Suspension For The Real WorldI'm in the process of buying a suspension for my '99 EK9 Civic hatchback. I've done a full Type-R swap (subframe, five-lug, blah, blah) and am looking at Progress. I remember you used this suspension on your original Civic Si project car, but couldn't find the suspension installment in my piles of old magazines. I'm hoping you'll share your memories of that car and suspension. Ed at Progress has been very helpful, but I am slightly skeptical about the spring rates he is quoting (450lb/in. front, 650lb/in. rear), even though the Series-1 coilover set-up will have custom valving for those springs.
I have a pretty high tolerance for a firm ride, but my VW background has given me a good idea of the difference between too much, not enough, track, autocross, street, and-most importantly-fun (the latter sorely lacking recently).
Reading your column has a lot to do with where I spend my money. I stopped listening to 'experts' (read salespeople, e-gurus, and 'that guy'). My local SCCA chapter has also been helpful, but your background in this unique situation would be really appreciated. I'm looking for a car biased toward aggressive street driving with some autocross thrown in for good measure. I also have an 80-mile commute to consider, though. It will be primarily street driving and I do want to retain some of the 'slop' that seems to be needed in a street car. It seems to me that too much suspension on a street car not only loosens fillings (especially on the roads here in PA) but also hurts handling on rough pavement.Bob FowlerSomewhere near Philly
The first concern with making a street car handle well is simply firming up the springs and dampers to keep body motions under control when you start throwing big speeds and big cornering loads at it. Second, with a front-drive car, you also want to reduce the car's natural understeer. That means shifting cornering loads to the rear tires by making the rear proportionally stiffer than the front (with some combination of springs, dampers and anti-roll bars). Lowering the car to get the center of gravity down is a distant third in importance. Lowering will only help if you've done the first two things well enough that the suspension doesn't bottom out with the lowered car's reduced travel. Keep these priorities in mind when making your suspension mods.
Your concerns about balancing street and track performance are well founded, but don't assume your VW experience will translate directly into the Honda world without a little math. The fundamental difference between the strut suspension on your VW and the double wishbones on your Civic is the motion ratio between the spring and the wheel. On a strut, the spring acts directly on the hub, so it moves with the wheel in a nearly perfect 1:1 relationship. For every inch the wheel moves, the spring will compress about an inch. That means a 300lb/in spring acts like a 300lb/in spring. How nice.
On the Civic, and most double-wishbone cars, the spring acts on the lower control arm somewhere between the outer ball joint (which moves with the wheel) and the inner control arm bushing (which doesn't). On the front of your Civic, one inch of wheel motion causes only 0.7 inches of spring travel (according to Ed at Progress, who has already measured this, unlike me). The spring rate obeys this same 0.7:1 relationship, so a 300lb/in spring only acts like a 210lb/in spring at the wheel.
Back in the November '99 issue (have you checked behind the toilet?), we installed the first-generation Progress coilover suspension. At the time, they were running 350lb/in. springs up front and 250lb/in. in the rear and we marveled at the smooth ride. In retrospect, we shouldn't have been surprised. With about a 0.7 motion ratio in the front, and around 0.75 in the rear, the actual wheel rates were only 245 front and 188 rear. It's no mystery why the ride was so smooth.
If you know what spring rates gave you an acceptable ride on the Volkswagen, you can translate those to Civic rates by comparing the natural frequency of the suspension. The natural frequency is determined by the spring rate (at the wheel) and the car's weight, and is effectively a measure of how fast the car moves when it hits a bump. This, in turn, determines how uncomfortable you are on that 80-mile commute.
The sprung weight is how much weight is on one corner of the car, minus the weight of the suspension itself. Since you're only using this formula to get a rough idea, it's OK to plug in rough numbers.
Since I don't know your Volkswagen, let's work from my Sentra instead. My Sentra weighs about 2600 pounds and has about a 60/40 weight distribution. That means the sprung weight of one front corner is around 780 pounds (half of the 60 percent of 2600 pounds that sits on the front wheels). Sure, the wheel, tire, hub, brakes, and all should take some weight out of that, but then you'll sit your heavy ass in the car and put some of it back in. Whatever, this is rough.
I was happiest with the ride when the front rode on 300lb/in springs and the rear rode on 225s. Running those numbers through the formula (assuming the Sentra's wheel rate is the same as its spring rate), we get a natural frequency of 1.94Hz in front and 2.05 in the rear.
Now, let's say the Honda weighs 2450 pounds and has a 61/39 weight distribution. That puts roughly 747 pounds on each front wheel. To get the same 1.94Hz front suspension, you'll need a wheel rate of 288lb/in, but with that 0.7:1 Honda motion ratio, the actual spring rate will be 411lb/in., remarkably close to what Progress was suggesting.
Their rear spring rate of 650lb/in, even with the Civic's 0.75:1 motion ratio, still works out to a natural frequency of 3.16Hz (assuming the corner weight numbers I made up are valid). That's painfully stiff for an 80-mile commute.
Don't take my rough numbers as a guide, though. Figure out what the motion ratio is on the rear of your Volkswagen (1:1 should work for the front, and for any strut suspension), and try to remember what spring rates you were comfortable with on that car. Then, using real numbers from your Civic, see what the Progress stuff will be like.
I prefer using the strongest possible rear anti-roll bar to balance the handling on a front-drive car, rather than the extra-high rear spring rate Ed was recommending. For track or autocross, however, the rear bar alone isn't enough to get maximum potential from your car. When we tested our softer Progress-suspended Civic, the rear bar was so big that the inside rear wheel stayed off the ground all the way around the skidpad. At that point, the rear bar is doing everything it can, and the only way to shift more load to the outside rear tire is to increase the rear spring rate. That, undoubtedly, is why Progress has arrived at those high rear spring rates. They probably work great on the track, and maybe California roads, but I wouldn't want to drive 80 miles of East Coast roads with that kind of rear spring rate.
This is the best guide I can give you, but it's still a rough guide. The spring rate is only one aspect of ride quality and road holding. The dampers are absolutely critical, and can have a huge effect on the actual ride frequency. Unfortunately, not only is there no simple calculation to account for the dampers, there isn't even a simple way to describe a damper's strength. They're subtle, complicated beasts and odds are they won't be exactly what you want, no matter who makes them.
Use your experience and that natural frequency formula to get the spring rates close enough, get the biggest rear bar possible (on a front-drive car), make sure the dampers are revalvable and that you have someone willing and able to revalve them based on your feedback. Good luck.