New bottom bracket or chainset?

mtb_mark_uk

Hi,

I've just bought a 2nd hand Cove Hummer that I'm building up with the parts from my existing bike. The problem is that the old frame has a 68mm conventional XT internal BB with Hollowtech cranks, whereas this one has external cups already installed on a 73mm shell.

The obvious thing to do would be to buy a new XT Hollowtech 2 chainset with integrated axle, but they are expensive and I'm quite happy with my old one!

My question is would it be possible to buy a 73mm internal BB and use my old chainset or would the chainline be incorrect?

thanks.

The Rookie

You can get 73mm versions of the (Octalink I presume) internal BBs

supersonic

As long as the axle length is ok, the chainline will be ok.

mtb_mark_uk

Hi,

Yes I believe it is Octalink, but it's been on the bike for years so I'm not 100%. It's basically Shimano's version of the ISIS standard.

By axle length, are you referring to the drive side offset? I wasn't sure if all 73mm BBs had the same offset or not actually, so an interesting point.

thanks.

supersonic

All cartridge bottom brackets have a stated axle length ie 113/118/121 or any other number. Use the same length as what you are using at the minute with the Cove.

cooldad

mtb_mark_uk wrote:

Hi,

Yes I believe it is Octalink, but it's been on the bike for years so I'm not 100%. It's basically Shimano's version of the ISIS standard.

By axle length, are you referring to the drive side offset? I wasn't sure if all 73mm BBs had the same offset or not actually, so an interesting point.

thanks.

Just to be pedantic, Isis is actually the non Shimano version of the Shimano Octolink standard.

mtb_mark_uk

No problem about being pedantic, I like to know if I've made a mistake!

Regarding axle length, if I now have a BB shell that is 5mm wider than before then wouldn't using the same width axle cause 2.5mm less axle to be protruding on the drive side? Or am I looking at this in the wrong way in that the 73mm would have the minimum amount of axle protrusion and the 68mm have an 'extra' 2.5mm to account for the difference?

nicklouse

Nope you use the same axle length to give the same chain line.

So get a 73mm BB with the same axel length as fitted. Unless the new frame requires a different chain line.

mtb_mark_uk

Sorry, I explained myself badly but we are in agreement. Just to clarify what I meant:

68mm BB Shell:

Drive side axle protrusion = (113mm-68mm)/2 = 22.5mm

73mm BB Shell:

Drive side axle protrusion = (113mm-73mm)/2 = 20mm

(This make the assumption that the same amount of axle protrudes from both the drive and non drive side)

supersonic

You assume wrong.

Get the same axle length!

mtb_mark_uk

Thanks for your advice guys. As a result I've also been able to do some "more informed" research on the net and understand what's going on now, in particular relating to the chainline, which I'd never considered before!

Cove have designed the 2010 Hummer around a 50mm chainline which requires a 118mm axle, using my M-752 chainset. If I use a 113mm axle then the chainline will be 47.5mm, which is usually the standard. I'm wary of this placing the chainrings too close to the chainstay and getting chainsuck and since I don't have a 113mm to try out, I'll play safe and get the 118mm.

I believe that my assumption was correct for the BB type that I'm using: BB-ES51. However, I've found a -K variation that has a greater offset on the drive side to account for chainsets with deeper splines, which would of course invalidate this.

Thanks again! I feel a lot happier about making the purchase now!

supersonic

The K and E options are for mounting chaincases and E type BBs, so will space further out.

nicklouse

mtb_mark_uk wrote:

Thanks for your advice guys. As a result I've also been able to do some "more informed" research on the net and understand what's going on now, in particular relating to the chainline, which I'd never considered before!

Cove have designed the 2010 Hummer around a 50mm chainline which requires a 118mm axle, using my M-752 chainset. If I use a 113mm axle then the chainline will be 47.5mm, which is usually the standard. I'm wary of this placing the chainrings too close to the chainstay and getting chainsuck and since I don't have a 113mm to try out, I'll play safe and get the 118mm.

I believe that my assumption was correct for the BB type that I'm using: BB-ES51. However, I've found a -K variation that has a greater offset on the drive side to account for chainsets with deeper splines, which would have course invalidate this.

Thanks again! I feel a lot happier about making the purchase now!

Your assumption was totally incorrect.

But as you say a different chain line is required for your frame so yes a different axel length I required. If the chain line was to stay the same then th axel length would have been the same.

Which is why the question was asked.

mtb_mark_uk

Ok, point taken that I've made a mistake somewhere, but that's why I'm posting on here - to learn.

For completeness can you let me know where the mistake in my assumption was? Could it be that the left and right axle protrusions are not symmetrical for *any* BBs, not just the K (& E?) types? When I remove my existing BB later on I'll measure them to see but until then I'm limited to conjecture.

mtb_mark_uk

Ignore that last post please. I'm trying to work out how to delete it.

supersonic

Some old BBs were that way, but nowadays almost all (well most are external), are equal ie 5mm longer is 2.5mm more at each side.

Shimano used to make some custom Octalink stuff for Specialized with deeper splines which required a different BB.