This post compares the advantages and disadvantages of 175mm and 172.5mm crank arms.
The Advantages of 175mm Cranks
- More Leverage
The longer the crank, the greater the leverage that the rider has over the chainrings. Consequently, one could make the argument that a longer crank makes it easier to pedal in higher gears.
That said, the length difference, in this case, is only 2.5mm or 1.44%. Thus, it’s highly questionable if this effect manifests to a noticeable level.
- More Suitable For Riders With Long Legs
In general, riders with longer legs prefer lengthier cranks too. One of the reasons for that is the knee angle. When a rider with long legs is put on a bike with excessively short cranks, the knee angle gets smaller.
Right before the pushdown of the pedal, the long femur positions the knee in front of the pedal/toes. When the crank is shorter, this effect is even greater.
The smaller knee angle places more stress on the knee when the rider presses down. Conversely, a longer crank opens the knee angle when the pedal is in the above position and reduces the stress on the knees.
However, this doesn’t mean that a longer crank is the solution to all knee pain. Sometimes the problem comes from the entire bike fit (frame size, saddle height, stem position…etc.).
The Disadvantages of 175mm Cranks
- Reduced Ground and Front Wheel Clearance
A 175mm crank is closer to the ground at the bottom of the pedal stroke and to the front wheel during the first 1/4 of the stroke.
As a result, the crank increases the chances of toe overlap and pedal strikes.
Toe overlap is a phenomenon during which the toes of the rider hit the front wheel when making a turn at a slow speed. Normally, this is not a major issue because it occurs only when the rider is pedaling and the speed is very low. However, it’s still an annoying experience.
It’s also possible to hit the ground with the pedal when cornering if the bottom bracket is already low. That said, if 2.5mm results in severe pedal strikes, the problem could lie elsewhere (e.g., the bike has been equipped with wheels that are too small or the frame geometry is simply off.)
- Smaller hip angle
The hip angle is the angle between the torso and the femur (upper leg). When the cranks are longer, this angle gets smaller and thus “closes”. A smaller hip angle is a bit more difficult to maintain and makes it slightly harder to get low and aero.
The difference is a lot more pronounced when the discrepancy between the cranks is greater. For example, if you go from 175mm cranks to 165mm, the hip angle will open much more and the extra comfort will be a lot more perceivable.
For that reason, many people choose to put shorter cranks on their time trial (TT) bikes. Time trial bikes require the rider to have a more horizontal back angle which is easier to maintain when the hip angle is more open and the rider’s knees aren’t kicking him in the chest.
The Advantages Of 172.5mm Cranks
- Extra Clearance
The shorter cranks will elevate the pedals and position them further away from the ground and the front wheel. This will reduce the chances of toe overlap and pedal strikes every so slightly.
- Better for Riders With Short Legs
When the rider with short legs is put on a bike with long cranks, the knee angle opens too much. As a result, there’s power transfer loss as well as additional stress on the tendons of the hamstrings. Hence why riders with shorter legs often feel more comfortable when pedaling with shorter cranks.
Everyone Is Different
Whether the above will manifest depends on the rider’s anthropometry, health, the bicycle, and the riding discipline. There isn’t a universal formula that fits all riders and purposes. Bike fit and performance are complex and require a lot of trial and error until one reaches an acceptable solution.
FAQ: Is the difference between 175mm and 172.5mm really that small?
Yes. Most riders won’t notice the difference between those two crank sizes.
It’s also possible to conclude that one size is better than the other because the change of cranks has triggered other changes (e.g., getting a bike fit) that mask the actual effectiveness of the new cranks.
That said, some people claim to notice a difference.
FAQ: Is it true that shorter cranks make you faster because the pedal stroke has a smaller diameter allowing for a higher cadence?
Not really. If shorter cranks always equaled more speed, the pros would be using them all the time.
The truth is that you will be faster on cranks that are comfortable and allow you to pedal for prolonged periods of time without experiencing joint pain. If those happen to be shorter cranks such as 172.5mm, so be it.
Or in simpler words, more comfortable cranks are faster cranks.
FAQ: Is it true that I can compensate for the bike fit change caused by a new set of cranks via the saddle?
Yes, to a degree.
If you go from 175mm to 172.5mm, the saddle will have to be elevated by 2.5mm.
If you go from 172.5mm to 175mm, the saddle has to be lowered by the same amount.
Summary: What You Need To Know
- The discrepancy between 172.5mm and 175mm cranks is negligible. It could make a difference in very minor cases, and it’s questionable whether the new crank size is the actual source of the improvement.
The theoretical advantages of 175mm cranks are:
- More leverage allowing riding in a higher gear;
- Better for riders with long legs
The disadvantages of 175mm cranks are:
- Reduced pedal clearance
- A smaller hip angle making it harder to maintain an aero position
The advantages of 172.5mm cranks are:
- More comfortable for people with shorter legs
- Increased pedal clearance
- Open hip angle making it easier to maintain an aero position.