When you are riding at very high speed and suddenly a sharp corner comes the first reaction would be slowing down the speed, applying the brakes and shifting in lower gear. But what if you want to shift 2-3 gears at a time while riding at very high speed? You can do it but at the cost of damaging the gear box and putting your safety at risk which arises due to engine braking force. In order to avoid such problems Slipper Clutch (also known as back torque limiter clutch) was introduced. The Slipper Clutch helps by allowing the clutch to partially slip until the engine speed matches to your own speed.
In normal clutches, the engine braking force is transmitted to rear wheel via chain drive or shaft drive which causes rear wheel to shake, jump or lose traction. This is the main concept behind slipper clutch to control rear wheel under hard braking and downshifting that causes the rear wheel to shake or lose traction. Especially in higher displacement bike where the engine braking force is massive and cause vital injury to rider if not controlled.
The first application of Slipper clutch in motorcycle was found on “Hogslayer” made by John Gregory and TC Christenson. (“Hogslayer” was racing bike and it holds many records on its name during 70’s decade. It was known for its technological innovation. The bike was able to produce 320bhp and top speed of 290 KMPH. Currently the final version of this bike is on display at the British National Motorcycle Museum.) Since then it has become tradition to use slipper clutch in Moto GP racing bike. Mostly all the high performance bikes uses slipper clutch. In India you can find it in Yamaha R1, Suzuki Hybusa, Honda VFRrrr1200F etc.
The actual difference between the slipper clutch and normal clutch is of clutch hub. In the above photo, the hub of the slipper clutch is disassembled. The base of clutch is on right side and the hub of clutch is on left side. You can see ball and ramp in the clutch base and ramp and engagement dogs in clutch hub (It may come with or without ball bearings, this one is without ball bearing). In normal operation, when the engine is trying to turn the rear wheel, the flat engagement dogs push on each other, and the clutch drives the motorcycle forward like a normal clutch operation.
During massive deceleration or hard breaking the torque forces the ramps together. To absorb engine braking force the ramps slide up on each other. This action lifts the hub up from the base, in the direction of pressure plate. It slides until the entire braking force is absorbed and then act like a normal clutch. This mechanism is called “spring loaded ball sliding up ramp”. The above process absorbs the engine braking force and partially disengages the clutch form the engine.
Clutch Stake height
Clutch stack height is more critical in a slipper clutch than in a normal clutch. The slipper clutch tends to work the best with a clutch stack height that is about 1mm more than the distance between the clutch hub base and the pressure plate.
Clutch plate order for slipper clutches may vary from the original method due to the clutch hub lifting off of the base. Some clutch plates includes special friction plate and judder spring apart from steel plate and friction plate to engage the clutch smoother. In such condition the first plate which is closest to the engine should be a friction plate. The second plate is a steel plate after that a special friction and judder spring comes. There after the above said order continue.
The following are the advantages of slipper clutch over normal clutch
- The slipper clutch reduces sudden forces on the inside of the transmission and hence, reduces wear and tear on the transmission
- The correctly installed slipper clutch improves performance.
- It can prevent disastrous rear wheel lock up in case of engine seizure or transmission failure.
- It also reduces work of suspension by absorbing engine braking force hence less bumpy ride while cornering.
- The rider does not need to concentrate on clutch operation which allows him concentrate on other things such as body posture, braking etc. while cornering.
Practically there are no such disadvantages of slipper clutch but hypothetically there are few as under
- It’s mechanically complexity. Some slipper clutches are really complex to install and deal with (not many of them).
- They are expensive (not against the saving they make by reducing wear and tear of transmission, less suspension movement and increased rider safety).
- Sometimes you really need the engine braking force to slow down the bike (but how many time) which can work against in case of slipper clutch.
- Slipper clutch can’t give desired result if not adjusted according to requirement.
- The last and somewhat little bit serious doing hard corner is a skill which rider develops over a time period. How much clutch to be released, how much gears to be downshifted, how to control engine braking force while cornering. With slipper clutch the rider is not able to develop such skills. As the slipper clutch gets more acceptances we may face loss of this skill (But this is the general rule of the world).
Most of the bikes produced in India are of smaller cubic capacity (CC), which are not that fast that they require slipper clutch. But in near future we may have bikes produced in India with slipper clutch.
- Mahavir Kothari
Images from: www.yoyodyneti.com