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-   -   Help with bb conversion! (http://www.bikeforums.net/bicycle-mechanics/926694-help-bb-conversion.html)

jerrymelons 12-18-13 10:01 PM

Help with bb conversion!
 
Hey everyone! I need some help with a problem I've come up with. I just purchased 76 Schwinn Varsity Bicentennial, (I know, not the greatest but I like it!) Anyway, everything on the bike is in pretty good condition. Except for the crankset.. It's bent. It looks like someone took a tumble and the crank took the damage. So while looking for a new crank, I'm faced with a dilemma. Find a used Schwinn crankset on ebay, with no guarantee that it won't be bent also. Or buy a conversion kit, bb, and crank. I would think that I might as well buy the conversion parts, and call it a day. Since most of the prices I see for the Scwhinn cranks are pretty expensive. The problem I have is, I have no idea what size of bb to get. The crank I'm looking at is this one http://www.amazon.com/gp/product/B00...A2TE9IQP68MWQU I know I need a 68mm, but I don't know how long I need to have it. (I'm very new to this, if you can't tell! Haha!) Of course, I don't want to throw a bunch of money at this bike. I just want something fun to ride around town with. Any help and advice is appreciated! Thanks!!!!!

Andrew R Stewart 12-18-13 10:15 PM

The supplier that the online shop gets these cranks from lists a 122 spindle length. Do understand, though, that factors beyond the control of the manufacturer sometimes throws a curve ball and suggested dimensions are not what can be the best fit. But start with a 122mm and see how it lines up. BTW you'll be spending about double to do the conversion to a cotterless crank then just replacing the one piece one. And remember if replacing with another one piece crank a Schwinn arm uses a 28TPI BB not the far more common 24TPI ones. Andy.

FBinNY 12-18-13 10:25 PM

If you buy the conversion kit, then you can buy just about any matched crank and ISO (1.37") BB.

Note the key word here is "matched". Crank offsets (the relative position of the arm and chainrings compared to where it fits the spindle) vary model to model, and have changed over time. Modern cranks use a shorter spindle for the same chainring position compared to earlier models.

For example since 1977 to 2001, Campagnolo record spindles have gone from 120mm to 102mm.

So I suggest you buy the crank and BB from the same vendor and ask that they confirm that they are a matched set.

jerrymelons 12-18-13 10:34 PM

Thanks for your quick replies! I checked on Sunlite's website and got these specs for the crank Steel 170x52-40 122 SQR JIS SIL 118x5B 1300g 2x8 I assume the 118 Is the spindle size I need? I'm sorry if these are dumb questions, but I'm a lot more used to working on cars, than I am bikes! I'm still deciding whether or not, I should do the conversion. I just needed some advice and help before I made the final decision.

FBinNY 12-18-13 10:41 PM

Quote:

Originally Posted by jerrymelons (Post 16341977)
Thanks for your quick replies! I checked on Sunlite's website and got these specs for the crank Steel 170x52-40 122 SQR JIS SIL 118x5B 1300g 2x8 I assume the 118 Is the spindle size I need? I'm sorry if these are dumb questions, but I'm a lot more used to working on cars, than I am bikes! I'm still deciding whether or not, I should do the conversion. I just needed some advice and help before I made the final decision.

I checked the prices for replacement 1-piece cranks and they're outrageous. However there were millions of bikes made with these cranks and they turn up daily in garage sales. If there's a bike co-op in town go there and odds are they have a dead frame with a serviceable crank, which can be salvaged. Otherwise, troll the garage sales in town until you find a cheap bike with a suitable crank.

The advantage of buying a donor bike, is that you'll have back up for other parts like the stem or possibly the seatpost.

So I don't know what the total cost of the adapter, crank and BB comes to, but I'll bet it's more than a donor bike would cost.

fietsbob 12-18-13 11:06 PM

I had a tubular OPC in the past, it was quite nice..

But yea the ones that let you install a standard BB is a good approach

No clue as to your mechanical chops , amazon wont help you with that,

other than sell you a book on mechanics to read.. (not a bad thing, Books )

onespeedbiker 12-19-13 01:46 AM

How badly is it bent? All the one-piece cranks I have seen are steel and I have repaired many a bent one-piece crank by simply bending it back to it's rightful place. Since they are steel they bend easily but you can bend them back just as easily and without compromising the strength of the crank.

surreal 12-19-13 06:01 AM

This comes up all the time. Personally, I think you're better off getting a new OPC, but I get it that some folks want the adaptor to use an upgraded crank. I can see how that makes sense, too.

But it makes less than zero sense to go to the trouble* and expense** of converting American to BSC just to install the worst crank you can find. An all-steel Sunlite? Really?

My suggestion? Take a deep breath, relax, and think about what you really want with this bike. I think FB's donor bike idea is a good one; if you want a 3piece crank, maybe look into getting something aluminum and decent.

hth
-rob

*** yeah it ain't a lot of trouble nor expense, but still way too much for that all-steel bottom-feeding 3piece

HillRider 12-19-13 08:05 AM

Quote:

Originally Posted by onespeedbiker (Post 16342223)
How badly is it bent? All the one-piece cranks I have seen are steel and I have repaired many a bent one-piece crank by simply bending it back to it's rightful place. Since they are steel they bend easily but you can bend them back just as easily and without compromising the strength of the crank.

+1 but if the bike took a hit hard enough to bend the crank, I'd be concerned that it may have distorted the bottom bracket shell too. Be sure the frame isn't damaged before spending the time and money on a new crank.

Wilfred Laurier 12-19-13 08:50 AM

Quote:

Originally Posted by HillRider (Post 16342521)
+1 but if the bike took a hit hard enough to bend the crank, I'd be concerned that it may have distorted the bottom bracket shell too. Be sure the frame isn't damaged before spending the time and money on a new crank.


this was what i thought

bent cranks are often caused by something other than a simple fall
like getting t boned by a car
which is very likely to have damaged the bike in other
more difficult to ascertain
ways

all that being said
if the frame is straight
and you like riding it
then make it the perfect bike for you
but be aware
that any money you spend beyond making it rideable and reliable
you will never see again

cyccommute 12-19-13 09:06 AM

Quote:

Originally Posted by jerrymelons (Post 16341886)
Hey everyone! I need some help with a problem I've come up with. I just purchased 76 Schwinn Varsity Bicentennial, (I know, not the greatest but I like it!) Anyway, everything on the bike is in pretty good condition. Except for the crankset.. It's bent. It looks like someone took a tumble and the crank took the damage. So while looking for a new crank, I'm faced with a dilemma. Find a used Schwinn crankset on ebay, with no guarantee that it won't be bent also. Or buy a conversion kit, bb, and crank. I would think that I might as well buy the conversion parts, and call it a day. Since most of the prices I see for the Scwhinn cranks are pretty expensive. The problem I have is, I have no idea what size of bb to get. The crank I'm looking at is this one http://www.amazon.com/gp/product/B00...A2TE9IQP68MWQU I know I need a 68mm, but I don't know how long I need to have it. (I'm very new to this, if you can't tell! Haha!) Of course, I don't want to throw a bunch of money at this bike. I just want something fun to ride around town with. Any help and advice is appreciated! Thanks!!!!!

I'm not sure where people finding for Austabula cranksets that are "outrageously" priced. A quick Googlization brings back all kinds of replacements for around $15 for a new one. Used ones are a dime a dozen You don't have to go with a "Schwinn" crank. Most of the Austabula cranks I've seen have removable rings that are just held in place by a nut on the back of the chain wheel.

I agree with onespeedbiker that you should try to straighten the crank arm first, however. It's bent now, how much worse can you make it? Use a long pipe and go in small steps. The Austabula crank is extremely rugged and the old Schwinn parts were pretty good quality.

FBinNY 12-19-13 09:32 AM

Quote:

Originally Posted by onespeedbiker (Post 16342223)
How badly is it bent? All the one-piece cranks I have seen are steel and I have repaired many a bent one-piece crank by simply bending it back to it's rightful place. Since they are steel they bend easily but you can bend them back just as easily and without compromising the strength of the crank.

+1.

This may be the best approach, even if you later decide to replace the crank. For decades it was SOP to straighten these cranks by simply sliding a pipe over them and levering them out. Any mechanic that worked on lots of kids bikes could do this in his sleep.

surreal 12-19-13 09:36 AM

#cycocommute I agree re: plentiful and cheap replacement one-piece cranks, but unless one goes with a Schwinn or an old Mongoose crank, all of the BB hardeware will need to be swapped out for 24tpi stuff. That being said, I'm assuming this bruiser is due for a new BB anyway. BTW, new OPC BBs start at, like, $6.

jerrymelons 12-19-13 02:40 PM

I've inspected the bike a little more closely and what I thought was the crank being bent, is actually the sprockets. So now I think I'll try to find new sprockets. But first I'll see if there's someway I might be able to bend it back. Thanks for everyones help and suggestions!

FBinNY 12-19-13 02:43 PM

Quote:

Originally Posted by jerrymelons (Post 16343764)
I've inspected the bike a little more closely and what I thought was the crank being bent, is actually the sprockets. So now I think I'll try to find new sprockets. But first I'll see if there's someway I might be able to bend it back. Thanks for everyones help and suggestions!

If you're good with a mallet, you can tap it back to reasonable shape on the bike. Use a cable tie around a frame tub as an improvised gauge. I prefer to bend them back with a pry bar of some sort as I feel it give more control.

Worst case, you can remove the crank, take off the sprocket and flatten it on a workbench. If you suspect the BB might need service (clean/grease) this is an opportunity to do so.

jerrymelons 12-19-13 02:48 PM

That's exactly what I was planning on doing. It looks like a pretty mild bend. If worse comes to worse, I may be able to have my dad take the sprockets to his work, and have them machine duplicates!


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