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# braking

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Is it normal that when braking the wheels that they don't rotate, the car at about 100 km/h (62 mph) spins on a not so smooth road? This is what I experience with my sim. Edit: spell [edited by - szinkopa on June 5, 2004 6:51:49 PM]

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From what I know...

In an ideal situation, if the car was going in a perfectly straight line, and all wheels breaked equally, the car would not lose control and would halt without turning or flipping over, no matter how much force is put in the braking.

However, if the car is turning, even just a bit, or if the forces applied during the braking is not the same for all 4 wheels... The distribution of force will provocate a rotation of the car around its center of mass... And then it will most likely lose control and flip over.

Your simulator could need some better simulation of some things... Do remember that when the 4 wheels brake, its not a single force that comes into play, but 4, one for each wheel, and those forces are applied at different points of the car, etc... You'll need to think about it and do some more reasearch.

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[edited by - Max_Payne on June 5, 2004 11:46:07 AM]

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Thanks Max.

I am aware of the 4 forces. I do so. 4 forces at the contact patches creating force and torque on the body. This is OK. No need to think about it any more

Well I emphesized that the road is not perfectly smooth, a bit rough. And I think from this that the vertical force (or holding force) at the tyres can be different. And the friction force depends on holding force about linearly. Based on this I think even if the car goes staight ahead, on a rough road it may spin a bit, because the forces at the 4 contact patches may be different, creating torque on the body.

This is my opinion. I think it''s logical, but I wasn''t sure, because I haven''t seen such situation in games yet.

Waiting still for others'' opinions.

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Although I've never tackled this particular problem I think I may have the solution. I assume you are applying the friction force for each tyre to directly oppose the direction of motion using only a single friction coefficient. This would assume that a tyre will resist motion in all directions by the same amount if its not allowed to rotate, ie with the brakes on. However imagine trying to drag a tyre around on the floor (without it rotating or tilting). You'll find it easier to move in the direction it would usually roll in than perpendicular to that direction due to the curvature of the tyre. Hence it is necessary to calculate the friction force in the tyre's perpendicular direction using a higher coefficient than for the roll direction. This should encourage the car to keep travelling straight while braking since the friction forces in the perpendicular direction will be stronger. Hope this helps

[edited by - motorherp on June 5, 2004 5:56:12 PM]

[edited by - motorherp on June 5, 2004 6:26:06 PM]

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quote:
Original post by Motorherp
Hence it is necessary to calculate the friction force in the tyre''s perpendicular direction using a higher coefficient than for the roll direction. This should encourage the car to keep travelling straight while braking since the friction forces in the perpendicular direction will be stronger. Hope this helps

The interaction of lateral and longitudinal tire forces is a great deal more complicated I''m afraid. There are entire books on the subject.

szinkopa, could you rephrase your question, I''m not entirely sure what the issue you''re having is. From what I could gather, you are getting 4 different braking forces from the different wheels because they all have a different load on them at any given point in time, and that is creating a torque and spinning out the car?

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The situation:
- 100 km/h velocity
- the car goes straight ahead
- the ground is not smooth, a bit rough

Then I apply a big brake, so that all the wheel stop rotating until the car stops. Then the car begin to slide with not-rolling wheels. But the ground is not smooth so the load on the wheel are varying in time, and are different. And that''s why the forces can create torque on the car and spin it.

So the wheel are not rolling ! The brake force does not matter, they are big enough to keep the wheels not rotating.

The another question would be that the friction coefficient of a tyre is the same in every direction or not? And if not, then how does it vary in the different directions?

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keep in mind that real cars have antilock breaking
as soon as a wheel stops spinning, it starts to skid against the road and you lose control
with antilock breaks, the car automatically releases the breaking force on a wheel that has skidded to avoid going into a spin and stuff like that

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Not all cars have ABS. Infact most that will be fun to drive..dont.

So the situation is all 4 wheels are locked up, and the car is sliding along with the loads changing at all the corners causing it to not slide straight?

This seems correct, the force any tire can generate in any direction, if its sliding or not, is dependant upon the load on it. Once you lock up all four tires you have basically given up any control of the car anyways, so the driver has no place to complain.
As for the seperation of long/lateral forces, I dont know any specifics. Last I remember reading was some website about Pacerjka(sp?) Tire forumlas and how it gets even more complicated when the two directions are combined. Infact I doubt there is any one rule to really explain all tires, it would seem to depend on individual settings.

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Thanks guys.

In Live For Speed the wheels are locked up when braking too. I will check if the car spins or not. The only trouble is that the road is smooth.
BTW in my opinion it has very realistic physics. I like it.

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Normal brakes don''t act on all wheels - just front axle I think. That''s why handbraking at speed is wierd - locks back wheels. If the ground is bumpy the weight transfer to front wheels means they have most determination of path. Any bumps will make left/right act more hence stopping the path being straight. Then it''ll spin any old way though you should be able to have some steering control - tyres should have more friction sideways than forwards even when locked I''d imagine due to the grooves running around them.

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