Choosing tires for street and wet track

[teamking]

All,

 

I am starting to formulate plans to make my Birkin street legal over the winter, and am trying to figure out what to do about tires. Currently, I am running Hoosier 20.0 x 7.5-13 A2500 front and 22.5 x 10.0-13 A2500 rear in a 35 compound as my track tires. I think at some point I will switch to 25 compound, and that will be my dry track configuration.

 

I'd like to find a setup that is street legal and doubles as a wet track setup. I'd like to not have to make any other modifications to the car if it suddenly starts to rain while I'm at the track-- I'd like to just throw on the new wheels/tires and be ready for the wet.

 

So, I'm thinking Toyo RA-1's are probably ideal. They're street legal, durable, and I understand they are quite good in wet conditions, particularly when they are at full tread depth. So, what sizes to choose-- I'm thinking: 205/60/ZR13 front and 225/50/ZR15 in the back.

 

The stagger of tread widths is identical at 23% more in the rear (the Hoosier fronts are 8" wide and the rears are 9.8" wide. The RA1 fronts are 6.9" wide and the rears are 8.5". Both ratios are 1:1.23). So, the handling balance of the car should be close to identical with the two types of tires. Also, the narrower RA1's should be less prone to hydroplane.

 

The stagger of the circumferences is a little different. The Hoosier circumferences are 20.6"f and 22.6"r, or about 10% more in the rear. The RA1 circumferences are 22.7"f and 23.9"r, or about 5% more in the rear.

 

So while the gearing will be longer in the street/wet setup, I'm not too worried about that. But the brake bias will also be affected, and this is where I need some help thinking this out.

 

Let's say my current Hoosier setup is ideally biased front to rear in terms of the braking, with the rears being 10% greater circumference. Now, I throw the RA1's on, where the rears are only 5% greater circumference. Essentially, I've made the fronts 5% bigger compared to the rears with the RA1s. If the front circumference is 5% bigger, that means the front tires will exert 5% more torque on the front brakes, which means that I will have just biased the brakes by 5% to the rear.

 

Now, since I'll be driving in the wet on this setup, I won't be able to brake as hard in the limited traction, and there won't be as much weight transferred off the rears onto the fronts, so the 5% effective rear brake bias increase of the RA1's is exactly what I want compared to the Hoosiers.

 

Does that logic make sense?

 

Next question: other than aesthetics, will there be any compromise in performance for the RA1's since they will be on 13" rims on the front and 15" rims in the back? On the Hoosiers (mounted on 13" rims all around) the rear sidewalls are much taller than the fronts. On the RA1's, the front sidewalls will actually be a little taller.

 

Thanks in advance for any advice you have for me!