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I'm wondering if it is better for your suspension to drive across a speed bump at a right angle (so that the two front tires and the two rear tires hit the bump at the same time, respectively), or at a skewed angle so that only one tire at a time hits the bump?

I've seen folks do both things and I'm wondering if there is a real significance to it or if it's just superstition.

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3 Answers 3

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Driving at a right angle will make the front and then the rear move further up and down than travelling over one wheel at a time, but that movement will be in one plane.

Driving diagonally puts more stress on the chassis as it tries to twist first one corner then then next. The car will not move up and down so much, but will move sideways a lot more.

So of the two options, the least stress on the car comes from driving straight.

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If you drive over it fast enough diagonally, the chassis doesn't twist at all; the bump just gets absorbed by one spring at a time. :-) –  R.. Oct 15 '12 at 23:45
    
The point being: the real reason to go diagonal is so you can go over the bump fast without slamming your bumper into the pavement due to the rocking. Not whether it's better or worse for the car. –  R.. Oct 15 '12 at 23:47
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There are several reasons why you should be approaching the bump at an angle.

Mechanical stress on the chassis

Let's take two approaches and study how they affect the chassis:

  1. Let's drive at the right angle (frontal). The chassis will always be stressed in one direction: front to back. The chassis is never stressed from side to side.
  2. Now let's try approaching the bump at an angle. The car is stressed in both directions, front to rear and side to side. The stress is evenly distributed to both axes but only if you hit the exact angle. This angle should be the one when your front left and rear right wheel align so that they form a line which is perpendicular to the bump axis (this is assuming that you are approaching with your left wheel).

It's very important to remember that you should switch the wheel you're first approaching the bump. Bumps usually come in pairs (at least here in Bosnia) so this should not be a problem. If the bumps do not come in pairs, you should randomize and try not to remember your last approach. This, on the long term, will ensure even distribution of approaches on both sides. A bit of mathematics can't hurt.

Mechanical stress on rims

Similar to the sidewalks, the bumps are better approached at an angle as this kind of approach eases the stress on tires and wheel rims. You'll never hear anyone suggest you take the frontal approach to these. Sidewalks are a bit different from bumps, but the same laws apply.

Your car bumpers are very low

If your front or rear bumpers are lower than usual, you should approach the bumps at an angle. This will allow you to pass over bumps more smoothly and lower the chances of hitting the bump with your bumpers.

I suggest you try riding over the bumps at an angle and see if you're more comfortable sitting in the car that way. If you are - your car most certainly is as well. It's the common sense.

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Interesting - I agree with your last paragraph, but that it is much more comfortable to drive over straight than diagonally. Which is why it is better for the car to drive over straight. –  Rory Alsop Oct 15 '12 at 22:18
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Your car will be just fine with either approach as long as you slow down to a reasonable speed before hitting the speed bump, and so long as your car doesn't bottom out (bottoming out meaning some part of the car other than the tires touches the ground).

The kinds of stresses put on a car by driving slowly over a speed bump are no more severe than the kinds of stresses that the car experiences while driving around town (around corners, over small bumps, etc).

The most common damage caused by speed bumps is damage from bottoming out. The parts of cars that typically get the worst of this damage are the plastic bits down low on the front of the car (front spoiler, belly pan), or low-hanging parts of the engine (oil pan) or exhaust. Bottoming out happens when you drive too fast, and the springs of the car can't lift the mass of the vehicle out of the way of the road quickly enough.

Approaching the bump at an angle (one wheel at a time) would make it more likely that you would bottom out, as you're then asking only one spring to do the job of lifting the car clear of the bump, instead of having two springs work together by approaching the bump straight-on.

In short, just slow down. If it doesn't sound or feel abrupt/violent, the car will be just fine.

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I don't agree "Approaching the bump at an angle (one wheel at a time) would make it more likely that you would bottom out". From my own experience, it's the opposite. There is a nasty dump at Costco and if I go over with 2 front wheels, I'll get bottom out sure. But if I go one front wheel at a time, I usually don't get bottom out. I have sport suspension if that makes a difference. –  Ray Cheng Oct 19 '12 at 4:09
    
@RayCheng, good comment. I don't doubt your experience at all. My answer was more targeted at the dynamic effects of going over a bump at speed, where the behavior of the suspension comes into play. If you go over very slowly, these dynamic effects don't come into play--it's pure geometry. If you have a very low car, or a car with a long overhang in the front, the geometry of the car may dictate that the only way you can clear the bump--no matter how slowly you drive--is to drive at an angle, which can effectively decrease the length of the front overhang. –  mac Oct 19 '12 at 16:10
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