How much to alter bike after bike fit?

I had a bike fit last week and whilst there wasn’t much suggested the one potential thing to spend money on was changing the crank length from 172.5 to 165. I have found an Ultegra R8000 (same model as my current groupset) chainset for £207. The bike fitter tested me out on the 165mm cranks and for the same resistance and cadence I was putting out 13w more and increased the pedal efficient from 55% to 61%. He said is would be good but not essential and that he sees many other riders who it would make a bigger difference for (he had someone in a few weeks ago and after moving to 165mm cranks his FTP went up by 50w the next day).

Is it worth spending the money for this kind of increase? or is it worth knowing about it and considering changing things up when there is wear and chainrings need replacing anyway?

Has anyone recently made any significant changes that resulted in big spends after a bike fit?

That is pretty much up to you to decide, honestly…you have the metrics (+13W, 6 pt increase in efficiency). Is that “worth” it to you for ~200 pounds?

I will say that a lot of people spend a lot more than that on wheels and other pieces of equipment and don’t see those kind of improved numbers.


My focus on bike fit is ergonomics.
In my view, reduced chances of injury triumph over any potential gains in power output.
In your case power gains and reduced chances of injury seem to be pointing in the same direction. I would start saving to upgrade within a year or so.

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I spoke to my bike fitter yesterday about this. His stance, as someone that does bike fits but doesn’t also try to sell parts, is if you have a problem today like hip pain that can be fixed by new parts like a shorter crank, then you should buy new parts. Otherwise, you don’t need to buy anything for a better fit.

My gravel bike has 172.5. My road bike has 165 just because that’s what came on the used bike. Both fit me and have their benefits.

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Thanks. My fitter said it wasn’t an essential change. He said some people will notice the feel immediately, I didn’t notice anything. Obviously I was a little more efficient but difficult to know in real terms what it would mean. Would it make any difference?

Found a set for £179 now with 52/36 chain rings. I currently have 50/34 - so could sell a 52/36 with 172.5mm cranks and probably make back £100 maybe more if I’m lucky… Maybe considerably less :man_shrugging:

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Thanks, makes sense. I didn’t feel any difference and bike fitter said it would be a big thing for me, but obviously my body likes it. I expect if I bought a set of might be able to get most off my money back but switching the cranks on to the new chain rings.

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That’s a fair point on the upgrades.

I’m only hesitate because it’s a new bike and I don’t really want to spend more money on it if I don’t need to. I need to look at the second hand market to see what I can sell my current cranks for. If I can claw most of it back might be worth it.

This is interesting. I never heard crank length can affect your FTP. I’m sure I could do with shorter cranks, but I’ve had sort of an “if it’s not broke” kind of attitude. Now you drop this on me :joy:

It is not that simple. For a given power, there is basic math in the crank RPM along with force at a distance (crank arm length) that gets to the power output. Longer crank = more leverage, but also needs faster velocity at the foot since it is traveling a longer circumference.

Claims to more/less power with different crank lengths are dubious to me since it is hard to eliminate placebo effect without some serious effort. Same level of reliability as non-round chainrings to me. IMO, take power claims related to crank length with a hefty dose of salt.

Using different crank lengths to deal with range of motion or similar fit issues are far more tangible to me and more worthy of implementation vs questionable short term power tests.


Totally agree. Wasn’t throwing numbers out there to make any significant claims, just giving context to my dilemma.

The trouble with bike fit and comfort is, unless it’s something that causes pain it’s difficult to feel the difference during just a bike fit. In my cases it wasn’t something I felt during the bike fit, couldn’t feel any difference but it may well be a long term fix for an issue that hasn’t happened yet. If that makes any sense

  • I get where you are coming from but… Would you change your saddle that is working and not presenting any obvious problems, nor you noticed any significant improvement while doing a short term test during a fit?

Main point being that unless you really have an expectation of a problem, I see no need to do some preventative change just in case you might have a problem down the road. Don’t fix what ain’t broke to a degree.

There is a lot of hokum around crank length that I don’t think holds water. But there are legit reasons to use different lengths for some people, but I doubt that crank length is a missing piece to the training puzzle for most.

If you have the money and time to experiment, have at it. But I doubt there is much hiding in the change that won’t become the new normal once the change is made.

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Some claims here sound very similar to those of roadies laughing/questioning/asking-for-proof on the aero improvements adopted in triathlon back in the early 2000s. Now some roadies don’t want to use their lightweight bikes even for climbing stages/races when their sponsors push them to.
Just saying.

Now, if bike fitting isn’t focusing on kinaesthetics there is something wrong with the bike fitter, and that is not uncommon, unfortunately.

I’m all for positive changes, especially if they can be backed with data. Regardless of the past, is there data supporting longer/shorter cranks for gains in power, efficiency or other improvements?

I liken much of what I’ve heard around crank length to non-round chainrings because there are some similarities to the claims with a general lack of non-mfg data.

The saying “there’s no free lunch” always comes to mind here for me. From a pure physics perspective, we have to produce forces at rates of speed (foot velocity, not rolling speed). Shorter crank is smaller circumference that then requires more force to hold the same power (assuming that is held equal). As such, we may reduce ROM with shorter crank, but we have to apply a higher force at the same cadence (or increase cadence at the previous force). Either way, there is an equation to be solved for final power output.

My issue here is then trying to evaluate any of those changes in a meaningful way, which means physiological evaluation and all the pitfalls that entails. Proper testing (included blinded crank length to the rider at the very least) along with numerous data points on the rider (HR, Power, gas exchange, etc.) are likely necessary to capture what I expect are single digit deltas in one or more of those areas. In short, not easy to capture gains/losses AFAIC.

Mixing in Tri related considerations related to running off the bike is a different consideration and I think well worth the time in that instance. May or may not be relevant to all tri riders, but likely more worthwhile there vs irrelevant to roadies.


I went with shorter cranks as per my fitters suggestion. I told him I didn’t feel any difference. There was a slight increase in power. He said, that because I don’t notice many changes, that I might be one that adapts quickly. Unlike some who notice 2mm of seat set back change.


Um, yes. That is just basic physics, as Chad explained above. If you keep the resistance and the cadence fixed, you have to increase the torque, ie the power you put out. If you had kept the power constant, your cadence would have had to increase. Your body probably just preferred to increase torque, instead of changing cadence. I’d be careful basing your decision on those numbers.

Also the “he changed crank length and his FTP went up” story sounds a bit like that rider had some sort of movement problem that stopped him from using his whole power in that position. FTP is an aerobic measure that is not directly related to acute force output (more, how long can you hold that force). (Also, as he presumably also had a bike fit, what’s to say that only the crank length change resulted in those improvements? Maybe it was saddle position, or bar height, or whatever?)

With all due respect to you and your fitter, a 13w increase is huge and an overnight increase in FTP of 50w strains credibility, and at the very least invites questions of methodology and accuracy.

If I believed an equipment change could produce an overnight 50w increase in my ftp, I’d make the change yesterday.

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I think the 50w change is a huge anomaly. The fitter said he could tell immediately from the bike setup and the rider that he’d be able to help him find an improvement following the fit.

It would be great to be able to borrow some cranks for a few weeks to see if the increased watts is a sustained thing at the same HR or just a one time thing based on physics, like others have suggested.

@Jonathan @Nate_Pearson
Is this a possible podcast question?

I’ve heard that a number of the pro peloton are changing to sorter cranks this season. What are the advantages of switching to a shorter crank, and are these advantages likely to be seen for most riders - or is it more down to physique and preference?


Maybe check this out? Forget the details but thought they’d covered it to some degree.

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Awesome grab! For a quicker jump, here is the linked blog article and snippet from the full video.

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