# How Does Traction Control Work?

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I want to find out how does the traction control system in car's work. I'd be really grateful if somebody explained it + added some equations :)

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if the computer thinks the wheel speed is too fast (IE slipping) it applies the brake to that wheel.

the idea being that the speed of the tire surface as it spins over the road will hopefully drop below the speed at which it hooks up and Ks as opposed to Kd applies. (Coefficient of static friction as opposed to dynamic).

coefficient of static friction

coefficient of dynamic  friction

"Physics" by Tipler

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Some cars also cut fuel also, so engine power output will also factored in if you are trying to simulate this.

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if the computer thinks the wheel speed is too fast (IE slipping) it applies the brake to that wheel.

the idea being that the speed of the tire surface as it spins over the road will hopefully drop below the speed at which it hooks up and Ks as opposed to Kd applies. (Coefficient of static friction as opposed to dynamic).

coefficient of static friction

coefficient of dynamic  friction

"Physics" by Tipler

OK! So that's what actually allows you to have realistic simulation on keyboard without a car that slips like crazy and can't start movement due to slip ratio around 1000000? If so, than this is good thing. I kind of think that it shouldn't be that hard to simulate. The fuel cut means that it reduces throttle, right?

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The fuel cut means that it reduces throttle, right?

correct. but they typically cut spark, not fuel, and its cause you hit redline, not cause you're slipping. and of course this is only on late model P.O.S. vehicles with closed loop induction systems and such.

REAL cars (IE race cars) don't have traction control.  race cars (like rail dragsters and such) will typically use progressive clutches, which lock up more and more as you go down the track. lockup too fast, and you break loose and your tires spin. lockup too slow and you're leaving tenths on the tree (not going as fast as you could). you want the force and mass to stay just under Ks at all times for max accel and no wheel spin.  once you look at the formulas it'll make much more sense.

and i'l tell you right now, there's no term such as "slip ratio" in the formulas. sounds like something somebody made up while trying to model slippage without bothering to learn the physics behind it first. PLEASE tell me you're not using bullet or its ilk. folks seem to have a lot of problems with many physics libraries when it comes to realistic vehicle physics. not sure why. physics was always one of my best subjects - long on theory, and short on formulas to memorize - so i always roll my own, and thus have never seen what the deal is with bullet etc.  all i know is that even bethesda can't seem to use havock correctly (oblivion and armor that falls though the floors). although i must admit i haven't seen such issues in skyrim or fallout 3. it would seem that a basic knowledge of physics is required to use a physics engine correctly.

race cars also typically have a rev limiter that cuts spark at high rpm. race engines are expensive, and you don't want to over-rev them and blow them up. but those will typically drop just one cylinder at random to keep the rpm at the whatever limit you set. not cut power to the coil(s) entirely until the rpms drop.

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The fuel cut means that it reduces throttle, right?

correct. but they typically cut spark, not fuel, and its cause you hit redline, not cause you're slipping. and of course this is only on late model P.O.S. vehicles with closed loop induction systems and such.

REAL cars (IE race cars) don't have traction control.  race cars (like rail dragsters and such) will typically use progressive clutches, which lock up more and more as you go down the track. lockup too fast, and you break loose and your tires spin. lockup too slow and you're leaving tenths on the tree (not going as fast as you could). you want the force and mass to stay just under Ks at all times for max accel and no wheel spin.  once you look at the formulas it'll make much more sense.

and i'l tell you right now, there's no term such as "slip ratio" in the formulas. sounds like something somebody made up while trying to model slippage without bothering to learn the physics behind it first. PLEASE tell me you're not using bullet or its ilk. folks seem to have a lot of problems with many physics libraries when it comes to realistic vehicle physics. not sure why. physics was always one of my best subjects - long on theory, and short on formulas to memorize - so i always roll my own, and thus have never seen what the deal is with bullet etc.  all i know is that even bethesda can't seem to use havock correctly (oblivion and armor that falls though the floors). although i must admit i haven't seen such issues in skyrim or fallout 3. it would seem that a basic knowledge of physics is required to use a physics engine correctly.

race cars also typically have a rev limiter that cuts spark at high rpm. race engines are expensive, and you don't want to over-rev them and blow them up. but those will typically drop just one cylinder at random to keep the rpm at the whatever limit you set. not cut power to the coil(s) entirely until the rpms drop.

I use Bullet for collision detection, gravity and I use it's function applyImpulse to apply all my calculations(friction, suspension, etc.) aswell as applyForce for drag.

I hope it goes well...

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I want to find out how does the traction control system in car's work. I'd be really grateful if somebody explained it + added some equations :)

I think the traction control has nothing to do with physics and there is no equation, it's just a "programmed" logic that handles the "too-much-slip" situation.

As said above applying brake and cut-off throttle are both used.

I've been only using the last one so far and quite happy with that. The main difference is when you are applying brake, different torques can apply on each wheel, while when cutting off throttle will affect both wheels.

OK! So that's what actually allows you to have realistic simulation on keyboard without a car that slips like crazy and can't start movement due to slip ratio around 1000000? If so, than this is good thing. I kind of think that it shouldn't be that hard to simulate.

That's something different. You should be able to drive with keys without traction control.

Seems more like low-speed-slip situation, when you divide with ground speed in your slip ratio calculation. Try capping the value between -1..1 or -1..10 maybe :)

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OK! So that's what actually allows you to have realistic simulation on keyboard without a car that slips like crazy and can't start movement due to slip ratio around 1000000? If so, than this is good thing. I kind of think that it shouldn't be that hard to simulate.

That's something different. You should be able to drive with keys without traction control.

Seems more like low-speed-slip situation, when you divide with ground speed in your slip ratio calculation. Try capping the value between -1..1 or -1..10 maybe :)

Thanks, good idea. Somewhere around -3...3 will work well I think;)

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There are two main mechanisms for traction control:

1) When only one wheel at a side of the differential slips and spins due to an excess of power.

The brake of that wheel is applied in bursts. This provides extra power to the wheel at the opposite side of the differential, allowing the vehicle to gain traction.

This traction control mechanism is sometimes named ASR (Anti-Slip Regulation). It typically uses the same circuitry as the ABS.

2) When the two driven wheels are slipping

In this case the system selectively cuts the engine power for reducing the output torque, allowing the wheels to regain grip. This is the typical TCS (Traction Control System).

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There are two main mechanisms for traction control:

1) When only one wheel at a side of the differential slips and spins due to an excess of power.

The brake of that wheel is applied in bursts. This provides extra power to the wheel at the opposite side of the differential, allowing the vehicle to gain traction.

This traction control mechanism is sometimes named ASR (Anti-Slip Regulation). It typically uses the same circuitry as the ABS.

2) When the two driven wheels are slipping

In this case the system selectively cuts the engine power for reducing the output torque, allowing the wheels to regain grip. This is the typical TCS (Traction Control System).

OK! What's the circurity of ABS? What type of formula does it use? Currently I'm using that I assign property to something whenever it's 0 and apply brake force, when it's bigger than 0 than I subtract 1 and not apply brake force. However, this isn't smooth enaugh.

For the second version I think it's done in the engine, right? Either limiting fuel or the spark:)

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