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Old 23rd January 2011, 20:21   #16
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Default Re: Spring spacer & Spring rate question

Got a nice explanation as to whats happening with the addition of the spacer and why it made the rear end bouncy in my car. This is what i wanted to know.

Coils spring rates are a function of the (unwound) length of the spring and its thickness. A longer spring of the same thickness will be softer, a spring of the same length with a thicker gauge will be stiffer.
On a SWB Zuk the rear springs are rated at 137lbs per inch, that means that it takes 137lbs of force to compress the spring 1".
On the front the spring is around 550lbs but because its mounted on the wishbone between the pivot point and the wheel the wheel has more leverage on the spring, the equivalent spring rate at the wheel is 137lbs, same as the rear.
When you remove a spring you will notice that it is partly compressed with the shock/strut at full extension, this compression is known as preload. If the rear spring is compressed one inch with the rear shock at full extension then the spring is exerting 137lbs of force against the shock (137lbs of preload). In order to compress the suspension a load of greater than 137lbs must be applied to overcome this preload.
If you fit 2" spring spacers without longer shocks or shock spacers then there will be 3" of preload so a force of (137 * 3) 411lbs is required before the suspension will begin to compress. This is why you get a harsh ride, the vehicle becomes skittish because it bounces off small bumps.
The amount of preload does not affect the spring rate, onve the preload is overcome it will still compress 1" for every additional 137lbs.
Another thing to note is that on a lifted Zuk the roll centre is higher which applies more leverage on the spring during cornering and acceleration/deceleration which gives a similar effect to a softer spring. Most lift kits come with stiffer springs to minimise the extra roll.
The other problem with running a lift kit without spacers or longer shocks/struts is that the suspension is almost fully extended, during cornering the suspension cannot extend any further and the tyre comes off the ground - not safe at all!

Learned: 1) the spacer doesn't change the spring rate of a linear spring, but may take out the soft part of a progressive spring due to pre-loading 2) spacer increases the pre-load which is what i was experiencing. Suspension preloaded and not enough weight.

This is like the same thing we have on our motorcycles which comes with a pre-load adjustable rear shockers which has a notched spring seat.

And how in the hell did the question i posted here in this thread end up here at Auto-India.co.in? Spring spacer & Spring rate question
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