28/08/2016
Managing the Useful Life of Your Clutch
Wheeling puts a huge strain on your clutch. The life expectancy of a clutch on a street vehicle can be 100,000 miles — but if you venture off the pavement frequently, you’ll be lucky to get 40,000 miles out of it. The good news is, there are measures you can take while driving to maximize that mileage.
Driving tips to protect your clutch
Most off-roaders do abuse the clutch to some extent, and that’s to be expected. It is possible, though, to make some small adjustments to minimize clutch wear, without taking all the fun out of your ‘wheeling.
First, don’t slip the clutch when you can avoid it. Also, keep your left foot on the floor when you aren’t actively shifting. Do not rest your foot on the clutch pedal or hover over it. Doing so causes the throw-out bearing to connect with the pressure plate, which leads to unnecessary wear. Even the slightest force on the pedal is multiplied at the clutch, which reduces the clamping pressure. The clutch could slip as a result.
When idling in place for long periods of time, on- or off-road, make sure you put the vehicle in neutral and release the clutch. Just this will extend the life of your clutch and, in particular, the life of your throw-out bearing.
Selecting a replacement clutch for your 4x4
Considering the way you use and abuse your 4x4, stock clutches generally aren’t adequate for frequent off-road driving. This doesn’t mean you need to replace the clutch if it still works. But know that your stock clutch will eventually give out, because off-road driving is inherently clutch-intensive. When that happens, be prepared to invest in a quality clutch rather than a cheap, rebuilt model. Buying the cheapest clutch you can find in the auto parts store will only end up costing you more in the long run.
A Centerforce engineer explains, "Our goal is to increase the torque capacity and durability of the clutch while retaining stock, or at least near stock, pedal effort. We get that extra capacity a few different ways. The first would be with improved friction linings, which we believe to be a large percentage of the game. The second would be in the design of the pressure plate. We use ball bearings at the pivot points of our premium pressure plate lines so we can offer more clamping pressure without more pedal effort. We also incorporate our weight system which uses centrifugal force to increase the clamping pressure as rpms increase. This effect starts as low as 500 rpm and increases rapidly with rpm. It's effective even at the low rpm ranges most four-wheelers operate in."
Setting up your vehicle
It’s really important to make sure your vehicle is geared properly. Weight, tire size, and terrain all impact the appropriate gearing for your 4x4. Even if the gearing is off just slightly, your clutch will face more wear and will break down that much faster.
You’ll know the vehicle isn’t geared correctly if you find yourself slipping the clutch to avoid stalling. As noted above, terrain is a factor here. Maybe you venture into unexplored territory on occasion and you have to slip the clutch to get across the more difficult landscape. That’s certainly reasonable. But if you are driving that way every time you leave the pavement, consider a gearing adjustment — unless you want to get stuck out on the trail with a burned-up clutch.
Notes on gearing
If you only drive your 4x4 off road, the appropriate gearing is more subjective. Your gearing should be low enough that your vehicle can idle up a shallow grade in low gear without stalling. Then, from there, you should be able to accelerate without issue. Rockcrawlers usually need the lowest gearing, because they need to climb more aggressive terrain, basically at idle, and then respond to acceleration from there.
"Rockcrawling is the prime example of where a good clutch becomes a vital element in the trail durability equation. However good the clutch though, it doesn't make up for having the wrong gears for the tire size, weight and engine power of a given vehicle."
All of this must be balanced with your budget and how often you drive your 4x4 on the road.
Clutch specifics
A clutch’s ability to link the engine to the drivetrain is based on three factors:
For on-road and off-road vehicles, the clutch must demonstrate a smooth transition between the released and connected states. Clutches that grab quickly are better suited for drag racing. These basically function like a toggle switch, such that the clutch is either connected or it’s not, and there’s no middle ground. This doesn’t serve you when you’re navigating an unpredictable trail and you need full control of your vehicle.
Appropriate torque capacity
The torque capacity of a clutch should be more than the torque output of the engine. The amount over and above the engine’s torque output compensates for clutch wear. Stock clutches tend to be 5% or 10% above stock engine torque. Aftermarket performance clutches are typically higher, at 20% or more above stock engine torque. The application determines the actual torque capacity of a performance clutch.
Static pressure/dynamic force/clamping force
Static pressure, dynamic force, and clamping force all basically refer to the pressure exerted on the disc by the pressure plate. Dynamic force is the term used for pressure plates that have centrifugal assist to increase their clamping force.
You can increase a clutch’s torque capacity by increasing the static clamping force. This is an inexpensive strategy, but it has drawbacks. For one, the pedal will be stiffer and will therefore require more physical effort to depress. This might not seem like a big deal until you spend a day on the trail working the clutch. Remember that fatigue can impact your driving, to the point of causing safety concerns.
Another drawback is that your clutch release components, particularly the throw-out bearing, will face extra wear. A small increase is acceptable, but a large increase may cause more problems than it solves. The additional force could result in premature wear of the engine’s crankshaft thrust bearing. Keep in mind that thrust bearings vary in durability.
Clutch lining
Different manufacturers use different terms to describe lining materials. The terms you might see include carbon, carbon fiber, ceramic, composite, metallic, and Aramid. All of these can be grouped into either organic or metallic materials, but this distinction may not be clear when you’re researching your clutch options.
Clutch linings function the same way brake linings do. When you combine a lining recipe with a particular surface area, you get a coefficient of friction or COF. This figure, expressed as a decimal, refers to how well the material grips to the flywheel and pressure plate. You’ll get a different COF if you increase the frictional component of the lining material or the surface area.
Organic linings consist of a fibrous material for structural strength, plus a metallic wool and fillers for friction. Fiberglass, carbon, Aramid, and Kevlar are some of the more common fibers used. These have replaced asbestos, which has been phased out by regulations. The wool is typically bronze, copper, or steel. A resin is used to bond everything together.
Some organic clutches will be labeled as metallic. This just means the material has a high metallic content. An organic clutch with a lot of copper or steel wool in it will generally be more durable.
Metallic and ceramic clutches share a similar manufacturing process. The materials are heated up until they are pliable. They are then molded under pressure. For metallic linings, also called sintered metal linings, the materials include powdered iron, copper, or bronze, plus carbon and fillers. This type of lining can grab quickly, which increases the wear on the flywheel and pressure plate.
Ceramic clutches are similar to the metallic ones. The main difference is that ceramics have a lower metal content. A ceramic clutch tends to transition more smoothly between released and engaged states.
Whether the lining is organic, metallic, or ceramic, the materials used, including any resin, impacts the COF and heat tolerance. A clutch with a very high COF will grab quickly, which impacts the drivability of your 4x4. With certain types of linings, the COF can be impacted by heat. Ceramic and sintered metal clutches are more resistant to heat, but they can lose COF if the temperature drops.
Heat tolerance
Off-road driving will cause your clutch to generate a lot of heat. This high heat environment will primarily impact the linings of your clutch. Performance clutches adjust for the added heat, usually by using better lining materials. An expensive metallic racing clutch, for example, might have a temperature rating of 1200 degrees. A good clutch for ‘wheeling does not need this level of heat resistance. Centerforce engineers tell us they target a temperature rating of 650-700 degrees. This is an upgrade from a relatively good stock organic clutch that might suffer under temperatures above 500 degrees.
The pressure plate and flywheel also impact a clutch’s heat tolerance. This is because the mass of each helps absorb and disperse heat.
Disc and pressure plate design
Most pressure plates for ‘wheeling applications are the diaphragm style. Older rigs sometimes have lever-style pressure plates, but these have less clamping pressure and require more physical effort to depress the pedal.
You can improve the pressure plate clamping pressure and torque capacity without significantly increasing pedal effort. Some options include:
As an alternative to the above methods, you could use segmented discs to divide the lining. This design is called puck- or puc-style. With this technique, a super-premium lining is used. The clamping pressure per square inch is increased because the pressure is concentrated on a smaller lining area. Race clutches use segmented lining on both sides of the disc. They’ll generally have a lot of torque and will grab quickly. That isn’t ideal or even appropriate for off-road driving, because you need to be able to slip the clutch in certain situations. A clutch that grabs too quickly won’t last very long in a 4x4 vehicle.
Centerforce engineers use a variation of this design as a compromise for the off-road driver. The Centerforce Dual-Friction clutch modifies the segmented design to create a smoother transition between released and engaged states. The design uses segmented lining on the flywheel side and a full lining on the pressure place site. This clutch is smoother than a race clutch, but still may grab too quickly for some types of off-road driving.
Another high-torque option is a dual-disc clutch designed with a second pressure ring between the two discs. This type of clutch will have a higher torque capacity, without a huge increase in pedal effort. Drivers of big trucks and diesel pickups sometimes prefer this design, although these clutches are typically pretty expensive.
Other clutch designs have a solid disc and no spring hub center. This design probably adds more problems than it solves. Spring hubs absorb chatter, so you’ll feel a lot of roughness without it. You’ll notice the difference more if your clutch lining has a high COF.
The flywheel
Flywheels transmit torque, dissipate the heat of friction, and dampen vibrations. Racers prefer a light flywheel and pressure plate because they need to rev their engines high. As a four-wheeler, you want your flywheel and pressure plate to be heavy. A weighty flywheel minimizes stalling when you are hauling at very low rpms.
Flywheels can be made of grey iron, nodular iron, billet steel, or aluminum. Grey iron is the weakest material of the three. The nodular iron flywheels wear better and are more resistant to shattering than grey iron units. Billet steel flywheels are very durable, but also very expensive. For this reason, most ‘wheelers will upgrade to a nodular iron flywheel when their OE flywheel or clutch gives out. Aluminum flywheels negatively impact low-rpm performance, and so are not well-suited to off-road driving.
A visual inspection of a flywheel may not reveal whether it is grey iron or nodular iron. Some, but not all, nodular manufacturers stamp their flywheels with an “N.” If you see no “N,” you’ll have to hang the flywheel and tap it with a hammer. The hammer’s impact on a grey iron flywheel will sound like a dull thud, while the nodular flywheel will make a ringing sound.
You will need to get the flywheel surfaced when you replace the clutch. This process takes off glazing and creates a smooth surface for clutch engagement. Keep in mind that some vehicles have a stepped flywheel, meaning the friction surface is higher or lower than the pressure plate mounting surface. As you might guess, the clutch won’t work properly if you don’t maintain the dimensions between these surfaces. Flywheel replacement is much simpler when the vehicle has a flat flywheel, such that the pressure plate mounting surface and the friction surface are on the same plane. Check your vehicle’s manual or ask your clutch manufacturer if you aren’t sure.
Breaking in your clutch
You will need to break in your clutch. The lining surfaces don’t meet up perfectly with the steel surfaces until their high spots are worn down. In other words, the lining will not initially make full contact with the steel surfaces. Therefore, your clutch won’t have its full holding power right away. Drive too aggressively and you’ll cause the clutch to slip, which leads to glazing of the disc surface, the flywheel surface, or the pressure plate surface. Glazing will reduce the clutch’s holding power.
Centerforce engineers tell us you need to drive your clutch at least 500 miles to break it in. This 500 miles has to be stop-and-go driving, with gentle engagement and disengagement of clutch. Do this right, and the high spots of the steel surfaces will be worn down so that the disc makes full contact with the flywheel and pressure plate. Also during this break-in process, microscopic pieces of the lining transfer to the steel friction surfaces, and that improves your torque capacity.
Clutches for custom applications
You may have trouble selecting the right clutch if you’ve swapped out the engine in your vehicle. The general rule of thumb is to use a clutch that’s rated for your engine. You may need to make adjustments for where and how you are driving, plus tire size, weight, and gearing. In some cases, you might have to install a smaller clutch than is ideal because of the size of the clutch housing.
If you aren’t sure, contact the people at Centerforce or another clutch maker and ask for advice. Centerforce engineers do have custom applications that are not listed in their catalog. They’d be the right people to ask when you aren’t sure what the right setup should be.
Centerforce clutches for 4x4 applications
Four-wheelers tend to choose from one of three Centerforce clutch designs, the Centerforce I, Centerforce II, or the Centerforce Dual-Friction clutch. The following specs are for a six-cylinder Jeep TJ, and the percentages refer to the increase over the stock clutch.
