Whuffo a dry clutch?

[sideways]

This is both a mechanical and riding question. Could someone, or someones, enlighten me as to why BMW uses the dry clutch in the boxers?

Since getting my RT, my first ever serious road bike, I've worked a lot on slow speed maneuvers ala BRC/ERC. The first thing I learned (from the acrid smell) is that the single plate dry clutch hates playing around in the "friction zone."

At the same time, minimizing clutch use doing the slow speed exercises seems to maximize the abruptness of throttle transitions both on and off. That abrubtness, I'm told, is a BMW hallmark involving both the fuel injection and the shaft drive. So the result is real slow speed tight turns are more difficult than they might be otherwise. Recently I've ridden both Harleys and Japanese bikes on which I can do the slow speed stuff much more easily.

It seems particularly odd that BMW would use the dry clutch on the GS, which might be likely to need a good amount of clutch slipping in tight situations in the dirt. Of course, the GS is not exactly a trials bike, and it virtually defines "adventure touring" so I guess the dry clutch must not be so bad.

Anyway, whuffo BMW doesn't go ahead and put a (more modern?) multi-plate, user-friendly, hydraulic clutch in the boxers, like for example, the new K bikes? Pardon the pun, but is it just inertia in the corporate gear box?

[RFW]

This is both a mechanical and riding question. Could someone, or someones, enlighten me as to why BMW uses the dry clutch in the boxers?

 

The real question is why on earth was the wet clutch invented? The reason initially was that it makes the motor more compact and simple. If you think about it, lubricating a surface intended to generate friction, is rather dumb.

 

More recently, wet clutches have created a lot of problems with the new more slippery oils. It was enough of a problem that a new oil spec had to be created for wet clutch bikes, that ensured the oil wasn't TOO slippery.

 

In short, wet clutches were not intended as a performance improvement at all; they were a sort of "kludge" to avoid the inconvenience of having to seal off the clutch chanber in a motor/transmission assembly that was tryingto be kept as small as possible.

 

Bob.

[steve.foote]

As I understand it, using a dry clutch eliminates the contamination factor inherent with wet clutches.

 

A couple of comments here:

 

1. Dry clutches have been around for a long time.

 

2. Most vehicles (non-motorcycle) use them.

 

3. Slipping the clutch during slow-speed manuevers is not forbidden. Yeah, you may get some burnt smell, but it's really no big deal.

[TowJam]

Anyway, whuffo BMW doesn't go ahead and put a (more modern?) multi-plate, user-friendly, hydraulic clutch in the boxers,...

Don't confuse "hydraulic" with wet/dry clutches. A hydraulic clutch usually refers to the clutch activation mechanism - not the clutch itself. All current BMWs use hydraulic clutches as opposed to cable-actuated clutches.

 

On a side note, my KRS was equipped with a dry clutch and was by far the smoothest shifting of any bike I've owned. (And as far as I could tell, it held up well in the heavy stop/go traffic I face in my daily commute.)

[ShovelStrokeEd]

Because a dry clutch, in this situation, longitudinal crank shaft, shaft drive makes the most sense from an engineering standpoint. The clutch, as designed, is more than adequate to the task and has the following advantages over a wet clutch.

 

No extra fluids to keep contained.

Typically, much lighter pull, although that is more a function of actuater than inherent in the clutch design.

Since it is dry, it is not sensitive to lubricant type.

Clutch debris does not contaminate the engine lubricant thus extending oil filter and oil change intervals.

The dry clutch allows the transmission to be lubricated by real gear oils rather than engine lubricant which, at best, makes a poor gear lubricant.

 

There are a few minuses as well.

Chiefly it is not as tolerant of heat as a wet clutch.

Can be grabby and, in certain applications (early Ducati) actuator was problematic.

In the case of the Guzzi and BMW and Gold Wing, the expense for transmission removal for service can be a bit high and quite high when compared to the service costs of a wet clutch.

 

That all said, unless you contaminate it with fluids, you shoud reasonably expect in excess of 50K miles and some have gone as high as 90K on their clutches with no problems.

[RFW]

In the case of the Guzzi and BMW and Gold Wing, the expense for transmission removal for service can be a bit high and quite high when compared to the service costs of a wet clutch.

 

That point is mostly not related to the type of clutch, but the engine layout. Most bikes with wet clutches have "crossways-mounted" motors, and therefore the clutch is naturally placed on one side where access is easier.

 

The reason that Guzzi, BMW and Gold Wing clutches cost more to service is because the engine is longitudinal, thus resulting in the clutch being sandwiched between motor and tranny, not because the clutch is dry.

 

Bob.

[Ken H.]

If you think about it, lubricating a surface intended to generate friction, is rather dumb.

Intially I struggled to come up with a answer to "why?" But I think you are spot-on. It's the wet clutch that makes no sense!

[Green RT]

That all said, unless you contaminate it with fluids, you shoud reasonably expect in excess of 50K miles and some have gone as high as 90K on their clutches with no problems.

I put 170K miles on the original clutch on an R60/5. Admittedly, the miles were mostly commuting and almost all freeway.