Welcome to part eleven of this racing concepts series. Remember: the most dangerous words in the English language are “I already know that.”
Here we go.
There are so many interesting downshifting techniques that can help a driver be smoother and more efficient: rev matching, heel-toe shifting, double clutching, skip shifting, short shifting, clutchless shifting, etc.
And while many young drivers dive into learning these techniques (which, in my opinion, is excellent!), many never stop to think exactly why they are downshifting at all.
Downshifting is the act of moving the transmission to a lower gear (obviously, right?).
More often than not, changing to the next lower gear will suffice.
However it’s not uncommon for a driver to drop two to three gears in one shift.
The purpose of downshifting is to place the engine RPMs in the torque band for optimal acceleration at the corner exit.
It should also be noted that when downshifting is required, it must be done after the application of brakes.
Downshifting before braking can cause an over-rev as the engine RPMs exceed safe operating parameters.
Downshifting should never be used to engine brake a racecar.
As a side note, downshifting under hard braking will not add braking force or reduce your stopping distance.
It will simply increase the probability of creating unwanted weight transfer, which can reduce the cars over ability to brake.
Imagine a racecar heading down a fast straight toward a tight right hand corner.
The corner is slow enough so that the driver will have to drop from 5th gear to a lower gear.
So how does the driver decide which gear will be best?
First, let’s look at a chart displaying the torque and horsepower curves of a theoretical engine (Figure 27).
As you can see, both power measurements rise to a certain point, level out, and then begin to fall away.
Since torque is a measurement of the cars ability to twist its own wheels and is directly proportional to acceleration potential, let’s focus on that one.
Clearly, this theoretical engine makes the most torque between 4000 and 6500 rpm.
Next, let’s look at the shift points of a theoretical gearbox (Figure 28).
As you can see by the green markings on the diagram, there are certain points in each gear where the engine can produce peak torque.
Now lets to back to the driver as he enters the corner. Let’s say he is on a warm up lap and is taking it easy through the turn.
When he reaches the corner exit, just before he applies the throttle, he is traveling at 47 mph. Ideally the driver will want the engine to be inside the torque curve on the lower end.
While doing 47 miles per hour, he should be at the bottom of the third gear. He can then proceed at maximum acceleration through the higher portions of the torque curve and then change to a higher gear.
Let’s say the driver has successfully warmed up the car and is headed toward the same corner at full bore.
This time as he nears the corner exit his car is doing 67 miles per hour. A reference to the gear graph chart shows that to remain in the torque curve, due his is higher speed, he must now be in fourth gear to achieve maximum acceleration.
As previously stated, some drivers prefer different methods when changing down more than one gear.
During a drop from 5th to 2nd, some drivers will drop successively through every gear as they brake and approach the corner (going from 5th to 4th to 3rd to 2nd).
Others may choose to begin braking, wait until they almost reach the corner entry, and then change directly from 5th to 2nd.
In theory, lowering the number of shifts (sometimes called skip shifting) reduces the chance of making an error.
NEXT WEEK: Rev-Matching and Heel-Toe Downshifting
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.