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RidingAgain

Just When You Think You've Seen Everything...

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Posted (edited)

So I've not done much with one-piece cranks and related bottom-brackets, but I've been working on a few kid's bikes that have one-piece cranks and thought I'd do a little research on what they're all about... And guess what... These cranks use wide diameter bottom brackets, and I thought that was that... But I found out that there is a converter that allows the use of regular bottom brackets... Which means, you can use different cranks... Which means... Possible weight savings and better drive. And this is good with adult bikes as well.

Pretty cool.

This is the adapter...

https://www.harriscyclery.net/itemdetails.cfm?ID=1335

Edited by RidingAgain

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Posted (edited)

that wide BB shell is so large purely because that is what it takes to fish a one-piece crank through it. It was common for most BMX bikes to use that size probably because manufacturers could make one frame design cheaply and equip the bike with a one-piece crank and Ashtabula BB. to put a conventional BMX three-peice crankset in that frame, you need what's called an "american bottom bracket" with the right size spindle and bearings. the bearings sit inside an aluminum spacer that fills the gap between the shell and the smaller bearing. most of the spindles are 19mm (classic Profile BMX) but 20mm and 22mm (good old Primo Powerbites!) were also common. the sprockets could be made with a center bore to match the spindle size, but could also be made larger for the imperial Ashtabula standard and a top-hat shaped adapter would size it down for the spindle.

the threaded bottom brackets we all know and love have been common on aluminum BMX race bikes for a long time before that and a few chromoly freestyle bikes adopted the design for a bit there, but it didn't stick for the freestyle world. I had a Premium street frame with a Profile BB for a minute there and it was quite fussy to get the bearing preload just right. it was easier once I faced the shell, but the spindle has to use a spacer stack that is just right.

Race bikes traditionally used square taper spindles and I think those are still around, especially for bikes made for smaller riders. it looks like BMX race bikes for adults have adopted external cups and 24mm spindles like mtn and road bikes, but I don't pay much attention to that.

As the market for BMX freestyle bikes matured in the late '90s(?), it became less and less common for bikes to come equipped with crappy one-piece cranks that kids would bend within the first week of ownership, the industry developed two alternative BB standards: "mid" and "Spanish". if I remember correctly, "mid" was a BB shell that allowed the bearings from an American BB to fit directly into the frame without the superfluous aluminum spacer (usually with the help of a block of wood and a hammer, because, BMX!) and the Spanish standard was a press-fit version of the Euro BB sans threads. Mid is probably the most common. something might have come out in recent years though.

you can put a crankset with a threaded BB on a BMX bike and I am glad to see that adapter is only $20. I was expecting it to be a lot more expensive. it's good to fit oddball parts together. but if you want a strong crankset that is going to hold up to BMX abuse, I'd go for a BMX crankset that will use a press-fit "American" BB.

keep in mind that if you take a crappy one-peice crank off a bmx bike, the pedals use a smaller 1/2" spindle, whereas all other cranks use a slightly larger 9/16" spindle. so the old pedals will not work in the new cranks unless you use some (sketchy, IMO) adpaters.

@Big_papa_nutscan probably recall some of the details better than I can.

Edited by mack_turtle
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1 hour ago, RidingAgain said:

And here a video on it...

the sound of bottom bracket bearing cups flying off a bike and bouncing across the floor brings up conflicted memories. maybe I missed it, but did he push those cups in dry? arg!

a lot of the time, I would have a hard time justifying the expense of putting a new crankset, adapter, and BB in a frame with an American size shell. those bikes are frequently junk. but if it's a nicer frame, or you it has nostalgic value, or you have the parts, go for it. if you're using a square taper BB, finding the correct length spindle to get the right chainline might be a challenge.

this would be a really good way to make an old bike into a functional mutant klunker. think about it: put some strong, tubeless 26" rims on a coaster brake wheelset, bolt those to an old beach cruiser frame, put something like a Surly Sunrise bar on it... this is the kind of stupid idea I would be pursuing if YBP was open right now.

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Posted (edited)

Great post, mack_turtle... Thanks for taking the time to make it.

And yeh... $20 surprised me too.

Edited by RidingAgain
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There is actually three sizes of bottom bracket shells I'm aware of for one piece cranks. The Ashtabula or American, a "mini" American (common on cheaper kids bikes), and Look makes one for their CARBON FIBER ONE PIECE CRANKS.

 

There are also square taper bottom brackets for Ashtabula bottom bracket shells.

Related reading:

http://www.gsportbmx.co.uk/support/rideuktech/bottombs.html

http://www.gsportbmx.co.uk/support/rideuktech/bbs again.html

http://www.gsportbmx.co.uk/support/rideuktech/bbs again.html

And a video:

https://www.google.com/url?sa=t&source=web&rct=j&url=https://m.youtube.com/watch%3Fv%3Dw2p7VJuZA2c&ved=2ahUKEwik7_GwpfbqAhVCSK0KHX-KARMQwqsBMAB6BAgKEAM&usg=AOvVaw25m3U7DmV-Xl5cMjZIhSA7

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Posted (edited)

I am so happy to see that George French' told G-sport tech columns are still around. that stuff is pure gold. I should save it for the a certain crowd, just in case.

Edited by mack_turtle

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