Mark Stonich's Short Cranks

Mark was asked to give his reasons for becoming such an evangelist on the subject of short cranks. His main reasons are:

The advantages for shorter riders are obvious. But on 'bents many ÔÇťaverage sized' people are finding that they do better on shorter cranks than they would use on an upright bike. Some find they provide a performance advantage. Others find they help with knee trouble. In my case it's both.

Unfortunately, commercial availability of short cranks is very limited at present, which is why Mark ended up experimenting with shortening and welding crankarms (several of which have been shown at previous meetings See http://mnhpva.org/meetings/July_02/Short.html).

Kid crank adapters for tandems could be used, but they give a very wide Q factor (pedal to pedal width) that may be uncomfortable and/or inefficient for many people. TA makes cranks down to 150 mm, but they are very pricey (about $200-$250 for just the arms, from various sources ) and not widely available. (These have since been discontinued. Mark)

Some BMX cranks are in the right length range and have 110mm bolt circles for the middle and outer ring, but do not take inner chainrings without modification, which is just what Mark has been experimenting with. He is drilling and tapping 58mm or 74mm bolt holes in Dotec BMX crank arms, which are available for about $29 in 155 mm length. These have an area cast on the back of the arms for 3rd rings. Mark says that they must be sold as touring or MTB cranks in markets where people average smaller than here.

To accurately locate the holes he uses a CNC'd plate (made by Todd Sorlie in his machine shop). It has fifteen 9/32" holes drilled at 110mm 74mm and 58mm. There is a centering hole that is a snug fit on a turned down BB spindle, which is bolted into the tapered square on the crank.

Mark also showed a tapping machine he made, based on an old drill press base, that ensures the threads go in straight. He described welding the extension onto the column while maintaining alignment. Every weld causes some warping, so he'd measure after each segment of weld to decide which bead to do next. He's satisfied with the result.

He has started to promote his adapted short cranks on the internet and has gotten a LOT of feelers at a projected cost of only $60 for two re-drilled arms. (They are selling well now, see Mark) You still need a bottom bracket (standard square taper axle) and a set of chainrings, such as the rather pricey Salsa rings shown here. Mark passed around a made up triple that looked quite professional, and said it would even be possible to do a quad (though that application is rather fussy about spindle length and derailleur specs).


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