Why “smooth” transmission shifts aren’t always better

People often describe a great automatic transmission as smooth, with gear changes that are barely noticeable. That is often what manufacturers aim for, and it comes at a cost. If the goal is power handling, longevity or track response, a soft shift works against it.

In most modern automatics, each gear is selected by applying a combination of clutch packs, stacks of friction plates and steel plates that a hydraulic piston squeezes together. Most shifts are clutch-to-clutch: one clutch, the offgoing, releases while another, the oncoming, applies. A dual clutch transmission shifts the same way, handing torque from one clutch to the other.

Some automatics use a one-way clutch for certain shifts instead, often in first gear. A one-way clutch locks in one direction and freewheels in the other, so it holds in first and lets go on its own as the clutch for second applies. Only that one clutch has to be controlled.

An upshift happens in two phases. In the torque phase, the oncoming clutch takes over the load while the offgoing clutch lets go, and the gear ratio has not changed yet. In the inertia phase, the oncoming clutch slips while engine speed falls to match the new gear, and the shift ends when that clutch locks.

The handoff comes down to timing. If the two clutches overlap too much, both hold at once and fight each other through the gears, which is called tie-up. If they overlap too little, engine speed flares up for a moment before the oncoming clutch takes hold.

A factory calibration built for comfort deliberately overlaps the two. The offgoing clutch drags a little longer, the oncoming clutch applies a little slower, and the handoff is spread over a longer window. That is what makes the shift feel seamless.

A clutch makes heat whenever it carries torque while its two sides turn at different speeds. That difference is its slip speed, and the energy it turns into heat is the torque it carries multiplied by the slip speed, added up over the time it slips.

In an upshift, the oncoming clutch slips from the moment it starts to take load. It slips at the full speed difference through the torque phase, then its slip falls to zero through the inertia phase. A longer handoff and a longer speed change both add heat.

The inertia phase can be worked through. Treat the rotating parts ahead of the clutch as one mass, and assume both torques hold steady and the car’s speed barely changes in the fraction of a second the shift takes. The heat then comes to:

The equation has two parts. The first, ½ I Δω₀², is the least heat the speed change can make while the engine is still driving the clutch, set by how hard the parts are to slow down and how much their speed has to change. It grows with the square of that speed change, which is larger the higher the engine speed at the shift.

The second part, T_c ÷ ( T_c − T_in ), multiplies that least heat because the clutch is carrying engine torque at the same time. The clutch has to carry more torque than arrives from the engine, or the engine never slows to the new speed. The closer the two are, the larger the multiplier, the longer the slip and the more heat it makes.

That heat works on the clutch and the fluid in three ways.

• Flash temperature. The plates truly touch only at many tiny high points. That is where the heat is made, and those points briefly run well above the temperature of the plate as a whole.
• Friction material. Prolonged slip wears the friction material and accelerates glazing, a hardened, polished surface that grips less.
• Fluid. The fluid carries the heat away, and heat speeds its oxidation, the slow reaction with oxygen that ages it. In a 2021 laboratory study, Farfan-Cabrera and colleagues found that oxidized automatic transmission fluid raised a wet clutch’s friction coefficient, its grip, and generally weakened its resistance to shudder, the vibration of plates that grab and release, which depends on friction rising as sliding speed rises.

A long, soft shift trades clutch and fluid life for refinement, and none of the energy turned into heat reaches the wheels.

The inertia is fixed by the hardware, and the speed change by the gear ratios and the engine speed at the shift. For a given shift, that leaves two things to work with in the speed change: more clutch torque, or less engine torque coming in while the clutch slips. A shorter handoff before it cuts heat as well.

A transmission asked to hold more torque, or to survive track use, needs a shorter slip and less heat. A performance recalibration often does the opposite of a factory calibration built for daily driving:

• Raise hydraulic pressure to the clutch packs, so the oncoming clutch clamps harder and stops slipping sooner.
• Shorten the overlap, so the offgoing clutch releases decisively as the oncoming clutch locks.
• Coordinate torque control with the shift, so engine speed comes down to the new gear without the clutch dragging.

More pressure means more clutch torque. How much a clutch pack can hold is:

A recalibration can change only the clamping force, through pressure. Everything else comes from the hardware and the fluid, and so does the most pressure the pump and valve body can deliver.

Less engine torque coming in is the other way. Here is the same shift with the clutch torque held at 1.5 times full engine torque:

In this example, halving the engine torque halves both the heat and the slip time, without any more clutch torque.

That is why disabling torque management is not the fastest way to firm up a shift. Torque management is the PCM and TCM, the engine and transmission computers, briefly trimming engine torque during a shift through spark timing and airflow. In stock form it is gentle, so the clutches do not shock the driveline. With no torque reduction at all, the clutch has to slow the engine while carrying full engine torque, the top row of the table. The key is to reshape torque management, so torque is cut briefly, the clutch locks quickly and cleanly, and then full power returns.

A performance recalibration can go further than stock, not by removing less torque but by removing more for a much shorter window. The speed change then takes less time and makes less heat without the clutch having to clamp harder, and the shift stays mechanically safe when pressures and timings are set correctly.

A quick shift done properly feels nothing like a harsh bang shift. It is firm and decisive without shocking the drivetrain, and it comes from coordinating:

• Hydraulic pressure curves
• Clutch overlap timing
• Clutch fill time, the moment it takes to fill the piston with fluid before the clutch starts to grip
• Torque reduction and restoration
• Fluid condition and cooling capacity

The aim is enough pressure and speed to keep heat down, without torque spikes that fatigue shafts, gears and differentials. Done well, the transmission runs cooler, the clutch packs stay healthy, and with more clamping force they hold more torque.

1. Harald Naunheimer, Bernd Bertsche, Joachim Ryborz and Wolfgang Novak, Automotive Transmissions: Fundamentals, Selection, Design and Application, 2nd edition, Springer, 2011. Shift elements, one-way clutches and shifts under load.
2. Richard G. Budynas and J. Keith Nisbett, Shigley’s Mechanical Engineering Design, McGraw-Hill. The torque capacity of friction clutches.
3. Harmen Blok, “Theoretical study of temperature rise at surfaces of actual contact under oiliness lubricating conditions,” Proceedings of the General Discussion on Lubrication and Lubricants, volume 2, Institution of Mechanical Engineers, London, 1937, pages 222-235. Flash temperature.
4. Leonardo Israel Farfan-Cabrera, Ezequiel Alberto Gallardo-Hernández, Manuel Vite-Torres and Jesús Gilberto Godínez-Salcedo, “Influence of oxidation of automatic transmission fluids (ATFs) and sliding distance on friction coefficients of a wet clutch in the running-in stage,” Friction 9, 2021, pages 401-414.

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