One of the most open ended questions in motorsports:
"What spring rates should I run?"
Outside of the highest levels of this sport (and sometimes even there) choosing a spring rate can be equal parts objective and subjective. Opinions and methodologies abound for choosing your springs. Talk to some race engineers, talk to a few top shops in club racing, read some forum threads; get confused.
Here, we'll look at the main influences of deciding your spring rates. Then we'll talk a little about balancing those factors to make a good decision. We'll abstain from equations for this installment and keep it simple. We'll come with the math later on.
These are your four main areas to work in when choosing or changing springs:
- Mechanical Grip
- Aerodynamic Platform
- Suspension Geometry
- Driver Preference
Generally, the more we press a tire into the road vertically, the more traction it will make. As you go around the track, the suspension moves, and this vertical load changes. Reducing the amount of load change and how quickly that change occurs is going to maximize the amount of traction made around a lap. The more the tire (unsprung mass) is separated from the chassis (sprung mass) the better our chances are of reducing this load variation. As the spring gets stiffer, our separation reduces, until you're in a go-kart. This is the underlying theme behind running soft springs to make more grip: the tire follows the road better.
Following this line of thought, we would reduce spring rates until we go slower. This has been a pretty strong basic approach for some time. However, like many other approaches, it is not without its limits. As you reduce spring rates, pay close attention to how the car changes in slower corners compared to faster ones, how the tire wear looks, and how it feels to the driver. Sometimes lowering spring rates can make for short run or track specific gains leaving the car lacking in race length performance.
Since reducing racecar weight allows the engine to push us around faster, and the tire doesn't have to work as hard to brake and corner, we had to find a different way to press the tire into the road more. Enter the air molecule and its friends; by manipulating their movement around the car, we can push on our tires without adding weight and go even faster. Thanks downforce!
In order to maximize downforce, we more or less have to fix the position of the vehicle body relative to the ground. To achieve a fixed position or the general range of position we want, a stiffly sprung suspension is required. This of course lies in opposition to the concept of separating the sprung and unsprung masses to increase mechanical grip.
Most of the time, we're going to have to find a happy balance. Rare is the car or situation where one contributor to lap time takes absolute priority over the other. This means you need to understand the general limits of what your tire will tolerate for mechanical grip and what your vehicle body needs to make downforce. It is recommended that you separate these in testing and find the extremes. Look at your notes and data a few days after to get an objective view, and then head back out with a plan. This should allow you to make a good choice on a compromise that will make the car and tire the happiest.
3) Suspension Geometry
Many times we're working with a car that was originally designed for the street. With completely different operating goals, tires, etc. When we go to race these cars, we lower them, change parts to add camber, reduce bump steer, and so on. Frequently, we'll end up with some "bad spots" in the suspension. Ranges of travel where we might lose camber, change toe, have a roll or pitch center migrate rapidly, or some other kinematic calamity.
If we're running around on softer springs, we're going to be seeing these "bad spots" more often. A solution of course is to increase spring rate so that the suspension never reaches this point of travel. The losses in mechanical grip can be offset by the gains in geometrical improvement.
To identify a "bad spot" look for issues the occur suddenly. For example, as the car enters a corner and rolls, we get sudden understeer. Also, look at the tires, excessive wear and temperature can be indicators of the tire "scrubbing" around on the track surface. If suspension geometry is varying substantially, we'll be rubbing the tire about the road like a pencil eraser, and not making a lot of grip.
This is a popular place for bump stops or some other secondary spring that comes into action as the suspension reaches a certain amount of compression. This allows us to maximize mechanical grip while doing our best to avoid "bad spots."
Similarly, we could use secondary springs to "come in" as the aerodynamic load increases. In slower corners, we're mostly using a softer spring rate and in faster parts of the track the downforce pulls the chassis down onto the stiffer springs. This can be cumbersome to set up for some, but if you have the tools and information it is effective.
Now it's time to throw rationality and quantitative analysis to the wind. Drivers don't necessarily care what some nerd made on a screen. They care what feels right, what gives them confidence, and what they can drive. A drivers will to push the tire to its limit and to do so for lap after lap can outweigh many physical parameters. After all, what benefit is a tool if no one is going to use it?
There have been numerous experiences and stories of drivers on the same team, with the same car, having completely different setups. All the while nipping at each others heels at the top end of the time chart and/or championship points. Write me an equation for that.
So while you're making an earnest effort to understand your tire, your aero, and your suspension geometry; pay attention to the driver's comments. We wouldn't advise tossing out the process immediately, but we would be remiss if we didn't listen. Teams have won telling drivers to "deal with it, the car is fast" and they've won giving the driver what they want. Again, it is a balance you'll need to strike on your own.
Putting it together
Springs are a big topic and seem to illicit pretty strong opinions. Even touching the surface made for a pretty long read. In short, you have to try a lot of springs and take a lot of notes
Here's a few basic points to hit every time you try a new set of springs:
- Lap time faster or slower?
- How did the tire look (pressure, temp, surface)?
- Where on track were the gains or losses in time made?
- Did the handling become more consistent through corners or less?
- Get some quantitative feedback from driver (understeer/balance x out of 10, bumps y out of 10, transitions z out of 10)
- Get some qualitative feedback from the driver (easy to drive, good feedback, etc)
Try as we might, there is no easy answer to selecting spring rates. If you want the result, you have to do the work. The good news is that paper is cheap, take a lot of notes and test smart. The rewards will be worth it. You'll know when it's right because your competition will start asking after your rates.
- The Olsen Motorsports Team