Clutch Basics - Bowler Transmissions

The clutch in a car or truck is probably about as exciting a subject to talk about as what gear oil you run in the differential, but it is actually a vitally important part of the drivetrain equation that you don’t want to overlook. This is the connecting point of the engine and a manual transmission, so if you want to make use of the power under the hood we need to be sure your clutch is matched up properly for your power level and driving style.

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There are a lot of opinions floating around about who makes the best clutch and what is the best clutch for you. Some of it will boil down to personal preference, but getting started in the right direction is the critical point, and from there you should take the time to research several different companies and options to see which one will be the perfect fit for you. This article should serve as an introduction to the basics of the clutches available and their various components. Our hope is to at least arm you with some basic information if you are starting from ground zero.

There are several factors to consider when determining which clutch system will work best for each application. You will need to know the exact engine & transmission, as well as the torque range of the engine along with the intended use of the vehicle. All of this information will allow you to pick a clutch kit that will give you the best holding power, ease of operation, and drivability. We developed this guide more specifically for the street driving and Pro Touring Auto-X enthusiast, but will be beneficial to anyone who wants to learn more about clutch systems.

We will start off with explaining all of the different components that make up a clutch system. Clutch kits will come with some or all of the components needed based on each application. Be sure when selecting a clutch system that you have all the parts needed and they will work with each other and with the engine and transmission you will be using.

Components that Make Up a Clutch System

1. 1. Pressure Plate - This is the pressure mechanism that clamps the disc to the flywheel to get the vehicle moving. Pressing in the pedal relieves the pressure from the clutch disc to disengage the engine from the drivetrain for shifting or stopping.
   2. Clutch Disc - The clutch disc is a flat plate with friction materials on both sides. As the pressure plate is engaged (pedal let out), the clutch disc is clamped to the flywheel. When the pressure plate is disengaged, (pedal pushed in) the clutch disc is unclamped. The disc is connected to the input shaft of the transmission by the splined hub, causing the input shaft to turn when the clutch is engaged, thus causing the vehicle to move.
   3. Flywheel - The flywheel is an inertia device that is bolted to the engine crankshaft. It has several functions including carrying the ring gear the starter uses to crank the engine, storing energy to get the vehicle moving from a standing stop, and providing the friction surface for the clutch disc to be clamped to.
   4.  Release Bearing - The release bearing is also known as a throw out bearing (T.O.B.). It can be a mechanical style or hydraulic. The release bearing is the actuating device that engages and disengages the pressure plate. When the clutch pedal is depressed, the release bearing applies pressure to the fingers of the pressure plate to disengage the drivetrain. When the clutch pedal is released, the release bearing retracts and allows the pressure plate to apply pressure to clamp the disc against the flywheel.
   5. Release Fork - The release fork is only used in applications that use a mechanical release bearing, it holds the release bearing, and pivots on a ball stud as the pedal is pressed in or let out. Pushing in the pedal pivots the fork towards the pressure plate and forces the release bearing against the clutch fingers, pressing them in to disengage the clutch.
   6. Pilot Bushing - The pilot bushing or bearing installs in the end of the crankshaft. When the transmission is installed, the input shaft tip inserts into the pilot bushing, which supports the input in the back of the crankshaft.

Friction Materials

Next we will discuss the different types of clutch friction materials. This is one of the most important factors in choosing a clutch. Choosing the correct type of clutch friction material for your application is essential in providing the correct amount of grip needed for the torque of the engine as well as providing the type of clutch engagement, noise level, and drivability you are looking for. You will also find that there are clutch discs out there that are made of a combination of materials in the list below. These types of clutch discs allow the user to achieve more than a single purpose when utilizing that particular style of clutch disc.

ORGANIC - Organic material is common to all stock clutch discs, and offers the best drivability but has limitations as operating temperatures rise. Under high loads accompanied by slippage, they fade because their coefficient of friction drops off. In addition, at high RPM and high temperature they tend to fail structurally. An organic clutch works best when used in conjunction with a stock type engine for normal street driving.
KEVLAR COMPOSITES - Kevlar material offers a higher coefficient of friction than organic material, but with some loss in drivability. As the coefficient of friction goes up in the disc material, so will the aggressiveness of the material on engagement. This means that some chatter can be expected with this material in low gear and reverse. Kevlar is compatible with stock flywheels and pressure plates, making it a good upgrade choice for later model vehicles. We do not recommend Kevlar for competition applications.
BRONZE METALLIC - Bronze metallic (paddle discs) is the most aggressive materials in terms of engagement. It offers extended life using reduced static pressures, and a quick, clean engagement. Bronze metallic is aggressive on the flywheel surface and should be used only with steel or nodular iron friction surfaces. On the street, this material will cause chatter on engagement, so we typically recommend this material for race applications.
SINTERED IRON - Sintered iron is well known for its ability to withstand some slippage and not lose its coefficient of friction. It is the material choice for high horsepower clutch applications for drag racing and truck pulling. A key feature of the sintered iron material is that it is maintainable. If the clutch is slipped excessively, the disc can be resurfaced and reused. This would be the most extreme material used and not recommended for street applications.

Flywheels

Another important factor in determining the clutch system that is appropriate for each vehicle is the type of flywheel needed. There are 3 main types of flywheels, nodular or cast iron (most commonly used in factory applications), billet steel, and billet aluminum with a steel insert. Each type of flywheel serves a different type of use. For most customers a cast iron or billet steel flywheel will be their best choice. Most street cars will benefit from a heavier flywheel to generate the inertia to get you moving. An aluminum flywheel will not generate as much inertia to move the vehicle, and thus it would be necessary to slip the clutch more on takeoff.

Some street vehicles may benefit from a lighter flywheel, but only if there is enough rear gear to help you transition the clutch smoothly without excessive slippage. The cast iron flywheels are best suited to vehicles creating up to 500 ft/lbs of torque, billet steel is necessary once you are beyond that mark, but can be used in all applications. Also note that the aluminum flywheels with the steel inserts are serviceable. The steel insert can be replaced when worn without the need to replace the entire flywheel. Steel flywheels are also serviceable by refinishing as long as there is enough material thickness available.

Contact us to discuss your requirements of clutch components and their functions. Our experienced sales team can help you identify the options that best suit your needs.

Single vs. Dual (Twin) Disc Clutches

There are 2 main styles of clutch kits used in hot rodding. Single disc and Dual disc. Each plays a specific role and can be used in a wide variety of applications. Single disc clutches are the most economical and widely used in street applications. They rely on one disc with a friction surface on both sides, which is typically the same unless noted otherwise. The single disc systems are larger in diameter to meet surface area requirements to provide enough grip.

Dual disc systems are designed for late model performance cars and newer engine swaps into older vehicles. Dual disc systems rely on two smaller diameter clutch discs sandwiched between the flywheel, a floater plate, and the pressure plate effectively giving the clutch twice as much surface area to grip against. Think of it like the Big Mac of the clutch world, the burgers are the clutch discs and the buns are your flywheel, floater, and pressure plate.

The purpose of these clutches is to provide smooth drivability for high power levels that was previously not available unless using aggressive friction material single disc clutches. The drivers of road-race inspired vehicles will benefit from the lighter weight of a smaller diameter Dual Disc clutch as it allows the vehicle to drive deeper into the turns before letting off the throttle, and accelerate back to torque range quicker coming out of the turns.

Typically the split from single disc to dual disc will be in the 500-600HP range. If you are unsure about which type of clutch to select based on your known horsepower or torque rating it is usually best to step up to the next highest level. It is not advised to back down to a lower rated clutch. There is enough variety in clutch systems that you should be able to find something to suit your needs without much trouble. As always, take the time to ask questions and feel comfortable with your decision. If you have any questions, Bowler Performance is always here and ready to help you make the best decision for your needs.

Stage 2

When to select a +Clutch+and+Flywheel+>+Clutch+Kits&fClutch-Stage=Stage+2">stage two clutch? The primary differences with a stage two clutch have to do with the type of disc offered; but typically the pressure plate clamp load is the same for stages one through five. A carbon-kevlar disc is considered an upgraded disc, and is generally offered in all stage two clutch kits; these provide, roughly a 50-75% increase in torque capacity. This disc requires a very strict break-in period, which can determine how long the clutch will last while installed. The kevlar material is much more durable than an organic disc, but is more prone to glazing caused by heat. Slipping the clutch excessively is something that is not recommend during the break-in, nor after. The kevlar material will naturally glaze when it heats up to a point, but will return to its normal state once cooled down. So it is important to remember not to constantly abuse the clutch disc which can “cook” the disc, thereby destroying it and potentially causing clutch failure. This type of clutch should be selected when you have more than all-motor modifications such as: cams, pistons, small shots of nitrous. A Stage two clutch is also recommended for supercharged applications making under 300whp.

Stage 3

K series applications don’t have many options when it comes to a clutch due to the type of disc that is used. The brands that do offer a stage 3 clutch are: Clutchmaster, and Action clutch. These discs are composed of a Kevlar material which offers an even higher increase in torque capacity than carbon-kevlar can. These disc require an extremely strict break in period to ensure you get the full life out of the clutch. The torque capacity is generally 75-125% increased over stock, applications that this type of clutch would be used in are advanced all motor builds that are still street driven. Supercharged and turbocharged setups making sub 350whp could also select this type of clutch for daily driving.

Stage 4

At the top end of single disc clutches are the and +Clutch+and+Flywheel+>+Clutch+Kits&fClutch-Stage=Stage+5">stage five. These clutches usually offer 6-puck cerametallic sprung/unsprung discs in a stage four and 4-puck cerametallic sprung/unsprung discs in a stage 5. Cerametallic clutch discs offer sharp engagement that aren’t meant to be slipped more than needed because it will wear down the surface of the flywheel, pressure plate, and clutch disc faster than intended. The torsion springs in discs that include them dampen the shock to the drivetrain when engaging the clutch, it is recommended for street applications to always use a sprung disc. The Competition Clutch stage four clutch has been the benchmark for higher horsepower turbocharged and supercharged applications for the K-series motor because of price and drivability. We recommend going with this level of clutch when in the 400-500whp range although we have tested it to hold even more horsepower.

Flywheels

The final step in deciding your clutch and flywheel set-up is what type of flywheel to run with the different clutches. Let’s start by discussing the weight of the stock RSX Type S flywheel, it is approximately 16 pounds. There’s only one reason stock flywheels are heavy, and that’s because car engineers design them to be that way. Car manufacturers want manual transmission vehicles to have a consistent and easy driving feel when stock. Increasing performance of an engine by lightening or reducing rotating mass is a relatively simple process, but there are trade-offs. Aftermarket clutch manufacturers usually produce two types of flywheels: lightweight and ultra lightweight. The +Clutch+and+Flywheel+>+Flywheels&fFlywheel-Weight=11.5+pounds">lightweight flywheels produced for the K series come in at a modest 11.5 pounds which frees up approximately 4.5 pounds and adds a little bit of top end horsepower but still maintains the lower end drivability. This is our most highly recommend flywheel weight because it doesn’t completely sacrifice the comfort of a stock weight flywheel but allows for better engine response and top end horse power. The +Clutch+and+Flywheel+>+Flywheels&fFlywheel-Weight=8+pounds">ultra-lightweight flywheels come in at 8 pounds and are made by a few manufacturers, these are approximately half of the weight of a stock flywheel. You will notice increase engine response and a moderate horsepower gain on the top end. With so much reduced weight in the clutch assembly drivability in stop and go traffic becomes an issue. The reduced mass requires more rpm to get the car moving as the clutch drags on the flywheel harder, because of the reduced weight. Another side effect of a lighter flywheel is commonly referred to as clutch chatter. It’s a sharp vibration in the clutch assembly that transfers as noise through the chassis and feels like something is vibrating loose. Clutch chatter can also be caused by increased pressure plate clamp loads and clutch discs used in the clutch kits, and is not always a result of the flywheel you have selected.

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