Originally Posted by
Duragrouch
Good explanation, but I would debate the bolded above; Chain driving toothed cogs is about as discrete, dare I say digital, as it gets. These are not frictional belts. Perhaps as you get to very small cogs, if not good teeth profiles, so less smooth tooth transitions, you can get very slight rotational speed variations, but in terms of displacement per rotation, it should still be same. Discuss.
Anything affecting tire rolling circumference, different story, that can change rollout for same gearing.
When you're dealing with Jr gearing, now only in track, a minute difference in the rear cog can mean the difference between skating under the line or over it. But even chainrings can have a small difference. Remember, the rear turns a little over 3x for the full rotation of the front that determines rollout. Tires matter as well, which is why many parents in track try to get their kids set up in the spring and not touch the bike again. It is possible to take 2 different 14t cogs and hold them against each other to find out the valleys don't match, whether different brands, different quality levels, or even just which was made when the tools are more or less worn. When you're dealing with number like 6.45m or 6.93m, your junior is already strong enough the gear is a hold back, and that 1/2" difference matters you test out different cogs and rings. For road, where it no longer matters, the difference of a 25c tire, something that was gaining traction in 06 as a racing size, the swap from a common 50t to a 49t might have been needed to avoid disqualification. Last year, in the first road race my kid did, practically half the kids were in danger of disqualification, but since the Jr's were racing with adults the officials decided to give the kids who went over rollout a time handicap to make it fair for the other Jr's. Month later the road rule went away and all the kids ditched the 14t for 11s. Had to deal with a parent at states who was dealing with it last year, his kids had the same bikes, same gearing and one failed, he'd bought 2 different 16t cogs due to stock issues, you had to hold them against each other to see the difference which was very small but at 16 valleys times 3 rotations the 1/4" difference means one fails and the other skates.