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Suspension Dynamics Pt. 2: Why Tuning Even Works

Posted 02-20-2010 at 07:21 PM by color0
Updated 03-08-2011 at 09:31 AM by color0
This week, we're going to go over some accepted "tuning rules" for Mini-Z, and explain why they work in the scope of the theory we went over last week, i.e.

Mx'' + Cx' + Kx = F(x).

Of course there are plenty of aspects of car tuning that don't involve the springs and dampers at all, and a whole other book of subtleties based on tire characteristics alone -- let me leave those for later articles. So without further delay, let's begin.

Tuning area #1: Springs

Rule #1: "Softer springs for bumpy tracks, and harder springs for smoother tracks."
This one should be intuitive: on a bumpy track where the wheels will be going up and down all the time, you don't want a very high K (stiffness), as the springs will be working very hard to return the wheels back to neutral, and as a result the load on the tires will be fluctuating like crazy! That leads to reduced grip, hence bumpy tracks require softer springs. But on a smooth track, if you don't have to deal with bumps, stiffer springs are just fine: a higher K means less total body roll, which means more grip. Note that you need a VERY smooth track to get more grip using harder springs -- after all, the load fluctuations are larger and faster with greater K, so you need to have very few reasons for load fluctuations (bumps) in the first place before stiffer springs will actually give you more total grip. However...

Rule #1b: "Harder springs give faster response."
A higher K means that the oscillations of the suspension will be faster, effectively making the car transfer its weight more quickly. A car that transfers its weight more quickly will "settle" into its cornering stance faster, which leads to the phenomenon we racers call "responsiveness". And oftentimes, a more responsive car will be quicker than one with more total grip, especially in Mini-Z where the corners are numerous and the transitions very fast.

Rule #2: "Softer springs for low-grip and harder springs for high-grip tracks."
Softer springs typically give a car more total grip and make a car easier to drive on any less-than-perfect surface: reducing K will reduce the speed at which the load on the tires fluctuates, which means the car provides a larger margin of error for you before the tires lose traction. This is exactly what you need on a low-grip surface to be able to maximize your car's potential on a slick track. However, high-grip surfaces are different: if you have too MUCH grip, then traction rolling will be literally just around the corner. Cornering speeds will also be higher, so you will be looking for a more responsive car that you can place more accurately on the racing line. Hence, you would want to use harder springs overall on a high-grip surface.

Rule #3: "Softer springs at one end of the car gives that end more traction."
We've worked this one out already -- with softer springs, the load fluctuations on the tires decrease and thus you get more grip and less responsiveness at that particular end of the car. So putting this on a real-life application, thinking about RWD Mini-Z's: since responsive steering and rear traction are both important, you will observe that the vast majority of racing Mini-Z's have a harder spring rate on the front wheels than on the rears. The harder front end allows quick transitions for getting through S-curves and chicanes, but the softer rear end makes sure that the rear wheels continuously have the traction to propel the car forwards. Of course, you will also find drivers that stiffen up the rear end such that it deliberately loses traction -- this rear-steer technique sacrifices forward traction for faster rotation in corners, and depending on the driver, can make for blazing fast laptimes or poor consistency.

Rule #4 (RWD only): "On the top shock, a stiffer spring gives more on-power traction, and vice versa."
If you're starting to get an idea of how the fundamental "suspension equation" works, then this one is intuitive. When you hit the gas, the car's weight will naturally shift to the rear. The top spring's job is to push down on the rear wheels, so if K is greater here, then the spring will push down harder for any given amount of acceleration. The spring pushing down harder when you hit the gas means more grip on the tires while on-power. But don't forget the converse of this example: when you lift off the gas, a stiffer spring will push down LESS on the rear wheels as the car's weight shifts to the front. So you'll get less grip on the tires when off-power, which usually manifests itself as A) lots of turn-in, or B) too much turn-in (spinning out).

Tuning area #2: Damping

Rule #1: "More damping means more stability and less response, and vice versa."
This one should be very obvious given last week's article. If C (damping) is high, then x' (the speed of the suspension working) is reduced, which means the actual body motions of the car are slowed down (it's connect to the wheels after all). The effect is slowing down the car's body motions is, clearly, less responsive and more stable handling. As we have implied all along, you need a balance of responsiveness and stability to have a fast car, which means that you can never go to extremes with damping settings. There will always be a mid-range "sweet spot" for a particular car on a particular track.

Rule #2: "The end of the car with more damping will have more consistent traction."
Damping, as we know, slows down the action of the suspension. This reduces the speed at which the load on the tire fluctuates, which means that in a situation where the loads are constantly changing (racing), it can actually reduce the total load change experienced by the tire, which means more grip to that tire. Real-life application: consider RWD Mini-Z's once again. Most racing RWD Mini-Z's have some form of damping system on the rear suspension, but none on the front. It's far more important that the rear end have consistent traction to power the entire car forwards throughout the course of a lap, than it is for the front end to have a perfect entry into every corner (although it's nice, of course... some of the fastest RWD Mini-Z's have front damping too for exactly that reason).

Rule #3: "Stiffer springs should be accompanied by more damping."
Let's look at the fundamental equation again and think. A higher K makes the suspension work faster. A higher C makes the suspension work slower. The suspension needs to work at an optimal speed for every particular track in order to extract the most speed out of the car, so clearly K and C must be adjusted in conjunction to keep the loads on the tires at an optimal balance. Real-life application: consider what happens when you have stiff springs in the rear but no damping (K high, C low). The car's rear end will chatter like there's no tomorrow, since the suspension's just working way too fast. What about C high, K low? By the time your car's settled into the corner, everyone else with good setups will have left you in the dust for the next one. In order to maintain a good balance of responsiveness and stability, spring stiffness and damping must go up with one another, with only a small window of adjustment to match your personal tastes.

And that is all for this article -- next time, I'll wrap up some loose ends with suspension tuning theory and finish this series off. I hope you guys and gals are understanding what I write about, because once you do, suspension tuning becomes far less of a mystery and more of an analytical process that will make "Test and Tune" sessions a heck of a lot easier (and fun!). Cheers!
Posted in Z Science
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