Bottom Bracket Problem

[rearwheel]

I thought there used to be a mechanics forum here?

Anyway, I have a 1998 Giant ATX 860 Mtn bike with 26" wheels. I have ONE large chainring on the front and 7 speed rear cassette. The front chainring is a single chainring with extra large teeth which ensures the chain doesn't slip off.

Anyway, I had to replace the bottom bracket. So, I removed the old BB, measured it, and bought a replacement one of the same size - 68 x 110mm.

The right crank arm is a Shimano and the new BB Shimano. The old BB was another brand 'X(?) and H Industries'.

After fitting the new BB and installing the right crank/chainring, the chainring now almost touches the driveside chainstay - too close for comfort.

There's already a spacer on the BB drive side and I don't want to put another in as it would be on the thread. (Maybe I'm wrong here?)

Furthermore, I put on the old BB but now that runs too close to the frame?!?!! So, I'm really confused now. Maybe there was some grit stopping the old BB from screwing on all the way thereby allowing the chainring to not be too close to the chainstay?

Is it an issue with the tapering of the BB? I thought it might be something to do with JIS vs ISO, but I'm not sure as I'm not entirely clear about the difference.

So, I guess the obvious solution is to go for a BB with 115mm width, which will push out the chainring to the right by 2.5mm, which should ensure sufficient space from chainstay and a good chainline.

Anyway, before I go reordering, I just wondered if there was anything I've missed? Cheers.

 

[syncrosfan]

Sounds to me like you have and adjustable chain-line BB on your old BB. Since you're running a single ring setup up front you will need to adjust the chain-line to work best for that set-up. Check if your old BB was ISO instead of JIS (that would push your JIS shimano chainset further outboard).

 

[rearwheel]

Oh, perhaps the old BB had an ISO tapering? Still, it doesn't explain why now it suddenly sits too far in when I reinstall it back on the bike?

 

[Rampage]

Take it out and hold them side by side the same way round and you should be able to see if the axle is shorter and if the axle is central or not central.

 

[syncrosfan]

"Still, it doesn't explain why now it suddenly sits too far in when I reinstall it back on the bike?" maybe you forgot a spacer that was on the drive side (it's ok to have one on the threads), or the BB doesn't have a lip to bottom out on then it's an adjustable chain-line one then the drive side cup simply was sticking out a couple of mm. Maybe your old BB was ISO and someone over-torqued the JIS chain-set on to the tapers now ensuring that a JIS spindle won't fit as supposed. Pictures of the whole set-up would be helpful.

 

[richbike]

Re:

Hi,
Im currently rejigging my old orange and am referencing this
http://i64.servimg.com/u/f64/14/44/52/87/sshot-10.png

if only I had my spanner and spacers with me

cheers
RIch

 

[FluffyChicken]

Re:

Its and MTB and we know its a 1998. Pretty much rules out JIS/ISO problems

TH industries is a common bottom bracket, they are FSA and make many for other people, they look the same and have an odd sized bearing in them.

113 is the next length up as 110 is the same near enough as 107 with respect to chainline.
115 is the same as 113 but the 113 will have the none drive side as the 110 you replaced.
You do have the ring on the middle ring.

-But what is the M number on the chainring.
-Measure it actual chainline
-A few mm from the frame is fine for a chainring.

See if you can remember if a spacer was fitted in the TH bottom bracket for an e-type bottom bracket style.

Pictures

 

[rearwheel]

Re:



Yes, I know the spindle lengths look different but they're not - both 110mm.

The Shimano one is brand new, the TH one very old.

The original problem I had was the left crank arm kept coming loose. Apparently, this is due to wear on the spindle and the crank arm hole which connects to the spindle.

 

[syncrosfan]

" Apparently, this is due to wear on the spindle and the crank arm hole which connects to the spindle."
Time to get a new chainset? I'm sure you can find some inexpensive replacement.

 

[FluffyChicken]

Re:

That is due to wear on the crank tapers, once it cones loose the square edges did in slightly as you pedal and every time you tighten it it it gets spread slightly wider and that causes it to come loose easier and ....

That would be my guess. If you put the crank on and tighten it does it bottom out and the big shoulder