Crank arm length

HBrook78

New Member
I recently purchased a2018 Explore e+3 and I pretty much love it. My only issue is that my right knee is a bit compromised WRT range of motion due to an injury. I have read that shorter crank arms can help with knee geometry problems. Are the crank arms for Giant e-bikes available in lengths other than 170 mm? I realize that changing crank arm length has a “gearing” effect.

Thanks in advance for any info.
 

6zfshdb

Well-Known Member
I have knee issues as well, sometimes involving my range of motion. I usually just lower (or in some cases raise) the seatpost until I can pedal without pain. The loss of leverage can be compensated by turning up the assist a bit.
 

HBrook78

New Member
I have knee issues as well, sometimes involving my range of motion. I usually just lower (or in some cases raise) the seatpost until I can pedal without pain. The loss of leverage can be compensated by turning up the assist a bit.
Thanks for the input. I have the saddle about as high as I can go without having my knee completely straight on the downstroke. I’ve found several pages on this subject and shorter crank arms are pretty much the standard recommendation. I have found some methods for calculating optimum crank length based on height, inseam, etc. When I use them, I come out with about 160mm for my answer.

It looks like Giant uses 2 “left side” cranks on this model, i.e. the chain ring spider is not integral with the crank arm as is the case on the right side with regular road bikes.

I plan to consult with my knee Dr. and bike dealer too. I’m due for another shot of Synvisc in my knee anyway.

I pretty much confirmed on my ride today that my issue is range of motion as opposed to force on the knee.

I will post whatever I learn here.
 

harryS

Well-Known Member
One of my bikes had a 160mm crank arms and I swapped in a longer 170mm. . A whole 10mm doesn't sound like much, but it allowed me to lower the seat by 1/2" inch.

In my case, I bought an inexpensive Lasco part which has the right side arm/sprocket as one piece, but your bike shop should be able to get you down to 160mm easily and to 155mm if needed.
 

HBrook78

New Member
One of my bikes had a 160mm crank arms and I swapped in a longer 170mm. . A whole 10mm doesn't sound like much, but it allowed me to lower the seat by 1/2" inch.

In my case, I bought an inexpensive Lasco part which has the right side arm/sprocket as one piece, but your bike shop should be able to get you down to 160mm easily and to 155mm if needed.
Thanks Harry. I can easily find 165mm left sides and I can find 160mm crank sets but, unfortunately, I would have to buy 2 to get 2 left arms. Attached picture shows that the right side crank arm on my bike is the same as a left side and the spider is completely independent. I will be working with my dealer.

It turns out that a 10mm change buys you 20mm at the top of the stroke because you have to raise the saddle 10mm to adjust for the shorter down stroke. You lose some leverage but a lower gear or a bit more boost fixes that.
 

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HBrook78

New Member
Update: I found some 6.25 inch (158.75 mm) crank arms (left & right). The trick is that they are parts for an exercise bike. They were not too expensive and appear to have bicycle standard interfaces for the pedals and crank shaft. We’ll see. I’ll also be careful to ensure they are robust enough before I trust them for anything but a test drive.

This is preferable to buying two left-side crank arms since left sides have a LH thread for the pedal interface and I would have to buy a second set of pedals to get another left pedal.

Plan B is probably to buy crankset and remove the integral spider on the right side (power tools!) or buy two left sides and press a steel insert into one of them to convert it to RH thread at the pedal end.

Dealer is talking with Giant but he did not seem optimistic.

I assume I’m not the only e-bike owner with knee issues so when I find a solution I will post it here.
 
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HBrook78

New Member
Update: Well the exercise bike parts didn’t work out. The online description was overall length, not hole-to-hole so they were too short. My knee Doc confirmed the range of motion on my right knee is 10 degrees less than my left. My dealer got me a pair of 165mm left side crank arms and another pair of pedals so I would have two left pedals. I asked whether the right pedal would want to unscrew itself and he recommended checking it periodically and if it was getting loose use some thread locker on it. He did not think it would loosen up in any case so no need to convert it to RH thread.

I put on the new crank arms & pedals, raised the seat about 5mm and voila! Problem solved. The gearing effect of using shorter crank arms is barely noticeable.

I sent an email to Giant recommending they offer crank arms for their ebikes in various lengths for those of us with worn knees and/or short legs. I will post the response if I get one.
 

HBrook78

New Member
Update....after a 12 mi ride with no issues, the right side pedal (with LH thread) decided to unscrew itself 3 miles into the next ride, destroying the threads in the crank arm. I put the 170mm crank arms back on, kept riding and pondered my next move.

Since one crank arm was stripped out anyway I decided to see if it’s possible to put a threaded steel insert in a crank arm (a la Helicoil). Turns out it is but the complete repair kit for both right and left crank arms is expensive. I was able to buy, individually, the drill bit, tap and RH thread insert I needed.

This worked, after some fiddling and cursing. I wouldn’t recommend this approach unless you are comfortable with drilling and tapping holes in metal. The good part is that I’m now able to use a RH thread
pedal on the right side with a 165mm crank arm so it won’t unscrew itself.

Knee doc gave me a shot of Synvisc the other day and I should be riding again this weekend.

FSA (supplier of crank arms to Giant) service rep said via email that crank arms for both sides should be available in various lengths. I replied, asking for part numbers and prices...now he has gone silent 😕.

If he ever gets back to me, I will post info here.
 

HBrook78

New Member
Update: FSA support guy responded to a phone call and he’s been really helpful. He explained that what is available as replacement parts is largely dictated by what the OEMs order for production. He did find me a pair of 155mm crank arms to try (at no cost).

In theory, FSA makes compatible crank arms in 160 and 165mm as well but none are in stock. Whether any are in stock depends on what the OEMs order. I’m optimistic that as e-mountain bikes become more popular, the supplier base for shorter arms will improve. The mountain bikers like shorter arms for ground clearance.

One other point: ebikes remain odd enough that you may have to speak with a human to find ebike-unique parts. FSA, for example does not list all of its ebike crank arms on its website.

FYI, Giant has yet to respond to my feedback WRT making a variety of crank arm lengths available.

Having fun figuring this out!
 

FRANKENBIKE

New Member
Hi, I am also wanting shorter crank arms around 150 mm, But have given up looking, And have decided to drill out the arms, Then use a 9/16 th. Tap and then cut off the extra length.
I have ordered the taps (L&R) from China and then I will find out if it does work !!!
 

FRANKENBIKE

New Member
Hi,
My left & right taps arrived from China (before this Mexican beer virus) But my modern weight reduced cranks have a groove on the inside, So no good, I went to a used bike parts store and dug through the old parts bins and found two crank arms that had "meat" on them. I was able to drill and tap both crank arms just fine and the cut off the extra length, grind and file into a round shape, And they look professional, Especially as the arms where polished aluminium. So, I now have 150 mm crank arms. I am now waiting for my crank arm remover tool to arrive from China. (maybe this year,?).