January 20th, 2017 by Matt Holden
Happy New Year! I hope everyone had a wonderful holiday season and, hopefully, everyone has been able to take what we looked at in 2016 and apply it to their sim-racing experience. Before we dive into the more complex components of the cars such as bumpstops, shocks, and alignments, let’s take a final look at springs and consider a few aspects we need to consider going forward.
We’re extremely lucky in sim racing with the fact that we do not have to bother with suspension motion ratios, however I don’t doubt that I may have to eat those words someday. My first experience with suspension motion ratios was when I began R/C car racing in the early 2000s. I raced primarily on a concrete road course, and I couldn’t afford a large assortment of springs. However, the chassis I had featured many different ways to position the shock and spring on the suspension. By simply moving the shock in or out on the suspension, I would wind up with a softer or stiffer suspension because of where the shock was located. This allowed me to adjust the individual suspension stiffness without changing the springs and correct the handling for changes such as cooler temperatures or any oils that may have been left on the track from the nitro-powered cars running on the same track.
Having a component of the suspension mounted inboard of the wheel creates what’s known as a “Motion Ratio” between the spring and the wheel. This motion ratio produces what is known as the “wheel rate”, and will come into consideration when we look at bump stops. For a big-spring stock car such as the NASCAR Cup or Xfinity cars, when the wheel travels one inch, the spring doesn’t travel a full inch. As a result, we wind up with a wheel rate that is much softer than the installed spring rate. We could have a 500 lb/in spring in the suspension, but the wheel only needs 300 pounds to move an inch, and thus has a 300 lb/in wheel rate.
Currently, there are no cars on the iRacing service that allow for the shocks, springs, or rocker arms to be moved. However, all of the cars consider the various motion ratios with the suspension components. For instance, we can have a very simple suspension assembly as in the Mercedes GT3 where springs are pretty much assigned and the relationship between the springs and sway bars can be effectively ignored. Conversely, we can look at the more complex assemblies in the higher-class cars. The NASCAR Sprint Cup car has three acting spring rates for both front corners: Sway bar, Main Spring, Bump Spring. Each of these components is mounted to the suspension arm at a different point, and thus has a different motion ratio with the wheel as well as the other components (The bump spring and the main spring do not compress equally). This is one of the reasons why a 5000lb/in main spring with no bump spring doesn’t behave in the same was as a 2000 lb/in main spring and a 3000 lb/in bump spring. On the road side, we can go even more complex with something like the Pro Mazda, with its adjustable rocker arms, or the McLaren MP4-30 and Dallara Indy Cars, that add third springs into the mix at high speeds to control body attitude.
For simpler cars, the motion ratios can be ignored completely since we have them locked at a certain value for all of us. More complex cars need to have this taken into consideration to make sure issues aren’t being brought in without welcome.
There’s a common misconception in the sim racing community (and the real-world racing community, in some aspects) that changing springs on a corner will change the load applied to the tire as a result of the spring change. This is not entirely true, and often leads to some confusing results on the racetrack, real or virtual.
To understand what happens, we need to break the suspension down and look at the parts. For this example, we’re going to ignore motion ratios completely and assume that the spring is mounted right at the wheel. Let’s say we are seeing 1000 pounds of load on our right-front wheel and we have a 500 lb/in spring. Our spring will be governed by Hooke’s Law in this situation:
F = k*x
F = 1000 lbs
k = 500 lb/in
x = ?
x = F/k
x = 1000/500
x = 2 inches
Now, let’s change that to a 600 lb/in spring on the same car at the same track:
F = k*x
F = 1000 lbs
k = 600 lb/in
x = ?
x = F/k
x = 1000/600
x = 1.67 inches
Note that the spring is governed only by three variables: Spring Rate, how much it compresses, and the external force acting upon it. When placed under load, the spring is simply going to compress until it can exert a force equal (and opposite) to the force compressing it. For our example, the external force is 1000 pounds, the springs will compress until they exert 1000 pounds and then the wheel will not travel any further. The spring cannot exert more than is applied to it, and it won’t stop travel until it has equaled the external force. It’s a very simple principle, but has been misinterpreted in the past. Changing the spring will not directly affect the load placed on the tire. The force is an external force governed by the car’s speed, track characteristics, roll center and CoG locations, and aerodynamics acting on the car.
What does change is how quickly the spring reaches the point of equilibrium. A very stiff spring will reach equilibrium quickly and with a short amount of travel, while a soft spring will allow movement before equaling the external forces. If our spring is stiff and the suspension too rigid, it may not deal with changes in load very well and the tire could lose contact with the track. If the spring is too soft, the driver may feel the car is too unresponsive, since movement in the suspension usually means forces aren’t increasing quickly at the contact patch. This is why some drivers prefer a stiffer car and others prefer a softer car, but if we leave everything else the same, the reality is that the tires are loading the same amount. It’s just a matter of how you get to that point.
Where the springs will change the load on a tire is when aerodynamics are considered. For instance, if we look at the MX-5 or the Legend car, there isn’t much to be found in the aerodynamics department, and the majority of our gains will be found in weight distribution and alignment. For cars like this, we can leave a set of springs in the car forever and tune the car simply by weight placement. On the other hand, if we look at a car that is heavily dependent on aerodynamics, springs will need to be adjusted to cope with differing levels of downforce.
At a higher speed track, it’s much easier to maintain the necessary levels of downforce to keep the car attached to the track. For this, mechanical grip isn’t necessary, but maintaining the car’s attitude is crucial to high speeds. Thus, stiffer springs will be needed. At a slower track, high downforce levels will be difficult to maintain, so mechanical grip is needed to fill in the space when the driver cannot rely on the air. Whenever we do something that increases the downforce level on a car, we will inevitably need to change the spring to respond. An increase in downforce will increase the external force acting on the wheels, and will in turn increase the spring’s deflection. Similarly, if we install a spring with less travel (stiffer rate, for instance), it could raise the car at-speed, reduce downforce, and that tire may lose grip.
Deciding when Spring Changes are Necessary
So with all this considered, when do we look at changing the springs? Most handling issues can be solved without changing the springs, but there are a few cases where it is necessary:
- Driver feels the car is inconsistent or unresponsive – Driver comfort is huge, and is a major factor in fast lap times. If the driver is not confident in the car, he or she is less likely to drive into a corner like their hair is on fire. A car that feels “floppy” to the driver needs to be stiffened up, and a car that feels too rigid will need to be softened. A car that is “loose” or “tight” doesn’t necessarily call for a spring adjustment. If the driver feels the car is consistent, it’s best to leave the springs and try a weight or shock adjustment, or aero adjustment if you have that at your disposal.
- Long run handling swings – Ever had a car that was awesome for five laps but “flipped a switch” and became terrible? That’s typically attributed to a suspension imbalance, where one end of the car wants to go off and do its own thing. This is almost always attributed to a roll stiffness problem, so it’s definitely a time where springs need to be changed. A simple check of the tire temperatures should show one tire excessively hot, while the two on the opposite end are rather close. The two tires with similar temperature belong to the softer end of the car. Either stiffen that end or soften the other end, and the balance swing should go away.
- Getting outrun between the turns – Drag will ruin your day pretty quickly, especially at a track where high speeds are a necessity. If another car is running the same speed as you are through the corners, but walks away from you on the straights, it could be that your car is simply producing too much drag. Whether it’s too much rake (rear higher than the front) or the front of your car is too high in the air, it’s unnecessarily robbing you of a few tenths, so take the time to trim the car out. If there’s too much rake, try softening the rear springs to let the rear drop at speed. If the nose is too high, soften the front springs!
- Digging up the track – The only thing worse than aerodynamic drag is drag produced when your car is in contact with the race track. Not only are you going to unload at least one of the tires, but it’s going to be like throwing an anchor out of the car. I can’t stress how important it is to fix issues with bottoming-out before ever fixing anything else on the car. If you’re hitting the track, stiffen up the springs before ever doing anything else to the car.
Conclusion & Bump Stop Introduction
We’re going to start looking at implementing bump stops and bump springs in the next few articles. These are going to bring out a level of complexity that can make your head spin as well as breaking a few of the sim racing setup rules of the past. Springs are not the “rough tuning” option that they’ve been billed as in the past, and it’s become a rarity to see springs being thrown around the garage at race tracks around the world. Understanding that springs are meant to absorb bumps and control body height, not manage weight distribution, is going to be key in grasping bump stop setups. It’s not a very difficult thing to understand, but it could cause you to look at things in a whole new way.
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