I'm surprised no one has mentioned and factored in the track of the car. If the tire centerlines are wider than the stock ratio or in a different ratio altogether, wouldn't that upset the math you've established?

The wider the centerline track, the more force it puts on the t-bar (has more mechanical advantage) and the more bar you need to compensate. Same with weight.

I would think before establishing a graph, adjustments have to be factored with respect to overall weight, weight bias and suspension layout, especially where the tires sit under the car (or outside of the car, in some cases ).

When looking at this chart again, I have close to "stock balance" which is 20/26 as opposed to my 19/26.

So what does "stock balance" mean?

that's pretty much the camp I'm in. And, i don't think you can simply go with the popular idea that stiffer is better. (for suspension)

Ultimately, you would have different bars for different tracks/surfaces. Tht is If you have a really smooth track surface, a kart suspension will do (read:next to none -- however much deflection you get out of the tires and frame flex)

Okay, so really Ultimately you would have constantly changing sping rate (active suspension)

But in general, bigger bars, on an already lighter car, are not going to be an upgrade for street use. Though, I suppose that we do now have ipods and kidneybelts . . . .so . ... :cool:

edit: kinda Capt.Obvious stuff . ..sorry for the ramble.

Island, don't you mean "Dr. Obvious"? After all, you didn't go to 4 years of medical school to be called "Captain". ;)

medical school !?

n'nah . .I've got a phd in "Evil." . .. .tho' very similar. ;)

By "stock balance", I mean if everything else is identical with the car and driver except the torsion bars, the car should have the same propensity to oversteer or understeer as the stock car. The suspension would be stiffer though. Of course this is a pretty simplistic approach that's not considering how the dampers, chassis, tires, driver, etc will react to the different bars.

My head hurts...again.

Alright, I have a bead on 28mm for the rear. Anyone have a set of 22mm for the front? Six milimeters separation from OEM is carried over to the upgraded bars - so given all I have are sport shocks all round (a factory option), I should be at stock balance, just more firm.

In short, let's get this over with!

Regarding the shocks tuned for diff bars . If you increase the bar size you can or are maintaining the ratio of spring rates F/R but the stock shock tuning (sport or otherwise) is matched to the original spring rate. When Bilstein custom valves for stiffer bars they increase the rebound damping more than the compression as the stiffer spring(bar) provides it,s own resistance to movement on the compression side. I expect the acceptable limit of increase for stock shocks is probably 20/21F and 27/28R. Any stiffer will benefit from revised valving.

As noted, so many variables, and maybe I missed it, but wheel/tire combinations should be thrown into the mix. The important thing to remember is that a suspension is a system, one in which components work with each other, not against, and only as good as its weakest link, so shocks, bushings, swaybars, torsion bars, tie-rods, ball joints, and tires all need to be evaluated within the context of what your end goal is. Done right, a suspension is more than just a sum of its components.

When doing a complete R&R on my targa, I wanted something more toward the DE end and sacrificing some comfort was OK. Maybe I've got extra-solid kidneys or the flexi-flyer nature of a targa with over 240K miles on it absorbs road bumps, but somehow I achieved both with the below combo. Car is also on a serious diet with little interior, Momo fixedbacks (save 20 lbs each) and no AC. I can drive from Boston to NY and not be tired getting out of the car and going to dinner or I can hit Lime Rock with confidence.

For what it's worth, my 84 has:
22/29s
Elephant polybronze all around
turbo tie-rods
Bilstein sports
stock sway bars w/ new bushings/droplinks
7x9x17 996 wheels on spacers
front shocktower strut

having driven the car for over 25K miles on this combo with several different wheel/tire combos from SO3s on 7x8x16s to Victoracers, Pilot SX's and SO3's on 17s, I wouldn't change a thing.

now if only the trans rebuild this weekend goes well, I can actually drive it again. :)

Unless you have adjustable sway bars, I don't think you'll like 22/28. It will most likely understeer more than a stock car. JMHO, but I don't see a 28mm bar working very well unless you could find a 21.5 mm front bar.

It seems like most people on this thread that are really happy with their set up have bars that maintain the stock balance or bars that slightly reduce the understeer. Yes tires, shocks and everything else affect the balance, but doesn't it make since to start with bars that give a good balance and work from there.

There are a few folks here that like the car even stiffer in the rear, but I don't see anyone going to a balance that makes the car understeer more.

One thing not mentioned so far (that I saw) are the cars that have a 3.6 (or larger than stock) engine transplanted in them.

Would believe that one more mm in the rear bar might help with the added weight from a heavier engine.

Comments?

Joe A

When I went to 30mm rear bars with Bilstein sport shocks it felt like the shocks were too light on rebound control. Noticeable in 4th and 5th gear sweepers when you hit uneven pavement or made large steering inputs the car would abruptly lift the inside suspension.This loaded the outside causing car to lose it's set and rock slightly left to right.

Imho revalved shocks and adjustable ARBs would make your set up sweeter.:)

Agree to many variables, car's, track and tires....

Joe--I've got a 3.6 in my SC with 22fr/30r torsions with adjustable sways set at loosest setting (in back) and revalved Bilsteins. It rides really nice on the street-not too rough, just tight and on the track feels marvelous.

BTW, I want to add that the 9x17s in the rear with 255s work much better than the 8x16s with 225s. Much more planted, and not sure if it's all about the extra rubber.

Given that, a 28 *may* work better with a 225 rear.

That, too. As stated, so many variables.

The 6 mm difference is a casual observation. However, I bet 5 or 7mm difference really throws the equation. So, even if you deduce this to pure science, it probably comes back to a certain range, and for the novice (me), 6 mm is probably a good enough rule of thumb.

this is a very good post, IMO. Worth reading.

If you have to have a simple rule of thumb I'll suggest it be; stay with what the Porsche engineers decided.

Well, since that seems directed at me, which engineers?

The ones that determined the design of the street machine, or the ones that hung the big meats out of the side of the 934?

I believe there were a lot of good posts here. 125's mathematical analysis showing what I believe is a progressive ratio (or, would that be a linear ratio?) is, indeed, a very good thought. At least I understand what he is saying, that a 6mm spread doesn't work exactly the same as you move up the scale.

By "rule of thumb," I was only suggesting that things fall back into a rough formula no matter how much you analyze the situation. It's kind like agreeing that bigger tires don't work all that well on the front. (I know, someone has done that, too. Some kind of extreme problem at a autox or something ;))

touché. :D

What he is saying is that it is VERY non-linear . . .making a rule of thumb not work here, as many may expect.

Now where can I find a steroe with volume that goes to eleven? ;)

Island, the only problem here is when we do not "stay with what the Porsche engineers decided" and throw a 3.6 in the back or turbo a car that was N/A from the factory. That mucks up the 6mm theory in some cases. Course we Americans are rebels at heart so this is nothing new!

Very good info as I am going to start upgrading my cars suspension this fall and was wondering what T-Bars to use.

Jon, Thanks for the input and I was leaning towards the 30mm bars in the rear.

My additional question is that my car is backdated, so has lost the weight of a "impact" bumper and reverted to a RS fiberglass rear bumper, losing 100-150 pounds in the process.

Believe that I will lean towards the larger (30mm) bar as I do not mind a rough ride.

Joe A

For sure. I'm just saying in refnc to dd's light 911. (he'll be fine with stock. (as a rule of thumb ;)