Dear Geek:
You guys are always talking about changing a bike's front ride height when you change tires. How do I do that, and how do I know how far to go, eh?
Tony Moonen
Dorchester, Ontario, Canada
PS: Love the bow tie!
When we change to a different brand or type of tire, we always account for any differences in the tires' diameters by changing the height of the fork tubes in the triple clamps. This is quite important on Yamaha R6s, as that particular bike comes stock with a 60-series front tire, and DOT race tires are most commonly available in the 70 series. Using a thin or cloth-type measuring tape, measure the circumference of the old tires before they come off, and measure the new buns after they are mounted. | 
Using a steel ruler, measure from the top of a fork tube or cap to the top of a clip-on or the triple clamp. If the new front tire is taller than the old, the front of the bike needs to be lowered to keep the chassis at the same attitude. Ideally, you would change your rear ride height to account for rear tire changes, but since most bikes don't have that adjustment you can take into account a change in rear tires at the same time. If you do have a rear ride height adjuster on your bike, you can accommodate the front and rear changes individually. | 
Using the measured circumferences (C), the fork tubes should be raised in the triple clamps by:
(Cnew front - Cold front + Cold rear - Cnew rear)/6.28.
If this value is negative, the forks should be dropped in the triple clamps. For our R6 the fork tubes need to be raised in the clamps by 10mm. Essentially, you are returning the bike to its original attitude, although it may sit slightly higher or lower overall. |
Calculate the new measurement for the fork tube height by adding or subtracting the appropriate amount. Lift the front of your bike using a stand that mounts under the bottom triple clamp--you'll need a good rear stand also. If your clip-ons aren't pinned to keep them at the correct angle, mark or measure their position. Loosen the top clamp pinch bolts and clip-ons. | 
Loosen the bottom clamp pinch bolts on one side and raise or lower the fork tube to the new measurement. Torque the pinch bolts, and repeat for the other tube. It is important that the two tubes be as close to the same height as you can make them, so that (among other things) you won't have trouble pulling your front axle out at a later date. | 
Tighten the top clamp bolts and clip-ons. If you have a fork tube mounted steering damper, check and adjust its clamp. New tires with different profiles will change handling even with the ride heights restored, and you may have to fine-tune your fork height adjustment after a test ride. |
This story was originally published in the February 2002 issue of Sport Rider.