I now have a 120/70-17 on my bike instead of a 120/60-17. How many mm do I need to raise the fork tubes to keep the front ride height the same? How does the numbering scheme for tire sizes work again?
Just a guess, but wouldn't it be better left alone? That is the size of choice for fronts from what I gather talking to Computrack last week. Wouldn't leaving it alone improve your turn in? I dunno...just blabberin. ------------------ Dan #180 NESBA ~ N.C. 96 CBR F3 Track 00 ZX9R Street [email protected]
Isn't a 120/70 taller than a 120/60? at least that's what it looked like side by side... maybe my eyes are going too... [This message has been edited by 3-dave (edited 02-04-2002).]
The second number is aspect ratio. 120/60; tire sidewall is 60 percent of 120 120/70 tire sidewall is 70 percent of 120. So the 120/70 is slightly taller then 120/60. Enough to effect turn in? yes. How much? I don't know, depends on the bike. According to my calculator there should be a 12mm difference in sidewall height for a total change of 24mm in overall tire height. Would you want to change the height of your fork tubes almost an inch? I don't think so. ------------------ Keith Snyder Ducati 748 theriderformerlyknownasbeatupducatiguy Nesba 199 WERA novice 199
3 dave, The 120/70 is the perfered size over the 120/60. Try it without changing anything and see what the bike is doing differently. Then talk it over with your tire guy or suspension guy to make any changes you might need. Roasted, as far as the math goes the difference is half, not twice the difference. David [email protected] www.bibmen.com
I am basically looking to be able to put the attitude of the bike back where it was if I find out it is too squirrely for me...
Putting on a taller front tire without changing anything else moves the forks away from vertical and increases trail, making the handling slower, less squirrelly. [This message has been edited by SV Bob (edited 02-05-2002).]
Well, I don't believe I want it to steer more slowly... But again, I was used to the bike with 120/60's. Now I have 120/70's. What is the adjustment to keep the bike the same so I can do what I did before... my brain is hurting again...
Measure the radius (or circumference and calculate radius) of both tires when mounted. Say the new radius is 9mm greater than the old, then drop the triples on the forks by that amount. On a naked SV I don’t like to have much more than 8mm of fork above the triple because of dust seals hitting the bottom triple. You can raise the rear, or increase the front sag if you have room for adjustment there.
Find the difference in radius of the old and new tires. THEN take this difference and divide by two because only half the tire sits below the axle. This gives you the change in ride height. Let's say the change in ride height is 4mm higher at the front. You can't just drop the fork tubes 4mm in the triple trees and get the exact same rake and trail as before. Better call GMD computrack and let them run the changes through their "what-if" model. FYI, see the RRW tire test article which used the GMD "what-if" model to adjust Mark Junge's bike to the same geometry. It's not as simple as lowering or raising the forks by the change in tire height(s).
Geometry 101: radius=half the diamater. sorry for the repeat, slow connection, plus it's not often I can show I actually know something. [This message has been edited by BC61 (edited 02-05-2002).]
I agree BC61 & SVbob Sorry I had a brain cramp and confused radius with diameter. This is why I made an Excel spreadsheet to calculate this by just pluggin in the new circumference. Measure the circumference (tape around the center line of the tread, mounted on rim) Then calculate the diameter (circumference divided by 3.1416) Find the difference in diameter of the new tire versus the old tire Then divide by 2 to get the change in ride height. Did I get it right I need a gold star for the day! [This message has been edited by Bruce Brown (edited 02-05-2002).] [This message has been edited by Bruce Brown (edited 02-05-2002).]
