Bicycle Mechanics - converting a 3piece BB to a 1 piece

I have an old walmart schwinn that im making a single speed out of and I wanted to know if I could Change the 3piece British/ISO size bottom bracket and Crank to a 1 piece crankset? Is this even possible?

I'm surprised a Wal-Mart Schwinn doesn't already have a one-piece crank. The bottom bracket you have should give better results and better interchangeability. It's a more precision component than a one-piece setup, whose principal virtue is that you can dismantle it with a screwdriver and a hammer! The Ashtabula one-piece crank has a larger diameter bottom bracket shell than a three-piece arrangement. I've heard of conversion kits to convert one-piece to three-piece, but can't imagine how you'd achieve the opposite without doing some welding. Anything's possible, but not always desirable or practicable.

I'm surprised a Wal-Mart Schwinn doesn't already have a one-piece crank. The bottom bracket you have should give better results and better interchangeability. It's a more precision component than a one-piece setup, whose principal virtue is that you can dismantle it with a screwdriver and a hammer! The Ashtabula one-piece crank has a larger diameter bottom bracket shell than a three-piece arrangement. I've heard of conversion kits to convert one-piece to three-piece, but can't imagine how you'd achieve the opposite without doing some welding. Anything's possible, but not always desirable or practicable.

You are correct. If the BB shell is the small threaded variety, it can't be converted to a once piece Ashtabula style. The reason a larger BB shell is required for the one-piece is for clearance so the crank can be inserted -- impossible to do with small BB shell.

Even if you could do it I see no benefit to substituting an Ashtabula crank on any bike. This forum gets many requests to go the other way to obtain better bearings and significant weight reduction. Another downside is that most Ashtabula cranks require 1/2"-20 threaded pedals so you current 9/16"-20 threaded pedals won't fit.

If you want to have a single chainring, remove the current rings, install which ever ring you want to use at the inboard position and get "single chainring bolts" at any LBS.

I just removed and replaced an Ashtabula one-piece crank & BB on my 3-spd. I love how smooth they spin - once adjusted perfectly. So I can see the draw to the OP. Might be primitive, but WOW! are they smooth.

I'm surprised a Wal-Mart Schwinn doesn't already have a one-piece crank. The bottom bracket you have should give better results and better interchangeability. It's a more precision component than a one-piece setup, whose principal virtue is that you can dismantle it with a screwdriver and a hammer! The Ashtabula one-piece crank has a larger diameter bottom bracket shell than a three-piece arrangement. I've heard of conversion kits to convert one-piece to three-piece, but can't imagine how you'd achieve the opposite without doing some welding. Anything's possible, but not always desirable or practicable.

:thumb:
I can theorize how somebody might do a one-piece crank to three-piece crank conversion, but, man, it would be alot of work. (Maybe on an old vintage bike you were in love with, and you were just determined to update the crankset???) A large diameter "press-fit" style of adaptor would have to be installed into the large BB shell, and it would have to have the appropriate threaded holes to accept a conventional screw-in (modern) BB assembly. If one had a mill, a lathe, and the proper thread cutting tools it could be done, but, wow, a lot of work:eek: for minimal gain.

:thumb:
I can theorize how somebody might do a one-piece crank to three-piece crank conversion, but, man, it would be alot of work. (Maybe on an old vintage bike you were in love with, and you were just determined to update the crankset???) A large diameter "press-fit" style of adaptor would have to be installed into the large BB shell, and it would have to have the appropriate threaded holes to accept a conventional screw-in (modern) BB assembly. If one had a mill, a lathe, and the proper thread cutting tools it could be done, but, wow, a lot of work:eek: for minimal gain.It's been done. You can buy one of these for $21.95:

http://www.sheldonbrown.com/harris/images/opc-adaptor-truvativ.jpg

I just removed and replaced an Ashtabula one-piece crank & BB on my 3-spd. I love how smooth they spin - once adjusted perfectly. So I can see the draw to the OP. Might be primitive, but WOW! are they smooth.
Is it really that smooth or is the crank so heavy it spins on sheer momentum despite mediocre brearings? :)

I just removed and replaced an Ashtabula one-piece crank & BB on my 3-spd. I love how smooth they spin - once adjusted perfectly. So I can see the draw to the OP. Might be primitive, but WOW! are they smooth.


???????

It's been done. You can buy one of these for $21.95:

http://www.sheldonbrown.com/harris/images/opc-adaptor-truvativ.jpg

Darn - look at that! Bet you it's Asian, too. Hat's off to whoever designed this little retro-device!:thumb:

It's actually a Truvativ item and yes, it's probably manufactured in Asia somewhere. Unfortunately, it costs more than what a lot of bikes with 1-piece cranks are worth.

Darn - look at that! Bet you it's Asian, too. Hat's off to whoever designed this little retro-device!:thumb:

I don't know who designed the first BB adapter, but it is certainly not a new idea. Back in the day, pre-bankruptcy Schwinn had a lot of unused frames with large BB shells that they couldn't sell because most people no longer wanted an Ashtabula crank. Schwinn upgraded those frames with BB adapters, and modern 3 piece cranks, and managed to sell them like that.

HillRider (http://www.bikeforums.net/member.php?u=38165) - Maybe it's both. Anywho, it's really smooth. But...

Retro Grouch (http://www.bikeforums.net/member.php?u=9509) - You won't see me installing an Ashtabula crankset/bottom-bracket on my vintage PUCH.

The simple fact is that if you hand me a bicycle, I'll make it the best bicycle it can be for it's genre.

Having overhauled a few Ashtabulas recently, I can see the appeal. They are durable and simple. They're particularly simple to work on. The Schwinn cranks were well made and reliable.

But no, I wouldn't want to do that kind of conversion, even if it were possible.

The Ashtabula crank is a wonderful design for situations where bicycles need to work hard with a minimum of attention, and that only by semi-skilled people with really basic tools. Like the battlefield, or the bush. However, some old Ashtabula cranks were really high-quality and beautiful to behold. We do the breed an injustice when we associate it purely with Wal-Mart cruisers.

If someone were to dare call my 3-spd. PUCH a "WalMart bike" - I'd drop a scorpion down their pants.

The Ashtabula crank is simple, robust, and easily serviced... it is also heavy and limits one to using 1/2 inch pedals which is fine on cruisers but can be a problem on bikes when you want to run clipless or toe straps and cages.

I do have some 1/2 inch road pedals on order... :)

Converting an English bb to an Ashtabula would be an epic bit of work and cannot see why someone would want to do this.

Now relegated to being used on the least expensive bikes they tend to be of lower quality than those cranks of old with high quality forged arms and good quality bearings.

There was also a conversion kit / adaptor that would allow one to use cottered cranks with an Ashtabula bottom bracket... I have a few of these.

Those old one piece cranks can be very beautiful and very smooth... I won't be converting this one.

http://www.ravingbikefiend.com/bikepics/ccmpathnewfork2.jpg

You can buy conversion adapters for letting your 1/2" Ashtabula crank-arms accept standard pedals. But they cost a bloody fortune. This is what I opted for in pedals. They can be opened for repacking/cleaning/adjusting. And they sure needed this fresh out of the box:

http://harriscyclery.net/itemdetails.cfm?ID=2343

I did attach toe-clips as well. Rides really smooth.