Suspension balance question

[Reiver]

I just re bushed the front end (A arms, sways b's, ball joint) of my '83 911 SC with stock rubber...during that process I checked my T bars and they were rusty/slightly pitted so replaced the stock 19.8 mm with 21 mm solid sway a ways (economical) as I couldn't find any stock T bars for sale that were not in the same used condition.
I also have a later Carrera 22 mm sway bar up front.
In the rear I have the stock T bars @ 24.1 mm with a Carrera 21 mm sway bar.
The spring plates are adjustable with new poly bushings.
Boges up front/ Bilstein HD's rear.

Are the 21/24 T bars an ok mix or am I too soft in the rear with this combo?

Spirited street...not a track car.

[Trackrash]

I think only you can answer your question. I think you should be fine with a street driven car.
However, if you notice the front end starting to slide during spirited driving, you may want to consider larger rear torsions to match the front.

[Flieger]

You will probably be fine with it until you drive something with proper torsion bars in it. Then you will realize the rear was bouncing and causing the car to pitch a lot over bumps. When you make the rear stiffer it gives a more level ride and more comfort.

I had 22/27, went to 22/29 saw an improvement. Then went to 22/31 and it was even better. Was going to do 22/32 but the 31 hollows I got for a good deal.

And yes it will push a lot. You will feel the steering be heavier in the turns.

[jpnovak]

that combination, as Max mentioned, is not balanced given your current weight distribution. The car will want to understeer more than a normal 911. Until you have larger torsion bars in the rear you could remove the front sway bar. This will soften the roll stiffness on the front of the car and allow a bit more balance - though not ideal.

For most street driving you will not notice the under/oversteer differences.

Once you are ready for the rears go with a 27 or 28. I personally would prefer the 21/28 combo but this also depends on tire setup, car setup and personal preference.

[patkeefe]

Please see post #7 in this link:
http://forums.pelicanparts.com/porsche-911-technical-forum/336374-22-27mm-torsion-bar-set-up.html

[Reiver]

Yup, good info and the T bar scale is excellent...looks like some larger diam. rears are in my future.

[Reiver]

Another question for you more experienced suspension tuners.
27's or 28's?
I've given lots of info but this...the car is lightened a bit and weighs 2480 w/full fuel tank. Some removed from the rear (all AC equipment/backdated heat) and cabin, all sound deadening, rear shelf/seats/RS door cards/perlon/ light weight cloth Recaro's....but you can only do so much with the lump out back so the front is lighter too (no spare/tools/ac crap/no water reservoir...but I did add a Carrerra oil cooler unit.
So...21's up front and 27/8's and why you'd suggest one over the other?

[winders]

I'd go 30mm and get some custom valves Bilsteins to match.

[Driven97]

Here's my chart.



Ride frequencies are based on my ~2650lb Carrera, but should be "close enough."

The big takeaway from both charts is that a seemingly small change in front torsion (OE 18.5 -> 21mm) raises the wheel rate from 139 ->231lb/in, 166% as stiff. That's a lot! So you should balance that out with a raise in rear spring rate for sure.

I'm on the same page as Winders in that we both think the cars are too front stiff from the factory, and have the opinion the cars are better when you raise the rear spring rate a higher % than you raise the front rate. I like 30mm rears with the 21mm fronts too. But again that's definitely an IMHO situation.

For street use only it likely doesn't matter much, but you're probably also underdamped up front. The HDs should be ok with a modest rate increase out back.

[Reiver]

Thanks Matt/Winders good food for thought...still thinking

[jpnovak]

Matt has the calculations. Think about this. The front changed 166% or 1.66X increased spring rate. To match the rear you need to multiply your 24mm spring rate by the same amount. If you do the math you will see that this falls right between a 27mm and 28mm. So you can tune the balance by going to smaller rear (27mm) for more understeer or 28mm (or larger) for more rear bias. Its not quite oversteer yet.

Normally I would use corner balance numbers along with wheel weight to find a spring deflection distance and tune the F/R balance accordingly. This allows you to compensate for changes in body (fiberglass bumpers/tail) compared to stock spring rates.

This is just a part of proper setup. Spend the time now and you will be rewarded in driving experience after you get a good alignment and Corner balance.

Set the car up right the first time and you will really be able to take advantage of the benefits of a 911 rear engine balance.

[Reiver]

jp....my thoughts once given the charts/info....not sure if I want to go with 27-understeer of the 28 etc....a fun deal...quick change rear plates would make this a fun proposition....not so much without tho.

[Bill Verburg]

I don't think you want to use % when you compare the effects of different t-bars, It's better to use the absolute difference in wheel rates in lb/in to compare the effects

[Driven97]

Should be ride frequency.

Frequency = 1/2π × √(wheel rate / sprung weight)

The 40/60% weight distribution of the 911 makes it so you can't go up linearly or you go up to much in front compared to the rear.

If you want similar balance to factory, the ratio of front to rear ride frequency should be the same. Of course there's a lot of other stuff that comes into play once it's all said and done, but it's a good frame to start from.

These cars have been around long enough that there's plenty of recommended setups. Pick one.

Or you can be stupid like me. I'm trying 23/36 next season (from 23/33) which puts me to square frequencies. All other things equal should be a perfectly balanced car once set, and help kill the understeer I had last season. In reality I may overshoot and have to play with sway bars. Playing / experimenting is most of the fun.

[Reiver]

Very interesting input from different angles...I wonder what 'system' Porsche used internally to decide upon the oem T Bars?

[winders]

Originally Posted by Reiver

Very interesting input from different angles...I wonder what 'system' Porsche used internally to decide upon the oem T Bars?

What setup won't get us sued......

[Reiver]

Originally Posted by winders

What setup won't get us sued......

I think that a bit cynical...they wanted a balance certainly between ride/sport but I think they did a fine job finding a sweet spot. When you re bush/ rebuild all the old bits it handles nicely....not on a track but then again that's not it's primary role aye.

[winders]

Not cynical at all. They build (too much) understeer into all of their cars because of law suits. Even the new Cayman GT4 has too much understeer stock.

[Reiver]

Ah, never heard that before.

[javadog]

Originally Posted by winders

Not cynical at all. They build (too much) understeer into all of their cars because of law suits. Even the new Cayman GT4 has too much understeer stock.

Don't forget they test the cars at much higher speeds, too. What understeers at 40 mph might oversteer at 140 mph.

JR