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. :)
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.
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
Thanks for the info guys!
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.
QuoteKinda, 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)...
QuoteWe were both typing at the same time... you beat me to the submit button...)
Oops, I hadn't noticed that :-[
QuoteThe 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!
Very helpfull for some of us... Thanks! :)
assuming adjustable ride height or shims you could raise the rear to compensate as well.... seems to work on my R6
okay... just say I want to drop my front end 2 mm. How much can I raise the rear to make up the difference?
Quoteokay... 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...
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)
First, mesaure the circumference of each tire on the rim with air in it, and of course at the pressure you ride on. Tires gain a little with air in them, some more than others.
Then, do the math to calculate radii for these measurements, using the formula:
r = c / 2pi.
The difference between front tires can be used directly. The rear will be different. On the gsxrs, the ratio works out to about 1 mm in shock height = 7 mm at the axle, as measured then calculated using radii of swing-arm pivot, shock centre, and axle in the middle of the adjustors. This should not be too different for CBRs, ZXs, and YZFs. Cheers
I would think that the fork angle would play a part as well... 5mm down on the fork wont lower the bike 5mm unless the forks are straight up and down...
Just a thought...
Does anyone know is there is a place that has the circumfrence or overall diameter of motorcycle tires listed? I know for car tires any of the major manufactures will have the overall diameter and loaded tire diameter listed in their product guides.
QuoteDunlop 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. :)
Drop your forks................but not too hard. Merry Christmas Shogu. Be advised, D-Bag club meeting coming soon. NO BOWLING ALLOWED!!!!!!