Commodore’s Garage #20 – Bumpstops and Crossweight

Following our look at the details pertaining to crossweight last week, we’ll now turn our focus to a situation unique to cars with bump stop suspension systems.  This is the first concept we’ll look at that will not apply to every car in some way, but while your interests may not fall into a category with one of the cars in question, that doesn’t mean you won’t be driving one in the future!  We’re going to look at what happens when bump stops engage, or come into contact with the suspension components and become an acting part of the suspension system.  We’re also going to look at bump stop gap, as well as packer settings, and how this affects what happens at the point of engagement.  Warning:  picture overload, but this is something that may be necessary.

Acting Spring Rate

The major reason for using bump stops (or bump springs) at all is to modify the suspension’s acting spring rate through travel.  In most cases this is to control heavy aerodynamic loads and prevent the car from coming into contact with the racing surface, but it also has the potential to drastically change the car’s handling characteristics if bump stop contact is not tuned properly.  For simplicity’s sake we’re going to eliminate rear bump stops in our example, but the principles that apply to the front suspension can also be applied to rear bump stops.

The term “acting spring rate” refers to the total spring rate acting on the suspension.  In a single-spring suspension, the acting spring rate it the rate of the installed spring. Since we have two springs (ignoring the sway bar) that will act on each corner of the suspension, that act in parallel, the rates of the two springs will combine to create the acting spring rate on a bump stop suspension system.  For these examples we’ll assume the following:  we have a 500 lb/in “main” coil spring installed and a bump spring with a linear 1000 lb/in rate.  (NOTE:  For a car with composite bump rubbers, such as the Super Late Model or the GT3 road cars, the same ideas apply, but they will not have a linear spring rate through travel.)  Our car will be arranged like this:

Initial Conditions: 500 lb/in main spring on both front suspension, bumpstops disengaged

Here, we have a symmetrical spring setup and for initial travel our bump springs will not be engaged (bump stops are crossed out).  This will remove the bump springs from consideration and our acting spring rate is 500 lb/in.  At low speeds with little aerodynamic load this will be our operating conditions and the bump springs can be ignored completely.  The front end will have a very low roll stiffness relative to full-load conditions because of the low acting spring rates.  Let’s now apply full loading from aero and the track characteristics and engage the bump springs:

Full Compression: Both 1000 lb/in bump springs are engaged, 1500 lb/in acting spring rates apply

Here, we’ve compressed the suspension to the point of bump spring contact and we now have new acting spring rates.  We’ll add the spring rates together to get our new acting spring rate of 1500 lb/in on each corner.  This new spring rate will keep the car off of the track at high speed, as well as produce a very responsive front end instead of the more dull response likely with the lower spring rates.  This also increases the roll stiffness of the front end, dynamically altering the balance of the car.

It’s important to note that this acting spring rate change happens instantly.  The spring rate for a suspension system changes at the moment of contact, and will similarly change again at the moment of release, where the suspension is no longer in contact with the bump spring or stop.  If the bump stop is not in contact consistently through a turn (the suspension is on and off the bump stop repeatedly), the acting spring rate can similarly change many times during a corner and produce a very unpredictable car.

For those of you interested in the math portion of the bump spring engagement, it’s a common mistake to only consider the acting spring and relate that with suspension travel.  Here’s another of my handy charts to go along with our example:

Click for Larger Image!

Here, we start with a 500 lb/in spring rate until around 1.25” of total suspension travel.  We’re eliminating the motion ratio for both springs, so at 1.25” of travel the main spring exerts 625 pounds of force.  At any point beyond 1.25” of suspension travel, we will be engaged with the 1000 lb/in and have an acting spring rate of 1500 lb/in.  However, if you notice from the chart, 1.5” of travel results in only 1000 pounds of force, not 2250 pounds as would be the case for a 1500 lb/in spring!   Were we to run a 1500 lb/in main spring and no bump stop, we’d actually see less travel and need to start the car lower due to the higher force with shorter suspension travel.

The missing 1250 pounds is found when the springs are looked at separately at this point in the suspension travel.  At 1.5” of suspension travel we have compressed the 500 lb/in spring 1.5” and it is exerting 750 pounds of force.  At this same point, we’ve only compressed the bump spring 0.25” since it engaged at 1.25” of suspension travel, and it is exerting only 250 pounds of force.  This is why having a main spring of 500 lb/in and a bump spring of 1000 lb/in does not produce the same results as a main spring of 1500 lb/in by itself.  At the point of contact, the force acting on the suspension will increase at the rate of the acting spring rate but will not be the same force as a single spring with the same rate.

Uneven Bumpstop Contact

Something that can prove to be both an advantage and a headache is whenever the bump stop gaps are set unevenly and contact occurs sooner on one corner than the other.  In most cases, the car will be set up to contact one bump stop before the other anyway, especially in oval racing, but if it’s set improperly this can cause unpredictable handling and it can very likely lead to a loss of control.  Let’s look at our example again, but mis-match the bump stop gaps.

We’re going to start again with the bump springs disengaged and an acting spring rate of 500 lb/in on both corners.  First, we’ll set the right-front bump stop gap very small relative to the left-front gap and see what happens:

RF bump stop contacts first: Increase in acting spring rate on RF, constant spring rate on LF

Here, we’ve engaged the right-front bump spring but the car is still riding on only the left-front main spring, so (instantaneously, remember) we’ve moved to a 1500 lb/in acting spring rate on the right-front while the left-front didn’t change.  If the bump stop rate isn’t that high, we can get away with this for short amounts of travel, however if there is a large amount of suspension travel this can be a problem.  Let’s break out the chart again!

With this chart, I’ve added a trace (in red) to represent the left-front corner.  We can see at the point of bumpstop engagement (1.25” of travel) the force on the right-front (blue trace) begins to gain force much quicker than the left-front due to the higher acting spring rate.

If we eventually engaged the left-front bump spring, and the bump springs were the same rate, we’d wind up with something like this where both corners are gaining force at the same rate. However the right-front will always have more force acting on the suspension at all values of travel in this configuration.  If we wanted the forces to equalize across the front tires, the left-front bump spring would need to be a higher rate.

Crossweight Implications

If the suspension travel were 100% in the vertical direction, we’d be okay in terms of weight distribution.  However, we do have to consider chassis roll when the acting spring rates change and what that will do to the sway bar and rear end housing (if the car has one).  Whenever one bump stop contacts before the other, the suspension will go through a change in roll stiffness and the chassis will inevitably begin a shift in dynamic crossweight until the other bump stop is engaged.  Following our same example, but looking at the front of the car, we’ll wind up with this:

Here, we’ve engaged the right-front bump stop earlier than the left-front bump stop again, which will start to cause the car to want to roll to the left (from the driver’s perspective).  This situation will likely result in an increase in crossweight, plus the force gain from the higher acting right-front spring rate.  Once the left-front bump stop engages, the dynamic crossweight will probably decrease, but the sway bar and rear housing could still be fighting the initial roll change.  This is often a very difficult problem to diagnose from the driver’s seat, mainly because the effect is very small initially.  Since bump stop gap differences can be fractions of an inch before motion ratios are considered, the shift in crossweight can be relatively small.  Still, it can manifest into a larger issue later in a run, resulting in poor tire wear and/or uneven tire pressure buildup.

The real-world method to avoiding this problem is simply to remove the main springs and allow the car to sit on the bump stops at the heights it would be under full compression.  Shims are then inserted or removed on the bump stop stack until the crossweight at full compression is equal to the crossweight at static heights.  As of now, we don’t have that ability in iRacing, but we can never rule that out as a future possibility.

Where do I start, then?

The best way to start setting shim height and bump stop gap is to keep them equal on both front corners, as well as equal front spring rates for both main and bump stop rates.  Testing runs should show whether or not one of the front tires get excessively hot over a few laps, and you can either add a shim to the other suspension (RF is hot, shim LF) or remove one from the hot corner.  In most cases, the ideal difference in shim height/bump stop gap will be small but it is something else that is driver-specific.  Once the difference between the bump stop gaps has been determined, adjusting them equally up and down can be used to fine-tune splitter or valence height at speed.  Should you decide to change spring rates at any point, you will need to change the bump stop gap/shim height to compensate for the change in suspension travel.  Even further, any change in ride heights will change the shock deflections, and thus the bump stop gap.  Always keep an eye on the shock deflections in a bump stop car to make sure your bump stop gap is staying constant through all of the adjustments.  If you change something on the car that changes the bump stop gap, but you do not account for it, you will have changed more than one thing on the car!

Bump stops are not simple at all, and it’s often the main reason why some racing teams just never have a competitive car.  Adding a new spring in the middle of the suspension’s travel is a mathematical nightmare, not to mention all of the things that carry a spring rate with them in the suspension.  If we looked at a real example, we’d need the motion ratios between the main spring, bump spring, and sway bar, plus the ride spring rate of the tire itself, as well as any extra springiness that could come from the suspension arms!  There is a lot to consider with these things, and they’re not something that can be understood overnight, but taking the information in a little bit at a time and testing each little bit as you go will work wonders in the long run.

Since this is article #20, there will be no new Commodore’s Garage next Friday.  We’ll be back on March 3^rd where we’ll dive into some telemetry analysis and start applying what we’ve learned so far.  I want to wish the best of luck to everyone who is running in the three NASCAR iRacing Series that start up next week, and I hope to see you on the track!
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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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