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Sigma Performance slipper clutches have been used in bike engined racing cars since inception , from a Ducati engined hill climb car to a winning Formula Jedi races in an Yamaha R-1 powered
open wheeler.
Recently we have put several R1-03 clutches into Global Lights cars and we now have student engineers using them in Formula SAE cars.
Rather than just have us spout off as to how everything works have a look at this report from the University of Illinois at Urbana-Champaign and their Formula SAE racing team, although
written from a car standpoint it sums up the way things work from either the car or bike point of view quite nicely!
Slipper Clutch Use on a Formula SAE car
Dan Reif, UIUC Formula SAE Team
Shortly after discovering the existence of Sigma Performance's slipper clutch, I immediately recognized the benefits it would bring to a Formula SAE (FSAE) car. For those who are not familiar with FSAE, it
involves the design, fabrication, and competition of a small purpose-built autocross car.
The rules of the FSAE competition purposely leave plenty of room for engineering innovation - featuring no minimum weight, an air-restricted engine, as well as the obligatory set of safety-related mandates. The engine's capacity is limited to 610cc's, and as a result many teams choose to use engines out of 600cc class sport bikes. These engines are the weapon of choice due to their light weight, high output, attached clutches and transmissions, and near bulletproof reliability (in stock form).
Formula SAE competitors are nearly universally undergraduate engineering college students - rarely are they highly experienced race car drivers.
This point alone makes it worthwhile to design a FSAE car to be as easy to drive for beginners as possible. We have found that one of the things beginners have the most trouble mastering is a racing downshift. To do this correctly one must pull in the clutch and rev the engine while downshifting. Then the clutch is let out while the rpm level is starting to drop off again. Further adding to the difficulty of this rev-matching is the fact that this whole operation must be orchestrated while braking hard and beginning to turn - most likely into a tight corner. As you may imagine, our totally inexperienced, but supremely confident, beginner drivers are able to master this technique instantly, and hence go on to achieve the most coveted motorsport glories within FSAE and beyond! For others, the consequences of making a mistake during the learning process include, over revving the engine, momentarily accelerating instead of braking, locking the rear wheels when the clutch is disengaged, hence upsetting the balance of the car and possibly inducing a spin, and, worse still, all the while introducing large shock loads into the drivetrain. Luckily there is a solution.
Installation of a Sigma Performance slipper clutch has eliminated all the
problems described above. The slipper clutch allows the driver to simply lift off the throttle, apply the brake, and push the car down into a lower gear. This also greatly reduces the consequences of downshifting too early. There is no need of rev matching as the drivetrain will allow the crankshaft to over rev until the crank and transmission input speeds are equalized. Essentially, the slipper clutch acts like a one way clutch with an adjustable amount of preload. Whenever the wheels try to drive the engine, ball bearings ride up on a ramp inside the clutch and automatically lift the pressure plate off the clutch pack, effectively pulling in the clutch for you. Then when you apply the throttle (on corner exit) the ball bearings come back down off the ramp and the clutch is seamlessly re-engaged. Due to the design, the engine is never allowed to rev farther than necessary without having the clutch automatically re-engage. This means that it is not possible for you to accidentally rev the engine up too high and have the clutch engage abruptly causing the rear end of the car to become unsettled.
Another benefit alluded to above is the fact that the driver has the ability to downshift as soon as he is done accelerating, effectively allowing him to have one less thing to worry about at the critical
corner entry time. You will also notice no mention made of needing to touch the clutch during the downshift. We have found that the amount of preload required for the slipper clutch to automatically
disengage is low enough that we are comfortable simply stuffing the car into the lower gear. At the time of this writing we have not noticed any abnormal problems that could be a result of doing so.
Another selling point of Sigma Performance's slipper clutch is it's weight.
Every sport bike engine needs some sort of clutch, and the slipper clutch provides a long list of benefits for only a handful of grams more that the stock clutch. Their clutches are a high quality anodized aluminum and lack the casting defects often found on the stock units.
By now the reader is probably thinking: "That sounds all fine and great, but what is it like to drive such a car?"
My answer to this question is that it is less work for the driver than any other similar car I've ever driven. The downshift is noticeable to the perceptive driver but it is never harsh in the way a poorly executed manual downshift is. To me, one of the greatest benefits is the ability to downshift when I begin my braking instead of at the end of my braking when I am starting to turn in. I've noticed that this enables me to focus on my brake and turn in points more, and that I make fewer mistakes when doing so. I've also noticed that it is easier to brake harder without locking up the wheels or free-wheeling while attempting to stab at the throttle. Another nice feature is that the preload seems to be just enough for the engine to keep revving above the range where you are in danger of stalling (in the unlikely case your car happens to have a less than optimal idle). On upshifts, the clutch functions just like the stock clutch, the feel is nearly identical and there is no special procedure needed to shift. In other words, you still have to use the clutch or risk harming the dogs in the transmission. Of course, in the end, all that really matters is that my lap times are lower!
Copyright Sigma Performance, 2005
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