Why is rotation important?

A few posts ago I talked about the R word. No, not R-comps, which is a favorite topic of conversation, because I think they’re basically useless. Also not racing, which is a great way to have fun, but also a way to gain bad driving habits by being too protective. Also not radios, which I have a hate-hate relationship with. Alas, the topic is rotation, and why it’s so important.

Two types of corners

You may have learned from “Driving in Competition” by Alan Johnson (or the people that quote him) that there are 3 types of corners:

1. In slow, out fast
2. In fast, out slow
3. Everything else (compromises)

Or you may think of corners in their various shapes:

• 90°
• Carousel
• Hairpin
• Chicane
• Esses
• etc

However, a more simple taxonomy of corners is this:

• High speed corners
• Low speed corners

Before I can describe more about these kinds of corners, I want to digress into a more general discussion of cornering.

Corners in general

In any corner, you have two competing desires:

1. Minimize the amount of time you spend in the corner
2. Maximize the speed coming out of the corner

If you look at any book on driving, you will see a figure that looks a little like the following. This shows various lines through the corner of various radii. The largest radius line (C) is often called the geometric line through a corner. The shortest path is depicted by line (A).

Line (A) minimizes the amount of time spent in the corner. Not only does the corner have the shortest distance, it also allows you to brake much later, which maximizes the speed on the way to the corner. If the finish line was placed just after the corner, a racer who drove line (A) would win compared to line (C). However, if the finish line is farther down the straight, line (C) becomes a better option because it has a higher speed. That is, unless the car has so much acceleration (like JATO rocket boosters) that it can surpass the speed of line (C) by the time the geometric gets to the exit. The point of this discussion is to show you that even though line (C) is what gets taught most of the time, there are also very good reasons to drive the shortest path. In other words, spending less time in a corner is sometimes better than having highest speed (and vice-versa).

High speed corners

In a high speed corner, the optimal strategy is to keep your minimum speed as high as possible. If a driver enters 5 mph below the optimal corner speed, they will be well behind the pace by the time they get to the exit. There is no way to make up speed in a corner. If you enter slowly and attempt to go faster, you will create understeer as the weight and grip shift to the rear of the vehicle. This will cause you to lift throttle at the exit to prevent yourself from going off track. So just when you’re supposed to be at full throttle (exit) you’re lifting.

A car with a bit of understeer is easier to drive through high speed corners. If the goal is to set your minimum corner speed as high as possible, you don’t want the rear stepping out on you as you enter a corner. You want to feel your way into the corner using the front tires as your guide to how much traction is available. Then, once you feel the right amount, you can maintain it with throttle. In other words, you’re taking a geometric line through the corner where the corner speed is both constant and as high as possible. In high speed corners, your motor is in high gear, and doesn’t have as much acceleration, so you can’t bail yourself out of a lower speed with your engine.

Low speed corners

In a low speed corner, you need to change directions as quickly as possible to minimize the amount of time you spend in the corner. At lower speed, your engine is more useful, and in order to use the engine, you need to get the car pointed straight as soon as possible. Here, oversteer helps because it allows the rear tires to provide some of the steering. Note that the oversteer must happen in the first part of the corner, during braking. Once you get on the gas, you don’t want the car sliding sideways. That will reduce both grip going sideways and your ability to accelerate. Oversteer and rotation should happen early in the corner when the speed is lowest. Then once the car is straightening out, you can add throttle and get a faster exit speed.

One of the phrases that goes along with rotation is backing up the corner. This is the concept that you do more stuff in the early part of the corner, where stuff includes inputs (wheel, pedals) and outputs (change of speed, change of direction). A rotation corner should have a lower minimum speed than a geometric corner. In order to go faster later, you need to go slower sooner. The path through a rotation corner has compound radii. That is, it’s not circular. The radius gets tighter to the point of minimum speed, and then gets larger towards the exit.

Another concept that must be addressed with rotation is yaw. During rotation, the car will have some yaw. That is, it will be rotating on an axis that goes vertically through the center of mass. If you’re not experiencing yaw, you’re not experiencing rotation. Again, that yaw must happen during braking, not accelerating. In contrast, in a high speed corner, you don’t want yaw because that will reduce lateral grip, which would reduce minimum corner speed.

More thoughts

You can drive low speed corners with a high speed strategy. In fact, you will have a higher minimum speed on a geometric line. However, if you rotate the car, you will get to full throttle sooner. Which strategy works best depends on individual corners, the drivers, and the vehicles.

Rotation is a form of controlled spinning. If you don’t practice oversteer recovery, you may end up in a full blown spin. It’s a good idea to practice trail-braking, rotating, and drifting during training sessions, but during a race, you may want to drive with more in reserve. Driving a rotation line uses more fuel, tires, and brakes, so it’s not the best strategy for endurance racing.

Putting a big rear wing on a FWD vehicle actually makes a lot of sense. If the car is set up to oversteer in low speed corners (as it should), the wing can pin the rear down in high speed corners, but it will have very little effect in low speed corners. You can get a FWD car to oversteer quite a bit by putting less grippy tires in the rear (change compounds, widths, tire pressures). There are lots of suspension and alignment things you can do too.