Changing pedal distance on cranks

I’m trying to get my wife’s new ebike to fit her. She is 69 yrs., 5ft. 1in. tall and has reduced mobility on one knee due to knee replacement. An ebike is the way to get her on the bike trails, considering the above.

With the seat set right down to the frame, there is an acceptable seat to ground distance for stability while not moving. I paid attention to this during the bike purchase, she is nervous about this as she has not been biking for many years.

The problem now is that the stock crank is 175mm and she has severe knee pain while trying to pedal. There is not enough range in the knee joint to accommodate the crank orbit. I think the solution entails a reduction in crank distance.

I checked with a local bike shop, they have advised that indeed the cranks will need to be replaced. Two issues here... new cranks are costly and secondly with a 5 spoke crank spider with 130mm bdc, the minimum crank size available is 165mm. My hunch is that 165mm will still be too long. We have an exercise bike at our gym with adjustable pedal cranks so I will work with this to see what size would get her dialed in (rather than taking the chance that 165 is the right size).

One question I had to the bike shop guy was would it be possible to drill and tap the existing cranks to a smaller size. Probably a cheaper option for me but this bike shop wants nothing to do with reworking them. New cranks are not cheap and I have no use for the old ones. I have a drill press and could purchase a tap set, but wanted to check here to see what you guys think.

Any experience with drilling and tapping cranks? I understand there is a weakening effect by doing this, but she is not a hard core grinder type of pedaller and the bike motor will do a lot of the work. The existing ones are aluminum, here’s a pic.

I just wanted to get any thoughts here as to whether this could be a viable option.

I haven't heard of drilling and tapping a crank to move the pedals, but I suppose it is possible. Remember the left pedal uses a reverse thread, so you may have trouble finding a tap for that. You are not locked into a 5 spoke spider with a 130 BCD if you replace the chainring also. I think your bottom bracket is a square taper if this is the same crankset and your chainring is 52T:

https://guide.alibaba.com/shop/prow...crankset-with-mounting-screws_1005699549.html

You could go with these to replace them for about $100:

https://www.amazon.com/Origin8-Allo...207011&s=outdoor-recreation&sr=1-1&th=1&psc=1

https://www.amazon.com/Origin8-Allo...71134&s=outdoor-recreation&sr=1-24&th=1&psc=1

You could go super low tech and just bolt on some delrin or wood blocks to one side of the pedals and use some grip tape for the pedal surface. Going this route, you can make it as tall or short as needed and make the top of the pedal surface even wider than usual.

You said she doesn't really crank on them, just kinda ghost pedaling. She would just need to make sure to rotate the now heavy side of the pedal up first. Or add enough counter weight on the bottom so the blocks stay up. If you get me?

The seat should be high enough that she gets almost full leg extension on the down stroke. That likely means she'll need to be in front of the seat to flat foot the bike.

You can talk to a bicycle recycler about a set of kids cranks too.

Not cheap, but ...

http://www.bikesmithdesign.com/Short_Cranks/

https://www.ilovebicycling.com/crank-arm-length/

I drill and tap my cranks. It's cheaper than having the local LB$ do it. And after you've bought the tooling nothing prevents you from continuing doing so. (friends, relatives, etc)

Thanks for responding with ideas. I’ll follow up on these resources and post the outcome of our chosen solution.

I got another one.
https://www.amazon.com/SINZ-Expert-Alloy-Square-Taper/dp/B07B8WKSWT

You can get these as small as 115mm, for all your clown biking needs.

Yea, I asked a local bike shop about the possibility of drilling and tapping the existing cranks. They were horrified, suggesting that they would not be strong enough to withstand light pedalling by a little old lady. No, no you need to buy new ones.

I’ve got a drill press and can certainly purchase a pair of taps. If I could build my own ebike, I should be able to do this. Only issue to determine is what the crank length should be.

Get a good ebike that doesn't need pedaling and then she won't have to. Even if she can pedal she'll need rehab for months before you get to the point where she's putting down any power...

In other words just rest your feet on the pedals and use throttle. Or have pegs.

Someone 5 ft tall shoud have 150mm crank arms. Actually, it is much easier to find in square taper cranks. Yet, to of the line spider cranks can be ordered the size you need. Drilling crank arms is OK if you can do it precise, and the arms are pretty straight thickness. Insert a long bolt in the actual drilling, to serve as a guide to drill the new length.

Bobw said:

One question I had to the bike shop guy was would it be possible to drill and tap the existing cranks to a smaller size. Probably a cheaper option for me but this bike shop wants nothing to do with reworking them. New cranks are not cheap and I have no use for the old ones. I have a drill press and could purchase a tap set, but wanted to check here to see what you guys think.

Any experience with drilling and tapping cranks? I understand there is a weakening effect by doing this, but she is not a hard core grinder type of pedaller and the bike motor will do a lot of the work. The existing ones are aluminum, here’s a pic.

I just wanted to get any thoughts here as to whether this could be a viable option.

Click to expand...

I'm not understanding going the cheap route on the cranks after spending what appears to be $3k+ on an ebike (Juiced ODK?). I might expect that if you went DIY for the bike, but generally if you buy off the shelf, it's to avoid this type of hassle.

MadRhino said:

Someone 5 ft tall shoud have 150mm crank arms. Actually, it is much easier to find in square taper cranks. Yet, to of the line spider cranks can be ordered the size you need. Drilling crank arms is OK if you can do it precise, and the arms are pretty straight thickness. Insert a long bolt in the actual drilling, to serve as a guide to drill the new length.

Click to expand...

I'd use the tap as the guide, but screwing it into the existing hole, and putting it in the drill chuck, then clamping the arm into the sliding vise. Then after it's aligned, replace the tap with the drill bit, then adjust the crank arm to the new position.

My mate drilled his out no worries for his kid. If there's enough metal where the new thread goes in it'll be fine. That 'too weak' BS is from your lbs not wanting to accept the liability....sure, I wouldn't if you were smashing dh on them...but you're not.
My mate had trouble getting the tap to start. Ended up tightly clamping an old crank so the hole acted as a guide for the new hole.
As long as you get em straight they should be fine.

Edit 'clamping'