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gears.. crank arms.. growing kids suck
justinuv
post Feb 18 2012, 10:06 AM
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ok... my 12 year old daughter is 5' tall. just measured her inseam. 27" last year she was riding 155 crank arms on 1 1/8 wheels with a 42/15 gear= 56 roll out. got out of the gate good.. and good speed around track.. when she gave a 100%. this year. put her on 1 3/8 changed gear to 40/15= same roll out-56. on same 155 crank arms. she is running a hyper expert xl frame. problem is this year. first crank is pathetically slow but after 30 feet almost spinning out? as far as i know.(which isnt much) 56 roll out is a big gear. are the crank arms too short??? i did use bike gear app. gear ratio 2.7 gain 4.2 with cadence at 80.. not sure where it suppoused to be... '

thanks for any help.
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brendellajet
post Feb 18 2012, 11:17 AM
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QUOTE (justinuv @ Feb 18 2012, 11:06 AM) *
ok... my 12 year old daughter is 5' tall. just measured her inseam. 27" last year she was riding 155 crank arms on 1 1/8 wheels with a 42/15 gear= 56 roll out. got out of the gate good.. and good speed around track.. when she gave a 100%. this year. put her on 1 3/8 changed gear to 40/15= same roll out-56. on same 155 crank arms. she is running a hyper expert xl frame. problem is this year. first crank is pathetically slow but after 30 feet almost spinning out? as far as i know.(which isnt much) 56 roll out is a big gear. are the crank arms too short??? i did use bike gear app. gear ratio 2.7 gain 4.2 with cadence at 80.. not sure where it suppoused to be... '

thanks for any help.

I assume you are using actual measured tire diameter. If not, measure both sets of wheels and recalculate. Small change in diameter has a measurable impact on roll out.

How long has she been on the new setup? May just need time to adjust.
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paulystx
post Feb 18 2012, 11:31 AM
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She is probably spinning out, crank length seems short at 155, but depends on her leg length. First crank would be helped with better mechanical advantage (i.e. longer crank) the ratio also seems small - most 12 year olds at our track are running at lest a 42-16 and some of our girls are running 42-15. I think you changed too much too soon. I would put her gearing back where it was and try that out first - then maybe move to 160 or 165 cranks. My 12 year old son just moved to 165s and is running a 43-16. Girls are different but i know that at my track that has a bunch of nag girls, they all tend run more gear than the boys the same age.
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justinuv
post Feb 18 2012, 12:44 PM
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QUOTE (brendellajet @ Feb 18 2012, 12:17 PM) *
I assume you are using actual measured tire diameter. If not, measure both sets of wheels and recalculate. Small change in diameter has a measurable impact on roll out.

How long has she been on the new setup? May just need time to adjust.


been on that setup for 2 races. using same 1 1/8 rims as last year. just put bigger tire 1 3/8
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justinuv
post Feb 18 2012, 12:47 PM
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QUOTE (paulystx @ Feb 18 2012, 12:31 PM) *
She is probably spinning out, crank length seems short at 155, but depends on her leg length. First crank would be helped with better mechanical advantage (i.e. longer crank) the ratio also seems small - most 12 year olds at our track are running at lest a 42-16 and some of our girls are running 42-15. I think you changed too much too soon. I would put her gearing back where it was and try that out first - then maybe move to 160 or 165 cranks. My 12 year old son just moved to 165s and is running a 43-16. Girls are different but i know that at my track that has a bunch of nag girls, they all tend run more gear than the boys the same age.


she does have longer legs for being short. 27" inseam. i didnt want to change too much too fast. hence why i changed gear. to keep same roll out with tire size change. was thinking maybe right move was to go to 165's?
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brendellajet
post Feb 20 2012, 04:43 PM
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QUOTE (justinuv @ Feb 18 2012, 01:47 PM) *
she does have longer legs for being short. 27" inseam. i didnt want to change too much too fast. hence why i changed gear. to keep same roll out with tire size change. was thinking maybe right move was to go to 165's?



In the last month I bought two sets of cranks and 4 chain rings & a new Answer mini cog. It sounds crazy, but unfortunately the best way to know if something will work is to try it and see if it works(quantify all your changes with stats). I know a lot of people say just to let them ride, but if your kid wants to win, thats easier said than done. Unfortunately I ended up with a set of cranks I did not need and you can only run one chain ring at a time, but with another little one in the lineup, they will get used. It took a little work, and some patience, but when you close in on the right parts, you will know it. My boy has been chasing a younger rider for the last month and a half, just could not catch him. Last night he put a sizable gap on him 3 races in a row(points race). I think we have a working combo now. Good luck to you!
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Jason F.
post Feb 20 2012, 06:14 PM
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That tire difference wouldnt make 2 teeth difference, no way, you need to bump her up a tooth if she was doing better before.

QUOTE (Jason F. @ Feb 20 2012, 04:13 PM) *
That tire difference wouldnt make 2 teeth difference, no way, you need to bump her up a tooth if she was doing better before.

My daughter is the same age but a bit taller, I have her on 165, so much better. And I am sure your daughter is totally spun out.
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jj
post Feb 21 2012, 08:39 AM
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Justin, rollouts aside, it's still early in the season and nobody has their wind up. Even kids.

Watch her hips as she's riding. Are they rocking back and forth? If she's all over the place, that'll tell you right then and there if the cranks are too short.
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tntoz
post Feb 21 2012, 12:45 PM
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What finally made it easy for me (especially when changing tire sizes) was a Rennen Tech Talk article in the October 2010 issue of the BMXer Mag.

George did a really good job explaining the why's of the formula and why it's not a good idea to use the "gear charts".

The formula is not tire diameter but the circumference that gives the actual rollout.
Rollout=Inflated circumference X Front Gear/Rear Cog.

So my son races on 1 3/8" tires that have a circumference of 64.5"
My daughter races on 1 1/8" tires that have a circumference of 63.75"

My son uses a 40/15 combo which comes out to 172" Rollout
My daughter uses a 41/15 combo which comes out to 174.25" Rollout

You mentioned your daughter used last year a 42/15 combo which would be a 178.5" Rollout. So compare that to the 172" rollout that the 40/15 combo you now run her on. It doesn't really match up to a 56 using the gear charts. A closer match would be a 41/15=176.3 or a 44/16=177.37. Again these figures are using my kids inflated circumference measurements, yours will probably be different.

That should take care of the spinout problem though and as far as the gate, maybe give her a few races to get in her groove before you change crank length but ya you might want to move up in length a little.

Again, this info was from the October 2010 issue of BMXer so its possible if you don't have it that the ABA site may have it archived. Hope it helps, it sure did for me when I moved my son up to the new tire size.
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MikeCarruth
post Feb 21 2012, 12:54 PM
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I gotta tell you, if you like experimenting with the options, and not having to buy a new set of arms to test all the time...grab a set of adjustable-length SS Cranks (sscranks.com).

My son ran them for two years (his racing is on pause at the moment), and it was great to be able to tweak the crank length in 2.5mm increments to see what works and what doesn't.

Best,

M
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