Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Suplex 5L: First Impressions

Mario when we come to Portugal at the end of May you will be most welcome to take the car for a run and try it for youself

Peter,
I think you will find that the improved cornering and lighter more precise steering is in part due to the fact that the exact loading on the rear suspension means that the corner weights are much closer to optimum than with less than perfect modern leave spring as fitted from new.
Maybe now you can understand why I am so pleased with my car and can use all the power the mighty 3.7 turns out.
Regards David

That's a very kind offer, Peter, thanks

This morning I had about an hour to play with and I set the dampers to the setting recommended for a "firm sporty ride"; 18 clicks down from full hard (as against the 8 clicks yesterday).

I took the car on a road I know quite well and I found that the ride remained pretty good or better on the small bumps and was improved somewhat on the large bumps. The amount of travel is limited so one can't expect miracles but the result was quite impressive. I did notice however that the initial roll on changing direction had increased a little. I will try it like that for a while and see how I go.

Cornering was impressive and whilst it is never going to equal a modern sports car it had me going as fast as I dared on a public road knowing full well I had plenty of grip in reserve. No loss of traction, no skipping on bumps, just nice and secure and reasonably comfortable. You notice the improvement in the suspension (by comparison to the fairly inflexible leaf springs) straight away in the way it soaks up the small bumps.

In fact later in the day I drove our light Subaru which also has a live axle and coils at the rear. The Morgan with the 5L was definitely better on the smaller bumps and it was only when the grater travel of the suspension came into play that the Subaru was more comfortable on the larger bumps. Naturally the Morgan would out corner the Subaru with ease but that was not what I was comparing. The Alfa Sportswagon, by the way, would be worse on all bumps, it has a much harsher ride.

My feeling is that if you give priority to comfort you would go to full soft (about the equivalent of the non adjustable Bilsteins). If you want a good compromise you would probably use 18 clicks down from full hard. If you want trackday type performance 8 clicks down from full hard is probably about where you would set it. There are a total of 24 clicks that you can use so there is plenty to play with.

The real test will be when we go on our next trip with a full load of luggage

Something not mentioned Peter, has the creaking bedspring noise vanished?

I have a Rutherford setup that took some fettling as my shock mounting points on the tubular bar were none standard.

But the access to oiling the leaf springs is a little restrictive to silence them and if the 5 link is quiet then I could well be tempted at some point.

I had not thought about it because I sprayed my leaf springs with chain lube. Even so I would get some occasional creaking. I have not noticed any creaking with the 5L.

Just a few comments on the 5L design philosophy as prompted by PaulJ, trying to avoid marketing and not making direct comparisons to the lovely ARP4.

Leaf springs in themselves add roll stiffness by the middle, where the axle is clamped, twisting relative to their ends in roll. This can not be ignored. So total rear roll stiffness with leaf springs is a combination of this twisting stiffness of the leaf springs plus the vertical stiffness of the leaf springs. See, leaf springs are actually quite clever!
So, if the leaf springs are removed and the same rear roll stiffness is required then the wheel rate (stiffness) from the new coil springs needs to be higher than that once provided by the leaf springs, so as to replace the roll stiffness previously provided by the leaf springs twisting. So ride will be harder.

The 5L uses two pairs of links that incorporate rubber bushes plus a Panhard rod. These links are unequal and nonparallel and so together with the Panhard rod they 'fight' in roll, there being too many constraints. (No restriction in bump when both wheels go up/down together.) But as there are rubber bushes in the top links these deform progressively to control roll stiffness, whilst still allowing small single wheel compliance to cope with degraded poor road surfaces. Thus a low initial spring rate (lower than any std Morgan leaf spring) can be used to give good ride, but then to cope with big bumps and heavy loads the springs are also very progressive to stiffen up when loaded. This stiffness progression then usefully also adds to the roll stiffness.
This configuration was developed to achieve good ride from a low initial single wheel rate but with a roll stiffness that significantly builds the harder the car is cornered.
I ran my car with the current spring rate but with prototype rubber bushes that were way too soft and it was nearly undriveable around medium speed corners with so little roll stiffness, like being at sea!
OK there is a lot more to it than this but PaulJ did hint the the 5L was "designed in a different way", and this is some of the background.
ALSO
I too was pleasantly surprised how the steering of cars fitted with 5L seemed to sharpen, as mentioned it must be that the car will respond to the steering input better with the rear axle far better located.
I hope this helps explain some of what was done to develop the 5L over three years with various cars and thousands of miles.
"Each to their own", "horses for courses", "pays yer money and makes yer choice", all as ever. Peter.

This is really interesting.

One question - where is the rear axle roll centre on your setup. Is it in line with the panhard rod or do the leading link pickup points lower it further?

Without a panhard rod, the roll centre on a standard car is rather high and especially so if the car is running neutral or negative camber at the front.

David - the rear instantaneous roll centre for the 5L is about 2 inches up from the lower flange of the chassis, this is maybe 1 inch lower than a leaf sprung car with no Panhard Rod. To get it any lower of course would mean the chassis end of the Panhard being below the chassis, a bit too exposed - or some other arrangement of links to replace a simple Panhard, space is at a premium of course.
ALSO
The suspension links are actually trailing links and do not affect the rear roll centre as the Panhard rod over-rules what those links would provide.
Cheers Peter.

Many thanks Peter!