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Road Crankarm Length

What if the key to more power or comfort on your bike was as simple as changing your crankarm length? While changing the length of your cranks might not be a necessary step for every rider, knowing what the differences can mean to your ride, and how they should be set up for your physiology are important factors if you’re searching for more power or comfort. A small adjustment to crank length can make a big difference in overall enjoyment of your ride.

15_TourAlberta_JDuncan 66

1. Crank arms come in different lengths!?
Crank arms come in a few standard lengths- in 2.5mm increments from 165mm to 180mm for most cranks. This range is slightly wider than the range found on mountain bike cranks, as the riders position is more static, and the size range has more to do with fit and comfort on a road bike, and less to do with ground clearance. While sizes outside that range are available, they are certainly not commonplace. The cranks your bike came with are likely 172.5 or 175mm, and they’re probably pretty darn close to perfect for you. It’s still important to know why crank arms come in different lengths, and what the differences actually mean.

2. But what’s the point?
There are two main reasons that cranks are sold in different lengths: Fit and power transfer. As we discussed in our article on mountain bike crank length, power transfer at low RPM’s is increased with longer crankarns, while shorter crankarms allow a rider to get up to a high RPM faster. As road riders typically spend the bulk of their riding time spinning faster than 60rpm (the point at which the benefit of longer cranks falls off sharply), the power advantage is really only felt in short accelerations and out-of-saddle climbs. The fit aspect of crankarm length is a more important factor to consider. Changing crank length has an affect on leg reach, upper body angle, and pelvic angle, all important factors in a comfortable and efficient fit.

3. What’s the difference?
Considering only the three fit factors we identified; leg extension, upper body angle and pelvic angle, the importance of proper crankarm fit is pretty clear- and there are more factors on top of these three! Make sure you visit your local LTP Sports dealer for a discussion of proper bike fit before making any serious changes, as they’ll be able to discuss the issue in greater detail, as well as help you with the fitting process. Let’s go over the three fit factors affected by crankarm length.

2A. Leg extension. This one is pretty simple, the shorter your cranks, the shorter your effective leg reach, and the higher your saddle has to be to give you proper leg extension (30 degrees of deflection at the bottom of the pedal stroke).

3B. Upper body angle. The angle of your torso is directly affected by the height of your handlebar compared to the height of your saddle. If you shorten or lengthen your crank arms, you’ll have to raise or lower your saddle to ensure that your leg-extension doesn’t change, and this will change the position you ride in, unless you also adjust your handlebars by the same amount.

1C. Pelvic angle. Here’s the big one, and the primary reason that you’ll want to adjust your current crank arm length. Longer or shorter cranks have a large effect on the amount that your legs move at your pelvis, which can fix or exacerbate a number of injuries or fitting issues. Your flexibility (and the tightness of your hip flexors, so don’t forget your stretches) dictates how far you can move your leg (at your hip) before you start moving your pelvis as well. Remember that you want your pelvis to stay in one spot while you ride, planted on the saddle, your legs and upper body moving around it. Not very flexible at the hip? Shorter crank arms mean that at the top of the pedal stroke (when your thigh is closest to your torso) your pelvic angle is lower, and you don’t have to reach ‘up’ as much. Someone with inflexible hips and long crank arms might find themselves rocking their pelvis at the top of every pedal stroke, which is both uncomfortable and a huge sap on power output.

There are two other ways to ‘open up’ pelvic angle without changing your crank length- neither of which are very effective on their own. A rider can raise their handlebar height, or slide their saddle forward. Unfortunately, raising the height of the handlebars affects aerodynamics, balance, and pressure on hands/saddle, and sliding the saddle forward changes the riders pedal stroke, muscle balance, and effective leg extension.


So, do you think changing your crank arm length might make you faster or more comfortable on the bike? The best way to find out is to get fitted by a professional, but you can start the process by identifying whether you ever experience hip pain on the bike, whether you’ve been observed to ‘rock’ on the saddle, both of which can be factors of improper crankarm length. Head to your local LTP Sports dealer for fitting advice- they’ll also be able to direct you toward the following cranks if your ride would benefit from the upgrade.

313195-01RED 22 GXP CRANKSET
11 x 2 Speed Road

• Exogram hollow carbon arm with integrated 4-arm spider and hidden bolt
• X-Glide-R AL 7075-T6 chainrings
• 5mm thick chainrings provide added stiffness for better energy transfer
• Designed to be used with Sram Red 22 front derailleur with Yaw technology
• Comes 1n 170 – 175mm lengths
• 714g


 

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