eli5: Why does a car spinout if the wheel is completely straight during hydroplaining?


Lets say you are traveling at a speed of 80, and there are no other variables like braking and acceleration, and you hit a giant puddle with the wheels COMPLETELY straight, why would the car suddenly jerk in one direction causing a spinout? Shouldn’t the car just continue in the direction of the wheels regardless of traction loss?

In: 4

Well, yes. The car will still mostly keep moving in the same direction. Cars, however, are not balanced. There might be more weight on the front or rear. This means the tires are not equally loaded. Even if the water and the road underneath are perfectly even (rare!), the front wheels might gain traction before the rear and that starts the front slowing down before the rear causing the spin. Or vice versa.

Unequally worn tires and road unevenness etc means the water is not equally distributed. So in any real life situation, the car is no longer in control of the driver but rather to external variations (roads are not made “flat” they slope slightly from the center towards the edge to promote water runoff.)

In your ideal scenario, sure, but in the real world you can’t ignore friction, acceleration, or the fact that your wheels are never perfectly straight. Your wheels don’t hit the puddle all at the same time. Your front wheels hit first. Now the friction is different on water (hydroplaning) than it is on the road. So your front tires are experiencing different forces than your back tire. If your car is a front-wheel drive and you were maintaining speed, this sudden change in friction would cause a momentary acceleration of those tires. Alternatively, if your car is rear-wheel drive, then it will continue pushing the front with high grip on the road while your front is no longer providing the same resistance it just was, causing your back to accelerate. When there is a difference in scceleration between the front and back, you will fishtail snd spinout.

At certain speed, the wheel can FLOAT on the thin film of water. It is the time when the car can slip to any direction.

Because it is unlikely that each wheel will encounter exactly the same depth of water that it is hydroplaning on, and it is unlikely that the roadway would be level… because roads are built with a crown to shed water.

That crown means that if there is standing water it is guaranteed to be deeper on one side than the other.

Aside from that, if the water depth is equal, the road is flat, no wind, no braking, steering or accelerating – then there would be no reason for it to spin out.

If this is to, as you say, settle a debate, then check out some YouTube explainers on a cars Traction Control and EBD systems. This will explain how different wheels could have different traction even under the same presumed conditions.