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Rear Cardan Breather Bolt

wallycycle

wallycycle

Tuned and Synch'ed
Joined
Apr 5, 2011
Messages
26
Location
Murrieta
Why does the rear cardan require a breather bolt? (please don't reply: so the rear cardan can breathe!). BMW doesn't have one. What is the consequence of not using a breather, ie just bolt the hole shut?

Thank you.
 
Well, if you think about it - when you heat a fluid, it expands. Now, if the container is sealed, one of two things can happen:
- The container distorts; or
- The container bursts (leaks)

therefore to stop your cardan from leaking it relieves pressure to the atmosphere and when the fluid cools down, it gets a breath of fresh air. Otherwise it would start leaking oil and you would not like that, I'm sure.

Hope this helps (can't comment on XXX though, who knows what the Germans have designed)
 
Hi Tony,

that makes sense and I was sort of thinking along those lines as well. I was just curious what exists inherent in the design that requires that. The transmission does not have the breather, at least not one like the cardan and I would think that the same principle would be involved.

Anyway, I won't over think it. It's there for reasons. Mine broke off in the cardan. I think the previous owner over torqued the thing to stop it from misting. I was lucky to get the broken threads out.
 
wallycycle said:
The transmission does not have the breather, at least not one like the cardan and I would think that the same principle would be involved.
Click to expand...

The transmission (or gearbox) does have a breather, it's connected to a hose that breathes under the tank, usually cable tied to the frame beside the airbox.

Peter Roper was offering a superior similar breather hose arrangement for the cardan at one time, dunno if they're still available. It's something you could easily knock together yourself if you're concerned about the possibility of water getting in.
 
thanks for that information.

I still wonder why the BMW doesn't have a breather on the rear cardan like Guzzi
 
Thanks John,

I'll take your word for it that it's there on Beemers.

Like an Easter Egg hunt. I couldn't find it looking at the fiche.

Happy Easter!
 
Sorry to drag this back up, but didn't think it warranted a new topic.

I've been thinking about this, and discussing it with someone on the GuzziRiders Forum.

My (our) thoughts are these.
The Final Drive housing has the 'concertina' Rubber Boot on the top, and another Rubber Diaphragm at the front, to allow for the movement inside the Housing, and to keep the Oil in obviously.

So why have the little vent plug?
It's not a Breather really, all it does is allow a very small amount of air to pass due to the slight warming of the Drive unit, and how hot does that actually get?
Not very I'm guessing.

If it did actually 'breathe' any significant amount, I'd expect to see some Oil misting/staining around the base of the outlet.
Anyone ever seen that?

I reckon the Rubber Boot and Diaphragm are more than enough to allow for the slight pressure changes within the Housing.

I'm going to block mine off, and keep an eye on the Boot.
 
The boots do not keep the oil in. Strip them back and take a look. They are there as somewhat decorative but protect the carr arm pivots from crap getting in. They are steel tie wrapped top and bottom to seal in the oil and seal out the water. There is no direct path to the oil on the carc other than seals and the breather.

The carc gets hotter than you think. Just take it for a ride for a long time in hot weather. Not hot enough to burn u as that is the function of the oil but hot enough for the oil to expand.

The oil level is low enough that it never reaches the breather but put too much oil in the carc and it will come out there. This is no different that any breather on an industrial or automotive gearbox. A sealed gear box is possible (BMW tried) but has its own problems (spontaneously bursting into flames) and need to be designed to take the pressure. Basically you are making a pipe bomb and need to design it so it does not blow the seal or itself up.

If you plug the vent it will eventually come out the main seal as that is the next weakest point. That has been observed several times from people that have had their breathers plug. If you are worried about water a banjo bolt and length of brake cable routes to under the seat will take care of that. That being said I know several people that have not routed the vent and ride in rain often without issue. Submerge the carc and that may be another story.
 
canuck1969 said:
The boots do not keep the oil in . . . they are steel tie wrapped top and bottom to seal in the oil and seal out the water . . .
Click to expand...

Hmmmm, which is it then? ;)

Looking at this . . .
4180128149_54162141b3_z.jpg

There's nothing between the Vent and under-Boot area, it's an open 'chamber'.
The front is sealed by the Diaphagm, the top by the Boot (when it's fitted).

I take your point about the temperature, but I live in the UK, that won't be an issue . . . :rolleyes:

I've not had a look inside mine . . . yet.
Will be doing a full service next week, so I'll dig a bit deeper than I have so far (one month's ownership only . . . ).

canuck1969 said:
. . . it will eventually come out the main seal as that is the next weakest point. That has been observed several times from people that have had their breathers plug . . .
Click to expand...
Guzzi owners?
The BMW has a sealed FD unit (as in no Boot or Diapragm), so they definitely need the Vent
 
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