Lower Links 101

NorCal_Prerunner

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It would be sweet if every month or so, there could be some kind of technical thread on building a piece or part of a desert truck from start to finish. I was hoping to cover building lower links for a 3 or 4 link setup. I always see plenty of great parts completely finished, Bent, Boxed, TIG welded, Dimple Died, ect, but never get to see the full make-up of a part from start to finish.

1. You determine you actually need or want a 4-link setup on your truck.
2. You have the tools, fabrication skills, and aren't a complete dumb ass.
3. You don't have access to a mechanical engineering program such as Solid Works to design it.
4. Where do you start?

A: What materials are you going to use? Tubing type, diameter, thicknes? Plate type, thickness? Heim size, threaded insert? Bushing type for front pivot? Cool, all that crap out of the way.

B: How to determine the lenght of the lower link?

C: Things to consider when designing the lower link?

D: Some images of expoded views of different lower link designs. (Kris loves to post, so let's see an exploded view of one of the lower links he has designed)

There is so much more that goes into a 3 or 4 link setup than building lower links. Everything could not be covered in an organized fashion in one post. I hope this post provides educational info for some of us.

Looking for posts from people have successfully designed and built a 3 or 4link setup that worked well.

What do you think. And no, 1,2, and 3 are NOT describing me, I just thought it would be an interesting and informative post. Later, J_





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Kritter

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No lower links my man..sorry. Havent had the want (read, money) to take the time and design one...yet. V8Ranger might have some of his SW drawings or I know John Richer has some.

Kris
"Buy American before it's too late..."
 

Curtis Guise

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I think that is a good idea. And starting with a 3 or 4 link setup sounds good to me since I plan to start building one soon. I have some ideas from looking at other trucks but to know how to get the correct geometry and materials to use would be great. (from people that know what they are talking about)

If enough people would contribute to this idea then maybe Klaus could start a new topic on the board called "projects" or something like that.
 

ntsqd

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Some of the things that need consideration in a rear linkage design:

Roll Center Progression (where it is relative to the axle at any point in the travel)

Roll Axis Inclination (To avoid or minimize 'Rear Steer' when the axle is twisted relative to the chassis)

Instant Center Location (Anti-Squat/Anti-Rise Properties)

Pinion Angle Delta (How much and in what direction the pinion angle changes from bump to droop)

Wheelbase Delta (How much the wheelbase changes from full bump to full droop)

Those are just off the bottom of my head. Everyone will assign their own priorities to these, and what ever others I missed. Where's Bob ?

Perhaps we ought to start a new thread ?



TS

I used swerve around my halucinations, now I drive right thru them.
 

NorCal_Prerunner

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Looks like TS has most of the things to consider when building a lower link covered. So now let's some basics covered such as A, B, and D. I don't want this post to get hyjacked with buckets of super tech info yet. Let's just get the basics covered first. Remember this post is NOT how to completely build a 4-link, just a part of it. All of TS's comments are very important. In due time though. Although this is a complex task, we are not talking about a designing and building a new type of micro-processor for a main logic board, so I don't understand the trade secret comment. We're talking about a fawking lower link. Good information for everyone.

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CRAIG_HALL

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One thing I tend to see alot even on some fairly nice trucks is the coilover & bypass mounts being above the centerline of the front and rear pivots. This makes the arm rely on the front, which is usually a bushing to keep it straight,prematurely wearing out the bushing. the mounts should be below the centerline allowing the arm to stay vertical and not wear your shock heims or your expensive delrin bushing or that 95.00 rear heim, usually I see 1-1/4" heims. Tube arms are simpler than sheet metal ones but you need the vertical strength at the shock mounts so I seem to favor a sheet metal arm possibly out of .090 chromolly if the sides are tall and vertical or maybe 1/8" if you make a triangle and taper the sides up . Try not to leave an open pocket at the shock mounts so that water and dirt collect and rust away your arm. I have some arms drawn but, there on auto cad at work (sheet metal shop) Just some things I've seen on other arms I've only made tube ones Craig
 

Curtis Guise

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The only problem is that if you try and answer B (lower link length) then almost everything that Thom brought up will be a factor. Because the length of that one piece has something to do with the rest of the setup. So in my opinion it would make sense to figure out the geometry of the entire setup.

I guess what I am trying to say is that it might be hard to answer B without the thread getting alot of "super tech info". But that's ok with me.
 

NorCal_Prerunner

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I'll all for super tech, but let's get the basics out of the way first. I agree with you that B is too hard to answer without addressing a lot of other things. I wish there was a orderly way the board could sort the posts. Thanks for the reply JDfab.

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ntsqd

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Somebody measure their truck. It needs to be a representitive truck.
Best way to measure is to use a flat piece of level concrete, a plumb-bob, and some chalk. We need to know:

Center of Gravity: Let's use the common assumption - the crank centerline at the flywheel surface. We need to know how far back it is fron the front axle centerline and we need to know how high off the ground it is.

Wheelbase. Measure it, don't quote what is considered 'common knowledge.'

Eventually we'll need to know where the frame rail is, but those two numbers will get this going.



TS

I used swerve around my halucinations, now I drive right thru them.
 

FABRICATOR

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Has the first part of question #1 been answered? What is the need or application? This should determine priorities.

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NorCal_Prerunner

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I guess #1 hasn't really been answered, so here it is.

Need: Prerun, chase, low budget race truck, let's say 4 MDR races per year.
Application: Class 7/1450 Toyota standard cab.

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ntsqd

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You bring up a point that I see often overlooked. I call it "wobbly arms", arms that are designed so that the damper and/or coil-over loads are working to twist the arm over sideways. My only problem with putting the damper mount(s) below the Line of Action ("LOA", the straightline btwn the front & rear pivot points) is that I've yet to see one that didn't make me wonder what would happen if just exactly the right sized rock came along......
Others have pondered this b4 me and one result is the bent arms you sometimes see. Those have a deliberate bend in them so that the damper mount(s) are on the top of the arm structure, but are still below the LOA of the arm. I see those as a broken arm looking for a place to happen, or they are overbuilt and cause excessive unsprung weight.

If the loads could be known or predicted, then determining the arm's needed section modulus at any given point along it's length would be a rather simple exercise. Translating the section modulus into actual metal would be the hardest part.

TS

I used swerve around my halucinations, now I drive right thru them.
 

ntsqd

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You bring up a point that I see often overlooked. I think of them as "wobbly arms", arms that are designed so that the damper and/or coil-over loads are working to twist the arm over sideways. My only problem with putting the damper mount(s) below the Line of Action ("LOA", the straightline btwn the front & rear pivot points) is that I've yet to see one that didn't make me wonder what would happen if just exactly the right sized rock came along......
Others have pondered this b4 me and one result is the bent arms you sometimes see. Those have a deliberate bend in them so that the damper mount(s) are on the top of the arm structure, but are still below the LOA of the arm. I see those as a broken arm looking for a place to happen, or they are overbuilt and cause excessive unsprung weight.

If the loads could be known or predicted, then determining the arm's needed section modulus at any given point along it's length would be a rather simple exercise. Translating the section modulus into actual metal would be the hardest part.

TS

I used swerve around my halucinations, now I drive right thru them.
 

rdc

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Remember, what you accomplish by placing the load below the center line only reduces what you call "wobbly arms", on compression. On rebound your gona get the "wobbles." Its one or the other, unless you control the arm with a tight bushing at one end or at the point where the load is connected, as in this example.



Sorry the picture is sooooo big.
 

ntsqd

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True, but the whole load of the truck's rear axle weight + any transfered off the front + any dynamic loading during a landing is acting on it in compression, while only the unsprung weight's inertia + any energy still stored in the springs is acting on it in extension. I'll compromise in the direction of the greater loading.
Doesn't negate the need for a bushing with anti-twist properties, just reduces the loads trying to twist the arm.

TS

I used swerve around my halucinations, now I drive right thru them.
 

billymanfroy

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Looks like we've established that a front bushing is the smart way to go. I've talked to several people - ones who would know - and they all recommend bushings at the front for the reasons you discussed. Theoretically, you could get away with running heims on both ends, but only if the shock mounts were on the C/L of the two ends. But why? Bushings are cheaper, safer, and are probably lighter. The consensus was to run silica bronze for the actual bushing. Bolt thru hole size is up to you, but you gotta think bigger is better.

We are in the process right now of drawing up arms, mounts, links, etc. in Autocad 2000. As John Richer says on his site, that really is the only way to get your brain around what actually happens when the thing cycles, and also how long the UPPER links are, and especially where they are mounted in relation to all the rest. We're doing exactly as they did using 3/16" plate for the lower links (no tubing). I have always thought that the bow or kink in most lower trailing arms was a risk, but I've never seen one fail or heard of one failing. Baldwin tore his rear end out at Laughlin (and Baja 500 too, right?) but they appear to just use tubes to locate their rear end and don't use a traditional arm. Don’t know how to accurately compare the two, but you have to believe they use pretty beefy heims (~1 ¼”) like everyone else. I have a friend who runs an ex-corr pro2 truck, and I swear his lower links drag not only on the ground at times, but literally under ground sometimes. I always thought there must be a better way, but he's never had to do anything to them but repaint them (often). I know, I know, short course and desert is apples to oranges.

I think it’s important to think about, along with the questions already raised, how much travel do you really need? 35” in the rear with stock beams (or a-arms) in the front is silly, and will probably hurt more than it will help. I’ll keep you posted on this, because we are going to have 28” in the rear, and 16” in the front. Most people (whether racing or prerunning) probably think they are eventually going to go full-tilt and go 6-9” wider front and rear, coilovers & bypasses. (Don’t forget the added 200+ lbs for the “biggest-junk-yard-V6-I-can-get” swap) Why? Because everyone else does? According to some of the experts I’ve been talking with, you shouldn’t really need more than one 2.5” per corner on a 7-type truck. Of course, set up is more critical and less forgiving, but done right, you’ll be happy for years to come.

Here are some basics:
Sheet metal boxed trailing arms. Silica bronze bushing in the front (3/4” bolt, 1.75x.120” tubing) 1 ¼” heim on axle.

Upper links: Not sure, 1”x.120” 4130 tubing with 7/8” heims. or 1 ¼” x.120” 4130 tubing with 1” heims.

Shocks: King 2.5” x16 coilovers w/ reservoirs (dual rate)

1” speedway sway bar

70” track width (until we extend the front)


I will post some pics as we go, and hopefully so will other people. We’ll eventually have to pare this long rambling thing down to the bare facts so it’s useful, too.

Billy
 

FABRICATOR

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Both ends of each trailing arm must be able to swivel in all directions.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

billymanfroy

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OK, Chuck... I'll bite.

Would you please elaborate as to why you need heims at both ends? How do you set that up?

Thanks.

Billy

http://home.off-road.com/~jricher/images/white ranger/susprear/4linkmount5.JPG

http://home.off-road.com/~jricher/images/white ranger/susprear/fulllean.JPG
 
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