"Yes you need to be within 4% front to rear, and within 4% of spec (25"). "

I knew about the 4% front to rear. I was unaware of "and within 4% of spec which is 25 inches.

This means the OD of the tires cannot be less than 24" or greater than 26". In addition, the spread between the fronts and the rears OD wise cannot be greater than 4%.

 

Good advice. Here's a link where you can check your tire applications and see the % change it might make.
http://www.miata.net/garage/tirecalc.html

 

cjv, sent you a PM

 

Why do you need to have a certain OD tire...other than to keep the speedo accurate as long as the relative sizing between the two is within the 4% parameter?

 

ABS and VC couplings. In addition it will affect over and understeer.

 

its such a small change in ackerman I doub't your average to advanced driver would notice any change in handling.

I also can't see any effect to abs or vc systems by a consitent f/r change in size. can you please elaborate as to why you say this...

-dave w

 

Who is saying consistent. I have used 265/35-18 fronts and 315/30-18 rears. I have also used 275/35-18 fronts and 335/30-18 rears.

A one inch difference in OD has a 42 revolutions per mile difference in tires. More than that detrimentally affects the ABS. If you want to make corrections for oversteer, two tricks that help are shorter fronts and wider fronts.

I have tried using other combinations where I have definitely noticed the affect on the ABS.

 

Chad...you lost me here...how can wider fronts help cure oversteer?

My point was simply that if you change the OD of the fronts and rears equally and in the same direction, then the ABS and VC issues do not crop up.

Also, I am no engineer...how does increasing or decreasing the OD affect ackerman?

 

John,

It has to do with the tire footprint on the road. A taller tire places more tire lenght wise on the road. To get a good expanation on oversteer/understeer corrections go to Roger Kraus Racing www.rogerkrausracing.com , then go to Technical Sheets, then go to understeer/oversteer.

 

There is something else to consider guys. The computer that runs PSM has assumed constants in its algorithm. These constants are gravity, vehicle weight, tire diameter, etc. The PSM program must know how fast the car is going, via the speedometer, to accurately function.

Here is an example.........

If you install larger OD tires on the front and rear while keeping the percentage difference front to rear the same, the computer will "see" a slower than real speed. Now, lets say you go into a corner too fast and the tail wants to go out................
If the computer thinks you are going slower, I would have to assume it will initiate PSM in a differnt manner than if the "seen" speed was accurate.
Intuitively, this could have devastating effects if you are relying on PSM.

BUT, since none of us rely on PSM, this whole point is MOOT!!

 

I'd agree is PSM made a single correction, but it makes hundereds per second each based in the results of the last...I think it would work it out no problem...similar to correcting for variing road conditions, traction coefients, etc....

-dave w

 

I misunderstood and thought you were referring to wider rather than taller front tires as a cure for oversteer.

 

Speaking of tires, I'd like to get a little more rubber on my HRE's. I currently run the Yoko 235/35 & 335/25 on HRE 545R 19". Since I'm having the car raised a bit this week, what is the aspect ratio I can run which will be closest to stock OD? TIA

 

John,

Wider up front will also put more rubber on the ground. This will also help to correct oversteer. Check out the Kraus website for more information.

 

Chad...not to be dogmatic but how can putting more rubber up front help to sure oversteer?