In your mind what's the difference?

there's an upward force on the chassis all the time from the shock and from the t-bar, reduced. these 2 forces are * ~.9 and summed to get the actual wheel rate less stiction and tire spring rate


no one is arguing for high spring or damping rates, of course the worse the roads, the less spring is desired and of course the damping curves need to match the spring rates and ride frequency

and of course a race car is different because of the higher loads and generally smoother surface conditions

no body here is advocating 23/30 t-bars for a street car

that's why I include a range of possible spring choices which need to be complemented by suitable wheel and tire choices to get a car that handles well for it's intended environment

much of the data in the table is from my car , weight, weight distribution unsprung weights, in use these are complemented by 8 & 9.5 wheels 225/255 tires w/ just under 300hp for use on the relatively smooth found where I live. The package works well for me, Your needs may be very different. I do get to drive many differen,t mostly trackcars, and have found the sweet spot for what I drive where I drive it.

The ride freq on mine is ~2.1/2.3, my 993 track car w/ some halfway decent aero is ~3.3/2.9, this is great for WGi , LRP etc and is ok on smooth local roads but the 911 is a much more street friendly ride. If the roads were bad I'm sure that less spring and taller side walls would be preferable

Tunable shocks are great, unfortunately most of us run what we bought, I've considered buying MCS for mine but the cost benefit just isn't there for my usage

I have used tunable shocks on my 993 as do most of the track guys I run w/. They are wonderful when dialed in but there is a learning curve as to settings and pressures to run for variable conditions. I've spent days helping Cup guys set theirs up.

similarly adjustable sways are great tuning tools but if the basic spring rates and wheel/tire selections are close you don't need to tune much if at all. I know that
I rarely if ever touch mine

My biggest concerns when it comes to toriosn bar 911's is that they are sprung way to stiff in the front, and way too soft in the rear. For years people have been adding a couple of mm of torsion bar to both ends (ie, 21mm front, 27mm rear) and making the problem much, much, much worse.

It's a matter of front/rear balance, more than stiffness, but if one was to look at the incorrect info that Bill's posted, they'd think that if anything, the rear is too stiff and the front too soft. For anyone doubting this, walk out to the garage, open both lids of your 911, and press down on the latch panel on both ends. Tell me which end moves more, then tell me which end needs to be stiffened or softened with respect to the other.

My other two main concerns are lack of suspension travel, most people are riding around on the bumpstops (in the front, especially), and finally, the horrible bump steer curves that the suspension design gives us need to be addressed.

Again, most people aren't actually checking suspension travel (or lack thereof), or measuring bump steer and correcting it.

Why do I bother with this? I should maybe just mind my own business and keep my knowledge to myself, but I like giving back to this community, and I'm telling anyone that will listen:

The vast majority of these cars (stock and modified) could be VASTLY improved with some simple tweaks. Following conventional wisdom and setup advice wont get you there.

Well this thread got hijacked from the OP. All the math above just gives you a baseline. Real world for that car and driver you can adjust for, just takes time and money. Shock options for these cars, mines a SWB, are very limited.

Clean the spindles and put them on. Please report back, brake setup and all. If you knew what I paid for my 3.5 spread struts you would be happy. I thought about a GT3 front and sorta wished I had. No raised spindles but custom front shocks. I have Q's about BV's A1/A2 and CG versus RC.

I'm excited this thread got hijacked! It's rare that us kids get to stay in the living room when the adults are talking ;)

And challenging the status quo is what it's all about. When rebuilding my suspension years ago, I used a "sport" recipe from Elephant's website that recommended 21/27 Tbars. What I found was a lot of understeer that I've been trying to dial out with ride height (rake) and sway bar adjustments. Had I had the math presented in this discussion, I would have chosen different Tbars (and yes, I know Bill has posted these figures before). While these discussions get spicy from time to time, this type of discourse is great for folks watching from the sidelines.

Thanks, guys. I'll go to my room now.

OP has an 80 930. Someone can chime in but at least 21/27's. My guess is 28 rear's but I'd like to know as that's what I have and haven't pulled it apart yet to measure.

Shoo, start adjusting your drop links if you haven't already. I ended up putting a lighter front sway on. Mine handles about as flat as you can get.

Back to the hijack--it's too much fun and we've gone too far to abandon it now.

Bill you asked me: "In your mind what's the difference? there's an upward force on the chassis all the time from the shock and from the t-bar, reduced. these 2 forces are * ~.9 and summed to get the actual wheel rate less stiction and tire spring rate."

Yes. And the combined spring rate of the shock and a nominal t-bar is: 1 lb/in + 120lb/in = 121lb/in (I'm leaving out the 0.9 correction for the geometry for simplicity), as Jonny stated; NOT 42 + 120 = 162. Remember, we are talking about spring rate, not spring force. Jonny measured the spring rate of the shock alone, and determined it was less than 1lb/in. Jonny thinks of it as 42# of preload. Practically speaking, if you changed from a front shock without internal gas pressure to a Bilstein, it would raise the front of the car about 1/3". You would then correct the height by unscrewing the T-bar adjuster about one turn.

As for adding the spring rate of the tires, that is not simple addition because the tire spring rate is in series with the suspension spring rate, not in parallel. The algebra for the combined "k" for springs in series is the same as for electrical resistances in parallel, and is always lower than the k for the component springs separately.

However, tires compress very little compared to the suspension, so the effect of the tires on the overall spring rate can be ignored in most cases (maybe not for off-road vehicles that run a few PSI pressure in big balloon tires).

Jonny, I share your concerns about the differences in wheel rates at the front and rear of our cars. Furthermore, given the car's roughly 40%/60% weight distribution, the wheel rates should also be proportionately higher for the rear than the front, so that the suspension frequencies are about the same at the front and rear. There's gotta be a reason that Porsche engineers chose to make the rear suspension significantly softer than the front. I've questioned it for years, but haven't come up with the answer yet. I suspect it was part of the tradeoff to make the handling reasonably predictable without too much oversteer under a variety of circumstances; given that the 911 has the mass of the engine in its ass.

FYI, I put 21mm bars in my front because I was following Bruce Anderson's recommended 21/27 bar combination for street cars with occasional track/auto-X use. However, I put 26mm (Turbo) bars in the rear because that's what I had at the time. I have a set of 27mm bars that I plan to install this winter. Even with the 26mm rear bars, I went up 3mm in the rear, compared to 2mm in the front, and allowing for the geometry, I did stiffen the rear more than the front.

As currently set up, I can feel that the front suspension responds faster/stiffer than the rear when I go over a bump. I'll see how it feels with 21/27. If I still feel like the front is too much stiffer than the rear, I can go back to the 18.8mm bars, or maybe try 20mm bars.

I'm not so sure the majority of these early cars can be "VASTLY improved with some simple tweaks." I contend that the OE design and balance was the result of multiple tradeoffs, thus, changing some of those characteristics will adversely affect other characteristics.

Hey Tony, out of curiosity, what T bars would you run if you had it to do over again? I have a stock ‘82 SC with an original creaky suspension I’m about to overhaul this winter. I have sprung for a a KW V3 setup based on forum feedback, adjustable F/R sway bars, and a few Elephant rubber bushings to keep my ride descent, but I could never seem to get clear T bar guidance due to the many variables involved. My old SC is only sport street driven, maybe a DE event on occasion. I will probably get the Rebel bump steer kit as well.

I was kinda set on the old 21/27 guidance, but I held back for some reason. I’m also running larger 7” F and 8” R Fuchs with Michelin PS’s, which transformed the ride and and handling even on my worn out suspension…

Thanks, I really enjoy the expertise that the forum members openly share. I’m currently thinking about turbo rear 26mm bars and keeping the front stock 18mm bars…

That statement is probably a bit hyperbolic, yes. But it's my humble opinion, and I'm going to stand by it, simply because the majority of these cars as they've passed through several owners are:

- Riding on the bumpstops/lack of suspension travel. Our eyes have gotten used to a "euro" ride height that is probably an inch or more lower than actual euro ride height).

- Too stiff in the front, too soft in the rear. They come this way from the factory, and the de facto recommendations from Moreland, Anderson, et al make the imbalance problem worse. If you're on or near the bumpstops (see point one) the spring rate is spiking towards infinity as you load the suspension.

- Too much bump steer. The curves were crappy from the factory, but a byproduct of the basic suspension design and some tradeoffs. Lowering the car (see first point) moves you from the crappy part of the bumpsteer curve to an even worse part of the curve. This applies equally to the rear end!

The majority of torsion bar 911s have all three of these problems. As you state, fixing one without the others can exacerbate the remaining isses. Luckily they're still generally great to drive even as they are.

More later, I have carbon bits to make! But I'll leave with this which I think explains part of why we've ended up here:

I'm good with the hijack. It's all pretty relevant information.

To close out the original point.... I sanded down the spindles. I removed all the coating and it seem a little more. I replaced the wheel bearings so I used the old wheel bearings to test as I went along with the sanding. Once the old bearings could be slid onto the spindle with medium force, I stopped. The hub with the new bearings went on fine and I'm in the process of wrapping it all up now.

I'm going to be running 30MM torsion bars in the rear and 21mm in the front. I have a 22mm front sway bar and 20mm rear.

I'm in Germany so it is a Euro ride height 930 (911 turbo).

Based on Bill's data, the 21/27 Tbar setup on a SC/Carrera would lean towards understeer. This is potentially compounded by lack of suspension travel on a significantly lowered car (with stock spindle height). I'm vacillating between leaving the bars alone and installing the KW spindles or switching to 20mm bars.

This is a different set of data than the first set posted, and actually makes a lot more sense, especially from a ride frequency standpoint. Still high on the wheel rates but at least the front/rear balance is close enough to allow comparison, and shows a bit more plainly how much softer the rear is compared to the front.

I'd stick with the stock bars in front (or replace w/19's if you are worried at all about grooving/wear) and go 27 in rear, maybe even 28 if your car is heavier. Raise spindles, ensure you have travel (cut bump rubbers if necessary), fix bump steer.

I have 2 data sets for t-bar rates
Steve Timmons

http://forums.pelicanparts.com/uploa...1732563387.jpg


from Will Ferch
http://forums.pelicanparts.com/uploa...1732563126.jpg

here is Will's methodology
Torsion bars
By Wil Ferch – 1/22/2000
Listers:
Every once in a while, questions come up on torsion bar sizes , and of
the suitability of certain size bars. I've compiled the following information
that can be used for all pre-G50 911's.
Torsion bar spring rates can be determined by the formula:
K= 1,178,000 ( d)*4 / (L) (A)*2
-d is to the fourth power, and A is to the second power.
-d is diameter of torsion bar
-L is torsion bar length
-A is the lever arm length
A fellow Rennlister, Joe Winn, measured torsion bars he had for sale,
and the following is for the FRONT bar:
-length of 611 mm ( or ~24" )
-spline length at each end of about 1".....this makes for an
"effective" length of 22" , assuming full length spline engagement as
installed. Also, I'm using a lever arm measurement of 12"...the distance from
the torsion bar centerline, to the brake caliper centerline. With this , I
come up with:
SIZE ( mm) SPRING RATE ( Lb/in)
18.8 110
21 173
22 210
23 250
24 296
25 350
Using the same principle for the REAR bars, we have:
- bar length of 626 mm ( 24.65 ")
- spline length at one end of 19 mm, or 0.75"
- spline length at other end of 26 mm, or 1"
- "effective" bar length of 22.9", assuming full spline engagement
- lever arm of 18.5"...centerline of torsion bar to centerline of
rear wheel.
With these, I get:
SIZE (mm) SPRING RATE ( lb/in)
23 100
24 120
25 140
26 165
27 191
28 221
29 254
30 294
31 332
32 377

I have no idea which is more accurate\

I'm a big KW V3 fan for these cars. I like the black bodies from a stealthiness perspective. Don't love the step of having to remove the coating from the spindles tho.

My 2,678 pound '87 M491 is running 19mm raised spindle KW V3s, stock torsion bars (18.8mm F & 26mm R), Eibach Anti Roll Kit sway bars F&R (on their softest settings), refreshed F & R rubber bushings, turbo tie-rods with ERP spherical ends with custom machined bump steer spacers, 16 X 7 & 9 Fuchs Evolution wheels shod with 205/55 & 245/45 Yokohama A-008P tires, all of which was corner balanced, bump steered and aligned by Jae Lee / Mirage Intl.

The car feels totally dialed for my use case: Mildly Wife Unfriendly touring car / road rallying canyon carver. The car was SUBLIME over 600+ miles of Texas Hill Country Rallye use (350 FAST miles on Friday and 250 mellower miles on Saturday), mostly on chip seal "paved" roads. The front end bottomed out a handful of times over high speed undulations, so perhaps I could have increased compression a click or two...

http://forums.pelicanparts.com/uploa...1732841163.jpg

http://forums.pelicanparts.com/uploa...1732842179.jpg

http://forums.pelicanparts.com/uploa...1732842179.jpg


I couldn't be happier with the above setup, but I'm sharpening things up even further for my '89 M491 hot rod build and I'd appreciate feedback here on TB selection...

http://forums.pelicanparts.com/uploa...1732842662.jpg

~2,4XX pound '89 M491 running...
19mm raised spindle KW V3s,
Rebel S Racing (RSR) Spherical Bushings F&R,
Quaife Quick Rack steering rack,
RSR Steering Coupler (solid aluminum),
22mm effective rate Adjustable Hollow Sway Bars (thru-body front),
Turbo tie-rods with RSR Adjustable Bump Steer Kit,
16 X 7 & 9 Fuchs Evolution wheels shod with 205/55 & 245/45 Yokohama A052 tires.

Intended use is aggressive road rallying / canyon carving. I am leaning towards 21mm F & 28mm rear bars. Keeping the weight and intended use case in mind, and that this is a wide body application... What are y'all's thoughts on 21 & 28 torsion bars?

Hi Frank!

Gosh those are a couple of beauties!

Your Gemini baseline car sounds like it's pretty dialed! Vents is lighter by a few hundred pounds, but is a good share of that weight loss from the front?

Your baseline setup would feel a little stiffer already in vents, due to the lighter overall weight (and a slightly more hard-core bushing setup). If you want to sharpen things up that extra bit you're on the right track, but your stock 18.8mm/26mm Tb's have wheel weights of 120lbs/in and 156lbs/in, respectively. Those lbs/in wheel rates are based on actual specs from a torsion bar manufacturer.

Going to 21/28 would be 202/210 ft/lbs and a percentage increase of 68% front and 35% rear, which is what I've been on about! Doesn't make sense in the least.

Depending on the weight balance, I could see leaving the stock front and going 27mm or 28mm in rear or 20mm front, 28mm rear, maybe even going 29 rear if you find 28mm too soft and the suspension isn't controlling the weight of the car.

If the front has good suspension travel and has the bump steer corrected, there's no real reason to stiffen it, but there are good reasons to stiffen that rear up a bit and keep it from moving too much. This is the only real way to keep the bump steer at the back to a minimum, especially on a car as low as yours. I think stiffening the rear of Gemini with 27mm might even take it from sublime to.... whatever is better than that LOL.

I meant to mention for the OP's benefit, and you might find it a useful check, the KW V3's have an external bump stop and it's really easy to pull it down the shock shaft by hand to see how much free travel you have on either end (actually worth checking all four corners!). If the bump stop/dust boot only moves a 1/2" or so when you pull down on it, you might consider raising the ride height slightly. It's surprising how little travel you may have and also surprising how small increases can make a big difference in ride quality and handling.

PS - You're going to love the quick steering rack. I put one in Project Heavy Metal and have one waiting to go in the Rot Rod.

Yes, I'm quite smitten with them. Also playing with "Apollo," a 75k mile stock-ish '87 M491 right now that has Bilstein HDs all around and new rear spring plate bushings...

[Apollo and Gemini in their Luft 10 liveries]
http://forums.pelicanparts.com/uploa...1732884992.jpg

I'm trying NOT to install KWs on Apollo, due to the cost. Their current $4k price tag is tough as I paid less than half that for a set in 2020 and half that for a set in 2021.

Anyway, thanks for chiming in here. I was looking for some divergent thinking / devil's advocacy / intellectual gymnastics on this topic and this thread has not disappointed. I've been rereading the Torsion Bar Help thread quite a bit and posted my same question there last night.

Regarding Vents's weight distribution, the front will be a bit lighter than Gemini's due to: Euro crush tubes, AC delete, smaller 24 Ah Antigravity battery, your CF protection plate, 100L fuel tank (so no spare tire or jack, smaller tool bag) and fresh air blower delete. The rest of the car has been lightened via Pole Position Carbons, sunroof delete, rear seat delete, AC delete, heat delete, radio delete, titanium muffler, etc. Final weight and F&R balance remain TBD.

I'm not afraid of stiff, per se (especially since I have two other, stock TB cars that the wife will ride in), but given public road conditions around here, I'd like to start with more softly sprung and go heavier in the future as needed. So, perhaps stock front w/ 27 mm rear is the better baseline to start from.

I sold my ~1,785 pound track spec Lotus Elise of eighteen years back in Feb and I miss the steering. So, I'm very much looking forward to the Quaife Quick Rack in Vents.

ok, I've come around to you way of thinking sort of.

the front spring force at rest is the same as the corner weight so w/ say 420# corner weight the 42# of Bilstein force adds !0% to the spring rate, this decreases in % as the spring is further compressed

I agree tire spring rate is a tough one to quantify but it does make a difference, the avg of the 15-17 " tire universe that applies to a 911 is 3-4% compression when moving

just think how a 185/70 x15 rides compared to a 205/50x15, the taller sidewalls flex a lot more and also are less able to limit slip angles

similarly bushes make a difference

after looking more closely at the 2 data sets, the differences are ~.75 front and ~.9 rear , these #s correspond to the motion ration f/r so it appears that, Will F included the motion ratio and Steve T. did not.

They agree quite well when that is factored in

When I got got the wheels rates from Sway Away I checked them against Wil Ferch's calcs and they were sufficiently close to give me some assurance that they weren't miles off!