I have a 2008 CBR1000rr. It came stock with a 190/50 rear and the gearing at the track was great. I got race rubber in a 190/55 rear, and the gearing is now off quite a bit. What would get me back close to stock? 2 teeth up in the rear? Hoping for a quick answer. I can probably measure and figure out a formula, but that would take time
The diameter (and possibly the circumference) of the tire increased by 5% (50 vs 55 profile). I would add enough teeth on the back to negate the 5% increase in tire size. Whatever your current ratio is, increase it by 5% and you should be good. I think. What is your current sprocket set up? Find that ratio and increase the number of rear teeth until the ratio is 5% higher than you started.
Thanks for thinking this through for me, you probably saved me a headache. 2 teeth up in the rear wins!
This logic fails my sanity check-o-meter. 50 to 55 is not a 5% increase, it's 10%, and that's per side, but it's only of the tire height, not the wheel. Switching to rad for simplicity: 17" rim = 8.5" rad tire height, 190/50 = 3.74" rad tire height, 190/55 = 4.11" rad 8.5+3.74 = 12.24" (190/50, total wheel+tire radius) 8.5+4.11 = 12.61" (190/55, total wheel+tire radius) 12.61 - 12.24 = .37 .37/12.24 = .0302 = 3.02% increase in rad/dia/circumference/gear ratio On the flip side a 55 might grow a little more at higher speeds, further reducing the gear ratio, so maybe 5% isn't a bad number, but still... the logic bugged me.
All the math in the world doesn't make two tire mfgs make their 190/55 tires the same size. Only way to know for sure is to measure the circumference of the old and the new tire...at speed.
Dude, I was a music major dropout. I couldn't even succeed at something that dropouts did for a living. Math is beyond me sometimes. If I had typed 10% would that have made it better? I like how you went with it, it makes a lot of sense to me (the music school dropout). Yet I still got close to the 5% magic number...
Hehe... It would have passed the sanity check-o-meter, but then I would have had issue with the fact that the change neglects the rim dia. From this exercise, I'd say it's probably safe to conclude that a 50/55 change within the same tire width alters gearing by 3-5%. Once you change tire width, then I'd have to re-run the whole process. No. I'm not going to do it. I'm going to resist. My wife is already making fun of me. And now that I see my article quoted, it's a decrease in gear ratio as mechanical advantage is lost. Simply put, taller tire needs more rear teeth.
55378008 Simply put, taller tire needs more rear teeth. [/QUOTE] Let's Calculate the vast amount of time involved in this group Master-calculation session...but quickly, time is of the essence. Just put the sprocket on there and run it. It's not that deep. I would recommend a leak down test and getting the rear sprocket blueprinted for sure. It's the only way to really know what's going on back there.
