bottom bracket size question

[adempewolff]

Hello,

I think the time has finally come to replace the noisy bb that I have been meaning to replace for 4 years or so (it finally starting sounding like a nest of mice this spring). The problem is that no matter how hard I google for information, I cannot figure out what type/size of bottom bracket my 2005 Trek 4500 uses. I don't have the tools to take it out and look at it (I have a family member who is a bike mechanic and will help me replace it, but I'd like to order the part first so I can just make one trip to see him) so I was hoping someone on the forum could give me some (probably common sense) advice on which size I should order.

I think that one of these Shimano UN26 units is probably what I'm looking for, but I'm at a lose for what size to order. Anyone have any insights? Does Trek generally use a specific size in all of their mtb frames? Thanks in advance for any suggestions!

 

[tafi]

You will not know until you pull the crank off, whether it will be a square taper or an octalink (splined). You also need to bear in mind that the axle needs to be of a specific length (end to end) to allow the cranks to clear the frame. So it will need to be measured.
You will definitely need one with english thread (if there is an option for that).

If you can't do either of these then you need to go to a bike shop. If you do, you'll likely be up and running again within a few hours and it is usual to pay a reduced labour rate if the part was purchased from the same shop. I don't know where you live but here you could probably walk in and walk out again half an hour later having parted with a total of ~AU$60 fully fitted (including ~$45 ish for the BB).

The alternative is for you to take several days to figure out the required dimensions, another several days for the ordered BB to arrive and when that's done you'll still need to pay someone to fit it for you.

 

[adempewolff]

It's definitely a square taper, I could tell that just by taking the cap on the crank off and looking in. You are right though, that the devil is in the dimensions. I probably won't save too much time/convenience doing it myself, but my LBS in the past has taken a long time to do repairs, and not actually fixed the problem I brought it in for. For example, when I first felt the BB start wearing out ~3 years ago, I took it in and asked them to replace it, but they said there was no problem. They were just pedalling with it on the stand, for which it ran smoothly. Problem was when you actually weighted the crankset and really dug in, it made all sorts of noise (not a huge problem, but annoying as hell). On top of that old noise, there are now much worse problems, so I think now they would definitely agree that it needs replacing, but I'm still a little sore about them not doing what I asked them to do back then.

Luckily, I don't need to use the bike for another 2 weeks and I should be able to borrow a crank puller and bb tool sometime this week to check the dimensions, and then again once to install the new part. I was just hoping someone else who'd replaced the part on a model close to mine might have some insights so I didn't have to borrow the tools twice.