?

In reply to Vigo: good information. thank you.

In reply to jrw1621: sounds like my interpretation of the rules was wrong. It came with 13" wheels. Still has the oem steelies and hub caps on it.

In reply to jrw1621: Are the CRX front torsion bars adjustable (as some Mopar were)? If so, doesn't that fall under 'According to stock rules you can adjust the suspension within the limits of the OEM parts...'. Or do they spec a ride height?

Then, in stock you would be expected to run 13". You can switch to alloy but the rims you switch to have to be 13" and can not be wider than the width of the stellies.

I was doing this from memory and that is not very good...
http://scca.org/documents/Solo_Rules/2010_scca_solo_rules.pdf

13.8A: No you can not cut springs.
13.8B: Yes you can adjust within the limits of the OEM parts.

13.4: Your can change the width of your rims by +/- 0.25 of an inch.

jrw1621 wrote: I was doing this from memory and that is not very good... http://scca.org/documents/Solo_Rules/2010_scca_solo_rules.pdf 13.8A: No you can not cut springs. 13.8B: Yes you can adjust within the limits of the OEM parts. 13.4: Your can change the width of your rims by +/- 0.25 of an inch.

Without digging all the way through the rules to find out... are you sure about being able to change the width of the rims ..?? IIRC, it's the offset that doesn't have to be exactly that of the stock rim....

Could be true... Whatever 13.4 says

After checking, you are correct, rim size must be the same, offset can change by +/- 0.25 of an inch

jrw1621 wrote: I was doing this from memory and that is not very good... http://scca.org/documents/Solo_Rules/2010_scca_solo_rules.pdf 13.8A: No you can not cut springs. 13.8B: Yes you can adjust within the limits of the OEM parts. 13.4: Your can change the width of your rims by +/- 0.25 of an inch.

As I remember, you are correct.

The stock 13" rims on a CRX Dx are 5" wide and can be replaced by any OEM Honda 13", 4x100 alloy used on Civics, CRXs, Preludes and Accords from '80 to '89. IIRC, the OEM alloys on the '85 CRX Si were the lightest, but they are hard to find.

With such an underpowered car, I wouldn't bother with staggering f/r tire sizes. Get all four in the same size and rotate them between events; you'll maximize wear and save money. Just make sure you adjust tires pressures as you'll probably run about 40psi in the front and 20psi in rear.

As for size, a 185/60-13 works really well on 5" rims, but a good tire shop should be able to competently mount 215/50-13s on the same rim. Both of these sizes are shorter than the stock 175/70-13s and provide slightly better gear ratios. FWIW, 185/60-13s were the favorite size on all three of my 1st gen CRXs.