Leaf spring

[LaurentGT6+]

I have 2 leaf spring, one with 6 bands and the others 7 bands. Wich one would you use?
It's a GT6+ with rotoflex.

Laurent
PS -18C and with the wind factor down to -27C. But +18C in the garage.

[AndreGT6]

Yeah its freaking cold here too.

Once I get the garage settled, it will be time to make some progress on my car.

[smallg]

It's summer down in the best part of the world. What's all that white stuff?????

[JohnD]

Laurent,
By "bands" do you mean blades or leaves to the spring?

GT6 Mk1 - 8 blades
Mk2-3 - 6 blades

Spitfire MkII-III 7 blades, so I suspect you have 1 GT6 spring and 2 Spitfire springs

I'll send you a Word document that lists the specs of all the models' springs, as far as they are known.  That should help you ID which is which.

John

[LaurentGT6+]

Thank you John, and yes I mean blades. The 7 blades was installed on a 69GT6 also. This is why I was asking which one am I suppose to use. Also thanks for the document.

Laurent

[CharlieB]

You can measure the spring rate approximately by standing on it & measuring (or asking someone else to measure) the deflection. Stiffer = better?

[AndreGT6]

I know my spring was a monster, compared to a friends GT6, hence I had too much POS camber.
Did the drop link mod and a diff spacer and achieved my desired neg camber.

A.

[LaurentGT6+]

If I stand on them the 6 blades,it will go down about 2 inches more.(weight 200 pounds) It's a lot softer then the 7 blades but at rest they have the same arc. So which one is the best stiffer or softer?

Laurent

[Richard B]

Stiffer, don't forget the extra weight at the back of a GT6 compared to a Spitfire.

[AndreGT6]

Yeah if you have camber issues, a diff spacer can help you there.

A.

[JohnD]

"Stiffer = Better"?
Not necessarily so!

GT6 Mk1 spring rate=305lbs/inch
Mk2-3 = 185lbs/inch

Vitesse also went down from Mk1 to 2, swing axle to rotaflex, 270-215
And Spitfire went Mk 1-3=166
to Mk IV, 1500=110lbs/in  - but they had the swing spring - as the Mk3 GT6 had.

It depends what you want to do in it.  Andre chose a stiff spring for his Rotaflex(?) car, I presume for performance.  I do for my racecar, but that will rattle your fillings anywhere that is not racetrack smooth.    

John

[LaurentGT6+]

So with the road that we have in my part of the world I should stick with the soft one.

Laurent

[2.5 Chris]

smallg wrote:

It's summer down in the best part of the world. What's all that white stuff?????

I nominate ALL comments to do with warm weather, & sunnier climates, be banished to room 101.
Im off to look at my Thompson holiday brochure and get my fake tan out. ::) ;D

[AndreGT6]

JohnD wrote:

"Stiffer = Better"?
Andre chose a stiff spring for his Rotaflex(?) car, I presume for performance.  I do for my racecar, but that will rattle your fillings anywhere that is not racetrack smooth.    

John

No rotoflex in my MK1 :)

But yes I've been aiming for a lot more performance with the GT6.
Also a bit of working with what you have, but I'm happy with my GT6's stance.
You can never have enough rear camber, so I've been told :)

[GT6Steve]

cjm wrote:

I nominate ALL comments to do with warm weather, & sunnier climates, be banished to room 101.
Im off to look at my Thompson holiday brochure and get my fake tan out. ::) ;D

Can you say "Vegas, babee...",  I thought you could.

It's low 38F and high 58F here today so it may not be the sunny clime you're after.
And I'll bet the Brits don't get the quote above although Roto will.  A childrens show , Mr Rogers, taught new words to kids with that line.

"Can you say Dichloroflouroethane?  I thought you could".

[rotoflex]

The rear springs for GT6 MK2/+/Mk3 rotoflex & late GT6 Mk3 swing spring (non-rotoflex) are constructed differently.  I think I have examples of the 2 types in storage & will get a photo later today.

[hypoid53]

As I have one of these springs for my 1972 GT6 Mk3 rotoflex currently stripped down I thought it might be useful to post this picture.

Incidentally can anyone  tell me if it is a good idea to paint these leaves as an anti corrosion measure? I intend graphite greasing between the leaves but am unsure if the paint and grease will be compatible. Any ideas?

[chrisspe]

i would guess paint would increase the friction and wear away anyway,  if its covered in grease it aint gonna rust

[JohnD]

AndreGT6 wrote:

You can never have enough rear camber, so I've been told :)

Andre,
I first fitted the Silverback with long halfshafts.  With the swing spring, and a spacer, it came out with a negative camber of four degrees.  Looked 'wicked', innit!
The first time I had it on a rolling road, both the apprentice AND I had to stand on the car's back step, as only two thirds of the tread (185/60 tyres on 6" rims) was in contact with the rollers and the tyres were slipping.
When one shaft  broke, I fitted short half shafts -> 1-2 degrees negative - and things were much better, both on rollers and the road.

John

[rotoflex]

There were 3 different springs fitted originally to the GT-6 line:
1.  The original, pre-rotoflex GT-6 (a.k.a. 'Mk1)
2.  The rotoflex MK2/+ and early Mk3
3.  The swing spring (late Mk3)

--
Here's the 'Mk1' spring, a.k.a pre-rotoflex:  8 leaves


--
Here's the rotoflex spring, in the MK2/+ and early Mk3:  6 leaves
So if your spring with 6 leaves looks like the one below, that's the right one for a rotoflex Mk2:



--
Here's the swing spring, in the late Mk3:  5 leaves (and they're fat leaves, too)


---

Here are a couple of photos of the rotoflex spring (top) & swing spring (bottom)



That central pivot bracket thing in the middle of the swing spring does come off, but I think you can see what it looks like underneath from the swing spring drawing.

The swing spring has a front & back orientation which must be observed.  I don't recall the same being true for the rotoflex spring.

If you've acquired a used replacement spring, it might be worth thinking about why it was taken out - they sag on the driver's side & must be replaced (or re-arched, if such a thing can really be done acceptably).

The drawings are from the factory shop & parts manuals.  You can get them on the Heritage Motor Centre CD.

[starman]

JohnD wrote:

"Stiffer = Better"?
Not necessarily so!

GT6 Mk1 spring rate=305lbs/inch
Mk2-3 = 185lbs/inch

Vitesse also went down from Mk1 to 2, swing axle to rotaflex, 270-215
And Spitfire went Mk 1-3=166
to Mk IV, 1500=110lbs/in  - but they had the swing spring - as the Mk3 GT6 had.

It depends what you want to do in it.  Andre chose a stiff spring for his Rotaflex(?) car, I presume for performance.  I do for my racecar, but that will rattle your fillings anywhere that is not racetrack smooth.    

John

John, et al.-

110 lb/in is the wheel rate when using the swing spring, not the semi-elliptical spring rate of the swing spring.  For an accurate apples-to-apples comparison, spring rates should be compared with spring rates.  I've actually tested early Spitfire fixed springs, Spitfire swing springs and one GT6 rotoflex (GT6+) spring, and I've measured 160 lb/in for early Spitfire fixed springs (vs. the factory spec 166 lb/in value), 180 lb/in for the one GT6 rotoflex spring I have (vs. the 185 lb/in value from Thomason) and 320 lb/in for Spitfire swing springs.  Note that in his "Guide to Originality", Thomason says that 110 lb/in is the wheel rate, not the spring rate.  So, let's convert wheel rate to semi-elliptical spring rate and compare.  Spring rate = 2 * wheel rate/(motion ratio)^2 = 2 *110/(41.25 inch laden spring length/50 inch track width for the "long axle" later Spitfire mkIV & 1500)^2 = 323 lb/in.  This agrees with my measured value, especially if you consider likely tolerances of measurement and production spring variance (3 lbs out of 320 is less than 1 percent).  Moreover, the unladen arc of the swing spring, measured from bottom surface of the bottom leaf to the centerline of the chord connecting the spring eye centers, is 1.625 inches (it's 2.625 inches from bottom of bottom leaf to the floor with the unladen spring sitting on the floor).  If it did not have significantly more vertical stiffness than the Spitfire fixed spring, which by my measurement has an unladen arc of 4.375 inches to eye chord/5.375 inches to floor (compare with the quoted 135mm = 5.3 inches), then the rear of the later Spitfires would be much much lower than they really are.  Moreover, as a sanity-check, I've calculated the spring rates of all the small-chassis Triumph fixed springs based on factory manual information about spring composition using the canonical semi-elliptical spring rate equation and compared the results with spring rates quoted in the same factory manuals, and my calculated rates agree reasonably well with the factory manual rates.  But even if you ignore these calculations--the test measurements and the correct interpretation of the published data speak for themselves--the Spitfire swing spring has about twice the vertical spring rate of the earlier Spitfire fixed spring.

That said, I concur that stiffer is not necessarily better and it depends on what one wants to do and what suspension one is using.  In the case of the swing axle cars, rear roll stiffness is an issue.  Reducing rear roll stiffness reduces rear lateral weight transfer, which suppresses jacking and the consequent development of positive camber "wheel tuck" as it's called (as well helps prevent the inside rear tire from unloading enough to lose grip and spin, thus robbing drive torque from the loaded outside tire). So, reducing roll stiffness of a swing axle rear is good. However, increasing vertical stiffness reduces the rate of camber change and hence limits the total amount of camber change for a given load, which is also good for swing axles. The problem is that with a conventional spring, these are competing objectives; reducing roll stiffness requires reducing vertical stiffness, and increasing vertical stiffness increases roll stiffness. Engineers at Standard-Triumph simultaneously reduced roll stiffness and increased vertical stiffness with the innovation of the "swing" spring. The factory switched from the "fixed" spring to the swing spring in the later Spitfires and GT6s (all mkIV and 1500 Spitfires have a swing spring, as do GT6 mk3s from KE/KF20001 onward). The trick of the swing spring design is that only the bottom leaf is actually affixed to the differential while the rest float and pivot, or "swing" to a limited degree about a center bushing. Roll stiffness is reduced significantly because only the fixed leaf contributes wholly to roll stiffness while vertical stiffness is actually increased because not only do all the leaves participate in vertical stiffness but also the two models of swing spring provide greater overall vertical stiffness than the fixed springs they replaced.  BTW, Thomason quotes a 75% reduction in roll stiffness with the Spitfire swing spring his "Originality" book; I calculate a 60 to 65 percent reduction--either way, the swing spring yields a significant roll stiffness reduction over the more vertically compliant early Spitfire fixed spring.  And recall, that because the swing spring provides reduced roll stiffness, the factory equipped swing spring cars with stiffer front anti-roll bars to maintain overall roll stiffness, albeit with a different distribution (more up front, less at the rear).

[Piero franchi]

I would sugest you look at the whole rear end as a whole entity.

The early spits had what I would describe as a free moving rear
The rotoflex had a lot of load built into its movment, hence the lighter spring rate.
It's not just the added weight of a gt6 that counts, as the mk1 gt6 had a different spring rate to the rotoflex
Any one using sliding shafts on a rotoflex spitfire or gt6 might want to take that into account.

[starman]

The early Spitfires have swing axle rears with fixed springs.  The later Spitfires and the late GT6 mk3s have swing axle rears with swing springs.  The GTg rotoflex cars have multilink independent rear suspensions with fixed springs.  I don't know what you mean by a "free moving rear" on the early Spits Piero, but I agree that you have to look at the whole rear end as a whole entity.  Yes, the GT6 mk1 fixed spring rate is higher at 305 lb/in than the rotoflex GT6 spring rate at 185 lb/in, but they have two entirely different suspensions--the former being swing axles and the latter being a multilink independent rear.  Moreover, don't forget converting semi-elliptical spring rates to individual wheel rates to get the real rates where the rubber meets the road.

[Richard B]

Now the question I have is; What spring (and rate) should I fit to a Spitfire MkIII with Rotoflex wishbones, vertical links & CV driveshafts?


Spitfire MkIII or GT6 MkII or a decambered GT6 MKII


A Standard GT6 MkII is too high, a 3/4" lowering block helps.

[Piero franchi]

What I ment was the rear suspension is free to move with little or no force needed  to worry about other than the force to move its own weight.

On a rotoflex with dough nuts, there is a force needed to compress the doughnut so as to allow the drive shaft to increase and decrease in lengh.

It's this added force that I belive accounts for the differance in spring rates.
The weights of the gt6 stayed prity much the same, so there was no need for a spring rate change, neither the less the rate did change.

The camber of a mk1 gt6 was linked to the none use of a lower wish bone and no sliding shaft via the doughnut.

Solid shafts are the problem not so much the spring rate or its camber.

Now a spit running a rotorflex system using sliding shafts, in my oppinion, should use the same spring rate as intended for that car, decambered if you wish.

A gt6 running rotoflex and sliding shafts should use a spring rate as per mk1, decambered if you wish.
As I said. This is ny oppinion, but it makes sence to me.

I am of course talking leaf spring and not coil over springs