The whole point of going sideways...is to have at least one wheel at a higher height so that your car isn't at the lowest point.

Think of going at an left angle...the FR wheel goes up with the speed bump...while it's on its way down, the FL wheel is on it's way up...when the FL wheel is on it's way down, if you measured your angle correctly...the RR wheel should be on it's way up...and when the RR wheel is on it's way down, the RL wheel should be on it's way up...that way at any time going over the bump, at least one of your wheels is higher than the ground level.

The same idea applies for driveways and ramps.

If you went straight on, there's that point when both your front wheels is off the bump and your rear wheels isn't on the bump yet...so that's the minimum height...where if you're too low, you get exhaust and sideskirt scrapage.

Hope that helps! Good Luck!