Wrong size BB

[losemoneyg]

I took in my old and worn cup type BB to my lbs and had it measured for replacement. I was told the size and was sold hat I thought was the correct BB. After installing NEW BB I am unable to shift to the largest chainring. Confused, could the mechanical have mismeasured?

[bobtravers]

On the old BB, check the shaft for model number... Compare it to the new BB to see if it's the same...

[DaveM]

You may just need to do a slight adjustment on the front derailleur. BB's that are the "same size" can vary by a couple mm depending on the offset, spindle shape, etc. It would be pretty unusual not to have to readjust the derailleur after changing out even identical BBs.

If you can't get it to adjust or you can visually see that the crankset is sitting a lot farther out than it used to be, then I'd open it up and measure to make sure they didn't mess up.

[losemoneyg]

(08-07-2011, 09:11 PM)bobtravers Wrote:  On the old BB, check the shaft for model number... Compare it to the new BB to see if it's the same...

on the old BB it says TH industries CR-BV 2003, 3A 03. I replaced it with a shimano bb un54 68 x 118, which is what the mechanic at my lbs told me was the correct size. I am unable to find the TH industries specs online, but plan to take it to another bike shop to have the original measured again.

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(08-08-2011, 01:20 AM)DaveM Wrote:  You may just need to do a slight adjustment on the front derailleur. BB's that are the "same size" can vary by a couple mm depending on the offset, spindle shape, etc. It would be pretty unusual not to have to readjust the derailleur after changing out even identical BBs.

If you can't get it to adjust or you can visually see that the crankset is sitting a lot farther out than it used to be, then I'd open it up and measure to make sure they didn't mess up.

I actually tried pushing the derailleur over as far as i could with my hand and I couldn't get it even close. The outside of the derailleur cage could barely reach the big chain ring. The derailleur was at full stretch as the pivot point could no longer extend. I'm gonna take the original BB back to another bike shop and have it measured and see if they come up with the same measurement as the first bike shop.

[DaveM]

It may actually be a BB problem then. When you do the measuring, make sure to pay attention to the "offset" and well as overall length. (Offset being the amount that the BB spindle is longer on the drive side than on the non-drive side.) Even on two spindles of the same overall length, different offsets will put the crank in a very different place.

Also, I'm assuming you backed out the outer limit screw on the derailleur before your test? Sounds like you know what you're doing, but just in case.

[losemoneyg]

(08-08-2011, 02:50 AM)DaveM Wrote:  It may actually be a BB problem then. When you do the measuring, make sure to pay attention to the "offset" and well as overall length. (Offset being the amount that the BB spindle is longer on the drive side than on the non-drive side.) Even on two spindles of the same overall length, different offsets will put the crank in a very different place.

Also, I'm assuming you backed out the outer limit screw on the derailleur before your test? Sounds like you know what you're doing, but just in case.

I wil ask the mechanic if there is any offset on the BB. To my untrained eye it looks to be the same on both sides, but will confirm tomorrow. Yes I had had adjustment screw backed out, thanks for checking

[RobAR]

Bob and Dave... methinks you guys missed something? Or maybe not? If I read the original post correctly, losemoneyg swapped a Cup and Cone BB for a Shimano UN-54 Cartridge BB.
While it is true that 118 mm is 118 mm, a lot of younger mechanics don't realize that it's just not that simple when replacing the old axle designations and you can't adjust a cartridge L and R (on a UN). If his old axle was a 118, there were probably 3 or 4 'centerings' for that length to account for crankset and chainstay clearance.
I would suggest looking for a mechanic who knows how to measure it before he sells you a new one.

Edit: Let me ask this. Does the chain even try to climb at all and just wont go, or can you not even get it to touch the ring?

[DaveM]

(08-08-2011, 04:39 PM)RobAR Wrote:  Bob and Dave... methinks you guys missed something?

I think we're saying the same thing. What I'm calling offset, you're calling centerings. But yes, even if the overall length of two BBs is the same, they could still put the crank farther out or closer in because the spindle is not of equal length on both sides. It's unusual for the offset to be more than a few mm. So I'm a little surprised that it would be enough to move the chainrings out of reach of the deraileur's range of motion, but...could be.

[RobAR]

It's very possible, Dave. And not uncommon to hear "offset" used as that is the proper term. Very cool.
Cartrige BBs came about after much Industry Standardizing. Yeah... Right.

[DaveM]

"Bicycle Industry Standardizing"...Hah! That's a good one.

[RobAR]

Old joke, I know. Sorry. Like Tom Petty, it just keeps on.

[nfmisso]

Hi Losemoneyg;

Given that ii is already installed, measure how much offset you need from the current position to get the FD working properly, and then get one with that much less length time 2x (usually but not always - see "standardization" above). At the shop, double check by comparing a 118mm with what you calculate will work.

[losemoneyg]

(08-08-2011, 04:39 PM)RobAR Wrote:  Bob and Dave... methinks you guys missed something? Or maybe not? If I read the original post correctly, losemoneyg swapped a Cup and Cone BB for a Shimano UN-54 Cartridge BB.
While it is true that 118 mm is 118 mm, a lot of younger mechanics don't realize that it's just not that simple when replacing the old axle designations and you can't adjust a cartridge L and R (on a UN). If his old axle was a 118, there were probably 3 or 4 'centerings' for that length to account for crankset and chainstay clearance.
I would suggest looking for a mechanic who knows how to measure it before he sells you a new one.

Edit: Let me ask this. Does the chain even try to climb at all and just wont go, or can you not even get it to touch the ring?

I was thinking the same thing but as I am not a mechanic I put my trust in the one I spoke with. I went to another bike shop and got the same measurements but was told that I may have not tighten the crank arms far enough. I don't think I could tighten them beyond the torque limits, so I think the bottom bracket is the wrong length. There went $25, I guess I will have to purchase a shorter one, but how much shorter????? The outside cage of the derailleur is just past the outside of the big chain ring, so the chain doesn't move from the middle chain ring. I will try nfmisso's suggestion

[losemoneyg]

I got a 1mm size difference from the second mechanic which i would think isn't very much. 118 from the first mechanic and 117 from the second. I decided before i purchase another BB I would tightening down the crank arms as much as could and I got it too work. It's not perfect as there is no room for any adjustments as the derailleur is stretched to its outer limit. thanks for everyones suggestions and thoughts.