I built this one setup.  It was awesome, if I’m allowed to say that about my own work.  Purpose-built specifically for the virtual high-banks of the Daytona International Speedway, it was meant to run in one, and only one, race of the 2015 NASCAR PEAK Antifreeze Series.  I built it knowing that once the checkered flag fell in that race, it would immediately become obsolete.  Still, it didn’t stop me from turning over 700 laps by myself on the track, filling my iRacing folder with over three gigabytes of telemetry data, and even researching race car aerodynamic properties.  It worked perfectly, was balanced mechanically and efficient aerodynamically.  The setup was put on only one car in its unaltered state and finished in eighth place.  After that, it was archived.

While the next race (Las Vegas) was run on the same build, I went straight into testing for the new rules package, one that would swap out bump stops for the relatively newer bump springs (or secondary springs).  These bump springs showed-up and turned the garage on its head.  Some NASCAR PEAK Antifreeze Series teams have gotten the hang of them, others have struggled to find a good package that works for them.  I try to avoid “setup guide” articles because there are guys on iRacingNews who do a pretty good job of it.  But I wanted to throw caution to the wind this time, and maybe go into a bit of detail on what makes the springs so incredibly different than the bump stops, and why it is throwing a curveball to a lot of teams.  So if you can survive a few paragraphs of “engineer-speak,” then you should walk away from this with a better understanding of what is going on with these things.

Force vs. travel graph for the RSW 530 bump stop “puck”. (Image from RE Suspensions website)

Let’s start with the old-style bump stops.  These were little pucks (or donuts, in some cases) made of rubber, polyurethane or whatever kind of material teams thought would do the trick, placed on the shock shaft.  Originally, they were used to actually stop any bump travel (compressing shock) at a certain point and keep the car off the ground.  Eventually, engineers started seeing that they could be used for handling purposes, which caused manufacturers to start making all different kinds of bump stops in many different shapes, “hardnesses” and color.  The graph above shows the force vs. travel trace for RSW “donut”-style bump stops, with each line representing a different hardness for that product.

If we look at the lines, and know that the slope of the line at a given point is the current spring rate of the bump stop at that point, it’s easier to put it into terms we can use for iRacing.  As you can see, they all increase in force, and rate, exponentially.  For instance, on the RSW Red bump stop, the stop is exerting ~325lb at 0.30″ of compression, but it’s almost 550lb at 0.40″.  If we simplify it (heavily, and I mean heavily), the bump stop had a rate of 1083lb/in for the first 0.3″, but was 2250lb/in for the next tenth after that!  Bump stops used for the NASCAR Sprint Cup series were much stiffer still, and it’s this inconsistency over small compression changes that caused some teams to be awful one week and dominant the next week.

Image of a late-model bump spring in action. The black cylinder at the top is the shock body, the silver pieces are the bump spring cups. (Image from www.bumpspring.com)

Eventually, someone probably got really pissed-off at bump stops being so finicky and began making tiny springs; that, or they took a valve spring and put it on the shock instead.  Thus, the bump spring.  It’s mounted in the same location as a bump stop, but since it’s a linear spring, it doesn’t have the bump stops’ rate inconsistency.  It’s easier to work with because of that . . . in theory.  There’s no real need to go farther in-depth because it’s pretty simple.  It’s just a tiny spring.

From the 2013 Texas NPAS race run on the rules package with minimum ride heights and rubber bump stops

Where a bump stop could have potentially exerted 5,000lb of force at, say, 0.25″ of compression, that exact same bump stop could exert 10,000lb at 0.3″ of compression.  This can be a nightmare at some tracks, especially bumpy ones.  Aerodynamically, we want the nose to be as low as possible all the time, so our setups for the NASCAR PEAK Antifreeze Series have changed heavily with each set of regulation changes.  In 2013, we had bump stops and a minimum ride height rule of 5.50″ on the front of the frame rails.  Since we lacked enough rebound in the front shocks to keep big springs down on the track, the main springs would typically be 300-400 lb/in on both corners.  We’d then install bump stops that were to the drivers’ liking (in Nick’s case, this was 1-3 on stiffness), and then shim-up the front end until it stopped hitting the track.  That was it, basically.  The picture above from the 2013 Texas race shows the cars before the ride height rule was eliminated.  In the case above, the springs were 350 and 300lb/in.  Since the shocks exert almost no noticeable rebound force, we had to do this to keep the nose low.  It worked pretty well:  We won six races that year.  Now let’s fast-forward to last season…

From the 2014 NASCAR iRacing.com Pro Series run without minimum ride heights but with rubber bump stops. This particular car was in a Charlotte test.

This image shows Brian Day’s car in a test at Charlotte during the 2014 NASCAR Pro Series.  Notice how the splitter is much, much lower than Nick’s was (and Ray Alfalla’s, too) at TMS, and it stayed that way around the entire track.  With the ride height rule removed, we were able to slam the car down, compress the bump stop in the garage, and “let the big dog eat” as they say.  This was really easy to do, and by adjusting the packer shims we could fine-tune the splitter height to where it needed to be.  Since it was already compressing the bump stop before the car ever moved, the bump stop was already exerting a great deal of force, so any extra force from a bump in the track kept the splitter from striking the pavement.  It was kind of like driving a go-kart:  There was no acting suspension in the front of the cars, they were pretty solid.  We used the sudden rate spikes to our advantage, and by mid-season could have the front suspension of any car figured out very quickly.

Because the ride height rule was removed, front spring rates skyrocketed since we didn’t have to worry about the nose popping up on the straights.  We could run 3000 lb/in springs if we wanted to, and that we did.  Mind you, we couldn’t get quite up to what the real cars were running because of the rebound limitations, but they were stiff.  The stiffest front springs we ran were over 5000 lb/in at Charlotte, and we never went much below 1000 lb/in.   Then, the nose would be ratcheted down onto the bump stops and paired with the heavy corner spring.  Mechanically, this is suicide, but the high spring rates from both the coil spring and the bump stop kept the nose from dipping low and striking the track.  If we ran the splitter at 0.2″ on track and it hit a big enough bump, the splitter would strike the pavement, and that’s bad.  Mmkay?

Pulling the nose down onto the bump stop caused it to exert a lot of force under no load.  Imagine the examples above, and let’s say that the bump stop was compressed 0.25″ and exerted 5000 lb/in.  At that point, the nose was a little higher than it should have been, but vertical loads made up for that.  Whenever that happened, the bump stops exerted way more force over a very short amount of travel, so it kept the splitter very low and very consistent.  Again, this is suicide in terms of mechanical grip, but the aerodynamic advantage was enormous.  There were many tracks where Nick could qualify without lifting off the throttle by a whole lot, if at all.  If we had the car sitting correctly on the track, it was incredibly easy to drive, if not somewhat terrifying.

From the 2014 Brickyard 400, the winning #24 car was run with the suspension package that we currently have on iRacing: No minimum ride height, allowing bump springs.

Now, with all this knowledge we had, it got tossed out to make room for the new bump spring.  These things aren’t all that new, actually, as they’ve been in the NASCAR Sprint Cup Series for a few years.  The picture above is from the 2014 Brickyard 400, and you can be certain that this car either had at least one coil bump spring or bellows “dish” style bump stop on the front shocks.  They were on short track cars long before that, even.  Another of our GFR drivers, Alex Scribner, actually used steel bellows washers on his Late Model a few years ago, which is where I first heard of them being used.

It’s important to note that a lot of setup developments stem from short-track racing.  Remember the inconsistency “feature” I mentioned from bump stops?  That is one of the main reasons many Saturday-night Late Model teams set their cars on fire after trying to switch from Coil-binding to bump stops.  They couldn’t really get a good feel on what was going on, and many never got it at all.  Eventually though, someone thought to just take a tiny spring and replace the rubber things on the shock, and it took off, eventually finding its way into the bigger series of NASCAR and later, iRacing.  The bump springs eliminate the inconsistency by using a linear-rate spring, so whenever they’re used, teams know exactly how much force will be exerted for each unit of travel.  Where a bump stop may be 5000 lb/in at 0.2″ of travel, it could be 10,000 lb/in at 0.3″, and so on and so forth.  A bump spring that is 5000 lb/in at 0.2″ of travel will be 5000 lb/in at 0.3″, so it’s much more consistent, and doesn’t have the giant force spikes of the rubber pucks and donuts.

So what’s the problem? For NASCAR teams, they can get it done pretty easily with simulations and wind-up with a much more consistent, balanced platform to set-up their cars.  For us on iRacing, it isn’t so simple.  We basically relied for two years on the bump stop’s rate-spike “feature” to keep the nose off the pavement, so removing that has forced us to completely re-engineer the front suspensions.  And to add to the headache, we have no idea of the bump stops rates on the cars that were good, so we don’t have a starting point!  Add-in the multiple ways these things can be used, and it’s a pretty big mess when you sit down and try to make sense of it.

Despite all that, I want to add that I am not complaining.  Quite the contrary!  I’ve spent loads of hours running laps by myself and analyzing the data that comes from it, then setting a new goal from real-world data and seeing if it works in the sim as well.  When it does, it’s awesome, and when it doesn’t…well it’s frustrating but I’m forced to re-think things.  It was pretty obvious that everyone except for two cars in the NASCAR PEAK Antifreeze Series was on the back foot playing catch-up in the early rounds, but I think the gap is closing.  If the Charlotte race was any indication, it should be a fun summer stretch, and I’m willing to bet that we’ll be seeing some really good sim racing at the next few tracks.

Alright, that’s enough of the engineer-speak.  Normal programming should resume shortly… If you want some extra reading, here are a few links that have some good information:

-RE Suspension’s Bumpstop page.  Check out all the different kinds of bump stops available, typically used by Late Model teams:  http://store.resuspension.com/home.php?cat=328
-RE Suspension’s Bump Spring selection.  Here’s a link to their selection of Hypercoil bump springs: http://store.resuspension.com/home.php?cat=404
-Also a link to see the metal “bellows-style” bump springs I mentioned:   http://store.resuspension.com/home.php?cat=336
-The TL;DR version of how bumpsprings can replace bumpstops:  http://bumpspring.com/


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One Comments

Good read Matt,

I have plans of running the NiS open next year and I think of myself as a setup newbie so I have tons to learn. It’s articles like these that just make me want to read more and more 😉

Yannick Van Broeck
May 28th, 2015 at 6:19 pm

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