The physics of cars spinning in crashes

I’ve been watching a few videos of racing car crashes and it seems to me that once the car becomes airborne it spins faster. Is this an optical illusion? If not, what is the mechanism for this? Is it due to the engine still running and the torque the engine produces?

Most cars spin laterally (yaw) and not about the axis of its engine’s crankshaft. Engine torque is directed around the crankshaft, whose spin axis is 90 degrees from that ‘lateral spin axis’ of a spinning car. So engine torque would not contribute to the spin speed of a laterally spinning car. It would cause an airborne car to twist (or, ‘roll’ — if thinking roll / pitch / yaw of a car).

More: the axis of an engine’s crankshaft is typically collinear with (parallel with) the long axis of a car. Modern compacts and mid-engine cars frequently have transverse mounted engines, and their crankshafts point to the car’s left and right.

In the first case, the torque would cause the car to roll (again, roll - pitch - yaw), and in the second case it would cause the car to pitch nose up or down.

In both cases it would not significantly contribute to a car’s yaw.

My guess is that cars spinning on the ground are usually decelerating as they spin. When they go airborne their rate of spin doesn’t decelerate very much, so they look as if they are going faster since we’re subconsciously expecting to see that deceleration.

Can you post some links to videos, just so we are all talking about the same phenomenon?

Spin is produced when a force is applied from an angle perpendicular to the direction the car is traveling.

Since race cars on an oval track have to negotiate a major turn at each end of the track at the highest speed that they dare travel, they sometimes lose the rear end because of the centrifugal force being applied in a direction perpendicular to the car.

Here’s an example of “yaw spin”:
https://www.youtube.com/watch?v=lV0f5lZW96k

[quote=“kenobi_65, post:7, topic:821161”]

Here’s an example of “yaw spin”:

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Thanks. In this case, I’m pretty sure the car spins because the final impact that sends it into the air is off-center. Nothing to do with internal mechanisms of the car.

The engine isn’t going to be making any torque once the car is in the air, at least not for very long. To the extent that it does, the driveline will accelerate with extreme rapidity up to redline RPM, since the vehicle’s mass isn’t holding anything back (think of how fast the engine revs up on your car when you stomp on the gas while it’s out of gear). The torque-producing ability of the engine isn’t what matters here, it’s the rotating inertia of the crankshaft and flywheel (and the rest of the driveline) - and that’s minimal compared to the rotating inertia of the entire chassis, which is to say that you can subject the driveline to gigantic RPM changes while inducing only tiny changes in chassis RPM.

If you watch professional motocross riders, you can get a sense of what’s possible as far as reorienting a motorcycle while it’s in the air on big jumps. If the bike is in a nose-down attitude, the rider can pin the throttle, accelerating the rear wheel; the reaction force causes the nose of the bike to come up. Conversely, if the nose of the bike is too high, the rider can stomp on the brake, and the reaction force will cause the nose to come down. But these are pretty modest effects: again, the entire chassis has a lot more rotating inertial than the rear wheel and/or engine, so the transfer of any amount of angular momentum from the wheel to the chassis will result in a big RPM change for the wheel/engine, and a tiny RPM change for the chassis.

To the extent that a race car’s chassis actually changes rotation in mid-air with contacting any other solid objects, this is almost exclusively due to aerodynamics. This is particularly true of race cars running at 150+ MPH, where aerodynamic effects are very strong. Crashes often happen in close proximity to other cars, resulting in invisible turbulence that can cause the airborne car to tumble unpredictably. Here are a bunch of race crashes that involve tumbling; in many cases, you can see that they become airborne after yawing or cresting a hill, due to unexpected direction of the slipstream, nothing to do with engine RPM or torque.

Yes, perhaps spin was the wrong word. Rotating around the drive shaft is what I meant. Rolling in aeronautical terms.

My sister’s husband and their sons love to watch car racing. They were watching a race on TV when I stopped and they told me it was a “great race”. I grabbed a beer and sat down with them to watch.

Not a single crash! I told them that I had assumed “great race” = “many crashes” and they were shocked.

Okay. When I think of a car spinning I usually think of yaw,

The drive shaft likely contributes to roll, yes, but how much it does, and if it’s observable, perhaps not much. At speed, the wind drag would have a significant effect on an airborn car.

I’ve noticed crankshaft-induced inertia rotating a vehicle before — when riding my motorcycle, a BMW with the boxer engine. When at a stop and I blip the throttle, the bike pulls (rolls) right. I could discern, then that the crankshaft rotates clockwise when viewed from the front of the bike. It was an interesting phenomenon.

I too would like to see some examples. Are you looking at open wheel cars? Stock cars? Cars with wings and/or ground effects? Are these collisions with fixed objects or other cars? From what era?

Pretty much all sorts, but most normally standard type cars. For example go to the 3:20 mark in this clip.

It exists, yep - I’ve got a BMW boxer bike too, and I’ve noticed the same thing. But the effect is very small. With the gearbox in neutral, if you snap the throttle wide open, you’ll develop full rated torque, which averages about 100 Nm (80 lbf-ft) as you sweep across the entire RPM range. This will last for about 1/4 of a second before the engine starts bouncing off of the RPM limiter, at which point it will be making zero average torque. During that 1/4 of a second, the the rotational acceleration of the crankshaft causes a reaction torque to be applied to the chassis, 100 Nm. Where as the crankshaft and flywheel weigh a few pounds and their masses rotate at a relatively small radius about their axis, the chassis weighs approximately 600 pounds and its mass moves at a much larger radius from the axis of rotation. Result? Crankshaft hits 8,000 RPM in one direction, and the chassis (if unaffected by gravity or your bracing efforts) hits a few RPM in the other direction. It also gets reversed as soon as you close the throttle and let the crankshaft decelerate back down to idle; during decel, the crankshaft is exerting a torque in the other direction, which tends to slow the rotation of the chassis back down to zero.

That car looked like it was going slow enough for aerodynamics to not be a major factor in the tumbling; it looks like the car was only under the influence of the ground and its own momentum. The tennis racket theorem may be in effect here, as the car fits the requirement of having three distinct moments of inertia; the result is that end-over-end rotation would tend to devolve, without outside influence, into yaw and/or roll rotation - the latter coinciding with the driveshaft. Here’s an outer-space example of such craziness.

Also, once the tires leave the ground, you lose an awful lot of friction with Mother Earth.

I believe the yaw induced to the Indycar in the above example was due to it climbing over the wall and impacting the wire fence. At over 200 MPH, anything going up against that wire mesh fence is going to experience a huge moment applied several feet away from its center of mass.

And hence the energy from the engine has to be expressed somehow.

And I suspect that this contains a hint to the answer: In general, harder impacts (higher impulse) will make a car spin faster about whatever axis, and the impacts that send cars into the air will be very hard indeed.

Yes, the wheels will spin faster.