Final 4 Link Calculations

[etrnlbzz]

After much deliberation, tons of tweaking (on the calculator), WAY to many days of reading Mud posts at work, employing my father (elec/mech engineer), generally upsetting my wife........etc. This is what I've come up with. You guys are by far the experts in this area and those of you who have had the chance to build a 3/4 link I would love to hear from! Anyway, before I order material (2.25" 7075 bar stock) I would like to throw myself into the pit and see what you guys have to say.

 

[kidglove13]

What kind of cruiser is this for?

 

[bustanutley]

I would shoot for a lower squat number, but that's just me.

 

[Mace]

Can you get the numbers a bit less fuzzy?

AS numbers look (can't really tell) a bit on the low side for what you have.

 

[etrnlbzz]

Sorry, I got excited. This is a 68' FJ40. I have a 08' 4.8L, 4L60e, 3speed t-case. SOA front. Not a daily driver but I will be driving it long distances so I'm going for streetablility.

I will get a clearer pic put up tonight.


(I would shoot for a lower squat number, but that's just me. )

This is what I'm curious about, how will a lower squat # effect the ride.

 

[bustanutley]

I can’t articulate the principles and thoughts on the subject as well as others. StrangeRover on POR is the man at explaining this stuff

Pirate4x4.Com Bulletin Board - View Single Post - Link suspensions for dummies?

Roll center seams kind of low, you may need to run a sway bar.

 

[etrnlbzz]

I'm planning on running an AntiRock by Currie. Any others you might suggest?

 

[etrnlbzz]

Clearer Calculations. Keep the replys coming!

 

[etrnlbzz]

Might as well keep asking questions since I have this thing going, I'm thinking of using air shocks on the rear with the sway bar. Anybody have any experience with the streetability of these and there longevity vs coil overs? I will post some pics of the build shortly, I haven't really chronicled it as the progress is really slow. But I have a few banked up now.

 

[BellyDoc]

I'm curious. You're reporting your CoG to a pretty high degree of accuracy. How did you arrive at your number?

Also, it's possible that your rolling radius isn't realistic. You've listed 35" tires. Unfortunately, many 35's aren't truly 35". Have you measured?

In general, the validity of this model's calculation is, at best, as good as your input. Have you played with using data ranges? For example, if you're not sure about your actual rolling radius, you could run the numbers as if it's an inch less or more and see how different the result is, with no other variables changed.

You'll find this model to be very sensitive to some variables and not so much to others. That's why some people build a degree of adjustability into their link designs - different bolt hole choices, for example.

 

[BellyDoc]

... oh... and another thing...

I was unpleasantly surprised to find out how many things are completely unrelated to this model.

The variation in pinion angle that comes from articulating the suspension in ride travel or twist isn't depicted in this model. That's not a failing in the model, but a failing in my level of understanding before I started using it.

Pinion angle is, in many ways, more important than antisquat. Antisquat may relate to how it drives, or even climbs obstacles, but pinion angle relates to whether or not you're going to snap U joints.

Another thing is rear/roll steer. The more triangulated your lower links are, and the longer they are and the more level they sit at ride height, the less you're going to see the rear end steering when it goes into twist.

With basically parallel lowers that are already at a down-angle, you're potentially going to notice your rear end coming off the line you're driving with the front end, when your rear suspension twists.

This is because the side that droops will also hinge forward relative to the other, and your wheels will no longer be parallel with the central axis of the vehicle.

 

[Gumby]

2.25 bar stock?

heavy.

 

[BellyDoc]

At 0.098 lbs/cu. in., I calculate 4.67 pounds per linear foot for solid 7075 round stock, 2.25" diameter.

At 0.290 lbs/cu. in., I calculate 4.78 pounds per linear foot of 2"x.250" wall DOM 4130 chromoly.

The weights are comparable... with a slight weight savings favoring the solid aluminum.

 

[notaf-ingjeep]

... oh... and another thing...

I was unpleasantly surprised to find out how many things are completely unrelated to this model.

The variation in pinion angle that comes from articulating the suspension in ride travel or twist isn't depicted in this model. That's not a failing in the model, but a failing in my level of understanding before I started using it.

Pinion angle is, in many ways, more important than antisquat. Antisquat may relate to how it drives, or even climbs obstacles, but pinion angle relates to whether or not you're going to snap U joints.

Another thing is rear/roll steer. The more triangulated your lower links are, and the longer they are and the more level they sit at ride height, the less you're going to see the rear end steering when it goes into twist.

With basically parallel lowers that are already at a down-angle, you're potentially going to notice your rear end coming off the line you're driving with the front end, when your rear suspension twists.

This is because the side that droops will also hinge forward relative to the other, and your wheels will no longer be parallel with the central axis of the vehicle.

I have to agree here. This is why I chose to mount both my brackets lower on the frame, sacrificing ground clearance. By doing this it helped with the rear steer, as it lowered the center tracking. This allowed for the axle to be at the pivot point at ride height. So......., the curve it travels on, it gets closer to the t.case output, in both directions of travel. You have to imagine a vertical line drawn through the frame straight to the ground at the t.case output. As this is the point I'm measuring to from the axle. So..... as the axle twists it moves forward at both ends at the same time. This keeps it more square with the frame under articulation. If I had mounted them higher in the frame it would have raised the center tracking. This more than likely would cause the axle to move away from this vertical line when under compression. It would still get closer to this vertical line when under drop. So when articulated you would have one end of the axle further away and one end closer to this vertical line. When twisted the axle isn't square with the frame, resulting in rear steer. It is hard to explain any better than that, I hope it made some sense.

Pinon angle is very important as well, so in any case make your brackets adjustable. I wish I had made more adjustment in my lower brackets before I welded them on. It is a lot easier to see how it all works in reality than on paper.

Your overall design looks similar to mine.

 

[BellyDoc]

Here's a link lesson I learned the hard way. A lot of people here will say "duh! Of COURSE!" ... yeah... well until I actually had metal cut AND WELDED, I didn't get it. Now I do. And it's responsible for one of the many re-re-re-do's in my project.

Here's a doodle:

In these drawings, we're looking at link mounting points from the side. The arcs are on the axle end, and the fixed points are the chassis ends.

In the drawing on the upper left, the vertical separation between the pivot points on the axle are identical to the spacing and separation of the chassis ends. The linkage "parallelograms".

That is, when the axle goes up and down (red vs. blue), the pinion angle doesn't change relative to the chassis. The pinion angle is depicted by arrows. The arrows that are in the up (red) position and down (blue) positions are parallel to eachother.

In the top right drawing, the axle mounts are the same, but the chassis mounts are much closer together. In this case, when the axle swings up and down, the pinion angle tends to point TOWARD CENTER at all times. Although this would be better for the U joint between the driveshaft and the pinion than the extremes in drawing #1, it has it's own draw backs. For a rear axle, this would be fine, but for a front steering axle, this would mean that you're throwing away your caster angle when the thing droops. This would mess with steering.

The third, bottom, illustration shows my particular screw up. I limited the vertical separation of link mounts on the axle in order to allow for the greatest possible up-travel. As a result, I ended up with the vertical separation of link mounts at the axle end being LESS than at the chassis. Because of this, the axle swings in the same way as one of those rocker-glider chairs. Just like how those chairs tip back and forth while they swing back and forth, this link geometry tips up and down as the axle swings up and down. It actually ACCENTUATES the pinion to driveshaft misalignment... a worst of all possible outcomes.

There is no adaptive reason to ever build this geometry that I can tell. The reason I bring this up is that if you're going to make adjustable link geometry by having multiple bolt holes for the chassis ends of the uppers, then you should be mindful of this possibility. You need to make sure that all of your bolt hole choices make the chassis end vertical separation less than or equal to the axle end separation. Any bolt holes that open this distance up to a value greater than the axle's is to be avoided.

 

[Sputnik40]

That was an extremely helpful post!

 

[notaf-ingjeep]

Here is a quick representation of my center tracking as my cruiser is now. A good point of center tracking in relation to my frame was a top priority to me while designing it. As this is how the suspension travels in relation to the rest of the chassis.
I was also very concerned with good squat characteristics, and achieving the proper ride height to match my front "soa" shackle reversal, suspension.

 

[etrnlbzz]

I'm curious. You're reporting your CoG to a pretty high degree of accuracy. How did you arrive at your number?

Also, it's possible that your rolling radius isn't realistic. You've listed 35" tires. Unfortunately, many 35's aren't truly 35". Have you measured?

In general, the validity of this model's calculation is, at best, as good as your input. Have you played with using data ranges? For example, if you're not sure about your actual rolling radius, you could run the numbers as if it's an inch less or more and see how different the result is, with no other variables changed.

You'll find this model to be very sensitive to some variables and not so much to others. That's why some people build a degree of adjustability into their link designs - different bolt hole choices, for example.

I haven't bought the tires yet, they are simply going to be the size I would prefer. As far them being dead nuts 35" that’s a variable that I don't have a whole lot of control over. My calculations were made be measuring and re-measuring the CG of as many of the components of the vehicle and then entering them into the calculator and tying those moment arms into the calculator CG cell. I'm not claiming any of this work, my father has spent hours going over the calculator and compiling all of this information. He added a spread sheet to the main page that is now tied into the rest of the calculator. I will be happy to send it to anyone interested.

At the moment, I'm maxed out on my lower links in relation for the ability for them to be adjusted down I have one hole above on the lower link frame side that i can go up with. I have built adjustability into all of my brackets after reading Belly docs write up (which was *^%* awesome!).

From your Pic (nota&%jeep) it looks like our rigs will be similar.

As far as using alum vs steel, it's less a weight issue and more of a material characteristics issue. Also, instead of spending $200 + on bungs and jam nuts (currie $25+tax ea x 8) I thought i would buy the alum have a buddy bore and tap it and have spent similar $. I can't get a hold of tube 7075 but the bar stock is just up the street...well Hwy anyway.

Thanks for the posts guys, I will pass this info on to a higher power (my father) and get his noodle grinding (he's retired......and bored).

 

[etrnlbzz]

Heres a screen shot of the weight/distance spread sheet thats tied into the CG calc. Sorry, I tried to get it clearer but you get the idea.

 

[White YJ]

I played with these calculators a lot when I designed my front 4 link. However when it comes to actually putting this in your rig remember there are going to be some variables that you cant control. Things get in the way and you will have to make some compromises. I know I did with mine and I am still VERY happy with how it turned out. It is important to know the basic principles, how to apply them to YOUR rig and not a set of lines on a spreadsheet that spit out numbers that everyone likes.