merckx89

Need some help fast here. I have a teammate who is a junior and his first race is tomorrow. Getting everything ready and I find out he has 650cc wheels when I thought he had 700. Does anyone know the junior gear rollout for a 650 wheel, or if not the diameter to enter into a gear inch calculator to find out the rollout?
Thanks

twahl

Wheel size isn't relevent, the rule is for the rollout. They define it as a gear ratio for 700s for convenience, but when it's measured they don't look at the gearing, they put the bike in the highest possible gear and measure how far it travels with one complete rotation of the crankset.

twahl

Crap. I answered that poorly. Sorry. Is there a reason you can't just measure a rollout?

merckx89

Well, I don't want to leave it up to a mistake that I might make doing the rollout. I would feel much better if I could see the numbers and be sure about it, instead of having his parents drive him all the way out to the race, and then not be able to race or be DQ'ed. I know how to do the rollout, as I was a junior last year, but one, I'm not with him right now, and won't see him till tomorrow and thats putting it off a little long, and two I would just rather be able to enter the gears online and be sure.

rm -rf

I don't race, but I found this pdf.

You measure out 26 feet and roll the bike backwards in the tallest gear for one revolution of the cranks. See the paragraph labeled 3. for the method.

26 feet is 7.93 meters. See Sherman's gear calculator. Plug in your cogs and chainrings, and select a 650 wheel size. Then scroll down to see the rollout in meters. It has to be less than 7.93. It appears you need a 53-16 or 50-15 to stay within the limits. But, like the other posters said, it's the actual rollout that counts.

Edit -- I see you know how to do it already. So the calculator should get you in the ballpark.

merckx89

That is perfect, exactly what I was looking for. I was trying to do it on analyticcycling, but could not enter 650, only the actual diameter in mm.

rm -rf

That's funny, I just found out about Sherman's calculator in another thread today.

merckx89

So on another note, what is 650b and the other ones that were listed on that page, I've never heard of them before.

AnthonyG

650c has a wheel diameter of 571mm + tire size. Assuming 23c then thats 571+23+23=617mm. 617mm x 3.14 = 1.94 m per wheel revolution. If his top gear is 48/12 (guess at junior gearing) then that's 4 revolutions x 1.94m giving a total of 7.76 meters.

I don't know what the gear limit is but that's enough to work it out. There could be some variation in tire size to stated values so when your close the the limit it could be an issue.

Regards, Anthony

Trsnrtr

Don't trust a chart; you must roll it out to be sure. Many refs don't have a sense of humor and rollout is all that counts, not charts.