Tilton carbon clutch

Power and packaging are the keys to F1 performance, but one isn’t always good for the other. Putting 800 BHP through a clutch the size of saucer sounds like a recipe for disaster. And so it was until Mac Tilton got on the case. Keith Howard reports

The he troubles for Mac Tilton began on Sunday June 21, 1987. It was an occasion for celebration too. Senna had just won the GP in Detroit, his Lotus 99T comfortably ahead of Nelson Piquet’s Williams FW11B. This win would assume historic importance for being the last ever by Team Lotus. But for Tilton Engineering and Formula One in general, it had another significance altogether. In its first race appearance (Lotus had only used it for qualifying at Monaco) Tilton’s carbon-carbon clutch had proved itself a race-winner, marking a step change in racing clutch technology. Within a year the sintered metal clutch — the previous state of the art—would disappear from grand prix racing forever.

Well before that, though, things turned ugly. Within six weeks of the Detroit triumph, AP Racing launched its own version of the carbon-carbon clutch, to protests from Tilton that its patent had been infringed. The ensuing bitter legal action between the two companies would drag on until 1994, by which time Mac Tilton — despite his eventual victory — had had a bellyful of Fl ethics. A year later, he retired to pursue his other great interest, target shooting. It had all been so different earlier in the 1980s. Tilton Engineering, far from being a competitor, was AP Racing’s North American distributor, manufacturing release bearings, clutch housings and flywheels to complement AP’s products and provide more complete application packages to Stateside race teams. Hitco, the US manufacturer of carbon friction materials, was anxious to develop its racing business and asked Tilton if he would approach AP Racing on its behalf. He did, and AP began attempts to make the first carbon clutch.

When it failed and abandoned the project after a couple of years, Hitco — still determined that carbon could be made to work in racing clutches, as it already had in Fl brakes — approached Tilton again to ask if he would take up the challenge. He agreed and, in his own words, “made all the same mistakes as AP” trying to get the carbon clutch to work. Eventually he concluded that the material just didn’t have the compressive strength necessary for rivetting it in place like conventional friction materials.

Instead, an entirely new method of clutch construction was needed: one that made no attempt to pin the carbon but would engage with it over a large surface area to reduce compressive stresses. What Tilton came up with was the finger or lug drive, still a key feature of carbon clutches today. One end of the clutch comprises a metal ‘basket’ with extended longitudinal lugs which engage with cut-outs in the periphery of alternate carbon plates in the pack. Between these, and running within the lugs, are a second set of carbon plates, keyed to a central hub. Clamping of the complete plate pack, accomplished by the usual diaphragm spring, locks the basket and hub together to transmit drive releasing the spring load uncouples the plates and removes the drive. The clutch can be mounted either way round: with the basket connected to the crankshaft and the hub to the gearbox input shaft, or vice versa.

Having developed the carbon clutch to this design and tested it on a variety of cars, from stock cars to Indy Lights — all in secret — Tifton announced the product at the Olympia Racing Car Show in early January 1987. Then he visited everybody who was anybody in F1, to persuade them to try it. Many were reluctant, but Ferrari showed an immediate interest and Lotus almost bit his hand off. The Honda RA166-E turbo V6 was mounted so low in the new 99T that it demanded the use of a 5.5-inch diameter clutch rather than the 7.25in unit then standard in F1. Lotus was worried that a sintered clutch of this diameter simply didn’t have the necessary thermal capacity, so it commissioned a 5.5in carbon unit from Tilton on the spot.

Ferrari’s interest was more in the carbon clutch’s low inertia, apparently because it was experiencing crankshaft vibration problems with its turbo V6. Although it asked Tilton for a standard 7.2 Sin diameter unit, it was still substantially lighter than the sintered alternative. Both teams received their first units by March, and both raced them for the first time at Detroit. Concerned that the unfamiliar characteristics of the carbon clutch could cause a driver to stall on the grid — “You had to bring the revs up a little higher and be pretty abrupt with it,” says Tilton — he lobbied the teams hard to practise standing starts, something he found them curiously reluctant to do.

He also preached that drivers should slip the dutch during the parade lap to get some heat into it — something that was anathema with conventional clutches. Senna, with his customary thoughtfulness, grasped the requirements immediately.

Once the F1 drivers had learned these idiosyncrasies, they liked the carbon clutch. Because the plates were immune to warp, the clutch rarely dragged on the starting grid, and its take-up was damped slightly by the material’s reduced coefficient of friction at low temperatures. A little slip occurred initially as the carbon heated up. The lack of plate warp also meant that clutch actuator displacement could safely be reduced by the fitment of a smaller master cylinder, which improved driver control over the biting point.

And because of its reduced inertia, the carbon clutch also made gearshifting easier.

Today’s Formula One clutches are actuated by powered hydraulics and the action of the clutch paddle is mapped by an ECU, but the carboncarbon clutch itself remains essentially the same, albeit even smaller. This season AP Racing is supplying all but two of the 11 teams on the grid (Jordan and Sauber both use Sachs) and its latest design is just 97ntin in diameter, about 50mm long and weighs only 1.2kg. Despite which it will withstand 15 or 20 successive full-blooded Fl starts, provided it is given adequate time to cool in between. As for Tilton Engineering, it still manufactures a range of carbon-carbon clutches — but not for F1.