1994 Mazda MX-5 Miata: Suspension Ruminations
May 11, 2010
One difficulty with modifying suspensions is that, on the surface of things, doing it right looks deceptively similar to the way an ape would approach it. There's some amount of additional spring rate and maybe some thicker stabilizer bars, and some stiff dampers for good measure. Ook ook.
However, things become much clearer when you go back to the basics. Those basics aren't spring rates or the diameter of the bars, either -- those things are actually the outputs of the suspension modification process, not the inputs. What we're actually interested in when we talk about the fundamentals of revamping suspensions is ride frequency and roll couple.
Come join me on the other side of the jump where we'll skim the surface of this suspension nerdery a bit and apply it to Project Miata.
In a way, the stiffness of your car's coil springs are only as high as the suspension geometry allows them to be. You'll notice that Dan Edmunds points out the suspension's motion ratio in his suspension walkarounds. It's important, and here's why.
A strut-type suspension has a high motion ratio since it locates the spring very close to the wheel, so the coil spring "acts" stiffer. The double-wishbone setup locates the spring further inboard, giving it relatively low motion ratio because the wheel acts like a longer prybar, effectively making the spring softer. More mechanical advantage, in other words.
Take two cars that are identical except that one has a double wishbone suspension and the other has a strut-type setup. Same weight, spring rates, everything. If you were to drive each car over a speedbump, the strut car would ride much more firmly than the other (forget about dampers for a moment). This is because of the suspensions' differing motion ratios and the resulting effect on ride frequency.
Back to our bouncing cars -- the frequency at which each car bounces is its ride (or bounce) frequency, and it's one of the first things you should decide when modifying your suspension.
The higher the ride frequency, the firmer the ride. Plain-jane modern street cars will be in the 1.0-1.5 Hz range for comfort. Modern stock high performance cars generally hover closer to the 2.0 Hz range. The rear ride frequency is typically about 15-20% higher than the front to keep the rear end's activity in sync with the front -- after all, the rear encounters each bump after the front does.
Because Project Miata is to be a dual-purpose street/track car, a ride frequency roughly in the 2.0 Hz ballpark is what we're after, up from about 1.15-1.20 Hz stock. We want quicker responses and more capability, but not a punishing ride.
That's great and all, but the spring rates that provide the desired ride frequency don't necessarily provide the right amount of roll stiffness.
Enter the stabilizer bar. The effectiveness of a stabilizer bar depends on its diameter, the length of its arms that twist the bar and the location of the endlink that pushes on the arm. If you also know the stiffness of the bar's steel, you can calculate the bar's stiffness, and from there you can dial in the right amount of roll stiffness.
The "right amount" is up to you. For us, well, we like the stock Miata's neutral handling balance, so we'll stick with a setup that emulates the stock front/rear roll stiffness relationship, aka roll couple distribution.
At this point, you can try to figure it all out yourself, measuring the suspension geometry and selecting springs based on your desired ride frequency and bars to provide the desired roll couple. Be advised, there are a lot of assumptions in there and you will be turning a lot of wrenches to get it right. And we haven't even started to talk about suspension tuning considerations like shocks or suspension travel.
Fat Cat Motorsports
Through the magic of the internet, Shaikh from Fat Cat Motorsports found our Project Miata and offered to help. He's done all the math, trial & error and tuning tweaks -- in the process creating a detailed Miata suspension calculator -- and has come up with a bolt-on set of coilovers for us based on the principles outlined above, and more. As you can see here, they're now on the car.
More details in a followup post.
Jason Kavanagh, Engineering Editor @ 175,611 miles.