bombay digital > boxster > projects > pss9 suspension upgrade

by Trygve Isaacson, ©2005

After 90,000 total miles, including something like 10,000 on the track, my Boxster was surely a deserving recipient of a refreshed suspension. I decided to upgrade the parts rather than merely replace them with stock components.

Don't Fix What Ain't Broke

I've been very happy with my car's handling. The Boxster is incredible to begin with, and the factory 030 suspension option takes it up another notch. Given the possible wheel fitment problems that can come with lowering the car and changing suspension geometry, I have been reluctant to open that potential can of worms. The car works really well, and my class records in the local region PCA time trials are still sticking, so why change it?

Cause For Concern

The problem is that at the last three track events, I've noticed something that feels wrong in the car's handling. And it just kills my confidence and ability to go fast in certain spots on the track. A prime example of the problem is how the car behaves in the section of Buttonwillow Raceway after exiting Cotton Corners and up to the braking point of the Bus Stop. Under constant acceleration, after a short-shift into 4th gear, there is a right hand kink turn (at about 80MPH), followed by a sweeping left turn (something over 90MPH) that ends as a brief braking zone. It's almost like a super high-speed slalom, and the car needs to feel solid as a rock through the side-to-side transitions, or it gets quite hairy! I noticed the last time I drove Buttonwillow that the car did not feel stable enough to take the right hand kink flat-out; I just could not do it. And even at the lower speed, on one lap the car did a kind of unnerving twitch through the left hand sweep. It almost feels as if the outside rear corner is drooping—and it is definitely not the tire. The likely culprit would seem to be aging shocks or springs or bushings or something. It isn't noticeable (to me at least) on the street, but when you are cornering hard at 90MPH, there it is, that little unsettling something. Others have pointed out that 10,000 track miles is a lot to expect of a suspension, as is 90,000 total, and that this stuff does not feel like brand new forever.

Add to this the fact that my car has stayed stock, while my buddies have been moving up the performance parts curve, and I have an extra incentive to refresh the components with something a little more aggressive in the handling department! For example, take my friend Kevin, with whom I've been attending a lot of the same events for the last 6 years. Two years ago, I was consistently turning faster lap times; one year ago I was struggling to match him; now he is beating me handily. I find myself further down the grid. It's gotta be the car! Right? :)

New Components

I decided to go with a proven setup, now that others have endured the "R&D" headaches. The Bilstein PSS9 kit is proven at this point to perform well and fit the car. At their softest setting the PSS9's should be a little stiffer than the US 030's were, which will make them fine on the street. At the track I'll probably dial them up stiffer. The TRG camber plates will allow more negative camber and eliminate the problem of uneven tire wear, without requiring the car to be dropped too low (which would look ridiculous and cause tire fitment issues). And finally, the TRG sway bars will also help to reduce body roll, and are adjustable as well.

Installation

I was fortunate to have fellow Boxster owners Alan and Ed to help with the install. Actually, "help" is a poor description. They have done this suspension work before and knew just what to do for every little thing. I was really just the helper. I was also the guinea pig for a couple of Alan's new tools. He welded four stools to stand the car on with turntables for alignment, and acquired a Super String alignment kit. This plus his scissor lift is enough to align the car right there in the garage!

"Highlander" (fellow frequenter of the Boxster Racing Board) has a great web page documenting the procedure for R&R'ing the suspension components (except for the sway bars, which are pretty straightforward although I had a little trouble bolting one of the bushings back in place).

It took the three of us about 8 hours from start to finish, including removal, replacement, and aligning.

The only issue was with my rear 285-18 street tires on the 10x18 wheels. I think that the PSS9 springs extend lower over the strut that those on the 030 did (the 030 spring "perch" was completely above the tire). On the left rear wheel, the inside tire sidewall just touched the adjustment ring below the helper spring on the strut. On the right rear wheel there was no contact but it was very close. To remedy this in the short-term, I'm using a pair of 1/4" spacers. The long-term solution is to replace the 285 tires with 265 or 275. Although 285 looks nice there, having that large a delta from the front tire sizes (225) is surely increasing understeer (not that it matters much on the street). Moving to 265 will still fit the 10x18 wheels fine and will provide the clearance needed.

My track tires are 245 front, 255 rear, on the original factory 17" wheels. This is better balanced than the 225-285 street tire combo.

Before And After Settings

Ride height was measured from the center of the wheel up to the bottom edge of the fender. Measuring from the ground would introduce tire diameter difference (tire size, air pressure) into the equation, preventing good car-to-car comparisons.

Toe was measured across the wheel diameter.

The additional front camber obtained by the camber plates should help greatly with tire wear. With the stock suspension (030 or otherwise), you can only get about 1 degree of negative camber; this means that with hard cornering on the track, the outer edge of the tire is being beaten to death doing all the work, and it wears out far more quickly. By dialing in 2 degrees of negative camber, the tire will sit more flatly on the surface while cornering and should finally give me even wear at the track; not to mention better front end grip. This will mean the street tires wearing more on the inside because while driving straight there is more inward lean, but this hasn't been a problem for the rear tires at the same setting. The rear tires have always worn evenly because they've had sufficient negative camber to begin with.

The Verdict On The Street

There's not much to say here, except that the car feels fine and the ride is not noticeably harsher than the US 030. I have the PSS9's at soft, the rear sway bar at soft (it has two positions), and the front sway bar at medium (it has five positions). It's hard to say without doing a back-to-back comparison, but I think the car has more precise turn-in.

The Verdict On The Track

(Following up later after having lots of experience on the new suspension: The improvement is noticeably felt behind the wheel and the concrete proof is that my lap times saw a 2- to 4-second improvement, depending on the track. That's a huge difference.)