Configuring gears for junior triathletes: 5.70m and 6.20m development

[ffox88]

Hi Guys'n'Gals,

my teenagers (13 + 15y.o.) are entering a triathlon race this summer where there are strict rules about the maximum "development" (length covered by bicycle with one complete pedal stroke). The maxima allowed are 5.70m and 6.20m, according to age groups. In this particular race, it is apparently not allowed to block chain rings with the set screws, like I have been doing so far. That means I might have to reconfigure their entire gear system. I have found a perfect combinations chainring--cassette--tyre that gets me just under the limits
32T -- 10/12-34 -- 25-622 = max devel. 5.68m
32T -- 10/11-34 -- 23-622 = max devel. 6.18m

The question is how do I get there? Both bikes currently have a shimano 105 3x10(11-32) system. Can I get away with switching the cassette in the back and going to 2 chainrings in the front? Could I then keep the left levers? I think the derailleurs can handle sprockets up to 34T.
The other question is: where do I even buy shimano-compatible cassettes that start at 12T?

Or for the creative types: how would you configure this?

thx in advance

Chris

 

[JonBuoy]

10 speed Shimano cassettes starting at 12T are available. For example: https://www.merlincycles.com/shimano-deore-hg500-10-speed-cassette-1-61919.html

 

[N0bodyOfTheGoat]

https://sheldonbrown.com/gear-calc.html is an oldie but goodie for gearing

At a super quick glance from the cassette choice window, SRAM do 10-speed 12-32, can't see 12-34 option.

I've got Shimano 12-36 9-speed on my '20 Marasa hybrid, with a 26-36-48 triple.

 

[andrew_s]

If you've been blocking out a chainring using the limit screws, the simple solution is to continue to do so, but to also remove the blocked chainring.

You'll need either a set of single chainring sleeve bolts or a set of chainring tabs (though, being the sort of person who never throws anything away, I just cut the tabs off a worn out chainring).

You should be warned that the bike checkers will conduct a roll-out test rather than work out the development from the tyre/chainset/sprocket sizes, and as tyre size doesn't necessarily correspond with what's printed on the tyre, you could find that the bike gets disallowed at the start line if you've tried to get as close as possible to the limit.

 

[DCLane]

You should be fine with two chainrings and a 12-other cassette.

+1 to the above post about checking. It's always worth measuring the distance yourself and doing a roll-out. I've been there with son no. 2 and we were a tiny bit over due to changing the tyre to a different make.