Replacing crank / BB

[apey75]

Hi all,

I've got an old bike that I've decided to finally give some TLC - can't bear to part with it. It's a year 2000 Giant ATX 830 mountain bike.

Not my bike, but this is what it looks like:
Giant ATX 830

It sports a 68mm, English thread BB, and a 3x8 Diamond taper crankset. I've measured my chainline to be approximately 49mm - 50mm, from the centre of the frame to the centre of the middle cog on my crankset.

My BB is cracked, and I've managed to strip the threads on my left crank arm trying to get the arm off to inspect the BB. Some time ago I also got rid of my front derailleur. Don't need it - I am happy with 1x8 speeds.

Hence I would like to switch to a single chainring (36t) crankset, and matching BB. However I am confused!

I would like to get the following (ISIS) crankset:
Truvativ Holzfeller OCT 1.1 DH

Some questions:
1. Would any ISIS BB do, or do I have to get a Truvativ BB?
2. What length BB do I get in order to preserve my ~ 50mm chainline? Most of the ISIS BB's I can find online (such as This One have a length notably longer than 100mm.
3. Whilst I've done my best to research this properly, I am very much a newbie bike mechanic, so any tips/issues to note/feedback would be much appreciated.

PS. I am aware the crankset is 9-speed and the rest of my driveline is 8-speed, but from what I've read this should work and I'm willing to take a bit of a gamble.

Thanks for any input....

[nfmisso]

your link is to a number of cranksets; some doubles, some singles. That said Truvativ are not very forth coming with their specifications.

Please note that the Truvativ Holzfeller OCT 1.1 DH is NOT ISIS.

You need the matching Howitzer Bottom Bracket from Truvativ/SRAM.

Mixing 8 and 9 speed is no big deal. I have 9 speed cranks with 7 speed freewheel and a 9 speed cassette with 7 speed cranks - no issues.

[apey75]

Ah, so the howitzer is a propriatary BB interface! Thanks for that - wasn't aware.

So regarding the chainline, is this something that the manufacturer should be advising? eg, given BB type X, of length Y, and crank type Z you end up with chainline length of <blah>.

As an alternative, should I just get a BB that is of the same length as my current BB and hope that I end up with a similar chainline?

(10-19-2013, 09:15 PM)nfmisso Wrote:  your link is to a number of cranksets; some doubles, some singles. That said Truvativ are not very forth coming with their specifications.

Please note that the Truvativ Holzfeller OCT 1.1 DH is NOT ISIS.

You need the matching Howitzer Bottom Bracket from Truvativ/SRAM.

Mixing 8 and 9 speed is no big deal. I have 9 speed cranks with 7 speed freewheel and a 9 speed cassette with 7 speed cranks - no issues.

[nfmisso]

(10-20-2013, 12:22 AM)apey75 Wrote:  .......
As an alternative, should I just get a BB that is of the same length as my current BB and hope that I end up with a similar chainline?

Definitely not - BB and crankset should match to get the correct chainline. The geometry of the cranks have as much to do with chainline as the BB axle length.

The matched set is usually set up for the standard of the type of usage it is intended for.
http://sheldonbrown.com/chainline.html

[apey75]

Ok, got it. Had a read of that page and it provides some clarity. I've also decided to go down the Shimano route and get one of their single-chainring cranksets. The only catch is that it's a 10-speed. I'll give it a shot. I've read that it should work, but if it doesn't, I'll replace the chainring only with an 8 or 9-speed version, keeping the cranks and BB.

Cheers for the help!

(10-20-2013, 09:42 AM)nfmisso Wrote:  

(10-20-2013, 12:22 AM)apey75 Wrote:  .......
As an alternative, should I just get a BB that is of the same length as my current BB and hope that I end up with a similar chainline?

Definitely not - BB and crankset should match to get the correct chainline. The geometry of the cranks have as much to do with chainline as the BB axle length.

The matched set is usually set up for the standard of the type of usage it is intended for.
http://sheldonbrown.com/chainline.html

[Joe_W]

It should work, the inner dimensions of the chains do match - the 10spd ones are narrower on the outside, the pins protrude less etc. Regarding the single speed crank - there are chain rings that have a special shape so the chain cannot drop (as easily) form the chain ring. If you do not have one of those, use a chain guide.