Limiting rear derailleurs for youth racing

MikeWW wrote:

Yes-just adjust the limit so 50:18 is the lowest gearing

You mean highest. 50/18 is the high limit.

John.T wrote:

MikeWW wrote:

Yes-just adjust the limit so 50:18 is the lowest gearing

You mean highest. 50/18 is the high limit.

OK-think you know what I meant

action dan wrote:

My son is racing tomorrow in a youth cat c race and he has to have limited gears, he currently has 50/34 12-25 chainrings and rear cassette. He can't do more than 6.05m in one revolution, so looking at a gear table he could go 50-18 to be within the limit. Now the Big Question! for me to screw the gears is it a case of running it in 50-18 then adjusting H limit screw so it wont move further down the range or is it more complicated? Sorry for the long explanation and the crap techincal knowledge! :oops:

Íf he's going to be racing a lot, you'll want to look at different gears (a smaller big ring and a cassette that goes to 15 or 16 as the smallest). Can you even screw in the limit ring to the 18, will it go that far? 6.05 rolls out as about 45-16 so unless you want to dock him half his gears, it's worth getting new ones.

Fwiw I ride a 50/34 with 12-25 as a fully grown adult, if your son is U12 (Youth C) then he definitely should have bigger gears to encourage him to spin more. He'll be gear-restricted til he's 18 so might as well get used to it now. That's the whole point of gear restrictions, to save growing knees... ask the other parents what they have on their kids' bikes to get an idea. The Youth B rider in my club, one of the top guys in the country, has a 45/36 on the front and a 15-25 on the back and has no issue beating most of his older clubmates up every climb and even on sprints.

As JohnT said, high gearing = big chainring / smallest sprocket combination.

hammerite wrote:

Any idea where I can get a 13/34 (or something with a reasonably low gear) 8sp cassette?

We got Jnr and Isla Bike Luath (26") for Christmas, current highest geared is 36x11 with 152mm cranks. He's got one season left as an under 10, we'd need an 8 sp cassette with 15 as the largest sprocket size for this - impossible to find! So I can deal with limiting off the higher gears.

But for u12 I'd like to get a more permanent solution - 36x13-34 would be idea as then he'd still have a low gear for when we go out for longer/hilly rides.

You might find a MTB cassette to do that, but more likely you'd have to go with making it yourself. The late Sheldon Brown has written a good article on it and his shop in the USA sells them too. http://www.sheldonbrown.com/k8.shtml

For example, if you want a 13-34 8-speed, you can start with n 11-28:

* Replace the 11 and 13 with a top-position 13.
* Add a spacer and a 34 after the 28. (You don't need to buy the spacer, because you can use the spacer that came between the original flat 13 and the 14.)

hammerite wrote:

Any idea where I can get a 13/34 (or something with a reasonably low gear) 8sp cassette?

13-30 or 13-32 are as close as you get, without "megarange" jumps:

13-15-17-19-21-23-26-30
13-15-17-19-21-24-28-32

Take a 13-26 and either an 11-30 or an 11-32

- Undo the 2mm Allen bolts holding the 13-26 together.
- Drill out the heads of the rivers holding the MTB cassette together, from the 15T side. (That's easier than grinding off the squashed heads from the big side).
- Punch out rivets without bending the cogs (rest big cogs on soft block of expendable wood?).
- Assemble required cassette onto freehub, using small cogs from the road cassette (leaving out the 15) & big cogs from the MTB cassette. Given a choice, use more cogs from the MTB cassette.

Shimano 8-speed spacing is about 2x5.1mm, 5x4.8mm (Not uniform 4.8mm as Sheldon Brown says). So a 0.2-0.4mm shim after the 15T (as well as the spacer) might give slightly better shifting arround the 15T. But that's a fine point.

We have a few such mashup cassettes. They shift fine. Can't tell the difference between shifts across the "join" & other shifts, even when trying hard to find one.

If you need a bigger top cog than a 13 you could
- get a 9-speed 14T seperately, and add a shim after it to make up the spacing up to ~5.1mm
- get a Marchisio Top cog (available up to 18T) http://www.highpath.net/highpath/cycles/plural.html