What does Suspension Bottom Out mean?

In this brief article, we will discuss the suspension bottoming out, the causes for bottoming out and steps to prevent it.

What does Suspension Bottom Out mean?

Suspension Bottoming out means that the suspension has exhausted its bump travel and that the bump stop has fully compressed.

What happens if your suspension bottoms out?

• When the suspension bottoms out, the impact is significantly experienced by the passengers and highly uncomfortable.
• Bottoming out causes loud suspension ‘Thud’ noises
• In some severe cases, bottoming out could result in damage of the MacPherson Strut top mount and bearing or even damper internal damage, depending on the intensity.
• Repeated ‘bottom out’ events would significantly reduce the service life of the suspension

What causes the suspension to bottom out?

A vehicle suspension’s main function is to absorb road impact loads by allowing the wheel to travel vertically upwards and downwards and using the spring’s elastic energy in the process.

A vehicle suspension is designed with a stiffness characteristic  and a certain amount of built-in suspension travel that will accommodate the maximum load that the suspension is likely to encounter. Ideally, the suspension spring’s stiffness is adjusted such that the maximum anticipated suspension load would occur before the suspension has hit the bump stop.

The instantaneous road load exceeds the maximum load at which the bump stop gets compressed, the occurrence is termed as a suspension bottom out.

In the figure above, the blue line shows the curve of load vs. deflection of a car’s corner suspension. The straight-line (Linear) portion of the curve is due to the steel spring. Notice how, after the bump stop contacts, the curve changes from linear to non-linear. The reason being that the bump stop rubber has a non-linear characteristic and is designed such that it increases the suspension load within a short travel.

How to prevent Suspension Bottoming Out?

• One way to prevent Bottoming out is to increase the ride height at normal load. This would give the suspension more length to travel before bottoming out.

The disadvantage with this measure is that the rebound length would remain the same and hence increase the overall suspension stroke. This would require component changes, like a new spring and a new shock absorber that can extend further in order to accommodate the new rebound length.

• Another way is to increase the spring stiffness of the spring so that for the same amount of deflection, the suspension load would increase significantly before bump stop contact. The disadvantage with this method is that it would detriment the ride quality since the springs have now become harder.
• There is a third method that could be tried out. When road impacts happen, they do so at a certain frequency. The suspension’s response to frequency inputs is handled by the shock absorbers. This method involves using a shock absorber that would generate more damping forces at those frequencies where bottoming out is most likely to occur (usually at lower range). Here again, the major drawback would be that the suspension ride would feel much more harsh at other driving speeds as well.

Conclusion

In this brief article, we have discussed the suspension bottoming out, the causes for bottoming out and steps to prevent it.