Rear spring rates

Simonpfhc · Post by **Simonpfhc** » Fri Jan 30, 2015 10:09 am

Hi all,

The discussion around this subject had already started in another thread, but thought it deserved its own........

Anyway, in the other thread, one of the regular posters commented on the fact to avoid the SNG Barratt pre-assembled rear shock/spring units as they gave a ride height way too high. The first sign that the springs could be wrong was that there were more coils in the spring and it used a thicker gauge wire. I ordered a set of separate springs and shocks from SNG that arrived just last week and also noticed the same differences to the original units. Any I have taken measurements from the new SNG springs and from my original units (pretty sure they are originals) and they have significantly different spring rates (using a spring rate calculator). The new SNG springs are about 313lbs and the originals 269lbs. Spring rate being defined as the force (weight) it takes to compress the spring by 1 inch. Fitting these new springs will definitely result in a higher ride height, but without knowing the corner weights of a S1 FHC, no way of knowing by how much.

I have contacted SNG who have no record of any other reported issues with these springs, but assured me that they will investigate. Will report back if I hear anything.

Cheers.

KingRichard · Post by **KingRichard** » Fri Jan 30, 2015 11:09 am

Hi,

I remember the discussion. I ordered new springs (and Boges) last year from SNG and the they were virtually the same as the one I removed from the car. No difference at all. I had (and still have) a ride height that is 1,5 inch to high at the rear, but after replacing the springs the ride is still to heigh. I didn't measure the spring rate (too dangerous and impossible to do at home) as I did recently with the springs in the carbs, to find out if the rate is correct or different from the original. The lenght is discussed before, and I mailed D. Manners and asked for the lenght of their springs, but as usual never got any answer from them.

At the moment I think my ride height is to high because of the tires I have. I have 185/70 instead of 185/80. I thought is was mr. Crespin (who?), who said it could be the diameter to small causing the high ride height. This is confirmed by Fred Puhn's book. I hope to replace my tires soon with new tires 185/80 (will try the Nankangs) and hope to see a difference in ride height and confirming the theory of the tires.

Richard

Post by **Heuer** » Fri Jan 30, 2015 5:12 pm

From the Service Manual:

When Norman Dewis was testing the E-Type he had a choice of springs (identified by red, yellow, blue, green and white paint blobs on the coils) to choose from. He signed off on 'Red'! Maybe Jaguar Heritage can advise?

Simonpfhc · Post by **Simonpfhc** » Fri Jan 30, 2015 7:10 pm

Yes, those figures tally with the measurements I got from my original coils. It equates to about 263lbs. The big difference with the SNG springs is the wire diameter of 0.475 which gives a much higher spring rate.

Simonpfhc · Post by **Simonpfhc** » Fri Jan 30, 2015 7:16 pm

Hi Richard,

The scenario you have would in fact give you a lower ride height by using the 70 profile tyres as they the tire will have a smaller measurement between the edge of the wheel rim and the ground (70 being a 'lower' profile to 80). You may be referring to the amount of tire you are seeing under the rear wheel arch?? This will look like it is a bit high, but the car will actually be lower to the ground.

Have a search and read about tire aspect ratio which is the 70 and 80 numbers you are referring to.

Cheers.

Post by **mgcjag** » Fri Jan 30, 2015 8:14 pm

Hi All......there has been a lot said about ride height over the years.......looking at the jaguar manual there are definate measurements for the front dependant on what model you have.....but there are no specific measurements for the rear..........as far as Im aware once the front is set to its specific correct height by the torsion bar setting then with the sill line level the rear is at the correct height......but with standard suspension there is no way to adjust other than fit height adjustable shocks or spacer shims to tne springs......Steve

adam · Post by **adam** » Sat Jan 31, 2015 8:54 am

Hi I have a new set off springs from David Manners, the coils measure up at 12.20mm dia but they are powder coated and the length is 27cm.I am in the process of painting new boge shocks and the dust shields, but I will post pics and measurements when I fit them.

Adam

Simonpfhc · Post by **Simonpfhc** » Sat Jan 31, 2015 9:15 am

Hi Adam,

Use this simple spring rate calculator:

http://faq.f650.com/FAQs/ShocksSpringRa ... Calculator

It will tell what spring rate your springs are. Post the results, it would be interesting for comparison.

Cheers.

PeterCrespin · Post by **PeterCrespin** » Sat Jan 31, 2015 9:23 am

Points to note:

Wire thickness is a fair guide but ignores material spec and treatment. Springs made of 1/2" thick solder would have a lower rate than 1/4" steel. There are also many grades of steel and types of heat treatment.

Other things being equal, more coils means a softer spring, not stiffer.

Colour markings are not 'absolute' like, say, resistor colours. I think different makers can specify their own markings for hard, medium or soft versions of a spring, or other specific rate codes?

Ride height is a fitting outcome not a spring property. A shorter spring could leave the car sitting higher and a long spring could leave a car sitting lower.

The same springs fitted to two different cars could still result in different ride heights due to other factors such as tyre and damper sizing.

The only real comparative test is to measure compression under a known load and for most people that means fitting them to the car. Which is a bit late to find there's a problem.

There are motorbike shops with spring/damper test facilities but the E-type springs might be a bit oversize for some machines?

Pete

MarekH · Post by **MarekH** » Sat Jan 31, 2015 9:43 am

Dear Peter,

Ride height and Damper size? I'd say that the spring simply settles due to the weight of the car and the damper is irrelevant to this. After all, if you want, you can take the the front dampers off and leave the torsion bars on and the height won't change.

Also, ride height has to partially be defined by some sort of spring property, otherwise they'd never "go off" - the fitting hasn't changed.

kind regards
Marek

PeterCrespin · Post by **PeterCrespin** » Sat Jan 31, 2015 10:03 am

MarekH wrote:Dear Peter,

Ride height and Damper size? I'd say that the spring simply settles due to the weight of the car and the damper is irrelevant to this. After all, if you want, you can take the the front dampers off and leave the torsion bars on and the height won't change.

Also, ride height has to partially be defined by some sort of spring property, otherwise they'd never "go off" - the fitting hasn't changed.

kind regards
Marek

Agree totally in principle Marek, but you are misconstruing what I actually wrote using careful wording.

Damper sizing is important in terms of perch distance from eye centre, at either end, inverted or normal.

Front scenario is not comparable since spring and damper functions are separated so the car sits fine mechanically on TBs with no dampers fitted. The equivalent IRS scenario, with equivalent outcome. would be making 8 dummy spring attachrment eyes and plugging them into either end of the four coils to support the car sans dampers.

Yes of course spring rate affects ride height but you cannot derive ride height from simply knowing the spring rate. Any change over time in the properties of a given set of springs may alter the ride height over time, but the ride height of two different cars can be very different even if you fit identical springs to each.

adam · Post by **adam** » Sat Jan 31, 2015 10:19 am

Hi using the calculator 251. I don't think the dampers have anything to do with ride height they only effect spring movement, only if the damper eyes where closer or wider apart than standard would the height be altered by the dampers.

Adam

johnney · Post by **johnney** » Sat Jan 31, 2015 12:23 pm

Regarding the rear suspension of an E type, four dimensions, three fixed and one variable dimensions control ride height.

1: The distance from the upper damper spring seat to the top mounting eye on the damper. (On standard dampers this dimension is fixed).

2: The distance from the lower spring mounting seat to the lower damper mounting eye. (On standard dampers this dimension is fixed).

3: The compressed length of the chosen spring. (This dimension for the purposes of checking ride height on a stationary car is effectively fixed and determined by its free length and spring rate).

4, The effective radius of the tyre when loaded with the weight of the car (This dimension is dependant on the size of the tyre, its aspect ratio and inflation pressure.

So in short, yes the dimensions of the dampers do contribute to the ride height.

adam · Post by **adam** » Sat Jan 31, 2015 2:30 pm

Hi, its only the spring and the way its mounted to the damper, and eye to eye length that effects ride height. Damper on its own can be moved to any position you want and left there, it has no rebound quality without the spring. If bottom spring mounting was moved closer to top eye, as on early cars with a bottom spacer, causing the spring to be more compressed or adjustable shocks to shorten spring length, that would increase ride height. Unless I was to carry 10 bags of cement around or tyres where flat I wouldn't expect ride height to change much, eventually springs will settle overtime but that's another story.

MarekH · Post by **MarekH** » Sat Jan 31, 2015 2:47 pm

Adam,

What I think we all are trying to say, in different ways, is that the eye to eye length isn't relevant - it's the top eye to top of spring plus bottom eye to bottom of spring which affects the ride height. And no amount of spacers will cause the spring to become more compressed - that stays the same because the weight of the car stays the same against the same spring. In your example, the height changes because the bottom eye to bottom of spring has just been changed by your spacer.

kind regards
Marek (and no doubt Pete and all of the others....)

PeterCrespin · Post by **PeterCrespin** » Sat Jan 31, 2015 3:13 pm

MarekH wrote:Adam,

And no amount of spacers will cause the spring to become more compressed - that stays the same because the weight of the car stays the same against the same spring.

There is one exception I can think of but it's for a few motorbikes and bicycles, not cars.

jag68 · Post by **jag68** » Sat Jan 31, 2015 5:26 pm

A coil spring is just a straight torsion bar wound in a coil. It's spring rate is a simple function of wire diameter and length. The more coils it has over a given circumference the longer the wire and the lower the spring rate; that is a longer torsion bar has a lower spring rate than a shorter bar at the same wire diameter. This manifests itself when hot rodders cut out a coil on a spring to lower their car and find it now has a harder ride. The important part from a design perspective is that the bending of the wire does not exceed its elasticity - that is it doesn't go beyond the point where the steel become permanently deformed. This is established by what material the spring is made from, it's heat treatment etc. In theory as long as you don't exceed this limit the spring can't sag. In practice we all know this happens after years of use - probably from some form of fatigue. My experience with E Type torsion bars is that they take a set after years of use, making reinstallation after they've been out of the car for a while tricky. The front of the car continues to sag if you set them to the manual specs. The main determinate of spring rate is wire diameter and it affects the rate by a factor of 4X.
The overall height of the coil is important to ride height if the weight bearing down on them is equal. Two springs of unequal height but the same rate will give quite different ride heights because the starting point of the spring compression from the weight is different. In other words a 10" coil and a 8" coil of the same rate are equally compressed by the weight of the rear of the car, but the car with the 8" spring started 2" lower.
I don't know if this is very helpful with ride height problems. I wonder where some of these springs today come from and how consistent the manufacturing process is. I have in the past purchased springs by spec from manufacturers like Eibach in the US and found them to be both inexpensive and what they are supposed to be.
Accurate determination of spring rate requires a very accurate machine. You virtually need a caliper to measure deflection to ensure it is equal spring to spring. A tenth of an inch difference on an 300 pound spring gives a significant difference in measured rate. In other words you can't do these measurements on a bathroom scale.
A good aticle on springs is here: http://insideracingtechnology.com/eibach1.htm

PeterCrespin · Post by **PeterCrespin** » Sat Jan 31, 2015 7:38 pm

jag68 wrote:..spring rate is a simple function of wire diameter and length.

That is an over-simplification really. Indeed, you say as much yourself:

jag68 wrote:..established by what material the spring is made from, it's heat treatment etc.

Your statement that "The overall height of the coil is important to ride height if the weight bearing down on them is equal" is only meaningful if the coils are the same rate, which depends on all four factors above, plus free length.

Like you, I (and probably most vendors) struggle to predict reliably the ride height delivered by a given set of out-sourced springs. This is not only because DIY measuring of wire thickness, coil length and numbers of turns is only part of the story, but also because even supplying warranted length and rate loose springs would still leave some customers unhappy because their ride height was screwed up by other factors (even assuming it was measured accurately from the correct datum points, which often it is not).

The best option to me (if there is something wrong that needs fixing) is either buying warranted original-spec spring/damper combos so that all dimensions are factory standard, or fittiing new or existing springs to dampers with adjustable spring perches so you can dial in your own settings.

abowie · Post by **abowie** » Sat Jan 31, 2015 10:22 pm

The springs from SNGB are .475" wire, 2.9" diameter. I'm not entirely sure what the definition of a "free" coil is ie where you start and finish. I'm going for 10.5 which gives a rate of 292lb/in.
My old 3.8 springs are .432", 2.9" diameter and 9.5 coils if I use the same counting method which equals 220lb/in.

jag68 · Post by **jag68** » Sat Jan 31, 2015 11:00 pm

Actually Pete the material used and it's tempering have only a very small effect on spring rate. If you have a look at engineering specs for the various types of spring steel material that could be used for car springs there is very little difference in their elasticity. In fact the material used today in virtually all car springs is something called "oil tempered wire", which is a low carbon spring steel that is cold rolled over a die to make the spring. It's then heated to 500 degrees f for half an hour, but this is only to relieve stress induced by the cold rolling, not to temper it. It is of course unknown where our suppliers are getting their springs made, or what they are made of, but it's unlikely I suspect that it's made of some steel that is outside stuff commonly used, and hence with an elasticity index different from the usual stuff.

Looking back at my post I frankly don't know what I meant to to say about the spring height in the sentence you referred to. My comparison of the 8" and 10" springs did at least have the "same spring rate" caveat in it. All I can say was that the post was interrupted by lunch. But spring height by itself has nothing to do with spring rates; while other factors have a very small effect, it's all about wire length used (often referenced by a calculation based on free coil numbers and coil diameter) and most importantly wire diameter.

I agree with you on adjustable spring perches. But sometimes it's not enough. My last set of springs from a well know N.A. supplier gave me a ride height so high that it couldn't be adjusted out. I went back to the old springs. Use of an adjustable torsion bar reaction tie plate is also essential to avoid pulled out hair.

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