Choosing The Right Circle Track Clutch
There are many considerations when choosing a clutch that best meets your needs. The selection of clutches on the market is quite large, with a wide range of diameters, friction materials and disc counts. The general rule for selecting a racing clutch is to choose the smallest clutch diameter allowed by sanctioning body rules. After you find your selected clutch diameter you should determine how many discs it takes to meet your engine's torque capacity then add one extra disc for heat capacity and durability. Below is a list of factors to consider when choosing the right clutch.
- How clutch selection effects durability: How often do you want to look at the clutch? Do you not want to look at it for another two seasons of racing? Or, are you going for the championships and are willing to perform the extra maintenance required on the lightest assembly available? These are two different clutches. Adding an additional plate to the clutch may increase initial cost by 30% while doubling the life of the clutch. Another option to adding a plate is to select a larger diameter clutch. Removing one plate from the clutch will increase engine performance, but long-term clutch life may be reduced. In short, there is a trade-off to be made between weight and maintenance intervals.
- Choosing a clutch that fits your sanctioning body rules: It all starts with what clutches are legal for use in your racing series. Most sanctioning bodies rules specify a minimum clutch diameter. The minimum clutch diameter rule is based on the diameter of the friction/driven discs, not the diameter of the entire clutch. In addition, rules typically specify acceptable clutch friction materials. Racing clutch friction materials, as far as the rules are concerned, are divided into two categories: metallic and carbon/carbon.
- Understanding what limits clutch performance: Torque capacity and heat capacity are the two main factors that limit clutch performance. Torque capacity refers to the engine torque that the clutch will hold before slippage occurs. Torque capacity ratings among clutch manufacturers cannot be directly compared. Racing clutch manufacturers do not have an industry standard with which they set clutch torque ratings. Generally, a Tilton OT-Series clutch does not slip until the torque is 50% above the rated torque capacity, making the rating rather conservative. On the other hand, another clutch manufacturer may rate a clutch at the torque level it starts to slip in an effort to provide a more impressive rating. One can usually use the torque ratings to compare clutches from the same manufacturer, but not from different manufacturers. Heat capacity refers to the amount of heat the clutch can withstand before damage or failure occurs. Whenever a clutch is engaged (slipped) it generates heat. The friction discs and floater plates of the clutch absorb the heat. The more mass a clutch has, the more heat/temperature it can absorb. Just like brakes and tires, higher temperatures do more damage. A clutch with an extra disc can last much longer since it will be exposed to lower temperatures, as a result of its better heat absorption capacity. Carbon/Carbon has the ability to withstand the highest temperatures before being damaged. The racetrack is usually easy on the clutch. It is the paddock area where the clutch must be slipped, raising clutch temperature and that causes the most damage. Even if horsepower levels are equal, a heavier car will require more material (to absorb heat) than a lighter car.
- Choosing the proper friction material: Tilton offers three clutch friction materials: Race, Rally and Carbon/Carbon. Tilton Race clutches feature a sintered metallic friction material. The friction disc is very thin, yet has excellent wear resistance and withstands fairly high temperatures. Tilton Rally clutches feature a cerametallic friction material. Compared to the Race clutch material, it is much thicker and is smoother engaging. Because of this feature, auto-crossers, drag racers and drivers who occasionally venture onto the streets often use these clutches. Tilton Carbon/Carbon clutches provide the best of many aspects. They are by far the lightest of the three types, superior at withstanding high temperatures (will not warp from heat) and have the smoothest engagement characteristics. Although the initial purchase price is higher, they can cost less per mile to run, especially when you consider that their smooth engagement can help increase the life of the transmission and other driveline components. In addition, carbon/carbon clutch rebuilds are relatively inexpensive. For these reasons, Tilton Carbon/Carbon clutches are becoming more popular in high horsepower street cars, as well as racing, where other clutches fail to live up to the owner's expectations.
- Why you need to set a pedal stop on the clutch pedal: A clutch pedal stop needs to be set to avoid over-stroking the clutch’s diaphragm spring. If the clutch diaphragm spring is over-stroked, the clutch may want to reengage and/or reduce clamp load (reduce torque capacity). In addition, the piston of a hydraulic release bearing assembly may over-extend and come out of its base.
- How to set a pedal stop:
- Raise drive wheels and support the car on jack stands.
- With the engine off and the car in gear, slowly depress the clutch pedal until the driveshaft (open differentials) or wheels (limited slip or locking differentials) turn without clutch drag.
- Press the pedal an additional .25” of travel (measured at the foot pad) and then lock the pedal stop in place.
Following this procedure will allow the clutch to cleanly release, without damaging the clutch.
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