The trail in front of you gets steeper and you don’t want to lose your momentum, so you start pedalling like a maniac. Then you hear a loud clunking noise and everything comes to an abrupt halt… the next thing you know you’re flying over the bars! What happened? Chances are you jammed your crank into a rock or root, which is exactly why shorter cranks are one of the current hot tuning tips. But what is the ideal length? Does size really matter in this case?
Over the last few years there has been an ongoing debate about optimal crank size. While this mostly applies to road bikes, in the case of mountain bikes some sort of standard has been set. Small frame sizes (S) are usually fitted with 170 mm cranks, while you are more likely to see a set of 175 mm cranks on bigger frame sizes (M, L, XL). Now manufacturers are applying the same standards to e-mountainbikes. But is this right?
Here’s the idea – why short cranks on an E-MTB?
With an e-mountainbike we can now ride steep and technical sections which would be impossible to clear on a traditional mountain bike. However, there is a common problem: pedal clearance. If your pedal gets stuck, you can crash. Until now, classic mountain bikes were specced with relatively long cranks in an attempt to find an optimal balance between ergonomics and power transfer, but with e-mountainbikes efficiency becomes a secondary issue thanks to the support of a powerful engine. We could also just raise the bottom bracket, but this would also compromise downhill handling qualities. That’s why short cranks seem to be the ultimate solution to improve ground clearance while preserving other important riding characteristics.
The 347 mm bottom bracket on our Rotwild E+ test bike is relatively high. However, once we add the weight of a rider onto our 160 mm-travel bike, 30% sag will cause the bottom bracket to sink by a further 48 mm. As a result, with 175 mm cranks the total distance between pedal and ground will shrink to a mere 124 mm. Shorter cranks will add some valuable millimetres of ground clearance and might prevent you from crashing or being thrown off the saddle.
Testing different crank lengths
In this test we compared three identical Miranda cranks, the only difference being their lengths: 150 mm, 160 mm, and 175 mm. We put all three cranks through a predefined test- track which includes technical climbs, long flat sections, a typical fire road uphill, and a trail downhill section. We also fitted the cranks with a power meter and compared the data recorded on a 150 m climbing section.
The classic: the 175 mm crank
When first getting on the bike, the long crank arm feels comfortably familiar and the pedal strokes are incredibly efficient. On normal paths or roads, the 175 mm cranks didn’t cause us any problems and made it easy to build up speed while maintaining a cadence between 70 – 80 rpm. On our technical climbing section we jammed the crank once into a step and we had to pick our lines carefully while trying to keep a good rhythm. After changing from a 160 mm to a 175 mm crank, we were left with a strange feeling of pedalling towards the front.
The in-betweener: the 160 mm crank
If you were blindfolded and put on a bike with 160 mm cranks without knowing it, you would hardly notice a difference. The overall ride feeling is very similar to a 175 mm crank and the stroke feels nicely rounded. However, what really caught our attention is the additional ground clearance. Even though we got only stuck once with the 175 mm cranks, the 160 mm version just gives you extra freedom and the confidence to forget about scanning the terrain for lines or managing the pedals. On approaching a climb, the cranks still manage to deliver good amounts of speed. We didn’t notice any differences to the 175 mm cranks on downhill sections.
The extreme: the 150 mm crank
Woah, now this is something else! The first strokes with the short crank feel very strange indeed. We feel like we are pedalling in a “downward motion” whilst missing a certain fluidity. The good thing is, this feeling goes away after a while. Really noticeable is the higher pedalling cadence, with an average increase of ten to fifteen rpm. At equal speed we are running at least one gear lower than with the other setups. However, the short cranks really manage to excel in the rougher trail section scattered with massive roots and rocks, where we could keep a steady stroke while delivering good and even amounts of traction. Having said that, the bike’s gear ratio reached its limits on very steep gradients. This can be easily fixed by swapping the 36 t chainring with a smaller one. All three setups failed in one particular section of our test track, but in this case the tires, not the cranks, were the limiting factor on the very slippery rock passage. Shorter cranks result in a smaller standing platform, which leaves us feeling rather uncomfortable on descents. The shorter cranks also require a higher saddle position, which restricts the range of movement on the bike.
A day on the bike with your mates – one is left behind
If you are used to riding with your mates and they all run a classic 175 mm setup, you should be forewarned before deciding to switch to 150 mm cranks – you’ll be left behind! Obviously the right gearing will get you to the top of every climb… just more slowly. A test with the Garmin power-meter pedals makes this clear: over a stretch of 150 metres, the 150 mm crank requires on average an additional 25 watts of power to reach the same speed with the same cadence and gear ratio. This means you have to change into a lower gear and pedal faster; this way you won’t get as tired. The difference between the 160 mm and the 175 mm cranks was significantly smaller.
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Downhill with the long cranks
When comparing the 175 mm and the 150 mm cranks directly we also noticed a clear difference on downhill sections. Due to the short distance between the pedals, the standing platform is noticeably smaller.
On descents a shorter crank will make you feel less anchored between the wheels, while at the same time when cornering you won’t be able to lower your centre of gravity as much because the outside pedal can’t reach as far down. Additionally, on the upper side the elevated saddle position restricts your range of movements. The difference is not dramatic, but it’s definitely noticeable.
What’s the optimal crank length for e-mountainbikes?
As with all bikes, the optimal crank length also depends on body proportions. In our specific case the test has proven 160 mm cranks to be the optimal e-mountainbike solution. Still delivering a very natural pedalling feeling on flat sections, they only require a little more power (or a higher cadence) on climbs while offering an obvious advantage in terms of safety on technical uphills. The popular 175 mm cranks might be a very comfortable option for tall riders with long legs, but they aren’t really offering any more advantages. Finally, we consider very short cranks (≤ 150 mm) to be more of a niche-product aimed at a handful of riders and for a limited range of applications in very technical terrain, while presenting a number of disadvantages on trail rides.
Words & Photos: Christoph Bayer