There's finesse in using the clutch and there's finesse in picking a replacement for the one you finessed to death. The clutch is a very hard worked part of a manual trans-equipped 4x4. On a street rig, getting 100K miles out of a clutch is fairly easy. The life expectancy for a clutch installed in a 'wheeling rig... who knows? Maybe 40K, perhaps even less on a rig that 'wheels a lot.
In this installment of "Finesse," we'll talk about getting the most from a clutch. Given the alterations most 'wheelers make, and the way they use their rigs, few stock clutches are up to the task. We're not recommending immediately tearing your rig down, but when the time comes to change the clutch, give it more serious attention than just going down to the auto parts store and buying the cheapest rebuilt you can find.
"Our goal," a Centerforce engineer said, "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."
DRIVING TECHNIQUE
Are you a clutch abuser? It's not easy to 'wheel hard without some clutch abuse and that's why a better clutch is often needed. The ideal, of course, is to slip the clutch as little as possible. If your rig is set up right, you won't have to slip it all that much.
The general rule is to keep your left foot planted on the floor between shifts. Don't "hover-foot" the clutch pedal. Even the slightest pressure will engage the throw-out bearing (T/O) with the pressure plate release fingers and cause wear. Every pound of pressure on the pedal is multiplied many times at the clutch, reducing clamping pressure and potentially allowing the clutch to slip. Even in town, when idling stationary for long periods in traffic, pop the trans into neutral and let up on the clutch. Every minute you do adds to the life of the clutch, especially the T/O.
IMPORTANT VEHICLE PARAMETERS
Above all, your 4x4 needs to be correctly geared for its weight, tire size and for the terrain in which it 'wheels. The farther away from just right, the less life you can expect from your clutch. Any clutch. It's as simple as that.
If you have to slip the clutch a lot to avoid stalling on the trail, you aren't geared correctly for the terrain. We gotta be a little reasonable here too. You could be set up perfectly for where you normally 'wheel, then go farther afield for a oncein- a-while foray into harder terrain and have to slip the clutch a bit. Nobody will call the clutch abuser's hotline. If, however, you are continually stinking up the outdoors with the smell of toasted clutch, you may find your face on a website and a GPS bracelet on your left ankle!
We've covered gearing several times in "Finesse," so we'll only hit it very briefly again here. For combined street and trail performance, use the equivalent ratio formula in the nearby sidebar. Compare the numbers from your original tire size to your current or projected size. If your axle ratios were taller than 3.55:1 (2.76, 3.07, etc.), use 3.55 as the current ratio and you'll get better trail performance results because the taller gears usually resulted in marginal performance even with the stock tires. Because the numbers from the formula will not match the available ratios, go to the next lowest available ratio for the best trail performance and least amount of clutch abuse.
Gearing strictly for trail performance is more subjective, with rockcrawlers needing lower gearing than most others. With your foot off the clutch, a stick shift trail rig should be geared low enough to idle up a shallow grade in low gear, low range, not stall, and have enough grunt left to accelerate from there. A hard-core rockcrawler needs to be able to climb more radical stuff, more or less at idle, and still have the grunt to accelerate. You have to compromise with your wallet, of course, as well as your non-wheeling environment, and that may mean delayed gratification of your big-tire dreams.
"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."
CLUTCH PARAMETERS
Clutch performance, namely the ability to connect the engine to the drivetrain, boils down to three things; the ability to handle heat, torque capacity and driveability. For street driven and 'wheeling rigs at least, it needs to exhibit a smooth transition between the released and connected states. "Grabby" clutches, which are kinda on and off like a switch, result in neck-snapping, finesseless takeoffs. These are fine for a "rev-n-dump" dragstrip takeoff but not for a tricky trail maneuver where you need maximum control.
TORQUE CAPACITY
Torque capacity should at least equal the torque output of the engine, but the clutch manufacturer usually gives it a bit of extra capacity to compensate for clutch wear. Torque capacity can be a hard specification to find. In scanning the few OE ratings I have at hand, there is an average of about 5- 10 percent over stock engine torque. The aftermarket performance clutches for stock applications I can find seem to be around 20 percent over stock engine torque, or more. The actual capacity is tailored to the application.
CLAMPING FORCE
Clamping force is the amount of pressure the pressure plate can exert on the disc. It can be rated as static pressure or, with pressure plates that have centrifugal assist to increase their clamping force with rpms, dynamic force (static plus the centrifugal assist). Increasing the static clamping force is one way to increase the torque capacity of the clutch. By itself, it's often the cheapest way, but this brute force approach may have tradeoffs. The first is pedal effort. The beef in your left leg will determine how sensitive you are to that issue. More than once, this writer has worked the clutch enough in a day of hard 'wheeling to get the "Jelly-leg" syndrome. Fatigue is an important consideration in 'wheeling that can lead to errors.
The second part of that issue is the extra wear that occurs to the clutch release components, the T/O especially. There's room in any clutch system for some increase, but a huge increase should be carefully considered. The extra force required for clutch release on a pressure plate with a high static rating can cause the engine's crankshaft thrust bearing to wear out prematurely. Some engines have very robust thrust bearings, many do not.
LINING MATERIALS AND TYPES
The discussion of lining materials is a vital one but opens a can-o-worms because each manufacturer seems to have different marketing terms for their own particular recipe. You'll see the terms metallic, ceramic, composite, carbon, carbon fiber, Aramid, and many others. It really boils down to two categories, organic and metallic, but those divisions are not always clear when reading a catalog.
The end result for combining any lining recipe with a given surface area is a coefficient of friction (COF), or the amount of "grip" the material has on the flywheel and pressure plate. Increase the area or the frictional component of the lining and you get a different COF. Brake linings operate the same way and use the same types of materials. COF is expressed as a decimal, such as 0.25.
An organic lining is a composite of materials designed to give structural strength and a frictional component. Fibrous material provides structural strength and metallic (copper, bronze or steel) wool and fillers make up the frictional component. It's all bonded together with a resin. Asbestos used to be the organic clutch structural mainstay, but it has been regulated for health reasons. These days, you'll see fiberglass, carbon, Aramid and Kevlar fibers in its place. The COF will vary according to the materials embedded, but heat tolerance will be limited to the resin that holds it all together.
You will sometimes see organic clutches with a high metallic content called "metallic." The term "semi-metallic" is probably more accurate. Most organic clutches have some copper or steel wool-like products embedded into them. Some have a lot, which adds to their COF and durability.
Metallic and ceramic clutches are very similar in how they are manufactured. The materials are heated to very high temperatures... almost to melting... and when soft, molded under pressure. Metallic linings, also called sintered metal linings, are a composite of powered metal, carbon and other fillers melted together to achieve the desired COF. The metal can be iron, copper, bronze or something else. Metallics tend to be very grabby and will cause more wear on the flywheel and pressure plate than a non- or semi-metallic lining. Linings generally called ceramic have less metallic content than those called metallic. As a result, they may be a bit less grabby.
In all types of linings, the exact mixture of the materials will dictate its heat tolerance as well as its COF. The potential downside of going too far with increasing the COF is making the clutch grabby. Bear in mind that the COF changes with heat according to the type of lining used. Ceramic and sintered metal clutches are the least effected by high heat, but may lose their COF at low temps.
HEAT CONCERNS
Heat is a given with a clutch and in a harsh 'wheeling environment, you will generate a lot... even if you are driving well. The linings tend to suffer the most and the soonest. To counter that, you substitute better lining materials. A good OE organic clutch lining can tolerate temps up to about 500 degrees Fahrenheit. Some OE and many cheap aftermarket replacements are much less than that. A good organic performance clutch will handle 700 degrees. In fact, according to Centerforce engineers, 650- 700 degrees is the target temperature rating for all of their performance clutches. A high dollar metallic racing clutch can be built to handle up to about 1200 degrees. The pressure plate and flywheel are part of the heat equation as well. The mass in the flywheel and in the pressure plate works as a heat sink to absorb and dissipate heat.
DESIGN ISSUES
There are many designs of disc and pressure plates, but only a few translate into the 'wheeling world. The overwhelming majority of pressure plates these days are the diaphram style, though lever style units (see illustrations) are seen on older rigs. Diaphram styles can generally offer more clamping pressure with a lower pedal effort, so if your older style is upgradable, it's usually worth the effort.
Pressure plate clamping pressure and torque capacity can be increased in ways that do not increase pedal effort, or increase it only in small amounts. There are various methods of improving the pressure plate to do this, including decreasing the friction at the pivot points in the pressure plate, and/or increasing the leverage there, and adding a centrifugal assist device to increase clamping pressure with increasing rpms.
Another way is to use segmented discs (a.k.a. "puck" or "puc" style), which divides the lining into evenly spaced segments. A super-premium lining is used, and because of the reduced lining area, the clamping pressure per square inch is greatly increased to allow for the pressure to be concentrated on the smaller lining area. A race clutch will typically use a segmented lining on both sides of the disc and thus will tend to be a very grabby unit. They are not suited to situations where a lot of slipping is needed because they will generate a lot more heat and will wear faster in that situation. They are designed strictly for torque capacity.
A compromise to a segmented lining is the Centerforce Dual-Friction disc, in which a segmented lining is placed on the flywheel side and a full-face lining on the pressure plate side. This adds to holding power while reducing grabbiness and chatter. They are still not well suited to situations where lots of slipping is needed.
There are also dual disc clutches which double all the available surfaces by sandwiching a second pressure ring between two discs. Two discs do not quite double torque capacity, but you can have a greatly increased capacity with a minimal pedal effort increase. Commonly used on big trucks, they have recently found favor with high power diesel pickup owners, but their expense is high and applications are limited.
Though you can buy solid discs without a spring hub center, you don't want one. Talk about chatter and spline-splitting roughness! The spring hub absorbs a lot of harshness you never even feel, especially if you have a lining with a high COF. It must be robust enough for the application.
FLYWHEELS
A flywheel is more than just a big hunk-o-iron. It's the engine driven friction surface that transmits torque. It's also a heat sink, as mentioned earlier, to dissipate the heat of friction. The weight of a flywheel is also a vibration damper and an inertial storage device. A racer likes a light flywheel and pressure plate for high revving, but a four-wheeler likes these parts on the heavy side. The inertia of a heavy flywheel helps an engine avoid stalling in low-rpm lugging situations.
Some OE flywheels are grey iron. The better OE and aftermarket flywheels are nodular iron, which tend to wear better. High revvers like them because they are less inclined to shatter. The top tiers are billet steel flywheels. The cost effective advice for most 'wheelers is to upgrade to nodular iron if or when you have to replace the clutch or the flywheel. Wheelers generally avoid aluminum flywheels because of the loss in low rpm performance.
It's difficult to tell the difference between a grey iron and a nodular flywheel. Some nodular manufacturers conveniently cast an "N" into the unit. Otherwise, if you suspend the flywheel from a hard object (a long punch clamped in a vice) and rap it lightly with a hammer (not on the friction surface, huh!), the grey iron will have a dull thud and the nodular will have a ring.
Getting the flywheel surfaced at clutch replacement time is another vital element. First, it removes glazing and leaves a flat surface for maximum clutch bite. If your rig uses a stepped flywheel, where the pressure plate mounting surface is on a higher or lower plane than the friction surface, you must maintain the correct dimension between these surfaces in order to achieve proper clutch operation. This process is a bit easier if your rig uses a single plane, or flat flywheel, where the friction surface and pressure plate mounting surface are on the same plane. When in doubt contact the clutch manufacturer or your vehicle's shop manual.
BREAK-IN
Yeah, clutches need break-in. The kiss of death to any clutch is to glaze either the surface of the disc or the surfaces of the flywheel or pressure plate. Because the surfaces of the lining are not perfectly mated to the steel surfaces, they actually are not making full contact until they wear off their high spots. That means a new clutch won't have its full holding power initially and if you push it, it may slip. Slipping causes glazing and glazing reduces holding power. The other benefit to break-in is that the lining will microscopically transfer material to the steel friction surfaces and that adds to torque capacity.
So how long does it take to break in a clutch? According to Centerforce, a clutch needs to be driven through at least 500 miles of stop-and-go use to become fully seated. Five hundred highway miles doesn't count. It's the gentle engagement and disengagement and the controlled slippage of getting the vehicle moving that wears off the high spots and allows full contact between the disc and the steel surfaces of the flywheel and pressure plate.
CUSTOM APPLICATIONS
Engine swaps can complicate your clutch needs. In general, if you use a clutch rated for the engine, you end up in the right ballpark. You should factor in the other vehicle parameters like tire size, weight, gearing, etc., especially in situations where you are putting an engine from a light duty car application into a hard working truck. You may also find that you cannot fit the bigger clutch because of a tight bellhousing.
In these cases, don't guess. Pick up the phone and call Centerforce, or whichever performance clutch builder you choose. Centerforce tells us they have custom applications that aren't shown in their catalog. The odds are good they have seen your swap before, but even if not, who better to figure out the right setup than guys who deal with clutches every day
CAPTIONS
The design of the pressure plate will help dictate the combination of clamping pressure and pedal effort. On the left, an arrow shows a standard pressure plate fulcrum. On the right is a patented, Centerforce ball bearing type, which will reduce pedal effort at any clamping pressure.
Let a clutch get like this and your face is on the clutch abuser's hotline ASAP... or should be! This guy either slipped his clutch way too much, put too much horsepower to it, or made some kind of an installation/adjustment error.
These are the two style of clutches you may see in 4x4s. On the left is a diaphram type, which uses a Belleville style spring. On the right is a lever, or Borg & Beck, style clutch that used coil springs. The diaphram type has the edge in pedal effort and trouble free operation.
A puc style clutch disc concentrates the pressure plate clamping force onto a much smaller area. This makes for increased torque capacity but reduced driveability. The number of pucs will dictate both of these factors.
Centerforce Clutches for 'Wheelers
Centerforce offers three clutches of particular interest to four-wheelers, but has many more for towing, high performance street or race applications.We'd like to generalize about the specs on the various designs, but because they vary so much from application to application, we decided to present specs from just one application, a 6-cyl. Jeep TJ, to give an apples to apples comparison. The increases over stock for another application could be more or less than these.
Centerforce I - This set can be looked at as an enhanced stock type clutch. It delivers a performance and durability edge for a stock machine used within its limits, but at an economical price. The Centerforce I will exceed the OE clutch for performance and durability by a significant margin but it's not a true high performance clutch. Engines with big power increases should go to a Centerforce II.
Dynamic Clamp Load: 34.2 percent over stock at 3000 rpm
Static Clamp Load: Average 12 percent over stock
Disc COF: Average 11 percent over stock
Centrifugal Clamp Load Assist: 2000 rpm - 4.9 percent over stock
3000 rpm - 11.2 percent over stock
4000 rpm - 19.9 percent over stock
Centerforce II - While some applications use the same premium disc as the Centerforce I, the upgrade in holding power comes mainly from a pressure plate with ball bearing pivots and a full array of flyweights. As a result, clamping pressure has been increased with only minimal pedal effort increase. This clutch is a good choice for built rigs, or stock rigs that are worked hard. It won't give you jelly-leg and is smooth enough for finesse on the trail. This is the best choice for a 'wheeling machine because it has a full faced lining that makes it smooth, it can tolerate a lot of heat and it wears well.
Dynamic Clamp Load: 46.5 percent over stock at 3000 rpm
Static Clamp Load: Average 12 percent over stock
Disc COF: Average 11 percent over stock
Centrifugal Clamp Load Assist: 2000 rpm - 10.4 percent over stock
3000 rpm - 23.5 percent over stock
4000 rpm - 41.8 percent over stock
Dual Friction - This clutch is about as close to a race clutch as you can get in a clutch that has near stock pedal effort. It uses a segmented lining on the flywheel side for maximum holding power and a premium, full-faced lining on the pressure place side. It's a marginal choice for a 'wheeling machine, especially a rockcrawler. It's more mannerly than a fully segmented disc, but still not suitable for lots of slipping.
Dynamic Clamp Load: 64.52 percent over stock at 3000 rpm
Static Clamp Load: Average 12 percent over stock
Face Loading PSI: 55 percent over stock
Disc COF: Average 15 percent over stock
Centrifugal Clamp Load Assist: 2000 rpm - 10.4 percent over stock
3000 rpm - 11.2 percent over stock
4000 rpm - 19.9 percent over stock
A dual disc clutch has long been a part of the big truck world and has now begun trickling down to the performance diesel markets. They operate much like a single disc clutch but have a floater plate between the discs. The advantage is more fiction surface. The disadvantages are cost, weight and a potential towards grabbiness if not properly tuned for the application.
ANATOMY OF A CLUTCH DISC:
Lining: Organic shown.
Lining Rivet: Attaches the lining to the drive plate.
Splined Drive Hub: The torque connection to the transmission input shaft.
Damper Spring: These springs are the main connection between the lining and the drive hub. They absorb the torque load and dampen out tiny speed fluctuations that might be felt as vibration.
Drive Plate: The plate to which the lining is attached. It spins freely around the drive hub and is connected directly only by the damper springs.
Stop Pin: Limits the travel of the damper assembly when the damper springs are fully compressed. This shouldn't happen much if the correct clutch disc is used.
Radial Groove: Wipes off dust, allows gasses out and cooling air in.
Cover Plate: Holds the damper assembly together and provides structural integrity to the damper.
Marcel Spring: This is a wavy spring, or cushion, between the linings. It provides a cushion for smooth engagement and reduces chatter.
