How can powerful cars easily do a burnout, but also take off from the line without spinning their tires?
In: Engineering
Ignoring the TCS (computer stuff), here’s the deal, OP:
The driveshaft of the car is typically able to deliver far more energy to the wheels than they need to turn. This extra value fades off as you approach higher speeds and the rotational velocity approaches the amount required to turn the wheels at that given speed, given those specific traction conditions.
All modern vehicles have some form of Traction Control System (explained in other replies, but basically a computer system that will slip the clutch a bit) that detects when there is too much torque for the speed at which the wheels currently are turning. With TCS, the rotational rate of the driveshaft will be retarded to provide as much power as it can (with varying margins based on the car’s design and intent, sports cars push the envelope much more) without overpowering the tires and breaking traction.
Most, if not all, of these modern cars also feature some means of disabling TCS. You can shut the computer off, or at least that function. It will no longer manage the torque at all and trust the driver’s judgment. So when you hit the gas and slip into gear, there’s nothing affecting how much torque is traveling through the driveshaft.
One other thing that can allow for spinning, even with TCS enabled, is “dumping the clutch” on a manual. For this, I need to ELY5 Manual transmission:
Manual cars work by transferring energy across a clutch, which is kinda like a bowl with a plate of sandpaper snuggled inside of it. The bowl and plate are connected to the engine output (camshaft) and drivetrain input (driveshaft). When you disengage the clutch (push the pedal down), a spring holds apart the plate and bowl, which keeps them from contacting; because of this, you can rev the engine freely with no resistance (don’t do this, by the way, it’s kinda bad for the engine). When you engage the clutch (by releasing the pedal), the plate and bowl come together and the one “grabs” the other and rotates it by frictional force.
So normally, because you don’t want the shock of all of the engine’s rotational energy hitting the drivetrain at once (and snapping it or jerking the car), you sloooowly let out the clutch, and the plate kind of “bites” the bowl partway about halfway up the pedal’s travel, allowing for transfer of some (but not all) rotational energy. This is called the “bite point” by most English-speaking manual drivers (unsure if the term is shared across languages), and is a significant part of manual transmission operation (at least in a smooth manner) Normal manual shifting involves letting this process happen in a controlled manner so that the driveshaft can “catch up” with the engine.
Now, back to spinning wheels and your OP: if you DON’T let it out slowly and just let the clutch smash shut, it’ll instantaneously transfer all of the energy. We drivin’ aficionados call this “dumping the clutch”. Since the TCS is expecting you to drive not like an idiot (remember, I’m discussing wheelspin with TCS enabled), the massive torque spike can not be accounted for quickly enough and you’ll start spinning your wheels. The TCS will recover eventually and start angrily berating you with its dash blinker as it slips the clutch and regains traction, but until then you’ll be burning rubber.
**VERY TL;DR:**
* – Most cars would spin the tires if not regulated in any way.
* – Computers can (and in normal conditions do) stop cars from spinning tires
* – It is very much possible to control and prevent tire spin (loss of traction) manually (clutch bite/management in manuals, throttle control in automatics) even without said computers
* – Drivers can disable the computer or take actions that are mechanically unsafe to spin them anyway.
Comes down to the tires. A burnout is typically caused by a shock-load of torque, which causes the initial loss of traction. Once slip starts, that threshold torque to maintain slip is lower than the threshold to go from grip to slip.
If you were to put arbitrary numbers on it, a performance tire can maintain grip so long as you apply less than 200 torques. Stay at 199 and you’ll never spin. But once it breaks loose, it only requires 150 torques to keep slipping, and you have to drop below that to regain traction. So a driver looking to maintain maximum acceleration is going to work the throttle to maintain 199 torques.
Depends on the car and the driver.
If you mash the go pedal on a 1st generation Viper, it’s going to spin rubber because there’s nothing to interrupt the cable from your accelerator to the throttle body.
If you mash the go pedal on a 5th generation Viper, as long as traction control is on, a computer will take care of figuring out how much throttle to give the car.
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Older cars did struggle with this exact issue and it was mainly down to driver skill, clutch and throttle balancing to maintain traction.
Newer cars with faster computers and faster controls can limit so many variables to certain parameters, and then monitor those parameters and change them if they detect slip on a certain tyre.
The earliest tech example is two step limiting. A control which allowed the engine to rev to a certain rpm ( an rpm where dumping the clutch wouldn’t cause a burn out, but a launch ) before the actual red line or cut off limit.
New traction control systems use mainly clutch bite (automatics). Where essentially if slip is detected then clutch pressure can be relieved. Combined with a little bit of braking pressure, and throttle adjustments.
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