Converting a stationary bike to clipless pedals

[RPS]

I meant to convert an old Tunturi stationary bike to clipless pedals last year but instead ended up using my trainer for the winter. This year I’d like to get around to it because my wife can use the stationary bike easier than a trainer. The stationary also takes up less space and doesn’t put wear on my bikes. I’d like to do the conversion as easy as possible if I can find the right parts so I’d appreciate any suggestions.

Of the available options (excluding buying a new stationary bike) I’ve already excluded using pedal thread adapters because the Q factor on the stationary is very high. I measured it at roughly 178 MM, and adapters would add to this considerably.

Although it would be an easy solution, I’m not aware of any ½ inch SPD pedals. Another easy solution would be one-piece cranks with 9/16 threads, but those are rare and I haven’t been able to find any. Harris stocked them at some point but they are all gone now.

Where I need help the most is in trying to determine what type of BBKT shell the stationary has. If it is the same as BMX type bikes then maybe three-piece BMX cranks would work. I’ve read that 3-piece BMX cranks use standard 9/16 pedals, but I have no idea how the BBKT shells compare. The stationary bike has a BBKT shell that is approximately 50 MM OD by approximately 75 MM long. I haven’t taken it apart yet because I don’t have the tool I need.

The best option may be using an adaptor (TruVativ American-to-Euro Conversion ???) and install standard three piece road or MTB cranks with SPD pedals. This would improve the Q factor, and would allow me to use 170 or 172.5 MM cranks. Because I have extra parts lying around this may be easier and less costly than it first appears. As above, what I don’t know is how many different “standard” shell sizes there may be or which one I have.

Any thoughts on other options I’m not considering, or information on BBKT types and sizes?

Thanks in advance.

[oliver1850]

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[RPS]

Oliver, my present one-piece cranks don’t appear to have a lot of extra metal around the pedal threads. I’m not sure if other replacement models would have much more, or if I’d be taking a chance with those too. There is also the problem and cost of having to buy 9/16 taps in RH and LF -- I may have the RH if I can find it, but don’t have a LH. I’d also need the right size drill bit -- and drilling the hole perfectly square isn’t necessarily easy either.

[kramnnim]

I'm pretty sure the one piece/Ashtabula style bottom brackets are all the same size, so the adapter sounds like the best solution... If I had a frame with that type of BB shell, I'd compare it to your measurements...but I don't have one on hand.

[AndrewS]

This link has enough data to tell what you have, I think:
http://www.sheldonbrown.com/opc.html

I think your 1 piece to 3 piece plan should probably work.

Another method, depending on what sort of stuff you have available, would be to overweld the outside area around the pedal, then drill and tap. The downside to that is finding a reverse pitch tap that is okay to use on steel.

Or, you could weld or epoxy a chunk of metal with a 9/16" pedal hole next to(rather than on top of) the current hole.

Or, take a cheap set of clipless pedals (old Looks might cost you ten bucks on CL) and turn the axel down to 1/2".

OR, grind the axel down until you can cram it in and weld the whole mess together.

All of this really depends on how much money you want to spend futzing with the trainer. I have a stick welder and a grinder, so that would be the way I'd lean. This is just a trainer, so it really doesn't have to be structurally perfect.

[rwsaunders]

Be careful if your stationary bike is like a fixie. I asked my LBS once why the Schwinn Airdyne didn't have toe straps like other indoor bikes. He claimed that the liability of not being able to remove your foot quickly from the pedal outweighed the benefit of the strap.

[Dekonick]

You mention not buying a new trainer... what about a used model? Revmasters can be found for $300 - $400 in good shape.

[oliver1850]

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[rugbysecondrow]

I might be misunderstanding, but would something like this work? http://www.benscycle.net/index.php?main_page=product_info&products_id=300

[kramnnim]

I might be misunderstanding, but would something like this work? http://www.benscycle.net/index.php?main_page=product_info&products_id=300

He needs something with a 1/2" diameter pedal axle.

[rugbysecondrow]

Oops, I got lost in the fractions. I would think a shop that sells spin bikes might be able to help. A local gym might even have a pair from an older bike that they might sell.

[RPS]

Thanks for all suggestions; I appreciate the help and information.

I was able to take it apart and measure it more carefully. It is not the standard or common Ashtabula one piece crank which reportedly has a shell inside diameter of 51.3 MM.

The inside diameter of the shell on the stationary is 45 MM which, based on a little research, seems to be the less common one-piece "Fauber" style crank. The only difference between mine and the standard is that the Fauber BBKT shell is listed at 68 MM long and mine is only 64 MM long. This may be due to the cups on the stationary bike being much thicker than anything likely to be seen on a regular road-going bike. Since weight is not an issue, the cups are made of thick heavy steel instead of thin metal like those of bikes. The shorter BBKT shell probably compensates for the thicker cups so that standard cranks can work.

Of interest is that the one-piece crank can't be removed without first removing the cups. This seems crazy to me. Between that and the drive side bearing being shot now I'd like to replace the crank and BBKT assembly even more or the entire thing is going in the trash.

As a last effort to save the exercise bike I'm going to try fabricating my own adaptor from an old discarded steel bike's BBKT shell; maybe using the Fauber cups (with a little work) as ring spacers. If that doesn't work it's adios.

Thanks again,
Rick