General Questions on Rumble Strips

[superberkut]

Why is it that a car will attain a higher top speed around a corner when running on a rumble strip/kerb? It never occurred to me, but is it a real advantage over extending the asphalt portion of a circuit by a few more feet to allow the cars some leeway?

 

[Boundary Layer]

erm, what?
Curbs aren't placed there to let drivers go faster, they're there to prevent drivers from cutting corners too severely. That's why they are rumble strips - they're serrated so they will shake the bejeezus out of the car and cause fluctuating levels of grip for the inside tire.

In general, a driver will hit the curb at the apex of a corner as the 'in' part an out-in-out line because by it's very nature that is typically the fastest way though a corner and most efficient use of the trackspace. It widens the turn.

You can sort of imagine here, that if the yellow dot were pulled even nearer the apex, the radius of the arc formed by the red line will become larger whereas if the yellow dot is pushed farther away the radius of the vehicle's trajectory through the turn becomes smaller. Wider arc = better because the centripetal force that must be applied to the car (from the tires) is less for a given velocity.

 

[superberkut]

Well, I thought I worded it that way, but the question of the physics of the cars gaining speed on it probably showed in my phrasing.

 

[Duke]

It's not that it actually speeds you up. However, if the curb is not too high, and you clip it with the inside wheels right at a good apex, it actually helps the car rotate and avoid understeer. The curb has to be just high enough to slow the inside wheels a bit without getting too airborne and unsettling the chassis. That lets you carry more speed through the corner. I usually try to stay off the exit/trackout strips unless they are very low and smooth, because they tend to make the car wash out more.

 

[niky]

If it's the right kind of kerb, it's okay to clip it... but some kerbs will unsettle the car. For some, like me, it'll make a nasty "klunk" as your strut bottoms out (not a pretty sound... not something you want to hear from a daily driver that's supposed to get you home afterwards)... for others, it means a nasty trip into the grass.

 

[TheCracker]

Curbs are probably best off being utilised on slower or medium speed corners. If you hit an inside curb in a quick corner you are more likely to upset the car and run wide onto the grass (or gravel) on the outside. In the wet, curbs, which are usually painted, are best off being avoided all together, they offer no grip whatsoever.

 

[Duke]

Understand also that we're ONLY talking about the kind of depressed curb that has a very low angle to the face of it. If it's the kind of curb that you parallel park next to, you want to avoid it at all costs.

One other thing the apex curb is good for is a landmark to let you know just where you are in relation to the apex. Just nudging the edge of the curb can give you a touchstone so you know you're hitting your apexes every time around, even if you don't ride up on them.

On the street my family always makes fun of me for hitting curbs, but I do it on purpose. Most divided streets have a raised median that slopes down to flush at the ends so trucks won't clip them. If I'm making a turn from one street to another, I like to just edge the low curb or gutter to let me know where I am on the lane.

 

[exigeracer]

If the arrows on the rumble strip are pointing away from you, they'll speed you up. If they're pointing towards you, avoid them because they will really slow you down.

 

[Kylehnat]

^

On the street my family always makes fun of me for hitting curbs, but I do it on purpose.

Doesn't that leave horrendously ugly swipe marks on your tires and wheels?

 

[SolidSnake15]

If the arrows on the rumble strip are pointing away from you, they'll speed you up. If they're pointing towards you, avoid them because they will really slow you down.

Remind me to never drive with that guy.

On the street my family always makes fun of me for hitting curbs, but I do it on purpose.

I plan on doing that when I finally start driving.

 

[niky]

Understand also that we're ONLY talking about the kind of depressed curb that has a very low angle to the face of it. If it's the kind of curb that you parallel park next to, you want to avoid it at all costs.

Hey, i hit those curbs all the time... cures understeer... but leaves a nice nasty mark on your rear rim... ...oh, okay, maybe it was just once or twice...

 

[Duke]

Doesn't that leave horrendously ugly swipe marks on your tires and wheels?

Which is why I carefully said this above:

Understand also that we're ONLY talking about the kind of depressed curb that has a very low angle to the face of it. If it's the kind of curb that you parallel park next to, you want to avoid it at all costs.

 

[Alfaholic]

General rule of keeping speed in a car is the less you're turning the steering wheel, the more speed you can carry, and the better you can accelerate. Running over the inside kerb means you can turn in earlier but also use less steering input as you don't have to turn as sharply. You can therefore take the turn faster because, as described above in previous posts, you have effectively increased the radius of the curve (i.e. makes the curve less tight). This assumes running over the kerb doesn't upset the car's balance or break any bits, and is something for the racetrack. Sometimes, if you REALLY cut the kerb tightly, you can drop the inside wheel off the inside and into the grass, which is often slightly lower than the kerb. The wheel will then hook onto the kerb, using it a bit like a railway track, allowing you to carry more speed again. You can experience this basic principle on some corners on The Nurburgring in GT4, where the tarmac has a distinct depression on the inside of some bends, particularly around the first part of the complex. Drop a wheel into the depression there and you'll feel the car being helped around the corner. Mark Hales described using this to his advantage in some race or another. Unfortunately I can't remember where he was racing or what he was driving.

Don't try that at home.

Like Touring Mars, if it's a graded kerb, not a vertical one, or a low mini roundabout, I also like to just nudge it when driving past, just for practice in anticipating where exactly my car's wheels are. It's a challenge, like hitting a particular catseye.