Crank length should be considered when buying a child’s bike as this can have a major effect on cycling. The Carrera blast came spec’d with 165mm cranks which for Lewis was far too long (I use 165mm XT’s). This was noticeable as he would bob when spinning under light load. Long cranks also affect saddle height which in turn affects body position over the bars. There are a number of pages on the internet to provide the information about leg and crank length to what is suitable. I recommend reading this page on bike dynamics and this one on highpath.net
The Carrera comes with a 22/32/42 triple crank with a 12-28 7sp cassette and is fitted with a no-name cone and cup bottom bracket – 68 X 122.5 square tapper. I fitted a SRAM 12-32 cassette at the same time as the new crank to give Lewis lower gearing.
There are a number of cranks to choose from if I wanted a minimum of 165mm but any lower was a struggle. BMX cranks are a good bet for small sizes but these are mostly single rings with a BCD of 110 which would limit the ring size to 34. Sinz cranks do a double but again with a 110BCD.
I eventually found a suitable crank made by thorn sold through SJS cycles which at 150mm was not totally perfect for Lewis’s size (would have preferred 145mm) is significantly shorter than the 165mm unit sold with the bike. The Thorn crank is a low profile that can take triple rings with a BCD of 104/64. In order to keep the correct chainline of 50mm I used a Shimano square taper UN-55 68x113mm bottom bracket.
The rings on the crank are 4 bolt SLX M670 with the 32 ring modified to fit (some of the plastic around the bolt hold had to be removed) held in place with NC-17 bolts
Having the shorter crank has given better pedal clearance, better position with the saddle raised and no more bobbing. There was also a significant weight loss too
old
BB – 312g
Crank – 938g
new
BB – un55 304g
crank – 766g
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An alternative to buying a short crank is to get your current cranks shortened – this however can only be done with certain types.
There is Bike smith design and are based in the USA who can supply SRAM S600 crankset in various lengths from $109. I think it works out about £120 posted with import tax, VAT and admin fees to the UK. In the UK there is Highpath Enginnering who charges £45 to shorten your own cranks. Highpath no longer do work due to health issues. Ben Cooper at kinetics will also shorten your own cranks depending on the type.
MTB mumblings 