Welcome to twelfth part of this racing concepts series. Remember: the most dangerous words in the English language are “I already know that.”
Here’s where it all gets very interesting. Let’s do it!
Let’s take a moment to consider a shifting concept called rev-matching (rev being short for engine speed in revolutions per minute, or RPM).
Rev-matching allows for rapid downshifts that, when done correctly, minimize weight transfer and wear of the engine, clutch components, and transmission.
To perform this technique properly, we first have to understand what we’re trying to avoid.
If you simply push in the clutch pedal, change to a lower gear, and then release the pedal you’ll experience a forward weight transfer.
This is because the car is being forced to decrease its speed because the lower gear has caused engine braking to suddenly increase.
While the engine speeds up to balance out the lower gear, extra weight will remain forward.
In racing we often need to downshift, but we want to avoid any excessive weight transfer that can reduce traction.
Rev-matching is the anticipation of higher engine speeds that occur during a downshift.
A theoretical car is driving at 35 MPH in third gear with engine RPMs at 2000. The driver knows that if he changes down to second and continues on, the engine RPMs will increase to 2700 to compensate for the lower gear ratio, causing forward weight shift.
So, to avoid the unwanted weight transfer, the driver completes the following actions:
- Pushes in the clutch.
- Briefly hits the throttle (called blipping) to manually increase the RPMs to 2700
- Shift to a lower gear
- Quickly releases the clutch when the RPMs reach 2700. The quick stab of the accelerator (the blip)
eliminates the engine’s need to balance itself to the new gear.If done correctly, zero weight transfer should be felt and maximum traction will still be available.It may take you a while to become proficient at rev-matching. It is, however, very important.
Remember: less weight transfer is faster.
Obviously, not all cars have the same gear ratios or engine control units.
Shift points will vary between vehicles, as well as an engines response time while blipping the throttle.
Take some time to find where yours are and learn to be consistent with them.
You may remember in the previous post when I stated that downshifting should occur after the application of brakes.
But you obviously can’t begin braking for corner, release the brakes for a shift, and then go back to braking–that would be a weight transfer nightmare.
Trust me, I see this speed killing technique all the time in autocross.
The trick to staying smooth is using all three pedals at the same time.
The technique is called heel-toe downshifting.
And, while it’s probably the more difficult parts of performance driving, it is a necessary skill to possess if you want to be the fastest guy on the track.
Heel-toe downshifting is simply rev-matching and using the brake pedal at the same time.
Keep in mind that you cannot left foot brake into a corner with the intention of downshifting.
Imagine that you’re approaching your braking mark and consider the following steps:
- When you reach your braking mark, begin firm braking with the ball of your right foot.
- Depending on your engine speed at the braking mark, you may have to be patient for the RPMs to drop as the vehicle slows.If you begin shifting too soon the lower gear may cause a very unnecessary over-rev and damage the engine.
- After the engine speed has dropped enough, begin to push in the clutch pedal with your left foot. Begin changing to the lower gear.
- As you complete changing gears, use the ball of your right foot as a pivot point to rotate the heel of your right foot over to the throttle.Without releasing brake pressure, blip the throttle with your heel.
- When the engine speed increases to the correct RPM, release the clutch pedal while you continue braking.
- If another downshift is needed before the corner, repeat steps 1-4. If not, continue braking normally until the turn-in mark.
I feel that I should restate the fact that downshifting under hard braking does not provide any added braking power, but can actually hinder it.
Since there’s nothing to be gained, don’t force your downshifts earlier and make the engine jump up to the red line during your shift.
This only adds more wear to the motor and may upset the car’s balance. Instead, brake a little longer and time your shift so your RPMs are lower, away from the red line.
Remember that your goal is to end up at the bottom end of the torque curve.
Or, if you must change down two or more gears, you could wait even longer and make one shift from the gear you’re in to the one needed to make maximum torque at the corner exit.
Remember, corner exit speed is your first concern.
NEXT WEEK: Traction Circle
The next part in this series is only a week away. Go back and read through this again. Make sure you truly understand what you just saw, because the series progressively builds as it continues.