# Too fast into a corner: should I brake?

Thought about this during a recent road trip. Suppose I'm driving in a corner and realize I went in too fast, and I'm worried that the tires are about to slide. Should I slow down?

Assume the corner is constant-radius and I'm gliding with no wind resistance. So the only way to slow is by using the brakes. Unfortunately this will add to my acceleration, which makes a slide more likely. But it also slows my speed, reducing centrifugal force and making a slide less likely. So which effect dominates?

I tried to work this but it's a little too complicated for me.

## 3 Answers

Your tires have a maximum force they can provide before they start slipping significantly. This maximum force becomes a maximum acceleration.

At any point before the turn, you can apply this maximum force via braking to reduce your speed. As you mention this will reduce the necessary centripetal acceleration needed to navigate the turn.

If the turn is truly constant radius and you have no slop, then it's too late once you reach it. Either your tires have sufficient grip to navigate, or they don't. Braking would reduce the maximum turn you could make without skidding.

For a real path, you'll have some space to play with. If you have the ability to shave the corner a bit at first, you can brake while putting in a bit of turn, then decrease the radius after some speed has come off.

• So basically, whatever you do, don't brake and don't turn more into the curve. Then hope that both are possible ;) Jun 25, 2023 at 8:42

Most passenger cars are designed to understeer- that is, their front wheels lose grip before their rear wheels do, in a too-sharp turn- and the car stops turning and plows forward.

Now note that the load on the tire contact patch will be a vector sum of the side force that is turning the car and the braking force that is slowing it down. If you get into a turn and discover that you are about to overcome the tire grip with the side load and you hit the brakes in response, the front outside tire will almost certainly break loose at that instant and the car then stops turning and plows forward on a straight path that takes it out of the turn and sends it off the road.

This means that if you are pushing the car too hard into a turn, you are basically screwed: hitting the brakes will not save you from a skid.

• To have some degree of braking: there is the option of operating the parking brake. According to the wikipedia entry: "In most vehicles, the parking brake operates only on the rear wheels". If the rear wheels lose sideways grip the car will oversteer. That may give the driver better odds than in the understeer case. Jun 25, 2023 at 5:46

You're both right. I did this as a point mass moving in an arc, with tires exerting force mv^2/r against the road and about to slip. Using the brakes adds some force to this and causes the slide (before the velocity has a chance to decrease).