MO Wrenching: Adjusting Ride Height To Account For Tire Diameter
Yes, changing tires can alter chassis attitude and handling
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:
[This article was adapted from Evans Brasfield’s book 101 Sportbike Performance Projects. Learn more about it here. Read the MO review here.]
Like most of the best happenings in his life, Evans stumbled into his motojournalism career. While on his way to a planned life in academia, he applied for a job at a motorcycle magazine, thinking he’d get the opportunity to write some freelance articles. Instead, he was offered a full-time job in which he discovered he could actually get paid to ride other people’s motorcycles – and he’s never looked back. Over the 25 years he’s been in the motorcycle industry, Evans has written two books, 101 Sportbike Performance Projects and How to Modify Your Metric Cruiser, and has ridden just about every production motorcycle manufactured. Evans has a deep love of motorcycles and believes they are a force for good in the world.
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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.
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.