Crank & BB selection for road conversions
 

I'm converting an old road frame for a friend, and I'm a little lost on crank / BB selection. The donor bike is a 1984 Trek 420. So if I pull out that crank and toss it, what choices are up to me in terms of dimensions for the new BB, and what is driven by the frame. Next question being how do I choose a compatible crank/bb combo. I've searched here and looked around Sheldon Brown's site, but I'm just not finding what I'm looking for.

Thanks!

Also, thanks to whoever recommended Pokez in San Diego for veg. mex. food. I finally made it down there for a tofu, potato and mushroom burrito last night. Superb.

depends on which rear hub you're using. why toss the original cranks? if you put the chainring in the inner position and use the same bb there's a chance you will have to do no homework whatsoever.

edit: in reference to your actual question, most people find that 107 or 110 mm bottom brackets do the trick. again, depends on your cranks and rear hub.

I was planning on buying that $135 CXP22/formula wheelset.

try it with the original components (even if you're planning on replacing them, just for experimentation's sake) and look to see if the chain is straight. if it's too far to one side, measure the bb axle and get a longer or shorter one.

edit: this is all assuming you can't/don't want to do the measuring/math described on sheldon's site about chainline.

edit2: this thread has some useful info on measuring chainline as well.

the harris cyclery website crank section has info on what crank plus what length BB spindle will yield what chainline. another good resource is businesscycles.com(net?), as well as searching this forum. those formula hubs you're looking at have a 42mm chainline, which is a track standard. you want a crank and BB combination that will give you 42mm--either on the inside or the outside of the spider if you're using a double, or else you want track cranks with the appropriately matched BB. k?

I suppose my question wasn't appropriately phrased, as the answers aren't really what I am looking for. What I am trying to discern is what constraints on BB dimensions a frame provides. If I have said frame, how do I know what BB dimensions I need? I'm referring to all the other dimensions besides length.

Maybe I am making the question more difficult than it needs to be. So I choose a wheelset, which will have some chainline, and I need the same (or close) chainline up front. That is determined by my crank/chainring/BB length. I get that. My question I suppose is that given my frame, how do I know what BB's will and won't be compatible?

If I look on www.thebikebiz.com for example, I see the Sugino RD crankset, and they recommend a TH7420 BB with it. If I assume that the chainline will be appropriate since it is a track setup, that takes care of that question, but how do I know if that BB fits my frame?

Thanks in advance, I know this is simple.

ah. BB width and threading. most bikes have 68mm wide bb, english threading. there's also italian threading, which means that one of your bb cups will always unscrew itself, and there's french threading, which means you'll never be able to find the part you're looking for (different # of threads-per-inch and whatnot).

my BB, for example (a shimano UN-52 or -53, i forget which. a very solid and reliable and dependable and simple and affordable BB), is measured by saying it's 68 x 107 english threaded.

follow?

I follow the dimensioning scheme. What I don't follow is how I know what my frame is, and which new BB's are compatible. Is the style of BB something that affects compatibility as well? ISIS, square taper, etc.

Style of BB depends on cranks. Since your bike is a trek road bike, I would say it is english-threaded with a 68mm shell. That is all the frame dependency there is.

That's what I was looking for. Thanks!

So I just get a standard 68mm english threaded BB, and the length will depend on the crank/chainring and the chainline I want.

Next question is where and what people suggest for getting such a thing. If I want a 42mm chainline, and I want to spend like $50 or less. Used is fine. :-P

One thing to watch out for, especially with something like the Sugino RD cranks that use a 103mm bb, is that you will have clearance between the chainring and chainstay. On a lot of touring/commuter style frames the chainstays are made to accommodate wide tires and you may find that larger chainrings (44t and up) may rub on the stay. If the original cranks are still on the bike, heres the easy way to measure.
1. With calipers if you have them, or a good metric ruler if you don't, measure the distance from the middle of the middle ring to the closest part of the seat tube.
2. Add half of the diameter of the seat tube and you will have your current chainline.
3. Subtract 42 from your chainline to see how far off you are.
4. Now, measure the distance from the inner ring to the chainstay and see if you have more clearance than the number you got in part 3. If not, you'll have to live with an off chainline, or repeat the above with the outer ring to see if you can make that work.

i'd recommend bulletproof cranks and a rocket ring, given your budget. your chainline will be 1.5mm off, but that's not bad. check harris cyclery: http://sheldonbrown.com/harris/crank...ct-double.html for bulletproof info. since there's a 39mm chainline w/ a 107mm BB spindle (square tapered--ISIS, octalink and whatnot are other spindle-to-crank interfaces), a 110mm spindle will probably get you pretty close to a 42mm chainline

Thanks for all the information. I appreciate it.

Regarding chainlines produced by BB/crank combo's, is that just trial/error, or is that information provided with the crank information?

Sheldon brown has a database of cranks and chainlines... check that out to get a ballpark. At worst, you'll have to swap out the BB once to get it perfect.

The 107mm and 110mm bottom brackets have the same length spindle on the drive side. The extra 3mm is added to the non-drive side. It looks like 40mm is about as close as you can get unless you go to a longer bb and use spacers. I wish shimano had a simple database of the driveside and non-driveside lengths of all their bottom brackets. Maybe its hidden on their site. off to search.

Also, you can find bulletproof cranks for pretty cheap on ebay.

I just bid on a rocket ring, Buy-it-now'd a set of bullet proof cranks, a lockring and cog. Now to grab a BB from sheldon, and that wheelset and I'm good to go!

Thanks again for all the help.