Front & Rear Springs

[Black Cat]

I see a number of people are using 480Ib springs on there cars. I'm looking to replace mine but was thinking more like 330Ib  for a fast road use but a comfortable ride. What are your thoughts?

I have a new set  of adjustable Spax dampers front and rear and will be using 13" wheels.

Any advice would be greatly appreciated.

 

 

 

[JohnD]

I have 600lbs/in fronts on SofS and earlier this year I drorve Marcus' GT6 that has 1000lb springs.  Remarkably comfortable!

As he will say (sorry, Marcus!), the dampers should be revalved for much stronger springs.

John

[Clive]

What car, and what use? Do you regularly take passengers (ie I do some serious distances with the lady wife) who may object to harder suspension?

FWIW my spitfire is "fast road" and the engine weighs the same as the std 1300 engine. I run 480lb springs with Konis. Suspension is firm but not uncomfortable unless the roads are BAD (Scottish borders last year was horrific, but stuff fell out of a friends stag and that is a really well built car)

Rear springs are a black art that nobody seems to discuss. My spit has a CV rotoflex rear, but I think it needs to be a little stiffer having watched the wife driving it round Curborough. That is something I will attend to over the winter....

[Black Cat]

Yes, should have said, it's a 1977 spitfire 1500,  had it a few months and only done a couple of hundred miles before striping it down. Just got the chassis back from the sand blaster, and ready to start the rebuild. Got most of the parts from Moss Bros but they don't do the springs anymore, well not at the moment.

Your right about the wife, she is due to go into hospital next month for a back operation and I would like to take it on  long runs when it's back on the road so might be best if they weren't too firm.

If I did go for 330lbs on the front, what would be best for the rear?

 

[Clive]

The std rear spring works well...I think your plan is sound.

Don't set the shocks hard. 3-5 clicks from softest is a good place to start and experiment. Too hard, and the car really suffers.

Most important is geometry. DIY is not hard and is rewarding knowing you understand what is going on. Plenty of guides to measuring toe, camber and castor. Using basic equipment...(string, tape measure, and plumb line on a board....

[JohnD]

If 'er not indoors but in your car has a back problem, stiffer springs might not be the way go.  One person's firm is another's agonising.     New coil springs are cheap and  easy to install - I'd say wait until your partner is better, and see how the present ride suits her.

This table from the Courier many years ago lists the various models' springs, and as you can see the Courier van had by far the stiffest, as a load carrier.

 

Front & rear suspension specs.htm

[Mjit]

12 hours ago, clive said:

Don't set the shocks hard. 3-5 clicks from softest is a good place to start and experiment. Too hard, and the car really suffers.

And sometimes in the most unexpected ways.

Had real issues with my 2.5PI tramlining in the motorway left hand lane lorry tracks when i first fitted adjustable rear dampers.  Softend them off a few clicks and the problem went away.

[Clive]

12 minutes ago, JohnD said:

If 'er not indoors but in your car has a back problem, stiffer springs might not be the way go.  One person's firm is another's agonising.     New coil springs are cheap and  easy to install - I'd say wait until your partner is better, and see how the present ride suits her.

This table from the Courier many years ago lists the various models' springs, and as you can see the Courier van had by far the stiffest, as a load carrier.

 

Front & rear suspension specs.htm

John, the troubleis that some (most??) of the new rear springs are not at all good. I fitted a brand new, british made one just before the CT "border raiders" event last summer. It was returned for a refund (no quibble) and binned from the one event, on road though plenty of poor road surfaces. But that was 1000miles only. Spring had sagged, bushes were shot.

I am not certain the table is actulayy correct either, or at least consistent. The spit 150 rear spring is more likely 110lb per side, but allows a lot of roll. And some springs do not seem available at all....

Best springmaker from my enquiries are Owen Springs. If I can work out what I actually want in terms of free arc and strength, I will give them a call!

But a good condition std 1500 rear spring is very good, and pretty compliant. And works well in a spitfire.... A late GT6 swing spring would be an obvious upgrade, but would have to be made, nobody stocks them at all.

 

[JohnD]

I accept no responsibility, none at all for the quality of remade springs, so don't drop them at my door.

And that chart has been around the last 25 years.  I've published it many times since it appeared in the Courier, and asked for corrections/additions.  Yours is the first!  Do you mean that that the 1500 should read "Spring rate 220 lbs/in"?  The Tegleriser site agrees with the Courier, so I'd be glad of corroboration before I insert that.   

John

 

 

[Clive]

John I was speaking to Mik Davies a couple of weeks ago. The penny dropped then....He had a spring made by Owen Springs that was effectively a 1500 spring but no hump and therefore swing. He was saying it is a very stiff spring. That does not make sense when you compare the rates on the above table. 

If a spitfire mk3 spring is 165lbs fitting a 110lbswing spring would be VERY soft, but it isn't. And looking at the number of leaves plus their thickness, it adds up to something approaching that of the herald coupe @200ish lbs. I wonder if how therate was measured changed from overall, to just one side? That would make some sense.

And regarding other websites etc, I suspect they get all their info from the same place?

Another odd one. Is the GT6 rmk1 rear spring really almost twice as stiff as a mk3 spitfire? bearing in mind the mk3 spit is now the one supplied for the mk1 GT6 by all suppliers.

And my comment was not criticising you, just pointing out the grief with trying to choose a rear spring. Things really are not as straightforward as that table suggests.

And finally, a friend dropped a spring off to me. From its appearance (6 leaves, the outer clamp wrapped around rather than a bolt) suggests Mk2 gt6. But the free arch is about 25mm higher. He can't remember how/when it appeared in his garage....but that spring is a mystery!

[JohnD]

Clive,

I don't think that the physics of springs work that way.   As you will know, the stiffness of a spring is the force required to deflect it by unit distance.   

Stiffness = Force/deflection

And, it depends on the length of a bar, its cross-sectional area and Young's modulus:

Stiffness = [Area x E (Young's mod]/ Length 

Consider an ordinary ruler, as a transverse spring model.    Clamp it to the table by one end and 'spring' the free end.  Now halve the length.  For the same deflection you must apply much more force - the shorter spring is stiffer.

Measuring the stiffness of one side of a swing spring  would yield a higher value than the whole spring.     That is why I'm reluctant to accept your reasoning.

Jhn

[NonMember]

John,

If you measure the spring rate of one side of a Mk3 Spitfire rear spring, by clamping the diff down and lifting one wheel, you will get the spring stiffness determined by Young's modulus for half the length of the spring, because it's clamped in the middle. If you do the same test method on a swing-spring car, you'll get the stiffness for the whole length. It will appear to be half the rate that you would get for the same spring in non-swing mode.

I think Clive's point is that such a measurement method, if carried out by somebody not aware of this, would give the stiffness of half a fixed spring but the whole length of a swing spring. That could account for the apparent inconsistency. Admittedly, he referred to changes in the measurement method that contradict my reading but I put that down to poor explanation 😛

[Clive]

Hmm, yes john,you are right. As the springs are clamped in the middle they act as 2 separate springs. So yes, the whole spring will be twice as strong as measuring just one side. 

So maybe the swing spring was measured as a whole spring that was tested as a single, whole length, halving the rate. Or due to teh nature of having 2 leaves (I think) fixed, and some that pivot, some combination? Could explain why the figures do not seem to tally.  

And yes, a long day here. Bad backs are unpleasant, especially when I am supposed to be earning a living, but more worryingly have a spitfire to work on....

[Colin Lindsay]

1 hour ago, clive said:

Another odd one. Is the GT6 rmk1 rear spring really almost twice as stiff as a mk3 spitfire? bearing in mind the mk3 spit is now the one supplied for the mk1 GT6 by all suppliers.

 just pointing out the grief with trying to choose a rear spring.

When I converted the MK1 GT6 to swing spring I used a 1500 Spitfire spring, which was woeful. Bottomed out far too easily.

I bought a NOS late GT6 swing spring from Chic Doig and it was an entire transformation, despite being far thinner and straighter than the hefty curved 1500 version, so looks are very definitely deceptive. 

[Clive]

Yes, the GT6 version is about 20% stronger so would make quite a difference. And it also depends on whether the spitfire spring was new OE.  I have a nasty feeling that the new srings we are offered today look correct, but are not the same specs as originals. A sadly familiar story....

I bet Chic hasn't got any left....

[JohnD]

The point of the swing spring was that it was weak in roll, but stiff in bounce, because only the main leaf acted in roll.

Any way, I can find a copy of the OE WSM on line - for the earlier car, but not the SWing spring.   That gives the spring rate for the Mk3/4 (as above) as 166lbs/in.     If someone has the 1500 manual then we may remove all doubt.

John

[Clive]

I understand there are 2 versions of the spitfire spring (short and long shaft versions, but struggling to find the 2 part numbers) plus the GT6 version, which was for long shafts. 

Some more info here, including some calcs for rates at the wheel and so on...

http://auskellian.com/paul/links_files/springs.htm  but that looks to be based in some of the same old data... Yes, the later manuals are rather elusive. The vitessesteve site has loads of earlier stuff....

[Gully]

What always surprises me about that table (and the GT6 workshop manual) is the amount of positive camber for the front. Not convinced that with wider, modern radials that's ideal. Always looks a bit much anyway - been thinking about reducing on mine for about 5 years!

Gully

[Clive]

Agreed. 1-2 degrees negative is a more sensible idea.

 

[johny]

If it helps Ive just used a formula Ive found on line to calculate spring rate and it seems to tie up pretty well using the figures in the table so I think the rate indicated is the number of lbs that have to be applied to the centre of the spring for a deflection of 1".....

[Black Cat]

Well, I didn't expect such an in depth response.

Just to clarify, I already have a new rear leaf spring for the spitfire from Moss Bros. (Part no. 159640Z) I have the Spax adjustable shocks for both front and rear, its just the springs that I don't have as Moss Bros didn't have them in stock.

Moss Bros advised 330Ibs springs. I was just looking for a general opinion as to whether these would be suitable for a fast road car with a reasonable comfortable ride and if so is there a specific size length of spring I should be looking for?

 

[Clive]

Sorry, it all got a bit "involved"

Yes, 330lbs are a good all round choice (my opinion...)

From the old Triumphtune manual, the ones to go for are the old TT4301 with a fitted length of 7.25" (std is 180lb, fitted lengthof 8")

Now, how that correlates to open length, I am not sure. around 9"??? But if you have adjustable spring pans that will help set the final ride height by a little.

 

Edit, google TT4301..... (or TT4302 6.875" fitted length) seems they are still available!

http://www.jagspares.co.uk/Abingdon/partdetailalternate.asp?partno=TT4301

[Black Cat]

Thanks Clive for confirmed the 330Ibs would be suitable for my needs.