Short cranks and saddle height.

[rjb]

I am experimenting with 150mm cranks on my shopper. I usually ride 170mm cranks on all my other bikes. So should I raise my saddle on the shopper by 20mm to compensate for the shorter cranks or leave as was when it had 170 cranks.
I've tried raising the saddle, it's a strange sensation but then it's uncomfortable when I stop as it's a longer reach to the ground.
What do you short crank people think.

[Paulatic]

Raise the saddle 20 mm. Yes the ground will now be 20 mm further away if you can’t reach it lean the bike.

[LollyKat]

Years ago, for the sake of my dodgy knee I went from 170mm to 160mm - I had to raise the saddle 10mm and to keep my leg extension the same as before, otherwise both my knees would have felt worse. I raised the bars too, so that my general riding position was the same as before. Yes, it was a longer reach to the ground but I always eased off the saddle to put a foot on the ground anyway. What I found more irritating was that the reach to the down-tube gear levers became a bit of a stretch. These days I find 165mm the best compromise.

150mm seems very short for an adult unless there is some sort of injury problem. Why are you experimenting?

[mattheus]

150mm seems very short for an adult unless there is some sort of injury problem. Why are you experimenting?

Is 150mm very short? If we scale upto human heights - and switch to cm! - 150cm and 180cm people are not all that rare (about 4'11 and 6' respectively. I think).

There have also been studies showing that even average height adults should use much shorter cranks than the de-facto 170mm.

[rjb]

Couple of things prompted this switch. I changed the wheels on my Dawes Kingpin from 440 to 406 as tyres in 440 size are getting difficult to find and virtually not much choice. This has lowered the bottom bracket so shorter cranks compensate for this and makes a better fold and I wanted to try them. I am happy to spin the cranks not a pusher of high gears and I am 5'6".
I am not going far on them round trips of 20 miles at most but I find I am riding it more as I like the ride.
Short cranks seem to be popular with recumbent riders so hoping for some feedback.

[LollyKat]

Is 150mm very short?

Note that I said it "seems" short - maybe it isn't, but it is unusual and more likely to be seen on kid's bikes. Your scaling up of adult heights to 150 - 180cm doesn't take into account that the inside leg length is only about half the total height. Your examples should be (I think!) 135cm and 195cm, or 4'5" and 6'4". 6'4" is not so uncommon these days, but 4'5" is.

Shorter cranks make spinning easier, and when I used 160cm my bad knee definitely was happier. It is quite hilly where I live, though, and I missed the leverage of the 170cm crank. 165cm has turned out to be perfect. (It may sound like princess and the pea stuff, but I'm very sensitive to position - i.e. my knee protests - and may move my saddle if changing shoes.)

The OP seems happy with the result - excellent!

[andrew_s]

It is quite hilly where I live, though, and I missed the leverage of the 170cm crank.

Changing the crank length also changes the effective gear ratio.
Going from 170 to 160 crank length will change a 70" gear to 74.5", which is enough to notice.
Ideally, you'd lose a couple of teeth of the chainrings at the same time, but that's not always easy.

[mattheus]

Is 150mm very short?

Note that I said it "seems" short - maybe it isn't, but it is unusual and more likely to be seen on kid's bikes. Your scaling up of adult heights to 150 - 180cm doesn't take into account that the inside leg length is only about half the total height. Your examples should be (I think!) 135cm and 195cm, or 4'5" and 6'4". 6'4" is not so uncommon these days, but 4'5" is.

Scaling is scaling - the "half height" factor doesn't matter. It is built-in to my calculations (the exact numbers were just a happy accident!)

If leg-length is about the same %age of height for most adults (and you seem to support this), then I stand by my maths on this occasion. But there may be other factors I have missed, so feel free to point them out!

[LollyKat]

But crank length is related to leg length, not overall height. A crank that's "too long" makes the knee bend too much at the top of the stroke and causes injury, while one that is "too short" leaves the feet spinning round without much forward motion. If I was a foot taller because my back was a foot longer, I wouldn't need a different crank length.

[fastpedaller]

There's maybe no correct answer to this. Somewhere there may be a crank length for each of us which is ideal (but can our 'ideal' also change depending on what we're used to?). Suffice to say a crank 220mm long would be too long (hit the ground!) and a crank 50mm long would be too short!
I time trialled on fixed gear for several years on 150mm cranks, and it felt odd at first, but if I changed to my other bike with 170mm cranks that felt odd after riding the 150's . I rode up to 12 hr time trials on 150mm cranks with fixed wheel. If I'd used 170mm cranks then a rough calculation suggests my feet would have travelled an extra 7000m or so during the race

[helijohn]

As a casual infrequent cyclist, I can't understand what difference crank length really means - it would seem a shorter crank requires more effort so can someone explain it and why raise the seat????

[Mick F]

Seat height is dictated to bottom run of the crank.

As a rule of thumb, sit normally on the bike (supporting yourself somehow) and place your heels on the pedals.
Pedal backwards.
The saddle is the correct height when your legs are fully extended in line with the seat tube at the bottom of the pedal stroke.
Shorter cranks, therefore, would make your saddle higher.

Rule of thumb. Not gospel, but it'll get you in the right ball-park.

[Tigerbiten]

As a casual infrequent cyclist, I can't understand what difference crank length really means - it would seem a shorter crank requires more effort so can someone explain it and why raise the seat????

The energy used while cycling can be split into two parts.
The first is how much effort it takes to press each pedal stroke down. the faster your cadence then the less energy is needed per pedal stroke. It's harder to press the pedal down when you're standing still vs when you're moving. Shorter cranks also gives you less leverage so more energy is needed per pedal stroke.
The second is how much energy it takes to spin your legs around. This goes up at the square of the cadence. Shorter cranks means your feet travel less far so you need less energy to spin your legs at speed.
Putting the two effects together, if you have a naturally slow cadence then you're slightly better with longer cranks than if you have a quicker cadence.
The reason for going to shorter cranks is that with a slightly quicker cadence it puts less stress/strain on knees while maximizing your pedalling efficiency.

The reason for moving the saddle up is that the pedal-saddle distance is fixed by your leg length. Shorter cranks moves the lowest point of the pedals in relation to the BB upwards. So you need to move saddle upwards to match this.

YMMV .........

[helijohn]

The first is how much effort it takes to press each pedal stroke down. the faster your cadence then the less energy is needed per pedal stroke. It's harder to press the pedal down when you're standing still vs when you're moving. Shorter cranks also gives you less leverage so more energy is needed per pedal stroke.
The second is how much energy it takes to spin your legs around.

Gosh I learn something daily - mind you I am still working it out.

[LollyKat]

Think of it as being a bit like climbing stairs. A deep tread requires more effort but if you have the strength you can climb faster, though you will tire sooner. A shallow tread needs more steps, easier to climb but slower, though you will be able to keep going longer. At least, I think that is a reasonable analogy....