Comment: Tyre Size Calculator

Submitted: Sunday, Jul 13, 2014 at 13:44

ThreadID: 108722 Views:5962 Replies:3 FollowUps:19

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Andrew J8

Why is the measured tyre size (approximately) 5% lower than the online calculator?
I measured the driven tyre of my car, and also a family members car. My tyres is a Good Year, and the other car is using the Yokohama brand. My tyres measured 95.2% lower than the online calculated diametre tyre size, and the other car's tyre was 94.2%. There is appropriate air pressure in tyre, and i checked the wooden ruler with a tape measure to see if there were any innacuracies. There were no anomalies to explain the roughly 5% difference.
Does the tyre really expand that much due to centrifugal force when driving? Somehow i am not convinced the tyre would contort that much. Does anyone know how much the tyre may expand in actual use?
Perhaps it is just the bean counters (ie Accountants from the tyre manufacturers) "robbing" the consumer by giving us less tyre?
I need to know this, because i am calculating gear ratios, top speed, and torque at the wheels for an electric car conversion. Depending on which car and motor combination i use, knowledge of the tyre size could be near critical to my calculations.
Thanks

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Reply By: Ross M - Sunday, Jul 13, 2014 at 16:29

Sunday, Jul 13, 2014 at 16:29

Are you only using the stated tyre size as the diameter or a online calculator as the diameter?

The real diameter of a drive wheel is the axle to ground distance which takes into account the squash of the tyre and therefore the axle is closer to the ground tan any manufacturer or calculutor will indicate.
Always flattest at the bottom!

AnswerID: 536014

Follow Up By: AlbyNSW - Sunday, Jul 13, 2014 at 16:48

Sunday, Jul 13, 2014 at 16:48

There was an in depth discussion on this on another forum and I like you Ross held that view initially but that is incorrect
The distance from the axle to the ground is largely irrelevant, the circumference of the tyre determines the driveline calculation in the same way as an excavator track does. Does not make any difference as to the shape of the track, one revolution is one revolution

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Follow Up By: Allan B (Member, SunCoast) - Sunday, Jul 13, 2014 at 17:44

Sunday, Jul 13, 2014 at 17:44

That's right Alby. A bit hard to get your head around it when we were taught that the circumference is derived from the radius x 2 x pi. But that only holds for a true circle.

As you say, "one revolution is one revolution" and that advances you by the length of the circumference.

Where the "effective" radius (axle to ground) comes in is calculation of axle torque applied to the ground......... I think. lol

Cheers
Allan

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Follow Up By: Ross M - Sunday, Jul 13, 2014 at 20:27

Sunday, Jul 13, 2014 at 20:27

It is torque at the wheels which the OP asked about. I agree, a turn is a length of tread surface.
However: exaggerated situation.
A vehicle in sand although using the additional length of footprint and same tread length also take advantage of the smaller lever arm of the deflated tyre to provide more mechanical advantage to the grip of the tread.
Both factors play their part.

While climbing up a hill in a 4wd with big wheels it isn't so much the tread length as the radius arm of leverage which makes it work harder.

I imagine the amperage requirement for an electricmotor vehicle feels the same about it.

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Follow Up By: mikehzz - Monday, Jul 14, 2014 at 07:31

Monday, Jul 14, 2014 at 07:31

Alby, I would have thought that the circumference was less on a flatter tyre because the flat part at the bottom is a straight line not an arc? An arc is longer than a straight line between 2 points. The flatter the tyre the longer the straight line section and therefore the bigger the difference from a theoretical circle. So effectively, a flatter tyre has a smaller circumference.

FollowupID: 820091

Follow Up By: mikehzz - Monday, Jul 14, 2014 at 07:37

Monday, Jul 14, 2014 at 07:37

Further to it, where does the extra circumference go? The rubber flattens out at the bottom making the tyre wider. That is how you fit the same amount of rubber in a smaller circumference.

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Follow Up By: AlbyNSW - Monday, Jul 14, 2014 at 08:21

Monday, Jul 14, 2014 at 08:21

Mike think of it like a fan belt
You can hold it in a perfect circle or distort the shape through any number of pullies but the overall length and single revolution remain the same

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Follow Up By: mikehzz - Monday, Jul 14, 2014 at 08:40

Monday, Jul 14, 2014 at 08:40

Mate, you aren't taking into account Newton's 6th law of stretchy substances... :-)

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Follow Up By: AlbyNSW - Monday, Jul 14, 2014 at 08:51

Monday, Jul 14, 2014 at 08:51

Hahahahah

I think I was down the back oval smoking something during that lesson
Come to think about it, I don't remember any of the lessons... hmm!

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Follow Up By: Allan B (Member, SunCoast) - Monday, Jul 14, 2014 at 10:13

Monday, Jul 14, 2014 at 10:13

Well, there is nothing like "Eating the Pudding" to get its "Proof".

I could not get my head around the math of this "effective" tyre diameter/circumference thing so I did an experiment.

I measured the advance of my vehicle with one full revolution of the wheel. At 36psi tyre pressure it measured 375mm from axle to floor and advanced 2350mm for one revolution. At 8psi it measured 290 from axle to floor and advanced 1820mm for one revolution.

Don't ask me where the extra circumferential rubber went.
I don't explain the wizardry, I just do it! LOL

PS. I haven't seen the "in depth discussion on another forum".

Cheers
Allan

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Follow Up By: mikehzz - Monday, Jul 14, 2014 at 10:18

Monday, Jul 14, 2014 at 10:18

I think that emulates Newton's experiment when formulating his rule for stretchy substances (a little known rule to be sure). The tread stretches sideways out when it becomes wider there bye making it shorter in the direction of the circumference.

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Follow Up By: AlbyNSW - Monday, Jul 14, 2014 at 10:34

Monday, Jul 14, 2014 at 10:34

Dam you Alan, lol
I have only just got my head around changing to accept this theory and you go and do that!
Firstly you have obviously got too much time on your hands and arn't you an engineer? you should be able to work this out with a scientific calculator or something without getting a practical real world answer hahahah

If you could now do the same experiment at speed so that centrifcal force comes into play without hurting yourself that would be great

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Follow Up By: Allan B (Member, SunCoast) - Monday, Jul 14, 2014 at 11:45

Monday, Jul 14, 2014 at 11:45

Yeah Alby, I do have too much time on my hands, but soon will be off to The Deserts. I'm an electrical & instrumentation engineer, what do I know about scientific calculators?

Now I could repeat the experiment at speed but will need someone to run alongside to count the revolutions. You wanna volunteer? But I warn you, the Troopy can get up to almost 80kph with a tailwind!

Now another thing (Oh gawd, I'm sounding like The Bantam!)___
If I need to refit a tubeless tyre I put a ratchet strap around the tyre circumference to assist with bead seal. This causes the tyre to expand laterally to seal, but it is noticeable that the tyre circumference is significantly reduced. From just tight, the ratchet strap pulls about 200mm more.

Cheers
Allan

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Follow Up By: AlbyNSW - Monday, Jul 14, 2014 at 12:11

Monday, Jul 14, 2014 at 12:11

Ok ditch the calculator and get out a test light or something and have another go.

Off to the deserts eh? I can see it now, sitting out under the stars at night and instead of being marvelled and mezmerzsed by the marvels of creation, you will be looking at your wheels and searching for an answer.

I won't be out there until September so it will have to wait for me.

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Follow Up By: Ross M - Monday, Jul 14, 2014 at 18:45

Monday, Jul 14, 2014 at 18:45

Alan
In your "travel the tread" experiment, you experienced tyre tread squirm which happens to all tyres. Flatter causes more heat by some hidden process.

That slip, and at high speed there is quite a bit of slip, hence the wear rate attributed to high speed travel.

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Follow Up By: Allan B (Member, SunCoast) - Monday, Jul 14, 2014 at 22:31

Monday, Jul 14, 2014 at 22:31

Like I said Ross................
"I don't explain the wizardry"....... I'll leave that to you maybe?

Cheers
Allan

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Follow Up By: mikehzz - Monday, Jul 14, 2014 at 23:05

Monday, Jul 14, 2014 at 23:05

I'll step in and field this one Allan. Tyre tread squirm = hidden process heat / theoretical circumference. It generally equates to 5%. Ipso facto QED. I think that explains the wizardry involved.

FollowupID: 820123