How to determin ideal wpring rates

BlueCoyote

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Did the search, bought the books by Herb Adams and Carrol Smith. Did lots of reading on the subject, but have yet to find a way to determine the ideal spring rates. Basically all I found was "use soft as possible" and " as long as the springs are stiff enought to prevent bottoming out, they are adequate..."
But since I do not have a big R&D budget, need a place to start. Have several non-OEM springs set options from various vehciles that will phsicaly fit, all with differing rates.
Application is a 1981 Toyota 4x4.
Any thoughts or suggestions?


Who are you calling Coyote ugly?
84 Toyota p/u Rokrawlr
86 4rnr
80 Toyota MDR Project
 

Kritter

Krittro Campbell
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Re: How to determin ideal spring rates

Mx" + Cx' + Kx = F(t) from that DE you can find k which is the ideal spring rate for whatever forcing function you subject your system to...a set of whoops could be synonymous with a sine function and so on. M is your mass, x" is acceleration, C is damping, x' is velocity (speed in your case), k spring rate, x distance, F(t) forcing function. Write a program in excel that models your system and lets you tinker with the variables and see what you come up with. I guess there is no "ideal spring rate" but if you want to shoot for one or a range, that would be the way to do it in few minutes without getting dirty. Pick up a book on vibs or mechanical systems and it will go into great detail on how to use the above DE.

Kris
 

Billy_the_Kid

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Re: How to determin ideal spring rates

Wow, my method isn't quit as mathematical as Kritter's, but it always got us close.

I say, if you're gonna spend a bunch of money on bypass shocks, then put 'em to work. My philosophy was always to let the shocks do all the work. We always put just enough spring in the car/truck/buggy to get it at ride height with the shocks removed. Then you can work with the shocks for the ride to keep it from bottoming out. And also gain a little ride height with shock pressure.

I've heard both sides of this argument (let the springs do all the work and use the shock just to control it) but in my opinion that only works if you have progressive spring packs/rates so that it works in soft as well as heavy impacts.

Bill Schmitt
 

1992f150

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Re: How to determin ideal spring rates

swayaways website tells how to figure out spring rate.

Azusa: shame of the foothills
 

BlueCoyote

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Re: How to determin ideal spring rates-starting point

Thanks All...

If i understand the SwayAway formulas, for a solid axle leaf spring vehicle the only formula to use would be for Wheel Rate. (SInce there is no motion ratio or Angle Correction Factor).

WR=(Sprung Weight/0.4* Wheel Travel)

If I understand my math correctly the formula indicates that for a given spring weight (vehicle corner weight), the rate should be adequate to support that weight at 40% of wheel travel (the 60% compression and 40 % droop theory).

So if the is the case should I build a spring to have a theoretical rate to suspend the vehicle using the 60 / 40 rule. Does this sound correct?

Who are you calling Coyote ugly?
84 Toyota p/u Rokrawlr
86 4rnr
80 Toyota MDR Project
 

partybarge_pilot

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Re: How to determin ideal spring rates-starting point

Wiegh the front of your truck, divide by 2, minus unsprung wieght. Take this figure and divide it by the disired amount of sag in inches and it will give you your spring rate with a theoretical zero preload at full droop. On a stiaght axle toy your probibly going to be in the 150# range.
 
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