What's the plastic sleeve inside the bottom bracket for?

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roubaixtuesday
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Joined: 18 Aug 2015, 7:05pm

What's the plastic sleeve inside the bottom bracket for?

Postby roubaixtuesday » 15 Mar 2019, 9:15pm

Just fitted a replacement bottom bracket on a 105 crankset (sorry, not sure the right terminology for the proliferation of bottom bracket types).

There's a cartridge bearing on each side, hollow axle through the centre attached to the RHS crack.

The bearings screw into the frame, and there's a rather flimsy plastic tube that connects them.

What's it for? It seems entirely superfluous as the bearings screw very securely directly into the frame.

mattsccm
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Joined: 28 Nov 2009, 9:44pm

Re: What's the plastic sleeve inside the bottom bracket for?

Postby mattsccm » 15 Mar 2019, 9:29pm

Keeps the crap from the inside of the bearing in theory. Ok with lumps of mud :shock: but doesn't do much with water. I slap a lot of toffee like waterproof grease around the joints of mine. Seems to work.

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Mick F
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Re: What's the plastic sleeve inside the bottom bracket for?

Postby Mick F » 16 Mar 2019, 9:05am

It's to stop bits of rust and detritus from inside the frame getting into the BB bearings.
Mick F. Cornwall

9494arnold
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Joined: 21 Jan 2007, 3:13pm

Re: What's the plastic sleeve inside the bottom bracket for?

Postby 9494arnold » 16 Mar 2019, 10:21am

What Mick said. Plus I believe you get condensation inside the tubes in certain condition, so stops damp falling down into the bracket.

roubaixtuesday
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Joined: 18 Aug 2015, 7:05pm

Re: What's the plastic sleeve inside the bottom bracket for?

Postby roubaixtuesday » 16 Mar 2019, 11:56am

Thanks all. Seems a bit pointless to me, but I'm very sure those nice Shimano people know a lot more about bike design than I do...

mercalia
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Re: What's the plastic sleeve inside the bottom bracket for?

Postby mercalia » 16 Mar 2019, 3:40pm

dont you bet on it. what a silly design, so much for progress and saving a few grms :roll:

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foxyrider
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Location: Sheffield, South Yorkshire

Re: What's the plastic sleeve inside the bottom bracket for?

Postby foxyrider » 17 Mar 2019, 10:30am

mercalia wrote:dont you bet on it. what a silly design, so much for progress and saving a few grms :roll:

Nothing to do with weight saving, it dates back to the late 70's / earl 80's when BB's were two cups, loose balls and an axle and frames didn't use plastic cable guides with the accompanying hole to fit them. It was a regular occurrence to find the BB full of water that had made its way in through the seat tube and contaminating the bearings as well as swarf/rust from the frame itself.

The simple fitting of the plastic tube, which can be as flimsy as you like as it serves no other purpose, worked / works fairly well at doing this. In lieu of the real McCoy i've used cardboard for the same job although it's crap at keeping the wet out, the dirt is kept at bay.

Looking at a fairly new Focus branded FSA chainset I have, that has a similar arrangement between the bearing cups to keep crud and wet out.
Convention? what's that then?
Airnimal Chameleon touring, Orbit Pro hack, Orbit Photon audax, Focus Mares AX tour, Peugeot Carbon sportive, Owen Blower vintage race - all running Tulio's finest!

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: What's the plastic sleeve inside the bottom bracket for?

Postby Brucey » 17 Mar 2019, 10:36am

perhaps controversially, I think the seal may be 'too good' in that the O-rings provide a gas tight seal to the cups. This stops water that might be sloshing around the frame from getting at the bearings, but it also causes there to be a significant trapped air volume in the BB assembly. This 'breathes' (with temperature and pressure changes) via the bearing seals, thus dragging water/crap in through the seals and into the bearings.

cheers
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