Stiff or Soft Suspension Explained

In this brief article we are going to discuss the importance of suspension stiffness, how stiffness affects vehicle behavior, and how stiffness modifications can be done to a vehicle.

When it comes to the ultimate in ride quality, or handling precision, a car’s suspension stiffness plays a primary role in establishing the car’s road feel or behavior. Every other suspension parameter like wheel rate, damping, subjective and objective ride & handling is optimized around the suspension stiffness.

What is suspension stiffness?

Suspension stiffness is used to describe the spring used in a car’s wheel suspension. It is also known as spring rate. The spring rate is defined as the force that the spring exerts, is proportional to its change in length, when the spring is compressed or stretched. Spring rate is expressed in the unit Force/unit length ,e.g 1 Newton/millimeter (N/mm). So a 10 N/mm spring feels harder and more difficult to compress as compared to a 1 N/mm spring. Therefore you could say that a 10 N/mm spring is stiffer than a 1 N/mm spring, OR, a 1N/mm spring is softer than a 10N/mm spring.

How stiffness affects vehicle behavior?

Stiffness affects vehicle behavior in the following manner:

Vehicles that carry heavy loads, in almost all cases, would be fitted with stiffer springs to compensate for the heavier weight of the vehicle. A softer spring for a heavier vehicle would compress the spring far too much, causing the suspension to the ‘bottom out’. 

As a thumb rule a stiffer spring is fitted on a heavier vehicle and a softer spring fitted on a lighter vehicle. So you could say that, in a general sense, the stiffness designed for a particular car suspension would be proportional to the weight of the vehicle that the suspension needs to support at a given height.

But you then might wonder why vehicles of the same weight feel a lot different in behavior. Vehicle ride and handling are influenced primarily by the spring. A stiffer spring rides harder as compared to a softer spring. At the same time, a harder spring, in a general sense, provides better handling behavior ( but there is the influence of front vs. rear suspension stiffness relationship as well). Shock absorber Damping values also play a major role, but they are decided only after finalizing the spring and tuned around that spring.

So for a particular weight, it is possible to have slightly different stiffness and yet the same suspension height. Here the spring pre-load is either increased or decreased to match the given suspension target height, but at the same time varying the stiffness. It is indeed a delicate balance to be struck between the conflicting requirements of “good ride” and “good handling”.

Wheel Rate

Wheel rate is derived from Suspension spring stiffness but has a few more elements to it. Wheel rate is defined at the overall stiffness at the tire patch of the car taking into account the effect of Spring stiffness, Bushing stiffness and Tire Stiffness.

The significance of Wheel rate is that, when tuning your suspension, apart from merely springs, the kind of bushings and the sort of tire you use are also contributors to stiffness and the overall feel of the vehicle.

You can do this simple experiment on your car. Check the tire pressure on your car tires currently and not how the ride comfort feels. Now increase the pressure by 10% and now drive the car. You will notice a big difference in feel. With higher tire pressure the stiffness of your tires has increased and you have an increased sense of road unevenness.

How to improve suspension on a car?

There are many ways in which you could improve your car’s suspension. The actual question is, ‘What aspect of your car’s suspension behavior are you trying to improve?’

Do you want better:

  • Ride Quality
  • Handling
  • Improvement in both
  • Off-roading capability

Depending on your requirement, the modifications to your suspension would change. 

  • For better Ride Quality:
    • Softer rate aftermarket springs to suit your vehicle could be an option that you could try out. 
      • One thing to remember is that if the springs are too soft, suspension travel increases and then your car might end up hitting the bump stop more often. Beyond a certain limit, a softer spring might actually run your ride quality.
      • One more aspect where you need to pay attention to the front + rear spring combination, because ride comfort is a composite effect of the two. 
  • Softer Suspension bushings should be definitely tried out. Look for aftermarket bushings that have a Shore ‘A’ hardness lesser than the original bushings.
  • Look for different tires that offer lower sidewall stiffness and result in a softer ride.
  • No ride quality improvement exercise is ever complete without damper tuning. As a sequence, it would be best to tune your dampers last. You would require aftermarket “Adjustable Gas Pressure Shock Absorbers” with which you can try out different softer settings. A word of caution here. A damper setting that is too soft may allow your suspension to hit the bump stops too often during normal driving.

You will need to conduct multiple trials with the different available choices in each of the suspension elements in order to arrive at the setting that gives the feel you are looking for.

  • For better Handling:
    • Low profile tires and appropriate wheels do contribute greatly to the handling feel of your car. They offer greater stiffness and more lateral grip which is needed while negotiating hard corners
    • Stiffer springs do help. But here, you need to see which combination of front + rear springs would best suit your purpose
    • Lowering your car’s suspension using modified springs and dampers or a lowering kit
    • Harder Suspension bushings should be definitely tried out. Look for aftermarket bushings that have a Shore ‘A’ hardness much higher than the original bushings.
    • Damper tuning is a crucial part of Handling. Use ‘Adjustable Gas Pressure Shock Absorbers’ that will give you harder settings that improve the response of your car while cornering

What are the advantages and disadvantages of lowering my car suspension?

Suspension lowering is probably the most common way to modify your car to handle better on a race-track.

Suspension lowering has the following advantages and disadvantages:

PROSCONS
Enhanced road feedbackHarsher ride
Stiffer suspension rateNot suitable for rough roads
Reduced roll while corneringFaster Tire wear
Improved handlingLesser suspension travel
Improved aerodynamic performanceReduced wheel envelope and ground clearance
Better tractionStandard jack not usable
Reduced  risk of rolloverExpensive
Sporty looksWarranty could be void

How do I lower my car suspension?

Suspension lowering requires the following modified items to be swapped with your car’s original suspension:

  • A shorter spring and with higher stiffness because of the reduced suspension travel
  • Modified bump stops
  • A Damper with the reduced travel and shorter shut-length
  • Modified Toe and Camber settings

Where can I lower my car suspension? 

You can either do the lowering by yourself if you have all the right tools and equipment, OR, you could employ any Auto garage, since it is not too complicated.

Can I make my car suspension softer?

Yes, it is possible to make your car suspension softer. A softer suspension is generally intended to make your car’s ride quality better. In other words, ideally, you don’t want to feel the road bumps inside the vehicle. While that is not practically possible, because you will feel at least a small percentage of the road irregularities.

A good Ride Quality can be achieved in multiple ways depending on what type of suspension your car has. In a generic sense, your suspension stiffness needs to come down, but within practical limits. Please refer to the options you can try in the previous section.

If you are still not satisfied with the ride quality even after you have made modifications, there is one alternative to change your entire suspension system to Air Ride.

Famous luxury car brands like Mercedes, Bently, Rolls-Royce, etc. pride themselves with the best ride quality because their models have air suspension instead of steel.

How to soften suspension on a truck

The approach would be the same as that followed for softening a car suspension. It is discussed in an earlier section of this article.

The only difference from a car is that in the case of a truck where the rear suspension is most probably leaf-spring, you need to fit lower-rated aftermarket leaf springs.

If you are still not satisfied with the ride quality even after you have made modifications, there is one alternative to change your entire suspension system to Air Ride.

Stiff or soft suspension for off road

For Off-road capability you would need a slightly softer suspension. Following are the factors to keep in mind while tuning your suspension

  • For Off-roading capability, the first thing you need is to increase the height and overall travel length of your car’s suspension. In articulated condition, the suspension would need to travel more in order to maintain tire contact. This means that you would need to replace your entire suspension with an aftermarket solution that provides the Linkage geometry required to achieve your increased travel requirement.
  • Softer springs as compared to the standard springs would be an advantage. The reason being that, during Off-roading, the main objective is to maintain tire contact on uneven surfaces at all 4 wheels. Keep in mind that these springs should suit your new “Off-road” suspension geometry. It would be preferable to get matching springs as a package along with your aftermarket suspension linkage assembly.
  • Shock absorbers with increased travel as well as softer settings. It would be preferable to get matching shock absorbers or Coil-overs as a package along with your aftermarket suspension linkage assembly.

Conclusion

In this brief article we have discussed the importance of suspension stiffness, how stiffness affects vehicle behavior, and how stiffness modifications can be done to a vehicle.

Please feel free to ask in case of any questions or comments.

Frequently Asked Questions (FAQs): 

 

Does car suspension soften over time

No, the car suspension is most unlikely to soften over time. What does happen is that you get used to the ride quality of your vehicle and you tend to settle-in.

What is possible is a phenomenon called suspension ‘Sagging’. Over a long period, due to fatigue, the spring tends to settle at a position lower than it was installed in. 

Do heavier cars ride smoother?

  • A heavy car has a lower payload-to-vehicle weight ratio. This means that adding extra occupants would not impact the ride frequency much. As an effect, the damping requirement for this vehicle’s suspension would not change much because of a change in payload. 

In a lighter car, adding more occupants will make the ride feel too soft while too few occupants will make the ride feel too hard. The lighter car therefore finds it difficult to have an acceptable ride quality within the shock absorber’s Damping range.

  • Humans are most sensitive to the car’s ride frequency in the longitudinal direction. A lighter car is more susceptible to longitudinal disturbances. So the effect is that the car’s acceleration and deceleration can cause a much bumpier ride. This situation is worsened when the luggage in the boot causes the vehicle to pitch up.
  • In a lighter car, an additional payload causes a vehicle to sit lower because of spring compression. This causes the center of gravity to shift in the downward direction. 

A heavier car lowers much less as compared to a lighter car. Hence, the heavier car can be designed to have a lower center of gravity.

  • Unsprung weight has increased in modern car designs. They now have bigger tyres and wheels, more advanced brakes, and more complicated suspension items. A lower ratio of sprung-to-unsprung weight makes the ride harsher while going over bumps and potholes. Additional weight to the body ( or sprung weight) helps keep the tires more firmly planted on the road since the springs now apply more compressive force from weight. So a heavier car, in this case, not only has a smoother ride, but also a better grip.

How to test coil springs?

Coil springs can be tested in multiple ways. Coil springs are tested for multiple reasons as well

  • By OEM spring manufacturers 
    • to evaluate the following quality attributes: friction, wear, and service life
    • to assess the coil spring’s ability to maintain its original shape or tendency to ‘SAG’
    • to determine pressure uniformity between a coil spring and the support. This result is utilized to assess the spring’s performance consistency, durability, and proper balancing of load
  • By Racecar builders to test consistency & accuracy

Coil springs are tested using the following methods:

  • Pressure Mapping
  • Multi-channel spring testing
  • Durability Cyclic Testing

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