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

[roubaixtuesday]

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]

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

[Mick F]

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

[9494arnold]

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

[roubaixtuesday]

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]

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

[foxyrider]

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

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.

[Brucey]

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

[bigjim]

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

Which is why I strip mine and overpack with grease. leaving as little room as possible for air to penetrate. Shimano don't seem to put much grease in there.

[freiston]

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

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.

I thought that mercalia's comment wasn't about the design of the plastic sleeve but of modern hollow-axled bottom brackets.

[Gattonero]

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

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.

The sleeve from Shimano HT2 bottom brackets does also work with Campagnolo Ultra-Torque & Power-Torque BB types too, just add a few wraps of PTFE tape at the ends. Been riding the same bearings for 3 years now still spinning perfectly with no contamination. Have used also on some chap's cyclocross bikes, and those are ridden through lots of mud, it does indeed work although regular strip&clean has to follow after a few rides.

[foxyrider]

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

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.

I thought that mercalia's comment wasn't about the design of the plastic sleeve but of modern hollow-axled bottom brackets.

Nothing new about hollow BB axles, I was using them back in the seventies but you wouldn't know because they had a bolt in both ends! Much like hollow wheel axles there is a payback in strength as well as weight so a win win. We might have different bearing arrangements and fitting devices but there really isn't anything new about the sleeves or hollow axles.

[freiston]

I thought that mercalia's comment wasn't about the design of the plastic sleeve but of modern hollow-axled bottom brackets.

Nothing new about hollow BB axles, I was using them back in the seventies but you wouldn't know because they had a bolt in both ends! Much like hollow wheel axles there is a payback in strength as well as weight so a win win. We might have different bearing arrangements and fitting devices but there really isn't anything new about the sleeves or hollow axles.

Hence me specifying modern hollow-axled bottom brackets

[Brucey]

modern sleeves have 'o' ring seals and BITD they didn't have this feature.

cheers