Tuning front/rear weight distribution

[Coaster]

Like the topic title says, I have questions about front/rear weight distribution and what to expect when shifting weight one direction or the other. Some background info first...

I have Bilstein PSS9 coilovers on my 2003 996 C2 Cab. The ride height was set twice by indie performance shops, the last time eight years ago. Springs have settled more since then and the latest measurements with new tires suggest it is time to adjust the height again.

The car is mostly a weekend driver and has never been on a track or in an autocross event. It sees a lot of freeways, on/off ramps, and a regrettably small amount of canyon driving. My driving style is spirited but not as aggressive as you might see on a track.

Ride height has been set low since installing the PSS9's and now I'm thinking of using factory X74 specs as a starting point for the new ride height and alignment.

Looking at Porsche's ride height specs, I see different front/rear distribution between different C2 models:

• USA Standard and Sport - 5mm lower in front
• RoW Standard and Sport, and X74 - 15mm lower in front

The indie shops I mentioned kept the height low but always set the front/rear difference close to the 5mm USA spec. I always assumed the USA spec was a variant from the Porsche design ideal, something Porsche had to do to meet DOT bumper height requirements. I'm leaning toward trying the lower front of the non-USA specs.

Questions:

1. What difference in handling should I expect by lowering the front an additional 10mm?
2. If I keep the front height 10mm above X74 spec, shouldn't I also use less front negative camber than called for in the X74 spec?
3. Are these differences inconsequential for the type of driving I do?

Comments, opinions, and personal experience appreciated. Educate me please!

Edited May 10, 2014 by Coaster

[Ahsai]

10mm is a significant difference you could feel and it should make the car handle even better. I upgraded from US stock to x74 with x74 alignment. The x74 totally transformed the car with much sharper handling with much less roll. The stance looks much nicer and aggressive too.

IMHO, you got PSS9 and it will be a waste not to take advantage of lowering the car from the US spec. With lower gravity, everything will be better, more responsive, less roll, better acceleration and braking, etc. I think it's good to start with the x74 alignment spec with its ride height as well and see how you like that.

You just need to be a little careful going down steep curbs to avoid scraping the front bumper bottom.

[Coaster]

Thanks for the feedback Ahsai.

I went ahead and set the ride height to x74 specs and I'll get an x74 alignment this week. I'm looking forward to seeing how the handling changes.

The shops that did the two previous ride height adjustments set up the car low but always maintained that 5mm difference front to rear. I'm wondering why. Lowering the front shifts weight forward doesn't it? With front-engine rear-drive cars, moving weight forward is supposed to increase understeer. Is that rule flipped for rear-engine cars where most of the weight is aft?

[Ahsai]

The 996 are biased towards understeer so any extra traction at the front will help that's why most racers are after wider front tires and more negative camber in the front. With the front lowered, the floating feeling at high speed should be gone.

Lowering the center of gravity will generally improve everything (of course permitted by the geometry permitting up to some degree) as it reduces weight transfer under acceleration and deceleration.

Really those racers should chime in but at least that's my understanding and experience as well.

[Ahsai]

Forgot to answer your question. Yes, lowering the front will bias more weight to the front (rake), which will increase the traction in the front hence less understeering.