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jpsantell
08-20-2013, 06:11 AM
It my be obvious to most of you but is the bottom bracket on my sons Mongoose 20" BMX bike the same size a a regular bottom bracket that will take a 3 piece crank? Is it English threaded? I haven't cut it off yet and don't want to waste the time if it won't take my Shimano components.

Ticktock
08-20-2013, 07:56 AM
If its a one piece crank, no. If its a three piece crank, almost certainly--but there are traps with BB threads. One piece cranks have a a bigger shell to allow the crank to pass through.
Adaptors are available, but I have never used them.
Check the threads before cutting, remembering there are left and right hand threads involved, so remember which side each bit came from.
Steve G
Beijing.
By the way, its not a silly question--its been asked before, and others have admitted finding out after cutting--better to ask first.

darnthedog
08-20-2013, 08:31 AM
And if all else fails- take the mongoose components out and attempt to mount your shimano components before cutting anything. If it does not fit- fear not as Brad does sell that part now. And you don't have to weld all the excess holes you will get from cutting up the mongoose bb.

darnthedog
08-20-2013, 09:38 AM
After retreading the title a couple times I wanted to state for the record, and I am quoting for an old college professor of mine" The only stupid question in the world is the one you didn't ask"
In other words according to him; There are no stupid questions.

Not knowing is not a crime- not asking allows for continued not knowing. What I know I know from asking questions and getting answers. Some times I get the answers from reading, other answers I get from sharing. The big deal is to ask and the keep asking till you have no questions. Hopefully I will take unanswered questions to the grave.
So ask your questions the worst anybody can say is I don't know.

Radical Brad
08-20-2013, 07:42 PM
There are no stupid questions here!

Here is a bit of info if you have not found it already...

http://atomiczombie.com/Tutorial%20-%201%20Piece%20Bottom%20Bracket%20-%20Page%201.aspx

http://atomiczombie.com/Tutorial%20-%203%20Piece%20Bottom%20Bracket%20-%20Page%201.aspx

http://atomiczombie.com/Tutorial%20-%20Bottom%20Bracket%20Bearings%20-%20Page%201.aspx

Brad

Tradetek
08-22-2013, 12:06 AM
Actually it depends. BMX cranks come in both single and "triple" cranks.

Multispeed bikes only come with multi-part cranks.

BMX bottom brackets and shells are NOT compatible with multispeed mountain and road bike bottom brackets and shells.

Bill

farst1
08-24-2013, 09:23 PM
Actually it depends. BMX cranks come in both single and "triple" cranks.

Multispeed bikes only come with multi-part cranks.

BMX bottom brackets and shells are NOT compatible with multispeed mountain and road bike bottom brackets and shells.

Bill

Bill, not true about multispeed mountain bikes. They do come in one piece cranks. The donar bikes that I purchase at K-Mart and Big W in the $80 - $100 price range are 21 speeds, all with one piece cranks.
Leigh.

Tradetek
08-25-2013, 12:35 AM
Seriously? Can you post a pic of the inside and outside of the right crank. I'd be interested in seeing how the sprockets are connected.

Thanks

Bill

FrankCrank
08-25-2013, 12:56 AM
...I've only ever seen one chainring on a one piece crank, so I too would like to see a photo of the 3 chainring version ........

Ticktock
08-25-2013, 02:29 AM
I can't remember where I saw the pic, but it was only recent, and I'm sure I saw two (maybe three) chainrings on a single piece crank. Thought it was unusual, but was busy with other things, so did not mark the page, If I can find what I think I saw I will post it, as it did get noticed as unusual.
Normally I would say the same thing, as I have never seen one in real life.
Steve G
Now the brain has gone into longer term memory mode, and is totally confused.

EDit--Took the easy way out--Googled " one piece crank and triple chain rings" Sheldon Browns site indicates they are (or were) available, so its not unrealistic to expect to see a few around, but I suspect , only in the USA.

farst1
08-25-2013, 11:31 PM
1piece crank with 3 chainrings. 18 speed not 21 speed as earlier quoted.
http://i1288.photobucket.com/albums/b481/farst164/Onepiece_zps2f608a2f.jpg

farst1
08-25-2013, 11:39 PM
There are 3 pressed metal rings riveted to each other. They then are afixed to the crank in the same manner as a 1 piece bmx crank. The cranks are the same.
http://i1288.photobucket.com/albums/b481/farst164/onepiececrank_zps6574300f.jpg

FrankCrank
08-25-2013, 11:53 PM
...thanks for showing the pics farst1, you live and learn eh :). One piece cranks are ubiquitous around here, and have one on my trike, which was made using an old clunker donor. Was amazed at how heavy the crank was, and with only one chainring. With three.....well, guess you don't want to drop it on your foot.......ouch.

Ticktock
08-26-2013, 09:34 AM
Hi Frank,
As I have had a bad run with square tapers working loose, it would not take much to convert me to a onepiece crank!
Any hints on solving that problem would be appreciated, as it has now happened on three different bikes, and about fives sets of axles and cranks. Mr Lee says I'm too strong for the cranks!
Dropped half a bed on my foot last week, and its still a bit sore.
Steve G

FrankCrank
08-26-2013, 11:05 AM
....that's a strange one Steve, just to happen once, but several times is most peculiar. Mostly people have a hard time just trying to remove a crank from the square taper, even with the crank extractor they can take some persuasion. If it's locally made stuff, maybe the tolerances are a bit off - best I can offer on this one....sorry....

bambuko
08-26-2013, 11:53 AM
...Any hints on solving that problem would be appreciated, as it has now happened on three different bikes...
Buy a decent set of cranks (I mean spend money) - there such a variability in a quality of tapers, that at a cheaper end of the range they just a piece of shi*e.
I am also assuming that you are not doing "mix and match" - JIS and ISO are superficially very similar...
Some of the cheap spindles have barely taper on them...
Do the test - blue the cranks and see how well they match the spindle - you will be surprised

Tradetek
08-26-2013, 11:38 PM
Second Bambuko on this one, but also add that either the bolt itself is not quite the right size, or simply are not tightening it enough. 3 sets is either a chronic installation error or a bad run of parts.

Tradetek
08-26-2013, 11:39 PM
BTW farst1, thanks for the pick, but is the crank arm actually one solid piece from pedal bolt hole to pedal bolt hole?

farst1
08-27-2013, 05:16 AM
BTW farst1, thanks for the pick, but is the crank arm actually one solid piece from pedal bolt hole to pedal bolt hole?


It sure is one piece, from hole to hole. Even takes the same pegs as a1p bmx crank.

Ticktock
08-27-2013, 09:03 AM
What is a bit strange is that they don't come off easy unless they fall off! And its only the left side, and is a regular problem--I just keep it tight, and carry the tools to fix.
Bambukos idea is the best--blue the cranks-.
But the two standards is a new one on me, any more info, as if it is the problem, then we should all know about it.
Steve G

bambuko
08-27-2013, 10:04 AM
...But the two standards is a new one on me, any more info, as if it is the problem, then we should all know about it.
Steve G
well... as always, Sheldon Brown is a good place to start:
bottom bracket interchangeability (http://www.sheldonbrown.com/bbtaper.html)

Ticktock
08-27-2013, 11:40 AM
I found more info on JIS and ISO tapers, which was new to me, so a check in that direction is next on the list of things to do .
It could be the problem, as all cranks have come from the LBS, but all axles have come from the local mechanic, so its a good clue that Bambuko is correct.
We will see in due course.
Steve G
PS Just seen your post, Bambuko---Sheldons page was where I found out what the difference was. Thanks for the link. His pages have the answer to most common problems.'

bambuko
08-27-2013, 01:15 PM
Having said all this, I guess that it is probably something more simple - not enough torque when tightening crank retaining screw.
When the taper is of good quality (like say for example R8 on my milling machine) it doesn't take a lot, but on bicycle crank the spec is surprisingly high, see:
Bicycle Specific Torque Values (www.parktool.com/uploads/files/blog/torque.pdf)

305-391 inch pounds for crank bolt

and... when you mix your cranks and spindles, there might be (there is) a combination where, however hard you tighten the bolt, the crank bottoms on the taper base and remains loose (or not tight enough).

Ticktock
08-28-2013, 09:23 AM
Mine are tightened way past that figure! Suspect the mixture is causing the problem, and , as you say , something is hitting bottom before it really tightens.
Further investigation required.
Thanks for the new info on cranks--will be more careful what gets used next time.
Steve G

worker1234
09-05-2013, 10:08 PM
I have 3 one piece cranks with 3 chainrings on each crank. When I started collecting bikes I got no 3 pc cranks so I never cut these up. They can't be that rare or I wouldn't have 3. My bikes were all free.