BB 118 - 122.5, will it cause a problem?

Copied this from my post in the road section as I'm generally in this section and though it's a road bike I can't see that's relevant here.

I need to replace the BB on my 80s Dawes Sterling. The old one is cup and cone 68x 118 and I have a UN54 which is 68x122.5.

Will it work?

I guess it will affect chainline and the front mech but will 3.5mm really cause a problem?

All help much appreciated!

If on a road bike, possibly as you're potentially going to be putting higher torque through the outer rings.
Depends on how you ride and how flexy the BB shell is, as you'll be putting more leverage through it. You might find it's fine to fit & align, but doesn't work when pedalling in a real world situation!

However.. You need to replace the BB, you have a BB - try it & find out?

You should have no problem. The torque is the same (full weight on one pedal). Chain tension will actually be lower than MTB due to the larger chainring.

You might find that the chain wants to slip off in bottom gear when back pedalling - the only way is to try.

I'll give it a try! Thanks guys

give it a go but i think its worth getting the correct size

if you dont want to bung out £15 on a new one, you could always stick a wanted ad on here - i bet someone has one sitting in a box somewhere you can have for a couple of quid

Huh? some strange comments on this thread.

Bear in mind Torque is a vector unit - it needs to be qualified by a direction.

Where does the OP say he is using a bigger chain ring or cranks? If he isn't chain tension is unaffected.

He is also not putting more torque through the chain rings - Torque in this direction is unchanged as its nearly totally dependent on the chain ring diameter and the crank length.

Torque acting along a line of the BB spindle at 90 degree to the crank rotation is changed and is at its maximum with the cranks at the top/bottom of their rotation

T=f * r

1. Assuming f (pedaling force) is unchanged and r (BB spindle length) changes by 2.5mm.
2. Assuming for a ease of calculation that the original BB spindle is supported by the bearings/cups at 10mm along the spindle:
r = 0.01m for the original and r = 0.0125m for the new BB.

Which gives us a 25% increase in Torque along this line and thats without taking into account the length of the pedal spindle which would reduce it from this value.

In reality the calculations are more complex and I would guess a much lesser increase.

Too bad the postage costs are roughly the same as the price of the BB itself, otherwise I'd swap my (new in box) 68x118 UN-55 for your 122.5

secret_squirrel":160odlib said:

Where does the OP say he is using a bigger chain ring or cranks? If he isn't chain tension is unaffected.

He is also not putting more torque through the chain rings - Torque in this direction is unchanged as its nearly totally dependent on the chain ring diameter and the crank length.

Click to expand...

This is hence why I referred to "If on a Road Bike" [as opposed to a mountain bike] - I should have made that clearer :facepalm: . I was therefore assuming more torque from my assumption that the chainring would be bigger, but my experiences of the same situation relates solely to MTBs.

secret_squirrel":160odlib said:

Which gives us a 25% increase in Torque along this line and thats without taking into account the length of the pedal spindle which would reduce it from this value.

In reality the calculations are more complex and I would guess a much lesser increase.

Click to expand...

Then it all comes down to whether this causes any problems or not in terms of transferring the torque into the frame at the BB shell, and whether it twists, and whether that causes any knock-on effect, hence trying it is almost always the best option.