Rear shock absorbers

[Hans Efde]

I think mine are around 100lb, it drives fine on those but does bottom out occasionally. That said, I ain't the lightest and the boot is usually full of crap.

I just did the maths on mine. It is 2000N/110mm = 18.18 N/mm. According to the web this translates to 104 pound/inch, so pretty close to Riches springs. Mine don't bottom out, but I do have rubber donuts on the shock absorber on the chromed part (shaft?) inside the spring. Also I have installed the upper bump stops as discussed earlier on this forum. It is the perfect setup.

[Alan D.]

Thanks for the advice guys. I've wound off the little bit of damping which I had on the shock absorber. It has improved it, but it it still not perfect. I'm going to reduce the ride height by winding down the spring. At the moment the spring is compressed about 1.5 inches or 38 mm. so with a 75 pound per inch spring rate, I need about 100 pounds force to make an initial movement on the spring. If my logic is correct! So by winding an inch (25.4 mm)of the height setting, I should reduce the initial force required to move the spring to about 25 pounds.

The front is set quite high at the moment, so I will need to reduce that height correspondingly. But I've lent my suspension pump to my brother in law and he is about 150 miles from me. So it will be a couple of weeks before I retrieve the pump. I'm going to the new Manchester Classic car show later in the month, so I'll call in then and collect the pump.

Alan

[Stuart]

Alan it's best to start with the damping on the lowest setting first and work your way up slowly, although you may find that wound right down is actually best.

[Rich]

From memory my dampers are on or close to minimum settings.

[Geoff Butcher]

Moving the spring platforms has no effect on the spring rate. All it does is alter the ride height!

[Alan D.]

Moving the spring platforms has no effect on the spring rate. All it does is alter the ride height!

Even if the spring is compressed?
If you had a 100 pound spring, it takes 100 pound to move the first inch, then another 100 pounds to move the second inch, so that's 200 load altogether to compress the spring 2 inches (50.8 mm)

Where am I going wrong?

Alan

[DavidL]

Moving the spring platforms has no effect on the spring rate. All it does is alter the ride height!

Even if the spring is compressed?
If you had a 100 pound spring, it takes 100 pound to move the first inch, then another 100 pounds to move the second inch, so that's 200 load altogether to compress the spring 2 inches (50.8 mm)

Where am I going wrong?

Alan

I can't explain it terribly well, but I'll have a go at saying how I see it. This may be a flawed point of view, I'd welcome comments from those who know better.

Measure the length of the spring when it's on the car. Measure the maximum distance between the spring platforms.
The difference between the two is the range of adjustment you have on the platforms over which you are only adjusting the ride height. However, once you go beyond this range, any further adjustment will not result in an increase in ride height since you've reached the upper extent of travel on the shock absorber.

[DavidL]

Moving the spring platforms has no effect on the spring rate. All it does is alter the ride height!

Even if the spring is compressed?
If you had a 100 pound spring, it takes 100 pound to move the first inch, then another 100 pounds to move the second inch, so that's 200 load altogether to compress the spring 2 inches (50.8 mm)

Where am I going wrong?

Alan

I can't explain it terribly well, but I'll have a go at saying how I see it. This may be a flawed point of view, I'd welcome comments from those who know better.

Measure the length of the spring when it's on the car. Measure the maximum distance between the spring platforms.
The difference between the two is the range of adjustment you have on the platforms over which you are only adjusting the ride height. However, once you go beyond this range, any further adjustment will not result in an increase in ride height since you've reached the upper extent of travel on the shock absorber.

I should have said, "measure the distance between the two spring platforms with the shock absorber off the car, fully extended".

[ACourtney]

The adjustable platforms can be used to raise or lower the car and whilst this won't affect the spring rate, raising the car by pre-loading the spring will affect the effective wheel rate.

If you set the platforms so that they just touch the ends of the spring unladen then this will be the "natural" ride height. So with a spring rate of 100lbs/in, then the spring will deflect by 1in for every 100lbs of weight at that corner. In fact the corner weight of a Midas rear end is likely to be somewhere between 200 and 300lbs, so to keep the maths simple we'll say 200lbs and a 2inch static deflection.

If you want to lower the car, then adjust the platforms further apart. The corner weight is unchanged (well not noticeably) so you still have 2in of static deflection. The downside of this is that you are using up damper travel, increasing the likelyhood of the damper bottoming out. Also there is a risk of the spring becoming unseated during droop, should you become airborne, as it is no longer held by the platforms when the damper is fully extended. This can be dealt with by limiting the droop travel with a check strap, or tie-wrapping the end of the spring to the seat.

If you want to raise the car then you adjust the platforms the other way, pre-loading the spring. So if you preload the spring with 2inches of compression you will now find that the body sits 2 inches higher, but you will now have no droop travel in the rear suspension as the damper is held at the outer end of its travel. If you pre-load it further you won't gain any more ride height as the damper can't get any longer, but the wheel rate looks higher: if you have 3 inches of precompression the spring is pre-loaded to 300lbs, so it is not going to compress any further until the total corner load exceeds 300lbs. Consequently the wheel is not going to move with respect to the body when you hit small bumps and the back end becomes bouncy. This is best avoided on a road car, although it is commonly used on track cars where they want to keep the ride height constant for underbody aerodynamics. If you need to raise the rear, then it might be better to put thicker packers between the top the damper and the bodyshell.

[Alan D.]

Thank you David and Alistair,

You have both confirmed what I already thought. I have some 60 pound helper springs, that are 2 inches long. If I can get time this week I'm going to try and fit them. The extra 2 inches in length will allow me to wind down the spring platform. I can then insert the helper spring and maintain my existing ride height. so I will have a 12 inch long main spring @ 75 pounds per inch and a 2 inch long helper spring @ 60 pounds per inch. This should reduce my overall spring rate, which in turn will give me compliance on the small bumps. The only problem I can foresee, is that if I cannot get the platform low enough, the the 75 pound spring will crush the 60 pound spring!
I'd already bought the helper springs last week, as this was the train of thought I was pursuing, then doubt sets in! (old age)

I've used helper springs before. I had 0 pound helper springs, as this takes up the slack on full droop and stops the spring unseating. I think this is possibly a better solution than a check strap or tie wraps.

I will report back, when I get them fitted.

Thanks to all for the advice.





Alan