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.

measuring tire diameter

While it’s possible to measure tire circumference with the wheel on the bike, it is much easier on the balancing stand.

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:

Calculating ride height change

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:

calculating ride height changeHere 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.

[This article was adapted from Evans Brasfield’s book 101 Sportbike Performance Projects. Learn more about it here. Read the MO review here.]

  • Old MOron

    I don’t like turning wrenches, but I like math.
    Cool stuff, Evans.

  • john phyyt

    Or you could fit the new tire/s and just adjust to the new feeling and new grip. There is definately a range of opinions about “feel” . I would guess one click of spring preload would probably do a similar thing.
    It wasn’t so long ago when someone was defending using car tires; I would guess that for them 6mm wouldn’t be an issue.

    • Evans Brasfield

      You certainly could adjust to the feel, and people who haven’t taken the time to dial in their chassis wouldn’t care either, but for some riders, adjusting the ride height is just part of the tire changing process, like balancing the wheels.

  • Curtis Brandt

    Hi Evans. Thanks for the reminder. Now if I could only remember to measure circumference on the next new tires I put on the bike, I’d have the baseline I needed! Measuring the old tires just before taking them off doesn’t seem right – I have chased setup on old tires before, with undesirable results…

    One thing – is there a slight problem with your centimeter to inch conversion in an example above? You wrote: “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 –1.5 in?” Shouldn’t the last 1.5 in really be 0.24 in?

    Thanks again for a cool article.

    • DickRuble

      He probably meant 1.5 cm or 0.6 in. In 1998 or 1999 a NASA Mars probe had a similar failure.

      • Evans Brasfield

        Thanks, Dick, but it was just a huge, overlooked typo. Sliding the fork tubes 1.5 inches in the triple clamp? That’s crazy big, and I should’ve noticed the mistake.

        Thanks, Curtis, for pointing out my error. It has been fixed.

        • Old MOron

          Ha ha ha, yes 1.5 inches is huge, especially compared to my dick.

    • Evans Brasfield

      Yes, valid point on old tires. They will be slightly different from fresh ones. If you keep good records, then you can go refer to the new tire number when you change tires.