Shifting gears is one of the most satisfying things you can do in a car. But not always. A ropey shifter or a gooey clutch can suck the joy out of driving a manual and lend credence to Porsche’s position that its dual-clutch trannies render them obsolete. To learn the secrets of satisfying shifters, we interviewed the engineers for two of America’s biggest handfuls, the Dodge Viper and the Ford Mustang Shelby GT350.

The ideal shifter has short throws, moderate effort, and a positive, unambiguous feel. Good clutch feel means satisfying, progressive weight with linear, easily modulated engagement. Ford defines the latter via 17 different parameters, including total pedal travel, pedal force, and the disengagement point. But the two key pieces of the puzzle are effort—both its peak and the linearity of its rise—and the modulation travel, or the pedal swing from zero to full engagement. Here’s how they put it all together:

Good clutch feel gets harder to achieve with powerful engines as the clamping force the clutch needs to generate rises with engine output. As clamping force grows, the clutch-pedal leverage needed to overcome it increases, and this greater mechanical advantage can diminish feel. ROY RITCHIE

Overcenter Spring

To reduce clutch effort, Ford uses a small coil spring built into the pedal lever. Halfway through the pedal’s travel, the spring goes overcenter (becomes active) to cut the total effort from about 70 pounds (without a spring) to a maximum of 35 pounds. A clutch effort this light is unusual in a car with 526 horsepower and 429 pound-feet of torque. But Joe Johnson, FCA driveline development engineer, says that if not carefully calibrated, overcenter springs can harm linearity. This is especially true when the changeover point at which the spring starts to reduce effort is near the clutch’s engagement point. So the Viper goes without such assistance, requiring a stouter 47 pounds of peak effort.

Shifter

A tight shift linkage limits shifter slop. “Our linkage [shown below] uses all solid bearings until you get to the handle,” says Ford powertrain engineer Jeff Albers. “In the vertical part of the shift lever, we have a soft isolator to limit vibration, but it’s in a tight cup with limited travel.” Positive feel is primarily the responsibility of the detents, spring-loaded plungers that engage shallow depressions on the shafts that hold the shift forks in place. “We wanted that sense of force building and falling when you select a gear,” says Albers. “The shape of the detents and their spring forces are critical to that.”

Right: Unlike the Viper's, the GT350's shifter is a separate part bolted to the transmission and attached to the body through a flexible connection to absorb vibrations. ROY RITCHIE

Direct Input

The Viper’s shifter, on the other hand, is a simple beast. “Our powertrain is mounted so far back that the shifter comes directly out of the gearbox and ends up right where it needs to be,” says FCA’s Joe Johnson. This makes for the most direct shifter possible, but at the price of the lever vibrating with the Viper’s big engine. Johnson says he likes the trade-off.

Moreover, Johnson tells us the Viper’s transmission has no detents. He says: “You are actually feeling the synchros engaging the gear side profiles as they slide home. It’s about an eighth of an inch of motion in the gearbox and less than half an inch at the shifter ball.” In lieu of spring-loaded detents, the Viper has springs that center the shift-fork shaft and the shift lever in the neutral position. Interaction between the synchronizer and the shift collar holds engagement in each gear.

ROY RITCHIE

Dual-Plate Clutch

Both the Viper and the GT350 use dual-plate clutches, with the two driven (or friction) discs splined to the transmission’s input shaft. An additional intermediate plate, basically a second, slimmer flywheel for the friction discs to grip onto, nests between the friction discs. Smaller discs have less rotating inertia [see “Dual-Mass Flywheel,” below], and each plate increases the clutch’s torque capacity. Motorcycles and F1 cars use six or more driven plates in their clutches, which can be as small as four inches in diameter.

ROY RITCHIE

Cushion Springs

Engineers manipulate the width of the clutch engagement point with cushion springs. These are wavy steel plates between the friction pads on the clutch disc. Wavier springs make for a wider engagement because additional motion is required to fully compress them.

ROY RITCHIE

Dual-Mass Flywheel

A dual-mass flywheel uses two rotating plates coupled by a polymer band. Each plate vibrates at a specific frequency, and as they respond to the rpm variations in the crankshaft caused by the engine’s firing pulses, the flexible coupling absorbs vibrations, reducing shifter wobble. Ford’s Albers says this eases shifter movement, as less vibration means his team can remove the coil springs normally used in clutch discs to absorb torsional vibration, shrinking the disc’s overall diameter. “This also reduces their rotational inertia, which is very important with an engine that revs over 8000 rpm,” says Albers. “On the old Boss 302, it was hard to pull a gear at 7500 rpm because there was so much inertia carried by the transmission’s input shaft.” Lower rotational inertia makes for quicker shifts, as it lessens the work the synchros have to do to match the speeds of the input and output gears. The Viper uses an aluminum flywheel—Johnson claims it’s the only factory-installed one in North America—with about one-third less rotational inertia than a dual-mass cast-iron unit.

From the March 2016 issue

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