Rate, Force and How They're used in Motorsports
In this post we're diving into a little more detail on rate, force, and how they're used correctly and incorrectly in motorsports. We're specifically setting out to focus on how the terms are used, and how a high level view of the ways rate and spring force can contribute to performance in engines and suspensions.
Rate: Spring Rate, Rate of Growth, Sales Rate, Damping Rate, and the list goes on and on. But what does it all mean? Specifically for this article, we want to cover a top level synopsis on rate used in valve spring, suspension spring, and torsion bar applications. Spring rate and spring force are two very different things, and so many times are mixed up in the field by customers and users. We hope you can use this as a very basic guide to understand the terms, and to help improve your setup by giving you ways to look at things differently.
Here's a basic definition.
Rate: noun, 1. a fixed ratio between two things. 2. a quantity, amount, or degree of something measured per unit of something else.
Force: noun, 1. strength or energy exerted or brought to bear : cause of motion or change : active power. 2. https://en.wikipedia.org/wiki/Force
Shown above is a typical graph that shows a force output and the force rate calculated off of the same force shown. Notice the shape of the curve, whereas the force is a progressive force, and the rate is linear, rising at an equal value for each displacement.
What does all of this mean to you?
In Valve Train Systems spring rate and spring force are both important factors. Spring load is what's used mostly to spec out springs on specific applications, and spring rate is typically ignored by a lot of folks. Installed load is the load at which the spring is captive statically, meaning the force applied at a specific height. This is the force applied to keep the valve closed. Open load is the designed or static load for when the valve opens to the peak position of the camshaft.
What is static and dynamic force? Simply defined, static is the spring in the non moving state. Dynamic force is the excitation force or additional energy applied (moving). Why does static and dynamic force matter for springs and components? Well, static is the force designed as the spring sits and doesn't move. Consider this the baseline force which coincides with the base circle of the cam. The spring is designed with these factors knowing it will (statically) operate between these two working heights, the "static" min and max stress. Dynamically, the spring force is much more active, possibly causing over stress situations, or an amplification above design intended stress. See the chart below, showing a simple relationship between typical valve spring conditions.
So for valve-train - managing your force is one factor in improving life of a valve spring- trying to stay out of the "red" surge area as shown in the graph. See previous Blog post about improving spring life, this is some added detail to a key note highlighted in that post.
How does force and rate apply here? Simply put, you have an applied force/movement coming from the mechanical input from the cam and engine system. This generates motion and speed into the valve/valve spring- how fast the system opens generates jerk/inertia (energy). The spring is the counteracting force for inertia management, whereas rate and force are what's typically used to counteract the explosive nature of spring surge, from input energy. The spring open load is what "counteracts" that motion and used to control and close the valve. Rate is how much force increases during the motion, a lower rate will not have as much open load, and will not be able to withstand certain applied energy, where a high rate will amplify the force increase, increasing the counteracting force more rapidly.
A simple analogy to make, is a train traveling down a track with no brakes, it's coming up to a barricade, and you need to slow that train without destroying it. I know you just had a flash back to middle school story problems. But imagine putting a spring in between the train and the barricade, how much force and how much travel do you need to slow the train? Yes, it depends on many factors, but a lower spring rate would mean you need more space to slow it, a higher spring rate means you're slowing it faster, but inertia may rip it to pieces based on the high counteracting (opposite) force. The end all is the barricade, which ultimately has the highest rate and stopping power. Using a hybrid rate/force is a way to manage this - reducing the required space which slows the spikes associated with dynamic surge or a velocity change. A progressive or 2 rate approach, would allow for an initial slowing, and increased rate as the deflection/travel increases until the train is stopped. This is done in suspension with an external shock damper, and easily tuned to do so.
Considering that the valve spring is typically an un-damped system, you're relying on the spring force and building force (rate of increase) to counteract the input. This counteracting force allows for the open point "return" or an attempt to not overshoot the open point - over shooting can use up the distance to coil bind (the proverbial barricade). Ensuring you have enough rate, and enough clearance to coil bind allows for the "overshoot" ramp, allowing the system enough response time to control the applied energy. Having too little rate, and not enough room to coil bind, the spring impacts itself at bind, causing a force/stress spike, and can cause many detrimental issues. ** note the red dynamic curve in the above chart. While tightening up the clearance to bind helps manage this, it's not a great long term solution, and points to not having enough load, or too much mass.
Types of rate: As mentioned above, there are varying methods of introducing force into a system, and varying methods to control and slow that force. As it applies to pure rate and force, here's a few examples of what the different types of rate mean.
On suspension systems, the common terms are opposite of what is typically used in valve train. Suspension springs are listed (sold) with rate, length, and dia, and "rate" is widely used when choosing or setting up your suspension. A better term would be load/force at a given height, this provides the working numbers to what your chassis really needs. Spring Rate is what you need to use to calculate this ride height force (installed load).
The suspension system is generally a secondary damped system using a shock absorber, so two main factors apply, force to hold the vehicle at height, and rate change during the travel to help the dynamic force. If you have a required height, a specific force is needed to set the vehicle at a given ride height, you can choose varying rates and spring free-lengths to get the same outcome.
For instance using a linear rate-14 inch tall coil over coil spring at 100 lbs/in would achieve 200 lbs compressed force at 12 inches, and a 16 inch tall coil over at 50 lbs per inch achieves the same force at 12 inches.
The only difference is how much the spring traveled to achieve the required ride height load. This is typically called pre-load. Or how much travel the spring has before a zero load condition on a rebound stroke. In this case the 16 inch spring has 4 inches of travel and the 14 inch spring has 2 inches before a zero load condition.
DO NOT be afraid to mix and match rates, and free-lengths. Especially if your favorite brand doesn't have what you're looking for. They typically can help you get the right load using different options.
For performance, once you achieve the ride height requirement by FORCE, you need to look at how good your damper setup is before you choose your net rate. A lower rate spring, without a double stack (dual spring) as a secondary rate increase, requires a higher rate spring to help compensate for the dynamic vehicle motion and weight.
For suspension systems choosing a higher rate to compensate for a bad damper isn't a good idea. Using a good damper allows for a much better scenario to control motion compared to using spring rate and rate change to control your suspension. Trying to keep your spring rate as low as possible gives you more tuning options on your damper valving and will improve your ride quality. However, in extreme situations like desert racing, using springs to help the damper will avoid adding a lot of heat and fatigue into the shock.
This all depends on application and how much room or travel you have to use in your chassis design. On Off Road, dirt circle track, rally, and motorcycles, they use a dual, or triple spring stack. This allows for multitude of spring rates to be selected and adjusted via a coil over secondary nut. Whereas, pavement stock car, street car, typically have lower travel requirements and use a progressive rate spring, or a secondary bump spring. The force at height still applies here, but due to space constraints it may be harder to choose a taller spring with a lower rate.
Secondary to that, on a street car or travel limited chassis you have less space to apply a secondary spring rate change device. Below is an example of a two rate spring setup coil over shock. The shock has a "slider" in between two springs which allows the springs to link and travel along the shock body. A threaded adjustable nut is placed on the body to stop the slider from moving thus enacting the secondary rate spring. This action causes a two rate condition (as seen in the above chart, blue line).
Torsion bar systems (Not Sway Bar) but a torsion bar used as a primary vehicle spring. Commonly used in late model pickup trucks, military tanks, and sprint cars, torsion bars are an elegant space spacing solution but come with a unique set of challenges. When it comes to rate, torsion bars exhibit something coil springs don't which is a true linear rate at the instant of deflection (Torque).
The below chart shows an example of how a torsion bar typically applies rate and force under deflection, and compares a coil spring rate deflection curve. Note that the coil spring has a lower rate, then builds gradually, while the torsion bar has an instant rate climb.
Also, notice in the above graph how the orange line (torsion bar) starts to decline in rate. This is due to the torsion bar typically not having a natural hard stop point designed into it (without other mating components). Unlike a coil spring which has a mechanical stop when pressed solid - the torsion bar can continue to twist until failure. This graph shows the area of plastic deformation, yield, and eventual tensile failure in a torsion bar. This happens when the torsion bar is used in an over stressed situation, once it reaches the plastic deformation state, the force curve starts to decline. On a coil spring, the inherent design is such that once it reaches coil bind - it's designed to not exceed the design stress that would allow for plastic deformation (spring sag or spring set). Some suspension spring manufacturers have coined the term "solid safe" which means the spring is designed to be below the over stress range allowing it to be run to bind without sag. Typically, higher travel suspension springs, have higher stress which are more likely resulting in a sag condition and bowing without the proper processing, materials etc.
Digressive spring rates- generally speaking a digressive spring is a coil spring or torsion bar that is used in an overloaded state which means that it is "losing" rate on deflection. This term is one that is used erroneously when associated with a mechanical steel spring used axially. While a shock absorber can achieve this with the right setup in valving and design, a spring can't for a long period of time. As the chart shows, the spring/torsion bar that is showing the "digressive" rate is in a yielded state, which means for the applied force, you're actually deforming the mechanical properties and it will not be allowed to return to the same position. This can also happen in a coil spring system when the spring is collapsed, and the wire slips over the next wire axially when at coil bind. Doing this can cause negative stress that could cause the spring to fail. Below is an example of that off axis loading (buckling) whereas dimension "C" shows the spring being compressed in a matter off the Base center, or off axis. Continued use will force the coil to bind and over stress (yield) the section buckled.
I hope this helps answer some questions, and gives you a step up to helping understand terms, and how you might benefit from using them.