Edit: Now with updated graph
So, what the heck does a manufacturer mean when they offer a ‘Sport Suspension’ and is it something you actually want? While I haven’t examined every version available, themes have carried through various makes/models, so what follows are safe generalizations. I even throw in a dyno chart!
OEMs give us lots of specs to get us warm and fuzzy about a car but the majority don’t affect your everyday, commuting-to-work driving experience. How they decide to set up a suspension does. They assume an average commute-only driver just wants a comfortable car. The enthusiast will opt for an (expected) upgrade via the sport version (with infinite colorful names). And if they happen to have a hard-core race version, that is another level all-together. What might feel fun on a 5 minute test drive (and help sell the sporty version) could get annoying (or literally painful) with ownership. Even more so with a disgruntled passenger (“I told you not to get this car!”). I believe (just as I’m writing this) that car makers know they have very little time to close a sale, like a first impression. If they can’t get your attention to begin with, they won’t capture it with pretty brochures or slick commercial spots. Your test drive experience is what will likely sell a particular package.
Fast forward to your first few months with this car; if you commute, most of the time you’re pointed straight and you really don’t want to be jostled all over the place. As I like to say (and have made a video using suspension potentiometer data to prove), there’s no such thing as a smooth road. The dampers and suspension are always working.
Even if most of one’s commute is a mountain road with 99 turns in 4 blissful miles, one still has to cross intersections, deal with potholes and other mundane events. Having a car that doesn’t beat you up is important, even if you’ve cultivated an immunity to the effects of poor damping. Why poison yourself to begin with?
In many sport suspensions, what you get is ‘the Illusion of Performance(IoP).’ I’d trademark it but would rather focus on the Perfect Ride! That IoP gives your body the sensation of activity – remember that we only sense acceleration, not velocity – but a damper with sharp edges on its force profile will cause time-varying load on the tires. The effect is being jerked around, a change in acceleration over time, like being on a rollercoaster.
An ideal suspension needs to soften the edges of the road, so the tires maintain contact and you get a human body-friendly ride (via muted vertical accelerations) plus solid lateral grip (minimal change in contact patch load during cornering). If the suspension designer felt its buyers would associate roughness with speed (which younger drivers – myself included – usually do), then it’ll emphasize creating jerk via more low-speed damping). For a more sophisticated audience or more expensive car, the low and mid speed will change more smoothly (still not necessarily optimal, esp. due to less compression than the chassis could use) they will typically add more high-speed rebound while keeping high-speed bump lower.
For this article I’ll make reference to 2 suspension options available on the 99-05 Mazda Miata, with a third introduced for 04-05 years. Standard was a twin-tube damper made by Showa, then a ‘Hard S’ package which used a Bilstein monotube and ostensibly stiffer and/or lower springs. From 04-05, the Mazdaspeed Miata came with its own package that had a 1mm larger front bar, 3mm larger rear, and stiffer/slightly shorter. The dampers had been tuned even more aggressively than the Hard S but were otherwise dimensionally identical.
The graph at the top of the page shows the various rear dampers only, but the fronts follow the same trend. A few observations: notice how the standard suspension has a much more smoothly varying shape, a more constant slope from 0 to +/- 2 in/sec (negative = bump, positive = rebound in this graph and all the ones we’ll share). The slope determines how much jerk the tires and you experience. The Hard S is 50% stronger in compression @ 1 in/sec and the MSM another ~15% on top. The difference in rebound and ratio of bump to rebound is what determines the degree of jacking down. At 1 in/sec, where small, repeated movements (like any rippled road surface) will affect the dynamic ride height, the ratio is a little less than 1:1 R:B for Showa, then 1.5:1 for Hard S and about 2:1 for MSM. The ‘sportier’ suspension specialize in more immediate steering feedback, yes (turning the wheel results in movements up to ~3 in/sec at the damper in the Miata’s case). But that degrades ride quality and road holding as well. Notice that the mid and high-speed damping isn’t very much different. In fact, the Showa has a strong slope for both bump and rebound, so it would tend to resist bottoming out better than the ‘sportier’ OE, Bilstein-based suspensions! One could also argue that the Bilsteins will blow-off better, which is true but I don’t find the amount of damping to be objectionable and in fact one could almost do a rally setup which was the inverse of the OE curves and have a wicked fast, comfortable car. Yes, I’ve done this! Yes, we’ve built this for customers. How stupid fast do you want to go?
I want to make it very clear that 99% of all complaints of poor ride have to do with jacking down via excess rebound damping, potentially combined with frequent engagement of the front bump stops which gets worse due to excess rebound/jacking down.
Jacking down can be an automakers best friend for numerous reasons. First, you get an additional ‘jerk’ when the bump stop acts as a supplemental spring. Two, the front end will get stiffer as the bump stop engages, increasing weight transfer across that axle and inducing more understeer. So even if you chuck the car into a turn (a novice driver won’t be trail-braking), it won’t want to turn. At least, not as eagerly as you’d like. Jacking down exerts a self-corrective effect on the driver. You are going slower in turns, you’re actually driving 50 though it feels you’re doing 80 (‘wow, what a sporty ride!’). And if you have bigger sway bars, then that jacking effect causes a coupled (cross-axle) time-varying load on both tires! Holy understeer, Batman!
I don’t fault them for doing this. Putting a very capable car in the hands of an inexperienced driver could be a bad thing. But they don’t tell you that this IoP is what they’re up to and that lack of fine print has bothered me since I learned these Truths. Pricier vehicles get better suspensions though it seems there’s always room to remove a bit of understeer, to have a bit more grip and sure-footedness, a bit more confidence. This isn’t including active suspension … although we did tune a Nissan GT-R last year using the OE Adaptronic Bilsteins. Results were very good and we could retain the Soft/Sport mode settings. It was a 2009 GT-R and the damping was definitely more biased to jacking the front down.
In the next article I’ll illustrate a few setups that have strong high-speed rebound and what effects you’d notice with that. This will include accelerometer traces showing the strong downward (negative) accelerations which are very hard on one’s body. I’ll also continue discussing the effects of bump stops on ride and handling.
HOMEWORK! For fun, check how close your front dampers (strut or shock) are to sitting on the bump stops. Report back in the comments section! I know for certain the Mini Cooper rests on the front bump stops, the Mitsubishi Evo and Subaru WRX essentially do the same.