Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Softer rebound springs

Temptation and idle hands got the better of me and I have followed Sid’s lead above and cut 20mm from the rebound springs of my ’90 Plus 8. My initial thought was to remove sufficient length from the rebound spring so that it just touched the underside of the stub axle with the car static. To this end, having removed the rebound springs I reassembled the suspension, replaced the wheels and let the car stand on all four wheels so that I could measure the length between the underside of the stub axle and the bottom lug. It was immediately obvious that without shortening the main springs the front of the car would sit too high, so Plan B was to remove just 20mm from the rebound springs thus still pre loading the main springs a little bit. I used a cutting disc in the angle grinder to slice through the spring laterally (slowly so as not to produce too much heat) to give a fairly even contact face. After reassembly the car sits about 13 or 14mm higher at the front, not really noticeable.
First impressions are of a significantly less harsh ride on uneven surfaces and possibly a smoother ride on good roads. The steering was already surprisingly light and doesn’t appear to have changed. The proof of the pudding ride wise will be next time my wife is a passenger...
I followed Spanner Juggler’s procedure for removing the rebound springs, very straight forward, thanks Colin. My only addition to the procedure would be to arm yourself with several pairs of disposable gloves and copious amounts of rag before you begin. There was 20 years’ worth of grease packed into and around the springs which transferred itself to hands, tools, floor, overalls and door knobs!
To me the result is well worth the effort.

I have a Rover Plus8.
When she is at rest I can rotate the rebound springs without any struggle, indicationg that they are pretty near the point of transition between tension and free.
On jacking the car they compress under the weight/travel of the wheel/hub.
I do not know the poundage of the springs unfortunately.
This tells me that on going over bumps the upper spring does the work.
The rebound spring does the work on depressions/potholes.
Obviously as the wheels change from these two extremes there would be some interaction, the upper spring expanding to push the wheel back towards rest position and vice versa for rebound conditions. Also, the hub will travel past rest position under influence of either spring. Dependant on the magnitude of the bumps/depressions.
Excess downward travel induced by main spring would impinge on rebound spring and vice versa.
A very dynamic scenario.
The video shows real time spring effects on what appear to be reasonable roads where upward movement of the wheels seems the more common. Very interesting!
The predominant spring is the main/upper.
I am not a suspension expert and am only trying to get a handle on the sliding pillar system based on what I see. Hooke's Law? Yes , covered that in Physics. Other influences like mechanics,friction, motion, momentum etc all add to it so it can become complex. Add in tyre and chassis effects......

Just before my recent trip to Northern California I installed a "MULFAB" spring and thrust bearing in My +8 Bitsa ('98 chassis). I did the same routine. Installed Mulberry Spring with out rebound spring, measure and cut to fit. This is about as good as it gets for Me and My +8 Bitsa. However on very rough roads it was "BANG", "CRASH". On very rough roads "Suplex Kit" was more compliant. As I did not have the ability to switch from one system to the other My comparison of the two systems is rather difficult.

When you cut a spring you increase its stiffness.

When you do not cut the rebound spring the rate is added to the main spring which stiffens the whole system. Suplex gets by this by adjustable ride height on the dust cover.

Button

Yes I agree, you are right.

On my Plus 4, when the car is jacked, the upper spring compress fully the rebound spring. If you cut the rebound spring it implies that in rebound position as you said the stiffness of both springs are added and in this case you could get a stiffer system with less rebound travel when a rebond occurs.

But when the suspension is in normal position or compression with a free rebound spring you get only the stiffness of the upper spring.
The choice of a shorter rebound spring with less stiffness perhaps could seem more appropriate.

I guess that Suplex is a smart answer to solve this problem.

you chaps might be interested in seeing this vid of the front suspension when the car is on the move.

sliding piller video

I concur re thepart about how many springs touch the hub at the same time. Also adding that if both are in tension then they will oppose each other with pre tension pushing against each other. All the videos showing the rebpund springs during road use show the range of motion of the hub, often rising way above the rebound spring. There are also times when the rebound spring appears constantly in compression. It is a shame there is no matching footage of the main upper spring as both need to be viewed to give a clear picture of interaction and how it varies when on smooth, bumpy surfaces and also during cornering.
Without actually checking and going from memory, my main springs are under tension when at rest as I cannot rotate them, unlike the rebounds which can be rotated without much effort.
One thing I notice is when hitting a deep depression in the road(pothole or drain etc) there is a clear thump. I wonder if the rebound spring is then fully compressed and the cross brace/chassis having to absorb the motion. The rebound springs compress a fair bit anyway with the car jacked up so a sudden drop of the wheel in a hole plus the push of the main spring aiding gravity on the wheel will compress the rebound to binding. A case for a bigger pitch in the rebound coils with modified spring rate poundage?

More likely the main spring becomes fully compressed and/or the damper is bottoming out. This is where the additional seemingly light rising rate spring in the heart of the Suplex system comes in to play, it is designed to increase the spring rate as the sliding axle approaches the end of its upward movement. Good and correctly set dampers will also aid a lot in this type of maximum bump event. They also slow down the rebound stroke so the system should not top out too violently. Conversely to what you may expect the Suplex uses a fairly hard rebound spring. This is to reduce body roll in cornering.

Don't forget that as the wheel lifts and the upper spring starts to force pressure on the lower spring the rate rises quite sharply in rebound.