It is a system. Nothing in isolation. That said;

1) Driver
2) Tires
3) Shocks
4) Springs/Torsion Bars
5) Sway Bars
6) Bushings

You posted this just for discussion I presume. Obviously, most that rebuild their suspensions do so with the end in mind. It doesn't make any sense to have your shocks re-valved with worn out bushings, etc., etc. When you start, you tend not to stop. I started a wheel bearing job recently only to finish with dropping and rebuilding, replacing and upgrading the front suspension. Again, this is incomplete because the rear has only been partially done.

We can spend tons opf money on our passion. You must define and draw a line somewhere. Just dont bastardize the car and make it unsuitable to your intended use.

Scott
73 911
74 914

you should keep in mind that the repalcement of tie rods, ball joints, rear bearings, bushings or torsions individually or together will require corner balance, ride height and alignment adjustments after the work. I

t is best from a money standpoint to do these repairs (if needed) at the same time.

Spot on!

F1 cars are designed to be under weight and tungsten is the weapon of choice for ballast. This is added to tune the C of G as far forward and low as possible. It usually designed into the area just above the legality plank just behined the front wheel centreline. The plank is not wood but 10mm thick fibreglass with a specific density of between 1.35-1.45. Sometimes teams will go as extreme as having the centre section of the front wing machined from solid steel or fit removeable blocks of tungsten inside the front wing.

Years ago before this loophole was closed the plank was fitted with 2 different types of fixing, 1 type of fixing could be flush with the roadside of the plank and this area was limited. The other type of fixing was 2mm underflush and wasn't limited by area, designers designed +5kg fixings just using this loophole to get ballast even lower.

Also before the C of G rule change to engines there would be a huge drive to get the crank centreline as low as possible, some teams were down below 50mm above the bottom of car.

All electical boxes, pumps, hydraulic manifolds, pipe runs, electrical cables are run along the bottom of the car where ever possible as this weight soon adds up.

Even the driver sits as close to the bottom of the car as possible, his seat will have a cutout for his bum so he sits on the floor which is sually less than 6mm above the legality plank.

The vertical C of G position on a modern F1 car is between 200-250mm (8-10" in old money).

So in answer to your question should you lower your seat then yes go as low as you can until you can't see out the front, it will only do good.

Apologies for my rambling and hope this helps,

I'm on it. I don't know why they were raised to begin with, but that is how I bought the car. Very interesting read about the F1 teams and their resourcefulness!

I, being short, like to have the seat up a tad higher than some racing Porsches because I can then see both fenders, which gives me a better idea of where the front wheels are in relation to the curve's apex. This makes me feel more comfortable and therefore faster. So, the small penalty in CG height (luckily I am light :)) is more than offset by the improved "driver factor".

When installing Recaro shells last year, I removed both manual sliders and mounted them as low as possible and as close to centerline as possible.

Shells vs. stock seats reduces 40 pounds minimum.

I believe lower C/G is just as important as lower weight.

My seats could be lighter but they grip me so well and look so cool.:)

RetroMod - Retromod Images - Early*Starter

Don’t even get me started on Factory Recaro Sport Seats from the late ‘60s and ‘70s.

I was seriously offended the first time I removed one of those seats. While very cool, it takes two strong gorillas on steroids to handle one. I forget the actual weight but they take the “Sport” out of the car. I never understood Porsche’s decision to use these.

If you have performance in mind, there are more modern seats. I have an original pair of Recaro Sport Seats setting to watch TV in. My son used to play video games in one; race helmet on, 6-point belt, box fan in his face at age 12. :D

They are fine in a show car depicting originality. For a light-weight, low-CG 911 there are better choices. Don’t throw them away (or sell), find another use.


My ‘daily’ ‘79SC has a pair of original 'high back' leather Sport Seats. I think they are only 1/2 of the weight of the early Racaro version. A good 'sport seat' is half again lighter weight and accommodates better (5/6-Point) seat belts.

Seats are VERY easy to change. The key is having a use or storage for those not in use. With care and practice you can swap a pair of seats in 15 minutes on a Saturday morning.

Best,
Grady

...Unless you have a roll cage to crawl aroun and over.;)

my Recaros are about mid-20s for wt. IIRC right -

while too lazy to look that up, I was so excited by the Cg discussion that I did troll thru some old notes -- here are the rousing tidbits:


Cg is at about level of tail lights in back for a shell w no powertrain (so
can't be higher than that if motor is in car, as it sits lower - esp.
the crank and oil pump)

at front, almost exactly halfway between top of front trunk panel
(where double groove is for rubber seal)
& suspension plate that bolts to car
(with no gas in tank or people sitting in car, so prob. no higher than
that)

--------------------

Bob, author of Pano article on handling in June Pano, said the 20" CG
ht. figure is just a guesstimate, they did not measure a 911 and it
did not come from any factory source (I have never seen any figure for
it in the factory specs either.) He thinks it might be a little lower
in actuality, may be 18".

Center of gravity is at 60% of the car's length

Center of Pressure is at 25-30% of the car's length

- both from
http://www.tfd.chalmers.se/~lelo/rvad/reports/rva2002_gr06_porsche911.
pdf p.6 Side Force section -- no primary citation in this student
paper

Barnard, RH H. 2001. Road Vehicle Aerodynamic Design.

------------------------

On a shell, the balance point is near the B pillar.

Well, if you have a cage there should be suitable light-weight race seats. Equipped for proper support in a crash with the best 6-point seat belts and HANS. :D
Not morning swaps.

Best,
Grady

The car came with lightweight Recaro "race" seats in it. 5-point harness ready, bolsters, etc. But they are lifted off of the mounting rails by four 1 and 1/4 inch aluminum "barrel" spacers. I want the spacers out, but just from an initial "once over" it looks like I am going to have a serious time trying to get the seats mounted back on the rails without the spacers, as I can't pull the seats back far enough to get to the front mounting holes. With the spacers on, I can just barely come from above to get to the front mounting bolts, without them the seat will be absolutely flush with the rail. Anyone know if I can un-attach the underseat spring (that controls forward and back movement) just to pull the seat far back enough to mount it, and then re-attach the spring? If I can, that would work.

Google Brey Kraus - A great source for modified seating systems, including Recaro. Sounds as if you may have a one-of-a-kind seat mounting. Correctly mounted seats are always good. Not to sound preachy, but you may want to think about messing around with that spring. 1.25 inch AL barrel spacers don't sound right, either.

While a low CG is an unquestionably good thing, I'd not rank it too high on the scale if practicality of improvement is a factor in the ranking.

The horizontal location of the CG is easy to determine from corner balancing data.

But I have yet to find data from anyone who has actually measured the vertical CG height. Maybe one of the guys who Randy took notes on did with their stripped tub, but it doesn't sound like it - not really quantified.

It sure would be nice if someone did actually measure CG height. You need it for no end of handling programs. The theory is simple, but rather precise measurement is needed. The details make it hard, as you need to lock the suspension front (and rear - don't want droop if you lift other than by the wheels. I tried once on my corner balance scales, but couldn't lift far, didn't attend to these details, so the result of the equation was garbage.

Maybe someone could measure roll under known cornering Gs, and work backward from suspension specs to get CG height?

As for a mass centroid, well fuggedaboudit. Porsche must know where the CG is (witness the experiment Grady describes), but wherever they recorded it is not easy of access.

In addition, lowering the CG is not easy at all. Not at all like an F1 car, as you don't have a blank design slate upon which to sketch.

Those heavy seats probably have a center of mass at or below the CG already? Sure, you can swap out glass for Lexan (or remove altogehter), delete sunroofs or substitute plastic for the steel roof, and get rid of a rather small amount of stuff (hot and cold air handling) from the upper part of the car. But not much, really. And the roll cage is apt to have a CG that is higher than where the car started, so just staying even may be victory there?

Much easier to remove weight overall, and to move things you need up front and on the right (for the track). With minimum class weights, much of the weight you can save (light hoods, for instance) can end up as ballast low down, which is good. Offset the roll bar CG.

And none of this is practical for a street car, where it isn't going to be noticible or matter anyway.

A track change (I understood what you meant) is simple in comparison, even if it might not matter too much. So we space our wheels out. A lot of bang if you have room to do it for very few bucks.

But I'd go with Grady's experience on tires first, and then overall weight. A list like this assumes that everything adjustable is adjusted to perfection, and we are looking more at design (tires excepted).

Walt

1. Can you post an equation or a procedure?

- there is some math in an old UpFixin' but IIRC, it involves the vertical Cg and tilting the entire car at various angle with the suspension bound.

2. No - I have never seen that.


And, Walt, I am shocked - shocked - that you would say that substituting in a ceramic clutch disc and gun drilling the trans. mainshaft and engine crank does not constitute "practicality." Shocked!

1) The horizontal CG location falls right out of corner balancing data. If you are 40/60 F/R banance, the CG is 60% of the way back from the front axle to the rear axle. Ditto for side to side. Really simple algebra. I think this typically puts it somewhere just behind the seat, and not too far to the driver's side of the centerline.

CG height is a bit more complicated. All the usual books have the formula for CG height. Puhn, for starters. Google for "cg calculator" promptly led me here:

CG Height Calculator.

Too bad the Upfixn author hadn't tried this on a 911, and given a result, though. I keep trying to get this to be a subject for one of our region's tech seminars, but the shop owners assert (probably correctly) that I am the only person ever to bring this up as a suitable subject.

The math part is just some trig and geometry: if tilting the car 30 degrees puts X more weight on the front wheels, and wheelbase is Y, what height of the CG will bring it that much closer (normal to the earth) to the front axles? The result is sensitive to small changes in angle for the angles which are in practice involved (30 degrees is way too steep to be practical - you'd need your scales up on tall stands). Hence the need to remove any suspension variables. And have the driver in the seat, too, all strapped in (have you noticed how sticking an arm out the window can change the corner balance scale readings?).

Here is a sketch someone (Burgermeister?) sent me when I raised this issue a while ago. Illustrates the concept.

http://forums.pelicanparts.com/uploa...1263599123.jpg

You need the horizontal wheel weights and wheelbase, the fronts again only when jacked up (you can calculate what the rears are, since you have not affected total weight), and the angle. Plus front axle height. If front and rear are different axle heights, maybe you want to compensate for that, too? But I think that comes out in the wash, and wheel base is very long relative to axle height differences.

The Robinette website suggests that a 10" rise in the rear was enough to get a good CG height, but my personal experience was not so good. I'd have to find my notes for what a floor jack under the rear of the engine while on scale pads gave me, but plugging the data into one of these calculators gave a result which was not to be believed. Locking the front suspension is not trivial, and I hadn't done that. So maybe 10" is enough if you get everything right. I can't remember which angles the tangent is most sensitive to - large ones, small ones, or those around 45.

Walt

Tan() gets very big away from 0

thx for the info!