Formula to convert coil rates to torsion rates ?

[slow car]

Hi,
Has anyone found a conversion chart/algorithm that takes into account early 911 suspension geometry and can convert coil spring lb/inch to torsion bar sizes solid or hollow sizing ?

ie 300 lb front 500lb rear coils = 23mm front 29mm rear torsion bars????ect.

Would love to see it.

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[rowingone]

The coil spring manufacturers would have that info . . . maybe send a few emails or make a few phone calls.

[rowingone]

or this;

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
33 427

[Bill Verburg]

Originally Posted by slow car

Hi,
Has anyone found a conversion chart/algorithm that takes into account early 911 suspension geometry and can convert coil spring lb/inch to torsion bar sizes solid or hollow sizing ?

ie 300 lb front 500lb rear coils = 23mm front 29mm rear torsion bars????ect.

Would love to see it.

The best analysis that I've seen was done by Dr. Steve Timons, He is the guy behind Instant G


https://instant-g.com/OldRoot/Data/911CoilConv.html

note that coil overs operate directly on the wheel hub assembly, while t-bars operate through a lever arm, there is also the extra complication of the different motion ratios of the wheel and coil though this last is relatively small. The figures for this motion ratio the I;ve seen vary but are ~.9 +/- for the front

from the data here it appears that a 20mm t-bar is roughly equivalent to a 207#/in coil,

from comparing the above to known 964/993 springs it's in the same ball park

just for example a typical 993 track front coil is ~400#/in which is roughly a 23mm front tbar @ 355#/in

[stownsen914]

Bill's numbers seem to agree what I heard years ago from a racer who told me that a 21 mm front torsion in a 911 = a 300 lb/in spring.

[slow car]

Wow.... that is a vast contrast to the chart above.
has anyone swapped T/B for C/O and maintained the same ride height and has had real life results to back up these charts?
Thanks very much for the replies

[Bill Verburg]

Originally Posted by slow car

Wow.... that is a vast contrast to the chart above.
has anyone swapped T/B for C/O and maintained the same ride height and has had real life results to back up these charts?
Thanks very much for the replies

Will Ferch calculated rates, he used the correct formula per Millikin but I am unsure of his methodology , I haven't been able to replicate his results

Dr Timmons teaches Vehicle Dynamics at U Del and runs a small but very competent race shop, I trust his results more

I ran your #s and compared them to my track only 993
your front using Dr Timmons data is roughly comparable to my 993 front in net results the back would want ~100#s net more, 34mm rear would get you most of the way there

more than that and the rear ride frequency gets bigger than the front, generally a poor idea

you don't really mention wheels, tires or sways they all affect the net result

[3rd_gear_Ted]

Originally Posted by slow car

Wow.... that is a vast contrast to the chart above.
has anyone swapped T/B for C/O and maintained the same ride height and has had real life results to back up these charts?
Thanks very much for the replies

Dr Timmons headed up a group of engineering students and had a dynamic torsion bar test rig set up to get the data presented in the chart.
Just plain hard to do.

Students from Cal Poly came to a CFOS event @ Fontana with him to relate to the G-body 911 track cars in the real world.
That was a great day at the track.

[slow car]

Originally Posted by Bill Verburg

Will Ferch calculated rates, he used the correct formula per Millikin but I am unsure of his methodology , I haven't been able to replicate his results

Dr Timmons teaches Vehicle Dynamics at U Del and runs a small but very competent race shop, I trust his results more

I ran your #s and compared them to my track only 993
your front using Dr Timmons data is roughly comparable to my 993 front in net results the back would want ~100#s net more, 34mm rear would get you most of the way there

more than that and the rear ride frequency gets bigger than the front, generally a poor idea

you don't really mention wheels, tires or sways they all affect the net result

Hi Bill ,the sways are adjustable 22mm Tires are 225x 50x15 front soft 255x 40 x17 rear medium.
thanks for the input everyone.

[Bill Verburg]

Originally Posted by slow car

Hi Bill ,the sways are adjustable 22mm Tires are 225x 50x15 front soft 255x 40 x17 rear medium.
thanks for the input everyone.

Those sways are very effective and very tunable

15 front 17 rear?

You are going to be limited by tire grip

w/ a 930 fendered car I would expect 9 & 11 w/ 245 and 275 tires at least

if all 15 or all 17 something like 245/40 & 275/35 R7s

if all 18 something like 245/35 & 285/30