waykooljr

I got this from the tire calculator at 1010 tires...

When changing tire sizes, we recommend staying within 3% of the diameter/height of the original tire. Any more than this and you face the risk of brake failure.

I haven't been able to find anything more on this through web searches, is there validity to this statement?

What raised this question is a thread on this forum, members are using tires that exceed this recommendation. They have acheived a certain look by doing it but I wonder if it isn't safe.

Thanks

jayel579

I think it mostly has to do wth keeping your speed-o accurate. If you go witha smaller diameter tire you are going tobe travelingslower then what your speed-o is reading and visa versa. Changing the overall diameter of your tires can also change your gear ratio in the rear end.

F1Fan

Hi waykooljr,

More fiction. What happens to SUV's and 4X4'srunning 34"-40" tall tires and beyond? They have small brakes and I see they seem to stop just fine with no exploding brakes.

With regards to the S197 the limit in front if you keep your inner fender wells (a good idea IMO) the tallest tire you can possibly fit is atire industry standardmeasurement 28" tall tire pertire maker specs. I have a 27.7" tall tire and it barely fits in the fender well without rubbing. The rearcan go taller but a lot depends on wheel and offset.

HTH!

waykooljr

Thanks for the replies!

I was aware of the speedo getting thrown off, that goes for the odometer as well. I'm not sure if it's taller or shorter but it will show more or less miles on your odometer accordingly. I know we can use a tuner such as Xcal2 to set tire size, would this correct the odometer as well?

Good point F1fan, about SUV's and 4x4's, that crossed my mind and I found the same statement from 1010 tires mentioned on a truck forum during my searches but it wasn't mentioned if it was valid or not.

Thanks

Norm Peterson

Not brake failure per se, but the ABS system works off of wheel revs per mile.

Staggered tire sizes do have the potential for making the ABS activate a little bit sooner on the end with the smaller diameter tires than necessary (since they'd be dropping rpms faster than the taller tires would). It's possible that significantly different tire revs per mile could trip the ABS light and deactivate that system.

FWIW, 3% represents a little more than the percentage that full tread depth is to tire radius and just about covers the range from a new full-tread tire at the 'large' end of actual OD and the OD of a 'small' tire worn to the wear bars. IOW, you should be able to replace two tires only without worry as long as they're the same nominal size.

It's also at least theoretically possible to program ABS to assume that any absolute rate of change in wheel rpm beyond a certain value represents wheel lockup about to occur and again invoke ABS early (or even put you into 'ice mode'). Again, this tends to penalize the use of shorter tires. However I doubt that the S197 is set up this way, or at least not too tightly so. Otherwise people like Sam Strano would be having huge problems, given the shorter and far grippier rubber that they're running at auto-X.

Edit: you might find something at www.brakeandfrontend.com - it's the online version of an automotive repair industry publication.


Norm

waykooljr

Thanks Norm

I did some digging around at www.brakeandfrontend.com

As you said concerning ABS...

Excerpt from http://www.babcox.com/editorial/bf/bf60240.htm

Wheel-speed sensor readings are also affected by wheel and tire size, specifically the overall diameter and circumference of the tires. Oversize tires with a larger diameter and circumference will turn slower and give a slower speed reading than ones with a smaller diameter and circumference. Because the ABS system is calibrated to a specific original equipment tire size, vehicle manufacturers warn against changing tire sizes. Plus sizing is OK as long as the overall tire diameter and circumference remain the same. But, changing tire sizes significantly may have unpredictable consequences as far as the operation of ABS/traction control/stability control systems are concerned.

Many ABS systems have a means of disabling the ABS system if a single tire is replaced with a smaller, "space saver" spare. The difference in the size of the spare would confuse the ABS system. Once the tire has been repaired and the spare removed, it’s important to remember to reactivate the ABS system.

FWIW, from another article...

From http://www.babcox.com/editorial/bf/bf50628.htm

Big rims can really look great on a passenger car too. If there is a change in ride height front to rear it can affect caster and steering. An increase in wheel and tire diameter can affect braking for all vehicles.

Although, it doesn't go into how it affects braking.

Thanks again y'all!