Does a 150cc bike really need ABS?
What is the optimal crank length on an eMTB?
The trail goes uphill, you don't want to lose your momentum, pedal hard. Suddenly there is a crack and you go over the handlebars! What happened? The pedal may have touched a stone or a root. So that this does not happen more often, you get the tuning tip "short cranks" more and more often. But what is the right length and in this case, for once, is shorter really better?
It feels like the correct crank length has been discussed for years, but so far almost exclusively for racing bikes. A certain standard has become established for mountain bikes. Small frame sizes (S) have 170 mm cranks, everything larger (M, L, XL) comes with 175 mm cranks. Most manufacturers then also transferred these dimensions to eMTBs. But is that correct?
The idea: why short cranks on the E-MTB at all?
With the e-mountain bike you have the opportunity to ride uphill paths that could not be ridden on a classic mountain bike. However, a problem often pops up: the ground clearance of the pedals. Getting stuck on the pedal can lead to a fall. Traditional mountain bikes have always had relatively long cranks in order to achieve an optimal compromise between ergonomics and power transmission. With e-mountain bikes, at least the issue of efficiency only plays a minor role thanks to the powerful motor. Sure, you could just increase the bottom bracket to make room for the pedals, but that would lead to poorer and unsafe downhill handling. Short cranks therefore seem to be the most suitable way to achieve more ground clearance when pedaling without negatively affecting the rest of the driving characteristics.
Our ROTWILD E + test bike already has a relatively high bottom bracket of 347 mm. However, if you sit in the SAG on the 160 mm bike (30% of the travel), the bottom bracket height is reduced by 48 mm. With a 175 mm crank, there is still an effective distance of 124 mm between the pedal and the surface. With a shorter crank you can gain the valuable millimeters that can prevent a fall.
Various cranks in a practical test
For our practical test, we compared three identical Miranda cranks in three lengths: 150 mm, 160 mm and 175 mm. With all three cranks, we undertook a defined test lap with technical uphills, long flat sections, classic forest roads and trail descents. We also compared the cranks with a watt measurement system on a defined, 150 m long slope.
The classic: The 175 mm crank
Everything feels very familiar the first time you step up. The step with the long crank is effective. We had no problems on normal paths and roads and were able to make good speed with the 175 mm crank. On average, we rode the crank at a cadence of around 70–80 revolutions. During the technical uphill, we touched down on one step and had to pay close attention to the choice of lines and the right timing when pedaling the entire time. When we later switched back from a 160 mm crank to the 175 mm, we also had the impression that we were stepping too far forward.
The middle thing: The 160 mm crank
If you were to step on a bike with a 160 mm crank blindfolded and without knowing it, you would probably not notice the difference directly. The driving feel is very close to that of a 175 mm crank, the step is also pleasantly round. On the technical uphill, however, the plus in ground clearance is already clearly noticeable. Although we only got stuck once with the 175 mm crank, with the 160 mm version all testers dared to pedal much more smoothly and had to pay much less attention to correct pedal management. When starting up on a hill, the lever is also sufficient to simply get the bike going. There is no noticeable difference on the descent.
The extremes: The 150 mm crank
Whoa, how's that going? The first turns of the crank with the very short cranks were extremely unusual. We felt like we were pedaling down rather than in a fluid motion. However, after a while this feeling subsided. What was noticeable was the cadence that was 10–15 revolutions higher on average. At the same speed, we usually drove at least one gear lighter than with the other two setups. It felt like we were able to pedal all the time and keep the drive on. However, we reached the limit with our translation in the steep terrain. The 36-tooth chainring would have to be swapped for a model with fewer teeth. At one point on our test track, every setup failed: on extremely slippery stones, it was not the crank, but rather the tire grip that was the limiting factor. On the descent, we found the reduced footprint rather uncomfortable due to the short cranks. In addition, the saddle height has to be increased with short cranks, which reduces the freedom of movement on the bike.
On tour with the buddies - one of them panting after them
If you often go on tour with friends who use a classic 175 mm crank, you should be prepared for one thing with a 150: You will be left behind! Of course, with an adapted gear ratio, you can climb all the slopes, but not that fast anymore! A test with the Garmin watt measuring pedals clearly shows: On a 150 m long test track with the 150 mm crank, an average of 25 watts more power was required to drive at the same speed with an identical gear and the same cadence. In short, this means: shift down and pedal faster so as not to tire too quickly. The difference between the 160 mm and 175 mm cranks, on the other hand, was significantly smaller.
|Crank length||Watt 1||Watt 2||Watt 3||Watt 4||Watt 5||Ø watt|
The long crank leads downhill
On our test track, we noticed a clear difference in a direct comparison between the 175 mm and 150 mm cranks, even downhill. Due to the smaller distance between the pedals, the dynamic footprint on the pedals is noticeably reduced. As a rider, you are less stable on the short cranks on the descent between the wheels and you can also lower your body's center of gravity less deeply when cornering, as the pedal on the outside of the curve can also sink less deeply. On top of that, the higher saddle extension with short cranks limits the freedom of movement on the bike. The differences are not huge, but they are clearly noticeable.
What is the optimal crank length for eMTBs?
As with all bicycles, the optimal crank length for an eMTB naturally also depends on the physical proportions of the rider. In this test, we found cranks with a length of 160 mm to be the best on eMTBs. You ride very naturally on the flat, you hardly need any more power or a hardly higher cadence uphill and still offer a noticeable plus in safety on technical uphills. The previous 175 mm cranks may be more comfortable for very tall riders with long legs, but otherwise offer no real advantage. In our opinion, very short cranks (≤ 150 mm) are an absolute niche product that may bring benefits for a minority in very technical terrain. With classic tours, however, considerable compromises have to be accepted.
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Text & Photos: Christoph Bayer
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