longer bike cranks = more power AND speed?

Factual Questions
1

Forgive my ignorance and confusion. THis doesn’t sound right, nor can I figure out if it would work or not. If I increased the length of the pedal cranks, I expect I would be able to increase the torque and therefor power going up hills. Additionally on my bike, at the top gear, I cant spin my legs fast enough to go faster. If I lengthened the pedal rods, would I be able to also spin my legs faster thereby increasing speed? I think the question is ‘can my legs biologically go at a faster rpm if they travel in a larger circle?’

I know there’s a physics issue where you sacrifice speed for low end climbing power, but biological legs don’t necessarily follow the physics rules perfectly.

2

Power is proportional to torque * cadence (crank RPM). A longer crank arm will increase torque, but this will only result in increased power if you manage to spin it at the RPM.

Also, the torque at the crank doesn’t really matter. You have a variable transmission between the crank and the rear wheel anyway. It’s the power that matters. (Changing the gear ratio changes the torque, but not the power.)

Absolutely not. It’s easier to spin your feet in small circles than large circles (at the same rotation speed).

The optimal crank length is determined by physiology, not physics. Your legs aren’t designed to move in 16" diameter circles, or 4" for that matter. The chain & gear shift mechanism is there to convert the optimal pedal speed for your legs into desired rotational rate of the wheel.

3

Its true to say that longer cranks give you more leverage, I have ridden on 165, 170, and 172.5 and yes you can turn a larger gear, and yes you can go faster - to a point. It may affect your endurance.

Longer cranks require you to turn slightly more slowly, and can affect your speed when climbing.

Shorter cranks mean you have to ride on a lower gear but it can be useful on banked tracks or on short criterium races where you corner hard and fast.

To work out what works for you, its really a question of measuring your power output against your endurance, and comparing different crank lengths.

The reality is that you can’t switch between crank lengths any more than you can change your saddle height or any other measurement, not quickly - you can move things a little at a time but its always going to be within limits that are imposed by your stature.

Changing between machines with different crank lengths is not at all easy, one day on one machine and the next on another. It takes so long to adapt that you haven’t got the time, not if you want to maintain your performance - you find you have to stick with one length.

I always find that if someone is telling me they cannot spin the top gear any faster, it means they are riding overgeared - they just do not have the power to spin any faster. I’ve seen this again and again.

Remember that the pros can’t spin top gear at maximum revs for long, and they are up at 40-45mph when they are doing it, in a bunch.

If you want to improve your cadence, then you need to train specifically for it, and that will mean using rollers - not fixed frame home trainers. There is a reason why pros use rollers - it conditions the riding style to spin the cranks.

I reckon on rollers you should be able to spin the cranks at 140 rpm as a constant rate, with intervals of increased rates of up to 180 rpm. If you work on this you will find that you learn to ride in lower gears and spin, you’ll also find that you can spin on easy hills without changing gear.

I should add that longer cranks also can mean that you find it can make breathing harder - you tend to find that your knees come up higher and this can make your diaphragm difficult to compress.

In absolute physics terms, well power is torque X revs (radians/sec if you really want to go in that direction) So if you lengthen the cranks, and then turn them slower, you increase the torque and decrease rpm, so the power output remains the same.

4

I’m on the tall side, and 175s are perfect for me. Most road bike come with 170s, which would be better for a guy of average or slightly taller height. I believe, though, that mountain bikes normally come with longer cranks, but that might be outdated info since it’s been about 10 years since I bought my last mountain bike.

Find what works best for you, give your size and the bike you ride. There is no one size fits all answer. When in doubt, go to a good bike shop and have someone there help you.

5

It’s true that there’s an optimal crank length for your physiology and biomechanics, as others have noted, but it’s a trivial fine tuning for the vast majority of people. Like if Tony Martin turned up to the world time trial champs, and his mechanic said ‘Sorry Tone, couldn’t find your 175 cranks, so I put some 170s on instead’, he’d just shrug his shoulders and say no big deal.

You (the OP) mention something that is far from trivial, however, in the business of propelling the bicycle - ‘I cant spin my legs fast enough to go faster’, ie cadence. We’d have to know what gear you’re using, but for a typical drivetrain there is no way the gear is the limiting factor here - it is your inability to spin it up. CasDave’s post lays this out - if you want to go fast on the bike you need to know how to deliver high revs whilst maintaining good form and power output.
Old Timers in my club tell me they used to ride the winter club runs on fixed 68, which feels like an absolutely tiny gear to me. They’d train their cadence in the off-season so they could spin at 120 all day long.

6

You can’t expect to be able to increase power indefinitely…
So you just keep making the crank longer ? 10 Hp, 20 Hp , 30 HP … your body can make 30 hp ?
The increase in mechanical advantage (increase in leverage - the perpendicular distance from line of force to the axis of rotation - at the crank ) , is , if your foots speed and force is constant, allowing the power to be delivered with a closer match to the torque/force required to maintain that speed up that hill. … so that you make use of your ability to deliver power.
The changing of gears is more about letting you supply your constant rate of power (since its a pain to spin your feet fast to deliver little power…) to a better matching drive train ratio … (the sum of all mechanical advantage or leverage in use…)

On the other side, if the mechanical advantage reduces too far, your power also reduces to zero since power = force * speed … if speed drops to zero, power also drops to zero… you are standing still, pushing hard, not going anyway, (your muslces take power even to produce force while you sit still, but its not EFFECTIVE power… Brake Horse Power is the term for power delivered from the wheels of the transport (car, bike,whatever) down to the drum which measures power… That is , power wasted internally (eg in friction in the hubs, chain, rubber of the tyres - they get hot right ? they burn power there ! - in your muscles pushing hard but the vehicle not moving fast ? - in your fast spinning of your feet for little useful effect ? - ) is not BHP … not effective power.

So there’s some of your confusion …assuming power your muscle makes (To twitch) is the same as the power put down to the road… no. .your muscles can twitch uselessly… Not contributing to BHP.

7

I’ve always thought that you want the top of the stroke to be as high as you can go without hurting your kneecaps, and the bottom of the stroke as low as you can go without hurting your hamstrings or making you rock in the saddle, and these two degrees of freedom translate into saddle height and crank radius.

8

There is a lot to be accounted-for in getting a good fit for efficient and powerful pedaling. Just lengthening the cranks may end-up doing more harm than good, which is why getting a proper fit is smart. This page, illustration 4, shows how to align you knee to pedal spindle. When you use a crank too long for your body, you may be harming your knee. It depends on the length of your legs, position of your seat, etc.

I bought a used crank set complete with chain rings I wanted for my touring bike (smaller, for climbing hills fully-loaded). I contemplated using the cranks that came with it (175), but stayed with my what I had since my body was used to it, and just installed the chain rings on my existing 170 cranks. My road bike has 172.5 - I can alternate between them without issue, but I felt 175 would be a stretch on the tour bike.

9

Weren’t there adjustable length cranks available someplace? It’d be great to do that experiment before settling.

10

I’m still baffled by the reasoning of the OP. How would longer cranks enable higher RPMs? What sort of reasoning led to that conclusion?

11

A quick search shows several after market cranks available. Interesting designs. I have encountered kids bikes that have 2 or3 pedal holes that can be used as the child grows. I would think a clever mechanical engineer could work-out the math of the length of leg bones, knee angle, and body position to identify correct crank length.

12

It’s not purely objective; riding style preference also affects optimal crank length. Some cyclists prefer deep strokes at lower RPM, others do better at spinning short cranks really fast.

Bike design affects it too. Shorter cranks work better on recumbent bikes, for example. (I use 155-160mm on my recumbent bikes.)