Normally when ordering a crank I’d always go for a 175mm, as this seems to be the most popular.

So I Google the topic and its now clear that crank Length is related to how tall we are, assuming our bodies are proportional.

Do the STW agree on this table found here
Height Crank Length(mm)
6’0″ (1828mm) 177.5mm
5’11” (1803mm) 175mm
5’10” (1778mm) 172.5mm
5’9″(1753mm) 170mm
5’7″ (1702mm) 165mm
5’5″ (1651mm) 160mm

I’m 5’9″ how much difference can 5mm make?
I can understand that shorter legs would be better turning a smaller circle than say some 6’6″ again assuming we are proportional..

Do the STW agree on this table found

In the grand scheme of made up bollocks it’s not that bad. I’ve seen worse.

It’s been pretty much shown that too long is bad (over use of knees/hips and so on) and too short is pretty much all in your head.

I use 170mm and find that I get less pedal strikes than I used to with 175mm. However, that could equally be due to different geometry, tyres, improving my riding, or indeed all be in my head! I’m 6’1 if it helps.

The small differences are easily cancelled out by the pedals and shoes you use.

A pic I just found online highlighting what I mean.

I was on 175mm until a couple of years ago. I now have 170mm on two bikes and 165mm on the third. I’m 5’10.5″ with long legs and all three pedal great. 5mm does make quite a difference to pedal strikes. Cadence tends to go up with shorter cranks, torque goes down, power stays the same, so you sometimes need to change to a smaller chain ring.

If you’re short man or average or shorter woman you’ll find that no-one makes cranks which are short enough for optimum pedalling and ground clearance – the range of sizes is far too limited compared to frame sizes.

In the grand scheme of made up bollocks it’s not that bad. I’ve seen worse.

Best sentence ever.I’ll be using that one. 😀

I can tell the difference between 175mm and the 170mm I swapped out on my Arkose, or at least I think I can. I’m long in the body and short in the leg so the cranks that came as standard felt too long.

With a borked knee, I have 165 on the left and 175 on the right. They don’t feel different, and no-one would notice without looking really closely.

I’ve got short legs 31″ inside leg and dodgy knees, so wondering if shorter cranks will benefit..

Not MTB but I got sent the wrong length cranks when I ordered a new chainset (ordered 170, got 172.5) and noticed it within about 10 mins of riding.

However, my 2 MTBs have 170 and 172.5 cranks and I couldn’t tell you which one is which without looking.

I’ve tried various but as long as you don’t have knee problems or similiar, I reckon you should just go with what you like. With mountain bikes it’s not just about pedalling, you’ve got clearance to think about as well but also stance- I had 165s on a dh bike and I didn’t like the foot position as much, it just felt a bit close. It’s a tiny difference and I can’t feel it from 175 to 170 but it seemed like 165 tipped it over…

so wondering if shorter cranks will benefit..

It deponds on how your knees are knackered. But most ways of knackering knees would make shorter cranks a good idea. Just remember to adjust everything else in accordance with the new crank length. Saddle height, reach, drop etc More critical on a road bike, but still worth casting an eye over for an MTB.

Best sentence ever. I’ll be using that one. 😀

Sorry, can’t take credit for it, got it from an email that a colleague sent me.

The small differences are easily cancelled out by the pedals and shoes you use.

Using thicker/thinner pedals/shoes won’t alter the crank length!

It only changes saddle height in relation to the top surface of the pedal.

I have a pair of 155mm Canfields. You bloody well notice those are short….

🙂

Going from 175 to 165 got rid of a hot spot in my knee ..

Noticed sod all difference going from 175 to 170 recently. Wasn’t really paying much attention to them though and never had problems with 175. I’m 5’7″

6’2″ and went from 175 (which i’d used for years) to 165. absolutely miles better and suits modern bikes a lot better. Shorter cranks are generally better on MTBs as the slacker seat angles compared to road bikes means your knee is a fair old way behind the pedal axle at the 3 o’clock position, 165 brings the axle back and allows you to make more use of your power without borking your knees and without smacking your pedals on the ground

There is no one rule fits all but there are two factors to help choose; leg length and leg power. Longer legs and more powerful legs can deal with a longer crank arm and vice versa. If you have leg length but lack power then you would want to reduce crank length a little to compensate with wanting a fast change in speed which too long a crank length may counteract. Like wise if you have the power but have shorter legs then you may want a longer crank to take advantage but no too long as to cause discomfort with a shorter leg.

This is the wrong way around. Shorter cranks require more leg strength to turn, longer cranks give more mechanical advantage making it easier to turn a hard/big/high gear.

The torque argument is rubbish, the difference in torque between 165 and 175 isn’t going to be perceivable IMO – (175-165)/175=0.057, so approximately 6% difference in torque.

Since power=torque X angular velocity

Taking a typical MTB cadence of 70rpm i reckon, to maintain the some power output on a 165mm crank as a 175mm crank would be changing from 70 RPM on the 175mm to 74 rpm on the 165mm crank with equal leg force.

All marginal stuff.

I believe how much better shorter cranks are for your knees, short cranks are a no brainer.

er cranks require more leg strength to turn, longer cranks give more mechanical advantage making it easier to turn a hard/big/high gear.

You need more power and/or longer legs to push the bigger circle.

6% change in torque is certainly perceivable. Going from a 34 to 32 chain ring is ~6% difference in required torque to climb a given gradient and most riders notice a two tooth difference in chain ring.

You do not need more power to push a bigger circle, you need to develop force over a greater range of movement but the force required is proportionally reduced. Power remains constant. This is basic mechanics.

When switching to shorter cranks you have two main choices to maintain equal speed – push harder but in smaller circles at the same rpm or push with the same force as before but in smaller circles at higher rpm (running lower gearing).

Think the crank length and leverage thing works on single speeds but with gears, if its not possible to push a gear, choose another.

Problem / argument solved ?

The small differences are easily cancelled out by the pedals and shoes you use.

A pic I just found online highlighting what I mean.

No no no no no…

As already stated, the thickness of the pedal / cleat / shoe / socks just affects the saddle height.

Regardless of crank length (or pedal/shoe/etc.) one adjusts the saddle height so that one’s foot is in the same position relative to the saddle at the bottom of the stroke (the straight-leg-heel-on-the-pedal thing and variations thereof).

The difference comes at the top of the stroke, where you see *twice* the difference in crank length: You need to lift your foot 20mm less using 165mm cranks compared to 175mm. Treating this as a linear relationship is too simplistic – it affects the acute angles that your knees and hips bend to, and as these angles get smaller, the stress on the joints increases rapidly, and the force you can apply decreases rapidly.

The leverage effect on gearing is just that – it’s worth about half a cog on a mountain bike, so you just select the gears to suit. There seems to be reputable data that says crank length doesn’t really affect power for a given rider, even over a huge range of lengths.

I was blissfully unaware of different crank lengths until I got a new bike and a painful knee. After months of trying to make adjustments to saddle position, ‘working through it’ until I couldn’t ride, resting, etc. I repeatedly found that riding my old bike was OK (once recovered), but that the new bike gave me knee pain. The breakthrough (for me) was when I twigged that the old bike had 170mm cranks, and the new had 175. I changed all my bikes to 165mm cranks (since I felt that 170 was only just OK, and I remembered the last bike I had in my ‘teens had 165mm cranks), and I haven’t had any knee problems since, but have ridden much harder and further. The feeling I get when riding a bike with 175mm cranks now is similar to that when walking up stairs two at a time.

I think if you’re happy with what you’ve got, then fine, there’s no reason to change, but if you have recurring knee problems and are a short-ar$e (I’m 5’6), then give shorter cranks a try.

I’ve had up to 180 (road) and down to 165 (pedal-strikey mtb) without really noticing the difference

Currently 165, 172.5, 175

the one biggish kneeish problem I’ve had happened well after I’d dropped the 180s

(6ft 1)

Well the table works for me as a 5’7 1/2″ guy I’m on 165s as the shortest generally available crank length. Anything longer just feels terrible.

I’m always disappointed when smaller bikes get specced with anything longer -shows a lack of attention to detail.

Sometimes a light bulb just flashes on reading a thread on here.

My left leg is 8 mm shorter than my right, I always set pedal height with my right leg at the bottom of the stroke duuuuuh I also often get the hot spot thing in the left knee, god how àç_èkin stupid am I.

As above the difference between 175 and 165 is about 6%

That’s roughly the same as the difference between a 19″ frame, and an 18. Hardly representative of the full spectrum of humanity.

It seems obvious to me that crank length must be a key element of bike ‘fit’, but it’s understandable that so many people don’t think there’s much difference really when the choice we’re offered is so bizarrely narrow.

(165 to 175 is barely enough to call fine tuning)

I’ve got 36″ legs, my (38yo) knees are happier with the 165’s that come off* my wife’s bikes, than they are with 175 that get fitted to more or less every bike ever made.

It does seem that most people seem to get on OK going ‘shorter’ than they’re used to.

(*we get cranks shortened to 140 for Ms wife’s bikes – I’d need 220mm cranks to see how awful 165’s must feel for her, no thanks, I’d probably quit biking if that was the best on offer)

The shorter the crank, the less you have to bend your knee.

Consider what happens when you do squats with a heavy weight. The part of the squat where your knees are approaching straight is easier than when your knees are sharply bent.

Shorter cranks mean you bend your knees less, so while the advantage of more leverage on a longer crank is obvious, it is trumped by your bio-mechanicals.

Most of this is only relevant to road riding where the rider is in the saddle all the time.

Mountainbike riders spend a lot of time out of the saddle which changes the whole story, and so longer cranks are usable because the body changes position to get the most effective leverage.

This is even more obvious in singlespeed riding, where some short folk can use long cranks despite that being all “wrong” in theory.

Thus, if you’re an in the saddle rider, crank length is very important.

If you ride out of the saddle a lot it is not so crucial – but if you have to do that a lot, maybe you’d benefit from shorter cranks.

That table seems a bit exaggerated,

I know a few tall roadies like to run 175mm cranks or longer, but most people 5ft10-6ft are happy on 172.5. I don’t think many people at 6ft would be using 177.5mm cranks. And it should be based on leg length not height.

Tried 170 on my MTB and my road SS is 170mm. Both feel wrong. Weirdly (in my head surely) it feels easier, like I’m spinning out of every gear sooner, I guess because I have to be in a lower gear to compensate for the shorter levers. Maybe it’d work if I gave it more time to acclimatize, but I’m 6ft so on the taller side and 175mm cranks are cheap and ubiquitous so I’m not inclined to bother.