And a lathe and lights! Congrats on some much needed upgrades Reid.
Ha ha! I was using the lathe in Thads shop
. It took about 45 minutes to make a reducer sleeve on his lathe. That same sleeve I could have made in my shop using the Mac Gyver method, but it would have taken 3-4 hours minimum.
I shared some pretty useful shock mounting data and formulas on another thread. I'll add it here for those who did not see it. It outlines the fundamental process I use when designing shock lengths and mounting criteria for our cars.
Originally Posted by
Glamisfan 
Reid, how do you figure the motion ratio? You state that 18" front travel with a 13.5" shock = .75 I divide 18 by 13.5 which = 1.33
A shock motion ratio is quantified as a
percentage of shock stroke relative to vertical wheel travel. In the example above, 13.5/18=.75, 0r 75%. The shock will compress 3/4" (75% of 1") for every vertical inch the wheel bumps. If you reverse the equation above as you have, the product is expressed as a mechanical leverage. That equation is good when working with fulcrums and lever arms.
For example, my front top a arm has a 25.3" long pivot axis (center of inner pivot to center of outer uniball/ball joint), while the shock is mounted 22.1" from the inner pivot. 25.3/22.1=1.14. Expressed as a ratio, it is1.14:1, which means that a vertical load of 1000 lbs. applied at the centerline of the uniball, will exert a force of 1,140 lbs. (1,000 x 1.14=1,140) at the shock/spring. The motion ratio comes in when the shock is not cycling at the same rate that the wheel is traveling.
The Popo's are a great example of this problem. Not only is there a mechanical leverage acting upon the coil over due to its mounting location on the a arm, there is also an inefficiency factor of the spring rate vs. wheel rate as a result of the shock being positioned at an angle relative to the pivot axis of the a arm. It is expressed as a product resulting from the SIN of the angle (angle being the angle the shock is positioned relative to the pivot axis of the arm). On the XP's utilizing stock shock mounting, these operating angles can be as low as 35 degrees. Look at most any RZR XP from the front, and the operating angle of the shock is painfully obvious. Most builders maintain the stock shock mounting locations so there are plenty of options available to the consumer regarding brands of shocks that will fit their car. Then, some offer extended arms, and motion ratios go through the roof as a result!
So, with all that being said, lets calculate just how much builders are asking of the coilovers they squeeze into their builds.
Let's throw out some hypotrhetical numbers. Lets say the front axle weight of a car is 850 lbs. Divide it by 2, and we have 425 lbs. weighing down onto one of the front tires. We'll use a constant of 5 g's, which quantifies the maximum expected impact load the front end might see during a race. So, 425 lbs. x 5 times the force of gravity = 2,125 lbs. That is the amount of energy, expressed in pounds, that one of the coil over shocks needs to resist. Keep in mind, that is a
wheel rate,
not a
spring rate. Assuming a linear progression, 2,125 lbs. / 18" of wheel travel = 118 lbs per inch. So, if the car had a live axle front end, and the spring/shock was mounted vertical, the spring rate would be equal to the wheel rate (a motion ratio of 1), and the spring would have to be rated at 118 lbs/inch. But, lets factor in the actual mounting position of the coil over, and see how much more spring we will need to resist our projected maximum impact load of 2,125 lbs. per corner (wheel):
The figures in parenthesis are approximate representations of the stock location and stroke shock.The other figures are approximations of what I am running on our new build.
Using approximate numbers from our new build, as outlined above, we take the mechanical leverage into account at 1.14 x 2,125 = 2,423 lbs.
(1.53 x 2,125 = 3,251 lbs.)) Then, the inefficiency of work being performed through an angle (average coil over to pivot axis angle is 48 degrees on our car,
35 degrees stock), which is 48 sin = .75. Divide that into one, and we have a 1.33 disadvantage.
(35 sin = .58. 1/.58 = 1.72 disadvantage)
So, we are needing 1.33 times
(1.72 times)more spring to resist our impact. SO, multiply the original calculated spring rate of 118 lbs. per inch by the mechanical leverage as a result of the mounting location of the coilover of 1.14:1, and we get 134.5
(118 x 1.53 = 180) Times that by the inefficiency of the operating angle, which was 1.33
(1.72), and we get a coil spring rate of 178.9 lbs per inch
(310 lbs./inch)! Divide 178 by 118, and that is a whopping 150% more work we are asking the coil spring to do
(divide 310 by 118 = 262%). So, we have a wheel rate of 118 pounds, and a spring rate of 178 pounds for our set up,
(and a spring rate of 310 lbs. for a stock shock setup). These numbers are using my front end as a model. If I used a stock shock mounting location
(and I did. In red), the inefficiency quotient would be far above double the work that the shock needs to perform because a fabricator was lazy. And people wonder why the Monster Mav cleaned house as heavy as it was. TYhe above example is what makes the Mav "BADASS"
Thanks to this lesson, I get to go back out and work til 2 a.m this morning, but it was worth it to educate the laymans, and even the experienced fabricator. Maybe we'll start seeing shock angles decrease as a result of this lesson, but I doubt it. Good. Another victory lies ahead for us then
!!
