DDK - Die Deutschen Klassiker

I am planning to replace the torsion bars all round as mine were a bit manky when they were removed. So, starting from a clean slate, what is the 'best 'combo of front and rear to give ride that would be good for touring as well as spirited driving (occasional track days) but doesnt mean i have to have all my fillings replaced after driving it for an hour.

I realise there are a lot of variables but i will be running
1. 7Rs at the back with 205/60 Avon
2, Deep 6 at the front with 185/70 Avon
3. Standard front and rear anti-roll bars for MY72 S
4. Rebuilt (to standard) koni struts up front and adjustable shocks at the back

Am also wondering about hollow vs solid bars. Lots of discussion on weight saves (6lbs across 2 rear bars) and response rates (some say better, some say no difference) but no real conclusion

Any recommendations for 'upgrade' of the bars or is standard the way to go? The RS was 19/27 i believe but i have seen 22/28 as a recommended combination.

Experience and thoughts welcome.

Andrew

One question to yourself:
What are you going to do with the car?
Nice sunday drives with a loved one, or crazy blasting in anger?

My 73T was built as a hillclimb car and driven to and back from the events.
Thus stock 73T front and 3 litre turbo bars to the rear.
Works a treat, but now it is a sunday driver and a bit harsh in the rear.

Pretty sure the 73 rs was same torsion bar sizes as the S and thus the E & T


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mostly normal driving (we live in the country so twisty potholed surface) but with some track day action. Want something that is firm but not harsh.

What size anti-roll bars are you planning to use?

A 1972 S should have 15mm bars both front and rear.

Standard torsion bars on the front and a 24mm of an SC might be quite good but dampers in terms of bump/rebound setting will influence transient behaviour.

15mm ARB all round - standard S

Standard S spec Koni dampers and struts.

I think 22/28 would give ride quality similar to a Victorian bedstead.

Standard Koni dampers are unusual in modern terms as the adjustment provided only affects the re-bound valving and has no effect on the bump damping and with such high spring rates it could just become very bouncy at low speed.

The increase in 'wheel rate' if you chose 22/28 would be around 80% on the front and 65% on the rear.

sounding increasingly like standard 19/27 is the way to go. Sounds suspiciously like the porsche engineers knew what they were doing......

Any thoughts on hollow vs solid bars?

from memory , original 1973 RS was 19mm / 23mm same as an S

27 isn't standard for a '72 car

26mm is the popular '930' rear torsion bar upgrade

I can't recall the dia of my rear bars except they came out of an early 3 litre turbo.
These went into the car when I installed the heavier 3.2 engine and box, both aluminium alloy.
Maybe the later 3.3 turbos are larger dia still?
Also the car had custom valved dampers all round, non adjustable, all spec'd and supplied by Bob Watson.
After another of his demon set ups the car was pure magic. That was in about 1997, today and 60k miles later all is well except I have Koni Sports on the rear set at 50% of damping range from when the car had a 2.4 drive train.

IMHO I would leave the bars stock and invest in some nice dampers, adjustable, that you might tweek when doing a track day.
Get the chassis well set inc corner weighting and the jobs done.

I would add that you can adjust damper if they have that option such that the car will be quite horrid and jarring.
Getting the right position on the adjusters can be tricky.
I've done all 3 of my race cars on a private day at a local sprint track with someone who knows what they are doing. Certainly paid off.

There is, of course, a huge amount of BS involved in the selection of a hollow torsion bar.

Hollow bars do save small amounts of weight maybe around 4kg for all 4 bars on a like for like basis with the weight saving on the rear bars being a bit greater.

If you uprate the bars the weight savings compared to standard bars reduces.

A small reduction in unsprung weight can help with damper control but in real terms I am not sure you would notice.

One manufacturer of Torsion Bars claims that the hollow bar is more 'reactive' than a solid bar due to the fact it has 'two surfaces'.

A very strange explanation.

The stiffness of a torsion bar is fundamentally controlled by it's Polar Moment of Inertia (PMI). For a given stiffness the PMI of both a solid and hollow bar will be the same.

If we deflect the two bars through the same angle then the energy stored will be identical.

When we remove the deforming force the bar will try to return to its original position and the force available will govern the rate of return.

If the force is identical and the PMI is identical then so will be the resulting acceleration.

The rate at which a steel spring can 'elastically' recover depends only on these criteria as the elastic wave can propagate through steel at it's speed of sound and the damper generally slows things down quite significantly.

The 27mm Torsion Bar from a late specification 3.3 Turbo fitted with a G50 Transmission just won't fit your car. It has the wrong spline count and is the wrong length.

The 25mm bars off the 3.2 Carrera with the G50 suffer the same problems.

The earlier 26mm Turbo Bars will fit as will the 24mm SC bars.

With a Mag engine it will ride quite well with 24mm bar which is a 20% upgrade in wheel rate.