Commodore’s Garage #7 – Selecting Springs

Last week we looked at how springs work, how they’re rated, and the different types of springs that may be available on our virtual race cars.  The big question, however, is always “What rates do I use?”  Older sims had flaws in the physics, so spring choice was simply based on what you could exploit to get as much speed as possible. Recent builds for iRacing have brought spring choice closer to reality, and choosing springs based on track characteristics is becoming more and more important.  Things such as aerodynamics, banking, and grip level play their part in laying out what a good setup will be, as well as what can get you into trouble.

The Car

Obviously, the biggest factor in choosing springs is going to be the car itself.  Lighter cars can use softer springs, while heavy cars need beefier springs to keep them from slamming into the race track.  In five years of working with Legend cars at Charlotte Motor Speedway and Concord Speedway, I never saw a spring that was labeled over 400 lb/in.  The car itself weighs only about 1300 pounds, so that was more than enough!  On the other hand, I’ve seen NASCAR trucks in recent years that had all four springs above 1000 lb/in!

Every car will have a spring range where it’s happy.  Too soft and it may flop around as you try to navigate even the slightest bend, but too stiff and the tires may never be effectively in contact with the racing surface.  Finding this “happy” range can be a drawn-out task, but narrowing down what you’re going to work with can really simplify the process down the road.  Some cars, like the Mercedes GT3, are limited heavily on spring choices, so it’s not that difficult to decide what you want, and it’s much harder to work yourself into a massive problem.

Legend cars have basicaly no aerodynamic devices on them, thus don’t react well to overly-stiff spring rates. While working at Charlotte Motor Speedway, I commonly saw rates in the 200-300 lb/in range on the flat 1/4-mile track.

The Driver

Drivers like to complain, and when something isn’t how they like it, they complain a lot.  Alan Jones famously left Formula One in the early 80s because the suspension systems were becoming too stiff and making it very hard on the driver following a ban on ground-effect skirts.  Drivers today are no different, and will not hesitate to let you know about it.  One of my teammates, Alex Scribner hates the feel of high-rate rear springs in his NASCAR vehicles, and will jump at the opportunity to swap them out for a softer spring.  I, on the other hand, prefer a stiffer suspension.

In my second article of the Commodore’s Garage, I mentioned how important it is to know what you’re looking for in a car so you don’t waste countless hours using things that just don’t work.  Now’s the time to put all that to good use!  If you like a more rigid suspension under your car, go for some higher rates.  Spring rates alone do not make a setup fast, so don’t be afraid to go against what the “norm” is if you simply do not like it.

Aerodynamics

Remember last week when I mentioned the difference between suspension movement in a Formula 1 car at different tracks?  We’re going to revisit that now, and I’ll again use the extremes to look at it:  Monza and Monaco.  Teams regularly bring their minimum-drag package to the Italian Grand Prix in Monza, but want a maximum downforce package for Monaco.  These changes in aerodynamic packages are not limited solely to the aerodynamic bits and add-ons to the car, they incorporate the suspension settings as well.  With low-drag comes low-downforce at Monza, and teams not only want to keep some amount of downforce in the car, but also want to keep the car sitting with an attitude that will produce the lowest drag possible.  Since mechanical grip is almost a non-issue, this calls for very stiff suspension, and almost no vertical suspension movement can be seen during a lap of the track.

Monaco, on the other hand, is relatively slow, run on public streets, and thus isn’t a great platform to produce downforce with.  The low speeds (Lewis Hamilton went through the Casino Hairpin at 45kph, or 28mph in his 2015 pole lap) of the track require the car to generate a lot of mechanical grip when the downforce falls away, so the cars run softer suspension than at higher speed tracks.

This is a big thing to consider when choosing your springs in any car.  You’re obviously not going to stuff some 1500lb/in springs in the Street Stock because it’s just going to bounce away, and at the same time you’re not likely to mount 50lb/in springs in the Williams because the downforce would crush them.

Furthermore, think about how much aerodynamics is going to be a factor at a track.  Aerodynamics are important at any track, but some tracks, like the example above, will benefit more from a bit of aerodynamic sacrifice and a little focus on mechanical grip.  Martinsville is commonly thought to be a track where aero does not matter, but teams still show up with the front of the car low in the corners trying to get whatever advantage they can.  If a car is moving through air at any speed, it’s being influenced by the air.

Track Loading

The next thing to consider is how much loading the track is going to produce.  For road racing, this is not a massive deal save for a few places such as Zandvoort, but it’s huge for oval racing.  For example, Dover and Atlanta, despite their differences in everything, produce very similar vertical loading.  For a general rule, higher banking will require stiffer springs regardless of the track’s length.  Bristol is only 0.5-mile in length, but its high banking angle produces very high vertical loads.  New Hampshire is one mile long, just like Dover, but vertical load is almost non-existent because of the low 7° banking in the corners.

It’s also important to consider how quickly the car will “load up” in the banking of the corner.  I mentioned Dover and Atlanta being similar, but Dover tends to produce the load much quicker.  Dover’s corner entry is almost like a drop, while Atlanta is smoother.  You can control a lot of this with shocks, but Dover may require slightly stiffer springs to keep the chassis from slamming into the pavement.

The only effective way to determine a track’s loading is to record telemetry for a track and look at the vertical G-loading for the car in the corner.  It can also be done with some simple math, but it’s necessary to know the exact banking of the track, the car’s speed in the corner, and the turn radius, which is difficult to find sometimes.  The vertical G value can be multiplied by the car’s weight, producing a load on the car.  This can be compared to other track figures, and help you get a starting point for your next race.

Banking Characteristics

A big thing to consider is almost always an oval-track problem:  banking.  There are some banked corners in road racing, but suspension asymmetry is not usually considered based on those corners like it is in oval racing.  How the banking forms in relation to the straight is a big factor in how the car itself loads up in the corner.  If we think about SMI’s Atlanta, Charlotte, and Texas Motor Speedway, they’re all the same length, they all have about the same banking (~24°) in the corners, but they’re all unique in how their banking is built.

Banking is formed in two ways from the most basic standpoint:  The top stays at the same height and the bottom drops away, or the bottom stays and the top rises.  Rarely does a track follow either of those exactly, but each track can be related in that sense.  Texas and Atlanta both have banking where the top rises more than the bottom falls, while Charlotte’s Turn 1 entry has the bottom drop away in an extremely similar fashion to Dover’s turns.  Tracks like Atlanta or Texas, where the banking forms more on the outside of the corner, will load the right-front corner more heavily on entry, and will typically call for a stiffer right-front spring in relation to the left-front spring.  Similarly, these types of turns tend to have a very flat corner exit (think of Texas’ Turn 2), and may require a little extra left-rear rate to keep the back end of the car planted leaving the corner.

A general rule of thumb is to increase the right-front spring rate as banking increases.  Whether you do that through the spring itself, or by other means such as sway bar asymmetry, is up to your own preferences, but this can help the tires last longer into a run.  Conversely, flatter tracks, such as Martinsville or New Hampshire, may allow you to run more even front spring rates, or even a stiffer left-front in some cases.  This is also hugely based on driver preference, since some drivers simply do not like having a stiffer left-front spring.

Bumpiness, Grip

Finally, we’re going to look at how bumpy and/or worn out the track is.  A bumpy track will usually require softer suspension and higher ride heights, while a smoother track can call for stiffer springs and much lower ride heights.  Similarly, older, more worn-out tracks will want softer springs, high-grip tracks will allow the use of stiffer springs.

We’d all like to have a lot of downforce at every track, but bumpy or worn-out tracks will often benefit less from aerodynamics than they will gain from mechanical grip.  If we install high-rate springs into a car at a bumpy track, there’s a good chance that the car can bounce around too much, the driver loses confidence in the car’s grip, and lap times suffer.  However, if we sacrifice some of the aerodynamics and install some softer springs, the car can glide over the bumpy sections of track and the driver has a more predictable platform underneath the car.  The same can go for a worn-out surface.  Chicagoland isn’t extremely bumpy, but the pavement is relatively old and has lost some of its grip.  We may want to install some softer springs for a little extra mechanical grip to keep the car from skidding when the tires can’t grip the track as well.

New pavement is always fun, and it’s not uncommon to see the biggest springs show up on newly-paved tracks.  These situations have almost no bumps and a ton of grip, meaning we can sacrifice mechanical grip for the sake of aerodynamics.  Extremely high-rate springs, aggressive shocks, and low ride heights can be used effectively, combining the best of aerodynamics and mechanical grip to produce some extremely high speeds.

Go forth!

I’ve intentionally kept spring rate numbers vague because I want everyone to go try to find what is comfortable for them.  If I were to say, “I used a 900 lb/in right-rear spring at Kentucky”, you may not be inclined to experiment with what would work best for you, and that’s not what this series is for.  I’ve given some guidelines as to what to think about, and when those are combined they can narrow down your choices considerably. Also, keep in mind that a set of springs doesn’t make a car fast by itself, there are many ways to go about this.  Never be afraid to try something new, and if it works, stick with it!  Try not to do the “setup dance” at a new track, but instead find something that is working well for you and adjust it to the new track.  Over time, with minor adjustments each week, you’ll consistently run well each and every week while you build up a notebook on what the car wants in certain conditions and what things don’t work at all.

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I want to dedicate this article to the memory of Mr. Jeff Dukehart, who passed away this week at age 51.  “Duke” was a member of Gale Force Racing in its early years, and was known for spending countless hours sweating the tiniest details of his car’s setup week after week.  He and some others left GFR to form another team, but we all continued to race together in the Class B series up until his passing.

He was a well-respected friend, and a great person to race with.  Rest in peace Duke, you will be missed.

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To keep up to date with The Commodore’s Garage, return to Sim Racing News every Friday afternoon and “Like” our page at https://www.facebook.com/CommodoresGarage

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