How to Protect a Motorcycle Wet Clutch (and How it Works)

Years ago, I owned a Honda 350 motorcycle with a clutch lever that frequently turned to mush and occasionally quit working altogether. I was constantly fiddling with the linkage, trying to take the slop out of the clutch lever. It never occurred to me that the oil used to lubricate the wet clutch could have been the problem. As we’ll see, oil plays a huge role in clutch feel and effectiveness. And, we’ll look at the best motorcycle oil for a wet clutch.

Honda motorcycle wet clutch
My old Honda 350 provided great memories – and terrible clutch feel.

How a motorcycle wet clutch works

First, a few basics.

As shown in the images of a dirt bike wet clutch below, the clutch connects the engine and transmission via a series of alternating friction and steel plates.

Motorcycle and dirt bike wet clutch.

The clutch basket (1) is attached to the engine. Friction plates (2) are attached to the clutch basket via splines along their circumference.

Motorcycle and dirt bike wet clutch.

The hub (3) is attached to the transmission. Steel plates (4) are attached to the hub via splines on their inside opening.

Motorcycle and dirt bike wet clutch.

The hub fits inside the clutch basket and the friction and steel plates are woven together, connecting the two components and creating one assembly.

Motorcycle and dirt bike wet clutch.

The pressure plate (5) squeezes the friction and steel plates together, binding the engine to the transmission and moving the bike. Squeezing the clutch lever relieves pressure, allowing the friction and steel plates to separate and rotate independently. Now the engine can idle without the bike moving.

Wet clutches are widely used in motorcycles and dirt bikes. The term simply means the clutch is lubricated with oil, as opposed to a dry clutch. Friction plays a big role in proper clutch operation.

To illustrate, imagine sitting aboard a motorcycle or dirt bike with the clutch lever activated, idling at a red light or in a starting gate.

The friction and steel plates are separated, allowing the bike to run without moving. The light turns green or the gate drops. As you let out the clutch lever, the plates squeeze together. The transition from the friction and steel plates spinning independently to becoming locked together is an example of dynamic friction. Once the plates are locked together and spinning in unison, they’re subject to the principles of static friction.

Huntersville Trail Dirt bike

Oil vital to clutch performance

Motor oil plays a vital role in both areas.

The formulation influences the dynamic friction you experience, which is best thought of as clutch feel.

Oils with incorrect frictional properties can result in inconsistent, or “loose,” clutch feel. This negatively affects your ability to confidently pull away from a red light without the bike cutting out, or start quickly and grab the holeshot in a race.

The oil also contributes to the holding power, or static friction, between the plates once the clutch lever has been let all the way out and you’re riding.

Oils with incorrect frictional properties can allow the plates to slip in some circumstances, which you’ll feel as lost power to the ground.

A powerful V-twin riding up a hill, for example, can generate sufficient load to cause the clutch plates to slip and the bike to surge.

The oil’s additive chemistry has the greatest effect on performance. Friction modifiers, added to some passenger car/light truck motor oils to maximize fuel economy, can decrease the coefficient of friction within the clutch pack and result in excessive slippage. Extreme-pressure additives, commonly used in gear lubes to protect against shock loads and intense pressures, can cause excessive clutch slippage and related damage.

Best motorcycle oil for a wet clutch

The key is to use a lubricant specifically formulated for wet clutches, such as AMSOIL synthetic motorcycle and dirt bike oils.

They contain no friction modifiers or extreme-pressure additives. They’re dialed-in with the correct frictional properties to promote smooth shifts and consistent clutch feel while guarding against wear for long clutch life.

AMSOIL Synthetic Dirt Bike Oil

Using the best oil for a motorcycle or dirt bike wet clutch will help you focus on riding instead of wasting time on unplanned maintenance.

Updated. Originally published July 19, 2017.

Comments

  1. My supplier gave me primary oil instead of transmission fluid for my 2016 Harley-Davidson. I started pouring before I noticed, so I finished putting it in.
    Do you foresee any issues running primary fluid in the transmission?

    1. Hi Howard,

      We recommend changing it as soon as you can. Using the wrong fluid not only may affect performance, it will jeopardize your warranty if it’s still in effect.

      Thanks,

      John

  2. I’m having trouble with heat here in Florida. Clutch feeling like it’s grabbing at a red light. Change oil it’s good again. Any suggestions?

    1. Hi Donald,

      We need more information about your bike and the oil you’re using to provide an answer. It’s best to simply contact AMSOIL Technical Services and talk to one of our tech reps. Call 715-399-TECH or email [email protected] and we’ll be glad to help.

      Thanks,

      John

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