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Tech Series

Gears l Tires l Friction Circle l Understeer
Springs l Brakes l Oil l Breathe

Shifting Gears

by Ruth Wolf

The racing gearbox is simple, compact and light weight.

Inside, the gears are in constant mesh with each other. All gears on both shafts are spinning when the clutch is engaged. Moving the shift lever moves the fork on a cogged sliding sleeve known as the dog ring. The dog interlocks its teeth into teeth on the side of the gear, transferring the torque from the input shaft to the output shaft through the selected gear set.

In a street gear box, to facilitate shifting, synchronizers and sleeves are used instead of a dog ring. When you move the shift lever, the synchronizer speeds up or slows down the speed of the gear to aid in smooth shifting. The synchro has a friction surface which makes contact first ,matching the gears rotation and locking the sleeve's teeth into the small teeth on the side of the gear. Shifting becomes effortless.

Street boxes use helical cut gears to reduce noise; racing boxed use straight cut gears.

All these extra internal parts add rotational mass (weight), that used up available horsepower.

transferring the speed in the motor...

Shifting gears changes the pair of gear ratios, and spins the output shaft faster (upshift) or slower (downshift). The speed in the motor is transferred through the clutch to the transmission input shaft through a selected set of gears to the output shaft, then delivered through the drive shaft to the differential and the driving wheels.

Gear ratio changes change the speed that the crank moves in relation to the driveshaft.

For a gear ratio of 2.5 the input shaft is turning two and a half times faster than the output shaft (the engine's rpms are higher). A ratio of 1 to 1 (usually fourth gear) has the input shaft speed and the output shaft speed the same. Overdrive gears have the output shaft spinning faster than the input shaft. Gears such as .85 are good for fuel economy, not for speed. Available gear ratios are plotted on a gear chart from the gearbox manufacturer.

Skills such as heel-toe, double clutching, blipping the throttle, are used to match gear speeds on the input and output shafts for smooth shifts without lurching and grinding. Smooth shifting is something you can practice everyday in your streetcar. It should be a natural unconscious motion.

The shift itself is a very precise, very positive, very quick flick of the wrist. The experienced driver shifts gears faster than the thought to shift.


The clutch connects the motor to the rest of the drive train.

When you come off the throttle, there is a moment when you can feel the tension from the motor driving the rear wheels change to the motor being forced by the rear wheels. This is the moment when you can easily shift out of the higher gear, blip to synchronize gear speeds and slide the shifter into the lower gear, without any grinding.

if you happen to lose the clutch...

The benefits for using the clutch are reliability and "mechanical sympathy". The clutch slips to cushion the impact of gear engagement. The experienced driver will not see any time advantage in clutchless shifting.

The benefits for learning the skill of clutchless shifting is that it frees the left foot for left foot braking. Also if you happen to lose the clutch during a race you are not sidelined.

Mostly it is important to learn to shift accurately, minimizing the time spent in neutral, losing rpm's. Forced shifts can lead to missed shifts or a spin from locking up the rear wheels, or blowing the motor from an over rev in neutral.


The rpm range where the engine has maximum torque to maximum horsepower is the powerband. Discuss redline limits with your engine builder to establish a workable rpm power range. There is a point where more rpm equates to less performance. You want to choose gears that when shifted at maximum rpm (maximum horsepower), the drop is to the rpm for maximum torque.

For road racing:

  • Select top gear for the longest straight first.
  • Intermediate gears are selected for the most efficient acceleration through the corner.

Ideally the ratios should be spaced so that it is not necessary to shift in a corner or just before braking.

  • At the point in the corner exit phase where you can give full throttle, the engine rpm should be near peak torque. If the engine redlines before the exit, the gear is too short. Staying in gear and increasing rpm will result in another discussion with your engine builder.
  • At the end of the straight, the selected gear should be at redline. Slow the car enough to not over rev when downshifting. Shift, add throttle smoothly. Too much torque applied suddenly will snap the car into power oversteer.
  • Brakes slow the car; downshifting is not an aid to braking. This practice was used when racing brake systems were marginal. Although in cars with stock brake systems (like showroom stock) as the brakes fade, downshifting becomes a last resort to get the car slowed.
  • If engine rpms drop below the power band the engine lugs. It takes a long time to regain rpms and lugging causes premature internal engine wear. Downshift.

Check clutch and shift linkage. Any slop translates into inconsistent shifting. Make sure that linkage does not bottom against anything, and there is no binding in the support bearing (but not too loose), and that the brackets do not flex. Avoid acute angles (15 degrees maximum). Clean and lube frequently.


By using a light weight, small diameter, low inertia flywheel-clutch assembly, the engine will accelerate faster. Angular momentum (the amount of inertia referring to rotation) is lower when mass is located towards the center, using less horsepower to move the part. The part will move faster.

Many flywheel-clutch problems can be eliminated by careful inspection and maintenance of the parts:

  • Clean all parts and use a new radius faced throwout bearing. Inspect the pilot bearing, trans shaft bearings and seals. Inspect the input shaft for twisting in splines.
  • Check the flywheel to crankshaft mounting for square. Use only top grade flywheel bolts or aircraft hardware.
  • Check that transmission receiver hole is concentric with the crankshaft, and that the locating dowels are installed, mark for alignment.
  • Make sure the clutch diaphragm spring is not depressed too far when releasing the clutch. Overtraveling the diaphragm shortens its life and can damage engine thrust bearings.
  • Do not drive the car on to the trailer.
  • Do not slip the clutch in the pit area - pop the clutch to get the wheels spinning and then release to bring the rpms up and ease the clutch while the car is rolling. Intermediate plates and pressure rings are easily warped by excessive slippage.


I'd love to hear from you. Let's talk about gears and other race tech stuff at: