What is the best replacement for BB square taper 118mm?

[rockyraccoon]

I need to replace the bottom bracket from my road bike. Bottom Bracket is 68 x 118mm (English Thread)

Could I use 115mm or 122.5 instead?

 

[mrandmrspoves]

You could -but best to replace like for like as you may not have enough clearance with the shorter, and shorter and longer will both affect your chain line and may cause difficulties with your front derailleur

 

[sidevalve]

Simple to do, but why ?

 

[Rickshaw Phil]

You could -but best to replace like for like as you may not have enough clearance with the shorter, and shorter and longer will both affect your chain line and may cause difficulties with your front derailleur

^^^^This.

Having thought I could get away with a different length and been proved wrong in the past, I can state that it is likely changing the bottom bracket length will cause shifting issues.

Fortunately the BB I bought turned out to be the exact length required when I upgraded my crankset recently.

Why do you want to change? 68x118 is a common size.

 

[rockyraccoon]

....
Why do you want to change? 68x118 is a common size.

Cost only. There are good offers from CC at moment. Unfortunately, the 118mm ones are costing twice as more .

My concern is that a shorter one (115mm) may cause the inner chainring to hit the chainstays. Another thing is whether the front derailleur will hit the seat tube when shifting onto the inner chainring.

 

[Rickshaw Phil]

Cost only. There are good offers from CC at moment. Unfortunately, the 118mm ones are costing twice as more .

My concern is that a shorter one (115mm) may cause the inner chainring to hit the chainstays.

That is a possible problem but will depend on the bike. It didn't do it on mine (trekking bike running 28-38-48 chainset at the time).

Another thing is whether the front derailleur will hit the seat tube when shifting onto the inner chainring.

That is also possible with some front derailleurs. In my case the derailleur didn't have enough adjustment to shift onto the small ring cleanly (I could just do it in testing, but it wouldn't do it at all on the road). In addition, because everything had moved inwards I had trouble with the chain wanting to shift past the big ring no matter what I did with the high limit screw.

I was trying to make a 113mm BB work, so it was a bigger jump than you propose. Getting the correct 118mm BB solved both these problems.

 

[ColinJ]

I wore out a second BB on my Basso and ordered the appropriate replacement. When I installed the new BB, the front shifting mysteriously improved. I checked the one I had removed and it was the wrong length! So ... make sure that you get the right length.

 

[rockyraccoon]

So ... make sure that you get the right length.

I'll pay more for the right length. Better safe than sorry.

Thanks everyone!

 

[Widge]

Sorry to butt in....but how does one KNOW whether the spindle is the right length for a new chainset?

Is it always the same as the original one (assuming that length is right)...or does it vary according to the chainset you have bought?

I have seen a Spa Cycles 'touring' triple (slightly smaller rings) that I might one day be tempted to buy, but the shop site 'stipulates' a shorter length than the one already fitted to the triple on my bike.

I find it all a tad confusing.

w

 

[Rickshaw Phil]

Sorry to butt in....but how does one KNOW whether the spindle is the right length for a new chainset?

Is it always the same as the original one (assuming that length is right)...or does it vary according to the chainset you have bought?

I have seen a Spa Cycles 'touring' triple (slightly smaller rings) that I might one day be tempted to buy, but the shop site 'stipulates' a shorter length than the one already fitted to the triple on my bike.

I find it all a tad confusing.

w

It varies according to the chainset and the instruction sheet or manufacturer website should state the size required.

To give an example my Raleigh was supplied with a Shimano chainset which required a 124.5mm spindle. The replacement Suntour one needed 118mm and the current Shimano takes a 113mm one. Despite 3 different lengths of BB the chainline has stayed the same due to the chainset design.

 

[Widge]

Thanks Rickshaw Phil.......so if the spindle on my current ch/set is 115mm and the shop suggests 110mm then I go with that and the chainline (and mech travel limits) will hopefully all work out?

 

[Rickshaw Phil]

Thanks Rickshaw Phil.......so if the spindle on my current ch/set is 115mm and the shop suggests 110mm then I go with that and the chainline (and mech travel limits) will hopefully all work out?

If the new chainset requires a 110mm bottom bracket the 115mm one will be a bit too long for it and the shifting won't work the way it should.

What is the new chainset? The manufacturers website will probably confirm what the shop says.

Once you fit the new parts you will still need to adjust the front derailleur to get the shifting spot on (I'd be surprised if the high and low limits didn't need adjusting) but yes it should be close.

 

[GrumpyGregry]

Then factor in the aspect that two identical chainsets on two identical bottom brackets can, and often do, give two different chainlines depending on all sorts of things like tolerances and assembly methods.

 

[rockyraccoon]

Then factor in the aspect that two identical chainsets on two identical bottom brackets can, and often do, give two different chainlines depending on all sorts of things like tolerances and assembly methods.

and frame design (e.g.: seat tube diameter) ??

 

[Rickshaw Phil]

and frame design (e.g.: seat tube diameter) ??

Shouldn't make a difference as chainline is usually measured from the centre of the tube.