Gear Bangers Part II

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In the last segment, we laid out the dilemma folks with stick shift street-strip cars face: The clutch must be capable of handling serious power. It must also be able to be modulated easily and, of course, used to your advantage. It must be driveable. And it must be reliable. That’s a tough set of parameters. And we pointed out that the best way to accomplish this is with a dual disc clutch. We also provided you with a little preview of Tilton’s new street strip ST-246 clutch. The attributes are many and we’ll dig into them right here.

Let’s stop for a minute and ponder this: The rotating components in a vehicle driveline include the crankshaft, the flywheel, the pressure plate, the clutch disc(s) and the gears inside the transmission as well as the driveshaft and rear-end pieces. All of these components operate at a relatively high speed (RPM). Because of this relatively high operating speed, the effect of the combined rotating inertia upon vehicle acceleration is considerable. When it comes to high performance, if the weight of that rotating mass is reduced, the basic assembly can spin faster. If the weight in that rotating mass is moved closer to the center of the mass, the entire assembly can spin faster. That simply means the car can and will accelerate more quickly. Sometimes this is difficult to picture, but in order to get a better idea of how this works, consider a figure skater beginning a pirouette with her arms outstretched. As the skater pulls her arms inward, the amount of inertia has been reduced. That causes the skater’s spin to increase in speed.

In the case of Tilton’s dual disc clutch, instead of incorporating a large, heavy 11.00-inch or larger Borg & Beck or Long style pressure plate, a much smaller, much lighter pressure plate can be used. Instead of using a large, heavy 11.00-inch or larger clutch disc, the system makes use of two small, lightweight 9.7-inch organic discs (to be fair, there are at least two other dual disc systems out there from other manufacturers that make use of sub-10-inch discs). There’s good reason to use more than one disc: surface area and clamping capacity. Dual discs increase the surface area of the clutch, almost doubling it over a conventional system. Since the surface area has increased almost exponentially, the demands placed upon the pressure plate spring(s) are reduced considerably. Basically, the reduction in the clutch moment of inertia also allows the use of a very lightweight pressure plate spring(s). While torque capacity remains high, reducing the force of the pressure plate springs means your left leg isn’t strained beyond comprehension.

Yes, Tilton could have made the clutch much smaller (for example, they build several different 5.5-inch diameter clutch assemblies for various applications), but you also need more heat capacity on the street – particularly in stop and go driving. Here you tend to slip the clutch, which generates heat. Something like a 5.5-inch diameter race clutch can’t manage the heat where a bigger clutch can. As a result, Tilton created a special high mass pressure plate and floater plate designed to manage the heat.

If you take a closer look at the ST-246 clutch, you’ll discover it features heavy duty straps that attach the pressure plate and floater plate to minimize noise and provide for a clean clutch release. Tilton uses six different drive strap locations – three that link the pressure plate to the cover and three linking the floater plate to the cover. Each of the drive straps measures approximately 0.030-inch thick. The pressure plate strap package uses two straps per assembly (approximately 0.060-inch thick) while the floater plate uses four straps per assembly (approximately 0.120-inch thick). In total, 18 straps are used.

You’ll also find aircraft and aerospace hardware used throughout (it’s rather amazing to discover a full complement of high-end NAS fasteners used in a street clutch or, for that matter, any other street-based product). Equally important is the overall weight of the package. On our home scale, the Tilton ST-246 clutch weighed a mere 37.5 pounds, including the flywheel (we’ll address the flywheel later – it plays a significant role in the assembly). In contrast, we weighed a conventional high performance 11-inch single disc diaphragm clutch setup complete with a relatively light weight billet steel flywheel. The flywheel weighed 25 pounds. The pressure plate and disc tipped the scale at 20.5 pounds for a total weight of 45.5 pounds. As you can see, the complete Tilton assembly is considerably lighter – eight pounds lighter – that’s a bunch.

We’ll stop right here for this issue. Next time around, we’ll examine some additional dimensions and at the same time, we’ll provide you with info on the available disc combinations, then finish our series with a look at flywheels. Basically, Tilton has you covered right up to 1250 or so lbs-feet of torque (yikes!). In the meantime, check out the accompanying slideshow for additional info:

Tilton is well versed in the design of small diameter racing clutches. Setups from the Tilton folks have won everything from F1 (no joke) to Score and almost everything in between. The key to the setup is this tightly packaged dual disc pressure plate.

As you can see, the clutch cover features a diaphragm spring configuration. Don’t confuse this with diaphragm pressure plates of old where the pedal would regularly stick to the floor! This setup is all-new technology.

The cover is machined from billet aluminum. Tilton uses a proprietary grade of materials in the construction of the entire clutch and much of it is accurately CNC-machined.

(Tilton Engineering Image): This cut-away view from Tilton tells the story. As you can see, the clutch cover, along with the pressure plate, might look a wee bit different from what you might be used to. More in the next photo:

(Tilton Engineering Image): The idea here is to provide not only max grip, but also maximum heat management. Tilton worked hard to make this package useable on the street.

There are a total of six straps holding the cover to the pressure plate and the cover to the floater plate. Each strap measures approximately 0.030-inch thick, and they’re bundled in groups by specific application. See the text for more details.

One thing that strikes you immediately when examining this clutch setup is the (unusual) use of aerospace hardware everywhere. And not only is it AN – much of it is NAS (basically, space ship stuff). Quality of construction is apparent everywhere.