Smaller chain ring or shorter cranks ... or won't it matter?

[Bristolian]

You have a large spread of available gears, no need to change it. There is also no issue with using the middle chainring - I've been cycling over 50 years and my big ring isn't used very often. The big ring is for downhills!

thank you for your input Sometimes I think there are too many gears. I'm not averse to riding in the middle chain ring and I often find I have to do that to cope with head winds or gradually rising roads. For me there is an issue, however, in that the increase in cadence required to maintain my road speed is generally more than my legs can cope with. For example, riding the big chain ring/small rear cog at my natural cadence of 75rpm gives a (theoretical) road speed of 27.3mph. To reach that same speed riding the middle chain ring/small rear cog requires a cadence of 96rpm ... and my legs just don't go that fast and never have done.

 

[Bristolian]

I have used 175 mm cranks and it felt like it was harder to keep on top of the gear compared to 170 mm

Thank you for your input . Your experience appears to echo that of others that have moved to shorter cranks.

 

[screenman]

Torque if you want to use the mathematical term. But remember that the maximum force that can be applied is also determined by the angle of the legs. Plus not a fixed value but varies through the pedal stroke. It is not necessarily the case that the average torque over the pedal stroke reduces when you use shorter cranks. We are also dealing with biology here.

I understand all of that, but that was not my earlier point.

 

[wafter]

The force that can be applied at the pedal will vary depending on the applicable angle of the legs. That angle will change over the period of a pedal stroke (and vary more for longer cranks) and is not a fixed value through the full pedal stroke. Plus the relative energy cost is lower for the shorter cranks. With shorter cranks it is easier to maintain a higher cadence. As per link above with shorter cranks you can put out higher power numbers.

Absolutely, however the component of force input will vary with crank angle not length (i.e. will still be 100% at 90 degrees past TDC, zero when the foot can't exert any force and follow the same relationship between them).

The stuff I've read on crank length has suggested that there's a fairly flat optimum range somewhat below what's traditionally offered on most stuff.

Personally I like shorter cranks as I find them easier on my joints while they give more clearance with mudguards and the ground..

 

[Dogtrousers]

And a 50t big ring is certainly not a waste IMO. I spend most of the time when on the flat or downhill in the big ring. On the flat I'll usually be 3-4 sprockets up from the smallest (11t), depending on wind (and my level of tiredness). On even a fairly gentle downhill, I will usually end up using the 11t sprocket.

And I will still usually be in the big ring on hills up to about 4-5% so it really isn't a waste (I am a 65 year old non-competitive cyclist).

That's entirely subjective. On my 50/34 I don't use my 50T much at all. I spend 80-90% of the time in the 34. I'll probably get a subcompact when I next redo my drivetrain.

Others are free to differ.

 

[Ming the Merciless]

Absolutely, however the component of force input will vary with crank angle not length

You are missing the point thst the range of the angles the legs moves through are different for different length cranks. That affects the effective torque thst can be applied across the pedal stroke. You can’t ignore the biology.

 

[wafter]

Torque if you want to use the mathematical term. But remember that the maximum force that can be applied is also determined by the angle of the legs. Plus not a fixed value but varies through the pedal stroke. It is not necessarily the case that the average torque over the pedal stroke reduces when you use shorter cranks. We are also dealing with biology here.

It it's rawest form it's force - force applied to the pedal which becomes a torque as it acts upon the moment arm of the crank about the crank axis.. output from the chainring as a tensile force to the chain which creates another torque about the rear wheel's axis via the sprocket, the torque applied to the wheel finally acting as a force at the wheel's contact patch with the road.

The biomechanical angle I'm less well educated to understand

 

[wafter]

You are missing the point range of the angles the legs moves through are different for different length cranks. That affects the effective torque thst can be applied. You can’t ignore the biology.

Granted, while as a standalone system force components about the crank remain the same but I appreciate that changes in leg angle / factors specific to the rider might affect both input force and its component at the pedal.

 

[Ming the Merciless]

It it's rawest form it's force - force applied to the pedal which becomes a torque as it acts upon the moment

Why are you stating sonething we both already know?

 

[Ming the Merciless]

Granted, while as a standalone system force components about the crank remain the same but I appreciate that changes in leg angle / factors specific to the rider might affect both input force and its component at the pedal.

Exactly you can’t simplify to a pure maths problem full of flawed assumptions. Doesn’t work that way and science as tested shows the biology of it matters hugely.

 

[Bristolian]

Hang on, you say you find it difficult to keep on top of gears in the 50T, but struggle with high cadence in the 38T, right?

But there is a significant overlap between the two. It's not like there is a set of high gears and a totally distinct set of low ones.

50/28 is 42" (low end of 50T) and 38/14 (high end of 38T) is 72".

Yes, maybe I didn't explain myself very clearly. So as not to change between chain rings whilst trying to put power through the pedals on a slope I often make the change from 50 to 39 early on. If I have been riding at my natural cadence of 75 rpm in the highest gear (50-11) I would need to increase that to 96rpm to continue at the same speed, which my legs simply won't do. Consequentially, I have to allow my cadence to drop off to just below 70 before changing to the 39 ring so that my legs can handle the increased cadence but whilst that results in reducing the force I need to apply to the pedals I can't maintain the increased cadence so it starts to drop and the cycle repeats itself.

 

[wafter]

Why are you stating sonething we both already know?

Because:

Torque if you want to use the mathematical term.

Force and torque are both equally valid terms in mechanics used to describe two different things; one is not a "more correct" equivalent to the other, as your post apparently suggests.


Anyway, I think that's enough internet for this evening..

 

[Dogtrousers]

thank you for your input Sometimes I think there are too many gears. I'm not averse to riding in the middle chain ring and I often find I have to do that to cope with head winds or gradually rising roads. For me there is an issue, however, in that the increase in cadence required to maintain my road speed is generally more than my legs can cope with. For example, riding the big chain ring/small rear cog at my natural cadence of 75rpm gives a (theoretical) road speed of 27.3mph. To reach that same speed riding the middle chain ring/small rear cog requires a cadence of 96rpm ... and my legs just don't go that fast and never have done.

I'm really not understanding your problem here.

You seem to be concerned that you can't ride at 27mph in the middle ring because the cadence is too high.

Well, that's what the big ring is for! If you actually need to go that fast - because you are extremely fit or going downhill - don't use the middle ring Problem solved.

If you find that changing up to the big ring makes your legs hurt then slow down.

If you are concerned that you can't find a gear that magically enables you to zoom around at nearly 30mph without your legs hurting well, that's probably because life's a bitch.

 

[Bristolian]

I find shorter cranks more comfortable and think they're easier on my crap joints but as mentioned they give less leverage so produce less torque for a given pedal load, so effectively give a higher gear ratio.

I think there's still potential benefit generally from reducing joint articulation.

In any case the range of crank lengths available commercially is very small - Shimano typically do 165-175mm which is only about 6% difference; meaning the pedal would require 6% more force for the same torque at the crank / equivalent to a 6% higher gear ratio; which on a wide-range cassette like an 11-32 is about half the difference between two adjacent sprockets.

Personally I'd look into how you're using the gears; spacing between 50 and 39T sprokets is pretty modest by modern double standards at a bit more than 25% (if shifting from 39-50T) so you should have a lot of overlap in available ratios on the cassette between both.

Personally my Fuji remains in the middle 36T chainring nearly all of the time which provides a perfect range with the 11-34 cassette, and I only use the big ring on fast descents. Your 39T is obviously even larger, however you may be a stronger rider / travelliing faster since you're on a road bike.

Are you using the whole cassette in the middle ring? When you hit a hill and down-shift from the 50T, are you also up-shifting on the cassette?

Hi Wafter, and thanks for your input I do try to use all of the cassette when riding in the middle chain ring but since the beginning of the year I have also been deliberately changing up into the big one at a lower cadence in an effort to develop my glutes and leg muscles - not all of the time but maybe on 1/4 of all my rides. This was recommended to me by a local college cycle team coach who I hope knew what he was talking about Initially, this was working but (like my weight loss) improvement appears to have stalled recently.

 

[Bristolian]

And a 50t big ring is certainly not a waste IMO. I spend most of the time when on the flat or downhill in the big ring. On the flat I'll usually be 3-4 sprockets up from the smallest (11t), depending on wind (and my level of tiredness). On even a fairly gentle downhill, I will usually end up using the 11t sprocket.

And I will still usually be in the big ring on hills up to about 4-5% so it really isn't a waste (I am a 65 year old non-competitive cyclist).

Our riding technique sounds very similar except that a 4-5% hill would have me moving down the gears and into the middle ring in fairly short order.