Commodore’s Garage #15 – Starting Spring Package
December 9th, 2016 by Matt Holden
We now know the basic ins-and-outs of the spring components in our race car, but we need to apply that to the car and get started with our race setup. “Where do I start?” is probably the most common question asked, and in almost every case, the answer is the spring package. We have to look at many different things that will affect the spring rates and consider all of them, eventually winding up with a spring package we are comfortable with. If the springs aren’t set properly, it will lead to handling issues during the race such as handling swings from imbalance, over- or under-worked tires, and possibly even a crash! This article and the next one will look at getting your springs arranged leading up to the holiday break.
Aerodynamics and Minimum Ride Heights
Most cars will have a minimum ride height defined by the series rules. Whether that’s 7″ or 7 centimeters, there is no getting around it while you’re in the garage. For the most part, ride height rules could be deemed more traditional than necessary, since most teams have found ways around the ride height rules. Overcoming the limitations presented by ride height rules is the first thing we need to consider when choosing our springs, and how we go about overcoming those rules can often be the difference between a leading car and a mid-field car.
First, we’ll look at an aerodynamically-dependent car, or one that relies heavily on downforce (or sideforce) to get it around the track. Formula cars, NASCAR’s Cup/Xfinity/Truck cars, Prototypes, and, to an extent, some GT and short-track oval cars. Aside from the NASCAR Cup car, we’re going to have ride height rules that keep the car elevated above what we want. For best aerodynamic performance, we want the car low, so we need to get around the ride height rules quickly once we’re on track and out of the inspection bay. We have three options in this situation: 1) Deal with it, 2) Bumpstops, or 3) Coil-binding.
The first option, “Deal with it” is the common choice for cars without bumpstops (or limited bumpstop options). If you don’t have the option to effectively drop the car and keep it from slamming into the track, the best option is probably going to be going with springs that will keep the car off the ground, but are soft enough to give the necessary mechanical grip to get around the track. A common example of this in the iRacing service would be the V8 Supercars. The Supercars generate a bit of downforce, but lowering the car for aerodynamic purposes can often hurt laptimes more than raising it up and sacrificing a bit of aerodynamics for mechanical grip.
The second option involves dropping onto bumpstops from aerodynamic load. This is very common in high-downforce racing cars, may or may not be adjustable in the garage screen. In these situations, softer springs can be used with the intent that they’ll collapse under aerodynamic loads and the car will ride on the bumpstops in high-speed corners, producing a lot of downforce. As the car slows down for a slower corner, it may rise off the bumpstops and use the soft springs for mechanical grip. Keep in mind that bumpstops are often very high rate springs, usually progressive, so a coil spring of 1000 lb/in (175 N/mm) will often be much softer than whatever the car was riding on as a bumpstop.
Finally, we have the dreaded coil-bind. I did an entire article on the basic mechanics of coil-binding here: https://www.iracing.com/commodores-garage-7-coil-binding/. For the crash course on coil-binding, we simply want the car to have an extremely soft spring rate so that, once it’s up to speed on the track, the springs collapse quickly from downforce loads. The result is that the entire spring is compressed to where all of the coils are touching each other in some way, preventing any further downward travel from the springs. This is extremely effective, however head-scratchingly complex to figure out and make it work properly. While coil-binding went out the door in Sprint Cup following the 2014 ride height rule elimination, it is still present in Xfinity and Truck, whether by full-coil bind or half-coil “pig-tail” binding. We don’t have the means for half-coil binding in iRacing, but full-coil is still a viable option in these cars.
Finally, we have the case where the car is not an aerodynamically-dependent workhorse at all. Cars like the Legend Car, the Modifieds, or the Lotus 49 all have very little aerodynamic dependence (or none at all). In these situations, the thing to consider is how far we need to drop the Center of Gravity to produce a good-handling car, if we need to at all. Again, it’s going to be a balance of advantages and disadvantages for these cars, so the only thing that will produce a good base to work from is old-fashioned testing. That said, none of these cars will behave well with excessive spring rates. Spring rates that would be considered extremely soft for the cars in the other three categories will be perfect for this range of vehicles. All we’re looking for here is mechanical grip, and that will come from softer springs.
Once we know what suspension system we’re working with, we should know how we could attempt to get around the ride height rules, if we can at all. With that known, we can look through our range of spring options to choose what springs to pair up with our aerodynamic needs. On some cars, such as the Mercedes GT3 car, this range can be extremely small, where it can be a relatively wide range on other cars, such as the BMW Z4 GT3 car. Driver preference can also be a factor, especially in whether or not the driver wants a stiff or soft car. If the driver wants a softer spring package, the car will need to be raised up higher to start, while a stiffer spring package will be just fine with lower ride heights.
The other major factor in spring choice is the track itself. Is it bumpy? Smooth? Are there hills? Is it flat as a board? While the aerodynamics and ride height rules for a car will determine the general range of springs to run, the track itself will narrow this down even further. Again, as is with every other thing, testing is the only way to determine what springs you like and what will work best for you.
A major track characteristic that will affect your spring choice is how much of the track is high-speed and how much is low-speed. If the track has a lot of high-speed sections, such as Monza or La Sarthe, you’re going to want to go with a relatively stiff spring package and set the car to generate a decent amount of downforce while having the car trimmed out for low-drag. Soft springs at these types of tracks will produce a large amount of vertical movement in the car, which can result in a lot of drag if the car starts moving excessively. Onboard videos from Formula 1 cars at Monza are filled with the sounds of the car skipping along the pavement, as well as virtually no movement from the suspension arms at all. The opposite would be somewhere like Zandvoort or Summit point, where slow corners dominate a lap. At these tracks it may be difficult to generate a lot of downforce, and you’re going to want to lean more on mechanical grip and soft springs to run a quick lap time. In general, you can think of the spring rates going up as downforce goes down as a general rule of thumb.
On ovals we would consider the same thing: Higher-speed tracks will need stiffer springs. Big 1.5-mile speedways will need a stiffer overall spring package than a short half-mile oval, and the same rule about downforce-vs-spring rate will apply. However, we have to further consider the amount of banking on the oval as well. Higher-banked tracks such as Charlotte or Bristol will produce very high vertical loads, and to keep the car from slamming into the race track we need high-rate springs. Small, low-banked tracks will see the lowest spring rates, and real-world rear springs under 200 lb/in are not uncommon in the upper levels of NASCAR at a track like Martinsville.
One of the rare things that applies to both road and oval racing is the bumpiness and grip level of the track surface itself. Smooth, high-grip tracks can allow much stiffer spring rates and a more “aggressive” approach to generating downforce. Old, worn-out tracks like Kentucky Speedway or Lime Rock Park will produce a need to deal with bumps to keep the car from jumping around and eventually finding its way to a retaining wall. Softer springs are necessary for these tracks to keep all four tires on the pavement, as well as reducing complaints from the driver. A track such as Kansas Speedway or Circuit of the Americas, where the pavement is very smooth and void of large bumps, doesn’t produce the need to deal with those bumps and a stiffer spring package can be used.
Putting it all together, forming a baseline
Once all of these options are considered, a general idea of what springs to use should be very apparent. For instance, if we’re setting up a NASCAR Cup car at Charlotte, we know that we don’t need to get around ride heights (soft springs aren’t necessary), we would like as much downforce as possible (stiff springs are desirable), the track isn’t extremely bumpy (stiff springs), and the banking is quite high (stiff springs). Looking at all that, we’re going to probably run a stiff spring package at Charlotte and attempt to generate as much downforce as possible while all but ignoring mechanical grip. However, Atlanta Motor Speedway has the same banking as Charlotte, is the same length as Charlotte, but is tremendously bumpy in comparison. We could take our Charlotte spring package to Atlanta, but it would likely bounce around and we’d have no grip. Instead, we need to adapt with softer springs for the bumps, which means we’ll need to raise the car a little bit to keep it from hitting the track, and we’ll sacrifice a bit of aerodynamic grip in lieu of some mechanical grip.
I tend to beat the testing horse beyond death, but I cannot stress how important it is to actually test various configurations. A lot of sim-racers have the idea that “testing” is simply where you go build a setup for a track, which it isn’t. To get what is necessary to build a setup week-by-week and know how to adapt a car for different conditions requires true testing. If you’re going to race a new car (or if you feel you still struggle with a car you want to drive), find a track you like, and try all kinds of different things to see what works, and what responds best to you as the driver. Try stiff springs, try soft springs, try a combination of both! Move the car up and down on ride heights, see if something clicks and you hit on something that you’re comfortable with. Once you have the car to your liking at that track, that setup becomes the “baseline”. Those springs are what you’re going to base every other setup on until there’s a major change to the car (such as aerodynamics, tires, or weight). Starting over with a new setup at a new track (or doing the “setup dance”) simply puts you back at Step 1 every single time, and you have to re-learn what works with that spring package. In many cases, one spring will apply to many tracks, and in some cases, it will apply for an entire season. So don’t over-think it, and never throw springs out just because you’ve gone somewhere new!
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