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Bottom bracket sizing for Trek 820

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getagrip

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Discussion starter · #1 · (Edited)
I'm rebuilding a 2005 Trek 820, and figuring out what bottom bracket size to go with has been difficult. I called a Trek store and they said they knew for sure that it was a 68, but they didn't know about spindle size.

I've got a buddy who has the tools to change the bottom bracket and crankset out, but we won't be hanging out again until the weekend. I really want to order my crankset and bottom bracket as soon as possible, so I came up with a way to measure it without taking the cranks all of the way off, but I don't know how accurate my measurements are.

What I did first was measure the distance between the out rings surrounding the part of the frame that holds the bottom bracket. I came up with 2 7/8". I then measured the distance from there to the short edge of the outside of the crank arms and came up with 1 1/4". I then measured the distance from the short edge of the inside of the crank arms to the edge of the spindle and came up with 13/32".

Doing the math I came up with a total of 5.375" (total distance from short edge of crank arm to crank arm) and subtracted 0.8125" (total distance from short edge of inside of crank arm to edge of spindle or 26/32") from that number to get the estimated length of my spindle: 4.5625 inches or 115.887 mm.

I used a metal measuring tape using the "English" system to get the measurements, but I'm just not sure how accurate my measurements were since I wasn't measuring in millimeters, and because of the tricky angles involved at the end of the cranks and the inside of the spindle.

That being said...how accurate do you think my measurements are?
 
#4 ·
getagrip said:
I did see a 68 x 115 mm bottom bracket:

http://www.jensonusa.com/store/product/BB309F06-Shimano+Un54+Bottom+Bracket.aspx

I believe that model will works with Shimano Alivio cranks - not sure about the Deore I'm looking at.

How do you know whether if it is square taper or not? Man there are a lot of technicalities! :madman:
Click to expand...
Check out sheldon brown dot com

There are lots of pages there that help you to figure out exactly what you have.

Another good site for help is the park tool dot com website. Scroll to the bottom of the home page and click on thr repair help link.
 
Discussion starter · #5 · (Edited)
For sizing, if I'm off, what would happen if I order a 113 mm or a 118 mm vs a 115 mm bottom bracket?

I did the math and that's less than 1/8 of an inch difference per side on the 113 vs the 118. It would seem like I would be safe going with the 118 - how much of a difference would 5 mm honestly make?

Also, here is a photo of the spacing between my cranks and the outer rings of the bottom bracket - its about 1/8 of an inch clearance. Not sure how much that matters, but it looks like I would have a little room to work with if the size runs a little small.
 

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#7 ·
Based on another recent thread on crank/BB selection I thought that the stock bottom bracket width was basically irrelivent. The BB width must match the crankset. I am also struggling with all this drivetrain selection stuff, trying to understand.:madman:
 
Discussion starter · #8 · (Edited)
jeffj said:
In my experience, 5mm could make a big difference. Even 2mm can make a difference.

Read about chainline here: http://www.sheldonbrown.com/chainline.html
Click to expand...
Wow. Thanks. That is crazy. Never would have occurred to me about how bottom bracket, chainline, and cranks can effect one another, even if the specs are slightly off by millimeters. I guess I'll have to take the cranks off and get a special measurement! Other than a crank puller to remove the cranks, are there any other tools I will need to pull the buttom bracket?

Wonder if I should I should just invest in a tool kit, but then again, its not like I'm going to be doing this very often! Would something like this do the job?

http://cgi.ebay.com/BIKE-BICYCLE-HO...TOOL-KIT-SET-REPAIR-NEW-/310326883948?pt=LH_DefaultDomain_0&hash=item4840ec766c

There is also this one - its only $10 more:

http://cgi.ebay.com/Home-Mechanic-B...ling-Tool-Kit-set-38pcs-/310326880109?pt=LH_DefaultDomain_0&hash=item4840ec676d

Ideally I'm going to try and get this done without buying any tools since this Trek rebuild project is already starting to get rather costly (and because my friend already has most tools - I just don't want to be a bother), but in the end, it will be worth it! :thumbsup:
 
#9 ·
Didn't you decide in another thread to get a new crankset?

See if you can find the manual for it. That should tell you what spindle size you need for the bottom bracket shell on your bike.
 
Discussion starter · #10 ·
Yeah, I'm going to definately get a new crankset and with it, a bottom bracket, unless its not possible spec-wise. I was hoping to have all of the parts this weekend...I'm just being impatient. I still have to get all the rest of the parts off, sand it, and then paint it, so I might be better off just having to wait an extra day or two, rather than trying to get the parts on a bike that is still wet with paint due to my impatience! I'm going to call Trek customer service tomorrow to see if they have any specs so that I can possibly get all of the parts ordered by tomorrow, which might get them here by the weekend. I still have my Windsor, so its not like I don't have a bike to ride.
 
Discussion starter · #11 ·
I called Trek this morning and it is a 124.5 mm. I hope they aren't too hard to find, but I wonder how that size will work with the crank I select. I'm still leaning toward a 48-36-26, but might go with a smaller size, maybe even a 42-32-22
 
#12 ·
If I had the legs for it, my cruising speed on a 44/11 ratio could be 26.6 mph. I have a pretty fast spin when I want to, so I could get the bike to over 35 mph, again if I suddenly developed the legs for it.

This is assuming a 1.5" slick tire on 26" wheels. Using a larger tire raises those speeds, and also the difficulty of reaching them.

So, for me, it wouldn't really be practical to use a Trekking crank on a bike I rode off-road. I can imagine putting one on a commuter because I'm used to pedaling all the time and there are some hills where I live that put my speed in the 30s. But it would be a little silly.

http://sheldonbrown.com/gears/

There are sometimes clearance problems with 48t chain rings, although it's uncommon on bikes of your Trek's age.

If I were working on an older bike, I'd probably get either the M-590 or M-582 crankset. They're the new style of crankset with the outboard bottom bracket, which I find easier to install and remove, and I could swear I can feel the difference in stiffness. Before you let sticker shock deter you, keep in mind that these ship with the bottom bracket, although you should read the description because some of the M-590s are takeoffs. It's certainly a more expensive option than finding an inexpensive crankset for a cartridge bottom bracket and the bottom bracket elsewhere, but it's not a huge difference in price, especially once you consider shipping, I think. You'll need a special tool for the external bottom bracket, and you may or may not need the shell faced. So these add some expense. I'd let the shop remove the old bottom bracket and face the shell.

If you decide you want to go with the larger rings, which will often be described as trekking rings if you want Google Shopping to find them for you, the newer Deore and Deore LX cranks I just mentioned are available that way too.

My bike has an SLX crank and it's pretty sweet. But I kicked myself a little when the LX cranks started showing up on Jenson for $90. The Deore is around the 'net for $75.
 
Discussion starter · #13 ·
Thanks again for the response. I just spoke to Trek again, and they said that the spindle size for a new bottom bracket is dependent on the new crank setup, rather than the spindle size of the current bottom bracket...which is based on the current crank setup. So, I think I need to decide on which crank to get first, then I need to figure out which bottom bracket size to get for that crank.

Thanks for the suggestions above on which cranks to get, but I think I'm going to go with my original plan with JensonUSA, because that is really a good deal:

http://www.jensonusa.com/store/product/CR703A00-Shimano+Deore+Crankset+M510.aspx

Also, after doing some thinking about it, I think a 44-32-22 setup might work best for me, and I'm guessing I should go with the square taper. The question is, what bottom bracket to get for that size crank? This is a bigger headache then I ever thought it would be, but I think I'm getting closer to the answer.
 
#14 ·
Shimano doesn't list it on their tech. docs anymore. Try emailing Jenson and see if they can tell you. The other thing you can do is buy the crank, and then take it to your LBS and buy the bottom bracket from them. They may not be crazy about it, but I think a lot of shops have realized that they need to at least act nice to a customer's face when the guy comes in with internet-purchased items, and they tend to make more money on accessories, clothing and service anyway.
 
Discussion starter · #15 ·
Well, I checked out http://www.sheldonbrown.com/bbsize.html and came up with a spindle length of 110-113 mm for the Shimano M440, 480, 510 series, so it seems like it would be safe to go with a 113 mm. One thing that concerns me is that if my current spindle length is 124.5 mm, that's a 11.5 mm difference, just short of 1/2 inch. Then again, it really makes me wonder how accurate Trek's specs are in the first place, because even though I'm quite flawless, its hard to believe my estimated spindle length measurement of 115.9 mm is THAT far off, but I'm open to the possibility.
 
#16 ·
I think you're not quite getting what's really going on with bottom bracket spindle lengths.

Depending on the vagaries of how a crank arm is designed, it could need the interface on the spindle to be at any arbitrary distance from the side of the bottom bracket shell. The designer could put the angled faces or the hard stop, depending on BB type, anywhere he wants them in the crank arm. Certainly there's a range of sizes that's more common, and a 200mm spindle would make no sense. But when we're talking about a few millimeters here and there, that's all about the design of the crank arm.

English-threaded bottom bracket shells come in two sizes - 68mm and 73mm. In the case of a 73mm shell, typically the crank will just call for 5mm more spindle. It's still designed to sit a certain distance from the side of the shell, so overall length is whatever it needs to be to keep that dimension.

Which is a long-winded way of saying that the overall length of the spindle you have right now is irrelevant to the size you'll need for the new crank.
 
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