Which bottom bracket????

[Grant]

So today I decided that I would attempt to take off the crankset, front chainrings and pedals from my Specialized Crossroads Sport.

'Easy enough' I hear you say - well it was my first attempt at doing it and with the help of a close to hand book I managed fine with little problems.

However, I decided that the crankset/chainrings needed to be replaced (bike was new in 2004) - so went to local shop (with crankset/chainrings - Shimano Neave 201 FC-C203, 48Sx38Sx28S hyperglide???) and they handed me a new one (Shimano Duel Sis Index FC-TX70/71???) which was not exactly the same as the old but very close. Now, the chap said to me that because the new set was slightly bigger than the last set I may have to get a new bottom bracket.

So heres where you folks come in:

1. How do I know if I need a new bottom bracket?
2. If I do need a new one, how do I know what size to get?

The old bracket is a Shimano BB-UN-25, 68mm shell, 113mm spindle, sealed bearing cartridge.

I don't want to take the entire bike to the shop for them to do it as I would like to try and do it myself but unfortunately I need a little help on getting the sizes etc.

I should add that I don't have a huge technical knowhow but I would be most grateful for any help.

[hubgearfreak]

I don't want to take the entire bike to the shop for them to do it as I would like to try and do it myself but unfortunately I need a little help on getting the sizes etc.

i have painted my own frames, built my own wheels and see no shame in getting the shop to fit BBs or headsets.

they will have a tool (a tap) for cleaning out the threads if necessary, and have much more experience in getting these fine threads started without crossing them. in addition he will be able to get the chainline correct (as it needs to be, i guess it's a 21 speed derailer setup so probably not so critical) the crucial thing is that if you fit it to the old BB and it catches the chainstay, you'll lose some paint and may even bend your inner ring.

if you (as you say) don't have a huge technical knowledge - or even if you do, you need to know when you're beaten

[andymiller]

Sounds to me like you need to find a better and more helpful bike shop.

I don't understand why they didn't simply sell you some new chainrings (though in fairness it might have been because buying the new crankset might have been the cheaper option). Changing the chainrings (ie using the chainrings from the new chainset on the old cranks) might be your simplest option.

The chainset should have come with instructions which would give you the information you need on BB axle length etc. If it doesn't have a look on the technical documents section of the Shimano website.

Replacing a BB is a perfectly simple operation - though you will need to buy a special tool.

[gaz]

.

[hubgearfreak]

The chainset should have come with instructions which would give you the information you need on BB axle length etc. I

isn't this information dependant upon the frame?

[Steve]

Your technical skills are obviously up to taking the old chainset off so I would have thought the first thing to do would be to fit the new chainset & see if everything works OK (although you won't be able to take it back to the shop then). If, once fitted, the small chainring is too close to or even rubbing the chainstay, then you need a longer BB; obviously the converse also applies. You will probably need to adjust the front mech as well, to get smooth changes & stop the chain falling off.

If you can't get all the above to work, then I would suggest you take the bike to the shop & seek further advice!

[Peyote]

Sounds to me like you need to find a better and more helpful bike shop.

Yep, I think I'd agree with that.

I don't understand why they didn't simply sell you some new chainrings (though in fairness it might have been because buying the new crankset might have been the cheaper option). Changing the chainrings (ie using the chainrings from the new chainset on the old cranks) might be your simplest option.

I've used some six and seven speed cranksets that have the chainrings rivetted to the crank spider, so it is impossible to simply change the chainrings. The whole right hand crank needs to be replaced when the chainrings get too worn. Tends to be on the cheap and cheerful ones that cost less than £20 to replace though.

[Steve]

The TX70 does in fact appear to be one without replaceable chainrings, as far as I can tell from the picture at

http://www.parker-international.co.uk/P ... uctID/6491

& the recommended bottom bracket is the same as the one you have already. But it is just possible that you will need a longer/shorter one, although I think it rather unlikely.

[Grant]

Thanks very much everyone for your replies - much appreciated.

I did forget to mention that the old crankset/chainrings were one complete unit and so replacing the rings only unfortunately was not an option - sorry.
So infact the shop was just replacing what I had taken to them or as close to what they had. In fact there is only two bike shops where I live and the one I visited is by far the best and most helpful.

I did put the new unit on the bike yesterday. The botton chainring doesn't touch the frame or any other part at all when cycling but it is quite close. However, when changing up to the larger chainrings the chain wants to jump which as Steve points out means the front mech needs adjusting - that I haven't done before.

So this morning it was down to the shop with bike and the chap (different one from last visit) is going to have a look at it. He is not convinced it will need a new bb but he will have a look and see and then adjust the front mech.

I will know what to do for the next time.
Once again - many thanks for your replies.

[andymiller]

The chainset should have come with instructions which would give you the information you need on BB axle length etc. I

isn't this information dependant upon the frame?

Yes but the instructions take the form of - 'if you have a 68mm BB shell and a 135mm OLD then you need an axle length of x mm', etc

[tjm]

The chainset should have come with instructions which would give you the information you need on BB axle length etc. I

isn't this information dependant upon the frame?

Yes but the instructions take the form of - 'if you have a 68mm BB shell and a 135mm OLD then you need an axle length of x mm', etc

finding those instructions isn't easy though. I did a pretty extensive search when getting a new BB & chainset for an old bike and found very little info on shimano kit. In the end I got a stronglight impact triple chainset and recommended BB from SPA. As it turns out the chainline is not right (more like 48 at the front vs 43 at the back) but I am not sure a shorter BB would work without the crank boss hitting the BB housing. And in reality I have a nice crisp change so it is not really a problem.
I reckon the only reliable method of BB sizing is to start somewhere and then go up/down in length as required.

Toby