Q & A: Street Class Gas Pressure & Valving Changes
After opening up our Q & A this week, we’ve been flooded with a bunch of interesting topics to cover. So we’ll start with the most difficult one, of course.
Sang Yi asked "Is running higher gas pressure to supplement spring rate a valid strategy for softly sprung Street class cars? Also, would you valve your shocks differently for Street class, compared to classes that can utilize stiffer springs?"
It's a valid thought. Not how I'd approach it, but a valid thought. WRT "spring rate", the only rate that is added from a damper is the gas spring (how far the rod displaces oil into the gas volume). This is really small (single digits in lb/in), basically a rounding error.
Gas FORCE in extension is the bigger effect. It's a roughly constant force equal to pressure times rod area, usually somewhere in the tens of pounds per corner depending on the rod. Constant force is preload, not rate. Think about it like a preloaded main spring. You aren't increasing the rate of the spring, you're changing where the corner settles. On soft stock springs that's a real ride height change (a few tenths of an inch), and in Street you can't adjust a perch to put it back down.
The other thing to understand is that since it's the same constant force on both sides of the car, it adds nothing to roll stiffness. Both rods are pushing out just as hard at full roll as they are sitting in grid, so there's no difference in force side to side resisting roll. The "extra spring" never shows up in the place you actually wanted it.
What gas pressure DOES change is where you operate in the travel, which mostly means bump stop engagement. More gas force means the car sits higher and touches the stops later and more gradually. Less pressure does the opposite, and bleeding pressure down to sit lower (or running a shock like a Koni Yellow without this extension force) and get onto the stop earlier is a real tactic on some platforms. So think of pressure as a position tool, not a rate tool. If your platform wants to stay off its stops, it's one legitimate way to get that.
IMO though, supporting the platform is a problem better solved with strong compression damping, for a couple reasons. First, damper force scales with shaft speed, so it shows up the instant the car starts to move (exactly when a soft spring hasn't built any force yet) and goes away when you don't need it, instead of holding the car and the CG up all the time. Second, you can run a much smoother ramp-in of the force to load the tires less abruptly, something I'd find very important when dealing with generally understeer-biased cars that are very prone to shock-loading the front tires.
If I'm setting up a car that isn't expected to make use of bump stops within the rules, and has reasonable balance, I'd expect to use quite a bit stronger compression damping overall than I would for the same platform with a strong main spring, especially at the low speed. You can get away with it because the tire only feels the total of spring force plus damper force. When the spring is contributing so little, you can add low speed damper force that would be too much alongside stiffer springs and the total load driven into the contact patch is still reasonable. This will in turn impact how I’ll end up valving the rebound to impact entry, exit, and transitional behavior. Without stops in the picture, this doesn’t function differently than with most cars, but will also be a (less severely than compression side) stiffer valving (WRT damping ratio, not necessarily actual force) than you’d likely run with stiffer springs.
This gets tricky in practice for a couple reasons. First, you may be prone to making the car more understeer biased in general by running a larger front bar to help camber control / add roll stiffness in Street class. Running a load of compression up front will tighten up the car on entry, exacerbating the natural balance problem inherent to most Street class cars. You'll have to decide what trade-off you're willing to make between platform support and the ability to initially rotate the car when necessary transitionally.
If I’m setting up a car that is expected to engage the bump stops, it can quickly become an endless rabbit hole worth its own article (or short book). With the options to tune around whatever sort of bump stops are legal (using foam, rubber, bump springs, with varying density, height, shape, and rate), you've now got a wide variety of variables. More compression force keeps you off the stops longer / engages them more slowly. More rebound keeps you on them longer / disengages them more slowly. Gas pressure moves where all of that starts. The bump stop you choose, spring rate, bar rates, and the net resulting effective wheel rate through its travel also impact those things. Also worth remembering that compression damping only holds the car up while the suspension is moving. In a long steady sweeper the spring, sway bars, and the bump stop are doing all the work.
Okay, so you want the car to have a greater effective spring rate in the rear to get the car to turn, so run a really stiff bump stop, right? Well, not necessarily. Easy to get the car out of phase front to rear, resulting in undesired and unpredictable handling and driving dynamics. The stops don't engage instantaneously, and the effective wheel rates don’t change at the same time, or even at the same point in travel front and rear, so now you're tuning around two different spring rates (both in ride and in roll / pitch / dive) per axle, and your car will shift balance / dynamics as you engage and disengage them. A change in sway bar on either axle also will impact how the stop engages, so it becomes likely that something will behave counterintuitively, like softening a front bar forcing the car into greater understeer, as you're deeper into the stops on that axle. As such, you'll have to make compromises here as well.
TLDR: It's complicated. Street class is MUCH harder than ST, because you're given way fewer options to chase balance and pace, those options require much stronger tradeoffs, and everything interacts (gas force moves stop engagement, stops change what valving you want, valving shifts balance, sway bars change when and how the stop is engaged), frequently making it less clear of a path forward. Look forward to a follow-up on some of the nuances of bump stop tuning.