If a car goes over a bump which creates a greater force than the spring can absorb before bottoming out, then the remainder of the force will be sent to the chassis. Below is the equation for determining the force which would be transmitted to the chassis if the suspension were to bottom out.


The force that the chassis will receive from the bump is equal to the force of the bump minus the force taken by the spring.

In the previous example, the stock spring would absorb 400lb of force, as would the lowering spring. The cut stock spring would only absorb 200lb of force, meaning that a significantly larger force would be transmitted to the chassis when going over large bumps. A car's chassis is only designed to handle a certain amount of force, and the cut lowering spring would be sending a greater force to the chassis.
Because the cut stock spring's ability to absorb energy before bottoming out is so low, the suspension would bottom out much more often than with the other springs. For example, a 300 lb bump force would not bottom out the stock spring or the lowering spring, but it would bottom out the cut spring. Ride quality would suffer as a result of bottoming out frequently.
All materials suffer from fatigue (from the French word fatigué, meaning "tired"), which is failure caused by cyclic loads. In the case of a cut spring, it is placing loads on the suspension and chassis far more often than the car was designed for, which over time could cause component failure, which could be very dangerous.
Effect on Handling

Cut springs also negatively affect a car's handling. When the suspension bottoms out, the chassis is unsettled by the sudden large force being sent to it from the suspension. Because a cut spring will be sending a very large force to the chassis, the car will lose grip more readily. This causes a reduction in handling predictability, and hurts performance on rough surfaces.

Fitment Issues

The ends of helical coil springs are made to be flat so that they fit properly into their retainers, and so the load is distributed evenly around the retainer. Cut springs have an angled end which concentrates the load on the retainer, and in some circumstances could pop out while driving.



Cut springs do not fit into the spring retainer properly.

Conclusion

Cutting springs to lower a car is an attractive option because it is cheap. It is also easy to get the exact desired ride height by cutting small sections until the ride height is correct. However, it cannot absorb forces from bumps correctly, which lowers ride quality, spoils the handling and causes unnecessary wear on suspension components. Cutting springs will in fact lower a car, but so would flattening the tires.