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Old 02-15-2007, 10:22 PM   #1
MadseasoN
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Changing aspect ratio of tires?

The stock front tire size for my F3 is 120/60/17. According to the chart below, changing that to a 120/70/17 will raise the front end by 1/2 inch effectively changing the steering geometry of the bike. The trail measurement (stock) for this bike is 3.7 inches.

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
120/60-17 2.8in 11.3in 22.7in 71.2in 890 0.0%
120/70-17 3.3in 11.8in 23.6in 74.2in 854 4.2%

If I wanted to raise my forks in the TT to compensate for the higher front end would I raise the forks 1/2" or 1". Reason why I ask is because the overall diameter of the tire is increased by 1" with the new tire size but the sidewall height is only increased by 1/2".


FYI - this is on a track bike and little changes like this will be noticed. I don't want high speed wobbles but I do want the bike to turn easily.
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Old 02-15-2007, 10:25 PM   #2
Patrick
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Dennis,
Measure from the axle to the ground with your 120/60 tire on it.
Install your 120/70 tire and take that same measurement, subtract the 120/70 from the 120/60 profile and lower your triple that amount.
Give me a call if you need any help.
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Old 02-15-2007, 10:33 PM   #3
MadseasoN
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Quote:
Originally Posted by Patrick
Dennis,
Measure from the axle to the ground with your 120/60 tire on it.
Install your 120/70 tire and take that same measurement, subtract the 120/70 from the 120/60 profile and lower your triple that amount.
Give me a call if you need any help.
Cool, thanks Patrick. In that case I'm guessing 1/2" since that's the sidewall difference.

The rake is 25 degrees. Would any geometry whiz's here like to figure out what the new trail would be if I left the forks the way they are? I heard anything more than 4" and the bike would be real hard to turn.
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Old 03-15-2007, 12:01 AM   #4
lb3tx
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a quick calc and it looks like a rake of 25.7 degrees (25.2 orig) and a tail of 4.0 inches (3.7 orig). I don't know what that means in terms of feel but those are the rough numbers.
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Old 03-27-2007, 09:23 PM   #5
RaZ
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http://www.papkegroup.com/superbikee...nematics1.html
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