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I see hundreds of exhaust scrapes on speed humps and things, or general gashes in the direction in which cars travel over them.
Does hitting the exhaust on bumps damage the car?
It's probably blindingly obvious, but how does it happen? I mean in my 2003 Opel Agila I've never hit the exhaust, and gone over humps at about 30mph (~50kph), and haven't bottomed out...
The bottom line is yes, if you drag your exhaust over the speed bumps, it will cause damage to your car. It will be mainly localized to the exhaust on the vehicle. Besides flattening out the exhaust pipes, you also put stress on the joints and the hangers for the different mounting points. If you hit the exhaust hard enough, you can cause further damage to the under side where the exhaust may be pushed up into other parts. If hit hard enough, it can also force damage up into the exhaust manifolds on the engine.
The damage to the speed bump could be caused from any of several reasons. It happens when the lowest part of the car is not high enough to go over the speed bump. It might be the exhaust which hits (most likely part on modified cars). It could also be other parts. For instance, I had an aftermarket torque arm on my 94 Camaro Z28. The mount for it hung low due to it's design which allowed the single exhaust pipe to go through without touching it. It was in the center of the vehicle. Since it hung low, then add in 1.5" worth of drop from lowering springs, this made it really tough to go over speed bumps without hitting anything.
The scrapes on the road you are seeing are probably people dragging hitches from their minivans that weren't designed for towing. I'm sure you have seen cars and vans with hitches sitting a little lower to the ground than usual.
As for hitting the exhaust on bumps, again this is normally cars that are low to the ground or have been modified (usually to be lower to the ground). If the only thing that is hitting is the exhaust, that's all that will be damaged. The extend of this depends on the situation, but you could potentially rip out a large portion of your exhaust system. Other than that, you'll probably just get some dings on the muffler.
