S60

I would like to install shorter crank arms for a child (stoker on a tandem). I have not yet taken the crank arms off, but I found the pictures below in another thread and my crank arms and chainring look identical. It looks as if the drive side crank arm is permanently affixed to the chain ring, so that I will not be able to replace the crank arm without also replacing the chainring. Am I correct, or is there a way to separate the crank arm from the chain ring and reuse the chainring with the shorter crank arm? I found shorter crank arms at a unicycle shop, as pictured below, but I am not sure if they will work. Thanks.

fietsbob

... Looks riveted on to me... , so. no.

mbbiker

The rings look riveted on so thats a no go BUT if your arms are as wide as they look you should be able to take them off and bring them to a machine shop and have them drill and tap a set of holes at 160 or 165mm or whatever length crank arms you wanted to put on. We've done that a few times for special needs customers. If it isn't safe with your crank arms the machine shop i would guess would let you know before sending out unsafe work.

Bill Kapaun

I'm not sure a machine shop would know what was safe or not for a bicycle.
Would they also have a 9/16"-20 LH tap for the L arm.

How would you plan on attaching the DS Unicycle arm to a chain ring???

You might be better served to find a short set of cranks off a 20-24" bike.

I salvaged a VERY CHEAP (qualitywise) 150MM set off a junk 24" bike

Bianchigirll 

The three rings are riveted together and the middle on is the base. the steel middle ring is, I believe the term is, Swaged onto the alloy crank arm. it is something like a press fit. I am not sure what your thinking about having a machine shop drill to accept new rings.

Dan Burkhart 

Don't think that's what he's suggesting. He's talking about drilling new pedal mounting threads. Never done this myself, but I've seen it done successfully.

Amesja

That is a very low-end crankset. On a regular bike replacing the whole crankset with another low-end crankset would be a simple (and inexpensive) process.

But on a tandem the drive train is much more complicated and your options are much more limited. You really need to talk to the mechanics at your LBS about the options you have. It can be done -but it probably won't be cheap as the options are limited.

Those unicycle crank arms are totally not the same thing and can't be used here. You will need cranks with "spiders" on them that mount separate chainrings -if you have a used bicycle cooperative you might have a better chance of finding something that you need although the knowledge to know WHAT you need might not be easily available. You don't want to mess with changing the gearing so the chainrings you find will need to be the same numbers of teeth unless the person who is helping you knows what they are doing with tandems.

Then there is the issue with chainlines (the distance the chainrings are from the frame tube) The chainlines of the tandems might be much different than what you can find off of regular bicycles. This isn't something that can be easily explained or helped online with advice to walk you through this. You need the active involvement of an experienced bike mechanic who knows tandems as you chose these parts to convert to shorter cranks.

Myosmith

Find a donor bike at a garage sale or second hand store. Make the swap, then resell the donor bike.

Bianchigirll 

RATS! I knew all those speed reading classes at Or Lady of Perpetual Sorrows would lead me to no good one day.

I thought he was say... Oh well it doesn matter.

Dan Burkhart 

Don't worry. Happens to me a lot too.

S60

Thanks for the thoughts. The tandem creates complications. As I observed this morning, the non-drive side is also a drive side as it contains a chainring that connects to the captain’s chainring. Both sides appear to be made similarly, with the crank arm somehow riveted to the chainring.

Looks like I have two options.

Option One - complicated and expensive, and probably won't work - find suitable sized tandem chainrings for both sides that have removable crank arms, then find shorter crank arms. The problem here is that both the left and right side will need spiders to be able to attach to the chainring, and with the left and right threading, that will likely be impossible to find in the shorter size crank arms I am looking for.

Option two - relatively easy and inexpensive - as suggested above (thanks Bill Kapaun), drill a second pedal hole on each crank arm, and tap the hole to accept a 9/16-20 pedal. I will need a right and left tap. Luckily Park Tool (picture below) sells a left and right tap set (Park Tool 9/16-20 pedal taps, click here), and they suggest a 33/64 drill bit for the hole. One of the comments on Amazon for the tap set says this is for chasing, not for creating new threads in a smooth hole, but I am going to give it a shot. Not sure why Park Tool would recommend the drill bit size if these weren’t suitable for creating new threads in a smooth hole (I am going to email Park Tool about this). Worst result is it doesn’t work but I have made my bike a little lighter. By the way, Sheldon Brown has a great write-up on taps here. Thanks again.

Dan Burkhart 

Tapping new threads in aluminum shouldn't pose any problems. Also, you might consider drilling and tapping for 1/2 pedal threads rather than 9/16. You might have better luck fitting a smaller hole in the flat area.
If it's for a small kid, the smaller shaft should be fine.

LesterOfPuppets

The other option is a child stoker kit from Co-motion. Kinda expensive @ $250 but you might be able to get some back upon reselling.

www.co-motion.com/index.php/product/co-motion-child-stoker-kit

Amesja

If you drill/tap you will definitely need to be using a drill press. If the hole is tapped at even the SLIGHTEST angle the pedal will not be straight.

If you have never used a tap before remember it is a slow and careful process. Use plenty of cutting oil. Cut about 1/8-turn of thread at the most at a time and then back off the tap and clear the threads of spoil. Then tighten the tap again slowly and cut another 1/8-turn of thread. Repeat. If you try and just bear down on the tap you will certainly break teeth and soon after the tap itself right off.

Drilling out a tap that is broken in a hole is nigh impossible...

Taps are VERY HARD tool steel and don't bend much at all. They shatter. Keep an eye on the tap and if you see it twisting between the cutting surface and the tap handle you are millimeters from disaster.

HillRider

This is the ususal method for letting a small child ride stoker on an adult size tandem. As noted they aren't cheap but sholuld be saleable when the child outgrows them. That said, I'd check a couple of tandem forums and see if a used kid-back crank set is available at lower cost.

Dan Burkhart 

Good idea. kids eventually outgrow them, so I'm sure there are used ones around.

Lawrence08648

You won't be able to get the threads 100% perpendicular to the crank. A slight offness and you will feel it when you pedal.

The easiest way, buy a new crankset and this crankset will have replaceable chainrings.

For a shorter person, you need a longer crank, not a shorter crank. A shorter crank you will need a longer leg, a taller person; a child is a short person with a short leg. A longer crank will come up higher. I'm assuming the seat is all the way down. Taller riders have longer cranks because they can generate more leverage plus it's to align their knees and the saddle. If the seat is all the way down and the distance is still too short to reach comfortably, then you need a longer crank that comes up higher.

bobotech

Buy a bigger kid.

couldn't resist.

bobotech

I'm confused by having a longer crank for shorter person. Sure the crank will come up higher but the crank will be WAYYY down at the bottom of their power stroke. As the child pedals, their feet would leave the pedal as they push down because their shorter legs won't be able to reach the pedal on a longer arm at the bottom of the compression stroke.

A solution for shorter riders is to raise the crank centerline closer to the seat with shorter arms.

reddog3

I'd probably advise against drilling your current crank arms, especially if you plan on returning them to the original length at some point. The extra hole will significantly weaken the arms. Also, crank arms are typically thinner further up from the ends, so you don't have the thread depth to support the pedal. Also, you need some method to mill a flat for the pedal to seat to. So... if you do shorten them, you'll be back to hunting parts when the little one does outgrow them. Might as well bite the bullet now.

There is this option- https://www.universalcycles.com/shopp...60&category=62

reddog3

Yeah, me too!

There is actually an auxillary bottom bracket to do just that- raise the bb centerline for shorter riders.

S60

No, I think I need a shorter crank. I am focusing on the down stroke. The shorter crank will make the pedal on the down stroke higher up, thus fitting the shorter leg.


A couple problems for me with the child stoker kit. First, it needs to be removed for taller riders, and I have two taller riders that also ride stoker, so removal and re-installation will not be pleasant. Second, I only need to bridge about 4 inches. Adding the child stoker will likley add a lot more than 4 inches of reach, resulting in the child sitting that much higher off the ground, and right now he can climb on and off by himself.

BUT, in researching the child stoker on the co-motion site (thanks for that great idea, and I see reddog3 just posted this), I came across another option that might work, and it's in between the cost of drilling new holes and the child stoker kit. And probably will have good resale value as well. It is called a Crank Arm Shortener. Between the few inches I will get on the crank arm shortener, and buying a lower profile seat, I should be able to bridge the 4 inches. Thoughts?

Lawrence08648

Bobotech, you are right on the down stroke, I am right on the upstroke however, my choice of a longer crank will fail and yours with a shorter crank will succeed. You are right, need a shorter crank, lower the seat. My mistake. I bow to you.

leob1

You aren't going to be able to drill the crank arm with any accuracy, unless you have at least a drill press and a jig of some sort(which you will have to build), or take them to a machine shop and explain what you want to do to them. A machine shop will mount them on a milling machine to hold them in place.
The crank arm shorteners look like your best bet. You can put them on and take them off, and the cranks won't be weakend by drilling holes in them.

cny-bikeman

There are options that don't require you to take chances with your crank arms.



https://www.medicaleshop.com/reha-par...-tricycle.html - About $40



https://bikesmithdesign.com/Short_Cranks/shorten.html - about $50