That Torsion Bar Question Again?

[DamianH]

OK, maybe not *that* torsion bar question.

I'm trying to understand the library of available torsion bars.

I have as a datum point that Series 3 torsion bars as standard settled on 0.75" / 19 mm. After that it all becomes a bit hazy. I understand that S1/S2 cars had larger diameter bars, but I see a variety of sizes quoted, up to 0.865"/22 mm. As someone else has accurately pointed out, this results in a large rate range for a given length.

Is there a definitive list of which diameters can be fitted on which cars?

Also, I'm trying to understand the whether the length of the torsion bars changes between the different cars. At first glance it's not obvious to me why a 5.3 litre V12 would be longer than a 4.2 litre I6 engine, and casually draping a tape measure from bulkhead to picture frame tells me the structure there is "about the same length" (within the precision of my tape measuring skills and the patience of various E-Type guardians). Are the torsion bars all the same length between all the cars? Are the splines interchangeable?

I don't want to engineer new torsion bars if there is one on the shelf I can use!

Thanks for any input.

[mgcjag]

Have a look here...Steve https://forums.jag-lovers.com/t/series- ... ars/436037

[DamianH]

That thread confirms 0.75" / 19 mm for an S3 car and confirms other bars are available, but not really any new information.

This post suggests they are different lengths, with the S3 bars being longer:

Just for the record, S3 torsion bars are over 4.5" longer than those fitted to S1/S2 although I believe the splines are identical (ignore the fact that the images show opposite ends of the bars and therefore different splines, they were just images I had to hand).

S3


S1+S2

There seems to be a lot of folklore about the V12 having "stronger" torsion bars but they are longer, so even at the same diameter they have a lower rate (GJ/l, where l is the length).

Wherever I look I can't find mention of smaller diameter bars than 0.75" on S1/S2, but that doesn't prove anything. I seem to be able to find plenty of mention of higher diameter bars on the earlier cars, though.

[MarekH]

It isn't really possible to easily interchange information from s3 with s1/s2 cars because their front suspensions are not interchangeable with each other.

The s1/s2 has the torsion bars terminating into a chunky horseshoe shaped plate (called "the reaction plate") to then take the load into the tub whilst the s3 dispenses with this and transfers the weight of the car onto two pieces of angle iron directly bolted to the underside of the car much further back roughly below where your knees are. That's why the bars are a different length. If you now want the same "spring" characteristics, given the cars are different weights, you'll have to play with the bar diameter as that is your only variable left.

kind regards
Marek

[DamianH]

Thanks for confirming the length difference.

What were the standard diameters of bar on the earlier cars?

I've already spotted the difference in motion ratio between the narrower and wider track cars, but it's really just the diameters I am trying to home in on. I can calculate motion ratio with the data I already have, but I can't be sure which diameters to plug in for the different cars to orient myself.

With both longer arms and a longer torsion bar, both effects soften the S3 wheel rate compared to the earlier cars *unless* the diameter was also less.

[mgcjag]

From memory early S1 cars had 0.77in diam bars...this was later increased to 0.780-0.784 as per service manual info....Steve

[dlgis]

SNGB sell 3 different ratings of torsion bar for the S1. 0.765, 0.860 and 0.835. Presumably 0.765 is standard and the other 2 are uprated

[DamianH]

SNGB sell 3 different ratings of torsion bar for the S1. 0.765, 0.860 and 0.835. Presumably 0.765 is standard and the other 2 are uprated

I've seen these. It wasn't completely clear to me which were standard and which were uprated, but I will go with the 0.765 as standard since it's usefully close to the previous assertion of 0.77 as standard.

I appreciate the input!

[mgcjag]

All the SNGB bars are aftermarket what do you mean when you say "standard"....I mentioned original dimension above depending on chassis number

[abowie]

Here's a couple of pictures.

[Bob.]

Hi Damian,
Prior to introduction of the larger diameter (0.780-0.784") torsion bars referred to above, production bars were rated at 100lb/in. The wishbone rubbers contributing a further 16lb/in.
(info from Jaguar documents dated 1962).
Bob
PS Have you seen CliveR's posts on jag-lovers? Sounds like you and he have similar backgrounds. He has been using RACE software to model S1/2 suspension.

[DamianH]

Prior to introduction of the larger diameter (0.780-0.784") torsion bars referred to above, production bars were rated at 100lb/in. The wishbone rubbers contributing a further 16lb/in.
(info from Jaguar documents dated 1962).

Bob, thanks for that information. My estimates got me to 117 lb/inch as an overall rate, which is shockingly close! There is a mixture of luck and good judgement there! I don't suppose you have a screengrab of the documents, do you?

PS Have you seen CliveR's posts on jag-lovers? Sounds like you and he have similar backgrounds. He has been using RACE software to model S1/2 suspension.

I am just discovering that, I've seen some of his very lucid discussions I think I need to connect with him!

[Bob.]

Facsimiles of the documents are in Peter Wilson's book "Strictly No Admittance". Probably be breaking some rules to post copies here.

[DamianH]

Facsimiles of the documents are in Peter Wilson's book "Strictly No Admittance". Probably be breaking some rules to post copies here.

It should arrive some time next week for me. Thanks for the pointer.

[DamianH]

Hi Damian,
Prior to introduction of the larger diameter (0.780-0.784") torsion bars referred to above, production bars were rated at 100lb/in. The wishbone rubbers contributing a further 16lb/in.
(info from Jaguar documents dated 1962).
Bob

My copy of "Strictly No Admittance" has just been delivered. I'm presuming you're referring to the information in Appendix 7. I note that it refers to a vehicle 4 WPD which is noted as having a modified body at the time of the report but is reported elsewhere as having started life as a standard steel-bodied car. I'll take a flier and presume the suspension wasn't changed before the testing described in appendices.

A lot of good information in here, so thanks for pointing me at it.

[DamianH]

All the SNGB bars are aftermarket what do you mean when you say "standard"....I mentioned original dimension above depending on chassis number

By "standard" I mean the ones that match the dimensions fitted as normal while the vehicle was being built. I don't care who the supplier is or was (I might distinguish them as "original" and "aftermarket" if I cared about that). I'm interested in the early cars as a going in point. Then I can track the journey from the original cars to the Series 3 in an engineering sense.

Something like this:

• 0.767" nominal (0.765"-0.770") diameter, 32 1/4" length - Early S1 cars

• 0.782" nominal (0.780"-0.784") diameter, 32 1/4" length - Later Six cylinder cars

• 0.750" nominal diameter, 36 7/8" length - V12 cars

• 0.835" nominal diameter, 32 1/4" length - Aftermarket uprated bars option 1

• 0.860" nominal diameter, 32 1/4" length - Aftermarket uprated bars option 2

The mid-sized bars are about 6% stiffer than the originals (106%). The Series 3 bars are only 63% as stiff as the originals, but they act on different length arms. Series 3 was a softer car, running a higher preload to carry the extra weight of the V12.

The aftermarket optional bars are 138% and 156%, respectively.

[Bob.]

I'm presuming you're referring to the information in Appendix 7

The production torsion bar figure of 100lb/in" is given in the document headed "Comparative Specification of Production "E" Type and Coomes "E" Type 18th. August 1962 " in Appendix 9.

0.750" nominal diameter, 36 7/8" length - V12 cars

It appears that S3 was introduced with ~0.830-835" diameter, 35 7/8" overall length bars (from measurements), but these were only used in about the first 480 cars. The setting up procedure and the part numbers changed after that.

All later S3s had 0.75" x 36 7/8" bars, There is no load in this extra inch which is an extension of the splines at the body end to allow the use of a pre tensioning tool for setting up.

It would be interesting to have your views on why this decrease of 54% in the spring rate might have been introduced. Dampers were changed at the same point but there was no change to the rear springs or ARB (unlike S1/2, fitted to front only on S3)

You may be interested that Jaguar quoted wheel rates for the S3 of 94.5lb/in front (0.75" bars) and 150lb/in rear.

[DamianH]

It would be interesting to have your views on why this decrease of 54% in the spring rate might have been introduced. Dampers were changed at the same point but there was no change to the rear springs or ARB (unlike S1/2, fitted to front only on S3)

Obviously I can't be definitive, but such changes are fairly commonplace. The first thing to say about spring rate changes is that they seem much more dramatic than they really are. The thing that matters is the frequency the car wants to move on its suspension, in cycles per second or Hertz. (They are identical; it's not clear to me why CpS isn't more commonly used as it's more readily understood. I guess the Hertz family get royalties or something.)

Ride frequency changes with the square root of stiffness for a given car. A 10% change in stiffness borders on undetectable even by skilled evaluators. 20% is about the "minimum meangingful difference", really. So a 54% drop only represents "three notches" in spring rate.

We can talk about a "baseline" frequency of about 1 Hz. This is about the rhythm at which we walk, and this is the primary reason car suspension frequencies are set here - we are well adapted to ignoring these motions. Less than about 0.6 Hz makes us seasick and between 4 and 8 Hz is particularly objectionable for making humans feel ill. All this is pretty well documented - see, for example ISO2631 or the NASA Ride Model.

There is thus a window between about 0.6 Hz and about 3 Hz where humans are fine with ride motions. In terms of setting the character for an automotive product, it's basically about some kind of equivalent walking pace. A luxurious car feels like a languid saunter on a summer afternoon. A track car feels like a sprinter pumping their legs at maximum effort. The base E-type is about 1.5-1.8 Hz, so a good sporty jog. The V12 as more of a grand tourer, is pulled down to more like 1.1-1.3 Hz, a rhythm more suited to American tastes - still substantially stiffer than the Cadillac boats of the era but more suited to the target market.

Damper development engineers can't resist tinkering with dampers incessantly, so it's no surprise that the dampers were re-valved.

You may be interested that Jaguar quoted wheel rates for the S3 of 94.5lb/in front (0.75" bars) and 150lb/in rear.

I am definitely interested - where is that from? I'm picking my way through a variety of books for sauch nuggets.

[Bob.]

Interesting background on ride frequency thanks.

The wheel rates come from the S3 launch press release.