Crank length

[Jdsk]

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/.

Thanks.

I'd read some but not all of those.

And it strongly supports going for what feels right.

Jonathan

[Jdsk]

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?

For typical designs with one driven road wheel... losses from longer chains, guide wheels and jockey wheels will be very small. Although of course it's wise to optimise alignment.

For a chain layout such as the KingCycle Wasp's the loss could be significant unless a lot of care was taken.

Similarly with more than one driven wheel with losses in diffs and from bad alignment (although that's technically after the wheel).

Jonathan

[Stradageek]

For typical designs with one driven road wheel... losses from longer chains, guide wheels and jockey wheels will be very small.
Jonathan

Agreed; but while chains, guide wheels etc. are not likely to be lossy, frame/boom flex could be a major factor.

When I rode DF racing bikes the stiffness of the rear triangle and BB was critical. On my best racing bike I could accelerate up hills in a way that I just couldn't on other equally light machines.

I'd love to be proved wrong (because recumbents would then truly be the bees knees) but I think we need someone with time to spare and some expensive power meters front and rear, in order to get to the bottom of this

[Blondie]

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?

Since this was averaged over 20 mins on a smart trainer I’d say no. The power is being measured at far end.

I also have local 5 min hills where my Strava PB on recumbent is 20 seconds faster than on my road bike.

[SRV]

Of course, if you fit significantly shorter cranks to a conventional machine, you'll have to raise the saddle and perhaps even the bars. It's enough to drive you round the bend.

[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?

Since this was averaged over 20 mins on a smart trainer I’d say no. The power is being measured at far end.

I also have local 5 min hills where my Strava PB on recumbent is 20 seconds faster than on my road bike.

Thanks for this Blondie, I shall be using this information in response those who look at me riding my recumbent and say "That looks hard", I've always maintained that it isn't but your data shows that it isn't

Out of interest, and apologies if you have already said, what type of recumbent were you riding to measure this FTP?

[Blondie]

This was on my Lightning P38. You sacrifice some aero for climbing ability. Having said that, observation when freewheeling downhill with mates on road bikes is that you’ll still outpace them. So still more aero. I’ve found that unless you live somewhere fairly flat, the speed with which you tackle uphills is a strong determinate of your average speed for a ride.

As to the that looks hard comment (on a recumbent) from other riders. I never quite understood that. It’s precisely as hard or easy as you are working, assuming you have enough gears not to be forced to work harder than you are comfortable with. If I’m working at a heart rate above 90% of max up a hill then that’s hard regardless of which type of bike I’m on. Similarly if I’m limiting my max HR to 75% uphill that’s a much more comfortable pace.

[SRV]

Correct me if I'm wrong, but isn't this post about crank length?

[kylecycler]

Correct me if I'm wrong, but isn't this post about crank length?

*checks thread title* - Yup! No harm in thread drifts, but from everything I've seen about crank length, there seems to be far less downside in going shorter than longer. Can't explain that but I think that's the point - maybe no-one really ever has.

Meaning, you might be running 175 mm cranks, go to 172.5 which might be more 'right' for your height, but if you go to 170 mm you'll not suffer for it and even if you then went to 165 mm you'd still get along just fine. Apparently it doesn't tend to work the other way.

No references, just an impression gleaned from reading a lot of stuff about it and not really learning an awful lot.

[swscotland bentrider]

I specified shorter (165mm) cranks on my recent bike because that way I would fit the larger frame option and have my saddle level with the bars. I went from 172.5 to 165 and they feel no different. Perhaps I reach for a lower gear a bit earlier than I would have but I don't notice that. My knees give me a bit less bother though!

[Stradageek]

Correct me if I'm wrong, but isn't this post about crank length?

Probably my fault, sorry. I often look at page one, then the least page of a thread to laugh at the amount of drift, this one is fairly typical. I think I caused the drift by finding someone who has actually measured recumbent drive train efficiency at last - and I got a bit excited.

Anyway; it seems that cranks that match the riders leg length continues to be a largely undiscovered country. I can only suggest that anyone who doubts the benefits of shorter cranks needs to come and see the grin on my wife's face since she discovered 130mm cranks

[Jdsk]

It's really helpful to know that there aren't massive effects identified by the evidence from the physics and the physiology.

That means that you can shift the emphasis to what works for you (and your machine).

But it isn't easy to swap cranks on your own bike... any advocates of adjustable systems to help with the decision?

Jonathan

[Marcus Aurelius]

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.

Absolutely bang on.

[Bmblbzzz]

Of course, if you fit significantly shorter cranks to a conventional machine, you'll have to raise the saddle and perhaps even the bars. It's enough to drive you round the bend.

Not necessarily. If you fit significantly shorter cranks it's probably because you've decided you need something shorter than the industry standard 170mm. You'll have a smaller range of hip and knee motion but the centre of rotation, the bottom bracket, remains in the same place. So you'll only need to raise the saddle if you've judged it by distance at full extension, ignoring the range of movement; and that's part of the problem.