Suitable glue-Carbon fibre BB sleeve

[bikes4two]

I am replacing the metal sleeve in my CF frame Bottom Bracket (thread on this here ) and am exploring the world of glues and epoxy resins to find something to use on this job.

As a basic requirement, the bonding agent needs to be:
- resistance to water, oil, and anything else that might find its way into the BB
- not necessarily fast setting - waiting 24hrs or more for a full cure is not a problem
- gap filling to a certain extent as the void around the BB sleeve will be irregular in places
- resistance to 'shock' (it's a tough environment down there!)
- good shear strength (for when I tighten up the BB)

So I'd be pleased to hear from anyone who has successfully glued things in a tough environment (not necessarily the specific area I'm looking at) and/or recommends for particular products.

Many thanks.

[Mike Sales]

I have built a boat from plywood, timber, glass and epoxy resins. The sea is a tough environment and I have sailed the cat as far as the Netherlands. So the build was sucessful. I bonded SS machine screws into the wood with resin.
For this job I would use resin with a high density filler. The various ingredients are available from boat supply shops. The quantities would be too great for such a small job, though. I would use the two part resins you can buy from hardware or pound stores. Metallic varieties are available.
The most important thing would be to make absolutely sure that the surfaces are clean, using a solvent and getting them dry. I guess the surfaces are already mechanically keyed.
Epoxy resin meets your criteria and bonds well to other substances like polyester resin.

[Norman H]

West system Epoxy should do the trick http://www.westsystem.com/ss/

For gap filling various fillers are available. There is good information about bonding metal deck fittings from The Gougeon Brothers book on boat building http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook%20061205.pdf

Hope this helps

[Dave W]

You probably want something with a little flex in it like "Penloc" http://www.panacol.de/fileadmin/panacol ... _v1_gb.pdf

Would the bottom bracket simply tighten up against the sleeve anyway so that the bond or frame is not under compression?

I can't quite get my head around your system but i would imagine over tightening would buckle the sleeve?

I guess the bond doesn't do much anyway - just enough to keep the sleeve in place?

[Brucey]

look for shear strength capability and compatibility with the other materials present. Be careful to observe the recommendations concerning bond line thickness.

If you have a significant gap you can use wet lay-up CF or glass to take the gap up. If you are using epoxy resin, you can build up the sleeve if necessary using (say) epoxy and CF or glass, and allow this to cure, then sand the assy to a good fit before bonding it in position.

If you do all these things then you should wind up with a sleeve where the bond sees loads that are primarily in shear, where it is best able to resist them. Loads in tension are not so well tolerated, and nor are loads of any kind on joints with a large bond line thickness. Fillers have their uses (and maintaining a minimum bond line thickness in a concentric joint is one of them) but generally

filler + larger bond line thickness = weaker joint

cheers

[Mick F]

I've been thinking about this .....................

I used to drive a delivery van for a car body refinisher equipment supplier. We sold all sorts of stuff including body panels, bumpers, valances, grilles, abrasives, fillers, polishes etc etc. One of the things that was very popular was Texflex sealant. It came in grey, black or white.

It went on very soft and sticky and dried off to a hard rubber quite quickly. You could use it as a filler or contact adhesive to hold wings in place, or between replacement body panels after welding to fill the joints.
If you used your fingers to apply it, it took days to get it off them! I've tried Googling it ........
.... and I think this is the stuff
http://www.cfsnet.co.uk/acatalog/_NEW__Texflex_PU.html

[Dave W]

It came out easy enough so my guess is there's hardly any need for anything too complicated, there's not even many surfaces to bond it too in the first place - i doubt it's under much load if everything is done properly.

[Brucey]

anything that isn't rated at about 1 tonne per square inch in shear isn't worth a light for a job like this.

This immediately excludes most sealants etc; they simply are not stiff enough to avoid generating peel stresses in most joints of this type.

Although the stresses are quite high, a single stress cycle is easy enough to cope with but if the adhesive bond is in any way inadequate the part will fail down the line; if this job is going to be any good it should be able to withstand millions of cycles.

BTW I should mention that I have bonded headsets and BB cups etc into steel frames (with oversize head tubes and/or stripped BB threads) using various epoxies and it has worked very well; I've never had one come loose, even though some of the parts were smooth chrome steel and the loads were just as large as those envisaged here.

The great thing about doing this is that in a steel frame, heating to ~135C degrades the epoxy, so disassembly is always possible should it be required later. You are very rarely any worse off by at least trying it.

cheers

[Dave W]

So where are the stresses? I could be wrong but to me the sleeve is simply that - a sleeve. In theory the bottom bracket and cranks could be set up without the bike at all.
What point is there in the bond apart from stopping the sleeve from moving sideways?

[Brucey]

really...?

Well when you stand on (say) the right pedal with the crank horizontal in a low gear (when climbing or starting off) you might get (roughly)

-200lbs force on one pedal. Resolving moments suggests that there is;

- over 400lbs vertical force downwards on the RH side transferred into the BB bearings and thence into the frame.
- an approximately equal upward force from the LH cup into the frame.

In addition the chain tension might reach ~400lbs too (if the chainring radius is half the crank length). This is resisted by a load in the horzontal plane of about 600lbs on the right side and similar load (in the opposite direction) on the LH side.

Adding these forces gives a net force of up to ~750lbs on each cup. All these forces are transmitted via the sleeve into the frameset.

Seated pedalling typically gives smaller forces but millions of cycles.

The bond needs to be very good indeed to withstand either let alone both.

The force trying to push the BB sleeve sideways is tiny by comparison with either of the above, and is easily resisted by any bond that is adequate for the other loads.

You will note that the forces reduce pro-rata whenever you can make the BB shell wider. This is one reason why later Madone models have a 90mm wide BB shell.

cheers

[tim-b]

Hi

The Praxis conversion BB uses an expanding sleeve design http://praxiscycles.com/pages/conversion

It doesn't solve your glue query, but it's a thought...

tim-b

[Dave W]

really...?

Well when you stand on (say) the right pedal with the crank horizontal in a low gear (when climbing or starting off) you might get (roughly)

-200lbs force on one pedal. Resolving moments suggests that there is;

- over 400lbs vertical force downwards on the RH side transferred into the BB bearings and thence into the frame.
- an approximately equal upward force from the LH cup into the frame.

In addition the chain tension might reach ~400lbs too (if the chainring radius is half the crank length). This is resisted by a load in the horzontal plane of about 600lbs on the right side and similar load (in the opposite direction) on the LH side.

Adding these forces gives a net force of up to ~750lbs on each cup. All these forces are transmitted via the sleeve into the frameset.

Seated pedalling typically gives smaller forces but millions of cycles.

The bond needs to be very good indeed to withstand either let alone both.

The force trying to push the BB sleeve sideways is tiny by comparison with either of the above, and is easily resisted by any bond that is adequate for the other loads.

You will note that the forces reduce pro-rata whenever you can make the BB shell wider. This is one reason why later Madone models have a 90mm wide BB shell.

cheers

I understand that but surely the carbon joint is designed to take those stresses rather than the sleeve? Where is all the fabulous bonding material from the factory - did it evaporate?

[Brucey]

which carbon joint do you mean? The BB bearings sit in cups and the cups are screwed into the sleeve. All the pedalling loads pass from the bearings to the cups to sleeve and then into the frame.

[NB Trek had to modify this design (making the sleeve stand proud of the carbon) because in some cases the end faces of the cups bore against the carbon frame directly when tightened and this caused problems.]

Anyroad up this part is quite highly loaded.

cheers

[Dave W]

Exactly

The frame is beefed up around the bottom bracket area to take the stresses, the sleeve is there because you can't thread carbonfibre can you? The sleeve is extended beyond the frame to stop the frame getting squashed when tightening the bottom bracket. The sleeve is probably a very close fit (it didn't fall out did it?) Therefore the bonding will probably be minimal, almost unnecessary and would require an adhesive suitable for bonding two different materials and probably need to flex very slightly due to differing expansion properties of carbon and steel.

I'm off to bang my head against the nearest wall.

[Dave W]

Also ........ I wouldn't think it would be a good idea to clean the bottom bracket area with any solvent whatsoever. Solvent will most likely attack the resin, delaminate the weave and make the area swell, maybe not immediately.