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
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
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
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
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.
WHEN TO SHIFT
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.
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.
THE RACING CLUTCH
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.
WISHING YOU GOOD RACING IN 1999
I'd love to hear from you. Let's talk about gears and other race tech stuff