Rear shock absorbers

[Stuart]

If you have two 60lb/in springs in line and apply a 60lb load, both will deflect by 1 inch.
That means your 60lb load has produced a total deflection of 2 inches - so your resulting spring rate is 60lb/2inches = 30 lb/in.

Yes got it now thanks

This I think is because the spring is just supporting the body, which is relatively light. the radius arm below the spring is unsprung weight. So I don't think we need monster springs on the back of the Midas

That's a good point Alan, and something I'd not taken into consideration. I'm still amazed that there is no static deflection, can the load presented to the spring really be lower than 150 lbs ?

[ACourtney]

First of all, thank you for all of your help. I've only speed read the post from Geoff. and Alistair. I'll read them more thoroughly tonight.

These are my thoughts from early in the morning.

If the 75 pound spring is fitted to the coilover with no pre-load and then fitted to the car. The car is then lowered to the ground with the full weight of the body on the spring, there is no deflection of the spring. This I think is because the spring is just supporting the body, which is relatively light. the radius arm below the spring is unsprung weight. So I don't think we need monster springs on the back of the Midas. When I sit in the car, there is a small amount of deflection. So we don't have 600 pound loads on the spring. The bathroom scales would measure the total weight on the footprint of the tire, but is that relevant to the whole spring loading?

Please feel free to correct me, I wish I stuck in at school!

Going up to the garages to start tidying up. I've not done any work up there for about two months. So time to make a start.

Alan

I would be surprised if there was no deflection in the spring when the car is lowered to ground, it would be interesting to know how much deflection there is. You may want to check both sides as one may be taking more load than the other indicating a static imbalance - which is normal as you really want the corners balanced for your most common driving mode e.g. with you sat in the driver's seat.

The weight at the tyre footprint should relate directly to the load on the spring, since the coil-over at the back of the Midas acts at the centre of the rear wheel axis. Again the load will probably vary from side to side and the ideal is to have them equal when you are in the car. So you may need a helper to read the scales.

[Geoff Butcher]

Surely it's impossible for the springs not to deflect at all with the car's weight resting on them? Either the suspension has seized or there's a pile of bricks under the floor...

[ACourtney]

Surely it's impossible for the springs not to deflect at all with the car's weight resting on them? Either the suspension has seized or there's a pile of bricks under the floor...

If the coils are pre-loaded - pre-compressed by the spring seats - then the weight of the car has to overcome that preload before they deflect more.

That makes me wonder if Alan is flattening the helper springs? If so then that would be around 120lb pre-load - 2inches at 60lb/in.

[Alan D.]

Hi thanks again for the comments, I've still not read all of the posts properly as yet. But just clarify the current situation, no helper springs are fitted at the moment. I have a 12 inch long, 2.25 inch i.d., 75 pound per inch spring fitted, which is compressed about 1.5 inches.

The question being: does the compressed portion of the spring act as a load, do I need to exert 112.5 pounds (75 pound spring rate X 1.5 inches distance compressed = 112.5 pounds preload) of pressure before the spring deflects?

Well at least I'm getting all of your brains working.

Sorry to our continental cousins for doing all of the measurements in imperial units.

Eventually I'll have to go through all of this again for the front suspension!

Alan

[ACourtney]

The question being: does the compressed portion of the spring act as a load, do I need to exert 112.5 pounds (75 pound spring rate X 1.5 inches distance compressed = 112.5 pounds preload) of pressure before the spring deflects?

The answer is yes, the compression in the spring is the pre-load ~112.5lbs. The spring will not compress any further until the load acting on the top of the coil-over (corner weight) exceeds that figure.



This thread has prompted a few questions, so as I was in my workshop this afternoon I decided to take some measurements from the various springs and dampers that I could lay my hands on.
I didn't have the tools to measure any spring rates, but of the six springs that are either off cars being rebuilt or second hand ones from stock, the were all about 325mm - 12.75in long

A new GAZ adjustable damper, with adjustable spring seats, had a maximum spring height of 12 inches. So any of my stock springs would have to start with 3/4" of pre-compression.

An AVO adjustable with fixed spring seats also measured 12 inches between faces.

A Spax adjustable, again with fixed spring seats, measured 11 inches between faces. So that would start with 1 3/4" of pre-compression.

All three dampers measured 15.25 inches, when fully extended, from the bottom bolt centre to the top face of the mounting washer. With the mounting rubber in place this would become 15.5 inches.


I also measured the travel to the point of contact with the bump-stop:

The largest travel came from the AVO at 4 inches.

The GAZ damper had 3.5inches of travel. This had a longer bump-stop, which looked more progressive, so it probably allows another 1/4 inch or so of travel of gently restrained travel.

The SPAX damper had just 3 inches of travel and the bump-stop looked more solid and less progressive than the AVO.


I was also concerned to see that people have experienced corrosion quite early on with the GAZ dampers. When I next meet up with Dave from Dampertech I shall try to discuss this point. I can see that there will be a limit to how thick the plating can be when the body has to have a screw thread that allows the spring seat to move freely and the action of the seat being wound up and down could wear through the plating at its thinnest points. However, it should be possible to passivate it without affecting the tolerances and the passivation will also help protect against wear. Failing that it would be worth investigating a different plating method.

[Stuart]

What's the spring rate of a 150lb spring and a 40lb spring? obviously until the weaker spring is coil bound.

The maths has frazzled my poorly functioning brain.

[ACourtney]

What's the spring rate of a 150lb spring and a 40lb spring? obviously until the weaker spring is coil bound.

The maths has frazzled my poorly functioning brain.

Yes once the weaker spring is coil bound the stiffer spring rate takes over, but up to that point the combined effect is actually softer than the weaker spring alone:

A 20lb load will compress the 40lb spring (more correctly 40lb/in) by 1/2in. However, add the 150lb/in spring above it and you get an additional movement of 20/150 inches, so the resultant movement is 0.6333"
So that gives a spring rate of 20/0.6333, or 31.58lb/in

[Stuart]

I guess 31.58lb/in isn't going to be much use.

I'm a bit older these days so perhaps I won't like 150lb/in. I'll probably try 140lb/in, but I'm a long long way off trying them on the road.

[ACourtney]

A weak helper spring is really only intended to stop the main spring becoming loose in droop. However, the figures I found above indicated that the dampers had maximum spring height at 12" (GAZ and AVO) and 11" (Spax) so if the spring is longer than 12" it is going to have to be pre-compressed, so a helper spring is not required.