Two questions; I'm running 135R15 Firestone car tires on all three
wheels of my '81 GL1100-EML rig. The tires on the rig have 135 R 15
72 S stamped on them. I want to run a larger diameter tire on the
drive wheel only, to "tweak the gearing" and reduce the RPM's,
especially at higher speeds. If I want to increase the circumference
of the tire by about 5%, how do I use the "135" and "15" to figure
out what I have for circumference, and then to figure out what I
should purchase?
Are there pitfalls to doing this? What about tire width issues?
Changes in handling? How do I make sure I have the same width tire
so I don't have rear swing arm clearance problems? Are there things
I should consider, if I want to get more mileage off the rear tire?
Second, there is an internet resource for Firestone tires like this,
and I was advised of its URL at one time. However, I cannot find
where I saved that URL on my computer, and I can't find the post in
the archives. Where can I purchase said tires?
MikeS
Coker Tire handles 135x15" Michelin and Firestone tires.
The 72 indicates that the molded height of the tire is 72% of the width of 135mm. The installed width of the tire will depend upon the width of the wheel used. For rear tires you must consider; the width: so as to not interfere with the driveline or fender. The heigth: so it doesn't bottom out on the fender, wiring or fender attachments.
Figure your increases by installed tire heigth or diameter. A 25" diameter tire has a circumference of 78.54 inches. If you go to a 26" tire the circumference is 81.82", which would give a total gain of 3.1416" or 4 percent. Rim width isn't factored in here so that must still be taken under consideration.
If possible check with the vendors technical dep't.
Handling shouldn't be a problem since most bikes use a wide variation of tire and wheel sizes between front and back. And the hack ususlly has a different size altogether. With so many dynamic variables at work when driving a sidehack rig, maybe a little minor tweaking would be needed at most.
Lonnie
