Quote:
Originally Posted by dogwood
Sorry Mr H.
I'm struggling with this one...
Wouldn't a longer spring wound up to make it stiffer (i.e. shorter)
have the same effect as a heavier one?
I'm happy with the ride height.
It's the bottoming out on humps and bumps I want to stop.
Sorry if I'm being thick..... 
David |
The stiffness is set by the spring rate alone. The length of the spring controls the ride height. Let's ignore the adjustable spring platform for the time being.
Let's continue the example from before- vertical spring with 400lbs sitting on it. The bottom of the spring is fixed to the wheel, and the top to the chassis. Let's suppose that the desire ride height is 5 inches. Let's suppose that it is found that if the spring is compressed to 8 inches then the ride height will be correct.
So the target is to get the spring length to 8 inches. If a 9 inch 400 lb spring is fitted then that will produce the correct height (in this example). If a 10 inch 200lb spring is fitted it will also produce the correct ride height (because it has been compressed by 400lbs, so it is compressed 2 inches).
Now the thing to consider is what hapens if some EXTRA load is imposed, suddenly, such as a bump in the road making the wheel want to go upwards. If that bump was to procude a force of 800lbs (extreme example to make the point), then the 400 lbs sppring would be compressed by 2 inches, but the 200lb spring would be compressed by 4 inches.
From that example we can see that the 400lb spring is indeed stiffer than the 200lb spring. If the ride height is changed using the adjuster then all that is changed is the ride height, the spring itself cannot be made stiffer. It won't even be made shorter, as the car will simply rise as the adjuster is wound upwards (until the shock runs out of travel).
The above example ignore the change in the corner weight that would happen on a car, in other words the weight on that corner would increase as the ajuster is wound up, but as both rear corners are being considered, that's ok.
It's a complicated subject, but the basic principles of springs are straightforward. Mike Capon, who contributes to locostbuilders from time to time, has written a spreadsheet that will enable the determination of the correct spring rate and length if one enters the variables such as the angles dimension and weights. Sorry, I don't have a link to it.