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We know from our experience so far that wet multiple disk clutches like these used on motorcycles have a way longer life that single disk clutches used in car engines. Why don't cars use the same design in their clutches?
Is it due to different engine torque and operation loads between motorcycles and cars?

Is it due to the more "aggressive" nature of motorcycle engines on lower RPM?

It all comes down to size.

There is one thing which makes a clutch work, whether wet or dry. It comes down to friction. There are two ways to achieve the friction needed to transfer the energy from the engine down into the drive train: clamping force and frictional area.

Clamping force is done rather easily ... if you need more, you put in larger springs. The problem with that is, the larger springs you put in, the more force it takes to overcome the springs. Many people experience leg fatigue in cars with stiff performance clutches when you are working the clutch in stop-n-go traffic. Not a lot of fun.

The second part of the equation is the surface area. While total frictional area of the clutch with the same clamping load doesn't do anything for total friction (see this question on Psylink.com), you have to make up for frictional area in a motorcycle due to needing to make it more compact. The way you do that is by making it multi-plated. Considering a regular MC clutch is in the 4-5" range, there needs to be a few of them to make up the difference.

As far as lifespan of a clutch goes, I'd suggest the final outcome of a clutch is more or less based on how the clutch is driven versus whether it is wet or dry. It seems the wet clutch has a longer lifespan, but really, most passenger cars with clutches today can last 150k+ miles without a hiccup if they are driven sanely. A bike may be able to do the same thing, I'm not quite sure, but again, if someone is riding a bike like they stole it, the clutch isn't going to last as long as if they use it sanely.

The thickness of the friction materials come into play for longevity. The wet clutches of a MC are relatively thin. This, too, is done for compactness. The friction materials used in cars are quite a bit thicker versus a MC. This aids the car in longevity as well.

Torque is a factor in all of this. An automobile, for the most part is going to be far heavier than a motorcycle. To compensate for this, the clutch material is placed further out on a greater circumference. This allows a smaller amount of frictional material the ability to transfer the energy without slipping. In effect they've placed a longer lever to move a heavier rock. With this in mind, the need for multiple clutch plates is abated. A MC engine is made with horsepower in mind. It can spin much higher. It's working life is spent at a much higher level than that of a car. Having the clutch with a smaller diameter allows the revs to occur quicker (less inertial forces to deal with) and be able to live the much higher lifestyle with less of a concern on the rider. Performance clutches in cars will also reduce their diameter for much the same reason, but will make up for it in other ways (such as friction material and clamping force).

This brings us down to wet versus dry. The main reason a MC has a wet clutch is to dissipate heat. There just isn't enough thermal mass in the MC clutch to be able to cope with the amount of heat generated. A car, on the other hand, has the mass of the flywheel and the pressure plate to deal with it. I do realize there are some bikes which use dry clutches. These take into account a lot of these considerations stated above. Most are used in performance applications, aren't expected to last as long, and are a lot easier to replace than the wet clutches found in most applications.

One last thing to consider ... automobiles do use wet clutches. They are used all over the place in automatic transmission. They are built very much like the wet clutches you find in MC's, too. Auto transmission can, if used with longevity in mind, last for the lifetime of the automobile.

I'm only guessing here but pretty much everything on a road car comes down to one thing; price. The manufacturing cost is likely lower if you just have a single, dry clutch plate.

I have changed the clutch on an Oreca Dodge Viper endurance racer and that was running a multi-plate carbon-kevlar clutch. Apparently this part cost £3,600. The clutch I've just fitted to my wifes old Golf GTI cost me less than £60 delivered.