Adjustable Spring Platforms
by Adian Yein Khalid
Spring Sag
When springs are designed and drop figures quoted, the engineers take into account sag after about 2000kms of use. For example the 40mm drop quoted for the Sapura Ledas takes spring sag into account, therefore upon installation actually you'll be looking at a 25-30mm drop, which will settle at 40mm after about a month of normal driving.
Even coil over's from Leda, RG, APM, etc sag after installation, there is nothing to get alarmed about. My Tarmac Rally RGs at 7kgmm took a little longer though, about 2 months, and I had to reset the alignment to cater for the slight change in toe and camber.Spring Pre load
Spring Pre load, is another matter.
Upon installation, the springs may be "preloaded" that is pre-compressed by a few mm. This "preloads" the spring by a certain weights, i.e. if the spring rate is 10kgmm, pre-compressing the spring by 2mm preloads the spring by 20kg.
Thus, if you pre load the all four springs by 20kg, for example, and you weigh 80kg, when you sit in the car, it will not drop down any further, even with your weight sitting in it. (20kg * 4 corners = 80kg). This is because up to 80kg in total has been "pre-loaded" and "pre-stressed" so anything less than 80kg sitting in the car will not alter the ride height at all.
However, if all the springs in the car were only preloaded with 10kg each, and you still weighed 80kg, when you sit in the car, the car will drop by 1mm on all four corners (40kg / 4 corners = 10kg each corner. As the spring rate per corner is 10kgmm, so 10kg per corner drops the car by 1mm).
This of course is a simplistic example, assuming that the weight distribution of 25% per each corner of the car, and that you sit smack in the middle of the car to distribute your 80kg weight evenly.
How is this done?
Adjustable Spring Platforms
Easy. Adjustable spring platforms! This is the original use of adjustable spring platforms, to pre load the springs at different corners to adjust the corner weights, not so much to lower your car! The amount the car is lowered should be dependant on the free-length of the spring, not adjustment of the spring platform.
Why do you want to pre load your springs?
You want to be able to adjust the pre load of your springs so that you can adjust the dynamic weight distribution of the car to equalize on the left/right on the front and left/right at the rear.Assume your Proton Satria has a weight distribution as thus:
You can obviously see that the corner weights are all over the place, and this will lead to a handling imbalanced, with the car being able to take right handers better than left hander (as there's more weight on the right hand side, so less weight will be transferred to the left hand side when turning right).
Also, as the weight bias is diagonal, lifting off and then accelerating again mid-turn will cause the car to be erratic and unstable.
By adjusting the pre load, we can cater for this imbalance. Recall how pre loading a spring by a certain weighted means that any additional weight in the car up to that preloaded weighted will have no effect on the ride height?
So, we could pre load the RF spring by 40kg, to make the 40kg difference from the RF dynamically disappear (actually redistributed to the LF). In the real world, we wouldn't add a true 40kg pre load, because the effect on redistribution of weight is dependant on where the weight actually is. If the extra 40kg is further towards the middle of the car, then we might have to pre load more, and vice-versa.
With our "lowering" adjustable platforms, we can achieve the same result by adjusting the corners which carry a heavier load to have a higher height adjustment, and the corners which carry a lighter load to have a lower setting. The weight will work itself out, and your car will sit flat and have closer Left-Right corner weights.
Hope this has session has been educational. Today's program has been brought to you by the letters F and U, and the number 103,842.
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