Crankset for youth racing bike? Need some advice

hockeyjunkie

Hello folks,

I'd like to tap into collective wisdom as I'm looking for an advice on setting up my son's race bike for the upcoming road season.

At the moment he's on Shimano 105 5800 groupset with 165mm cranks. He's turning 11 this year and so will be riding in a C age group.

I got a couple of headscratchers I'm trying to solve for:

1.One of the big races we're signed for requires 38x14 gearing for the age-specific rollout limit. His Shimano 105 compact chainrings are 50/34.

Shall I be looking into replacing the smaller 34t ring with 38t? Honestly I'm not even sure if such ring for 105 can be sourced... :?:

What I'm pondering on is replacing his bottom bracket entirely, moving to a single ring, which I can then alternate between 34t and 38t depending on the race and rollout limits. I also think that will help reduce the bike weight (he never uses the 50t ring anyway and it's restricted for racing).

2. His 105 groupset came with 165mm cranks. He did ok with them last year, yet I think that he could potentially benefit from improved cadence with shorter cranks.

He's 154 cm tall, and I so a rule of thumb suggesting 10% of height as an optimal crank length, which in his case comes to c.155 mm.

What do you guys think? Will 10 mm difference in crank length make a notable difference to his cycling? Is 155mm a right length for his height, or should we be looking even shorter, say 145 or 150?

In summary, I'm pondering on removing the 105 crankset entirely, replacing it with a single narrow-wide chainring (104bcd or 110bcd) and adding suitable youth crankset with c. 155 mm cranks. There's a company (kidsracing) that procures these kids-specific components, and it doesn't cost a fortune.

Thoughts, comments please?
Any advice would be much appreciated.

Thanks and have a nice weekend.

imposter2.0

There is only one roll out limit in the UK for youth c, which is 6.05m, so use whatever gear combo you need to achieve that. If he is already happy on 165 cranks, then moving to 155 is likely to make things worse not better imo. Presumably the other races you are referring to are foreign nationals like Assen or similar, in which case you may need to switch to a unique or specific gear combo for those.

hockeyjunkie

Imposter wrote:

There is only one roll out limit in the UK for youth c, which is 6.05m, so use whatever gear combo you need to achieve that. If he is already happy on 165 cranks, then moving to 155 is likely to make things worse not better imo. Presumably the other races you are referring to are foreign nationals like Assen or similar, in which case you may need to switch to a unique or specific gear combo for those.

I'm referring to an overseas race indeed, where the age groups and the rollout limit are different from BC. It's 5.78m, normally achieved through 38x14x23c. Can't get anywhere close to it with the 34c cog unfortunately, no matter what the cassette.

Re crankset length, I still wonder if shorter cranks will improve his posture and cadence - not fixing something that's not broken

imposter2.0

'improving' cadence is pretty subjective, as it presupposes there is something to improve - the important thing is can he produce sufficient power at a cadence he is comfortable at? If he is already competitive on 165s, then leave them on. With events like Assen, you just have to bite the bullet and switch the gearing.

StillGoing

hockeyjunkie wrote:

2. His 105 groupset came with 165mm cranks. He did ok with them last year, yet I think that he could potentially benefit from improved cadence with shorter cranks.

He's 154 cm tall, and I so a rule of thumb suggesting 10% of height as an optimal crank length, which in his case comes to c.155 mm.

What do you guys think? Will 10 mm difference in crank length make a notable difference to his cycling? Is 155mm a right length for his height, or should we be looking even shorter, say 145 or 150?

In summary, I'm pondering on removing the 105 crankset entirely, replacing it with a single narrow-wide chainring (104bcd or 110bcd) and adding suitable youth crankset with c. 155 mm cranks. There's a company (kidsracing) that procures these kids-specific components, and it doesn't cost a fortune.

Thoughts, comments please?
Any advice would be much appreciated.

Thanks and have a nice weekend.

If you think about it, it actually gives him 20mm difference at the top of the stroke giving increased clearance getting onto the power stroke of the pedal revolution. By reducing to 155mm, you'd need to raise the seat height by 10mm for the correct angle at the bottom of the stroke. Add the 10mm clearance from the top and you have 20mm increased clearance at the top of the stroke.

https://bikedynamics.co.uk/FitGuidecranks.htm

Dorset_Boy

49t outer chainring to use in BC races with a 14-28 youth cassette locking offer a couple of the smaller rings.

imposter2.0

Dorset Boy wrote:

49t outer chainring to use in BC races with a 14-28 youth cassette locking offer a couple of the smaller rings.

With a 49t ring, you'd need to lock off half the cassette. When my lad was youth C, he used a 42 outer with a 14-25 cassette, locking off the 14 and with the 15 as his biggest gear..

hockeyjunkie

Dorset Boy wrote:

49t outer chainring to use in BC races with a 14-28 youth cassette locking offer a couple of the smaller rings.

49x17x23c should in theory give an exact rollout limit of 6.05, however in practice this is a recipe for trouble. Slightly wider or overinflated tire and you're over the limit. Have seen too many kids getting time penalties or even DQs from marshals for overgearing... Best to be 10-15 cm under the limit to have some leeway for tire size

imposter2.0

The gear checker can reject a bike if it fails the gear check, but marshals do not have the authority to DQ any rider. Only the chief comm can do that.

hockeyjunkie

Imposter wrote:

The gear checker can reject a bike if it fails the gear check, but marshals do not have the authority to DQ any rider. Only the chief comm can do that.

More youth races now do selective post-race gear checks, to discourage any tinkering with the bike before the start (like adding tire pressure). Errigal youth tour, for example, did that and handed bunch of time penalties this year across age groups.

Experienced marshals can pick an irregularity between the cadence and the speed and report a suspicious rider to the comm

imposter2.0

hockeyjunkie wrote:

Imposter wrote:

The gear checker can reject a bike if it fails the gear check, but marshals do not have the authority to DQ any rider. Only the chief comm can do that.

More youth races now do selective post-race gear checks, to discourage any tinkering with the bike before the start (like adding tire pressure). Errigal youth tour, for example, did that and handed bunch of time penalties this year across age groups.

Experienced marshals can pick an irregularity between the cadence and the speed and report a suspicious rider to the comm

Post-event gear checks are commonplace at NYS and regional omniums and can be ordered by any BC comm at any other race at their discretion, so there's nothing unusual there. Errigal is run under Cycling Ireland rules - if anyone here is caught trying to cheat the gear-check (post event), they will probably get straight DQ, not a time penalty.

Gear/cadence differences are easy to spot on the track - not so easy to spot in a circuit race bunch. Subverting the gear restriction rarely gives an advantage in youth racing anyway.