What causes the clutch fluid to darken so rapidly?

[thought]

This isn't specific to our bikes.  Pretty much all cars which have clutch fluid also see it turn dark far more quickly than the brake fluid, and the temperature for both is about the same.  It isn't moisture either, since both systems have the same seals.

The real reason is that the rubber grommets/seals/o-rings (depending on the system) in a clutch piston move a far greater distance than the ones in a brake system.  The black rubber particles are therefore scraped off in greater numbers, darkening the fluid.

The brake system moves slightly, but is mainly transferring force as the pads don't move very far.  The clutch, in contrast, is transferring force, but over a greater distance as it disengages the clutch plates.

And now you know the rest of the story.

I have to say that I always really like your posts... always well thought out and well explained. 

[Bill in OKC]

If anyone has polished aluminum, it creates 'blackness' - on your fingers, on the rags.  The brake and clutch cylinders are aluminum and the seals polish the bores each time you use them.  I guess I use the clutch many more times than the brakes - that is the theory anyways.  YMMV

[Scissors]

To expand on these systems, some of the newer guys might not be aware that there is a rubber diaphragm on the brake and clutch reservoirs.  The purpose is two-fold:

1.  To provide a seal against the outside elements.
2.  To prevent differences in pressure between the brake/clutch system and the outside atmosphere from introducing water to the system, or causing fluid to escape.

For example, if the pressure within the clutch system decreases in comparison to atmospheric (either because the outside pressure increases, or you pull the clutch lever, draining some fluid from the reservoir temporarily), the diaphragm sinks downward as the outside air displaces the volume necessary to equalize the pressure on both sides.

Another way to think of it:  If you put a plastic bag over your mouth (and don't breathe through your nose) you'll see that attempting to breathe in brings the bag into your mouth, and attempting to breath out bulges the bag outwards.  Yet no moisture is exchanged with the outside air even though you are able to breathe in and out (slightly).  In some circles this is referred to as "kinky".

I've seen some bikes, including my own, where the diaphragm has dropped in the center down into the reservoir so that it forms a cup.  This is improper and should be corrected.  The diaphragm should form a "flat" shape composed of up and down pseudo-circular folds.  When looking at your reservoir, you should see the fluid at the bottom, then completely empty air above it, with the diaphragm "hidden" by the cap up in the top.  If your diaphragm is touching or below the surface of your fluid you need to either fix it, or think of another form of birth control.

[Howie]

If anyone has polished aluminum, it creates 'blackness' - on your fingers, on the rags.  The brake and clutch cylinders are aluminum and the seals polish the bores each time you use them.  I guess I use the clutch many more times than the brakes - that is the theory anyways.  YMMV

This is what I believe the problem is, plus, as scissors stated, the length of the stroke as compared to the brakes.  Hard anodizing, like they do on aluminum car brake masters would prevent this. 

[Scissors]

Almost forgot to also add:  One tends to cycle the clutch more often than one tends to cycle the brake, multiplying the effect of the longer stroke.