Commodore’s Garage #8 – Coil Binding

Last week I went over the various thing that can influence your spring choices for a given track.  How much banking, amount of bumps, and even the weight of the car can drastically alter what you need in your race car, but can those rules be broken?  Of course they can, and it’s not uncommon for them to be broken for the sake of a stopwatch.  NASCAR in the 90s and early 2000s was purely a fight for mechanical grip, but that all changed sometime between 2003 and 2005 (the title of “First” is debated) when a rather ingenious crew chief found a way around the minimum ride height rules and was able to harness the air like nobody before him.  This started a trend that still exists today, we know it as “coil binding”.

The Basics

In a coil spring, we have a limited amount of available travel, or the total deflection the spring can undergo before the coils themselves will start touching each other.  Once the coils touch, the spring is “binding” (the height of the spring in this condition is known as “block height”).  Regardless of the spring’s rate prior to binding, whether it be a 300 lb/in spring or a 3000 lb/in spring, the effective spring rate once bound can be thought of as Infinite for our purposes.  In reality, further compression would depend on the spring material itself because any more travel would come from literally squishing the metal together, kind of like you might smash Play-Doh in your hands.  That considered, racing springs are usually made of steel, and very tough steel at that, so the chances of the spring compressing any further past its block height are almost nonexistent, so it’s safe to say that a bound spring has an Infinite spring rate and can be treated as completely solid.

WHY?

In most engineering circles, if you design something with a sprung mass, but the spring isn’t strong enough to support it, you get made fun of.  During a design process, a lot of work goes into simply isolating and dampening vibrations, usually by choosing springs that will prevent harmful oscillations from destroying the product.  This goes for automotive suspension as well, with car manufacturers putting a lot of time into matching spring rates and damping rates to produce good handling and good ride quality.  So with all this considered, the question becomes:  “Why on earth would you bind a spring?”

The genesis of coil binding is both secretive and openly debated.  Some say that Michael Waltrip’s car in 2005 at Charlotte was the first to use it properly, others say it was going on a few years earlier when the
common-template bodies showed up in 2003.  I’ve also never gotten a straight answer as to whether or not Jeff Gordon’s 1997 T-Rex car was coil-binding or not, which would explain why it was so blisteringly fast compared to the rest of the field.  Regardless of when it started, it stems from a need to get around ride height rules, and nothing really beyond that.

Ride height rules were circumvented as early as the 1980s in Formula 1’s Ground Effect war, when teams would run soft springs to keep the car at legal heights while at a standstill, but utilized bumpstops to keep the car off the ground at high speeds.  NASCAR teams wanted something similar after bumpstops were banned in the early 2000s.  The mandated frame height rules were way too high to be of any value to aerodynamicists, who want the cars as low as possible.  Instead of stiff springs to keep the car up off the track, they opted instead for softer springs (usually just strong enough to maintain legal ride heights for tech) but allowed the springs to bind just before the car hit the track.  Thus, they clear ride height checks in tech inspection as well as have a really solid…no pun intended…aerodynamic platform on the track.

Developing the Coil-Bind Setup

With the introduction of the COT, NASCAR allowed bumpstops back into the picture but kept the minimum ride height rules.  In an attempt to save money by offering teams an alternative to coil-binding, it inevitably wound up costing a bunch of money in the long run.  Early setups in the COT looked similar to stuff from the 90s, and that first race at Bristol was known for high body roll angles and giant splitter gaps.  Eventually, the soft spring idea returned, and the bumpstops were used as the ride springs at speed.  While not an infinite spring rate, bumpstops were still very stiff because they want aerodynamics and don’t particularly care about the driver’s teeth while on track.  Stiffer bumpstops wound up producing sharp changes in wheel rate for small amounts of travel, and then we wind up with the “pigtail” spring.

The last major evolution in coil-binding still exists in the Truck series (and possibly Xfinity) series today, and is known as the “pigtail” spring.  NASCAR mandates that front springs have a “closed, ground coil end” and one “open coil end” (pictured to the right).  In a standard configuration, the open end (top of the picture) would sit in the lower suspension arm and the closed end meets the perch.  Teams began running angled spring buckets where the only part touching the lower mount was the open end of the spring.  This caused the spring to bend under aerodynamic load with little effort and effectively bound only half of the spring.  This allowed much stiffer rates to be run (upwards of 2000lb/in) but still got around the ride height rules.  The high rate spring allowed better control of the bumpstops, and produced a more consistent handling.  A picture of a spring binding in this configuration is below, with a link to a demonstration video in the description.

A “pigtail” front spring bound only one side of a high-rate spring. Pictured here is a 1200 lb/in spring in a pigtail configuration. Note how the spring bends and binds only one side, leaving the other half open to provide an effective spring rate after binding. Full video: https://youtu.be/RUrb4tm7AJs

With the removal of ride heights in 2014, the practice of coil-binding has vanished from the Sprint Cup garage since it’s now possible to run whatever rate springs you want, as well as get the aerodynamic platform for the best downforce.

Coil-binding in the iRacing Sim

iRacing has a very accurate coil-binding system built into the physics, however a lot of the major issues have been stripped away for the sake of the user.  The major thing to remember is that the iRacing springs will bind across the entire spring equally at the same time, so there’s no pigtailing going on.  This means that once you’ve bound a spring, you have no suspension left, so timing and rate choice are extremely important.  As of writing, all of the fendered stock cars can run a coil-binding front end, but the Sprint Cup, Xfinity, and Super Late Model offer bumpstop options instead.  There’s no right or wrong way to set the cars up, but some drivers prefer the bumpstops over coil-binding, so play around and see what you like better.

If you do decide to go the coil-binding route, it’s important to know the springs you’re working with.  Across the stable of big-spring coil-binding cars (not coil-over like the two Late Models), from the National car to the Sprint Cup, it’s safe to assume that any spring under 430 lb/in will bind under all circumstances.  Springs from 440-470 will bind in specific cases (like if you raise the nose and have a high vertical load), but anything above that will likely not bind and stay down for the straights.  It’s very simple to run tests to find out which springs can provide the travel you need, simply by starting with a baseline and changing the springs out one by one to see how early or how late each one binds.  Run a few laps on a pair of springs, change one (and reset the ride heights!!!), and go back out and see how much difference there was in ride height.  A general rule of thumb is that a softer spring will raise the nose of the car if you are binding them, while a stiffer spring will lower it.  For instance, if you have a 350 lb/in left-front spring and that side is hitting the track, swapping to a 330 lb/in should bind earlier, preventing the chassis from hitting the track.  This is backwards from the traditional school of thought, not a typo!

Below is a picture from a 2016 test where I charted the spring heights for the Class B car.  Both runs pictured were taken using the same starting ride height, but changing the left-front spring.  Both springs are less than 425 lb/in, but there is a tremendous difference in splitter height.

Testing is necessary to see how springs will affect your at-speed ride heights. In this picture, I tested two springs separated by only 100lb/in, resulting in a large different in splitter heights at the center of the corner.

Adjusting with Coil-binding

Once you’ve chosen your front springs to give you the proper ride heights and splitter clearance, you’re undoubtedly going to have to adjust things to get the car handling properly.  There are a few things to remember though:

• Avoid Sway Bar changes if possible: While balance changes with the sway bar are effective when you’re using freely-moving springs, sway bar changes while binding can mess up your aerodynamic platform by allowing too much body roll with a small change.  If the car does happen to roll up off of the left-front spring with the right-front bound, it can get extremely tight.  A natural thought would be to decrease the sway bar diameter, which would make that issue much worse.
• Remember that your spring rates no longer work: Like I mentioned, a bound spring is effectively at an “infinite” spring rate, and is no longer the rate represented in the garage.  Prior to binding, the rate would be in effect, but once bound it’s thrown out.  Because of this, front spring rate changes won’t actually result in an expected result because both the old spring and the new spring will still be “infinite” lb/in.
• Keep the same front springs week-to-week: Changing the front springs requires you to start completely over with setting the heights, sway bar, and alignment.  iRacing’s chassis don’t change much at all for a given track type, so it’s wasteful to change front springs when going to a different track.  What worked at Charlotte will work at Homestead, Dover, Texas, and Kansas, while your Richmond package should work at Martinsville or Iowa as well.  All of your major tuning adjustments can be done with the rear end without hurting the aero platform at the front of the car.

Finally, crossweight adjustments gain a new level of complexity when coil-binding as well.  If crossweight was adjusted properly (same ride heights, no preload changes in front ARB and rear housing), it will change how much travel is available in the front springs, which is where close attention to spring deflection comes into play.  It’s important to keep track of how much travel is available in a spring before and after a crossweight adjustment and to adjust accordingly.  If you aren’t sure how to find spring travel, take a look at the spring deflection numbers.  The first number is how much it’s compressed in the garage, the second number is how far it can travel before binding.  The available travel is the difference in these numbers, so if you have a spring deflection “4.55 of 6.10″, you have only 1.55” of available travel in that spring!  Changes in this value will result in the car’s height when the springs are bound, so pay attention closely.

Coil-binding is complex and the main reason why there was such a discrepancy between higher-funded teams and low-budget teams in the higher levels of NASCAR.  Luckily, we don’t have to go buy special springs or perches for our virtual racing cars because they’ve done all the hard work for us already.  Still, it’s a very detail-oriented aspect of racing even in the virtual world, and will reward those who pay attention to the most minute details.  Doing so will save you a lot of headaches, keep you from chasing handling issues that you may never find, and keep you running up front.

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Some images used in this article appeared various sources:
– #99 Ford:  www.motorsport.com
-Pigtail demonstration and video:  Endless Innovations
-NASCAR-style spring image:  www.eibach.com

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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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