What to do when ambushed by an ‘invisible’ bump

Many of our highways have large speed bumps that are unmarked and almost invisible, especially in places where cars are likely to be cruising at quite high speeds. When a driver is caught by surprise, what is the best technique to minimise the jolt, damage or loss of control? 

Several readers

Reduce speed as soon and as much as possible in the time available but, a split second before your front wheels hit the bump, release the brake completely.  Here’s why:

To judge the release moment optimally, you need to understand that when you brake hard the kinetic weight of the car (its centre of gravity) is thrown forward dramatically enough to “squash” the front suspension, significantly reducing the available travel remaining in the springs.

If they are still squashed when they hit the bump, they will be less able to absorb the additional shock, hit their solid bump stops, and fling the front of the car into the air. The risk of damage and loss of control is increased.

When the brake is released, the springs are no longer squashed and as they recoil will even stretch them slightly, so there is less weight, less force and more absorbent space for them to deal with the thump of the bump. Also, the wheels will be rotating freely, without braking resistance, so they will “ride” the bump more smoothly.

That’s the first and most essential remedy. If your reflexes and timing are good enough, as soon as the front wheels have landed on the far side of the bump, apply the brakes again…to throw the weight forward and therefore lighten the load the rear wheels have to cope with when it is their turn to go mountaineering.

Steer as straight as possible throughout. Your instincts will probably ensure you grip the steering wheel tight. Don’t lock the wheels.

Weighing up the best angles

The kinetic weight of your car, however slightly,  is moving around all the time – when you brake it moves forwards,  when you accelerate it moves backwards, and when you drive round a corner it moves centrifugally towards the outside of the bend.

So deliberately shifting the weight of your car applies not only to hitting an unmarked tarmac “log” at a higher speed than desired or intended – it applies to negotiating any obstacle severe enough to “bottom” your suspension. Like a deep pothole.

And it can be used to help lessen the blow to the front wheels, or the back, or on one side or…even in one corner by using a “flick turn”. That is a sharp but momentary turn to one side, almost immediately returning to straight.

For example, if your right-hand front wheel is about to hit a lurid pothole, you can lessen the blow with a flick turn to the right. The flick must be firm enough to throw the weight of the car onto the left-hand side, squashing the left suspension and thereby stretching the springs and reducing the weight on the other side, so the now “lighter” right wheel more easily rides through the hole. At normal speeds on cars with suspension stabiliser bars, the difference may be marginal. But it is still better than nothing.

The whole process takes about half-a-second. Hard turn, straighten.