So I read the entire thread and I already learned a few things just from reading, especially about roll centers. Seems to be a really cool software, thanks for all the effort Bob! I'm a little bit confused though regarding one particular topic and I was hoping you could help me understand it.
I downloaded the trial and started measuring my TLR 22 2.0. When I wanted to measure the front suspension I missed a place to enter the kickup of the front suspension, which in my case would be 25 degrees. I remembered a statement regarding this:
Originally Posted by BobW
It's the angle of the shock relative to the arm that is important. So if the total kickup on the arm is say 20deg and the shock is laid back 20 deg the angle of the shock relative to the arm in the side view is 90 deg and no correction is needed.
At first I was fine with the answer but then I started wondering and to be honest I don't quite understand how the kickup can be irrelevant. If the entire front suspension including the shocks is angled (i.e. the shocks and therefore also the springs are not perpendicular to the ground plane) then the spring rate of any spring will not have the same effect that it would have if it were perpendicular to the ground. From what I remember from my math class the angle of 25 degrees would result in the effective spring rate of the spring mounted in the car (i.e. with kickup) being around 10% less than the nominal spring rate of the spring.
cos alpha = adjacent side / hypotenuse
cos 25 deg. = 0.906
Not sure if that is the right way to calculate it. Maybe the effect is even bigger than 10%. But one way or another, the way I see it, the suspension will be softer with kickup than without.
There is another very interesting thread regarding the topic of suspension geometry and how to properly setup an RC-car. In there Fred talks about the effect of the kickup (amongst other things):
Originally Posted by fredswain
You aren't choosing your shocks oils on the bench. You are systematically determining what to use based on how the car drives. For me when I tune 2WD buggies, the fronts always end up with a heavier weight. Why? It's due to the front end rake. The more rake you have, the more oil weight you need. If we had no front end rake we'd probably end up with the same or near the same weight front to rear.
Think of it this way. A shock that is mounted straight up and down and is getting a purely vertical load exerted on it will be utilized 100%. You are fully using the spring rate and dampening rate. Now let's say we lean that shock over 45 degrees yet keep the vertical load. Suddenly the shock is only 50% effective. Our effective spring rate and dampening rate has decreased. How do we get the effective rate back? We stiffen the spring and increase the oil weight. Remember that the front of a 2WD buggy is laid back anywhere from 20 to 30 degrees thereby lessing the effective dampening ability.
We have 2 things to understand and those are the static dampening and the effective dampening. The static dampening/spring rate is what the shock is sitting on a bench. A 4 lb spring is a 4 lb spring. 30W oil is 30W oil. The effective dampening/spring rate is what it ends up being in the real world. Let's say we install that shock directly in the middle of the a-arm. Let's say the a-arm is 2" long. With the shock being at the mid point suddenly our 4 lb spring behaves like a 2 lb spring due to the leverage the arm has on it. Our 30W oil acts like 15W. What if we lean the top of the shock over 45 degrees? Suddenly our effectively 2 lb spring acts like a 1 lb spring and our effectively 15W oil is now 7.5W. See how this works? Again, please no one get too picky about the examples. They are there to get a point across without getting overly complicated. A 2WD has far more rake than a 4WD and will have stiffer oil as a consequence.
When you drop the car on a bench, hopefully it won't bounce at all! It should just absorb the impact.
(In the above post Fred uses the term "rake" which I believe is the same as "kickup".)
In the above post Fred states that a 45 degree kickup reduces the effective spring rate by 50%. So my above calculation is probably wrong. With Fred's logic, the effective spring rate would be reduced by around 28% and not just 10% in case of 25 degree kickup.
Now I would imagine that one could somehow either model the effect of kickup on the effective spring rate in the suspension geometry. I take it from your comment above that this was not done. Probably not so easy. But I see another possibility, which is to reduce the spring rates of all the front springs which are entered into the software by a certain percentage, depending on the kickup angle (exact formula to be determined, hopefully in this discussion). I believe that would take care of it. But in the video which shows how to use the spring rate calculator, a correction of the spring rate due to kickup is not mentioned.
Is there another way to deal with this...
...or am I totally off? Please help me understand this, thanks!