What is Suspension Travel?
In this brief article, we will discuss the Suspension Travel and the Suspension travel found in various types of suspension.
What is Suspension Travel?
Suspension travel is defined as the maximum vertical distance that the wheel can travel from the fully extended ‘Rebound’ condition to the fully compressed ‘Full Bump’ condition.
‘Bump Travel’ is defined as the distance traveled by the wheel center from Normal ride height to Full Bump condition.
‘Rebound Travel’ is defined as the distance traveled by the wheel center from Normal ride height to Rebound condition.
The ‘Bump travel’ is limited by the provision of Bump stop rubber blocks that compress and stop further travel before metal-to-metal contact. In most cars, the ‘rebound travel’ is limited by rebound rubber buffers inside the shock absorbers that stop the wheel hubs from dropping beyond the fully extended length of the shock absorber.
How is Suspension travel calculated?
Suspension travel is mostly dependent on the stiffness of the spring selected for the suspension. While designing a suspension system, ideally, a spring stiffness should be selected:
- ‘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.
Additionally, the springs must also be stiff enough to prevent excessive roll on corners, but this is not always required since roll stiffness can be specifically handled by the use of an anti roll bar and not increasing the spring stiffness.
The normal rule of thumb in design is to allocate:
- ⅔ of the total suspension travel to ‘Normal ride height -to- Full Bump’ and,
- ⅓ of the total suspension travel to ‘Normal ride height-to-Rebound’
How is Bump travel decided?
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. Bump stop hitting causes a great amount of discomfort to the occupants.
In order to control bump stop hitting is to make the spring slightly stiffer than its normal rate as the wheel gets closer to its upper limit, or bump stop. This is called ‘progressiveness’ of the spring rate. The progressiveness of the suspension rate can be achieved using
- A progressive coil spring
- A rubber bump stop buffer
Why is Rebound travel 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 travel 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 rebound travel 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?
In most common street applications, the suspension travel ranges in the following manner:
|Travel from normal ride height|
|Bump||75mm to 125mm|
|Rebound||40mm 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 normal ride height to both bump and 4” inches of travel to rebound.
- For track applications 3” of travel from normal ride height to both bump and 2” inches of travel to rebound.
Is it possible to Increase suspension travel?
Yes, it is possible to achieve more suspension travel. The situation where this would be required is if you intend your vehicle to have Off-roading capability. In Off-Roading, a greater wheel articulation would help you tackle various sorts of uneven terrain.
But compared to Decreasing suspension travel, increasing suspension travel requires comparatively more modifications. What is basically needed is:
- Increased Normal ride height, or, the vehicle must sit higher when unladen
- Increased Rebound travel, or, the wheel must drop further downwards.
In order to achieve the above, you would need to replace your vehicle’s factory suspension with an aftermarket Off-Roading Kit. Depending on the vehicle you own, you would need to look for a supplier that has compatible parts. One important point to note is that your car/truck was designed by the OEM specifically for the current ride height. So changing this setup cannot be done using ready-to-fit parts. You would need to make several chassis/underbody modifications in order to achieve your goal.
Superlift, ReadyLift, Icon Vehicle Dynamics, BDS Suspension and Rough Country are some of the many brands available in the market.
If you are trying to Lift your truck suspension, then some of the changes that would be required to the vehicle would be
- Modified Cross members that lower the support position of the suspension brackets. This may require you to remove the chassis bracket and maybe cut and re-weld the aftermarket parts onto them. Make sure you have a really experienced person to do this as it is a high-precision job.
- Modified front differential mounting brackets (in the case of a 4X4 vehicle)
- Long-travel Shock absorbers to replace the OE parts
- Longer travel coil Springs (if your vehicle has coil springs)
- Modified higher camber and long-travel leaf springs (if your vehicle’s rear suspension has leaf springs)
- Modified anti-roll bar chassis mounts and drop-links. Since the wheels sit lower than the original position, the anti-roll bar’s position will also need to be lowered so that it can function properly at the new height.
- Before starting out on the modifications, you need to decide whether you are going to someday go back to the original OE setup or not.
- If you intend to put back the OE parts, you need to be mindful of the original parts and store them properly as a set. Also, you may not be able to reverse some of the cutting and welding that you may do during lifting. So you would need to replace original OE brackets in those cases.
Is it possible to Decrease Suspension Travel?
Yes, it is possible to Decrease Suspension travel, or in other words, lower the car. The most common reasons for doing this would be
- To make your car look cooler. It is a known fact that lowered cars give the impression of sportiness to anyone looking at it.
- To make the car functionally perform better on the race track.
There are several aftermarket parts suppliers to choose from, when it comes to getting your car lowered. You need to choose a reputed company that has experience in modifying your particular car model, or brand at least. Or the other way would be to source each of the best aftermarket replacement parts from multiple specialist suppliers who are experts in manufacturing a particular part, e.g Shocks might be a specialty of one company, while lowering springs would be a specialty of another manufacturer.
The changes that would be required are:
- Lowering Springs: The aftermarket lowering springs would be of shorter length and be of a higher and more progressive rate in order to make the suspension work within the reduced travel.
- Aftermarket Coilovers or shock absorbers: The new lowered stance would mean a higher spring rate with lesser travel. The original shock absorbers were tuned for comfort and more travel. The new shock absorbers would require a different damping ratio to account for the new springs. With a shorter spring, the suspension arm would be able to travel more into the bump, closer to the bump stop. This would require a shorter ‘shut height’ in the new shock absorber/Strut.
- Modified control arms of higher strength would be required since the bump stops would be hitting more number of times.
- Low Camber leaf spring and axle mounts: In the case of leaf springs, you would probably need a new set of springs and slimmer axle mounts.
- UnderSlung Axle: For a leaf spring suspension, if it was an overslung axle, there might be potential to convert it to an underslung using an aftermarket kit. The axle will sit above the spring, thus bringing the chassis closer to the ground
- Torsion Bar Key: For a Torsion bar suspension, lowering can be easily done without any additional aftermarket parts. The only thing to be done is to readjust the zero rotational position of the torsion bar.
After-effects to expect when Lowering the Suspension
- Poor or harsh ride quality
- Underbody scraping when going over bumps or steeply inclined driveways
In this brief article, we have discussed the Suspension Travel and the Suspension travel found in various types of suspension.