Intermediate Topic #4: downshift timing

ATTENTION

This is the last weekly post. YSAR is moving to an event-based format. Race reports, experiments, reviews, etc. will now happen when they happen. This means YSAR will probably not be updated as frequently, but the content will hopefully be more meaty.

Back to our series in progress

This series of posts is aimed at you, the intermediate driver. Let’s identify and fix some common errors. If you’re not an intermediate driver, fake it till you make it. Last week we talked about driving a higher gear as a way to keeping momentum. Let’s get more complicated this week and actually perform a downshift.

The engine is not a brake

One of the big misconceptions I inherited from my father was that downshifting improves your braking. He told me that by feeding out the clutch gradually, I could use the friction of the drive train to slow the car down. While it is true that engine braking slows you down, it does more harm than good. Back in the old, old days when cars had drum brakes, engine-braking actually did reduce stopping distances. But today, with disc brakes, engine-braking just changes the brake bias from optimal to sub-optimal. In addition to longer braking distances, engine-braking also wears out the clutch and may kill the engine by over-revving it. There’s really no need to do it on a race track ever.

Downshitting is bad

Engine-braking in a FWD vehicle doesn’t do much harm, except as noted above, but in RWD it can make you spin. One of the terms I’ve come up with on this blog is downshitting. This is the act of downshifting, locking the rear wheels, and spinning. Although it occurs most frequently in corners, downshitting can spin a car going in a straight line. It’s a lot like grabbing the e-brake. Most downshitting is accidental. It results from putting the clutch in too soon.

The exercise: downshift later

Once you’ve pushed the clutch pedal, you’re mentally committed to the downshift, especially if you heel-toe. Your thoughts run something like this: “I don’t want the revs to fall, so I had better shift soon”. This is the source of downshitting, so we need to fix that. In this exercise, I want you to shift as late as possible. As you enter a braking zone your thoughts should be “brake, keep braking, keep braking, downshift”. Make it a game to see how late you can downshift. You can even shift after the corner. What? The video below is queued up to Tim O’Neil describing when to shift. He says “after the corner”. I’ll let his driving speak for him.

Wow, just freaking wow.

Telemetry Analysis

Let’s take a look at the difference between early and late shifting. The green line shifts much earlier than the blue. The top panel is gear. The second panel is RPMs. Here we can see some of the properties of early shifting. The highest RPMs occur when engaging the lower gear. Over-revving an engine causes excess wear if not outright destruction. It also causes the rear end of the car to drag (because the car is RWD) and re-proportioning the grip may induce a spin (and another chance at destruction). Lots of intermediate drivers, especially those using heel-toe technique, shift way too early and are consequently in danger of damaging their engine or spinning. If you have to ease the clutch out gradually, you didn’t match revs. That doesn’t mean the fix is to dump the clutch! Please don’t do that. Learn to match revs.

The blue line shows me shifting much later. In fact, later than I normally drive. This is a drill after all. I’ve gone through a surprising amount of the corner in the higher gear and then switch down after all the hard work is done. The speed and time graphs show there isn’t much difference in the outcome. The extra-late shift wins by 0.3 seconds. Shifting too late is better than too early because it’s much easier on the car. I’m still rev-matching because the engine is spinning very low by the time I want to engage the lower gear. If you’re not comfortable with heel-toe technique, you’ll have to ease out the clutch.