Which bottom bracket length is right?

[mercalia]

My Dawes 1-Down was sent out with the wrong BB - should have been 118mm according to Brucy's chart. I replaced it with the right one more by accident than design and the front changer works perfectly with the 118mm, before couldnt get to the small ring well, of the triple rsx 46/36/26, so it pays to get the right one

[mercalia]

Thanks for all the replies, I'm constantly amazed by how active this form is and how helpful everyone is!

Yes, I will clean the frame BB very well. My plan was to use WD40 and an old toothbrush. The shop has said that once I've cleaned it, if the threads need clearing then just to bring it back as he has a tap that will do it. The threads are in surprisingly good/clean shape though.
I will clean and test with the new BB before applying anti seize and torquing up.

I do have some proper anti-seize but I could just use silicon grease instead as I'm not sure what metals are present in my anti-seize paste. It's in a sample pot rather than the original container so I don't know what brand it is. It is a "Pure Nickel" type grease anit-seize so it has nickel, graphite and maybe even aluminium in it - would that be better, I have some copper slip spray also?

So you would all recommend staying with Shimano for the replacement BB?
My LBS said he once stocked the lower level Shimano BB's but had too many returns so has switch to VP for that level.

Just had a look on fleabay and can see there are UN25's, UN26's which I guess is as the original, and the ones the LBS had a high return rate on.
I have also found the UN55 110mm's on fleabay to, which is what I'll go with then.

I put in lots of copper grease - from any auto shop

[nsew]

Edit

[nsew]

My bad, calculated up the wrong way (115.00 as opposed to 113.0)


Nominal size---actual length------RH stickout-----LH stickout----Equivalent (symmetric) length

107---------------108.0---------------21.0-------------19.0--------------110.0

110---------------111.5---------------21.0-------------22.5--------------110.0

113................114..0..............23.5.............22.5..............115.0. (UN54)

115---------------115.6---------------24.0-------------23.6--------------116.0 (UN73 same)

118---------------118.5--------------25.6--------------24.9--------------119.2

122---------------123.2--------------28.6--------------26.6--------------125.2

[nsew]

You’re doing really well. I defer to Brucey and Gattenero on the merits of which grease. I use Shimano anti-seize in that area. The drivetrain is such a critical area where a mm here or there can throw everything out. I’m not sure about WD40 though, I believe it would leave an unhelpful residue. Muc-Off have an excellent lime green degreaser that can be found with discounts. Definitely go with UN55/54 they’re ridiculously cheap for a reliably great bit of engineering.

Thanks for all the replies, I'm constantly amazed by how active this form is and how helpful everyone is!

Yes, I will clean the frame BB very well. My plan was to use WD40 and an old toothbrush. The shop has said that once I've cleaned it, if the threads need clearing then just to bring it back as he has a tap that will do it. The threads are in surprisingly good/clean shape though.
I will clean and test with the new BB before applying anti seize and torquing up.

I do have some proper anti-seize but I could just use silicon grease instead as I'm not sure what metals are present in my anti-seize paste. It's in a sample pot rather than the original container so I don't know what brand it is. It is a "Pure Nickel" type grease anit-seize so it has nickel, graphite and maybe even aluminium in it - would that be better, I have some copper slip spray also?

So you would all recommend staying with Shimano for the replacement BB?
My LBS said he once stocked the lower level Shimano BB's but had too many returns so has switch to VP for that level.

Just had a look on fleabay and can see there are UN25's, UN26's which I guess is as the original, and the ones the LBS had a high return rate on.
I have also found the UN55 110mm's on fleabay to, which is what I'll go with then.

[Brucey]

worth mentioning that some shimano BB units still come with an aluminium LH cup that is a very slack fit on the cartridge unit. Unless you do something with it or replace it with something else, this means that the unit will probably work loose and/or destroy the DS threads in the frame.

Having seen several frames wrecked this way, it ain't a pretty sight.

My advice is to check the fit of the LH cup on the cartridge and if it is a loose (rattling) fit, do not install the BB with that LH cup . Even a used plastic cup would be a better idea than that.

cheers

[hondated]

Blow it ! worried now !

A couple of weeks ago I brought this CS


https://www.spacycles.co.uk/m2b0s109p3/ ... act-Triple


and the recommended BB and having yet to fit it now having read this thread I am not sure whether I should return the recommended BB rather than to coat it in copper grease and try to fit it and find its the wrong length.
Are you there Colin, if you are what is your advice or indeed any other forum member.

[NoIdea]

My advice is to check the fit of the LH cup on the cartridge and if it is a loose (rattling) fit, do not install the BB with that LH cup . Even a used plastic cup would be a better idea than that.

Yes, I read your warning about the loose fit. I do still have the plastic cup from the old one and will re-use that if it's a better fit.

[mercalia]

Blow it ! worried now !

A couple of weeks ago I brought this CS


https://www.spacycles.co.uk/m2b0s109p3/ ... act-Triple


and the recommended BB and having yet to fit it now having read this thread I am not sure whether I should return the recommended BB rather than to coat it in copper grease and try to fit it and find its the wrong length.
Are you there Colin, if you are what is your advice or indeed any other forum member.

I suppose depends on how wide the frame BB is? and how far out the chain stays stick out?

[fastpedaller]

IMHO it's not really critical with a derailleur whether the BB is spot on for length, as the chain may only run straight for a small %tage of use (otherwise you may as well run single speed, where the chainline is more critical)

[nsew]

IMHO it's not really critical with a derailleur whether the BB is spot on for length, as the chain may only run straight for a small %tage of use (otherwise you may as well run single speed, where the chainline is more critical)

I’d contend there is an optimal chainline for any speed bike. Why not obtain it? If the chain is only running straight or straight-ish for a small percentage of time then surely a change of gearing either at the front, back or both is required.

[nsew]

Blow it ! worried now !

A couple of weeks ago I brought this CS


https://www.spacycles.co.uk/m2b0s109p3/ ... act-Triple


and the recommended BB and having yet to fit it now having read this thread I am not sure whether I should return the recommended BB rather than to coat it in copper grease and try to fit it and find its the wrong length.
Are you there Colin, if you are what is your advice or indeed any other forum member.

I shouldn’tworry about that, other than the cost and hassle. That’s why we have bottom bracket boxes.

[fastpedaller]

IMHO it's not really critical with a derailleur whether the BB is spot on for length, as the chain may only run straight for a small %tage of use (otherwise you may as well run single speed, where the chainline is more critical)

I’d contend there is an optimal chainline for any speed bike. Why not obtain it? If the chain is only running straight or straight-ish for a small percentage of time then surely a change of gearing either at the front, back or both is required.

Gears are there to be used to (hopefully) make the process more efficient. The gain from changing to a more efficient gear is huge compared to the loss through the chain being out of alignment. If cycling is (only) about keeping a good chainline then we'd all be riding singlespeed. BTW how many sprockets have you on this bike? If you go into the big sprocket the chain may be 15mm out of alignment!

[Brucey]

the recommended chainline for particular cranksets is often 'wrong' for the rear hub/number of speeds in use, anyway. In point of fact the same chainline cannot be perfect for 5s, 6s. 7s. 8s. 9s. 10s. 11s in various hub widths, not without adjustment, anyway.

IME, an MTB chainset onto a 135mm 8/9/10s setup always results in a 'bad chainline'. It is OK on an MTB (you don't have much choice, with fat tyres and fat chainstays) but it is the sort of thing that drives me crackers on a road bike TBH, especially as it is avoidable and it comes with an enormous Q value too.

There are several considerations; first (prosaically) that the chainrings clear the chainstays, second that the FD is happily working in its intended range of chainlines, third that the chainline onto the rear sprockets is going to work at all, and fourth that the chainline doesn't result in over-use of inefficient gears, by you, with your gear ratios, for your style of riding.

The last point means that with a road triple (for touring) there is something to be said for making the chainline as narrow as possible, since you are likely to be spending much more time in the lowest gears than the very highest ones, and you ought not to be using the small-small gears anyway.

With a little cunning, on some framesets you can get the chainline down to about 35mm on the smallest chainring of a triple (although getting a FD to shift it is often quite another matter...), which leaves the other two chainrings about the same chainline as they would be on a double. With a 135mm 8/9/10s hub the chainlines on the two largest chainrings seem quite acceptable.

With a double things are a bit simpler, but there are still (often individual) choices to be made.

cheers

[nsew]

IMHO it's not really critical with a derailleur whether the BB is spot on for length, as the chain may only run straight for a small %tage of use (otherwise you may as well run single speed, where the chainline is more critical)

I’d contend there is an optimal chainline for any speed bike. Why not obtain it? If the chain is only running straight or straight-ish for a small percentage of time then surely a change of gearing either at the front, back or both is required.

Gears are there to be used to (hopefully) make the process more efficient. The gain from changing to a more efficient gear is huge compared to the loss through the chain being out of alignment. If cycling is (only) about keeping a good chainline then we'd all be riding singlespeed. BTW how many sprockets have you on this bike? If you go into the big sprocket the chain may be 15mm out of alignment!

Currently? Seven naturally. 13-30. Short cage (big big is off limits). 46/36/26. 135oln. 26” wheel. For this set up I’m using a 127mm bb. This gives a cracking chainline in all chainrings depending on terrain. For the next adventure I am hoping to successfully set up a half step + granny 42/39/24. 12-28, short cage, giving a gear range of 22” - 90”. Providing 17 or 18 useful gears. After Brucey’s comments and reading up on Q factor I will again try a 122mm bb in there.