Tire math.... can it be used to accurately calculate rolling diameter?

[OreoGaborio]

Can't switching make/models, but keeping the same size, still have an effect on geometry if one tire has a taller/pointer profile than the other?

According to this page: Motorcycle Tire Sizing and Designations, it seems that it can be calculated... however I was under the impression that in order to get rolling diameter you have to physically measure it.

[Chippertheripper]

Yes.

Probably.


I'm sure at your level getting the next (few) tenths is more of a math problem than most others circling the track.

[OreoGaborio]

I'm sure at your level getting the next (few) tenths is more of a math problem than most others circling the track.

Lol, nah this isn't a question related to my race program. This is about the ONLY THING that is MORE IMPORTANT than racing:

.............. I'm in an internet argument.

I've been under the impression for a very long time that two different make/model tires could still have different rolling diameters despite having the same size stamped on the sidewall.

[Speed serpent]

I'm not sure for motorcycle tires but on my truck some 31" tires will fit, others will not- tread depth. It might be that the size is to the casing of the tire, not including the tread.

[Chippertheripper]

Lol, nah this isn't a question related to my race program. This is about the ONLY THING that is MORE IMPORTANT than racing:

.............. I'm in an internet argument.

I've been under the impression for a very long time that two different make/model tires could still have different rolling diameters despite having the same size stamped on the sidewall.

I don't know about very different, but certainly different (new) tires of the same size will have a different circumference. What's the argument for acceptable deviation?

[OreoGaborio]

What's the argument for acceptable deviation?

Well, the discussion came up when someone calculated, to the MILLIMETER, the rolling diameter of a certain make/model, so I guess we'll go by his precision. One or two mm?

I thought comparable but competing tires in brand new condition could still vary by a few millimeters in diameter, enough that it'll change geometry a little bit.

[Speed serpent]

This site tells you what the actual diameter is and how many revolutions the tire makes in a mile: tiresize.com

Couple tires of same size but different dimensions:

[Chippertheripper]

Well, the discussion came up when someone calculated, to the MILLIMETER, the rolling diameter of a certain make/model, so I guess we'll go by his precision. One or two mm?

I thought comparable but competing tires in brand new condition could still vary by a few millimeters in diameter, enough that it'll change geometry a little bit.

Calculating? I'd say this argument requires measurements to resolve.
And I think that's where my horse runs out of gas in this race.

Regardless, a couple mm in circumfernce won't make much of a difference for the normal rider. Shit, even the motogp star probably won't FEEL much difference either.

[RandyO]

the answer to your question is NO, or certainly not with accuracy, different tire manufacturers profiles deviate too much as well as air pressure settings, when measuring tire diameters for calibrating the Sigma bike speedo I had on my SV650, I had one 120/60-17 that was larger rolling circumference than a 120/70-17

I found the only accurate way to measure effective rolling circumference was to put a paint spot on the tire, get on the bike and ride it in a straight line on a smooth flat surface (garage floor) and then measure the distance between the paint spot prints on the surface

I also found a significant difference between a brand new tire and a worn tire, the tire not only looses diameter from tread wear, it also squats more as the carcass gets thinner

[OreoGaborio]

Calculating? I'd say this argument requires measurements to resolve.
And I think that's where my horse runs out of gas in this race.

Regardless, a couple mm in circumfernce won't make much of a difference for the normal rider. Shit, even the motogp star probably won't FEEL much difference either.

Eh, this was was actually a clearance issue.

OP: "My bike has tire X in size Y... the mechanic put the same tire on it but size Z"

Someone else: "That's because size Z is precisely 12.29657y8237fblue MM larger in diameter"

Me: "Uhhhhh, no. That's not correct. Tire math doesn't work like that"

Someone else: "YUH HUH!"

[OreoGaborio]

I also found a significant difference between a brand new tire and a worn tire, the tire not only looses diameter from tread wear, it also squats more as the carcass gets thinner

Well that I definitely know for sure.

[RandyO]

Granted, that the circumference is exaggerated 3.14x the diameter, or 6.28x the radius, it was still significant, nearly 10% deviation

[Chippertheripper]

NO SAHHHH.

[RandyO]

Eh, this was was actually a clearance issue.

that was my first foray into tire measurements, wanting to know if I had to raise my front fender when I put a knobby Pirelli MTR60-Corsa that only comes in a 120/70-17 on my SV

I did find that it fit, but had to raise the fender anyway cause of the stones that the tread picked up

[RandyO]

By the time I experimented with the darkside on my V-strom, I had already learned that calculating tire heights by manufactures size was only a general indication of clearance and that I was gambling the price of a tire that it would fit

[obsolete]

Internet argument? Call the Hitler, literally and collect all those sweet sweet Internet points.

[PhilB]

Manufacturing tolerances are approximate.
Precisely to the exact millimeter -- no.
Within 1% of tire height (± ~3mm total diameter) doubtful.
Within 5% of tire height (± ~15mm total diameter) probably.

Measurement is the only way to tell, and even then that would have to be mounted and inflated to be accurate.

PhilB

[RandyO]

.

Measurement is the only way to tell, and even then that would have to be mounted and inflated to be accurate.

PhilB

and loaded, I found a measurable difference between rolling an inflated wheel and mounted on the bike and riding it

[Uncle Snake]

Since sidewall height is a percentage of the tire width, yes. Years ago I installed a new Pirelli Diablo on my zrx and measured the width. The tire was a 180-55-17 on the stock 5.5" wide wheel. The next tire that went on that bike was the same size only a Dunlop Qualifier. It actually looked wider from the rear so I measured it. 187.5mm. The Diablo measured 178mm. Also the profile has to be a factor as well as manufacturing allowable tolerance.

[Garandman]

Didn't find any manufacturer numbers on exact rolling diameter, but a millimeter would be measurement accuracy of better than a tenth of a percent.

[Kurlon]

Bridgestone usually gives the true tread width and OD of their tires as specs, and I've gotten them for Dunlop as well when digging last I recall. Where did you look?

[RandyO]

Bridgestone usually gives the true tread width and OD of their tires as specs

anything they publish is theoretical and subject to variations in air pressure and load they are carrying, for instance if you air down for a gravel road, the tire height and rolling radius will be significantly different than fully inflated

[Kurlon]

True, but if you want to do a paper comparison that data is better to go off of than the size bin (120/70R17) guestimate.

[RandyO]

True, but if you want to do a paper comparison that data is better to go off of than the size bin (120/70R17) guestimate.

not really, tire pressures and loads affect different carcasses differently, some tires are nearly as stiff as a runflat, and others squat to nearly flat by being 4-5 psi low

[PhilB]

True, but if you want to do a paper comparison that data is better to go off of than the size bin (120/70R17) guestimate.

I'd agree with this. Just be aware that it's approximate.

Can such math be used to get a rough idea? Yes.
Can it be "used to *accurately* calculate rolling diameter"? No.

PhilB