Will a 2.5mm crank difference be worth it?

Hoping for a little crank length opinion (a total worm hole, eh?). Currently I ride road/gravel 175mm D/A cranks. I’m building up a new bike and can use the exact same length. However, I’ve been reading generally that shorter may help provide a bit more power/efficiency. I’ve not had fit issues or repetitive injuries and although not fit by a pro, I feel confident that my current fit is pretty dialed in (after several years of road, gravel at about 5,000km a year).

Is it worth the high cost of experimentation to moving to a 172.5mm? Would that make any real difference in building up this new, dedicated road bike that will get about the same annual distance? Or, forget it and just go with what’s never been a problem in the past?

Definitely more the spinner, high cadence type here too.

5’11”, 158lbs, Cat 4/5.

You’ll never notice the difference…go with whatever you want.

I will add that at your height, a 175 crank would probably be right on the edge of what most “old school” fitting doctrines would use. Switching to a 172.5 will open your hip angle up a bit, which in turn would allow you to get a bit lower if you desired. So there could be some aero gains to be had, but I seriously doubt you’ll ever notice a change in efficiency.


My view is that if you dont have pain and are comfortable on the bike then it wouldn’t be worth it. I had larger cranks but I was getting sore knees. Changing to a smaller crank helped but performance remained the same. I’d say look elsewhere for gains which is based on my experience.


I’m 5’8" and went from 172.5 to 165, made a huge difference, was having trouble getting higher cadences previously and wanted an open hip angle for comfort, both improved significantly!

You’re not going to notice 2.5mm, I’d suggest finding someone with shorter cranks and trying them out, even down to 165s!


FWIW I have 175 on my MTB and 172.5 on my road bike and I don’t ever feel a difference. Longer cranks can in theory possibly lower your saddle and center of gravity for cornering, but can also clip the ground easier.


Get a retul fit and experiment with different lengths before you buy. I’m 5’10” and ride 165mm on my tri bike.


Gotta say I disagree 100% with this. When I started riding I was on 172.5 cranks and was always uncomfortable because at the correctly set saddle height My knees would hit my chest and I’d feel cramped. Went to 165mm and never looked back. VERY comfortable, opened up my hip angle and back of the knee pain that would come on once in a while (usually in long rides) disappeared.

Crank arm is arguably the most important fit element on a bike IMO. Stem length / reach is more on preference/aero/comfort and it changes overtime as you get faster more flexible etc. Your leg length or saddle height never changes and not up to preference however. Crank length has great effect on saddle height.


I’ve ridden both 175.0 and 172.5 mm on two similar bikes (one for commutes and bad weather, one for when I want to enjoy my riding), so I’ve had the chance to experience differences firsthand. I ride 56 cm frames and stand 183 cm tall.

Initially, I noticed a slight difference, but only when focusing on it, and after a few minutes of riding. If I were blindfolded, I wouldn’t be able to tell which bike I’m on. I’ve set PB’s on nearly all my favourte hills and segments since the swap, but I’d put that down to training rather than crank arm lengths. :slight_smile:


I would say that a pro fit is worth the cost, even if you think you’ve dialled it in. And that buying a new bike is the perfect time to do it, since it could well be no cost at all. Most shops that offer fitting services will also offset the cost of that fitting against the purchase of a new bike.

Of course that can also limit your purchasing options a bit, e.g. If you’re looking at Canyon or other online only deals then you’d have to pay for the fit. But if you happen to find a fitter with a good reputation who can also sell you a brand of bike that you want I would seriously consider it.

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I had a professional bike fit a few years ago. The bike fitter said I was the least flexible person he had ever fitted to a bike and put me on 165mm cranks from 172.5mm to open up my hip angle. I don’t know whether it’s made me faster but I’m a lot more comfortable on the bike and find it a lot easier to spin higher cadences. As an aside though, I once needed a replacement MTB crank whilst on a cycling holiday and only realised a few months later that the one I’d got was 170mm whilst the bike still had the original 175mm on the chainring side. Never noticed the difference!

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I ride 175mm on my road bike and 170mm on my TT bike, and manage fine.

However that was specifically for the purpose of getting more aero for short TTs, so if that isn’t a priority, make sure it’s a length you’ll be comfortable with long-term.

YES! You will 100% notice a difference. I thought “how can 2.5mm mean anything”, and when i went from 175 to 177.5 (I’m 6’5") it was like a WHOLE new bike.

Experiment. It’s worth it.



Generally longer legs can benefit from longer levers.

This article may help explain som elements of riding that can benefit from adjusting crank length:


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I normally run 175mm at 188cm tall. I picked up a pair of 172.5mm for my cross bike, only because they didn’t have the 175mm on sale. Couldn’t tell any difference at all. It’s such a small change. Maybe a little benefit for pedal strikes, but in my experience 2.5mm isn’t big enough to tell a difference. Going to 170mm, which you probably could at your height, may be a good choice.

Same here, going from 172.5 mm to 165 mm really makes riding more comfortable for me.

The only thing I’d say to the OP is the (s)he should aim for a difference in crank length of at least 5 mm, but I’d probably go directly to 165 mm. The difference is definitely noticeable. And according to everything I have read, going to a shorter crank length doesn’t really negatives attached to them.

Can you borrow a chainset from someone? It doesn’t need to be high spec for a few test rides, and as long as it fits your BB, most are interchangeable.

I’ve got 172.5, 175, and I think a 165 on my single speed bike (not measured it, but its a track crankset). They all feel fine. Admittingly, the single speed is hard to compare because both the drivetrain and the cadence changes make it feel quite different anyway. But it’s likely a personal thing, some people are really sensitive to the smallest changes.

The differences between your situation and the OP are significant.

  1. you had a very specific fit issue (knees hitting your chest) that needed to be addressed.

  2. You made a dramatic change in crank length. He isn’t.

Two very different scenarios.


Indeed, I’m not having knees issues and regularly ride hilly 3-6 hour climbs and flats. The new bike will be bit more “aero” bar position, but not ridiculously different.

I’ve read some good thoughts here and I think my approach will be to hold off on the expensive power meter cranks. I’ll try my current 175 on them first as I settle in to the new seat/stem/bar positions with dims taken directly from my current road/gravel bike. Then, if anything seems less than ideal I’ll try to borrow some 170s, perhaps, and play out that difference. Then, drop the big bucks as this settles in.

If need be, I’ll just buy a low-end 50/34 Shimano crankset at a shorter length and then sell or keep them. Hopefully, someone around Boulder I know has a pair of 170s 50/34 to borrow! I figure by going a bit shorter than the 172.5 I was thinking it’ll provide more noticeable difference to help me recognize any differences. Then the 172.5 would be the “balance” between them, or I’d still have the option to go back to the current 175 or lower 170.

Great insight and it’s helped me steer an approach, albeit a bit on the graduated, conservative side.

Adding to that, the OP is looking at drop bar setup, and many others relaying crank length changes are specifically talking Tri / TT setups. Those are two VERY different use cases and should be distinguished clearly.

Couple that with the fact that the OP seems to NOT be having any problems, and suggesting a change based on persons N=1 experience that is not really similar to the OP may be misleading.


I disagree with this. I am 6’1" and always rode 172.5mm cranks. Had nagging knee pains and when I got low I would be hitting me knees in my chest. I recently moved to 170mm and it’s like my knee pain is gone and the ride just feels more natural. Maybe it’s placebo but who cares if it is actually making a difference. Makes me wonder what 165mm feels like.

Edit: I know I had knee issues but crank length was never an option to fix those issues. The difference 2.5mm made for me overall was just a better feeling pedal stroke and more natural position.