One thing that many riders fail to consider when changing brands or models of tires – even those marked as the same size – is that the new tire will often alter the outer diameter of the complete wheel assembly. So, if you’ve meticulously set up your bike’s ride height to tune its handling or if you’ve just been happy with the turning characteristics of your motorcycle with the old tires, you might want to account for the change in tire diameter so that you can maintain the same chassis attitude.
If you stayed awake during geometry class, you know that, if the circumference of a circle increases or decreases, the radius does, too. In the case of motorcycles, the radius is the distance from the axle to the ground. Having the front radius increase while the rear decreases could turn your razor-sharp canyon weapon to a slow-steering dump truck.
While you can measure your tires’ circumference when the wheels are still mounted on the bike, the best time to measure the circumference is when the wheel is on the balancing stand. Wrap a metric measuring tape around the wheel, making sure that the tape stays in the center of the carcass. (A thin tape, like Race Tech’s Sag Master is ideal since it bends much easier than wide tapes do.) To get your baseline measurement, measure the properly inflated tire before you replace it. Then remeasure after you’ve mounted and inflated the new tire.
Since the circumference of a circle is equal to two times pi times the radius or c=2πr, to calculate the radius, the equation becomes r=c/2π. With calculator in hand, substitute the measurement for c and 3.1416 for π. You can shorten the equation by subtracting the new measurement from the baseline measurement to calculate the ride height change in one step with:
If the number is positive (the old circumference is greater than the new), the ride height should be raised by the calculated amount to make up for the shorter tire if you intend to keep your chassis geometry specs consistent. If the number is negative, the ride height must be lowered by this amount.
Suppose your old front tire measured 184.2 cm (72.5 in.) and your new tire was 188.0 cm (74 in.). Your front ride height needs to be adjusted –0.6 cm (or –0.24 in.). So, you would lower the triple clamp 6mm down the fork. Get it?
This method works great for bikes with adjustable front and rear ride heights. However, most stock bikes don’t have rear ride height adjusters. In this situation, the front ride height must be adjusted to maintain chassis attitude, making the equation:
Here R is the rear wheel circumference, and F is the front. (As in the first equation, a positive number means you increase the ride height by sliding the triple clamp up the fork.) Unfortunately, the overall height of the bike can’t be maintained because the rear stays at the same height. So, you’ll get a bike that should handle the same with either slightly more or less ground clearance.