Crank length query


#1

How much difference does 2.5mm make?! I use a Wattbike Pro to train on a lot during the winter and it has 170mm cranks but I ride 172.5mm cranks on all my road bikes. What affect does this have, if any?


#2

I wouldn’t sweat it, I doubt you’ll notice a difference. Any differences in power delivery between the two will likely be caused by the geometry differences between the Wattbike and your road bikes, not the crank length. I switched from 172.5mm to 170mm to try and get a better fit and I didn’t feel like my pedal stroke changed at all, it just allowed me to raise my seat up ever so slightly


#3

Ditto I don’t find it makes any difference personally. Interesting article on it: https://www.trainingpeaks.com/blog/the-benefits-of-reducing-your-crank-length/


#4

Crank length is highly subjective. Choose what you like and ride it. 2.5mm is small enough as to be quite negligible.

The one thing I remind people is to make sure you have your saddle height set correctly.

Assuming that you have the same saddle and position, as well as pedals and shoes, from one bike to the next; the common way to measure a saddle height is from the center of the Bottom Bracket (BB) to the top of the saddle (exact location on the saddle varies between measurement practices, but make sure you use the same relative location in all cases).

If you measure from BB, then crank length differences mean you need to adjust the saddle height to match that difference. You want the same effective length from the pedals to the saddle (again, assuming equal equipment from bike to bike).

So, your saddle height (measured from BB) on a bike with 180mm crank will actually be 10mm lower than the saddle height (measured from BB) on a bike with 170mm crank. That is because the longer crank is “lower” at the bottom extension point, with respect to the BB.


#5

Just to add to Chad’s post, crank length can also be used to help open up the hip angle if you or your fitter find it’s too closed. Personally, I find the hip angle “problem” perhaps the most relevant parameter to select crank length.


#6

True, it can be used to control hip angle. However, one point that often gets missed is the consideration of the front end height in conjunction with any crank length change.

  1. If you select a short crank to reduce total knee and hip angle flexion, that leads to a taller saddle height (as correctly shown in the pic).
  2. If you also leave the handlebar height unchanged, that effectively leads to a closing in the hip angle between the torso and thigh (also shown via the higher, but flatter blue torso). This is because the front end is unchanged, but the hips come up and close that hip-torso angle again.

So, unless you increase the handle bar height by the same distance as the saddle height increase, you may actually eliminate any “flex angle gain” by only adjusting the crank length.

It is important to take the entire picture into focus, and not just one part of the whole system. I find this detail is not examined as often as it should be, especially when crank length changes are applied as suggested.


#7

Absolutely correct. It’s just a piece of the puzzle.


#8

I have 2 bikes with 170 crankarms and one with 175. I can’t really tell the difference at this point. I like the 170 over long distances and the 175 for shorter fast group rides. That’s not because the 170 isn’t fast, but more because I feel like I get a little more leverage on the pedals with the 175. The 170 spins quicker and doesn’t require as much at the top of the stroke, but I still like the 175’s. In conclusion, there is hardly any difference. Riding on either one feels fine.


#9

I think it’s quite personal to your biomechanics. If you can feel the difference when riding and you think it makes a difference to your performance then I would look to make then the same, but if you don’t notice the difference I wouldn’t bother going through all the hassle.
I run 165mm cranks on all my bikes and I really notice any difference in crank length. I have disproportionately short upper legs / femurs and the short cranks make a massive difference to my pedaling comfort, especially on the turbo where the pedaling effort is more consistent. I also notice any tiny change to my saddle position (even a shift of a couple of millimeters will cause issues). I can be happily comfortable with a huge range of front end positions (and my various bikes have very different front end positions) as long as the back end is dialed in to my exact preferences.


#10

For me (1.84m, 93cm inseam) it made a lot, when I switched from 172.5 to 175mm. But I‘m sensitive to things like that.


#11

Thanks everybody. I guess I should get a bike fit! Never had one and I’ve been riding for many years. Perhaps I’ll treat myself for my 50th next year! When, incidentally, I plan to beat my 10,25,30 and 50 Mile PB’s - as I did this year since starting TR in January! Here’s to a cold UK winter training ‘properly’ indoors!


#12

I just installed 145mm cranks on my tri bike going down from 165. I’ll report back in a month how it’s going. I can feel the increase in comfort despite my seat being higher now.


#13

I have 172.5 on my TT bike and 170 on my road bike. I can tell a difference. Is it marked? Not really. A little ways into the first ride after switching, it doesn’t bother me; both bikes fit pretty well. I feel like I can spin faster on my roadie, but that’s probably just as much about the TT position itself.

If you use a power meter with a crank length input (Garmin Vector 3, et. al.), don’t forget to change it for best accuracy. Even then, it doesn’t make that much difference.


#14

Thanks I would be very interested to hear how you get on. I have 172.5mm on my TT bike and thinking of trying 165mm but not sure as yet.


#16

Thanks for the link. Good read and information. Looks like 2.5mm isn’t going to make a big difference then!


#17

How tall are you? Unless you’re over 6’0” i would definitely do it. Going from 172.5 to 165 was one of the best things i did. It’s not even a 1 cm difference in length. You won’t notice any power changes. But the difference in hip angle is amazing. Don’t forget when you do it, to bring the seat up by 8mm and forward a couple mm


#18

Brilliant thanks. I will give it a go.


#19

I striped a pedal on my MTB a few years ago on a 3/4 day trip and got a new single crank arm from a shop and used it quite happily for a number of months until I saw that it was a 172.5mm whereas the other (original) side was a 175mm.


#20

…a good example what a “macro-absorber” can handle with easy. :relaxed:


#21

What is your height? 145mm looks tooo short!