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Crank length is a very personal business, most people use what they first get with the bike and only change it according to fashion.
The only other catalyst tends to be knee trouble which bring the old wags out saying "Those cranks should be shorter."
I offer a table below based on second hand evidence (I have only ever ridden with an 88cm inseam so how could I test my
short cranks advice?)
Actually other things effect crank length selection. Longer cranks make your knee subtend a larger angle each rev, hence you may rev slower, or you may be able to push a bigger gear.
If its the latter, its a good move!
If you are used to one length and conclude you need a change of 5mm (or more) then you might consider a graduated
changeover.
This can get expensive, so ask around for swaps till you reach the target. You may find your stepping stone
is better than the target anyway.
Knee force is reduced by longer cranks, they are not inevitably harder on your knees.
The biggest factors in
knee trouble are past injuries (eg Rugby) coming back to haunt you, not keeping knees warm or failing to warm up
before an effort and overgearing.
I would say the last is the worst since it is so easy to avoid. (Cadence computers are cheap nowadays)
Many riders mix crank lengths according to the situation, I used to use 175mm in the winter (cheaper cranks!), 177.5mm in the racing season and have even
experimented with 180mm with no knee problems at all.
I started on 170mm and used to be a revver:
Now I am a much faster rider and still rev faster than my peers.
Miguel Indurain gradually increased his crank length until finally, when he used 180mm, he won the tour, five times!
One important aspect is saddle height. Some believe the saddle height should drop by the increase in crank length.
Some say leave it alone. I have found that for my circumstances I leave the saddle alone when I shorten the cranks for
the winter but would drop it 2mm for 180mm.
The only problem I find with 180mm is it takes a couple of weeks to get
totally fluid with the change and I wanted to use them for a one off event so it is not worth the bother.
Muscle memory is part of what makes a champion so messing with it via crank length alterations is asking for trouble.
Bear in mind that when you shorten a crank, if you leave the saddle height, then at the bottom of the stroke your leg will
be less streched but also the upper position will raise the pedal less as well:
Hence any saddle height change is going to affect one or the other and make any change greater.
23½" (60 cm) up to 24¾" (63 cm) 160 mm
24¾" (63 cm) up to 26" (66 cm) 162.5 mm
26" (66 cm) up to 27½" (70 cm) 165 mm
27½" (70 cm) up to 29" (74 cm) 167.5 mm
29" (74 cm) up to 31" (79 cm) 170 mm
31" (79 cm) up to 32¼" (82 cm) 172.5 mm
32¼" (82 cm) up to 33½" (85 cm) 175 mm
33½" (85 cm) up to 35" (89 cm) 177.5 mm
35" (89 cm) up to 36½" (93 cm) 180 mm
Over 36½" (93 cm) 182.5 mm
You may wonder why the main manufacturers only provide 170, 172.5 and 175 with the premium models coming in 177.5mm and 180.
It is a example of profits before ethics!
Campagnolo in particular should know and act better: When they brought out a new ceramic bearing version of the Record carbon chainset they promptly stopped doing 177.5mm in the Record and only now do it in the Super Record!
All I can say is shame on them!
The pros of course insist on the correct length crank so some even have to use old models or other makes with camouflage!
Others makes are more helpful, for example, Miche Monolithic Young Chainsets come in 125, 145 and 155mm!
We can order these as required.
Stronglight used to be very accomodating but even they seem to have come into line and dropped the wide range of lengths.
Steve Howells. BSC Hons.
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