Crank length recommendations

daniel_b

Gents,

I am 5ft 10, but with a (What I believe to be relatively long) 34" inseam, so a fairly short torso - although I have been working on my flexibility and core, which enables me to reach further now and employ longer stems etc.

I used to religiously use 175mm cranks, due to my long pins, but most bikes I buy (54cm) seem to come as stock with 172.5's.
My CR1 I built up a few years ago, I specced with 175mm cranks, and in a 52\36 chainset - this with a 11-28 is probably my favourite recipe, though if I had my way it would be a 12-28, or I could easily argue the case for a 50\34 with an 11-25 for tiny gappage.

Anyway, I digress, that was not the reason for my post!

I have another bike now, with 172.5, and since I built my CR1, i have spent a LOT of time on the turbo, and turned my grindy type riding, into a more spinny, cardio based affair, my average cadence now being usually 90, which is much kinder on my old knees!

So my question is around with that kind of spin preference, should I even be looking at a 170mm, as I read some articles that suggested if you spin, a shorter crank is advantageous.

Thanks

Dan

imposter2.0

Crank length is a function of leg/thigh length, bike fit and personal preference. It won't have any effect on your cadence, just the circle that your feet describe and the limits of your knee rise/drop. That's pretty much it. If you're happy enough with whatever crank length you are using, then there's very little reason to expect any change to make any practical difference..

poppit

There is some info here about crank length, it’s a bit track specific and long but interesting info, https://youtu.be/5i9DtIFi9pM

You just need to ignore the gunshots!

timmyotool

Reducing the crank length will reduce the effective gear, therefore it should allow you to increase your cadence.
I run 165mm cranks - 31inch inseam, personally it's done wonders for my hip flexors where I used to get pain and feel awkward at the the top of the pedal stroke.

I did a lot of reading about it and there's a lot of info about crank length around, some people are very definitive on the benefits of shorter cranks, but many say it there's no right answer for everyone.
It seems to be popular for time triallists as it can allow you to get into a lower and more aero position.
Vice versa it's said short cranks can limit your explosive power...

This is quite a good read:
https://www.velonews.com/2011/06/bikes- ... ers_178528

imposter2.0

timmyotool wrote:

Reducing the crank length will reduce the effective gear,

By a tiny, barely-noticeable fraction...

cq20

If you eliminate the variables (for the moment) and keep the chainwheel and the rear cog as constant I.e. a fixed system, then shortening the cranks will result in a higher gear ratio because for the same cadence, the pedal distance travelled is less and the leverage is reduced which leads to a higher pedal force to maintain the same road speed. In the real world I.e. with gears it will be tempting to use a larger rear cog to counteract the need for the increased pedal force. Because the gearing is now lower than before, the cadence will need to be increased to maintain the same road speed. In most cases the difference in gear ratios as a result of changing to a larger rear cog is greater than that achieved by changing crank length.

IME, with my stumpy legs, a move to 165 cranks overall resulted in a better / smoother pedalling action and my cadence has increased but that is as a consequence of using lower gearing. Overall, I prefer the shorter cranks but sometimes when climbing out of the saddle I get the feeling that my shoe laces have been tied together.

BikerGroveish

I’m about the same height and leg length as you and I use 170mm cranks and spin at about 90 rpms due to it being proven to be an efficient Cadence. I find that the relatively smaller pedalling circle with 170mm cranks is beneficial.

imposter2.0

BikerGroveish wrote:

I’m about the same height and leg length as you and I use 170mm cranks and spin at about 90 rpms due to it being proven to be an efficient Cadence. I find that the relatively smaller pedalling circle with 170mm cranks is beneficial.

Can you link to anywhere that offers proof that 90rpm is an efficient cadence? You trotted out the same old nonsense on another thread recently and failed miserably when asked for proof of this.

BikerGroveish

Imposter wrote:

BikerGroveish wrote:

I’m about the same height and leg length as you and I use 170mm cranks and spin at about 90 rpms due to it being proven to be an efficient Cadence. I find that the relatively smaller pedalling circle with 170mm cranks is beneficial.

Can you link to anywhere that offers proof that 90rpm is an efficient cadence? You trotted out the same old nonsense on another thread recently and failed miserably when asked for proof of this.

https://fitrecovery.wordpress.com/2014/ ... ling-fast/

Again, in case you missed it the first time.

https://www.trainingpeaks.com/blog/3-hi ... -training/

And here.

https://www.velon.cc/en/news/2017/01/ve ... -explainer

Where both a highly qualified Dr. and a professional racer ( Alex Dowsett ) tell you how it works.

Now obviously it depends on the power your making at that 90 rpms as to whether it’s effective, or flapping. There’s no point in wanging the pedals round at 90 rpms if your in such a low gear that your power is not great. But that said the 90 rpms is still biomechanically efficient, even if it’s not doing your speed any favours because your power is too low.

imposter2.0

Yeah, you posted that same tiresome nonsense before. Opinion is not the same as fact, unfortunately

daniel_b

Thank gents,

it's a weird one really, as the force required will be marginally greater with a smaller crank which means you are using more muscle than before, so presumably that might tire you out quicker!

When I started riding my cadence woud be around 70, and I was dead set on pushing a big gear, but am much happier, fitter, and able to complete longer rides and not feel fatigued at a cadence of 90 - admittedly a proportion of that can be attributed to improved fitness, but I personally believe for me it has made a positive difference.

Plenty to mull over.

svetty

As has been said the major determinant is your leg length - although my experience is that unless you are at the extremes you quickly get used to what you are currently riding.

Given you have a reasonably normal leg length I'd stick to normal cranks - 172.5 is as good a length as any

cougie

In my experience it doesn't make a noticeable difference.

daniel_b

Svetty wrote:

As has been said the major determinant is your leg length - although my experience is that unless you are at the extremes you quickly get used to what you are currently riding.

Given you have a reasonably normal leg length I'd stick to normal cranks - 172.5 is as good a length as any

Totally hear what you are saying, and you are very likely right!

Just there is a possibility I might end up switching the crankset on a brand new bike, which gives me the option to change should I so desire, so wanted to explore the possibilities, or hear other peoples findings, before I potentially reach that point.

keef66

Svetty wrote:

- although my experience is that unless you are at the extremes you quickly get used to what you are currently riding.

That's been my experience too. I have bikes with 165, 170 and 172.5 cranks and I'd struggle to tell them apart without reading the labels. Even on my son's skip-rescued Peugeot which has an old square taper Veloce chainset and a different length NDS crank which looks like a part from a Romanian tractor

imposter2.0

http://myworldfromabicycle.blogspot.com ... -just.html

timmyotool

One thing I couldn't find much info on what was the determination of a "normal" crank length. At some point 170-175mm was chosen but it seems a bit arbitrary.

It wouldn't surprise me that in a few years the standard changes (one way or another) a la bigger rear cassettes, wider tyres, less aggressive geometry.

BikerGroveish

timmyotool wrote:

One thing I couldn't find much info on what was the determination of a "normal" crank length. At some point 170-175mm was chosen but it seems a bit arbitrary.

It wouldn't surprise me that in a few years the standard changes (one way or another) a la bigger rear cassettes, wider tyres, less aggressive geometry.

Some of the peloton are already favouring cranks even shorter than 170 mm as we speak. We’ll all be using kiddy bike cranks before you know it.

BikerGroveish

Daniel B wrote:

Thank gents,

it's a weird one really, as the force required will be marginally greater with a smaller crank which means you are using more muscle than before, so presumably that might tire you out quicker!

The smaller pedalling circle will compensate meaning you won’t be tiring out more. Marginally more force but for less time per revolution.

Daniel B wrote:

When I started riding my cadence woud be around 70, and I was dead set on pushing a big gear, but am much happier, fitter, and able to complete longer rides and not feel fatigued at a cadence of 90 - admittedly a proportion of that can be attributed to improved fitness, but I personally believe for me it has made a positive difference.

Exactly. The proof of the pudding is in the eating. If you do a given ride and don’t concentrate on keeping the cadence up by using your gears more for example but instead just ride in your unthinking default style which is probably going to be a greater variation of Cadence, and probably lower cadence. Then repeat the ride concentrating on maintaining a higher cadence and trying to keep it more uniform, you’ll find you won’t be so fatigued, your average speed will probably be higher by virtue of the fact your speed variations won’t be as great and you’ll recover more quickly. The experiment relies on the conditions and route being as close to identical as possible though.

super_davo

I've got half of my bikes on 170 and half on 172.5 - and from a cadence / leverage perspective I can't tell the difference. If there is one you'll just compensate using gears.

Where you can tell the difference on is the fit of the bike - you can raise your saddle on the shorter cranks because your leg doesn't need to extend as low. I'm personally very sensitive to saddle height changes and definitely notice a 2.5mm or 5mm change. Higher on the saddle can mean flatter back, which is almost certainly the reason pros are shifting, particularly on TT bikes.

But the differences are small, so don't expect miracles, I wouldn't bother unless you need to change the cranks for some other reason.

cq20

BikerGroveish wrote:

Daniel B wrote:

Thank gents,

it's a weird one really, as the force required will be marginally greater with a smaller crank which means you are using more muscle than before, so presumably that might tire you out quicker!

The smaller pedalling circle will compensate meaning you won’t be tiring out more. Marginally more force but for less time per revolution.

This is wrong. You can't spent less time per revolution unless you are running at a lower cadence i.e pedalling slower. If you then want to maintain road speed you will have to select a higher gear ratio which results in even more force being required.

imposter2.0

For anyone who hasn't read the study I linked to above, here are the main take-outs from it...

higher cadences are generally NOT MORE EFFECTIVE

1. effect of crank length is small and significant only at extreme lengths, 2. 170mm cranks will compromise power of the tallest and shortest riders by at most 0.5%, 3. Pedal speed and pedaling rate interactively limit power, and 4th and most importantly: Cyclists can ride the crank length they prefer without concern of decreasing maximal power.

cyclists are more efficient pedaling at 60rpm cadence vs 100rpm cadence

(efficiency as in metabolic efficiency)

Data suggest that a relatively fixed increment of fatigue occurs with each maximal contraction

(which obviously debunks what milemuncher/bikergrovish says above, not that it really needed to be said)

crank length has no effect on fatigue, no effect on metabolic efficiencies and very small effect on maximum power

Lots of science/studies around this topic, but it pretty much all comes to the same conclusions as above.

cq20

super_davo wrote:

I've got half of my bikes on 170 and half on 172.5 - and from a cadence / leverage perspective I can't tell the difference. If there is one you'll just compensate using gears.

Where you can tell the difference on is the fit of the bike - you can raise your saddle on the shorter cranks because your leg doesn't need to extend as low. I'm personally very sensitive to saddle height changes and definitely notice a 2.5mm or 5mm change. Higher on the saddle can mean flatter back, which is almost certainly the reason pros are shifting, particularly on TT bikes.

But the differences are small, so don't expect miracles, I wouldn't bother unless you need to change the cranks for some other reason.

I certainly agree with that. Whether it is noticeable or not depends on where you lie on the fit spectrum. In my case, according to most charts, I should be on 160 cranks, so my current 165s are "too long" but not as much as the 170s I replaced. If I was a marginal case, I wouldn't bother.

This http://bikedynamics.co.uk/FitGuidecranks.htm
is interesting.

Canyon are one of the few bike companies that supply 165 cranks as standard on some of their smaller frame sizes.

PS There is a difference in gearing as a result of changing the crank length but it is normally much smaller (e.g. 172.5 vs 170 or even 170 vs 165) than can be compensated for by selecting a larger rear cog (e.g. 16 vs 17). If there is no change in cadence or gearing, it just means that you are pedalling slightly harder with the shorter cranks.

stevie63

I went from 172.5 to 165 and the main advantage that I have had that has not been mentioned is increased ground clearance in roundabouts and corners which means I can now pedal where I used to coast (I will point out though that I am only 5 ft 6 so the reason for going shorter was due to height)