Convertible rear springs - 5-leaf or 7-leaf?

[Tootall]

My 1275 Converted saloon is probably due new rear springs and the poly-bushed ESM kits look comprehensive and well-priced.

Before I press the 'Purchase' button, for 'spirited' road driving, should I be looking at 5-leaf springs or 7-leaf?

As the car was originally a 1963 2-dr saloon, I assume it has 5-leaf (it's in the lock-up and I haven't checked) but would 7-leaf springs be more suitable?

I don't want to create a roller-skate track car, I want compliant suspension that rides better than my wife's modern MINI cabriolet (shouldn't be too hard to achieve, as that has a dreadful ride - especially on its run-flats!)

For better or worse, the car has Owen Burton damper kits fitted at the front and the rear, which are now also getting pretty tired, so will receive a gas-damper upgrade soon... I'm thinking firm damping and soft-ish springing is the way to go.

For completeness, it's on 5.5x14 Minilights with 60-profile tyres and produces about 90hp at the wheels. It has a standard 4-speed Minor box and standard rear axle/diff, with front discs (another Owen Burton kit.)

Any advice?

[kevin s]

we have been looking into something similar for our 2 door which often carries a fair amount load and some light towing, we have decided on a 6 leaf spring which many others seem to have success with, it's a traveller spring with the bottom leaf removed.

[oliver90owner]

My 1275 Converted saloon is probably due new rear springs and the poly-bushed ESM kits look comprehensive and well-priced.

Wouldn’t it be sensible to determine if it really needs new springs, before jumping the gun?

[panky]

Before you do push the button try re-filling the damper with SAE40 oil to stiffen them up, makes a big difference but doesn't make the ride harsh.

[Tootall]

My 1275 Converted saloon is probably due new rear springs and the poly-bushed ESM kits look comprehensive and well-priced.

Wouldn’t it be sensible to determine if it really needs new springs, before jumping the gun?

I've been driving it since the mid-80s and it's my feeling that it needs better springing. However, if you have a cast-iron visual appraisal technique, I will bow to your greater expertise.

Can you tell me how, exactly, would you judge the remaining temper of the springs by looking at them? How would you determine whether or not the car needed new springs, other than if a leaf was actually broken?

They're nearly 60 years old and the car has not led a cossetted life... it bottoms out at both ends, when driven quickly... Damping will be upgraded to help with that - but I was looking for thoughts on whether Traveller-spec springing would result in too choppy/firm a ride in a hard-drivern convertible.

[Tootall]

Before you do push the button try re-filling the damper with SAE40 oil to stiffen them up, makes a big difference but doesn't make the ride harsh.

The lever-arm dampers only act as top-mounts for the oil dampers... they've had the valves removed, so I'm not sure that thicker oil will make any difference in this case...

[Tootall]

we have been looking into something similar for our 2 door which often carries a fair amount load and some light towing, we have decided on a 6 leaf spring which many others seem to have success with, it's a traveller spring with the bottom leaf removed.

Thanks Kevin... that sounds like a very sensible solution... go for 7-leaf and remove a leaf if too stiff...

I may well go down that route!

[oliver90owner]

Temper will not change. Hooks Law will apply to springs just as it does to spiral springs. Usual checks include ride height and deflection under load. All done with suspension mounting parts in good condition.

I posted because you simply assumed, without any evidence of checking. Perhaps it is time to change the engine? The gearbox? The differential? They, likewise may have had a hard life.

It is always sensible to check rather than guess.

[Tootall]

Temper will not change. Hooks Law will apply to springs just as it does to spiral springs. Usual checks include ride height and deflection under load. All done with suspension mounting parts in good condition.

I posted because you simply assumed, without any evidence of checking. Perhaps it is time to change the engine? The gearbox? The differential? They, likewise may have had a hard life.

It is always sensible to check rather than guess.

Engine has been rebuilt, gearbox is due replacement and diff will also be changed, to giver her longer legs.

I was looking for advice as to which spring set might be most suitable for the car and what I want from it.

But thanks, anyway.

[Tootall]

Temper will not change. Hooks Law will apply to springs just as it does to spiral springs. Usual checks include ride height and deflection under load. All done with suspension mounting parts in good condition.

I posted because you simply assumed, without any evidence of checking. Perhaps it is time to change the engine? The gearbox? The differential? They, likewise may have had a hard life.

It is always sensible to check rather than guess.

You'll notice, from reading my original post, that I said my car "is probably due new rear springs"... and then asked for advice on possible alternatives.

[les]

I would suggest you do one modification at a time, and see what improvements you get after each. Start with new springs, Kevin’s idea sounds a plan.

[Monty-4]

From my reading the stiffening of the rear suspension by using six or seven springs, while intuitive, could be the wrong approach to improve the handling of our RWD cars with a solid axles. The axle needs to be kept pliable to maintain good contact with the road for grip, and the standard dampers may be overwhelmed by the tougher springs. Harder suspension risks an uncomfortable, skittish ride, too.

The issue with Minors seems to be resistance to body roll due to poor damping, especially at the rear where they are oft ignored due to the relative difficulty of maintenance compared to the front. This F-R imbalance amplifies the problem as the front will go light as the rear dips first and further under cornering force. Reconditioned dampers with thicker oil or the conversion to SPAX is transformative with five leaves, in my experience!

[jagnut66]

Abby, my former 1275 two door, had 7 leaf springs and green poly bushes all round, she drove / handled very nicely with this combination.
Best wishes,
Mike.

[kevin s]

It's a complex subject, softer rear springs usually help with traction ( more weight transfer to the rear axle on acceleration) on the other hand the body roll on a live axle will tend to tranfer more weight to the outside wheel reducing grip, then there is the balance of roll stiffness front to rear , if you make the back to stiff you tend to generate oversteer, then there is progression, by having a variable spring rate or invoking the bump stops you change the spring rate with suspension travel and consequently the roll rates, damping doesn't really affect the steady state characteristics (ie driving round a fixed circle) but does affect transient behavour such as turning into a corner and keeping the tyre in contact with rough road surfaces. Bushes also have an effect in allowing geometry to change with travel and provide isolation, they can also give problems when they wind up and cause a sudden change in geometry as the wheel begins to slip.

My take on the our minor is the front which is a bit low on ours grips well and stiffens progressively as the bump stops come into play, the rear on the other hand rolls much more, is underdamped and breaks away suddenly which I think is the rubber between the springs and axles (we used to vastly improve the breakway on escorts by removing these). We are going fof 6 leaf springs, 40 weight oil in the dampers ditching the sadle pads and poly bushes, I'll feedback how it works.

[philthehill]

Kevin
If you ditch the spring saddle pads what are you going to put in their place?

Phil

[jaekl]

Kevin s I disagree with some of your statements. Weight transfer is analyzed with force diagrams. They only need to know the external forces acting on a body to establish the reaction forces, the weight on the wheels. Acceleration has the similar effect of shifting the center of gravity rearward as braking shifts it forward. This shift puts more weight on the rear wheels. The fact that soft springs will cause more deflection doesn't mean that there is more weight on the rear wheels. Cornering is more involved. The side force is working to roll the vehicle over and thus increases the weight on the outside wheel. The effects of the springs are an internal force and have no effect on the reaction forces on the inner and outer wheels. The advantage of devices to reduce body roll is to try to control the camber of the wheels by limiting the amount of suspension movement. Thus more important on cars without solid axles. Sliding pillar suspension is more like a solid axle than independent.

[kevin s]

I don't disagree and it's long time since I studied vehicle dynamics , my understanding was that there are 2 basic forces, gravity which is constant and always vertical plus lateral which is a function of the velocity and radius of the bend, both of these are considered as being applied at the CofG.
As the CofG in most cars and especially tall ones like a minor is above the geometric roll centre the lateral force makes the car roll but this also moves the cofg further towards the outside of the car so now the vertical component of the force has also shifted towards the outside wheel meaning the outside wheel will slip earlier. Much the same for pitch, the pitch centre being lower than the CofG so the CofG moves rearwards transferring mass to the rear wheels( we were told most car pitch and roll centres are close to the ground). So in practice lowering and anti roll bars as you state brings the CofG down and reduces all these effects, you need to be carefull though because lowering can effect the geometry and move the roll centre plus change squat and dive geometery. (Mcpherson strut suspension is susceptable to this)

[Chipper]

Minor Mania in London once built a Fiat twin-cam powered convertible, and reckoned they dramatically improved the rear end handling by fitting four-leaf springs, i.e., making the rear end softer.

From my own experience, 7-leaf Traveller springs and Spax adjustables (fitted by a previous owner) on a 2-door saloon resulted in an overly firm, choppy ride, even with the dampers set to their softest settings.

I find even my Traveller's 50 year old and probably rather tired 7-leaf springs to be somewhat overly stiff, unless I am carrying a heavy load in the boot, when it all smoothens out quite nicely!

I would imagine 7-leaf rears on a convertible would probably be very bouncy and harsh on the typical roads we have to endure lately...

[kevin s]

All of vehicle dynamics is a compromise, most people associate good handling with taking their car close to a race car so lower ride height, stiffer springs / dampers wider tyres etc, a race car tends to be driven on a smooth surface with no consideration to ride or noise though. In the real world lower ride height and stiffer springs rob you of suspension travel which can mean the tyre can't follow a rough road surface and if you just change the springs the geometry is all messed up. Lotus for many years have been considered leaders in vehicle dynamics, and on the Elans they used relatively soft springs with good geometry, roll bars and damping, Mazda followed a similar route with the NA MX5's, of course both of these had physics on their side being low and light to start with.

I think the problem with soft springs on a minor would be there is relatively limited travel so you would crash through into the bump stop, a re-designed spring with a dual rate could work well (basically a 4 or 5 leaf with a extra straight leaf on the bottom which comes into play as the spring deflects), If it's bouncing around damping could also be the issue, the lever arms had limitations particularly with heat management and fluid frothing when new, replacing the probably 50 year old mineral fluid with a modern formulation will make a big difference to even a standard car.

[oliver90owner]

.... replacing the probably 50 year old mineral fluid with a modern formulation will make a big difference to even a standard car.

Shirley, you are not expecting the oil has not been changed in 50 years! I would think most units havebhad attention in that time frame.