braking

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]

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.

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]

quote:

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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

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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?

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?

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

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.

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.

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.

quote:

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Original post by d000hg
Normal brakes don''t act on all wheels - just front axle I think.

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No.


On the subject of Live for Speed, I did not find the tire physics to be.. ''complete'' I guess. I noticed the RWD car, while completely underpowered, drove like it had 500hp. It seems the tires would give up too much lateral grip when even the slightest bit of longitudinal was required of them.

CombatWombat:

I like the FWD car in it. I played very few with the other.



Well this should be another topic: Which PC game has the best tyre model? Of course we cannot compare F1 with rally. So by category.

Applying friction forces in the tyres'' two directions need not be complicated as combatwombat implies. I suggest splitting the tyre''s force of motion into its roll and lateral directions first. Then make two seperate friction force calculations for both directions using their corresponding friction coefficients. When the tyre is closely aligned to the cars direction of travel its lateral force will be small. If the lateral friction coefficient is made much higher than the parallel coefficient, the lateral force should be insufficient to overcome the static friction thus preventing the tyre sliding in the lateral direction. However as the angle between the tyre and the direction of motion increases, the lateral force will increase and eventually overcome the static friction leading to the tyre sliding sideways. This way the car can be made to skid when cornered sharply but should remain stable when kept pointing in its direction of motion. In fact if you are also incorperating the weight shift of the car, then as the car decelerates the weight over the front tyres will increase giving them greater grip and improving the cars stability. For more info on static and sliding friction I suggest reading any core physics text book. Hope this helps

quote:

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Original post by CombatWombat

quote:

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Original post by d000hg
Normal brakes don''t act on all wheels - just front axle I think.

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No.

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Care to elaborate? I thought in FWD at least both gas and brake pedals affected front wheels only with Handbrake on rear. Could you tell me how these are set up for FWD,RWD and 4WD vehicles?

The brake acts on every wheel in each situation, as far as I know. Handbrake in street cars is for fixing the car while it''s standing, and it act on the rear wheels.