Calculating Revs / Mile and Determining Front-Rear Tire Compatibility

[red]

I know there are tire spreadsheets floating around, but in case you're curious about the math, I'm providing how to calculate both revs / mile and how to determine whether your fronts and rears will match up (i.e., play nicely with Traction Control / ABS). If the "rake" is changed too much on cars with traction control ('03 300B's and all 04's), you can run into issues. '03 300A's may have similar problems.

This is now captured in the Wiki (https://www.mymarauder.com/) under the tires section.

[JMan]

You're making an assumption on the actual tolerance of the Antilock/Traction Control system(s). There has to be a lower and upper delta allowance that we just don't know the specifics about. What we can do (as you have stated) is figure the proper ratio and keep track of the deviations used so far and in the future keeping in mind individual tolerances. Not being critical Frank, just aiming to de-myth-ify some.

Good work,

J

[magindat]

Or you could just look up the revs / mile on the mfg web site:

Rear 724
Front 759

From my own experimentation, I know a rake is built into the ABS system. I know I have exceeded that rake at 777 and 682.

If we survey folks who have exceeded (had problems) with changes and those who have not, we may be able to narrow down on the Delta Jake speaks of.

I have several combos to offer that DID NOT trip ABS.
782 and 713
770 and 713

[JMan]

That's what I'm talking about! If get enough input this can go *Sticky* or sumptin' like the Resource Room forum. It's great to have really sharp people like Red to initiate this stuff!

J

[red]

You're making an assumption on the actual tolerance of the Antilock/Traction Control system(s). There has to be a lower and upper delta allowance that we just don't know the specifics about. What we can do (as you have stated) is figure the proper ratio and keep track of the deviations used so far and in the future keeping in mind individual tolerances. Not being critical Frank, just aiming to de-myth-ify some.
Right, the tolerance is an assumption and based on what I've heard from both the dealership and Reinhart. I tested the assumption against what I've seen people on the forum have success with (e.g., 255/55R18 and 295/45R18).

If we are to make this more useful, we should build up a list of ratios that have worked on traction control cars *and* log the inflation pressures as well. Sidewall stiffness and tire inflation are factors that will affect static loaded radius and therefore true revs/mile.

[ctrlraven]

I just use http://www.miata.net/garage/tirecalc.html

My 255/55-18 kdw2 tires say one Rev Per Mile on their info and the tire size calculator above tells me different. I've always gone with the tire size calculator and it's been dead on. 255/55-18 = 694 revs per mile, I've been paced and radared to confirm it was the right Revs Per Mile and I was dead on.

[magindat]

Right, the tolerance is an assumption and based on what I've heard from both the dealership and Reinhart. I tested the assumption against what I've seen people on the forum have success with (e.g., 255/55R18 and 295/45R18).

If we are to make this more useful, we should build up a list of ratios that have worked on traction control cars *and* log the inflation pressures as well. Sidewall stiffness and tire inflation are factors that will affect static loaded radius and therefore true revs/mile.

I agree with the rest, but inflation pressure? Revs per mile, while calculable from diameter is a circumference measurement. The circumference or 'length of tread' doesn't change even if the tire were dead flat. Steel belted radials are steel belted for that very reason. I assert that the inflation pressure is irrelevant and will not affect actual revolutions per mile.

If, somehow, we could prove inflation DID affect revs/mile the skew factor caused by inflation would be actually less than the skew factor between the calculations and the actual manufacturer's spec.

Let's be practical, here.

Who's gonna build the chart? I have offered several scenarios known to work and to not work.

Input please? "Number 5 need input!"

[magindat]

I just use http://www.miata.net/garage/tirecalc.html

My 255/55-18 kdw2 tires say one Rev Per Mile on their info and the tire size calculator above tells me different. I've always gone with the tire size calculator and it's been dead on. 255/55-18 = 694 revs per mile, I've been paced and radared to confirm it was the right Revs Per Mile and I was dead on.

That works very well as an initial reference. Same calcs. However, MFG specs do vary, so I consult MFG specs as the final word.

I have changed my Xcal for revs/mile many times. Seems a variance of 10 or so makes no noticeable difference in MPH even when measured by radar.

[red]

Or you could just look up the revs / mile on the mfg web site:

Rear 724
Front 759

From my own experimentation, I know a rake is built into the ABS system. I know I have exceeded that rake at 777 and 682.

If we survey folks who have exceeded (had problems) with changes and those who have not, we may be able to narrow down on the Delta Jake speaks of.

I have several combos to offer that DID NOT trip ABS.
782 and 713
770 and 713

Rich, the manufacturer assumes a loading / inflation pressure, so even their numbers can be off for our cars. Their benefit is that they can gather empirical data to build an average.

The sidewall height (or series or aspect ratio) and stiffness/softness is another variable. So, a taller section height and software sidewall will result in a larger difference between actual radius and loaded radius (consider a truck tire).

I admit that I do not know the exact tolerance. I am basing my % change off of a number I've heard from several sources (including here). Also, I do not yet understand what the threshold of the ABS only Marauders versus the traction control / ABS Marauders. When you go to the dealership to get them to fix your ABS (if you've not done so already), can you pick their brain?

Another note: even if we know the exact tolerance, a user who under- or over-inflates their tires or has significantly different wear front-to-rear may experience problems, as they can be out of this range. So, the number that is derived must have a buffer to accommodate some amount of variations in pressure and wear, as our TC/ABS units do.

I'm all about using this thread to collect users experiences in order to help correct the formula.

Thanks!

[magindat]

Unfortunately, word is dealer can't fix it. I'll call the local dealership today to confirm.

There definitely has to be a built in buffer or Delta as Jake call sit for tire wear and varying inflation (though I still believe it's not really relevant). Our mission seems to be to find the outside limits of said buffer.

As far as empirical data for loading, etc. I can agree. However, Since our cars are the ONLY reason BFG makes our rear tires, I'm going with BFG's revs/mile as gospel. Although I have found through experimentation with the Xcal that 10 revs/mile is undetectable by radar at 45 MPH, so 10 revs/mile is obviously less than .5mph difference.

Another note" Performance tires have 10-12 32's of tread. My new KDWNT's have 10. So if we wear them down 85% That's 8/32's x2 (top and bottom) for 16/32's or 1/2" in overall diameter. In our r/m range (this 1/2 would make a bigger difference on a smaller tire and less difference on a bigger tire) this amount to 13-14 revs/mile just due to tire wear alone. So, we know that the buffer will accommodate AT LEAST this much.

Consider I nearly wore out my first set of rears which started at 713. You can figure they got to 700 while my fronts were 782 at half wear or 776.

So there are some more extremes to add to the data. Hopefully, we'll get somewhere.

Since I downsized my fronts. Maybe someone would like to swap fronts with me for a moment to perform a test. If I do not trigger my ABS, we'll know that I am close to the edge of the Delta envelope and we'll probably be able to define the upper limit.

[red]

I just use http://www.miata.net/garage/tirecalc.html

My 255/55-18 kdw2 tires say one Rev Per Mile on their info and the tire size calculator above tells me different. I've always gone with the tire size calculator and it's been dead on. 255/55-18 = 694 revs per mile, I've been paced and radared to confirm it was the right Revs Per Mile and I was dead on.

I disagree. That's the calculated revs per mile, not the actual. Your speedo will be showing faster than normal, unless your KDW2s are actually larger than specified by the manufacturer, which is sometimes the case. I've confirmed this with my GPS and speed signs using 255/55R18's and 695 revs/mile set using my SCT XCal2.

Additional note: the amount of speedo error will vary with speed (higher speed, higher error).

[red]

I agree with the rest, but inflation pressure? Revs per mile, while calculable from diameter is a circumference measurement. The circumference or 'length of tread' doesn't change even if the tire were dead flat. Steel belted radials are steel belted for that very reason. I assert that the inflation pressure is irrelevant and will not affect actual revolutions per mile.

If, somehow, we could prove inflation DID affect revs/mile the skew factor caused by inflation would be actually less than the skew factor between the calculations and the actual manufacturer's spec.

Let's be practical, here.

Who's gonna build the chart? I have offered several scenarios known to work and to not work.

Input please? "Number 5 need input!"

Yes, inflation. The rolling circumference (and rolling diameter) are affected by inflation and load. Here's a quote from allexperts.com (http://en.allexperts.com/q/Tires-2359/Rolling-circumference-tyres.htm):


"The actual circumference (diameter) of a free hanging (not touching anything) tire is different than the rolling circumference (Rolling diameter) because the tire deflects under load. Different inflation pressures and different loads will affect the rolling circumference, but as a general rule a properly inflation and properly loaded tire will have a rolling circumference about 97% of the free hanging circumference - a 3% difference."

3% is commonly accepted, though it will vary with height, sidewall composition, etc. My comments above still apply.

Concerning your comment about the tread length, you're assuming tires to have a perfectly rigid surface as a result of bands. The rubber still expands and contracts, changing with pressure, load, and temperature. Also, the design of radials creates a more flexible sidewall that contributes to squish and a reduced loaded radius.

[red]

There definitely has to be a built in buffer or Delta as Jake call sit for tire wear and varying inflation (though I still believe it's not really relevant). Our mission seems to be to find the outside limits of said buffer.

Agreed that we need to come up with a safe number. I'm comfortable with the revs/mile calculation, though people can opt to use the manufacturer spec.


As far as empirical data for loading, etc. I can agree. However, Since our cars are the ONLY reason BFG makes our rear tires, I'm going with BFG's revs/mile as gospel. Although I have found through experimentation with the Xcal that 10 revs/mile is undetectable by radar at 45 MPH, so 10 revs/mile is obviously less than .5mph difference.

This depends on the size of the wheel, but for ours, it's negligible. A 10 revs per mile difference with a tire that is about 700 revs per mile will result in about .64 mph error at 45mph.


Another note" Performance tires have 10-12 32's of tread. My new KDWNT's have 10. So if we wear them down 85% That's 8/32's x2 (top and bottom) for 16/32's or 1/2" in overall diameter. In our r/m range (this 1/2 would make a bigger difference on a smaller tire and less difference on a bigger tire) this amount to 13-14 revs/mile just due to tire wear alone. So, we know that the buffer will accommodate AT LEAST this much.

Consider I nearly wore out my first set of rears which started at 713. You can figure they got to 700 while my fronts were 782 at half wear or 776.

So there are some more extremes to add to the data. Hopefully, we'll get somewhere.

Agreed.


Since I downsized my fronts. Maybe someone would like to swap fronts with me for a moment to perform a test. If I do not trigger my ABS, we'll know that I am close to the edge of the Delta envelope and we'll probably be able to define the upper limit.

You can borrow mine the next time I see you.

Thanks for your input!

[red]

That's what I'm talking about! If get enough input this can go *Sticky* or sumptin' like the Resource Room forum. It's great to have really sharp people like Red to initiate this stuff!

Thanks for the compliment, J! It'd be great to get this stickied, though I'm not a fan of putting it in the Resource Room, since I want to make this information publicly available.