With thanks to Gordon Morrison for letting us use his original material
A hallmark of an advanced driver is the ability to maintain a vehicle as a stable platform. While we often focus on smooth steering and progressive braking, one of the most common sources of instability is the engine itself.
Truly advanced driving requires "mechanical sympathy"—the art of managing engine momentum to ensure that gear changes do not unbalance the chassis.
Understanding Kinetic Energy and Balance
Every moving part in your engine has mass and momentum. When you change gear, you are essentially asking the vehicle to reconcile the difference between the engine’s rotational speed and the road wheels' speed. If these are not matched, the car will "lurch" or "squat," transferring weight and potentially reducing the tyre’s contact patch efficiency. We call this pitching, a fore-and-aft movement caused by sudden acceleration or engine braking. What we are looking for is stability, a balanced car that is distributing its weight evenly, providing maximum grip and passenger comfort.
The Upshift: Timing the Decay
When changing up (e.g., 3rd to 4th), the engine speed must drop to match the higher gear. The secret is timing. If you re-engage the clutch exactly as the revs naturally decay to the required RPM, the transition is seamless. Re-engaging too early forces the car to "drag" the engine down; waiting too long forces the car to "pull" the engine up. Both scenarios cause an avoidable longitudinal jolt.
The Downshift: The "Rev Match"
Downshifting is more complex because the engine speed must increase. By finishing your braking first, your right foot is freed to press the accelerator while the clutch is depressed. This raises the engine’s RPM to match the lower gear’s requirements. The Benefit is that you avoid using the clutch as a "brake" to drag the engine speed up. You avoid excessive "coasting" (travelling with the clutch depressed for extended periods) and ensure that the car remains balanced.
Why Synchromesh and Reve Matching Technology Isn't Enough
Manual gearboxes (apart from very old classics) have synchromesh to prevent "crunching" gears, but they do not match engine speed to road speed—that is the driver’s job. Relying on a slow clutch release to mask a speed mismatch is a common habit that leads to instability and unnecessary wear. By mastering the rev-matched change, you gain the flexibility to select the correct gear at the precise moment it is needed, ensuring your progress remains swift, smooth, and unobtrusive. This is particularly important at speeds above about 20mph.
Some more recent manual cars will automatically rev match when the lower gear is engaged, but the timing of the gear change and lifting of the clutch is crucial for this feature to function properly.
Quick Reference: Managing Engine Momentum
To achieve a seamless transition, your goal is to synchronize the engine's revolutions per minute (RPM) with the vehicle's road speed for the gear you are selecting.
