What is Suspension Height?

In this brief article, we will discuss Suspension Height and the various aspects of suspension height.

What is Suspension Height?

Suspension height, also called ‘Ride Height’ is defined as the  vertical distance between the wheel center and a body reference point at a given loading condition. So suspension height by itself is not a complete description. Suspension height must always specify the loading condition at which the suspension height is measured. 

How is Suspension Height Specified?

The most commonly specified ride heights are:

  • Unladen suspension height, or the suspension height measured when there is no payload or occupants inside the vehicle
  • Laden suspension height, or the suspension height measured when the vehicle is fully loaded to its specified payload, including the occupants.
  • Bump Height, or the suspension height measured when the Bump stops have contacted fully metal-to-metal
  • Rebound Height, or the suspension height measured when the Shock Absorber Rebound stops have contacted 

There are other ride height specifications that some truck manufacturers specify, like “Overladen Suspension Height”, measured at the specified overladen condition as defined by the manufacturer.

The way Suspension Height is measured can be different for different car/truck/SUV manufacturers, depending on what reference point is used. Some manufacturers define ride height as the vertical distance between the wheel center and the periphery of the wheel arch or fender. Other manufacturers like BMW take the bottom of the wheel rim as reference instead of the wheel center. For this reason, Suspension Height is not a comparable quantity between two given vehicles.

What is the significance of Suspension Height?

  • Suspension Height is an important defining parameter for designing the suspension.
  • A vehicle Ride height specification by the manufacturer is an important piece of information used in inspecting the vehicle throughout its life. In case of any failures in either the springs or control arms, the suspension height could decrease.
  • In self-adjusting types of suspension like an air suspension, the reference ride height

How is Bump Height and Rebound Height calculated for a vehicle?

Suspension travel is defined as the distance traveled by the wheel from ‘Rebound height’ to ‘Bump height’. Suspension travel is, therefore, the difference between Bump height and rebound height. The amount of Bump Height decided for a given vehicle is a function of the suspension spring stiffness. There needs to be a balance between Bump travel and spring stiffness. 

A rough rule is that the stiffness and bump travel have to be both selected such that the spring stiffness progressively increases and can take a load of ‘2g’ or twice the corner weight before the point of full-bump travel is reached. The intention is that, while driving on normal roads, the car suspension should not be continuously hitting against the bump stops, either due to too much softness in the springs or less bump travel.

The stiffness of the spring selected for the suspension should be:

  • ‘Soft’ enough to give not too harsh a ride, but yet is 
  • ‘Stiff’ enough to absorb the energy of road shocks without hitting the bump stops too often. 

The normal rule of thumb in design is to allocate:

  • ⅔ of the total suspension travel to ‘Laden ride height -to- Bump’ and,
  • ⅓ of the total suspension travel to ‘Laden ride height-to-Rebound’

Rebound Height is decided by the fully extended length of the shock absorber. At the fully extended length the shock absorber’s internal rebound stop will restrict the suspension from hanging any further. Generally, the fully extended length of the shock absorber is proportional to the fully collapsed length of the shock absorber. Logically, the fully extended shock length would not exceed 2X the fully collapsed length.

There is yet one more independent factor that influences Suspension Height or Ride Height. And that factor is called ‘Vertical Wheel Offset’. Let’s take an example, say a truck’s laden height is 150mm from the fender. Now, with everything else in the suspension remaining constant, let’s say we add additional packing plates of 20mm thickness between the leaf spring and the axle seat. Now the suspension height has increased to 170mm at the same loading condition as before. So here we understand that bump travel and ride height can be decoupled from each other.

How can Suspension Height be lowered?

Suspension height can be controlled by reducing the length of the spring and keeping the stiffness same. The problem with doing this is that the bump stops will tend to hit more often due to that lack of available length for the spring to travel.

Now, in order to control the bump stop hitting the spring needs to be slightly stiffer than its normal rate. There are 2 ways to achieve this:

  • First simpler way is to simply increase the stiffness in proportion to the new reduced height
  • The second method is to increase the ‘progressiveness’ of the spring rate, or in other words, as the wheel gets closer to the bump stop, the spring gets stiffer. The progressiveness of the suspension rate can be achieved using 
    • a progressive coil spring having varying coil diameter at the start of the coil
    • Or a 2 stage progressive leaf spring (if the suspension is of leaf spring type)
    • A longer and hollow rubber bump stop buffer that gradually increases in rate under compression

Why is Rebound Height important?

Ideally, the suspension must allow the wheels to move up and down in such a way that they follow the unevenness of the road while maintaining the body to ride as level as possible.

So the Rebound Height should be such that the suspension is able to maintain tire-to-road contact at all times. The shock absorber rebound stop contact must be avoided, or, the wheels must not hang in air, since this is a source of discomfort to the occupants.

The suspension rebound height must also take into account vehicle roll during cornering. The inside tire during extreme cornering maneuvers must not lift off the ground. Tire lift off during cornering will drastically reduce lateral grip since grip is proportional to tire contact area and vertical load.

How much Suspension Travel do Cars have?

Unlike Suspension Height, Suspension Travel is a normalized quantity that can be compared between any 2 given vehicles. In most common street applications, the suspension travel ranges in the following manner:

Travel from Laden height
Bump 75mm to 125mm
Rebound40mm to 65mm

Suspension travel is different for different vehicle applications:

  • For Trucks and off-road vehicles, the suspension travel is usually 4” of travel from Laden height to both bump and 4” inches of travel to rebound.
  • For track applications 3” of travel from Laden height to both bump and 2” inches of travel to rebound.

Conclusion

In this brief article, we have discussed Suspension Height and the various aspects of suspension height.

In case of any queries or questions, please ask.

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