If shock absorbers are meant to absorb shock, why do we drive slower through pot holes?

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If shock absorbers are meant to absorb shock, why do we drive slower through pot holes?

In: Engineering

Your shocks have a limited amount of travel up and down. They can only absorb so much before they bottom out.

Because they can’t absorb more force than they are capable of.

Shock absorbers and suspension come in many different forms but they can all be understood by looking at a spring.

If you squeeze the spring between your fingers it compresses, and the force it contains pushes your fingers apart.

If you squeeze hard or fast enough you overcome the springs force and push the two ends of the spring together to make it flat.

You have made it absorb more force than it is capable of.

It’s the same with shock absorbers. If you squeeze them too hard or too fast you overcome their force and make them flat, which means they no longer absorb any shock and it all gets passed into the car itself.

So we go slow on rough ground to try to stop that happening.

One hard blow (above the material’s elasticity, and already in the plastic deformation range) will break them immediately, but they can take hundreds of thousands small blows in the elastic range before the material wears out eventually.

So you’re better off driving slowly over potholes since you won’t know how hard the blow on your absorbers will be.

Source: I’m a mechanical engineer specialised in material science and technology

Driving hard through potholes also buggers your tyres. I hit a pothole in the dark and rain a few months ago. Ruptured the tyre.

Very few cars hit anything hard enough to bottom out the suspension. While some lowered sports cars might, it takes a bit more impulse to bottom out most cars. Usually anything that could do that would also damage the rim and/or tie rods as well. Most cars have a surprising amount of suspension travel, and most only take up about half of that sitting under their own weight unless there’s something wrong.

However, most suspension isn’t as good as you’ll find on a race-tuned stadium/Baja truck or high-end luxury car either. That means something like a pothole that those vehicles would not even feel would send an uncomfortable jolt through as they only absorb part of the impact. It can also jerk the wheel and make for an involuntary swerve. Both of those are lessened at lower speed. Good suspension may absorb nearly all of the impact, but you rarely find that on any vehicle with a reasonable price tag. You’re more likely to have a vehicle with dampers that are meh and spring rates that aren’t exactly optimal, geared more towards longevity and production costs than performance.

That said, there are some nasty potholes that could cause the suspension to bottom out, and some cars are in such bad shape that the suspension is notably softer than it should be. There’s reasons why one should replace worn shocks/struts/springs. But for the most part, anything that will bottom your car out will likely cause enough other problems that bottoming out is not a big issue by comparison. Damaged tie rods, bent rims, launching the car, or simply transmitting excessive force to parts of the vehicle that are not really meant to handle that impact are bigger issues.

Because they’re not magic. In times gone past, if you drove your carriage over a pot hole, your wheel fell off and your carriage broke in half, then your horse ran away.

Now, you might break your axle or bend your tire if you take a pothole too fast. That is far preferable. But you still need to account for the fact that there are limits to your vehicle and what it can withstand.