ast2000

From another post the statement was made that lowering the ride height would improve handling. Has anyone done this and does it make a difference???

This seems like a free performance improvement.

Peter L

"Free" heh. It will make a difference in the way your car handles, lowering the ride height will lower your center of gravity. I don't know which post you are refering to but if it's the one where it says to lower just your rear to euro spec's then yes it can be done for free, providing you can do it yourself. There is an eccentric on the rear swing arm which will give you about a 1/2" adjustment (someone correct me if i am wrong) which is not much but i suppose you may notice a difference. In order to adjust more you must re-index the rear torsion bars, which is not an easy job as anyone who has done it can tell you, a lot of trial and error, search the FAQ's for instructions on how it is done, i think you can find some here too http://porsche-944-garage.tripod.com/ . In order to lower the front also you need lowered springs, to install them you must disassemble the strut assemblies, to do that you need som sort of spring compressor. Good luck!! let us know if you are going to try it.

Cobbs

The Eccentric 24mm bolt is a be-och to remove. You'll probably need a 1/2" impact wrench.



BTW I thought it could be lowered almost an inch without touching the torsion bars. Anybody...

Tom

The goal is to level the rocker sills which can be accomplished by the rear eccentric. (Also a check to see if it has already been done!) The rear bumper was raised for US regulations. It really does make a difference and makes the rear brakes much more effective and reduces weight transfer to the front. This isn't really totally free as it requires an alignment after but it isn't too expensive if you need the alignment anyhow. Changing springs and adjusting torsion bars is a different league entirely.

Danno

Well yes and no, because like everything else, there are pros and cons.

One must look at the dynamics of the car under cornering forces. Based upon the speed in a corner, there will be a certain amount of lateral load acting upon the car, causing a certain amount of weight transfer. This weight transfer acts upon the center-of-gravity (COG) of the car. The COG then pivots around the instantaneous roll centers between the front and rear wheels. This roll center is determined by the suspension's geometry. With the MacPherson front struts, the roll-center is usually about 3-5 inches above the ground. The rear semi-trailing arms have a roll center about 6-8 inches above the ground. So the car basicly teeter-totters on a roll-axis connecting the two roll-centers. Kinda like those blow-up clowns with sand at the bottom; they rock back and forth. So the car rolls over the roll-axis, it doesn't just rise up on one side and drop down on the other (there's some laterall movement as well.).

Ok, fine, so what happens when you lower a car 1"? Well you actually change the suspension geometry because the pick-up points on the body has been lowered, and the angle of the suspension arms have changed. But the outer pivot points and contact patches remain the same. What this does is lower the instantaneous roll-centers MORE than the 1" you lowered the car. So the moment-arm between the center-of-gravity and the roll-center has INCREASED. This gives the lateral cornering force more leverage to act on your car and the result is MORE BODY ROLL.

The proper way to lower your car is to install shorter springs and stiffer torsion bars, for two reasons:

First, is to limit suspension travel and prevent bottoming. If you had 4" of travel beforehand and now you only have 3", you need springs that are at least 33% stiffer to make sure you can absorb the same impacts as before without bottom out the car.

Second, is to reduce body roll. With the longer moment around the roll axis, you need more resistance on the outboard side to prevent the body from rolling more than stock.

So what's with this body roll thing anyway? Why are we trying to limit it? That's because the McPherson front struts don't dial in as much negative camber as needed to keep the tire vertical and the contact-patch flat. So as the body rolls, the outside wheel leans over more and more and overloads the outside of the tire. By limiting how much the body rolls, you keep the contact patch flat for optimum cornering.

So without going to stiffer springs, and not as pressing, the rear torsion bars, lowering the car will do you no good.

As for the rear adjustability, depending upon how your torsion bars are indexed, you may get stock ride-height with the eccentric adjuster to full-high or full-low. If your's is set to full-high or mid-range, you can still lower your car. But if your adjusters are already at full-low, you'll have to remove your torsion bars and re-index them.

ed devinney

Hear hear, Danno. The PO of my car lowered it to 25.5" from the wheel center to the fender lip, and with the stock 951S t-bars it's too low. On the track I'm starting to get some scraping of the rocker extensions under hard use.

Esthetically I prefer the rear of the car to be a little higher, too, but for my 944 I care only about performance. I'll be raising the rear this winter.

IceShark

25.5" from wheel center to fender?! LOL, you must give Explorers a run for their money on tipping over.

Cobbs, it depends on where the eccentric is. About 3/4" is as far as you can move without going to re-indexing.

marksportcts

25.5" wheel center to fender is more like a Freightliner. The overall diameter of the factory sized Porsche wheels is about 25"-26", giving it a wheel center to fender distance of about 15-16 inches.

Danno

I think he's referring to the fender-to-ground measurement. But 25.5" is pretty close to stock height (depends upon tire wear). Wasn't there some pictures rally 944s running around someplace?

Lemon Yellow 87 n/a

I'm planning on putting koni yellows on all four corners and lowering the front about 1-1.25" with the weltmeister 200lb springs. Are there any disadvantages to doing this apart from a harsher ride? I have the 16" design 90 wheels on, these should be better for the setup I'm proposing, right?

ast2000

WOW, thanks Danno for the explaination of the forces acting on the suspension. Do you any drawings or pictures that would help explain this?

I added a rear sway bar this summer and had to replace the bolt with the eccentric on it. It was fully one way (I don't know full up or down however). I think for now I'll simply try and learn to drive better rather than make any more adjustements.

Thanks
G

Danno

Ok, here's a quick synopsis. Most people mistakenly think that cornering forces act upon the center-of-gravity of the car, which then pivots the body above a point on the ground over the center of the car. Or they think it pivots over the outside tire. But the actual roll center is an imaginary point dictated by the suspension geometry. Check out the following diagram for a MacPherson strut front suspension:

To find the roll center, you do the following:

Great, so what good is finding the roll-center? Well, the force from cornering then pushes on the center-of-gravity (C.O.G.) and the amount of roll is determined by length of the pivot arm between the C.O.G. and the roll-center (resisted by the springs).

Here's what happens after lower a car from it's stock ride-height:

The A-arm now has a tilt to it at static ride-height and this geometry lowers the roll-center. However, as you can see from the diagram, while the C.O.G. may have been lowered by 1", the roll-center has been lowered twice that mount (2"). This gives the cornering force a LONGER lever arm to push the body sideways.

The result is that by lowering a car and maintaining the same spring-rates, you'll end up with MORE BODY ROLL than stock, not good. Well, why is that not good?

Looking at the geometry of the MacPherson front strut, you'll see that once the outside ball-joint rises ABOVE the inner A-arm pivot, the suspension dials in POSITIVE camber (the top of the tires lean outwards). This is because the ball-joint moves in an arc and after it rises above the inner-pivot, the arc moves inward, pulling the bottom of the tire in as well. Combined with the increased body-roll, you'll have a situation where the tire is no longer vertical and the contact patch isn't flat on the ground anymore (bad for cornering).

So the best way to lower a car is to use a DROP-SPINDLE. That maintains the suspension geometry at stock settings, while lowering the entire assembly as a whole (body & suspension). If that's not possible, then you can get shorter, STIFFER springs (see previous post) to limit the body-roll to stock or lesser amounts.

Be aware though, that weight-transfer under cornering is the SAME at the SAME cornering speeds, regardless of body-roll. The roll is just the RESULT of the weight-transfer, NOT that body-roll causes weight-transfer. If you were to weld your suspension shut with no travel (like a go-cart), you'll still have the same amount of weight-transfer as before (at the same cornering speed).

To optimally combat body-roll and detrimental camber changes, you'll need one of THESE suspensions:

An unequal-length double-wishbone suspension or its variants (4/5/6-link) allow you to customize the tire's camber-change curve so that as the body rolls, the tops of the tires are actually pulled inwards (more negative camber) to keep the contact patch flat on the ground (well a little more load on the inside is better because the outside tire edge gets pulled under).

Although car-makers who utilize these suspensions typically don't optimize them for all-out performance, rather for excellent performance combined with a comfortable ride (SupraTT, Ferrari F355/360, Corvette, etc.)

Piece of cake, eh? So I suggest not worrying about upgrading the car or suspension if you're just starting out with DE, autocross or on the racetrack. Work on the driver first because you'll see more improvements in lap times quicker that way.

For example, I've dropped about 30-seconds off my Streets of Willow Springs times the past couple of years. But suspension upgrades only accounted for 5-10 seconds of that. Big sticky R-compound tires on massive rims may only be worth another 4-5 seconds max. So suspension and tires can account for only about 10-seconds, the rest is up to the driver. Have fun!!!

ed devinney

Nice explanation, Danno - thanks!

I should engage brain before posting - even the syncro vanagon is only 19+" from center to fender and I look out over the tops of Suburbans :-) The 944 measures about 24" to the top of fender from the ground, rockers about 4".

Danno

Yikes! Ed, that 24" height is way too low (about 2" lower than stock). No wonder you're rubbing the rockers. This is especially dangerous in front because you can reach the rotation limit of your balljoint in hard-bump. Eventually you can rip out the balljoint, or break the pin, bad either way. About 25" all around is a good height I think.

keith

Danno - correct me if I am wrong here, but common wisdom seems to be that 13" from center of front wheel to top of wheel arch is about where you want to be (no lower), and then level the rear to that..

Measuring from the ground up doesn't really tell you much, as it doesn't take into account differences in tire sizes/heights...