BarryII

I'm building a crank shortener for an elderly man and I wanted to use a bolt that's smaller than 1/2" with a nut so I won't wear out the threads in the crank. I'm having second thoughts about that. Anyway, what's the standard strength of the threaded portion of pedals when they're 1/2" fine threaded? Is it like a grade 5 bolt or what?

AnkleWork

How would you wear out the crank threads?

How would a smaller bolt prevent that?

BarryII

One user needs the shortener and the other doesn't so I'd be switching back and forth, gradually wearing out the threads. I'm not sure now. Maybe I'll keep the crank shortener on and I'll use the last available hole while my father uses a nearer one.

The smaller bolt would have had a nylon bushing around it so it wouldn't press against the threads in the crank.

I've been reading about aluminum cranks breaking so I guess there's no standard for the bolts on pedals since there's no standard material.

AnkleWork

There's probably too much stress for a nylon bushing.
When used as designed, cranks are pretty durable. Maybe look carefully at how you intend to alter the way pedaling forces act on the crank.

Andrew R Stewart 

So as often is the case here there's more data with the OP's further posts. Now a second rider is involved.


If nylon is used as a bushing it will deform before the steel bolt will bend.


I know of no published specs to crank arm strengths WRT their pedal threadings if 1/2". But you might find such data. I suspect a Machinist's Handbook might be the first place I'd look. How you'd relate this data to your plans is my question. You seem to not be of an engineering backround... Maybe some simple experiments are in order.


In the field repeated removal of pedals tends to wear the threads at a slow rate how tight the pedals are tightened influences the depth the pedal spindle tends to dig into the arm. While with steel arms this gauging in is less, but that the cross section around the pedal eye is also less is a factor. Andy

FBinNY 

To the OP

NOTHING will wear out a thread faster than an undersized shaft, especially on with different threads, precessing in the hole. Either use the correct threaded stud or pass on the project.

If you're unsure consider -- You'll be threading the bolt in and out X number of times over the life of this project, vs a bolt slopping around once per pedal revolution.

CliffordK

Is this the exercise bike someone was talking about earlier? It is quite possible that it will not get enough wear to actually cause problems.

Any chance you could use standard 9/16" pedals? They would be a lot easier to deal with.

The MKS EZY pedals are really designed to do exactly what you're wanting.



They come in a number of different styles. So one person could have cleats, and next person flats. Changing would be instantaneous.

That said, a lot of people regularly swap pedals without problems (as long as they don't cross-thread).

BarryII

This is an exercise bike and the bolt won't be spinning in the hole. There will be a nut on it. I think I have my plan. First week or two, undersized bolt and I'll watch it carefully. After that I'll see. There will be a second and maybe a third attachment point that may be strong enough even if the bolt fails.

cny-bikeman

FB was not talking about the bolt spinning around, but rather the tendency it will have to rotate incrementally from precession (Google it). In addition the stress of pedaling will inevitably mean that the bolt threads will press unevenly on the crank threads. Bad idea.

FBinNY 

Just so you understand, think about how you'd try to remove a fence post in the ground. You grab it and pull it to one side, then to another, then to the side, working your way around in a circle as you enlarge the top of the hole into a cone and keep working this way until you've worked it loose.

This is EXACTLY what will happen to your pedal shaft in the crank. As the crank turns the force applied to the pedal shaft is essentially rotating in the crank. The outer threads will be the first to go, and the process will continue until you have an enlarges hole and loose pedal. You might be able to retighten the nut, but the threads in the crank will be mashed to death.

BarryII

OK, I understand precession. It's why there's reverse threading on the left pedal. I guess my way will speed that up and the nylon bushing wont protect the threads 100% but I think everyone's exaggerating the problem. One side of the crank shortener will have the bolt, but on the other end I could put two hose clamps. I did a preliminary experiment with one hose clamp and it was strong enough (for the short time I tested it) all by itself. Here it is so far (a 1/2 inch nut will go somewhere in the middle for the new pedal location):

bud16415

Yes it would be aprox. equal to a grade 5 fastener. Maybe 8000 lbs in shear or something like that.

Your design should work ok you can buy both RH & LH taps for these threads and drill and tap your arm you are adding.

I could think of a few more eloquent ways to build something to do this if you have a machine shop. But given what most might have at home that’s not a bad plan. I would tape the hose clamps and the crank all up good once I had it made so he won’t hurt himself on the sharp edges on both ends. And no long pants when riding as his cuffs might get caught on the long end on the upstroke.

joejack951

That's a steel crank arm. Why not have an experienced welder attach an additional pedal bung or two? With steel threads, there is absolutely no concern about wearing them out swapping pedals. The main thing that wears out threads in cranks is not installing the pedal tight enough and/or riding with loose pedals. Get that right and you'll be fine.

Those hose clamps frankly frighten me.

BarryII

I'm going to round the ends of the steel with a jig saw and file. I was thinking of gaffer tape over the clamps but I'm going to experiment with a piece of neoprene under them. When the clamps squeeze down, the sharp edges should be protected by the neoprene.

joejack951

The neoprene is going to extrude right through the slots in those clamps. If you are stuck on using hose clamps, at least get some good strong, solid band clamps like these: https://www.amazon.com/Ideal-Tridon-.../dp/B007Q4YCRK

Trakhak

Many recent studies have indicated that shorter crank lengths work fine for most people. I use a desk cycle during the winter months, and I have no trouble getting my heart rate up to the target range, despite the fact that the crank arms are about half the length of the cranks on my real bikes.

The OP is probably determined to come up with a practical way to switch effective crank lengths, but if it were me, I'd simply shorten the crank to the necessary length and leave it that way permanently.

fietsbob

One Piece cranks BB type can Be converted to square taper accepting Threaded BBs, then a modest cost set of cranks from Like J&B (Origin8)
can be bought. they are offered in 150->170

CliffordK

There are lots of options for someone who chooses to experiment a bit.

If that is a special chainring with a belt drive, then it may be difficult to adapt to a different crank spider, at least without some serious machining. And spacing must be perfect.

If an undersized bolt is used through the crank arm, then a nylon bushing may help. However, you likely would be better off getting an exact match to the original pedal threads if you don't want to bung them up. 1/2-20 should be a standard bolt size, although left hand bolts are harder to find. 9/16-20 is less common.

fietsbob

Belt on a stationary bike? yea I suppose .. regular V belt drive would do..

Given its for Exercise, Not Efficiency, since its not Going anywhere..

BarryII

New, improved hose clamp idea, except I think I'll put some kind of spacer within the hose clamp so the bolt ends don't press on the crank.

HillRider

One piece (Ashtabula) cranks are available in shorter than adult lengths since they come on many kid's bikes. Could you find an appropriate length complete crank instead of all of jury rigging the existing ones?

BarryII

I wouldn't want it in a child's size for myself anyway. My father may need a drastically shortened crank. I'm willing to go only a little shorter. I'd rather just finish what I started. Also, I read there's a kind of bearing that's hard to put back in a one piece crank and I don't want to be stuck in that situation.

79pmooney

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If I had the tools, I would sut the bolt flush with the nut, then file both nuts with a round file parallel to the crank arm so the channel created fits tight against the crank. That plus the pipe clamps would stop motion and help whatever you do at the pedal threads of the crank, esp if you choose to go undersize.

(I wouldn't actually use a round file. Way too much work! I'd use a Dremel and the right bit/grinder. But the round file description makes it clear what I am after.)

Ben

ThermionicScott 

Which one? OPC BBs aren't hard to service, you should give it a try so you're familiar.

BarryII

If I do that I'll use two nuts and only file the one against the crank. Maybe two half nuts.