norocksplease
October 30th, 2007, 14:22
I posted this over on Glamisdunes.com. Didn't get much response.
I was also curious to know if the Motion Ratio work based on the A to B distance between the shock bolt and the one on the fame. Or, on a "gull-wing" or an arm that has the lower shock mount lower than the linear path the arm follows to the spindle from the frame, is it the point on the line above the bolt? Or directly to the bolt that we use for that distance?
Here is a picture to clarify
http://i88.photobucket.com/albums/k171/norocksplease/arms1.jpg
There are 2 paths here...so how does this work?
Below is a piece Chopshop posted recently that KingGlamis wrote. THANKS KG!!!
I do have a few questions though, as there are more variables here. And with new technology, we have more things to be concerned with, dual rate is no longer the biggest and baddest.
So here I go:
FIRST: I'd like to ask, for the Angle Correction factor, is this angle at full extention? Full compression? It would seem that if you changed the preload or spring rate, the ride height (and therefore the angle) would change. So where do we measure this at?
SECOND: This assumes the mounting point is on the lower arm. What about the upper arm? Rhinos and 4wd trucks have the shocks mounted on the upper arm. Would we measure any differently? Or would the same measurement for the shock mount work.
THIRD: What about for solid axles?
THIRD PART B: What about leafs on solid axles?
FOURTH: How about Single rate springs?
FOURTH B: And triple rates? I've seen a few rails with King shocks and I believe I'm seeing a 3rd (very small) spring mounted up top.
And now for a whole different section. How about shock valving? How does one account for this in a mathematical manner? Or do you?
Here is a piece by KingGlamis explaining shock tuning. This is where my questions stem from.
One thing that baffles me is how many cars seem to have the improper spring rate on the shocks. Most of the time it's too stiff. I don't understand why this happens so often and this question comes up here on the board quite often. The formulas for figuring this stuff out are available on the internet, yet still many customers seem to be unhappy with how stiff their car came from their builder. I was just at a friend's house today trying to get his car to ride better and the springs are way off. But... the place that weighed his car either wrote down the numbers wrong or their scales were way off. So I'm going to have to get his car weighed again to get it right. Anyway, while helping him and showing him the math involved, I thought to my self, "Self, this would be good info to share with the fine folks at GD.com." So... here it is folks, I tried to write it in real-world-speak to make it as easy as possible. I know it's long but if you follow it step by step you can once-and-for-all get the right spring setup on your car. And yes, you might have to revalve the shocks after changing springs, but that is a totally different topic for another time.
So here it is, as written by yours truly, KingGlamis.
First off, a lot of people have the misconception that you find your combined spring rate in a dual-rate coil-over setup by adding the two spring rates (rated in pounds-per-inch) and dividing by two. This is not at all how you figure that out and will get you a number that is way off. Luckily though it's pretty simple. To figure out your combined spring rate, the formula is "S1xS2/S1+S2" where S1 equals one of the spring's advertised rate and S2 is the other spring's rate. For example, a 200 pound spring over a 300 pound spring you would figure out as follows: 200x300 divided by 200+300. This gives us 60,000 divided by 500, which equals 120lb./in. combined spring rate. That sounds low until you consider that the springs get 120 pounds stiffer every inch they compress.
So now you have figured out what your car or truck currently has on it, but how do you know what it should be? This gets a little more complicated but isn't that hard if you take it step by step. The primary calculation is figuring wheel rate (WR). Lets simplify it by focusing on one corner of an A-arm front suspension. You will need the sprung weight (SW) of that corner of the car, which can only be accurately found by using four race car scales, one under each tire. If you have not weighed your car in this fashion you will have to take an educated guess (not advised, if you're going to try to figure this out, take the time to get your car weighed). For the purposes of this article let's assume that this one front tire has 500 pounds of weight on it. The only other number you need is total wheel travel (WT). Let's say it's 20 inches. With these numbers written down you can plug them into the formula, which is WR=SW divided by 0.4xWT. So we have 500 divided by 0.4x20. This works out to 500 divided by 8, which equals 62.5. This is your wheel rate, which doesn't mean much at all until we do a couple more calculations.
The next step is figuring the motion ratio (MR), which is the ratio of leverage put on the shock depending on where it is mounted on the A-arm. The first thing you do is measure from the lower A-arm pivot point on the chassis to the lower shock mount bolt hole. This we'll call D1 for distance one. Then measure from the same lower A-arm pivot point on the chassis to the outer spindle pivot point. Basically the whole length of the lower A-arm from pivot point to pivot point. This we'll call D2 for distance two. The formula used for motion ratio is D1 divided by D2, then squared. So let's say D1=20 and D2=25. This would be 20/25, or 0.8. Now we square 0.8 (0.8x0.8), which equals 0.64. Note that if you are figuring out the MR of a rear trailing arm suspension, the measurements are: from pivot point to shock mount, and from pivot point to wheel centerline).
There's one more step before the final calculation. This is measuring the angle that your shock sits at as measured from vertical. You can get a cheap angle finder at most hardware stores to help figure this out. Once you know the angle you can figure out the angle correction factor, or ACF. For this you will need a scientific calculator, which most home computers have built in. The formula is the Cosine of the angle. So just type in the number of the angle and hit the Cosine button on the calculator. For this article we'll say the shock is at a 10 degree angle. I punch 10 into the calculator, hit Cosine, and it spits out 0.985. This is the angle correction factor.
It's now the moment of truth, to see what the spring rate "should" be and compare it to what is on the car. The final spring rate (SR) calculation is this: WR divided by MRxACF. We already figured out all of these numbers so we just plug them in. This is 62.5 divided by 0.64x0.985, which works out to 62.5 divided by 0.63, and finally, 62.5/0.63 equals 99.2. This is our final spring rate number. Now if you compare it the first calculation we did with the 200 over 300 pound springs, you can see that those springs are stiffer than needed because they work out to a combined spring rate of 120 pounds per inch. To get the actual combined spring rate closer to the 99.2 that it should be one or both of the springs will need to be changed. It's cheaper to buy only one set of springs so we'll try to figure out what combination will work using either the existing 200lb./in. or the 300lb./in. springs. Going back to that first S1xS2/S1+S2 formula and plugging in a few numbers, I see that 200 over 200 works out to exactly 100lbs./in, and so does 150 over 300. Given the choice it's my opinion that it's better to run the 150 over 300 because the 150lb./in. spring will give a nice soft ride in the initial several inches of travel while the 300lb./in spring will resist bottoming out a lot better than if we ran the 200 over 200 combo.
That's all there is to it folks. But again, I would like to stress that for these calculations to work, you REALLY need to weigh your car on scales that give accurate individual wheel weights. If you guess, it's only that, a guess, and you could spend money on the wrong springs, which of course you don't want to do.
I understand that suspension tuning is an art but, I would at least like some direction so that I can learn more
I was also curious to know if the Motion Ratio work based on the A to B distance between the shock bolt and the one on the fame. Or, on a "gull-wing" or an arm that has the lower shock mount lower than the linear path the arm follows to the spindle from the frame, is it the point on the line above the bolt? Or directly to the bolt that we use for that distance?
Here is a picture to clarify
http://i88.photobucket.com/albums/k171/norocksplease/arms1.jpg
There are 2 paths here...so how does this work?
Below is a piece Chopshop posted recently that KingGlamis wrote. THANKS KG!!!
I do have a few questions though, as there are more variables here. And with new technology, we have more things to be concerned with, dual rate is no longer the biggest and baddest.
So here I go:
FIRST: I'd like to ask, for the Angle Correction factor, is this angle at full extention? Full compression? It would seem that if you changed the preload or spring rate, the ride height (and therefore the angle) would change. So where do we measure this at?
SECOND: This assumes the mounting point is on the lower arm. What about the upper arm? Rhinos and 4wd trucks have the shocks mounted on the upper arm. Would we measure any differently? Or would the same measurement for the shock mount work.
THIRD: What about for solid axles?
THIRD PART B: What about leafs on solid axles?
FOURTH: How about Single rate springs?
FOURTH B: And triple rates? I've seen a few rails with King shocks and I believe I'm seeing a 3rd (very small) spring mounted up top.
And now for a whole different section. How about shock valving? How does one account for this in a mathematical manner? Or do you?
Here is a piece by KingGlamis explaining shock tuning. This is where my questions stem from.
One thing that baffles me is how many cars seem to have the improper spring rate on the shocks. Most of the time it's too stiff. I don't understand why this happens so often and this question comes up here on the board quite often. The formulas for figuring this stuff out are available on the internet, yet still many customers seem to be unhappy with how stiff their car came from their builder. I was just at a friend's house today trying to get his car to ride better and the springs are way off. But... the place that weighed his car either wrote down the numbers wrong or their scales were way off. So I'm going to have to get his car weighed again to get it right. Anyway, while helping him and showing him the math involved, I thought to my self, "Self, this would be good info to share with the fine folks at GD.com." So... here it is folks, I tried to write it in real-world-speak to make it as easy as possible. I know it's long but if you follow it step by step you can once-and-for-all get the right spring setup on your car. And yes, you might have to revalve the shocks after changing springs, but that is a totally different topic for another time.
So here it is, as written by yours truly, KingGlamis.
First off, a lot of people have the misconception that you find your combined spring rate in a dual-rate coil-over setup by adding the two spring rates (rated in pounds-per-inch) and dividing by two. This is not at all how you figure that out and will get you a number that is way off. Luckily though it's pretty simple. To figure out your combined spring rate, the formula is "S1xS2/S1+S2" where S1 equals one of the spring's advertised rate and S2 is the other spring's rate. For example, a 200 pound spring over a 300 pound spring you would figure out as follows: 200x300 divided by 200+300. This gives us 60,000 divided by 500, which equals 120lb./in. combined spring rate. That sounds low until you consider that the springs get 120 pounds stiffer every inch they compress.
So now you have figured out what your car or truck currently has on it, but how do you know what it should be? This gets a little more complicated but isn't that hard if you take it step by step. The primary calculation is figuring wheel rate (WR). Lets simplify it by focusing on one corner of an A-arm front suspension. You will need the sprung weight (SW) of that corner of the car, which can only be accurately found by using four race car scales, one under each tire. If you have not weighed your car in this fashion you will have to take an educated guess (not advised, if you're going to try to figure this out, take the time to get your car weighed). For the purposes of this article let's assume that this one front tire has 500 pounds of weight on it. The only other number you need is total wheel travel (WT). Let's say it's 20 inches. With these numbers written down you can plug them into the formula, which is WR=SW divided by 0.4xWT. So we have 500 divided by 0.4x20. This works out to 500 divided by 8, which equals 62.5. This is your wheel rate, which doesn't mean much at all until we do a couple more calculations.
The next step is figuring the motion ratio (MR), which is the ratio of leverage put on the shock depending on where it is mounted on the A-arm. The first thing you do is measure from the lower A-arm pivot point on the chassis to the lower shock mount bolt hole. This we'll call D1 for distance one. Then measure from the same lower A-arm pivot point on the chassis to the outer spindle pivot point. Basically the whole length of the lower A-arm from pivot point to pivot point. This we'll call D2 for distance two. The formula used for motion ratio is D1 divided by D2, then squared. So let's say D1=20 and D2=25. This would be 20/25, or 0.8. Now we square 0.8 (0.8x0.8), which equals 0.64. Note that if you are figuring out the MR of a rear trailing arm suspension, the measurements are: from pivot point to shock mount, and from pivot point to wheel centerline).
There's one more step before the final calculation. This is measuring the angle that your shock sits at as measured from vertical. You can get a cheap angle finder at most hardware stores to help figure this out. Once you know the angle you can figure out the angle correction factor, or ACF. For this you will need a scientific calculator, which most home computers have built in. The formula is the Cosine of the angle. So just type in the number of the angle and hit the Cosine button on the calculator. For this article we'll say the shock is at a 10 degree angle. I punch 10 into the calculator, hit Cosine, and it spits out 0.985. This is the angle correction factor.
It's now the moment of truth, to see what the spring rate "should" be and compare it to what is on the car. The final spring rate (SR) calculation is this: WR divided by MRxACF. We already figured out all of these numbers so we just plug them in. This is 62.5 divided by 0.64x0.985, which works out to 62.5 divided by 0.63, and finally, 62.5/0.63 equals 99.2. This is our final spring rate number. Now if you compare it the first calculation we did with the 200 over 300 pound springs, you can see that those springs are stiffer than needed because they work out to a combined spring rate of 120 pounds per inch. To get the actual combined spring rate closer to the 99.2 that it should be one or both of the springs will need to be changed. It's cheaper to buy only one set of springs so we'll try to figure out what combination will work using either the existing 200lb./in. or the 300lb./in. springs. Going back to that first S1xS2/S1+S2 formula and plugging in a few numbers, I see that 200 over 200 works out to exactly 100lbs./in, and so does 150 over 300. Given the choice it's my opinion that it's better to run the 150 over 300 because the 150lb./in. spring will give a nice soft ride in the initial several inches of travel while the 300lb./in spring will resist bottoming out a lot better than if we ran the 200 over 200 combo.
That's all there is to it folks. But again, I would like to stress that for these calculations to work, you REALLY need to weigh your car on scales that give accurate individual wheel weights. If you guess, it's only that, a guess, and you could spend money on the wrong springs, which of course you don't want to do.
I understand that suspension tuning is an art but, I would at least like some direction so that I can learn more