TB ratings in FT/LBS?

[pantera]

Why is it Torsion Bar information provided only tells you the diameter and not the spring rating in ft/lbs? If you have a coilover setup the MFG's provide coil diameter and more importantly the rating of the spring in ft/lbs.I understand the TB to be a rod and well, a coil is a coil. Each one still has to be measured in value in its compression or bending. Other than diameter what is the difference between a 22 vs a 23 TB in spring pressure?

Thanks, Dan

Edit: I also posted over in Autocross/Roadracing

[2.7RACER]

from an article by Wil Ferch,

torsion bar size ( mm) spring rate ( lb/in)
23 100
24 120
24.1 122
25 140
26 165
27 191
28 221
29 254
30 291
31 332
32 377
33 427

Thanks Wil,

[Jack Olsen]

One important correction: the equivalent rates are not the same when you're talking about front versus rear torsion bars:

Front Torsion Bars

18.8 mm = 110 lb/in
21 = 173
22 = 210
23 = 250

Rear Torsion Bars

23 = 100 lb/in
24 = 120
25 = 140
26 = 165
27 = 191
28 = 221
29 = 254
30 = 294
31 = 332
32 = 377
33 = 427

All the information is at this link.

[pantera]

Amazing Doug, one post and its reveiled. I asked many, no one knew.

Thanks for enlightening me Doug, Wil and Jack!


Dan

[2.7RACER]

Thanks to Wil Ferch and Jack Olson. Each man is a fountain of knowledge.
I'm still learning from these guys.

[ttweed]

Originally posted by pantera
Why is it Torsion Bar information provided only tells you the diameter and not the spring rating in ft/lbs?

I think the problem with this is that the T-bar can be integrated into different kinds of suspension design and construction in different ways.

For instance, the 911 rear T-bar is combined with a certain length of spring plate which gives a unique "lever-arm" value which has to be used in the calculation of "effective" spring rate at the wheel. The T-bars in the 924/944/968 are used in a different rear suspension design, and will yield a different effective rate for the same diameter of T-bar. There is a chart on the Paragon Products website that shows the values for each T-bar diameter in that application and they are slightly different from the values for the 911 application in Wil's chart above.

Dia. / Rate

18.5 / 48
19 / 54
20 / 66
21 / 80
22 / 97
23 / 116
23.5 / 126
24 / 137
24.5 / 149
25 / 161
25.5 / 175
26 / 189
27 / 220
28 / 254
29 / 292
30 / 335
31 / 382
32 / 433
33 /490


The length of the arm that is used to apply the twisting force to the rod is crucial to how many pds/in are required for a given diameter. This is not true with coilover springs, which are rated by simply compressing them vertically to calculate their resistance. I suppose there could be some engineering standard applied to rate torsion bars in the same way (i.e., a standard lever arm length used to calc the number of lbs. required to deflect the arm 1") but I have never seen it done.

The calcs for front bar "effective spring rates" on 911s would be different than for the rear bars as well, it seems to me, since they are installed longitudinally and integrated into an A-arm design that has a different "lever-arm" length to the wheel location. A 23mm front bar would resist movement at the wheel at a different rate than a rear bar of the same diameter measured at the rear wheel due to the differences in the suspension design.

To some extent this is also true of coilover springs as well, since they can be utilized in different ways in various designs, according to their placement. I believe this is why suspension gurus prefer to look at "wheel rates" rather than "spring rates" when evaluating chassis balance. The motion ratio of the wheel compared to the coilover spring has to be taken into account. That is why my head always starts to hurt when delving into the complexities of chassis dynamics. It is a "non-trivial" subject. There is a reason why good race engineers are highly paid! Let's not even start on roll centers, OK? My head will explode!

TT

[pantera]

Tom, thanks for furthur tech explanation and making adjustment to my post in other forum. Next time I'll try not to post same topic in two forums.

Dan