# 4-link calculator and some other cool stuff

Discussion in 'Center Of Gravity' started by Triaged, Nov 8, 2003.

1. ### Triaged1/2 ton status

Joined:
Feb 21, 2001
Posts:
3,808
0
Location:
CA (LA/OC area)
Update:
Just uploaded V1.5. Greg has helped me get it looking alot better with everything layed out in a more fluent manner. He also added some pull down menues for the link materials. If you click on the cell a little down arrow will pop up next to it. Just click on the down arrow and you will be able to pick from a list of choices.

V2.0 is being worked on (by Greg) that will draw a picture of your suspension geometry automatically. He has already drawn up one for the 2D anti-squat excel program he made and is trying to update it for the new 3D spreadsheet (which doesn't sound easy to me!).

I am working on making it cycle your suspension so you can see how some of these values change as your suspension compresses or droops (articulation would be nice as well but I have no idea even where to start on that).

I am also looking for some other people to help out.

I need someone who knows all this math stuff (most likely another mechanical engineer) to check my work and see if my assumptions are valid.

I would also like to have someone acutally use it to analyze their suspension with it and give any notes on how it works in real life.
-------------------------------------------
Disclamer:
I think this info is correct. If you use it and die (or injure yourself in any way) it is your own fault that you were stupid enough to listen to me.

I finally finished my 4-link calculator. To the best of my knowlage it is correct. It should not be used as a substitute for common sence but as a guide. It also might be fun to play with. It isn't too pollished up but it is there. It requires that you use my "coordinate system" which is based on the ground right under your rear diff...so when you measure stuff start from there.

"X" lengths are from the rear axle centerline forward
"Y" lengths are half (1/2) of the horisontal sepperation.
"Z" lengths are all measured from the ground.

You can end up wiht negative (-) values for "X"...that would just mean behind the axle.
If you do a Wishbone type link you will end up with a "Y" of zero (0) length. If you put in a negative (-) value for "Y" you are stupid because you won't be able to build it (the right link would connect to the left side of the chassis /forums/images/graemlins/rolleyes.gif). If when looking from the top your links are parallel make them "not" by some small # (even as small as .001") or it won't calculate your roll center or roll axis.
If you put in negative (-) "Z" values you just designed a plow...not a 4x4.

BTW: The cells are protected just to keep you from accidentally screwing it up. If you really want to change the formulas there is no password for the protection.

For those of you that care...

Assumptions:

1) Loads are calculated with 100% weight transfer.
2) Tire coeff. of friction of 1.
3) That you have 4 non-parallel links (if you use a wishbone don't forget to double the load to the center rod end)
4) For the F.S. (factor of safety) in bending you have half of the weight of the truck acting on the center of the link.
5) Your links are made of steel with a yield of 63,250 psi (normalized 4130)...if you want to use a different material you will have to find the spec's and plug them in on the 2nd page

As you can tell from this pic I have a 14B in the rear and a D44 up front /forums/images/graemlins/rotfl.gif

The rest of the list is below:
This one has some guessed info, some measured stuff, and some BS...measure up your springs and plug in the #'s.
This is some of the part #'s for parts I have used on my truck.
This file will calculate the stiffness of your swaybar in G's per degree. It is made for "fabricated" sway bars...not bent tubes. This is the "Really hard math" stuff and won't be much fun for anybody but...what the hell /forums/images/graemlins/thinking.gif

Edit:

This is an email I sent Greg answering some of his questions

------------------------------------------------------------

The F.S. stands for Factor of Safety. It is how much stronger than failure it it. The lowest F.S. is what will fail first (except for the bending). Ideally you would want to mess with the rod end rating, tube OD, wall thickness, and material (on the 2nd page in blue) to get them all around the same value (no use having a solid 3" diameter link and 3/4" rod end!). In this case a F.S. of 10 might be just a bit overkill...but you sure don't want a number as small as 1 or 2. It is also sometimes called a "Fudge Factor" by engineers because it is tossed in to account for all the stuff they didn't (or couldn't) include in the calculations (like dynamic shock loading in this case). There are also some more general guides for F.S. like "If this fails will someone die, just get hurt, or just property damage?" In this case "Someone might die" so try and keep the F.S. high...esp. for a street driven truck. The bending doesn't have as much of a "someone might die" factor because it will be (I hope!) at slow rockcrawling speeds.

In the case of bending you can see that the link is much weaker than in compression (this is the same reason that cages need triangulation). I think your best bet would be to shoot for something just over 1 and then try not to slam the links all over every rock you see.

The nice thing about a F.S. is that they have no units. At least not normal ones...they are in fact lbf/lbf (which cancles out to just 1). This way you can compair them with any other truck directly (just like AS values in % which is also unitless).

For views I don't see the need for a front view (but it might be cool to see). A top view and side view should be able to show everything.

For the vector algebra:
I took the link length and sepperated it into it's components (how much it went forward, how much it went up...etc.). I then devided each component by the overall link length to get a "unit vector". This is like a arrow pointing in the same direction as the link but it has a length of "1". From that I know in what direction the force in the link is and I just have to find the "magnitude" of it. To do this is I added up all the moments (another word for torques) and set them equal to 0 (zero). This is one of the statics equations that will tell you why something doesn't move (in this case the pinion doesn't point to the sky on acceleration...we just want to know why/how). From there I devide the total link force back into the components (don't know that it is needed for the rest of the equations but it didn't really take any more work...the equation I gave you before calculated only the force in the "x" direction...you can see how it is an OK estimate if the links don't converge too much but it will always be too small...hence giving you a F.S. of 10 to work with...using the more exact forces the F.S. can be reduced some).

Finding the buckling and bending loads was just a matter of "plug and chug" with some equations from an engineering book.

I guess that's about it in writting. You already know how to find the intersections of the lines you'll just have to go back through and figure out how I went about finding them (I actually used alot of your ideas to do this...they were alot cleaner than my origional work).

2. ### big pappa b3/4 ton status

Joined:
Jan 3, 2003
Posts:
6,328
0
Location:
I looked at all of your spreadsheets and it appears that you have put a lot of work into them.
It's just too bad I'm not a fabricator cause i don't know what tha "F_(&amp;" I was looking at /forums/images/graemlins/rotfl.gif

Joined:
Feb 24, 2001
Posts:
7,789
14
Location:
Phoenix, AZ
Very nice, thank you! /forums/images/graemlins/thumb.gif

Joined:
Feb 24, 2001
Posts:
7,789
14
Location:
Phoenix, AZ
BTW, you can't use "#" in a URL so you might want to change the name of that parts file...

5. ### Greg72"Might As Well..."Staff MemberSuper Moderator

Joined:
Mar 5, 2001
Posts:
16,183
2,397
Location:
Austin, TX
Build(s):
3
Somebody help me......

!! MY BRAIN IS SMOKING....MY BRAIN IS SMOKING !!!!

/forums/images/graemlins/angryfire.gif &lt;- Smoking brain emoticon?

Dan, that is WAY cool. As I mentioned in my last e-mail, I am convinced that I can roll all this data up into the "graphical version" which will be a lot more fun to use.

I think if this program does nothing else, it should help people understand some of the forces involved in the rod ends and how critical it is to size these parts correctly. This is my main worry with the "just go out and build it" crowd... for a trail-only rig, a broken rod end is an aggravation....but for a street-driven truck, a broken component is a high-speed CRASH!!! /forums/images/graemlins/eek.gif

I can't wait to see how people start using this program, and the designs that get posted up. Like any collaborative effort.....it sometimes will amaze you what others can do with your initial ideas. /forums/images/graemlins/thumb.gif

6. ### Bubba Ray Boudreaux1 ton status

Joined:
Jan 21, 2001
Posts:
20,716
2
Location:
Undisclosed Location
Real quick, on the Leaf Spring Calc, when measuring free arch, what are the proper points to take the measurements from? Bottom of the pack, or top of the pack to the top of the spring eye?

By the way, great stuff, thank you /forums/images/graemlins/thumb.gif

7. ### Greg72"Might As Well..."Staff MemberSuper Moderator

Joined:
Mar 5, 2001
Posts:
16,183
2,397
Location:
Austin, TX
Build(s):
3
BRB,

http://www.rpmnet.com/techart/leaf.shtml

It looks like it's measured from the spring eye centerline to the bottom (mounting surface) of the leafpack. You'd better read the source too, and see if you agree with my interpretation..... /forums/images/graemlins/deal.gif

8. ### DPI1/2 ton status

Joined:
Jun 22, 2000
Posts:
2,792
1
Location:
In the Burbs close to Tulsa, OK
Thanks for sharing your brilliance Dan! I really appreciate it. This will really save me and others a bunch of time when starting and fine tuning their 4 link.

9. ### Triaged1/2 ton status

Joined:
Feb 21, 2001
Posts:
3,808
0
Location:
CA (LA/OC area)
Greg is right about the free arch. I quess I should add a picture /forums/images/graemlins/rolleyes.gif...I just suck at making one myself (see posted pick above). /forums/images/graemlins/grin.gif

The intresting thing about measuring this way is you can see that a spring pack with lots of thin leaves will be alot thicker than a pack with just a few thick leaves. This will make it "look like it has less arch" and might be one reason they flex better.

10. ### Greg72"Might As Well..."Staff MemberSuper Moderator

Joined:
Mar 5, 2001
Posts:
16,183
2,397
Location:
Austin, TX
Build(s):
3
Dan,

F.S. = "Factor of Safety"

Is this a multiplier value to be used against the "Link Force" value?

EX.

"F.S. Rod End" = 9.44

Therefore the load to create failure of the rod end is (5825 x 9.44) = 55,000Lbs???

Interesting coincidence that the number just HAPPENS to be the exact value of BIG rod ends in the Aurora catalog??? /forums/images/graemlins/thinking.gif

11. ### Triaged1/2 ton status

Joined:
Feb 21, 2001
Posts:
3,808
0
Location:
CA (LA/OC area)
[ QUOTE ]
Dan,

F.S. = "Factor of Safety"

Is this a multiplier value to be used against the "Link Force" value?

EX.

"F.S. Rod End" = 9.44

Therefore the load to create failure of the rod end is (5825 x 9.44) = 55,000Lbs???

Interesting coincidence that the number just HAPPENS to be the exact value of BIG rod ends in the Aurora catalog??? /forums/images/graemlins/thinking.gif

[/ QUOTE ]

That's it! Looks like Watson put more thought into those rod ends than you might have realized! I thought it was a good sugestion so I stuck with it.

Now just try and see if you can get the F.S. for the rod end that size with a smaller rod end! /forums/images/graemlins/wink.gif

I updated the name of that file...thanks

12. ### Greg72"Might As Well..."Staff MemberSuper Moderator

Joined:
Mar 5, 2001
Posts:
16,183
2,397
Location:
Austin, TX
Build(s):
3
[ QUOTE ]
Greg is right about the free arch. I quess I should add a picture /forums/images/graemlins/rolleyes.gif...I just suck at making one myself (see posted pick above). /forums/images/graemlins/grin.gif

The intresting thing about measuring this way is you can see that a spring pack with lots of thin leaves will be alot thicker than a pack with just a few thick leaves. This will make it "look like it has less arch" and might be one reason they flex better.

[/ QUOTE ]

Here's a pic to save Dan from drawing anymore....

the straightedge is across the middle of the springeyes, and you can almost read the tape measure at the base of the springpack (where it sits on the perch)....

BTW -&gt; It's a 4" springpack with what appears to be 9" of "free arch"... Soft Springs, eh? /forums/images/graemlins/wink.gif

13. ### Bubba Ray Boudreaux1 ton status

Joined:
Jan 21, 2001
Posts:
20,716
2
Location:
Undisclosed Location
[ QUOTE ]
The intresting thing about measuring this way is you can see that a spring pack with lots of thin leaves will be alot thicker than a pack with just a few thick leaves. This will make it "look like it has less arch" and might be one reason they flex better.

[/ QUOTE ]

After going to the 'Way Back' file and a conversation I had with one of the guys at Deaver; I think I can make a call on that one since it wasn't directly said.................And you two better keep the conversation simplistic for my slow mind, y'all engineers might leave me behind /forums/images/graemlins/rotfl.gif

I think the reason for the thin leaves is one, the spring will retain the arch longer than a couple of thick leaves, but more importantly, even though there are more, maybe it's easier to get the thinner leaves to move compared to the thick ones, therefore even for you rock guys this might not back too much of a different, but for something that is hauling butt; the spring can start moving faster and will be keeping up with the terrain compared to what a thick setup will............

Sound right?

14. ### Greg72"Might As Well..."Staff MemberSuper Moderator

Joined:
Mar 5, 2001
Posts:
16,183
2,397
Location:
Austin, TX
Build(s):
3
Sounds right.

Really, my only experience is in plugging the numbers into the "classic" leaf spring rate formula (yet, ANOTHER spreadsheet I built a while ago).... it became obvious in a hurry that for a given pack thickness....more thin leaves stuffed into that height would provide a softer rate than a few thick leaves.

Sort of like comparing a soft-bound magazine to a single thick sheet of cardboard.... with all those individual sheets of paper in the magazine, is wriggles and moves with a high degree of freedom. The cardboard (even though it's still made of the same material, just thicker) can't do much of anything flexwise.

That's the best analogy I can think of at the moment.

15. ### Bubba Ray Boudreaux1 ton status

Joined:
Jan 21, 2001
Posts:
20,716
2
Location:
Undisclosed Location
One quick question on the Leaf Spring file, I just got done pluggin the numbers in posted by ntsqd in the Garage forum, but it's been a long time since I messed with an Excel file, how do I get it to calculate the spring rate once I've put in all the numbers?

16. ### Greg72"Might As Well..."Staff MemberSuper Moderator

Joined:
Mar 5, 2001
Posts:
16,183
2,397
Location:
Austin, TX
Build(s):
3
Use the "=" sign to indicate that you are defining a formula instead of just doing a numeric entry... Then you can click on each of the "involved" cells to add them to the calculation string....add your multiplications, divisions, and ^3s....(or whatever you want)

Once you hit enter, Excel will perform the calculation.

The best part is that every time you change one of the cells involved in the calc....the rate will be automatically recalculated. It will teach you the effects of each change in a HURRY!!!

Fun stuff.

17. ### Bubba Ray Boudreaux1 ton status

Joined:
Jan 21, 2001
Posts:
20,716
2
Location:
Undisclosed Location
Okay, just a few more questions.........

Load F? That would be weight of front end, right?

Loaded Arch looks like would be the measurement of the springs when on the vehicle, and therefore the deflection is the differance between the free arch and loaded arch.

Last but not least, stress? Is that something that I can measure at home, or does it need a special measuring doohickey?

18. ### Greg72"Might As Well..."Staff MemberSuper Moderator

Joined:
Mar 5, 2001
Posts:
16,183
2,397
Location:
Austin, TX
Build(s):
3
[ QUOTE ]
Okay, just a few more questions.........

Load F? That would be weight of front end, right?

Loaded Arch looks like would be the measurement of the springs when on the vehicle, and therefore the deflection is the differance between the free arch and loaded arch.

Last but not least, stress? Is that something that I can measure at home, or does it need a special measuring doohickey?

[/ QUOTE ]

BRB,

I'll take a crack at these.....Dan will probably followup at 2AM when he logs in here! /forums/images/graemlins/thumb.gif

Stated Load is probably just what the "vendor" says the spring rate is. Just a sanity check against the calculated value, I think.

Load F - I'll bet ya it's the weight on the spring. 1250 looks like exactly 1/4 of the 5000 Lbs of sprung weight that a Blazer probably has.... I don't necessarily think it's FRONT. Curiously my truck when weighed on a set of precise "race scales" had almost perfect 50/50 weight distribution... /forums/images/graemlins/thinking.gif

Load Arch - I think you've got this one right. The arch that's left over once you add the 1250 Lbs on top of it.

Stress - I don't think this is a "measureable" thing, so much as a way to indicate the stresses on the metal itself. Might be an indication of the springs "longevity"...higher values are probably going to sag or fail sooner because the stresses are closer to the limits of the material (in this case spring steel).

Something that can help a lot in Excel, if you don't already know this.... when you click on the cell you're curious about, the formula is displayed in the upper "text window" (duh, obvious right?) BUT!...If you actually move the mouse UP to that text window and single-click.....it will highlight and color-code all of the cells involved in the calc for you!!! It makes it a LOT easier to decode which other cells are being used, and makes it pretty easy to figure out the "meaning" of a calculation that's unclear.

/forums/images/graemlins/cool.gif

19. ### Triaged1/2 ton status

Joined:
Feb 21, 2001
Posts:
3,808
0
Location:
CA (LA/OC area)
Again Greg has it right...
Alot of those springs I got all the info (except for leaf thickness) from the tech section of CK5. I measured the springs that I have laying around and guessed at the leaf thickness for the rest.

The springs I actually measured are
1st gen 4" ranchos (might as well have an I-beam insted of springs)
52" rear (ones that came on my truck...other years might be different)
2nd gen 4" TC EZ-ride (if you notice the calculated value is off by a HUGE amount from the stated rate...I have no idea why)
63" chevy's

Joined:
Feb 21, 2001
Posts:
3,808