Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Roadster 5 link rear suspension re-visited

Following on from my post above, have just ordered the front RS kit from Dan who already has them in the post to me. He did a deal minus shocks and steering bearings as I have them already.

Reminder. The initial harshness of the original front suspension setup is due to the rebound spring and main spring being compressed together at ride height, their stiffnesses add together resulting in a very stiff initial spring rate. That is what causes the nose of the cars to bob up and down, that is the car bouncing on the front tyres at around 3 Hz.
To get rid of that initial stiffness and resulting harshness to thus get the front suspension to actually work the rebound spring must be only just kissing the slider/hub at ride height, you must be able to turn it by hand. This requires a shorter main spring and/or a shorter rebound spring or go for a main spring supported on adjustable spring seats.
Declared interest - Director of SSL, product range includes the SSL 5L rear suspension, SSL RS front suspension and alterative springs and dampers.
Regards Peter J Ballard. http://www.suspensionsupplies.co.uk/

What Peter says re. the rebound spring compressing the main spring, on the front suspension, makes perfect sense. In fact I can't help but wonder why MMC decided to put the two springs into contention with each other in the first place, or maybe it wasn't an actual design decision, rather something that just happened?

I had an extended ride across Salisbury plain in Peter Ballard car, "Molly", now in the ownership of my pal Ian.
With SSL's front and rear ends the ride was remarkable: no "Morgan nod", no crashing in and out or ruts: just composed.
Ian said that other than a slight tendency to follow longitudinal ruts, such as where a cable tench has been laid and badly repaired, it is remarkably good.

I had a long chat with Peter about the suspension on my Plus 8, which he and Dan drove extensively, in early 2014 and have some very good pointers as to the best way to refine the ride, without harming the roadholding. It seems my car had Koni/Erbach suspension later cars have non adjustable SPAX coil-over units. As the suspension is now 41000 miles run and upgrade is due, Peter agreed to recommend SPAX adjustable units, probably with progressive springs.

The service this November will be costly as the front upright road joints also need replacing.....

Still cheaper than a new Plus Six!

Ironic that in preparation to fitting my SSL RS kit I measured the length of rebound springs yesterday as best I could static with Wolf gaiters on. I then realised I could turn the nearside one just. When I checked the offside not only could I turn it easily, but found 2-3mm of vertical movement. This despite have Wolf lower steering bearings which take up an extra 3mm! I guess the main springs have settled although only 8000 miles covered. Maybe as I drive mainly solo it explains the difference as well.

This discovery threw out the window the premise that compressed rebound springs accounted for harsh front ride. Nevertheless expect a comfort improvement when kit is fitted.

Thanks to you all adding your observations for us all to share.
Additional tech. for those new to the Traditional Morgan front suspension...….

1. If one has "zero droop" on a suspension, rather than allowing droop as well as bump from static ride height, the roll stiffness is doubled. But zero droop causes the occasional knock as the suspension tries to droop after a compression. Thus some compliance in droop softens that, hence a rebound spring.

2. By controlling droop the front instantaneous roll centre moves from below ground to ground level. That lifting of the roll centre, reduces the 'roll moment' and thus increases roll stiffness.

3. If you have a gap between the slider/hub and the rebound spring you loose that initial high roll stiffness. If you have the rebound spring compressed you not only get harsh initial ride but the roll centre moves as you enter a corner so the balance of the car changes, not nice. Read http://www.britishv8.org/Articles/Morgan-Plus-8-Road-Test.htm "...……..and by the front-end suddenly going softish.." That is the slider/hub coming off the rebound spring! You need a main spring that can be preloaded to balance the corner weight and set the rebound spring contact.

4. MMC do know the effect of the rebound spring and do try to get the main springs the right length but there are so many weight variants they prefer the compressed rebound spring option, rather than a gap - playing on the safe side. But of course Morgan supply the RS as an option on new builds anyway.

5. Morgan have relied for decades on that high initial spring rate to get the car to turn in quickly by loading the outside front. However also for decades all other cars (saloon & sports) have softened the front suspension springs to get better ride comfort but then have increased the low velocity bump force provided by the damper to get turn in. Many Morgan dampers on production cars have very little bump damping so still rely on the high initial spring rate with compressed rebound spring, but then having to accept consequential the poor ride. However when one goes for a softer initial spring rate one must add low velocity bump force in the damper. So not any old damper off the shelf but a damper valved to give that specific required low velocity force to get the car to turn in but then the valves must open to give good ride - at the same time! This is what damper tuning is so much about. Fit a damper designed for a different spring rate and car weight and it will not be optimum degrading both ride and handling.

6. Many other makes of cars are now using dampers with internal rebound springs to give increased roll stiffness and better ride BUT with much smaller anti roll bars. The current Merc E Class is the best example. The first dozen or so M3W (& the demo cars) is another example with the SUPLEX coil overs, that is before production went to SPAX without any internal rebound springs and hence more roll. The early 5 door 'Mini's (also the BMW X3 etc) are among the worst; come off a full width speed bump and the suspension feels quite soft, drive along a country lane with undulations on the near side and your head is continuously nodded side to side, uncomfortable - ARB 'too' big! BMW are too going to internal damper rebound springs - Morgan were first!

A lot of ride and handling is indeed subjective, but there is a book full of 'rules of thumb' that gets one quite close to the required ride and handling characteristics before the detailed spring and damper tuning to suit the character of the car and the customers.

If you are still awake, thanks for your attention. Regards Peter.
Declared interest Director SSL.
Current SSL suspension projects include; Porsche 356C & 911S, Jensen CV8, FIA ACCobras, FIA MGB, off road rapid response 4x4 truck etc. as well as Morgans of course for road and track.

So for us that think We understand Morgan Suspension, do I source Front Shocks with adjustments for both Bump and ????? - Both Up and Down?

You can source dampers with one two or three (or more) adjustments.
1. Low speed bump up to around 2" per second that controls turn in, control over hard braking, long undulations etc.
2. High speed bump above 2" per second that is the ability to absorb road surface irregularities such as a very coarser surface.
3. Rebound.
OR set them in ratios.
Typically the ratio of low speed bump to rebound is around 1:3 for a road car to minimise bump forces but control oscillations, or 1:2 for a sports car to create more ability to turn in, or 1:1 for a race car where turn in rules over comfort.
The high velocity bump should be as close to flat as possible (so force not increasing much with velocity), some trick dampers can actually reduce force over say 2" per second.
The 2" per second is typical for a road car, maybe up to 3" or 4" per second for a race car as you hit bumps faster so the damper velocity is higher.

For critical damping so any oscillation dies in one cycle the Force:Velocity slope is 2xsqrt(mass x spring rate). For a road car use 30% critical, a sports car 70% and for a race car around 100%.

Make the suspension adjustable and they will adjust it wrong -- look what they can do to a Weber carburetor in just a few moments of stupidity with a screwdriver.

Sorry I hit send too early...……...
Then you need to consider damper types: twin tubes gas pressurised or atmospheric, monotubes gas pressurised or atmospheric & specials where the damping is also dependent on stroke etc. They all have different responses with respect to rate of change of velocity, ie how fast they give force for a velocity input. For a road car you want them to respond relatively slowly to give good ride over rough surfaces but for a race car respond quickly as you are going faster. Difficulty is to make a responsive damper like a gas pressurised monotube Bilstein give good ride but then work well when driven hard - it is done!
Any of the above damper types can give the same force;vel curves, but it is the transients and change in characteristic with heat that are the differences. Pays your money, takes your choice and do the road testing for the spring rate and car weight in mind. Hours of fun and many hundreds/thousands of miles of testing.

Cheers Peter.
SSL Director.

Following Peter's convincing arguments ended up ordering the matching Spax adjustables. Finished the fit late this afternoon with just enough time for a short test drive. Still need to tune them to the worst of Norfolk roads but gratifying to find in the short run that front now matches the back in ironing out most bumps. Car seems more chuckable as well with super sensitive steering.

Needless to say the second side was stripped and rebuilt in half the time of first although I did release spring tension with a jack. Would suggest it's ideally a two man job to replace using same method though. Bizarre that I did find a kingpin bump stop on second (offside). Left it out of rebuild for symmetry as definately not present on other side. Bump stop's on dampers with RS kit anyway.

Anyone want a pair of front adjustable AVO's, only 6000 miles from new