What Makes A Car (or motorcycle) fish-tail?

[Dari]

I'm looking for a logical (physics related) answer.

 

[silverpig]

Without actually checking, I'd say it was conservation of angular momentum.

 

[speed01]

Lack of friction.......

 

[Dari]

would it then help if you went in the opposite direction (or at the very least, turned the wheel in the opposite direction of the turn)? Would that stop it?

 

[WinkOsmosis]

What causes fishtailing is the rear end of the car having less lateral traction than the front end.

 

[silverpig]

Originally posted by: Jellomancer
What causes fishtailing is the rear end of the car having less lateral traction than the front end.

That's not what CAUSES it. You can have situations where the rear has less traction than the front and you won't fish tail.

 

[speed01]

No. Once you lose traction the vehicle is going to try to go in the same direction. If you turn in the opposite direction you allow the inertial forces to carry you in the initial direction of the slide but when you turn into the slide you impose an outside force upon the slide causing it to change direction and slide the opposite way. Thus resulting in the fishtail action of sliding from one side to the other repeatedly until the tires generate enough friction to overcome the lateral movement of the vehicle. Or something to that effect.....

 

[Zenmervolt]

Originally posted by: silverpig

Originally posted by: Jellomancer
What causes fishtailing is the rear end of the car having less lateral traction than the front end.

That's not what CAUSES it. You can have situations where the rear has less traction than the front and you won't fish tail.

Not bloody likely in real life. If there's insufficient lateral traction at the rear then almost any lateral force will cause a slide, and in most real-world situations there will be some sort of lateral force.

ZV

 

[thomsbrain]

dari, do you mean when you go fast around a corner, or when you burn out from a stop?

if turning, then turning "into the slide," which means turning the oppisite way from the way you were trying to turn originally will help stop the slide.

if burning out from a stop, it has more to do with the tires getting more or less traction randomly, and on cars equipped with them, the limited slip differential is constantly trying to juggle power back and forth to the grippier tire, making the car fishtail until it gets traction.

 

[silverpig]

Originally posted by: Zenmervolt

Originally posted by: silverpig

Originally posted by: Jellomancer
What causes fishtailing is the rear end of the car having less lateral traction than the front end.

That's not what CAUSES it. You can have situations where the rear has less traction than the front and you won't fish tail.

Not bloody likely in real life. If there's insufficient lateral traction at the rear then almost any lateral force will cause a slide, and in most real-world situations there will be some sort of lateral force.

ZV

Ever driven through a puddle? I do all the time here. Almost never fish tail though.

I'm still pretty sure it's conservation of good 'ol I*omega.

 

[raptor13]

My thought is (please prove me wrong if I am!) this:


Before fishtailing starts, a situation has to arise where the back end initially breaks loose. That could be mashing the gas in a rear wheel drive car and then turning or any other number things. Then, since a car in motion is - assuming it's set up reasonably well - stable, it will try to return to a straight line path. This effect can also be seen when a car "unwinds" from a turn, i.e. the steering straightens out if you let go of the wheel.

So this is the situation now: The backend is out of alignment with the frontend and trying to get back into equilibrium. It moves to the center but, because of momentum, moves past the middle to the other side. A pendulum effect is the result. That right there would be fishtailing. The amplitude of the fishtailing dampens to nothing because of friction if left to correct itself. The car would spin when an incompetant driver tries to correct the instability, over corrects, and ends up adding more instability.

And that's that.

 

[silverpig]

Yeah, we have to narrow down the situation here... Are you asking about a car accelerating and then fishtailing, or just having a stupid driver yank the wheel back and forth, or what?

 

[Dari]

I'm talking about when you're driving in the rain. Happened to me a month back. I was driving up Broadway when the car started to fishtail...scared the bejesus out of me. Thank God there was no one around for me to crash into. Now I think twice before speeding in the rain.

 

[silverpig]

Were you braking, accelerating, or just travelling at a constant speed? Coulda been hydroplaning combined with one tire in a puddle combined with a few other things.

 

[Tominator]

I can't tell you the physics behind it, but anytime one wheel is in a skid it attempts to pass those that are not. No exceptions. Uphill, downhill, no exceptions.

 

[WinkOsmosis]

I meant the rear wheels having less traction than the front AND not having enough traction. This is why FWD cars generally don't fishtail... less lateral traction in the front in all situations (except with the clutch disconnected).

 

[Zenmervolt]

Originally posted by: silverpig

Originally posted by: Zenmervolt

Originally posted by: silverpig

Originally posted by: Jellomancer
What causes fishtailing is the rear end of the car having less lateral traction than the front end.

That's not what CAUSES it. You can have situations where the rear has less traction than the front and you won't fish tail.

Not bloody likely in real life. If there's insufficient lateral traction at the rear then almost any lateral force will cause a slide, and in most real-world situations there will be some sort of lateral force.

ZV

Ever driven through a puddle? I do all the time here. Almost never fish tail though.

I'm still pretty sure it's conservation of good 'ol I*omega.

I usually get a bit of a twitch to one side or the other if the puddle does, in fact, induce a sufficient lack of traction. Most of the time the tire will sipe away more than enough water to maintain sufficient traction.

ZV

 

[silverpig]

Okay, "lateral traction" doesn't really make much sense anyways.

u (mu) is a scalar quantity, not a vector. It doesn't have direction.

I'm pretty sure it's caused by a change in engine rpm. The change in the angular momentum of the engine is countered by a gain in angular momentum elsewhere in the car.

If you know any race car drivers (I mean REAL race cars, not suped up mustangs), they'll tell you that their cars actually twist and start to turn when they hit the gas hard. This isn't because of lack of traction, but because of a HUGE increase in angular momentum caused by the high torque engine.

It's sorta like how leaning your bike causes it to turn.

There was actually a PC racing game with ultra realistic physics that incorporated this effect. I can't remember the name of it right now though.

 

[Tominator]

Originally posted by: silverpig
Okay, "lateral traction" doesn't really make much sense anyways.

u (mu) is a scalar quantity, not a vector. It doesn't have direction.

I'm pretty sure it's caused by a change in engine rpm. The change in the angular momentum of the engine is countered by a gain in angular momentum elsewhere in the car.

If you know any race car drivers (I mean REAL race cars, not suped up mustangs), they'll tell you that their cars actually twist and start to turn when they hit the gas hard. This isn't because of lack of traction, but because of a HUGE increase in angular momentum caused by the high torque engine.

It's sorta like how leaning your bike causes it to turn.

There was actually a PC racing game with ultra realistic physics that incorporated this effect. I can't remember the name of it right now though.

Maybe I used to be a 'real race car driver,' but not any more. The forces on a chasis brought about by torque or attitude of the chasis have little to do with loss of traction and the tendency to fishtail as the question was posed.

'Leaning your bike' does not cause it to turn, btw.

I cannot play any racing game without getting frustrated. 50% of racing is seat of the pants. No PC can duplicate that.

 

[silverpig]

Originally posted by: Tominator

Originally posted by: silverpig
Okay, "lateral traction" doesn't really make much sense anyways.

u (mu) is a scalar quantity, not a vector. It doesn't have direction.

I'm pretty sure it's caused by a change in engine rpm. The change in the angular momentum of the engine is countered by a gain in angular momentum elsewhere in the car.

If you know any race car drivers (I mean REAL race cars, not suped up mustangs), they'll tell you that their cars actually twist and start to turn when they hit the gas hard. This isn't because of lack of traction, but because of a HUGE increase in angular momentum caused by the high torque engine.

It's sorta like how leaning your bike causes it to turn.

There was actually a PC racing game with ultra realistic physics that incorporated this effect. I can't remember the name of it right now though.

Maybe I used to be a 'real race car driver,' but not any more. The forces on a chasis brought about by torque or attitude of the chasis have little to do with loss of traction and the tendency to fishtail as the question was posed.

'Leaning your bike' does not cause it to turn, btw.

I cannot play any racing game without getting frustrated. 50% of racing is seat of the pants. No PC can duplicate that.

There has to be some kind of sideways force to cause the car to fish tail. If you went perfectly straight on a piece of road, and then continued onto a sheet of ice, you'd continue going in a perfectly straight line.

Take a hula hoop and roll it out. It'll lean and turn. It turns because it attempts to conserve angular momentum.

 

[Mungla]

Ever been going 60mph on a sportbike and fishtailed? Ahh, what a rush.

 

[Tominator]

Originally posted by: Mungla
Ever been going 60mph on a sportbike and fishtailed? Ahh, what a rush.

It WAS a rush at 179mph until I crashed....I do not remember the 'rush' however...unless you mean skidding several hundred feet into the hay bails....god bless them!

 

[silverpig]

Heh, sounds kinda fun... if you know you're gonna live that is

Bah, I can't find anything about it on the internet.


Granted, you do have to lose grip with the rear tires, but there has to be some kind of force turning the car. If you're in a turn and it happens (oversteer), then it's obvious, but if you're just driving straight and you start fishtailing all of a sudden, it's something else.

 

[sharkeeper]

This guy wasn't as lucky. :Q

Cheers!

 

[Tominator]

Granted, you do have to lose grip with the rear tires, but there has to be some kind of force turning the car.

I cannot explain it, but if you lose traction on one axle you will start to spin. It is unavoidable.