Oversteer overanalyzed: weight transfer & brake bias

Whether you’re talking about drifting, e-brake turns, trailing-throttle oversteer, or Scandinavian Flick, oversteer looks awesome. It’s not always the fastest way around a corner, but it’s the coolest. Unfortunately, oversteer often leads to spins and crashes. Understanding oversteer will make you safer and faster, so that’s the topic for a few posts.

First off, let’s define oversteer. It’s pretty simple really. It’s caused when your tires are sliding and your front tires have more traction than your rear tires. Assuming your car has 50/50 weight distribution (half the weight on the front tires and half the weight on the rear tires), when you start to slide, the front and rear tires will slide equally. But this assumes you’re not accelerating or braking. As soon as you accelerate, weight and traction shift to the rear. This causes understeer. As soon as you brake, weight and traction shift to the front, which causes oversteer.

Wait-a-goddamn-minute, if accelerating causes understeer and braking causes oversteer, how the hell does drifting work? That’s totally different and has nothing to do with weight transfer. If you spin your tires really fast, the friction starts to disappear. The rubber begins to liquify and gasses build up at the tire-road interface. That’s some slippery shit. But even drifters initiate their turns with weight transfer. It’s the key to understanding oversteer.

So how does one add weight to the front of the car while driving?

  • Brake
  • Hand brake
  • Downshift (RWD only)
  • Trailing throttle oversteer
  • Downhill

Brake

Probably the most obvious way to shift weight forward is by pressing the brake pedal. But what isn’t so obvious is how much of the braking effort is being done by the front wheels vs. the rear wheels. Generally, the front brakes are designed to do more work than the rears because the engine is in the front of the car and the weight is also transferring to the front. Adjusting the brake balance between the front and the rear can make the front or rear tires lock up first. If the brake balance is too far to the rear, the rear tires will lock up first, which will cause additional oversteer beyond the weight transfer. How can you change brake balance/bias? That’s a topic for later.

Hand brake

The hand brake does double-duty for oversteer. (1) It transfers weight to the front (2) It causes the rear wheels to lock up. Grabbing a fist-full of e-brake is one of the most common ways to make a FWD car oversteer. There are a couple reasons for this. First, FWD cars can’t liquify their rear tires, so that’s out. Second, FWD cars have most of their weight forward anyway, so locking up the rears gets them to pivot very easily.

Downshift (RWD only)

Engine-braking a RWD car slows down the rear wheels only. This effectively changes the brake balance toward the rear. Downshifting and feeding out the clutch is therefore a good way to cause oversteer because the weight is transferring forward and the rear brakes are doing more work. A sudden pop of the clutch is a lot like grabbing the hand brake.

Trailing throttle oversteer

Driving at constant speed requires some throttle to counteract air resistance and mechanical friction. As soon as you lift off the throttle, the weight shifts forward. This is called trailing throttle oversteer (TTO). It’s basically a milder form of downshifting.

Downhill

If you’re driving on a downward slope, there is naturally more weight on the front than the rear. The car wants to oversteer simply because of the geometry of the track. Downhill corners are therefore the most prone to oversteering (and spinning).

Quiz

See if you can determine why these crashes happened.

Bad driving tip #1: rubbing is racing

No, no, he didn’t slam you, he didn’t bump you, he didn’t nudge you… he *rubbed* you. And rubbin, son, is racin’

That Days of Thunder quote may be the most recognizable racing quote of all time. That’s too bad, because even the slightest touch can cause cars to wreck.

Stay safe out there amigos.

Bad driving tip #2: drive impatiently

One of the things that makes racing so challenging is that there’s a constant conflict between being aggressive and being careful. Finding the appropriate balance depends on a lot of variables that are constantly changing. When should you push hard and when should you hold back? If you’re being held up by the car in front for what feels like too long, you may get impatient and do something unwise. The most common form of impatience is trying to steal the apex of the car in front. Let’s see what that looks like.

Another area where people get impatient is when a car spins in front of them. It looks like a great opportunity to gain a position, but spinning cars have a way of changing directions and crossing the track multiple times.

There’s almost always time to brake and come to a complete stop if necessary. But the urge to get past is strong, and driving around the trouble should be faster, right?

Cars reacting to spinning cars can be just as dangerous as the spinning car.

Patience grasshopper.

Bad driving tip #3: don’t check your mirrors

When two cars make contact, one of the drivers usually ends up saying “I had the line”. Well yes, there are rules that give one car priority over the other. As a very general rule, the first car that turns into the corner has right of way. Does that mean they can chop down on the trailing car? Yes and no. Yes, if you want to assert yourself as an aggressive driver, shutting the door on a competitor can be a smart move. The ensuing contact may be ruled in your favor, and that driver will probably avoid going door-to-door with you again. You win. However, there is an overarching rule that all cars deserve racing room, and if the officials say that you impinged too much on their space, the ruling may go against you. You lose. If you’re into amateur endurance racing, which is really the focus of this blog, then you lose even if you win because the other teams that didn’t get into these shenanigans all benefit whenever you pick up penalties.

One of the charms of amateur racing is the wide range of driver abilities. What looks like a driver aggressively shutting the door on another is often just lack of awareness. They had no idea the other car was there. The situation goes hand-in-hand with the bad driving tip #5, “always driving the school line”. Rank amateurs may believe that they are supposed to drive the line and let others work around them. But you can’t go outside-inside-outside if there’s a car on your door. You have to give the other drivers their racing room. Just like on the street, you have to check your mirrors before changing lanes.

In the first video below, a faster car blows by a slower car on the straight. They think they are in the clear, but they park it in the braking zone and then proceed to hit the slower car on the way to the apex.

In this next video, the slower lead car crosses the track in HPDE style and squishes the faster trailing car into a concrete barrier.

Here’s one more.

In just about every incident, there’s enough blame to be shared. In all three videos the car in front could easily have avoided the incident by checking their mirrors. The trailing drivers were the victims here, but had they realized how unaware the other drivers were, maybe they would have driven differently.

Bad driving tip #4: drive (too) nice

The road to Hell is paved with good intentions. The road to repair bills also. Your #1 job as a racecar driver is driving. While it’s a nice courtesy to signal other drivers that there is a danger ahead, such behavior requires you to remove one hand from the steering wheel and divert your attention from driving to signaling. In that moment your concentration shifts, bad stuff could happen.

In this first video, the driver sees the yellow flag and attempts to control the situation by alerting the drivers behind him. A fine gesture, and to be applauded. Unfortunately, he mishandles the steering wheel, which causes the car to drift to the left. The Volvo on his right gets spooked by other traffic (I think) and turns into him and what little space there was is now used up. This results in a multi-car crash. Alerting other drivers to the yellow flag ahead was a great idea, especially as the upcoming corner is blind and the flag station is hard to see. But keeping control of your own vehicle is job #1.

In this second video, the driver once again attempts to alert the cars around him of the upcoming yellow flag. But he’s so focused on waving that he runs into a stack of cars.

In this third video, the HPDE driver in the blue Z takes a very wide line while pointing-by 2 cars to the inside. Unfortunately, the driver is so focused on others that he doesn’t keep his car on track. The panic that ensues collects an innocent bystander.

In all 3 videos, the drivers are exhibiting a rare and treasured personality trait: kindness. But driving racecars on track is stupidly dangerous and requires a lot of attention. If you’re an expert, you can probably spare to split your attention between driving and other tasks. But if you’re not, focus on the driving as the #1 priority.

 

Bad driving tip #5: always drive the school line

In 1971, Alan Johnson published “Driving in Competition”. In that seminal work, he codified the three types of corners: Type I (leading onto a straight, Type II (after a straight), and Type III (connects other turns). The usual HPDE instruction focuses on the Type I corner. Here’s a charming picture from his book that uses Matchbox cars instead of an illustration.

The thing I find most interesting is point 3, which he calls the “balancing point”. Ross Bentley calls this the “end-of-braking”. In my book, I call it the “nadir” because if every corner has a top (apex) then every corner should have a bottom (nadir). Regardless of the terminology, this is the slowest point in the corner, and it occurs before the apex (point 4). Also note that it is described as the transition from braking to accelerating. The term trail-braking probably hadn’t been in use in 1971, but the fact that brakes are released during the corner entry means that trail-braking was part of Alan Johnson’s cornering technique, and probably many other drivers of the time. Mark Donohue is given a lot of credit for talking about brake release, but “The Unfair Advantage” was published 3 years later in 1974. I would guess that drivers were trail-braking from the beginning, because it’s the fast way through a corner.

In the HPDE world, students are taught the geometry of the Type I corner but not how to trail-brake, which is considered and advanced skill. I don’t want to get into teaching philosophy or this post will stray even further from the original intent. Let me just say that I completely disagree. So, back to the typical HPDE corner: start at the outside of the track, brake in a straight line, turn in to a late apex, track out to the outside. Oh, and if a faster car comes along, stay on the line and let them work around you. That may be fine for HPDE days, but robotically following this advice on a race track is folly. Here’s a trio of videos that will hopefully send that message home.

To be clear, these drivers aren’t at fault for the incident. They didn’t do the hitting, they got hit. But let’s say that the result is that the car is done for the weekend. It was supposed to be a weekend of racing with friends and it turned into a weekend of frustration and repair bills. You know what would have been better than following the line? Realizing that when you set up on the outside of the track, the shit-for-brains behind you sees that as an invitation to dive-bomb you.

Bad driving tip #6: on track praying

When driving a racecar, there is no place for faith. If you look at your brake lines and see they are cracked, you don’t blissfully head on to track because you have faith in the almighty that everything will be okay. The consequences of such an oversight could be harmful if not fatal to you or someone else. Similarly, if you’re a safety marshall and you see that someone has their harness straps under their HANS device, you don’t say a silent prayer for them while sending them on their way, you stop them right there!

OTP (on track praying) is a term I made up for spinning when dropping 2 wheels off track. Why is this called OTP? Because it’s like cracked brake lines or messed up harnesses: obvious and avoidable. You know you’re approaching the edge of the track long before you actually leave the asphalt. That moment you realize you’re running out of room is just like observing cracked brake lines. It’s time to do something about it. That something is not hoping everything will be okay. Yet that’s what a lot of amateur racers do. They keep the steering wheel and throttle fixed and hope the racing gods will take care of them. I have for news for you. The racing gods are unkind.

In the video above, the text reads “Once that left rear goes in the dirt it’s all over…”. Yeah, if you’re cornering at something around 1.0G and your outside rear tire can only sustain 0.6G once it hits dirt, you’re going to spin. But you don’t have to corner at 1.0G. There is an alternative. You could corner at 0.6G and then you wouldn’t spin. How can change from 1.0G to 0.6G? OPEN THE WHEEL. Go off track on purpose and under control. Don’t hope that you will stay on track. Hope is forsaken in these lands. Be proactive and do something about the problem before it becomes a disaster for you and other people.

The problem with OTP is that it’s nearly impossible to practice live. You can’t go to an HPDE session and constantly drive off track. And you can’t (legally) slide off the road on your daily commute. So what can you do? The least expensive is to role-play. You can do this entirely in your head. Imagine you’re about to go off track. Right before your tires hit the dirt, picture yourself unwinding the wheel so that you go off track in a straight line. For a little more realism, pretend you’re holding a steering wheel while watching the video above. Open the wheel to prevent the spin. For an even more authentic experience, get a simulator.