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TURBO TALK – CLUTCHES PART 2
This is a weekly discussion on "grey" areas in regards to turbocharging. I answer many of the same questions every day, one at a time on many confusing subjects that really are misunderstood. I want to help, so, we will open up 1 new topic per week to provide accurate information and explain how and why stuff works. This week‘s topic is Clutch Accessories for Big Power. We are going to discuss Lockouts and Slipper Clutches.
Most OEM clutches use springs or a spring system to supply pressure to the outer plate and friction stack. A great system, however there is an upper limit to its potential. There is only so much spring you can use.
The upper limit to a conventional spring clutch system in a dirt bike is about 65 hp. Beyond that hp point, the spring pressure required makes the lever incredibly stiff and the outer plate distorts under disengagement (pulling in the clutch lever), which causes the clutch to drag (remember that’s bad).
“Ok Kelly, if that’s the case, then what about street bikes that make 150+ hp?” The answer has to do with surface area. Increased surface area of the friction system enhances the performance and power delivery of the clutch. Street bikes use more friction plates and are usually larger in diameter. This increased surface area allows the clutch to transfer more power with less pressure.
What options are there if you are going to build a big horsepower bike? There are 2 primary systems that will get the job done and keep the clutch feel good and the first one is a lockout.
A lockout is essentially a centrifugal clutch, meaning it uses rotational velocity to apply pressure. In this system, outer weights apply additional pressure to the outer plate from a fixed point (remember that!) This system has a very high upper limit because pressure is only supplied from the lockout during clutch engagement (lever out). This is important because of one simple reason: There is no change in how the clutch lever works or feels. The clutch will feel identical and operate as if it were stock. You can even use stock clutch springs. Because of this, you can essentially apply as much pressure as you want. In many cases, up to 1000 lbs. or more. At that point, the efficiency of the clutch is maximized and clutch life expectancy becomes very long.
“Wow! A 1000 lbs. sounds like a lot…Will my clutch break under all that pressure?”
The answer is absolutely not. Clutches love pressure. More pressure = lower friction temperatures and higher delivery = longer life. This question is better answered with a hydraulic press. The fear is that the plates will “shatter” under all that pressure. So here is the test: Imagine you have a clutch stack in a hydraulic press. The stack has an upper and lower plate to even out the load (outer pressure plate and inner hub). Now crank away. The fibers and steels will compress tighter and tighter, but because the load is even they are totally fine with this. As a matter of fact, we tried this and what happens if you keep jacking is that the fiber material will literally smear into an even coat. The fiber squares start to get wider and touch each other and that occurs at about 40 tons of pressure. So 1000 pounds is a walk in the park for these guys.
The only downside to the lockout is space. This guy is relatively large and requires about 1.25" additional clearance. In most cases, this can be achieved with a a simple spacer.
The other Clutch Accessory option for big power is the Slipper. Rekluse is the most common these days. There are actually many different styles of slippers, however, for snow biking, Rekluse seems to be king of the Slippers.
How it works. The clutch has 2 main features. A preloaded stack spacing (this is what provides the slip at idle) and a centrifugal wedge plate that applies pressure from a sprung surface (different than the fixed point of a lockout). Ok, so here’s what happens. If you are at idle the clutch pack has a gap. There are clutch springs supplying pressure to the outer pressure plate, however there is an inner collar that supports the plate and provides the gap. This gap spacing can be experienced with any conventional system by having a clutch cable that is too tight.
The second feature is the wedge plate:
There are 2 styles of wedge plates, roller (ball bearings) or slider (EXP). Both work on the same principal, yet the slider systems offer a cleaner system with few moving parts. The wedge plate is sprung together and when the velocity is high enough, the inner weights will travel up a ramp causing the wedge plate to expand. This expansion, first takes up the preset gap creating clutch engagement (bike starts to move). The really cool part is what happens next. Remember those clutch springs on the outside, well as the wedge plate approaches full expansion, it over comes the springs and pushes the pressure plate out. This in turn, over compresses the clutch springs. For example, if you have a spring system that is supplying 325 lbs. of force on the pressure plate, the change in the installed height increases the pressure. What? Basically the wedge pushes back onto the springs making them shorter, therefore stiffer.
As cool as this system is, there is an upper limit…the wedge can only expand so much. For example, an installed pressure of 350 can only go to about 400-415. Definitely better than the conventional, however much lower than a lockout. Why? It is pushing against your springs and not a fixed point.
“Hey Kelly, I really like my Rekluse for snow biking. Can I have my cake and eat it too?” Yes. You can actually put a lockout on top of a Rekluse. You know when I told the guy at Rekluse that I was going to build one of these, he said that I was nuts. However, the 1st one that I built is on season #4 on the same clutch.
This setup is way cool because it gives you both a slipper function with the pressure capability of a lockout. There’s no limit on this bad boy! However there is some bad news. The new Rekluse EXP 3.0 has holes in the pressure plate right where the lockout weights go. There is nothing but empty space for the weights. The old style, full faced, Rekluse pressure plates still work, but the new ones are a no go.
Thanks for reading and questions are always appreciated.
This is a weekly discussion on "grey" areas in regards to turbocharging. I answer many of the same questions every day, one at a time on many confusing subjects that really are misunderstood. I want to help, so, we will open up 1 new topic per week to provide accurate information and explain how and why stuff works. This week‘s topic is Clutch Accessories for Big Power. We are going to discuss Lockouts and Slipper Clutches.
Most OEM clutches use springs or a spring system to supply pressure to the outer plate and friction stack. A great system, however there is an upper limit to its potential. There is only so much spring you can use.
The upper limit to a conventional spring clutch system in a dirt bike is about 65 hp. Beyond that hp point, the spring pressure required makes the lever incredibly stiff and the outer plate distorts under disengagement (pulling in the clutch lever), which causes the clutch to drag (remember that’s bad).
“Ok Kelly, if that’s the case, then what about street bikes that make 150+ hp?” The answer has to do with surface area. Increased surface area of the friction system enhances the performance and power delivery of the clutch. Street bikes use more friction plates and are usually larger in diameter. This increased surface area allows the clutch to transfer more power with less pressure.
What options are there if you are going to build a big horsepower bike? There are 2 primary systems that will get the job done and keep the clutch feel good and the first one is a lockout.
A lockout is essentially a centrifugal clutch, meaning it uses rotational velocity to apply pressure. In this system, outer weights apply additional pressure to the outer plate from a fixed point (remember that!) This system has a very high upper limit because pressure is only supplied from the lockout during clutch engagement (lever out). This is important because of one simple reason: There is no change in how the clutch lever works or feels. The clutch will feel identical and operate as if it were stock. You can even use stock clutch springs. Because of this, you can essentially apply as much pressure as you want. In many cases, up to 1000 lbs. or more. At that point, the efficiency of the clutch is maximized and clutch life expectancy becomes very long.
“Wow! A 1000 lbs. sounds like a lot…Will my clutch break under all that pressure?”
The answer is absolutely not. Clutches love pressure. More pressure = lower friction temperatures and higher delivery = longer life. This question is better answered with a hydraulic press. The fear is that the plates will “shatter” under all that pressure. So here is the test: Imagine you have a clutch stack in a hydraulic press. The stack has an upper and lower plate to even out the load (outer pressure plate and inner hub). Now crank away. The fibers and steels will compress tighter and tighter, but because the load is even they are totally fine with this. As a matter of fact, we tried this and what happens if you keep jacking is that the fiber material will literally smear into an even coat. The fiber squares start to get wider and touch each other and that occurs at about 40 tons of pressure. So 1000 pounds is a walk in the park for these guys.
The only downside to the lockout is space. This guy is relatively large and requires about 1.25" additional clearance. In most cases, this can be achieved with a a simple spacer.
The other Clutch Accessory option for big power is the Slipper. Rekluse is the most common these days. There are actually many different styles of slippers, however, for snow biking, Rekluse seems to be king of the Slippers.
How it works. The clutch has 2 main features. A preloaded stack spacing (this is what provides the slip at idle) and a centrifugal wedge plate that applies pressure from a sprung surface (different than the fixed point of a lockout). Ok, so here’s what happens. If you are at idle the clutch pack has a gap. There are clutch springs supplying pressure to the outer pressure plate, however there is an inner collar that supports the plate and provides the gap. This gap spacing can be experienced with any conventional system by having a clutch cable that is too tight.
The second feature is the wedge plate:
There are 2 styles of wedge plates, roller (ball bearings) or slider (EXP). Both work on the same principal, yet the slider systems offer a cleaner system with few moving parts. The wedge plate is sprung together and when the velocity is high enough, the inner weights will travel up a ramp causing the wedge plate to expand. This expansion, first takes up the preset gap creating clutch engagement (bike starts to move). The really cool part is what happens next. Remember those clutch springs on the outside, well as the wedge plate approaches full expansion, it over comes the springs and pushes the pressure plate out. This in turn, over compresses the clutch springs. For example, if you have a spring system that is supplying 325 lbs. of force on the pressure plate, the change in the installed height increases the pressure. What? Basically the wedge pushes back onto the springs making them shorter, therefore stiffer.
As cool as this system is, there is an upper limit…the wedge can only expand so much. For example, an installed pressure of 350 can only go to about 400-415. Definitely better than the conventional, however much lower than a lockout. Why? It is pushing against your springs and not a fixed point.
“Hey Kelly, I really like my Rekluse for snow biking. Can I have my cake and eat it too?” Yes. You can actually put a lockout on top of a Rekluse. You know when I told the guy at Rekluse that I was going to build one of these, he said that I was nuts. However, the 1st one that I built is on season #4 on the same clutch.
This setup is way cool because it gives you both a slipper function with the pressure capability of a lockout. There’s no limit on this bad boy! However there is some bad news. The new Rekluse EXP 3.0 has holes in the pressure plate right where the lockout weights go. There is nothing but empty space for the weights. The old style, full faced, Rekluse pressure plates still work, but the new ones are a no go.
Thanks for reading and questions are always appreciated.
