Crank length

[Stradageek]

For the practical outworking of all of this see the 'almost identical' thread viewtopic.php?f=23&t=146159

Cranks shortened from 170mm to 130mm with no change in gearing required and much improved speed and stamina

[Jdsk]

Another class of bikes that can benefit from short cranks is the recumbent.
You cannot get out of the seat to grind up hills, so you're better gearing right down to spin up them.
That's exactly the situation where short cranks can work out better on a long run.

Many 'bent riders prefer short cranks. But I'm not sure about that explanation. Because it isn't possible to stand on the pedals longer cranks would counter that with greater leverage...

Jonathan

[Jdsk]

I prefer not to describe the length of cranks as affecting gearing. That's because of the specific craft meaning of gearing as equivalent wheel diameter, and that value isn't affected. That's the gear inches meaning as in Sheldon Brown, below.

Jonathan

https://www.sheldonbrown.com/gloss_g.html#gearinch

Gear
The "gear" of a bicycle relates to the mechanical advantage of the whole drive system. In a low gear, the pedals are easy to turn, but you have to spin very fast to get any speed up. In a high gear, the pedals are hard to turn, but you don't have to make them turn very fast to make the bicycle go fast.
The gear of a bicycle depends on the ratio between the sizes of the front and rear sprockets, and the size of the drive wheel. If the bicycle is equipped with planetary gears, they also affect the gear. There are several ways of designating gears numerically. See gain ratios, gear inches, and development.


Gear Inches
One of the three comprehensive systems for numbering the gear values for bicycle gears. It is the equivalent diameter of the drive wheel on a high-wheel bicycle. When chain-drive "safety" bikes came in, the same system was used, multiplying the drive wheel diameter by the sprocket ratio. It is very easy to calculate: the diameter of the drive wheel, times the size of the front sprocket divided by the size of the rear sprocket. This gives a convenient two- or three-digit number. The lowest gear on most mountain bikes is around 22-26 inches. The highest gear on road racing bikes is usually around 108-110 inches. Unfortunately, the handwriting is on the wall for all inch-based measurement systems.

[Chris Jeggo]

For many years the CTC Handbook's advice on gearing was to have a 'normal' gear of ten times crank length and, for touring, a top gear of twelve times and a bottom gear of five times crank length. That is, with 170 mm (6.7") cranks, normal 67", top 80" and bottom 40". That is another way, different from Sheldon Brown's, of including crank length in your calculations.

That CTC advice suited me, for touring, for many years. In more recent years, however, since councils have started spending more money on steepening hills than on mending potholes, I have been using lower and lower bottom gears.

[Stradageek]

Another class of bikes that can benefit from short cranks is the recumbent.
You cannot get out of the seat to grind up hills, so you're better gearing right down to spin up them.
That's exactly the situation where short cranks can work out better on a long run.

Many 'bent riders prefer short cranks. But I'm not sure abut that explanation. Because it isn't possible to stand on the pedals longer cranks would counter that with greater leverage... Jonathan

It's a common misconception that the inability to stand out of the saddle is an issue on a recumbent - it isn't actually the issue. On a recumbent you push against a rigid seat and are able to generate much more force than just adding your body weight (plus a bit of pull on the handle bars) when standing out the saddle on a DF bike. I've broken chains on a recumbent when pushing hard but never on a DF.

In fact the amount of force you can generate on a recumbent becomes dangerous for the knees, which is one reason for spinning lower gears.

The other reason for spinning lower gears on a recumbent is to avoid losses associated with the flexing of a necessarily longer frame (or turning forces on a FWD recumbent). The top end recumbent racers also extol the aerodynamic advantages (less frontal x-sectional area) and the ability to tuck the pedals inside a fairing.

It's all very complicated

[Jdsk]

Thanks.

I know the theory about bracing your back, and I can do it, but I certainly can't keep it up, and I doubt that I ever got near the force available from honking.

Jonathan

[Bmblbzzz]

For many years the CTC Handbook's advice on gearing was to have a 'normal' gear of ten times crank length and, for touring, a top gear of twelve times and a bottom gear of five times crank length. That is, with 170 mm (6.7") cranks, normal 67", top 80" and bottom 40". That is another way, different from Sheldon Brown's, of including crank length in your calculations.

That CTC advice suited me, for touring, for many years. In more recent years, however, since councils have started spending more money on steepening hills than on mending potholes, I have been using lower and lower bottom gears.

40" doesn't sound low enough as a bottom gear for laden touring IMO. 80" sounds acceptable as a top but I'd probably choose something a little higher where possible. I guess this is one of the advantages of modern cassettes with many sprockets. 67" does sound a decent middle though.

[Stradageek]

Thanks.
I know the theory about bracing your back, and I can do it, but I certainly can't keep it up, and I doubt that I ever got near the force available from honking.
Jonathan

In this age of power meters we should be able to answer this question, witness the GCN semi-pro team trying recumbents and being exhausted despite generating only half the power they would produce on and upright (though still enough to beat a DF).

On a GCN recumbent hill climb test the mega fit GCN rider on a recumbent was easily beaten by an old codger on a Speedmachine (a very poor climbing recumbent) leaving me to suspect it's all to do with developing those 'recumbent' muscles?

Some nice experiments beckon

[Marcus Aurelius]

The length of the lever pushing the crank round makes a couple of mm difference in a comparable cranks length mechanically insignificant. The main advantages in reducing the crank length is reducing the pedal circle circumference. It gives an advantage if ( for one example ) you want to hold and maintain a more aero position whilst pedalling. Short cranks are also advantageous if you are riding a tight course ( like a traditional crit for example) and want to reduce the likelihood of grounding the pedal in a tight, leant over corner.

[Manc33]

Between 175mm and 170mm I'd rather have the extra 5mm ground clearance than any gains there might be from leverage. Your gears are there for that, just swap to another cassette if the lowest gear isn't low enough.

[Bmblbzzz]

Yep. Choose the crank length which suits your legs and the type of riding you do (and the type of bike you're riding). Choose the chainring and sprocket to get the gearing you want.

[Blondie]

Of course, crank ground clearance isn’t an issue on a recumbent. Otherwise you are doing it wrong LWB excepted maybe. I run 175mm cranks on my recumbent because that’s what came off my mtb bike when it was retired. It’d be interesting to try shorter cranks and see hat difference it’d make. But not so interesting I’m going to fork out for an extra crankset.

What I do know is that I can generate same power (FTP) on recumbent as my road bike. But it took a few months to get there after I’d first built my recumbent up. Road bike has 170mm cranks so slightly shorter.

[NickJP]

AFAIK, lab ergo tests using different length cranks find no statistically significant difference in power levels for crank lengths over a wide range of lengths. This article on Cyclingtips summarises things quite well: https://cyclingtips.com/2017/09/crank-l ... power-fit/.

[Stradageek]

What I do know is that I can generate same power (FTP) on recumbent as my road bike. But it took a few months to get there after I’d first built my recumbent up.

Aha! Though I haven't researched in detail you're the first person I've read who has made this comparison - but what I'd love to know next is whether you record a significant difference in power generated at the rear wheel for recumbent vs upright i.e. are the transmission losses significantly different on a recumbent?

[Bmblbzzz]

AFAIK, lab ergo tests using different length cranks find no statistically significant difference in power levels for crank lengths over a wide range of lengths. This article on Cyclingtips summarises things quite well: https://cyclingtips.com/2017/09/crank-l ... power-fit/.

Interesting studies but require comment: studies looking at power output are applicable to sports but ignore things like injuries which might be more relevant to utility or leisure cyclists; and studies on "trained cyclists" are probably not applicable to those most likely to be considering a change in crank length, either due to size or power output. As is noted towards the end of the article "cycling populations are changing". Nevertheless, some interesting ideas there.