Preload vs Spring Rate & Dual Rate Springs

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Preload vs Spring Rate & Dual Rate Springs

#1 Post by john » Sun Dec 28, 2008 1:48 pm

Reprinted with permission from Dix (At Maine Sledders)

I've never been a fan of running a lot of spring preload... if bottoming is an issue (excessive travel) get a spring with a higher rate (assuming your shocks aren't toast)... if the ride's too harsh get a spring with a lower rate (assuming nothing's bent).... it's really that simple... & they're not hard to change... 30 bucks on eBay will get you a motorcycle spring compressor set.

It always amazes me how people have no problem spending thousands on a sled... yet won't want to spend 60 bucks on a set of springs to make it right.

Walk through the pits at any NASCAR Modified event and you'll see the spring adjusters taped to the shock so they can't move from where they have it set when the car is jacked up, because the spring will literally flop around on the shock when they jack it.

I always shoot for zero preload on my setups (or at least as little as possible)... if I have to add much preload at all I'm pulling up BRP's Race Handbook and going through the spring listings to find a higher rate spring.

Or actually in my case, I found the setup that came with my sled a bit too stiff, after talking with Ross at Hygear & getting the spring rates I did the math & picked out a lower rate setup... the rear torsions were crazy stiff... probably the nuts for serious "big air" ditchbangers... but completely wrong for a boondocker... they are actually a set of Polaris springs that are stiffer than anything BRP even offers.

Examples are always good for getting a handle on concepts... so plug in some numbers to get the idea here... the numbers I'm using are just examples to keep the math simple... not actual rates that might be on your sled... also for this example we won't get into "dual" or "progressive" rate springs... just simple straight rate springs.

What amount of force is needed to compress a 100lb/in spring a distance of 5 inches with no preload vs. one that's been preloaded 2 inches vs a 175lb/inch spring with no preload?
(spring rate - preload in inches - force @ 0" in lbs - force @ 5" in lbs)

100 - 0 - 0 - 500

100 - 2 - 200 - 700

175 - 0 - 0 - 875
When you look at the numbers above it's becomes fairly clear that preload is really nothing more than a "crutch" and not the best way to do things... a higher rate spring will actually not only give better bottoming resistance than preloading the original spring... but will give a better ride as well... it's a win-win in most cases.


DUAL-RATE SPRINGS

Dual-Rate springs have become all the rage lately... and with good reason... they work.

Should you ever have to figure out the initial rate of a dual setup (or want to figure it out to create your own) it's really not difficult... here's the formula:
The initial spring rate of a dual-rate system is calculated as the product of the individual spring rates divided by the sum of the individual spring rates, according to the following formula:

Ki = (Km * Ks) / (Km + Ks)

Where:

Ki = Initial Spring Rate (of combined springs)
Km = Spring Rate of Main Spring
Ks = Spring Rate of Secondary Spring
The "Main" spring is the longer of the two, and the "secondary rate" of the spring will be equal to the main spring after the secondary spring has gone into coil bind or has hit it's compression limiter. (OK, so that part's slightly confusing)

But, here it is....

Main Spring = 100lbs
2ndary Spr. = 300lbs

100 x 300 = 30,000
100 + 300 = 400

30,000 / 400 = 75

So we have a spring that has an initial rate of 75lbs/in and a secondary or "final/finish" rate of 100lbs/in.

Raising or lowering the rate of the secondary spring will raise or lower the initial rate while the finish rate stays the same. While raising or lowering the rate of the main spring will raise or lower both rates.

The secondary spring is usually limited in the amount of travel it has in compression buy some method unless its rate is much lower than the main spring... since it's a higher rate spring it will compress less than the main spring for a given amount of force... in a "coilover" type setup like sleds the "spacer" between the springs usually reaches inside the secondary spring so it bottoms out against the spring perch.

The length of this spacer can be adjusted to control the exact "transfer point" between spring rates... in the attached photo you'll see two dual rate front springs, note how much the spacer limits the amount of the secondary spring's travel by reaching inside the spring.

If the secondary spring used is a much lower rate than the primary spring the secondary can simply be allowed to fully compress into "coil bind"... problem with this method is that you can't vary when this happens and the rate transfer point is dictated by the spring rates involved.



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