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Question regarding tire circumference

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TZDeSioux

Question regarding tire circumference
« on: May 16, 2005, 08:55:31 am »
Dunlop went from a 65 profile front slick to a 70 profile. Do I drop 5mm on the forks to make up the difference or is there some formula that I need to use to calculate the exact amount? Much appreciated.  :)
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H-man

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Re: Question regarding tire circumference
« Reply #1 on: May 16, 2005, 10:14:16 am »
Hey I remember that 12" action figure 8)

Well, you partly right.  Your tread title has part of the solution.

First the difference isn't 5mm.  It's 6mm.  Using a 120 front as an example, the 2nd figure represents the aspect ratio of the tire.  It's the percentage of the tires width (the 1st figure, 120) that is the sidewall.

But here's were the confusion comes in among different tire mfgrs.  There's no uniform way to measure the sidewall.  The sidewall starts at the tires bead and goes to the sipe (i.e., where the tread begins).  Where the tread begins on the side of a motorcycle tire is pretty tricky to figure out.  I wonder how they determine it for slicks.

So, what ya do is measure the circumference of the two tyres.  The difference between the two is the amount you should alter your forks in order to maintain the same geometry.
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Black Ops Racing
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dsb

Re: Question regarding tire circumference
« Reply #2 on: May 16, 2005, 10:16:39 am »
Move the forks half of the difference in tire height... ex. If the new tire is 10mm taller than the old, drop the forks 5mm.

Unfortunately you can't just go by the size, not all 120/70 tires are the same height on equal width rims. If you could, the diameter of a 120/70 17" tire would be:

120 * .7 *2 + 17" *25.4mm/in = 599.8 mm

as apposed to

120 * .65 * 2 + 17" *25.4 mm/in = 587.8 mm

which gives you a 12 mm difference, and you would need to move your forks 6mm...

HTH,
Dave
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TZDeSioux

Re: Question regarding tire circumference
« Reply #3 on: May 16, 2005, 10:33:10 am »
Thanks for the info guys!
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H-man

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Re: Question regarding tire circumference
« Reply #4 on: May 16, 2005, 01:47:04 pm »
Kinda, sorta Dave.

Because both terms have "+ 17 inches" in common, that part of the math can be dropped from both terms.  In other words, the wheel is a constant so it doen't need to be factored into the calculation.

That then leaves you with exactly what I'd written above.  That's why I wrote the difference is 6mm not 5mm derived from subtracting 65 from 70.

However, when you have tires from different manufacturers you can't be sure they are measuring from the same points in determining the aspect ratio.

That's why when millimetres really matter, a tire's circumference is measured.
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Black Ops Racing
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dsb

Re: Question regarding tire circumference
« Reply #5 on: May 16, 2005, 03:36:43 pm »
Quote
Kinda, sorta Dave.

Because both terms have "+ 17 inches" in common, that part of the math can be dropped from both terms.  In other words, the wheel is a constant so it doen't need to be factored into the calculation.

That then leaves you with exactly what I'd written above.  That's why I wrote the difference is 6mm not 5mm derived from subtracting 65 from 70.

However, when you have tires from different manufacturers you can't be sure they are measuring from the same points in determining the aspect ratio.

That's why when millimetres really matter, a tire's circumference is measured.


We were both typing at the same time... you beat me to the submit button...

The formula I gave gives you the diameter of the tire (which is the easiest to actually measure), the circumference is the measurement around the outside of the tire, otherwise known as the 'roll out' ( you would have to divide this measurement by 2pi to get the radius). The actual suspension adjustment is equal to the difference in the radius of the two tires... And I agree, the size marked on the side of the tire is only an approximation, otherwise you could just use the difference in the width * aspect of each (i.e. 120 * .7 - 120 * .65 = 6mm)...
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H-man

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Re: Question regarding tire circumference
« Reply #6 on: May 17, 2005, 05:54:08 am »
Quote
We were both typing at the same time... you beat me to the submit button...)


Oops, I hadn't noticed that  :-[


Quote
The formula I gave gives you the diameter of the tire (which is the easiest to actually measure), the circumference is the measurement around the outside of the tire, otherwise known as the 'roll out' ( you would have to divide this measurement by 2pi to get the radius). The actual suspension adjustment is equal to the difference in the radius of the two tires...


Excellent point!
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Black Ops Racing
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rotoboge

Re: Question regarding tire circumference
« Reply #7 on: May 17, 2005, 01:42:24 pm »
Very helpfull for some of us... Thanks! :)
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tstruyk

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Re: Question regarding tire circumference
« Reply #8 on: May 25, 2005, 09:19:30 am »
assuming adjustable ride height or shims you could raise the rear to compensate as well.... seems to work on my R6
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TZDeSioux

Re: Question regarding tire circumference
« Reply #9 on: May 26, 2005, 12:53:39 pm »
okay... just say I want to drop my front end 2 mm. How much can I raise the rear to make up the difference?
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dsb

Re: Question regarding tire circumference
« Reply #10 on: May 26, 2005, 01:02:20 pm »
Quote
okay... just say I want to drop my front end 2 mm. How much can I raise the rear to make up the difference?


Whatever the total is just split it between the front and rear... 2mm drop in the front + 4mm raise in the rear = 6mm net difference... Just remember that we're talking about a measurement 'at the wheel' ... putting a 4mm shim on top of the shock is going to raise the rear more than 4mm...
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Clarkie49

Re: Question regarding tire circumference
« Reply #11 on: June 26, 2005, 06:36:44 pm »
The 120/70-17 Dunlop 209 DOT race tire is a lot shorter than the 120/70-17 Slick...........

The 190/60-17 DOT tire is exactly the same height (in the centre of the tire) as the 195/70-17 slick.....

we all ride differently, we all look for a different setup, before making any changes I would ride the bike with the new tire so you can evaluate the tire before trying to make it work  ;)  while the 120/70 may be taller in the centre of the tire than the 120/65, the tire shape may actually make the bike turn quicker.   you may gain trail but a bike that turns quicker, but also gains stability mid corner

tire sizes and rideheight changes have an adverse effect of swingarm angle, your swingarm angle dictates traction more than a lot of people think about.  The best advice is to find a local GMD Computrack centre and send some time (and obviously money) with them, you will learn a lot about your bike and what changes can effect each component of a corner, the money you spend will come back to you ten-fold in the way of quicker lap times, fewer crashes and most importantly, knoweldge  8)
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