Nyah

This question is in regards to conventional crank design. I don't ride bicycles regularly anymore and have not done much distance cycling. The most recent cycling trip of considerable distance that I took was somewhere around 52 miles total over two days. Afterwards, I could feel that my right leg was considerably more fatigued than my left. I narrowed the cause of this discrepancy down to the fact that the crank arm of the right side of the bike has a bar (or pin) that drives the spider/chainring. I would like to know if this phenomena has a name or, if this bar/pin crank design has a name or, if there are any alternative crank designs which attempt to spread the driving force evenly between both of the rider's legs. Thanks!

Edit: The bike is a Trek 520 touring bike, which I bought new from a bike shop. The frame size is appropriate for my physiology.

BlazingPedals

It's possible that the left crank arm flexes a tiny bit more than the right by virtue of the spyder reinforcing some part of the arm; but I highly doubt that you could tell the difference by pedaling. And I don't see why lack of flex would give you a sore right leg. More likely you have uneven muscle development or you were pushing unevenly.

Flip Flop Rider

this is a tough one. Is one of your legs shorter than the other or have an injury that the other one does not?

rosefarts

I have occasionally been much more sore one one side than the other. Either in the leg or back.

It's a hip and hamstring stiffness issue. Also possible that it's a different cleat position on the pedals issue but I don't think that is your particular problem.

I highly doubt that the soreness is from the slight inherent difference in the left to right balance of the bike. More likely it's your own balance and flexibility that should be improved.

livedarklions

What makes you think it's the crank? Have you done similar rides on different bikes without this happening? Frankly, unless the arms are different lengths, I can't see how having the pin on one side has anything to do with making the effort different for your legs.

shelbyfv

Not your crank but it would be interesting to hear how you came up with that idea. Unique!

Nyah

No, for both questions. I have a regular running/walking gait.

CliffordK

Likely something that sends shivers up the spines of developers of crank based power meters (that use absolute flex to determine power input)

However, I agree, this is likely a training issue, or perhaps one leg had something like a sore knee throwing everything off.

Was the bike assembled from parts? Double check that the crank arms are the same length. If you use foot retention like clipless/cleats, are the cleats set the same on both shoes, or right for your feet?

Nyah

I seem to remember the injury being in my hamstring. As I implied in the other post though, I wasn't injured until after the ride. It wasn't a cleat issue for me, because I was wearing street shoes, with toe-clips properly arranged to the pedals.

livedarklions

Is this the crank in question?https://www.ebay.com/i/173739985391?...caAuzuEALw_wcB


I don't see how this is any more "imbalanced" than any other crank--one arm has to be on the drive side and the other on the non-drive side.

Not trying to be a wise-guy, but is your seat on straight? What about your handlebars? It sounds like you're riding off-kilter somehow.

Phil_gretz

You could to a sports physical therapist and have a full flexibility and strength rundown done on yourself. You'd be shocked what you'll discover. As others have said, it's likely some mechanical imbalance in you, rather than with the bike.

There are simply too many variables to say without a PT exam, but relative crank arm stiffness would be very, very far down on the list of likely contributors.

mstateglfr 

Take a picture of the bar/pin that you are referring to. I have no idea what you are trying to describe. A crank flexes on both sides. The bottom bracket attaches to each arm at the same location and if anything, the arm on the drive side is thicker(and therefore stiffer).

livedarklions

The link I posted in # 10 is the crank that comes standard with the Trek he identified.

shelbyfv

Has anyone figured out what the "bar/pin" is?

livedarklions

I think it's the bolt on the front of the non-drive side crank arm:

ThermionicScott 

The ideal crank wouldn't flex at all, so I'm leaning way harder in the direction of a fit issue here.

mr_bill

On the drive side, there is a pin that keeps the chain from jamming between the largest chainwheel and the crank. It transmits no torque.

On the non-drive side there is a safety-plate. Some people call it a pin. It serves two purposes. It is a gauge to aid in proper assembly of the two piece crank, and, if the crank arm bolts are not torqued correctly, to serve as a fail-safe for a few turns of the crank. It transmits no torque under normal operations.

So, double check:

Crank arm lengths. (Unlikely.)
Centerline of frame to cranks. (Unlikely.)
YOU.

We are asymmetric creatures, think lobster claws.

(For most people the duh moment is put your right hand over your right shoulder at the top of your spine behind your back. Then put your left hand behind your back above the bottom of your spine and reach up and grab your right hand. Then visa-versa.)

-mr. bill

Elvo

livedarklions

Honest question--if the issue is rider asymmetry, what can be done to address it? Is it a conditioning issue or are there mechanical fixes?

I've never experienced this, so I'm wondering if anyone has had to deal with it and what they did.

MoAlpha

The left front tire on my car went flat and I've narrowed down the cause to the extra weight of the driver and controls, which are on that side. Can anyone recommend a car without this obvious design flaw?

cgates66

I've been following this thread for a couple of days, and I feel compelled to weigh in. FairWheel bikes did a study a while back comparing crank stiffness, using a 200 pound load (I think the bar graph above is from that study, or maybe an earlier one). The range for deflection on the DS was between 4 - 6mm, and on the NDS was 5 - 7mm or so, depending on the model. An ideal 175mm beam, with a 5x25mm section would deflect ~4mm under a similar load. Anyway, the DS / NDS difference is ~1 - 1.5mm under a 200 pounds. It is highly improbable that anyone could do 200 pounds per pedal stroke for any extended period of time - think 30 - 60 pounds as more typical, and the deflection is linear with the load for these small values. That means 1mm under a 200 pound load would be more like 0.15 - 0.3mm under a "typical" load.

It is highly unlikely that a 1mm deflection difference would have any physiological effect, and even less likely that a 0.15 - 0.3mm deflection would have any effect.

Homebrew01

It's not plausible that a minuscule difference in crank stiffness caused injury/pain to 1 leg.
Plenty of more likely causes.

DrIsotope

On a Trek 520, frame flex at the bottom bracket is significantly more likely than crankarm flex.

For the arms to deflect, the OP would either need to be putting down a whole bunch of power, or have a really off-balance pedal stroke.

I think the simple answer here is the OP doesn't ride much, then did 26 miles a day on back-to-back days.

Conditioning most likely, with bike fit a close second. Frame/crank material not even in the running.

Caliper

A Formula 1 car avoids this design flaw, I suggest you upgrade!


To the OP... Yes, as an engineer, I am sure that I could calculate slightly more flex on the left side crank arm due to having to transmit torque through the crank axle to the drive side. However, that difference in flex is going to be minuscule, something best measured with tools a machinist would use. If you are riding in regular street shoes, I would expect the flex in the sole of your shoe to be larger than in the crank. I would agree with those above that this is a setup or bike fit issue. Our bodies are often fairly asymmetrical whether it is noticed or not.

mr_bill

About a year ago first time putting any weight on my leg:



Anyhow, most people are not that extreme obviously. Just keep riding is usually all that needs to be done.

But yes, in some cases you adapt the bike. See adaptive cycling forum here.


But in atrophy cases like mine, pros help. I miss my physical therapist honestly. I had to mindfully make sure that I wasn't cheating with my right leg.


Many crank power meters assume rider symmetry. We aren't. But you don't need a power meter.

The easiest real world way to find out is to ride on a quiet bike path or quiet road. Slight uphill useful, and nobbies help but road tires work too.

WAAAaaahWAAAaaahWAAAaaahWAAAaaah. (Symmetric power application.)
WAAAaaahWAAAaaahWAAAaaahWAAAaaah. (Asymmetric power application.)
WAAAaaahwaaaaaahWAAAaaahwaaaaaah. (Physical therapy to work that left leg.)
WAAAaaaaaaaaaaaahWAAAaaaaaaaaaaaah. (Single-leg training.)

And of course:

WAAAWAAAWAAAWAAAWAAAWAAAWAAAWAAA. (Spin class.)
BOOMBOOMBOOMBOOMBOOMBOOMBOOMBOOM. (Spin class bass line.)
KEEP IT QUICK COME ON. STARTING TO BURN A LITTLE? (Spin class instructor.)

Finally, some asymmetry is perfectly normal. Effects runners too. Before you think symmetric is obvioulsy better, Usain Bolt has an asymmetric stride.

-mr. bill