hsea17

Allow me to ask, since I am not very technical and wonder if it is technically / mechanically possible for a bicycle dealer / mechanic to install a crank set, but use different lengths on the crank arms? Yes, I'm aware I will need to purchase an extra crank arm (possibly one more full crankset if not possible to buy one crank arm only)!

Thanks & Regards
hsea17

blamester

​​​​​Yes.

JoeTBM 

Yes it is possible, sounds like the bike had either a bent or damaged crank and they just replaced it without checking the length. If you have any bike coops around, they most likely would have a proper size arm to match.

HillRider

Is that what happened or is it what you think you need?

Crankycrank

Yes you can mount different length arms. I have to ask why you would want to do that as if it's a solution to a rider problem or bike problem there may be better solutions.

Andrew R Stewart 

We see this every so often. Generally due to a replacement arm (almost always the LH arm) being installed after the bike left the factory. Most every time the rider has no idea. Rarely (thankfully) a rider with serious leg length discrepancy will use different crank arm lengths. Andy

Sy Reene

Yes, and if you're looking to do this, you may find a way to rationalize offsetting the cost of a power meter by getting a left crankarm based power meter crankarm, in a shorter or longer length than the right. eg. STAGES SINGLE-SIDED POWERMETERS

grizzly59

Easily done by accident

70sSanO

If you have a leg discrepancy, you can put an insert into one shoe or build up under the cleat.

My left leg is about a cm shorter than my right. It has caused right hip pain. I initially built up under the cleat. Now I merely take cheap EVA foam flip-flops and make an insert for my left shoes.

You may need to do both and a different crank arm.

Good luck.

John

RingSteel

Being a below-knee leg amputee, I’d be curious to try a shorter crank arm on the prosthetic side of the bike.
It might reduce the difference between the power levels of the good leg and the prosthetic leg?

Sy Reene

Of course, since a crank moves in a circle, doing what you did makes the arm effectively shorter at the bottom of the stroke, but flips it and makes it effectively that much longer at the top of the stroke. 1 1cm shim makes eg. a 175mm crank 165 at the bottom, but 185mm long at the top. May or may not be an issue.

70sSanO

???

What I did was make both legs as close to the same length. It has nothing to do with some convoluted transfer to crank arm length.

I’ll make it pretty simple. Ride a bike with one foot in a shoe and one foot barefoot. There is a discrepancy. Then put shoes on both feet.

John

hsea17

Thank you for the replies. I have different leg lengths as well as a hip (same side, right side) that has seen better days. The hip is a training injury from younger days and not related to cycling, but another sport, but which I have managed to cycle and ride with for years. But now as a still relatively active retire, it takes away the joy of slightly longer bike rides. Since I am now retired and have better time to try different position on the bike etc. I thought why not try, maybe I can find the solution now, even if it is a bit late in an active life, it would still be satisfying to find a solution to what I have been wondering about for many years. I previously tried both shims under the shoe and also thicker insoles and even the Shimano pedals with a longer axle, but to no avail.
hsea17

70sSanO

Have you had your legs measured?

Additionally, I’ve slightly lowered my saddle from a more optimal height to ensure I don’t hyper-extend my hip.

I generally don’t have any pain cycling, but the years have left me with some ongoing dull hip pain.

John

70sSanO

Someone who has experience in fitting and hip angle, at the top of the stroke, might suggest shorter crank arms in general to help your right hip.

John

Crankycrank

Also a leather saddle such as a Brooks or similar can be broken in to fit your body movement better than rigid saddles if the shorter crank by itself doesn't give you enough comfort.

Sy Reene

So, in other words, shimming your foot might not at all be the correct solution. Got it.

70sSanO

No. Shimming/inserts have worked great for me for over 30 years. In a couple months, Lord willing, I’ll be 70. I’m thankful I’m still riding road and mountain and active in other sports.

I have no clue the extend of the OP’s situation. I believe the OP has done some shimming already. I can only provide what I have actually done in a similar situation.

I hope he finds a solution regardless if it something I post or from someone else.

John

hsea17

I have tried the Bikefit cleat wedges for spd pedals and the shimano cleat spacers. Due to lack of qualified and good bike fitters in the region i live a bikefit its not an option but the bike dealer I use have also good bicycle mechanic so will try out the shorter crank on right side. Will post back when bike is ready and I have tested it out for a few rides.
hsea17

base2 

u = 2πr

A 165mm crankarm yields 103.6726 centimeters of foot travel with every pedal stroke.
A 175mm crankarm yields 109.9557 centimeters of foot travel with every pedal stroke.

(103.6726÷109.9557)-1=0.0571421036 In this case: The side with the shorter crank arm will be doing 5.7% less work. Say "hello" to strength disparities and all the complications thereof.

A block or a shim does not make the offending leg do a different amount of work. It aligns the work to be done to where the riders foot happens to be. This is the correct solution. (Edit: or at the very least, the best place to start when seeking the best combination of all compromises.)

I really do wish the whole different crank arm length thing would just die. (Edit: What I mean to say is it's just a terrible go-to as the first & only suggestion. ) It is for fitting a person to a bike in a static photograph sort of way with no regard to cycling being the dynamic activity that it is.

Kapusta

The OP has tried shims and they have not worked, so all the armchair second guessing here is really not useful.

As someone who has spent the past 40 years with a significant LLD (50mm) and the past 25 years experimenting with different cycling setups (crank length and shoe lifts) and working with a top notch bike fitter…. Here is my $0.02 about dealing with significant LLD:

A shim under the shoe works like a charm IF the leg length discrepancy is below the knee. It gets more complicated when the discrepancy is above the knee.

If the discrepancy is above the knee, and you try to make up for all of the difference with a shim, you end up with your foot AND your knee higher at the top of the stroke (compared to the longer leg) and the knee more deeply bent.

Is that a problem? It depends how much the difference is. For the bast majority of LLD cases - which are not too significant - No. Which is why shims work fine for most people. But in my case where the difference above the knee is 35mm… it does.

Using a different crank length to make up some of the difference reduces how deeply the knee is bent at the top of the stroke.

The problem you run into is that if you go too different on the crank lengths, the leverage changes and it is almost like being in different gears. So the short leg is in a higher gear (OK, not exactly, but it has less leverage) AND more deeply bent.

So it is a whole mess of compromises you need to deal with, and no matter what you do, something is not optimal. It just took me a few years of experimenting to see what worked the best.

And for me what works best for my 50mm LLD (35 above knee, 15 below) is to use a 40mm lift and a 5mm difference in crank length.

All this said, I have no idea what the OPs situation is.

Kapusta

To answer the OP‘s question directly:

Yes, it is easy enough to combine crank lengths on any crank I have used. Whether you need to buy 2 cranksets or one crankset and a spare arm depends on the model and availability.

Back when I was experimenting with different length combos, I was using a sqaure taper bb and a bunch of cheap crank-arms.

Years ago, SRAM worked with me on this. I called them up, told them my situation, and they offered to send my LBS a 170mm crank arm for free if I bought a 175mm crankset through them. I did that for two cranksets.

CliffordK

You might ask to get your topic moved to:

Adaptive Cycling: Handcycles, Amputee Adaptation, Visual Impairment, and Other Needs

Use the little red button if you wish to get it moved. If it doesn't work for a newbie, post here.

Hunt on E-Bay for single crank arms. Or check out your local bike co-op recycling center.

It may depend a bit on the type of crank you have on your bike.

"Square Taper" cranks were common 40 years ago, and still are around for low to mid range bikes. You may be able to use different brands, assuming the hole orientation is the same.

Many other types of cranks will be brand specific.

base2 

Kapusta You're right. I should've phrased it as something along the lines of: "...is rarely the solution." I thought of the knee placement on the leg thing after I hit enter & neglected to edit it in to my reply after the fact. Consider that factor hereby acknowledged.

As noted by people smarter than myself: All things engineering is an endless list of compromises.

hsea17

Here were many informative answers and many things to ponder and honestly I do not know what is the best solution if there is any solution at all but I can not lose more than the cost of trying different crank arm lengths so will try. On the bike now is a Shimano 105 R7000 (172.5 -50/34) crankset and a new full 170mm crankset will cost me approximately US 115, - However I haven't checked Amazon or eBay yet. Another possibility is a friend in USA who if the covid-19 quarantine get lifted here will visit in December and then I avoid shipping cost so can be he could bring with just one crankarm if I find on Amazon / eBay.
hsea17