maartendc

Hello there,

Since I am in the market for new crankset, I thought I would look into crank length. I currently run 172.5 mm on my road bike, and 170mm on my commuter. Never really gave me any problems, never even thought about it.

I am 5ft7" (168cm) tall and 78cm (30 1/2") inseam.
Most common methods out there seems to suggest I would need a 165mm crankset.

I am reluctant to try this because:
- Cranksets are expensive, I dont want to get it wrong
- If I do get it wrong, it might be harder to sell a 165mm because it is less common?
- I never really had any problems with 172.5mm or 170mm, so why fix if it ain't broken
- I am NOT looking to get a more aero position on the bike (which seems to be #1 reason for going smaller)
- I am more of a 'grinder', meaning I prefer low cadance riding in a harder gear rather than high cadance.
- I live in a hilly area, some people seem to suggest slightly larger cranks for climbing.

The only up side I could see to it is when riding in the drops: my knees to get slightly uncomfortably close to my chest when bending low in the drops. This is slightly better on the 170 vs the 172.5mm. This would potentially be even better in the 165mm.

So am I wrong? Should I get 165mm? Or just get 170mm and call it a day?

Thanks!

10 Wheels

It is about personal preference.

You choose the size you like.

1nterceptor

Buy used. Try, don't like, sell. Might get the same amount you paid.

Leisesturm

I would think it hard to sell a used crankset simply because most people just work with what is on their bike. The enthusiasts that build up framesets would rather buy new. That said, 165mm cranks are not at all rare. It is a very common size on mountain bikes.

Leisesturm

I hope it's more than that. What if you don't even know what you like though?

McBTC

You've hit on several considerations. It's not convenient to try something different because of the expense and because experimentation is not supported by the industry: cranks shorter than 165 are priced like a specialty items.


From what you might get out of it you probably do not have to worry about getting it wrong in the limited range you're considering. Assuming, however, that you could more easily try cranks even shorter than 165, I believe you would immediately see that the old saw about spinners versus grinders is no longer useful in describing riding technique.


For any given gear you might be putting a bit more power into a stroke when using a shorter crank -- which is like grinding -- you also will see increased RPMs -- like spinning. You spin more easily in any given gear with shorter cranks because the foot speed per revolution is reduced.


Losing a little leverage on hills because the cranks are shorter shouldn't be a big issue if you have a range of gears to choose from.

black_box 

See if you can find a bike local to you that you can try out? I tried out my ex-girlfriend's road bike with a 165mm and was sold on it (5'6", 29.5"). You may find it easier to spin on hills because there is less of a dead spot when your pedals are at the top or bottom of the stroke.

Another option, some fitting devices have adjustable crank arms. You might be able to bribe them to have a test run (if you don't want a full fitting done).

AnthonyG

Based on a 78cm inseam I do recommend that you try 165mm cranks. You will need to move your saddle up and back to get the most out of them. Apart from anything else it will improve your comfort.

Buy yourself some second hand cranks and try them out first before spending money on new ones.

Anthony

wphamilton

Bike fitters may find this a little weird, but researcher J.C. Martin has looked into crank length in "Determinants of maximal cycling power: crank length, pedaling rate and pedal speed." and other studies. You can find the full pdf with a little poking around.

The upshot is that we're best off with a crank length that is 20% of leg length, which is defined by the difference in sitting height and standing height. Given normal proportions, for people 5'7" to 6' (according to this research) the optimal crank length ranges from 150 mm to 160 mm.

It's interesting that it's so much shorter than the cranks almost all of us ride around on, but I think that it indicates that there's not too much risk in trying out a 165mm crank.

(we're talking about only a percent or two difference here).

bikebreak

If you have a shimano BB find a used ultegra or 105 crank in 165. you can resell it on ebay if you don't like it.


remember to move your seat up 5mm or it will feel cramped


I found 165 helped with keeping cadence up in bigger gears. Also helps if with longer crank arms your knees hit your chest when you get low in the drops.

ThermionicScott 

That's an interesting-looking study. I'll download the full text at some point.

My take on the crank length thing is that it probably ought to be proportional to your leg length, but as others and the study have pointed out, the adjustability of bike setup and cadence can make a wide range of lengths work for a person. One thing I've noticed for me (at about 5'8" with 32.5" PBH) is that 170mm cranks feel best, and 165mm works well but I get a little of that "shoes-tied-together" feeling when climbing a hill in a big gear. So I'm not really interested in going any shorter.

Another benefit of shorter cranks is less toe overlap, FWIW.

Leisesturm

If you have a frozen bolt (110" gear) to unstick, which wrench would you rather work with, a 14" plumbers wrench, or a 9" pedal wrench? Well it depends right? If you are strong enough, the 9" will allow you to work faster, but if you are not that strong then you need the leverage of the bigger wrench, or you are not going anywhere, and it really does not matter how long your arms are.

The 52/42 with 14-28 5sp was the de facto gearing of all bicycles sold in America, whether it was to Cat 3 UCI Federation racers or housewives. It may well be that 150mm cranks work best for typical human dimensions but an earlier supposition that shorter cranks help spin big gears is completely false! The opposite, as cranks get shorter, the gears should also. But they usually do not. So the cranks remain longer out of necessity.

And just as well, since the average untrained cyclist works at a 60rpm cadence, not at the racers 110rpm. 170mm @ 60rpm will do nicely in a big gear (and tailwind) to keep up a satisfying ~17mph. I also completely disagree that a 5mm difference in crank length is "huge". It is not. 10mm is about where you actually notice that something has changed, and to justify the term "huge" you need to be talking about 20mm, or more, of change. And while I am about the good work of putting myths out of their misery... there is little that crank length has to do with "dead spots" in the power circle. But if it did, it would be the longer crank, not the shorter one that made any perception of a 'dead spot' less noticeable.

It's just physics people. Don't ignore the physics. Don't ignore Occam's Razor. If it gets hard to (and it should be) fathom the difference between 170mm and 172.5mm, don't waste the time and money trying it. Think big... what do you imagine the difference between 170mm and 180mm... 200mm!! Do you imagine you have less leverage with the very much longer crank? Of course not. So, you cannot have less leverage with the 172.5mm.

And finally. If you change from a 170mm to a 175mm, the relationship of pedal circle to hip girdle has only changed for 1/2 of the stroke! If you raise your seat 5mm to compensate for a 5mm difference in crank length you have raised it too much. The other pedal goes as additionally far away as the other comes closer. To keep the same relationship to the new longer cranks as before you must raise the saddle only 1/2 of the amount of change in crank length. 2.5mm. I don't know... 2.5mm is probably the amount that your backside compresses when you sit down on the saddle. It is also an amount of change difficult to ... ... are you getting how meaningless tiny changes like this are? They really are meaningless.

NickFit0036

Crank length does depend on your leg length as well as what you are doing with the bike. I worked for a high end manufacturer and these were the crank lengths we installed on our bicycles by size for Men's bikes.

48cm-51cm - 170mm
54cm-56cm - 172.5mm
58cm-61cm-175mm

It also depends on your proportions and how you like to ride. If you are flexible (can touch the floor with finger tips on straight legs), more of a climber and not as strong (like you can't squat at least 1.5 body weight full range), you may prefer a longer crank. The pro of the increased leverage will outweigh the longer amount of time to complete the larger circle (5mm larger total between each crank length). If you ride in an aggressive position (bar at least 5cm lower than saddle, you would be riding a 51-52cm road bike. In this case, you would be on a 170mm crank. 165mm cranks are not easy to ride on. They require a lot more strength to get going and once you are up to speed you will be spinning your butt off. The only benefit to running a shorter crank than normal is the fact that you can run a lower saddle height, which means that your bars can also be lower, which also means that your entire body sits lower (more aero). If you are riding only on flat ground at high speeds where aero dynamics mean everything, you may want to try a set of 165mm.

Yes, I am a certified fitter. More info on me is in the link below.
Bought A Bike Online? I Come To You Build & Fit It. Road Mountain Tri

McBTC

Nope... a shorter crank length requires that you raise not lower the saddle.


While a length of 165mm or less may not be 'normal' as far as what is usually offered or commonly available in various size bikes, studies support the finding of many that shorter crank lengths are more efficient with 145s being enabling some riders to produce more power than using 175s.


Shorter cranks have a positive effect on cadence so if you've always liked the idea of being a spinner but pushing in beyond the 70s seems a bit uncomfortable, a shorter crank decreases foot speed per revolution so a higher RPM just comes naturally.


My personal experience is that shorter cranks also are better on the knees as there is less bending per revolution.

NickFit0036

You are correct technically in that on a shorter crank, since the pedal is closer to the hip at bottom dead center, you now have to raise the saddle to get the same knee angle. Many riders I work with that have tight hamstrings opt for a shorter crank and lower the saddle enough to close their knee angle at bottom dead center to 40 degrees. With this knee angle at BDC, I will generally set them up with the tibial tuberosity slightly behind the pedal axle at the center of the power stroke putting less stress on the patellar tendon. Also, at top dead center, the hip angle is also more open allowing for a lower handlebar height. So the lower saddle with the more acute knee angle at BDC along with the more open hip at TDC allows for less hamstring flexibility which in turn allows for a lower bar.

AnthonyG

"High end" bike manufacturers couldn't build a bike to fit short people to save themselves. Do you know why?

Its because they don't care. Bikes aren't made to fit small people because it would adversely affect the cost of manufacture and the profit margins. Its FAR cheaper to have people argue until they are blue in the face that they are doing the right think than to ACTUALLy do the right thing.

I'm a student of Industrial Design and Manufacturing economics. I do understand these things.

Ohh, I'm a student of the human condition too. Its dishonourable to admit you've been doing something so wrong for so long. Bikes with 700c wheels and 170mm cranks only fit people of medium or taller height. Shorter people need Road bikes with 650c wheels and about 145-150mm cranks.

Anthony

NickFit0036

Big time. They definitely do what they can to save money. How short do you think an adult would be to need 140mm cranks? That's the length that comes on 20`` kids bikes.

AnthonyG

The whole crank length paradigm is completely out of whack with cranks being just far too long for everyone.
140mm cranks are FAR too long for children's bikes with 20" wheels.

I grew up with and until recently owned 20" children's bikes with 110mm cranks. That's what the standard was. There were 22" wheeled bikes with 5" (127mm) cranks and 24" wheel adolescent bikes with 6" (152mm) cranks. Since small adults are roughly the same size as an average adolescent then 24" wheeled bikes with 150mm cranks is FAR closer to the mark than anything current.

I'm a 155cm adult who rides with 135mm cranks on a custom 650 wheeled road bike.

The other issue that is obscenely wrong with current frame design is seat tube angles. Steep seat tube angles of 74 degrees or steeper are a fudge to make frames with long cranks and large wheels "Work".

My custom 650c frame has a 71 degree seat tube angle. When you work out the geometry using shorter cranks the seat tube angles on small frames need to be relaxed. Not steep.

Its money. If your a production manager in a bike factory trying to keep manufacturing costs down the number ONE imperative is to reduce the number of different sized parts being used. If you can make two different sized bikes where the only difference is a little adjustment to the frame but all the parts fitted to those frames are the same then this is a WIN for keeping your manufacturing costs down.

Smaller wheels on a smaller bike? Forget about it.

Anthony

Leisesturm

Mm hm... erm... you can't both be right, just saying...

fietsbob

170 is commonplace so wont cost Much. That is if your tastes dont demand the premium components.

but you list fancy bikes.. Origin8 (J&B Imports makes low cost square taper 110 BCD from 150-170)




./.

Leisesturm

I don't know. If you are a trades(wo)man you invest in a toolkit with wrenches as small as 4" and as long as 14". In fact sometimes even 14" won't give you enough leverage, so you also have wrenches that are not much more than collars into which you fit any length of pipe necessary to give you the leverage you need, and in none of that is the length of your arms or the size of your hands or any aspect of your overall physique a consideration.

You can't have it both ways. In the same thread you tie crank length to rider height based on frame size but also tie it to intended purpose (i.e. climbing..) but which is it? You can't have it both ways. I am not as confused as you about it. I don't think rider height or leg length has very much bearing on crank size. It might, but the point is moot since the ease of obtaining cranks other than 170/5mm is not worth the hassle for 90% of people riding a bicycle today. Moot! However, for those who do persevere and do obtain short cranks and/or long ones, IMO to not also adjust their overall gearing of the bicycle or their use of the existing gear sequence, is only addressing half of the issue.

noglider 

I have bikes with various crank lengths. For me, the shorter, the better. I didn't believe one could notice the difference until I noticed some bikes just didn't feel right, no matter how many adjustments I made. It turns out the cranks were long. One of my bikes has 155mm cranks. I got it as an experiment, and I like them just fine. You may or may not notice the jump down to 165, and if you do, it will probably be a positive experience.

But to be complete, I will say that my thighs are probably short compared with my shins and the rest of my body, which would indicate I need shorter cranks than most people.

I have a pair of cranks of 160mm I'm not using. I don't remember, but I think the BCD is a common one. Let me know if you're interested.

McBTC

From my experience, the benefit of shorter cranks defies convention of suggesting crank length is based on leg length. In looking for a comparable analogy, convention would suggest that a taller person should have higher stairs, But, logic probably is no more accurate than assuming a taller person is faster because they can handle higher hurdles.

ThermionicScott 

Well, ideally, they'd have them! But you gotta make stairs in a size that everyone can use. It's rare that I don't take them two at a time, even descending.

McBTC

Interesting about the information below, if you translate preferred stair height to cranks, since the pedal stroke top to bottom is twice the crank length, based on stair preferences, the cranks ought to be a lot shorter than even 165mm since double that would be something less than 330mm but definitely a lot greater than 183mm during the power stroke. There aren't many studies on the subject but in the best known study on optimal crank length, 145mm cranks (coming in at 290mm top to bottom-- based on the stair analogy) enabled riders to produce the most power and crank lengths in the 180s were the worst. I believe even less than 145s is better for riders with range restrictions or whose preference is to maximize RPMs over watts.




Stairway risers and treads: acceptable and preferred dimensions.

Abstract

Stairway preference and acceptability were investigated with psychophysical techniques. A series of six experiments was conducted where subjects ascended and descended 19 sets of stairways with different riser and tread (run) dimensions. Subjects were instructed to identify stairways that they considered acceptable, and the one stairway they most preferred. The optimum riser was 7.2 in (183 mm), and the optimum tread (run) was 11 or 12 in (279 or 300 mm). These dimensions were acceptable to both males and females, young and old, and subjects of greater or lesser stature. Larger dimensions were not as acceptable to shorter subjects, and smaller dimensions were not as acceptable to taller subjects. The 4-in (102 mm) riser was almost totally unacceptable, and never preferred. The 5.14 and 9-in (131 and 229 mm) risers were acceptable to less than one-third of the subjects, and rarely or never preferred. These results are compared with existing practices and recommendations.


PMID: 15676777