Bicycle Mechanics - That thin bottom bracket driveside washer...

I have a Trek 5200, about 8 years old, that I just upgraded the original Ultegra cranks to new Dura Ace 7700 (9 speed) cranks and Ultegra bottom bracket. On my old setup there was a thin metal washer on the drive-side of the bottom bracket, between the actual frame and the BB cartridge. They did not replace this washer when it was installed at my LBS. I have all the old parts, including the washer. What does this piece do? Do I need to have them remove the cranks and BB to re-install this part? Thanks.

SAB

I have a Trek 5200, about 8 years old, that I just upgraded the original Ultegra cranks to new Dura Ace 7700 (9 speed) cranks and Ultegra bottom bracket. On my old setup there was a thin metal washer on the drive-side of the bottom bracket, between the actual frame and the BB cartridge. They did not replace this washer when it was installed at my LBS. I have all the old parts, including the washer. What does this piece do? Do I need to have them remove the cranks and BB to re-install this part? Thanks.

SABThey must be idiots. It's supposed to be there.It's for protection in case of chain drop.

Not necessarily, I have a RaceFace ISIS XC bottom bracket and it comes with a single aluminium spacer about 2mm wide. It says to install it on the drive side but I personally think whether or not you install it, and on which side, depends on whether you need it to get a perfect chainline.

Before you start abusing your LBS, just ask them if this is the case with your BB :).

That thin washer on your Trek is designed to protect the carbon frame from abrasion in the event that you throw the chain.

Thanks, guys. I wasn't sure if it was needed from a mechanical/structural perspective or if it was there for safety reasons. The bike rides great without it, but I think I'll go ahead and have them put it back on.

SAB

Not necessarily, I have a RaceFace ISIS XC bottom bracket and it comes with a single aluminium spacer about 2mm wide. It says to install it on the drive side but I personally think whether or not you install it, and on which side, depends on whether you need it to get a perfect chainline.

Before you start abusing your LBS, just ask them if this is the case with your BB :).You got a Trek 5200 that frame from the factory with that specific washer installed, for that specific purpose?

They must be idiots. It's supposed to be there.It's for protection in case of chain drop.

A "thin" washer is going to protect a frame from a chain drop? I think (and I could be wrong!) it's more for the chain line. Anyone else?

I think (and I could be wrong!) it's more for the chain line. Wrong. You need to bone up on Trek OCLVs and why they come with what from the factory.

bac,

Not sure about chain line....wouldn't the BB length do that?
I think it is there to protect the CF frame. There is also a stainless strip on the chainstays to protect that area from slap and if the chain gets jammed.

On my 5200 that stainless piece has become unglued and is now duct taped until I get a good answer to "glue" to the CF.

bac,

Not sure about chain line....wouldn't the BB length do that?
I think it is there to protect the CF frame. There is also a stainless strip on the chainstays to protect that area from slap and if the chain gets jammed.

On my 5200 that stainless piece has become unglued and is now duct taped until I get a good answer to "glue" to the CF.Hey, a guy that has one and knows. You are right about chainline.The 'washer' in question is too thin to have any meaningfull effect. Stick that stainless strip back on with good auto grade double sticky tape.

The bike did indeed come from the factory with that (very thin, 1mm) washer in place. I have had the bottom bracket replaced before and the service guys put that washer right back on. This is a new shop in a new city, so I'll have to have them correct things.

Wrong. You need to bone up on Trek OCLVs and why they come with what from the factory.

Hey, I said I could be wrong - I was just offereing my opinion.

However, help me understand how a 1mm washer protects the frame from chain rub? Isn't the chain wider than 1mm? Moreover, after the chain pops, wouldn't the chain flop around a bit, making the potential wear from the chain effectively even wider? For the record, I do own a 2002 5200 OCLV, and I just swapped out the drivetrain to the new Dura-Ace. There is no more washer. Will my frame survive?

bac,

Not sure about chain line....wouldn't the BB length do that?

Yup, it should. However, I've seen a few mountain bikes that needed washers/spacers mounted for a good, straight chainline. The washers were recommended by Shimano after a call from my LBS.

Bac re: avatar - aspect ratio.

Hey, I said I could be wrong - I was just offereing my opinion.

However, help me understand how a 1mm washer protects the frame from chain rub? Isn't the chain wider than 1mm? Moreover, after the chain pops, wouldn't the chain flop around a bit, making the potential wear from the chain effectively even wider? For the record, I do own a 2002 5200 OCLV, and I just swapped out the drivetrain to the new Dura-Ace. There is no more washer. Will my frame survive?

Well, I ain't no engineer, I'm just a humble bike mechanic, but here goes: The washer that I assume he is talking about has a diameter a few millimeters wider and sticks up from the bottom bracket shell. They're ugly. I assume the idea is that, when you drop the chain, the metal washer will keep the chain outboard of the carbon frame. In other words, it's the frame equivlent of the spoke protector. The difference is that if you chew up 8 outside spokes, the parts cost will only be a buck or two. If you chew up your carbon frame because you left off the factory installed protective washer, the replacement cost is likely to be considerably more.

That is exactly the piece I am talking about. I will definitely have them put it back on.

SAB

That is exactly the piece I am talking about. I will definitely have them put it back on.

SABGlad we got that straightened out.