Running 7x17 with 43mm offset with 215/45x17. Stock GP wheel/tire fitment is 205/40x18 on 7x18 with 52mm offset. If I add 8mm spacers, I'm effectively reducing offset to 35mm which effectively changes scrub radius from basically zero to slightly positive. Am I making handling worse by trying to widen track width?

 

If I were looking for the "best handling", or easiest to drive in and out of corners...combination in Solo type racing, the least amount of scrub as possible ! Only stands to reason, right ? Tire scrub is just that, "scrub", which equals the contact patch NOT making the turn smoothly, or accounting for the least amount of horse power to complete.

The larger the turn radius, the easier the "bad" radius will hurt the turn speed / handling. But still...see above.

Mike

 

Mike, I appreciate the input but I guess I need to rephrase the question. Is the amount of positive scrub radius caused by adding 8mm spacers significant? Will I notice it? Or...Is the benefit of adding wider track greater than the detriment of adding that amount of positive scrub radius?

 

Excellent question. I have not been able to find the answer.

Track cars are set up to have positive scrub so the front tires toe out under braking and help turn the car into the corner. Of course this makes the car feel like it wanders in the brake zone - because it wants to turn!

Street cars are set up with negative scrub for stability under braking.

Tendencies are reversed under acceleration, but more force (sometimes way more force) can be generated under braking than under acceleration. So acceleration affects are negligible.

Zero scrub is always undesirable since it generates tire squirm.

So, fit the wheels, tires, and spacers you want and take the car to the track. If wandering under braking is too much to handle you have too much positive scrub. Fit smaller spacers or wider tires to reduce positive scrub.

 

Alan, thanks for the thoughtful reply! Well, this morning, I decided to take the GP out to my favorite stretch of "test road" under the pretense of breaking in the new front rotors and Hawk DTC60 pads. I'm pleased to report no squirm or darting under straight line braking. Steering was just slightly heavier but response was just as quick, turn-in predictable. Evident was a more planted feeling in turns. So far, so good. What little positive scrub I may have seems certainly acceptable.

 

I was going to say I doubt you'd feel any negative effects unless you're driving the car 10/10 on the track. Happy Motoring!👍

 

My "test road" is very much like a road course and with little traffic. I'll be instructing at VIR in a few weeks so we'll see if there's any difference.

 

Sounds like fun, but I'm assuming you don't drive 10/10 while instructing!

 

I don't drive 10/10 even when I'm all by myself. I make it a point to always leave a little for the unforseen. They don't give trophies for HPDE instructor sessions, nor is it instructive to take a student out and scare the Hell out of them!

 

Viewing the mods on your signature, I don't see that you've installed an adjustable larger rear sway bar. If handling is your priority, a larger bar would promote oversteer, help eliminate a front-wheel drive car's inherent understeer, and would make cornering more predictable.

 

The first mod I made was a 22mm rear bar. They just don't give us enough room to list all the mods. Among others, I have poly bushings throughout but no room to list.

 

I also learned to drive in college, and it took quite a long time.

 

You are correct that adding 8mm spacers to your 7x17 wheels with a 43mm offset and 215/45x17 tires will effectively reduce the offset to 35mm, which will increase the scrub radius and potentially make the handling worse.

According to the reference titles below, increasing the scrub radius can lead to understeer, which can make the vehicle less stable and more difficult to handle, especially during high-speed cornering. Additionally, the wider track width can also affect the vehicle's handling by increasing the moment arm and reducing the turning radius, which can also negatively impact the responsiveness.

Therefore, it is not recommended to add 8mm spacers to your wheels without proper testing and evaluation of the handling performance. It is important to consult with a professional mechanic or a wheel and tire manufacturer to determine the appropriate wheel and tire combination for your vehicle to ensure optimal handling and safety..

 

As it turned out, I settled on 3mm spacers because any thicker would cause rubbing on the rear wheel arch trim under extreme load. The effect on scrub radius is minimal but I do experience a slight improvement in handling, primarily quicker turn-in.