Clutch 12

To read the earlier parts of this series, go HERE:
Part I
Part II

Double Your Pleasure

Given the single disc “old school” clutch package we discussed in the past two sections, we have to keep in mind that technology has relentlessly marched forward.  In most dedicated racing applications, clutches have shrunk dramatically in size.  Plenty of Pro drag cars function with positively tiny clutch assemblies. The truth is, it’s pretty common to find clutches with (get this) an outside diameter of 5.50-inches.  And yes, there are some “big” set-ups out there that measure anywhere from 7.00-8.00-in in diameter.  Typically, many of these little pressure plates incorporate diaphragm springs, but real key is the use of multiple discs (two, three or more).

A number of years ago, McLeod Racing picked up on the value of multi-disc clutches.  They were fully aware of the advantages of the technology, but the big question was how to mate the technology from professional racing to something that could be used on the street. The result of their research and development is their latest series of “RST” clutch assemblies.  Although  not designed as dedicated racecar clutches, the RST is a street car clutch you can take to the track. In turn, this makes for a pretty nifty combination for a dual duty street-strip machine.  Or even for a (rare!) stick shift bracket car.

McLeod has been investigating and manufacturing high performance dual disc clutch assemblies for years (pretty much ever since GM discontinued their Borg-Warner-built dual disc clutch option in the early seventies).  One big problem that faced those earlier dual disc setups was the need for a unique (and perhaps more important “expensive”) flywheel.  McLeod’s RST clutch package is actually engineered to work with a stock style flywheel (you can use an aftermarket flywheel as well and of course, for most drag race applications you’ll need one due to SFI requirements – we’ll get to that later).

Based upon a 9-11/16-inch diaphragm pressure plate, the RST clutch includes 2 organic discs as well as a steel floater plate.  By reducing the overall diameter of the clutch, the moment of inertia is reduced.  How important is this?  More than you might think:  You see, the rotating components found in the driveline of your car include the crankshaft, flywheel, pressure plate, clutch disc(s), the gears inside the transmission along with the driveshaft.  Each and every one of these pieces operates at a relatively high RPM.  Given the high operating speed, the effect of the combined rotating inertia has upon acceleration is high.  If the weight of that rotating mass is reduced, then the basic assembly can spin faster.  Bottom line? If the mass becomes smaller, the car in question will accelerate faster.  But that’s not the end of it:  If the weight in the rotating mass is moved closer to center of the mass then the entire assembly can spin faster.  Basically a double-whammy.  That’s why the Pros we mentioned previously have gone to smaller and smaller (diameter) clutch components.

Next time, we’ll wrap up our look at clutches (keeping in mind there’s plenty of choice out there).

This is the foundation for the McLeod RST line of clutches – it’s based upon two small-diameter (9-11/16-inch), clutch discs that function by way of a custom floater ring.

This is the foundation for the McLeod RST line of clutches – it’s based upon two small-diameter (9-11/16-inch), clutch discs that function by way of a custom floater ring.

 

With the RST package, each disc is a solid hub disc (unsprung) pretty much like you’d find in a hardcore drag race clutch. Solid hub discs such as this have the splined hub riveted directly to the disc carrier plate (the circular metal plate that carries the friction material). With a sprung, or dampened hub disc, the hub floats in a spring-loaded assembly on the carrier. When the hub is loaded (when the clutch is engaged), the springs help to absorb the load instead of transfering it directly to the carrier plate. This arrangement allows shock spikes from aggressive engagement to be dampened out to avoid possible damage to the drive train.  Sounds a-ok, but how can this be streetable?  We’ll explain later.

With the RST package, each disc is a solid hub disc (unsprung) pretty much like you’d find in a hardcore drag race clutch. Solid hub discs such as this have the splined hub riveted directly to the disc carrier plate (the circular metal plate that carries the friction material). With a sprung, or dampened hub disc, the hub floats in a spring-loaded assembly on the carrier. When the hub is loaded (when the clutch is engaged), the springs help to absorb the load instead of transfering it directly to the carrier plate. This arrangement allows shock spikes from aggressive engagement to be dampened out to avoid possible damage to the drive train. Sounds a-ok, but how can this be streetable? We’ll explain later.

 

This photo shows how each disc is built with a Marcel.  What’s a “Marcel”?  It’s the wavy material sandwiched between each side of the lining material on each disc. The Marcel is put in place to prevent clutch chatter by providing a certain amount of “movement” as the clutch is engaged. The Marcel serves another purpose:  The spring effect of the Marcel helps to prevent the lining from sticking to either the flywheel or the pressure plate (or both) as the clutch is disengaged. Marcel thickness varies for the application, but it does have an effect upon pedal feel.

This photo shows how each disc is built with a Marcel. What’s a “Marcel”? It’s the wavy material sandwiched between each side of the lining material on each disc. The Marcel is put in place to prevent clutch chatter by providing a certain amount of “movement” as the clutch is engaged. The Marcel serves another purpose: The spring effect of the Marcel helps to prevent the lining from sticking to either the flywheel or the pressure plate (or both) as the clutch is disengaged. Marcel thickness varies for the application, but it does have an effect upon pedal feel.

 

This is the floater ring that keeps both discs isolated.  The floater ring eliminates the chattering of discs.

This is the floater ring that keeps both discs isolated. The floater ring eliminates the chattering of discs.

 

You can see how the floater ring, complete with a clutch disc on either side bolts to the flywheel and in turn, the pressure plate bolts to it (on a pair of bosses machined into the floater ring).

You can see how the floater ring, complete with a clutch disc on either side bolts to the flywheel and in turn, the pressure plate bolts to it (on a pair of bosses machined into the floater ring).