About Crankarm Length
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  Posted October 6th, 2016 by Zdenko  in TOOLBOX | No comments yet.

Coaches corner

By: Joe Lindsey, Published in Bicycling magazine

What You Need to Know About Crankarm Length
Overlooked for years, crankarm dimensions are starting to play a pivotal role in many aspects of cycling.

Crank-Length-600x362Crank arms are made in many different lengths

For years, most cyclists assumed that crank arms came in one size: the one on their bike. Chainring choices aside, road bikes have long come with 172.5mm crank arms regardless of frame size, while stock sizing for mountain crank arms is generally 175mm.

Bike and parts makers have quietly started introducing more sizes in recent years because, says Ian Buchanan, master bike fitter and owner of top East Coast shop Fitwerx, ‘There’s tons of people out there for whom the standard-size cranks aren’t going to work best.’ Consider that crankarm lengths can differ by only a few centimeters, even as riders’ inseams and seat heights can vary 20cm or more.

Although stock options on new bikes are still fairly limited (you might find shorter crank arms on very small bikes or women’s models), aftermarket road crank arms now range from 165 to 180mm long, and that’s just from Shimano and SRAM (Campagnolo, sadly, doesn’t offer options outside of the traditional 170, 172.5 and 175mm range). Look further afield, and you’ll find options from a Lilliputian 130mm to a Brobdingnagian 220mm.

But with all this choice, how do you know what’s right for you? Should you go shorter or longer? And what are the effects of changing crankarm length on everything from your bike fit to gearing? To find your appropriate length, it is essential to work with a master fitter who has experience changing crankarm lengths. But if you’re curious, here’s what to know if you want to explore further:

Body size is a guide, not a formula.
It’s not a shock that proper crank length generally follows rider height. To start, fitters typically look at physiological dimensions like femur length and foot size, both of which are primary lever arms in a pedal stroke. But those body data points ‘only help me make a decision, they don’t drive it,’ says Zac Daab, co-founder of Cascade Bicycle Studio and former lead fitter for custom builder Seven Cycles. Other factors, like pedaling style, have to be considered as well. ‘What I look at is what a rider is doing in the pedal stroke, especially from the two o’clock to six o’clock positions, and particularly what the ankle is doing,’ says Daab.

Aches and pains can be a clue.
Back and knee pain are often signs of improper bike fit, and can point to crank length concerns. ‘If someone is still having knee pain after I do a fit, we may try to change pedaling trajectory with a different crankarm length,’ says Greg Combs, a master fitter at North Carolina’s VeloSmart. Buchanan says that a history of low-back or calf issues on the bike can be an indicator that a rider is on too-long a crankarm. But before you mess with crankarm length, adds Buchanan, take a look at other likely causes of pain and cramps, like turning a high-enough cadence or building core strength. Crank length changes may help solve long-simmering aches and pains, but they’re most effective when you’re doing everything you should be doing to be strong on the bike. 

gearThe crankarm that comes with your bike might not be the best one for you. PHOTOGRAPH COURTESY OF EMILYDICKINSONRIDESABMX VIA FLICKR

It changes gearing.
If you do end up changing crankarm lengths, it will change your gearing as well. How hard or easy an individual gear feels depends on three things: the combination of chainring and cog size that actually produces it, the wheel size it rolls on, and the crank length that turns it. The relative jumps between gears stay the same, but the overall range will feel slightly easier to pedal, or smaller, with shorter cranks and harder with longer ones. If you’re only changing slightly – say, from 172.5mm to 170mm – it may not be an issue. But if you’re a very short rider making a big change – like 170 to 155 – you may need to change your gearing to compensate for the shorter cranks. 

A related consideration: Since Shimano and SRAM only make crank arms between 165 and 180mm, going outside that range means you might also need to buy new chainrings, or even a different bottom bracket.

CrankLengthThis pictures shows how to measure crankarm length

It might change your bike fit.
If you’re only making a slight move, then adjusting saddle height to account for the longer or shorter cranks may be all you need to do. But if, say, you’re a 5’2’ woman who’s been sentenced to 172.5mm road cranks all your life and you’re moving to 165mm or shorter cranks, more substantive changes may be necessary. (No-duh note: This is just an illustrative example, not a crankarm length recommendation for 5’2’ riders.)

The main points to consider: handlebar height and saddle setback. Handlebar height is always set relative to saddle height. That 5’2’ woman moving to 165mm cranks will raise her seat about 7.5mm to compensate for the shorter cranks. So her handlebar will be lower relative to her new seat height, and she may want to raise it. Raising your seat height also moves the saddle back (the ratio varies slightly depending on seat angle, but it’s roughly 2:1) so your fitter may also push your seat forward slightly on the rails to maintain proper setback. Always make these changes in consultation with your fitter, and keep track of all original fit measurements so you have a baseline for comparison.

Finally, consider that you may decide not to make other changes. ‘A lot of triathletes who go to shorter cranks do it in part because it opens up the hip angle at the top of the pedal stroke as the seat rises,’ says Buchanan. With a more open hip angle, they sometimes find that they can keep the original handlebar position and comfortably produce power in what is now a more aerodynamic position.

There may be other fixes first.
Changing crankarm lengths may be an effective way to fix fit and ergonomic issues on a bike, but it’s not your first resort. Setting proper seat height is the absolute first step in any fit, says Daab. And if you’re looking at making relatively minor changes in crankarm length, your fitter should work with cleat position first. Sliding the cleats back on the shoe has the same effect as putting on a longer crankarm; a more mid-foot cleat position essentially adds to your leg extension at the bottom of the pedal stroke. That said some riders benefit from a mid-foot cleat position for other reasons, like reducing forefoot pain or tingling. Proper cleat position should be considered first from a standpoint of what’s best for your fit, and only second as a way to affect leg extension.

Bike-Crank-Arm-length-DeterminationBike Crank-Arm length determination

What’s available:
Need shorter or longer cranks? Here are the broadest stock options from each major component maker. Road cranksets are typically available in a wider size range than mountain bike cranksets.

Shimano Road: 165mm-180mm in 2.5mm increments (11s Dura-Ace)
Shimano Mountain: 165-180mm in 5mm increments (XTR, XT)
SRAM Road: 165-177.5mm in 2.5mm increments (Red, Force, Rival); 162.5-177.5mm in 2.5mm increments (Force 1)
SRAM Mountain: 170, 175mm (all groups)
Campagnolo: 170-175mm in 2.5mm increments (all groups)
Rotor: 155-180mm in varying increments (3D+ road only)
FSA road: 165-175mm in varying increments (SL-K Light, Gossamer Pro, Energy)
Most retailers do not stock cranksets outside of the standard 170mm-175mm range, but will be able to special order other sizes.

Need more? Zinn Cycles makes a range of cranksets for road andmountain bikes that start at 130mm and go all the way up to 220mm long.

Hope to see you on the road.

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