Rear Spring Perch Adjustment

[William2]

Sorry if this has been asked before but I'd like to get my head round it and understand it.
When adjusting the rear spring perches to obtain the correct road going ride height I assume that the spring is being gradually compressed and therefore the ride will be harder the more you increase the ride height. Is this the case?

[nmauduit]

no, as the bottom nut moves up (or down) the spring just follows and is only compressed by the constant approximately quarter weight of the body while you do the adjustment.

To be more precise, when you start adjusting spring individually, you may want to adjust them by approximately the same amount for both rear (or front) wheels to avoid imbalancing too much the weight carried by one wheel vs. the wheel of the other side and having a car not cornering symetrically left and right.

or much better do a corner weight setting (more work and requiring scales, though)

[William2]

If that is the case, why is it the more you screw the nut up the more effort is required i.e. it feels as though the rear springs are being compressed? I also have dampers fitted with limited droop.

[rgh0]

If you screw the seats up far enough until the damper reaches the end of its travel then you start compressing the spring and preloading it so the spring load starts going up above the weight of the car itself

cheers
Rohan

[nmauduit]

If that is the case, why is it the more you screw the nut up the more effort is required i.e. it feels as though the rear springs are being compressed? I also have dampers fitted with limited droop.

if you only want to change the height of the car and do not want to set the corner weights, you should raise each side by the same amount. When you raise only one plateform, you tend to lift the whole car and not just say one quarter of it, a bit like when you raise a table by lifting one leg. Since the "leg" is not rigid but mounted on a spring, the more you lift the more weight it carries (until there is zero weight left on the *other* wheel*s*), that is why the more you screw up the nut the more difficult it gets. When you do the other side it should balance out eventually.

Again, if you don't do corner weights with scales, you should at least accurately set the height of the plateforms so that they are equal for both sides (assuming springs, suspensions geometry and applied body weight etc. were identical to start with), if a wheel has a lot less weight on it it will not stick like the others.

--------edit for clarity (well...) : if one were to raise one corner so high that the car would rest on 3 (or even 2 if balanced) wheels, the weight carried when lifting the (other) wheel(s) would get about constant then. Anyway, that's obviously going too far, so remins hypothetical.

[Grizzly]

The type of Adjustable damper you will be using is not a classic height adjustable..... the classic version looks like the picture below and the height adjuster has nothing to do with the spring perch, to adjust the height you loosen the nut marked c and turn the strut assembly from the base, you don't adjust a-b.

fully-threaded.png (148.21 KiB) Viewed 2041 times

The type used on Elans from what i've seen still require the springs to be roughly the correct length/rate for the ride height you want, as Rohan says you can move the ride height up and down a bit but on a proper height adjustable coil over the a/b platform is solely for setting up the spring preload. On a race car you would want to adjust the corner weight without changing the spring preload.

It depends allot on how well the weight is distributed in the car, if it's distributed well you will get similar preloads side to side so shouldn't be a problem especially on a road car that won't really get used in anger.

[nmauduit]

The type of Adjustable damper you will be using is not a classic height adjustable..... the classic version looks like the picture below and the height adjuster has nothing to do with the spring perch, to adjust the height you loosen the nut marked c and turn the strut assembly from the base, you don't adjust a-b.

fully-threaded.png

The type used on Elans from what i've seen still require the springs to be roughly the correct length/rate for the ride height you want, as Rohan says you can move the ride height up and down a bit but on a proper height adjustable coil over the a/b platform is solely for setting up the spring preload. On a race car you would want to adjust the corner weight without changing the spring preload.

It depends allot on how well the weight is distributed in the car, if it's distributed well you will get similar preloads side to side so shouldn't be a problem especially on a road car that won't really get used in anger.

I don't believe preload change makes a difference once the car is on the road (unless it is huge, exceeding the car weight, but then there would be no suspension any more in one direction) : what matters is the dimension from where the setup rests (on the above photo it looks like the bottom of the C ring (the black ring below being fixed, mechanically stacked on the hub) to the bottom of the spring. That dimension is what you want to keep balanced from side to side if you want to replicate the stock setup (same springs on both sides resting at the same height above the hub).

[Grizzly]

I agree, the positioning of the fuel tank and the difference from full to empty would make more of an impact on a road car than half an inch more preload on one side than the other to get the ride height matching (it's why they get away with it)

I notice it on my race car though, if the preload is different side to side it has different characteristics when you load up the suspension going into left and right handler, i find the car's more neutral if everything side to side is balanced and the corner weights are done well.

[nmauduit]

I agree, the positioning of the fuel tank and the difference from full to empty would make more of an impact on a road car than half an inch more preload on one side than the other to get the ride height matching (it's why they get away with it)

I notice it on my race car though, if the preload is different side to side it has different characteristics when you load up the suspension going into left and right handler, i find the car's more neutral if everything side to side is balanced and the corner weights are done well.

what I meant was that I believe preload makes absolutely no difference in normal use conditions as the spring is always more loaded than just the preload, the only situation when a normal preload will start to be at play is when a wheel is about to get in the air.

[Grizzly]

You could be quite right on a road car (i don't tend to spend any time on mine in general) i wasn't saying the Perch type wouldn't work, i was saying it's not the most ideal way of doing corner weight/ride height/spring loading. Your basically compressing the spring harder on one side to compensate for more weight to make the ride height the same (if you set the corner weight or ride height with the driver and half a tank of fuel imagine the difference in weight side to side on a RHD car) you only get away with it if your setting the ride height to look nice when stationary.

From my experience things only start to get funky when you are transferring weight from side to side at speed, it can feel like you have a softer spring on one side.

[nmauduit]

indeed there are several ways to mess up the load on the wheels unless one does corner weights: one way to look at it is that for 1 single goal (ride height) one has 4 degrees of freedom (individual spring load by way of plateform height). If one wants to obtain say an horizontal car, that only takes out 2 degrees of freedom, so there are still two left to mess things up... To take an extreme exemple, say the rear is too low and one raises only one side so that the 2 adjacent wheels are barely touching the floor : the goal is achieved (car horizontal) but it's almost driving on 2 wheels.

My assumption was that a number of street car owners would rather not spend a day adjusting corner weights, yet would like the benefit of being able to adjust ride height (because of sagging springs or other reasons). In this case the way I would go is to assume identical springs for both sides and set each platform at the same height. If there is a pitch correction to do, adjust both plateforms (say the 2 rear, or the 2 front) of the same amount.

If there is a static roll tilt to be corrected, I would go into corner weights because I believe the difference between the front suspension and the rear suspension (geometry, spring rates) will not make for an easy factor to adjust height on one side only without risking to mess up the load balance on the individual wheels.

[William2]

Gentlemen, thank you for your comprehensive advice. From what you say it sounds like the spring perches sold for Elans will only give you limited ride height adjustment at the expense of stiffening the suspension the more you try and increase the height i.e. compressing the spring. In other words for a road going Elan they are a compromise solution. In my particular case I only went down the adjustable perch avenue because I wanted to fit 5J Minilite alloys. As Chris says, the Rolls Royce solution would be adjustable perches that can alter the length of the rear strut and another set to change the spring length. I have made sure that both rear perches are adjusted to the same position and set the TT dampers fully anti-clockwise to their softest setting. The car does seem to corner well on the road and although the ride is harder than the standard car (as you'd expect) I think I am happy with the trade off. At least the car battery position helps a little bit to distribute the weight at the rear.

[nmauduit]

From what you say it sounds like the spring perches sold for Elans will only give you limited ride height adjustment at the expense of stiffening the suspension the more you try and increase the height i.e. compressing the spring.

no, if what I wrote let you understand that I must have poorly expressed myself. To express it differently, and hopefully more clearly :

1) a spring is a spring, no matter what it rests upon (stock tube or adjustable perch). It won't get harsher or softer when you change it from one setup to the other. And changing ride height won't change the spring rate either (so if you have adjustable perches and raise your car by say half an inch, the suspension will be exactly the same, not softer, not harsher).

2) adjustable dampers is an other, independant setting. It should not affect body height (unless there is a spring like effect in a particular damper, like N2 pressure in a gaz filled damper)

3) ride feel is affected by both spring rate and damper setting. I understand there are different schools of thought for adjusting that combination. What I go for is reasoning in term of resonant frequency (conversely optimum dampening time for a given deviation), which entails harsher damping for harder springs.

I'm glad your current setting is satisfactory. The Elan is a well balanced car to begin with, so a setting with equal perch height side to side should not imbalance the car from a stock setting. If your car feels harsher than stock and you have stock springs, my bet is that the dampers are a bit harsher than stock dampers (i.e. perches setting has nothing to do with it).

keep it on the road !

[Grizzly]

William2, Indeed... it depends on what goal your trying to achieve, looking level at a show or out and out performance.

I'm not trying to argue only share what i've picked up from the experts who set my race car up, manufactures always tend to set their cars level with no one in it to stop customers querying it being different side to side, but for performance you need to set it up with a driver and fuel on board which is where adjustable ride height, pre-load and corner weight etc come in..

[Grizzly]

At least the car battery position helps a little bit to distribute the weight at the rear.

This is true, it's a shame they put the tank at one side though..... bit daft imho having some thing that changes it's weight so dramatically not in the center of the car and behind the back wheels.... but then i guess space designated where they fitted it.