Bicycle Mechanics - What's the shortest crank arms I can find?
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05-29-07, 04:45 PM
My wife has limited mobility in her knees. She has a comfort bike with a Truvativ crankset and 170mm arms but has trouble riding it. What are the shortest crank arms I could find, preferably using the existing press-on (square) bottom bracket. Thanks!
05-29-07, 04:57 PM
You can usually find several popular cranks in 165 (Shimano, etc.). The smallest I've seen is a 155--you can get it from Peter White (http://www.peterwhitecycles.com/carmina.asp) (and probably others).
BMX and recumbent bikes tend to have riders that
are interested in shorter crank arms. A search on the
BMX board here and a look at the popup ads for
bike shops might turn up something. I have seen
people on bent forums talking about cranks in the 145
range, or 1 3/8" shorter than usual. There are also
crank shorteners designed for children that screw into
the original crank at the pedal end and have an arm that
points toward the BB of various lengths into which the
pedal is screwed, shortening the crank. Might be easier
to find these and try them first. A google on crank shortener
turned up several at various prices. And finally, children's
bikes will have shorter cranks, and a perusal of yard sales
on a weekend or two might turn up a $10 bike that you
can recycle the cranks from. Chainwheels may be a problem
for some of these as they are more likely to be nonstandard,
apart from the ubiquitous sq taper which is usually obvious.
05-29-07, 05:48 PM
all the way between 140 and 175.. they're not great, but they're strong enough.
The Bulletproof cranks come in a variety of lengths and allow for double chain rings
05-29-07, 07:14 PM
I seem to remember Sheldon saying something about TA (a French company) making all of the sizes 140 plus.
05-29-07, 07:21 PM
The FMF are only good for 1 chainring.
The Bulletproofs will take two or 3 but nothing smaller than 34t.
TAs only go down to 155mm and are priced for lottery winners and CEOs.
A person with range of motion issues probably needs a low geared triple.
http://bikesmithdesign.com/Short_Cranks shows some of what I have. I may also be able to simply shorten her Truvativs.
I do short cranks for a living and have helped hundreds of people find the right solution for their problems.
Contact me at email@example.com or 612-824-2372
05-30-07, 01:46 AM
If I may humbly propose something for the gearing: if you use a hub gear system, like the Shimano Nexus (7 or 8) or SRAM Spectro S7, or best of all (if you can afford it) a Rohloff Speedhub, then the number of chainrings in the front is a non-issue, as all you need is one chainring, and one cog in the rear. For people that need low gearing, you might have a 34T or 32T in the front, and then a 22T sprocket ("cog") on the geared hub, for a low starting gear.
If budget is a concern, go for the Shimano Nexus 7 which can be had together with the wheel it's built into, for as little as US$ 80 on eBay. Maybe even less, by now (the low price of the Nexus 7 is probably caused by the newer Nexus 8 which offers a larger gear ratio in addition to one more gear).
Gear hubs offer the advantage of requiring nearly no maintainance, and your wife would have to concentrate on switching gears with only one hand, unlike with deraileur systems.
By the way, this thread is very useful to me, too, as I have relatively short legs, and struggle with anything over 170 mm. I think 160 would be ideal for me.
05-30-07, 07:10 AM
I may also be able to simply shorten her Truvativs.
+1 to shortening cranks. If not the Truvativs, just get a decent solid forged aluminum crank, drill, tap, and cut off the ends. Here are some cranksets that start off very inexpensive -- but go up to the normal prices: http://aebike.com/page.cfm?PageID=30&action=list&category=181&type=T&searchtype=attributesearch&searchDropDown104=&searchDropDown97=&searchDropDown282=&searchDropDown99=&searchDropDown2=&searchDropDown271=&searchDropDown171=&searchDropDown105=&searchDropDown80=&searchDropDown82=&searchFrom10=&searchTo10=&gobutnsearch=Search
Shortening is probably the easiest way to go, IMO. Aluminum is soft.
05-30-07, 08:01 AM
Though take it to a machinist that has a good mill so you can drill the pedal holes straight
05-30-07, 01:08 PM
SJS cycles has a shortened 125mm (http://www.sjscycles.co.uk/category-Cranksets-363.htm)
They sometimes have their own crank in 140mm size.
http://www.davincitandems.com/child.html are worth a call.
Crank shortening requires a fairly accurate jig. A couple of workshops already have one for this purpose:
In the UK Highpath Engineering.
In the US:
05-30-07, 01:23 PM
I put a 152 mm crank with triple rings on my wife's bike, because of her arthritic knees. It was a cheap square taper set meant for a youth's bike. It also required a longer BB to make the granny ring clear the frame, and move the front derailler down a few mm.
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