What Does Increased Crank Arms Accomplish?
 

I hope that I've posted this into the right sub-forum......

After recovering from an injury last year that had me lower my crank arm distance to compensate, I'm now considering slightly increasing the crank arm length on my 'bent trike's pedals. (I have an adjustable crank arm thingy.)

All else remaining the same, my understanding is that this would allow me to impart more power into my pedalling, making hill climbing a bit easier. Is that correct?

Other than requiring larger leg movement, are there any negatives to doing this?

Looking for a simple, straightforward explanation rather than illustrative formulas and equations.

Thanks!

There are pro's and con's for either. I never have use a 'bent, but I think that similar thoughts about the subject applies with them as well. Generally, I feel if you are a slower cadence type person, then longer cranks will seem better to you. And if you normally keep a higher cadence, then shorter crank arms will seem better. Of course longer crank arms give you more leverage to push a higher gear ratio. But shorter tends to let most people spin a higher cadence more comfortably, and that smaller gear they are pushing is just as fast.

With longer cranks your knees will go through a larger range of motion, and some of us don't have that range of comfortable motion or simply just don't like it. While obviously others do fine with it. There has been for many decades a thing about proportionally sizing cranks to leg length. However, IMO at best that only shows what the longest crank arm length is you should try, not what is the length that will be best for you.

If you find it not too expensive to try different crank lengths out to see what you like or what gives you the best performance, then you should. It certainly won't hurt anything. At worst, you might have sore knees with either. Not permanently though. If you aren't having any issues currently, then the change might not be anything to write about.

Thanks! I ride at relatively low cadence, so I think that I'll give this a try. More leverage is probably a good thing for an old guy like me...

I think more leverage on a longer crank on a diamond frame bike. Maybe not as much on a bent with your legs straight out.

Pedaling is pedaling, whether you're lying back or sitting up.

Longer cranks have more leverage. They also make your knees go through a wider range of motion, putting more strain on them. Check the recommended crank length for your height or inseam, than use that or the next one smaller.

No, I do not believe that is correct. The key thing to understand here is the difference between torque and power. A sustained climbing effort will not be difficult because a rider struggles to generate sufficient torque but, rather, because they struggle to generate sufficient power.

For the same pedal force and cadence, increasing the crank length will increase the leverage and the power output. However, that additional power still has to be supplied by you. The force that you apply to the pedal now has to travel around the circumference of a larger diameter circle. And that increases the input energy demand per revolution. So, all other things being equal, it will be more difficult to climb with longer cranks unless you reduce your cadence commensurately (in which case, by definition, all things are not equal).

There's just no free lunch in Newtonian physics.

The same principle works in reverse for short cranks. A pedal stroke is akin to a one legged, partial squat. All else held constant, a shorter crank will make the squat more partial and allow the rider to generate more pedal force (just as with a real, partial squat). However, because that force will now travel the circumference of a smaller circle, the net effect on power is no effect at all.

This linear trade-off is valid for the bike because bikes are very simple machines. A human body, on the other hand, is a vastly more complex machine. Muscle contractions are more efficient at different speeds, fatigue can spread out when more muscle motor units are recruited, etc. As a result, optimal pedaling efficiency is about dialing in the efficiency sweet spot of the body and not the bike.

I realize that you asked for a simple explanation sans equations. Unfortunately, this stuff cannot be grasped meaningfully without at least this much physics.

Maybe too simplified ?
'Shorter' crank length can be an advantage as one pedals at higher cadence - less leg movement makes the process and coordination easier, especially if your muscles are not trained to ride efficiently at higher cadences. Riders who have a wide cadence comfort range are less reliant upon the crankarm lever...
A longer 'lever'/crankarm, at the same gearing, allows a bit more force/torque applied. Since we're talking mm, the relative difference is very small. The leverarm increase, going from a 165mm to 175 crank is lest than 6% - not significantly noticeable at difficult climbing grades. If you are climbing at your max at 6 mph, think what a 6% improvement means...
If you are right at the edge of your power range, a 6% difference is not going to make any difference on a climb, if you're close to blowing up...
Pedaling is NOT just pedaling, recumbent vs upright (standing)....
When climbing a significant amount of 'force'/torque applied is in part your mass under the influence of gravity (if you're standing on the pedals)... If you're climbing recumbent, the force/torgue applied by can be signficantly in the 'Negative' value, relative to the vector of force/torque of mass/gravity...
Climbing, recumbent, is all about leg power applied... no gravity assist...
There's hardly any real climb which I don;t find myself occasionally getting out of the saddle to get the pedals turning better...

Suitable gearing is the 800 lb Gorilla for anyone,, light climber or gear masher...

Since you have adjustable cranks - give it a try... there's nothing like 'experiencing' ... and if you have a power meter on the 'bent', then you'll certainly see if crank length makes a difference.
Ride On
Yuri