Technical question about clutch discs made of aluminum

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Technical question about clutch discs made of aluminum
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Technical question about clutch discs made of aluminum

Technology question – what use are clutch friction plates made of aluminum?
Easier is really better?

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PS reader Frank Schulze asks: Aluminum clutch discs should reduce the rotating mass. Don’t burn them up too quickly and have to be replaced very soon? The PS technology expert Werner “Mini” Koch gets to the bottom of the question.

Werner Koch

07.02.2017

There are suppliers of aluminum clutch disks that replace the steel disks in oil bath clutches. Of course, these retrofit parts reduce the rotating masses in the drive, which theoretically allows the engine to respond more spontaneously and rev up more freely. However, these effects are unlikely to be measurable and certainly not noticeable in the form of better acceleration. We weighed standard, 150 millimeter steel disks from a common Japanese clutch with 100 grams per disk. With a clutch pack of seven steel disks, that makes a total weight of 700 grams.

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Sounds pretty good


Technical question about clutch discs made of aluminum


Werner Koch

The aluminum friction disks, which are already very light as standard, with the applied lining mixture.

In addition to the steel disks, there are also the friction disks with the clutch lining applied, which are already manufactured in large series from aluminum and weigh only 34 grams each. Why? Because steel has a specific weight of around 7.8 kg (weight per cubic decimeter). Aluminum, on the other hand, is 2.7 to 2.8 kg, depending on the alloy. When replacing the steel disks with aluminum ones with the same diameter and the same material thickness, which is mandatory based on the given dimensions, you would save around 460 grams in our example. Sounds pretty good and is of course a real word when it comes to a lightweight racing sandwich that has been slimmed down to the last gram.


Technical question about clutch discs made of aluminum


Werner Koch

This steel disk package can be replaced by aluminum components, but these can hardly be changed in terms of thickness and design.

Nevertheless, these 460 grams are only insignificant if you consider the entire package of rotating engine masses, which consists of the crankshaft plus alternator rotor, the primary gear and the large and heavy clutch gear made of steel including the clutch basket and the friction discs. To a small extent, there are also the gears plus transmission shafts, sprockets, but above all the large rear wheel weighing around 15 kilograms, including the brake disc. Not to be forgotten, even if it is not connected to the drive train, the approximately eleven kilogram front wheel with its large 320 mm brake discs, which also has to be set in rotation when accelerating – unless you hold it skillfully over the asphalt! Of course, with all the components listed, not only the weight, but also the diameter and the mass distribution are decisive factors for the mass inertia.


Technical question about clutch discs made of aluminum


Werner Koch

At 100 grams each, the conventional steel disks weigh around two thirds more than the aluminum tuning disks.

I stick to it: The lower masses due to the aluminum windows will not be proven in better driving performance. It is not for nothing that BMW even increased the weight of the crankshaft in the course of optimizing the S 1000 RR in order to enable smoother power output and better traction. In addition, the question arises of how well or poorly the coated aluminum disks get along with the standard friction lining in terms of metering and wear. There will be hardly any wear and tear in the conventional sense. The coating, which is only a few hundredths of a millimeter thick, is either complete or it tears. There is no measurable wear, such as on the friction lining. We do not know whether the aluminum panes warp like overheated steel panes in a wave or umbrella shape.

Counter-notification or consent?

If you now want to add the theoretical disadvantages to the lighter coupling, then it will be nitpicking, but also exciting. The clutch, which rotates in the opposite direction to the crankshaft, is one of the masses that counteract the gyroscopic forces of the crankshaft and the wheels, even if it is significantly reduced by roughly three times the speed. And as we know, counter-rotating, equally high inertia forces cancel each other out in their stabilizing effect, the centrifugal force. This improves the handiness, especially in quick change turns. For this reason, Aprilia built two counter-rotating crankshafts in the world championship RSV 250 engines. Many 500 series two-stroke engines were constructed in a similar manner, each with two crankshafts. And the backward-running crankshafts of the MotoGP-Yamaha M1 do this to counter the rotating inertia forces of the wheels. As I said, light clutch discs could theoretically even be detrimental to maneuverability. Just theoretically. Which opens the discussion. We are waiting for your counter notifications – or approval.

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