Home of the K5 Blazer/Jimmy including GM Trucks and Suburbans.
|05-31-2008, 06:45 PM||#1|
How to link the rear of a K5
I've been determined to link the rear of my truck for over a year now and am just getting around to finishing it.
My truck is pretty different from most of the K5’s on here but I am still using most of the K5 frame and the methods I used to attach the links and springs could be useful for any K5 owner who is considering linking the rear of their truck.
**If you don't care about how to design a 4 link, pics of my setup are at the bottom**
Why would I want to switch to a link suspension? This is what the rear springs that came off of my truck looked like :
These two videos show the axle wrap that I dealt with for the last few years (34% for language)
(I was open in the front then, the "bolt" that fell off was just rust :lol
(I don't know why the sound is off)
I chose to replace the leaves with a double triangulated 4-link. A triangulated 4 link will locate the axle front to back and side to side and allows the springs to simply hold the vehicles weight. Using spherical rod ends (abbreviated as SRE’s, usually referred to as heim joints) or flex joints a triangulated 4 link will practically eliminate any unwanted movement. This means that the axle won’t “walk” under the truck when turning, the springs can’t wrap or bend, wheel hop is all but eliminated (if you set it up right), you can achieve better approach/departure angles, etc.
On a leaf sprung suspension you are depending on the leaves to locate the axle front to back, locate the axle side to side and resist the torque running through the axle. Generally leaves do a fairly good job of all of this but the desire for bigger tires, more flex and better performance can exceed their capabilities (as you can see by the bad axle wrap and wheel hop I've experienced). Leaf springs also allow the axle to move around a lot under the truck because they use bushings, the steering on my front end (I'm still running leaves in the front) will actually move the front of the truck (rather than the tires) through the first bit of steering travel
This is roughly my setup and shows you what a double triangulated 4 link looks like. It simply means that all 4 links are at an angle when looking from above, like the picture here (red are the lower links, blue are the uppers):
As long as the combined angles of the 4 links are at least 40 degrees you won’t need a track bar (panhard bar) to locate the axle side to side.
So what do you need to know to design a 4 link?
Each motorsport (drag racing, desert racing, mud bogging, competition rock crawling, recreational rock crawling, etc.) all commonly use 4-link suspensions but their design is different for their different purposes. The focus of this writeup is for a recreational rock crawler.
First, you need to understand a couple terms for the most important aspects of a 4 link suspension:
Anti-squat: Anti-squat is the force that actually “picks up” the rear end under power. As the truck accelerates, weight is naturally transferred to the rear and causes the truck to “squat”. Basically the links have to resist the torque running through the axle shafts and exert an equal and opposite force to the frame which will actually push up on the body of the truck based on how the links are setup.
Roll Axis Angle: When a 4 link suspension flexes it can create what’s called “roll steer”. To describe it, lets say (when viewed from the side) your tire sits centered in the wheel well at ride height. When the suspension flexes, each tire may move forward or backward from the center of the wheel well based on the roll axis angle. A zero degree roll axis angle would keep the tires perfectly centered in the wheel well throughout its travel.
That’s a REALLY basic run through but those are the major design constraints to watch.
Resident Offroad Design rep
Last edited by 38377k5; 05-31-2008 at 08:46 PM.
|05-31-2008, 06:45 PM||2|
Alright, so where do you start with the 4 link design?
There is some good reading here on one approach to 4-link design (http://www.4wheeloffroad.com/techart...t_2/index.html). This is NOT the gospel on 4 link design, quite frankly there isn’t one.
I did not choose the paper and pencil method, in particular I feel it makes it difficult and tedious to make slight changes in your design. It takes two seconds to change the excel sheet or 10 minutes to recalculate everything on paper.
Here’s how I did it:
First, download the 4 link calculator here http://mysite.verizon.net/triaged/fi...arLinkV3.0.zip (props to Triaged and anyone else that worked on this calculator, its helped a lot of us )
Next you need to fill out the column in the upper left of the 4 link calculator:
Measure your wheelbase (center of hub to center of hub, front to back)
Tire rolling radius (actually measure it, its smaller than ½ the diameter of the tire)
Vehicle COG; measure from the ground to the top bolt on the bellhousing or the cam centerline, make sure to adjust for any ride height adjustments you plan to make with the 4 link
Vehicle Mass (don’t lie here, make sure you input the correct weight including you, your trail gear/spares/passengers)
Front and rear unsprung mass (This is the weight of your tires, wheels and axles, front and rear)
If you are making any tire size or lift height changes make sure to factor that into your design!!
Now you need to determine how high from the ground you will mount your lower link axle mounts (it’s the z column, lower links section, axle end in the calculator). A good starting point is the axle tube centerline, put that into the calculator.
Determine your upper link, axle side link mount dimension (z, upper links, axle end). This is important because it plays a large role in the stresses that your mounts, frame and joints will see. The more vertical separation you can get between the upper and lower mounts, the easier the suspension will be on your frame, joints and suspension mounts.
The general guideline for determining vertical separation is to multiply your tire size by .25 and add it to your lower axle side link mount measurement. For instance, I’m running 42” tires, so (.25*42) = 10.5. My vertical separation should be about 10.5”. Given that my axle tube centerline is at 20” I should set my upper link axle side mounts at 30.5” from the ground. Input your upper axle side mount dimension into the 4 link calculator.
To set the frame side mount for your lower links, a quick guideline is to point them up 5-10 degrees from the axle and see where they would contact the frame. This should result in a lower link frame mount that is a few inches higher than your lower link axle mount. Input that number into (z, lower links, frame end).
For now, set the upper link frame mount (z, upper links, frame end) at the same height as the upper link axle mount (z, upper links, axle end).
At this point you’re done with preliminary design from the side and you need to start looking at triangulating the links. Looking at the suspension from above:
Start with the horizontal separation of the upper links. If you are triangulating like me (which is probably the most popular way) and provided you are using a narrow K5 frame with wide full width axles you will want your separation here to be around 8”, so enter a value of 4” here (upper links, y, axle end).
I boxed the frame on my truck and mounted the uppers to the inside of the frame rails. I was assuming upper links that would be ~30” long (pretty standard) and input them into the calculator as such. If you’re playing around with link location and wondering how long the links are (or you’re too lazy to do trig) click the vector calculations tab at the bottom of the sheet and it will tell you how long the links are.
I set the lower axle mount (lowers, y, axle end) at 22”, i.e. my lower links are 44” apart from each other at the axle.
I mounted the lower link frame mount directly under the frame to triangulate the lowers.
Up to this point I left it pretty general so you can read through it and get your numbers pretty close for whatever ride height and wheelbase you want.
If you don’t feel like crawling under the truck and since I’m doing this writeup for a K5 frame, I’ll just go ahead and give you some kind of starting point and I’ll explain how to adjust everything the way you need it later in this article. These numbers assume the stock wheelbase:
Upper links: Frame end; x = 29, y = 12, z = 32
Axle end; x = 1, y = 4, z = 32
Lower links: Frame end; x = 40, y = 17, z = 24
Axle end; x = 3, y = 22, z = 20
These are guesses, you will need to eventually measure some things on your truck to get everything to fit the way you design it.
Resident Offroad Design rep
Last edited by 38377k5; 04-22-2009 at 11:45 AM.
|05-31-2008, 06:46 PM||3|
Now the calculator will have given you some numbers for antisquat, roll center, etc. based on what you have entered.
Here’s where the fun starts
Now go out and measure your frame and compare it to the numbers you put in the calculator. Since your attaching a new suspension system to an existing frame you will need to make some compromises and probably change the design several times before you can make everything work as good as it can.
Go back to your calculator and change values as necessary to accommodate your frame, lift height, etc. Think about how you might be able to attach the links. You may have to box the frame in places, make new crossmembers or you may be able to add brackets to the existing frame.
Now you’ve probably realized that nobody can tell you exactly how to put everything on the truck.
Here are guidelines to watch while designing your 4 link at this point:
From talking to many, many people with linked rear suspensions it seems that anti-squat percentages between 50% and 100% work well in the rocks. Too little anti-squat and your rig will want to flip over backwards, too much and the axle will try to drive under the truck and it will hop.
Keep your roll axis angle as close to 0 as possible. If your design necessitates a few degrees of roll axis angle it will probably be ok. Having lots of roll steer sucks.
The COG height and roll axis height should be close. They don’t have to be exactly the same but if they’re off by a lot you may be in trouble.
Some will recommend that your uppers be ~70% as long as the lowers, this helps keep the antisquat numbers consistent through bump and droop travel.
Equal length upper and lower links will keep the pinion angle consistent throughout the suspension travel.
You don’t need to triangulate the links like I did, many people run links that are parallel with the frame (straight) and only triangulate the uppers. Some people converge the upper links at the frame side and keep them wide at the axle.
If you’ve read this far, measured your truck and input numbers into the 4 link calculator you’ve realized that it doesn’t all work out perfect the first time. Or the second time. Or the third time. And then once you’re ready to bolt/weld everything under the truck you’ll realize you want to change it one or two more times.
|05-31-2008, 06:46 PM||4|
Once you've decided on a design that you can make work its time to construct links, a truss, brackets/mounts/gussets and get this stuff under the truck.
On a 4000-5000 lb. rig you should construct your lowers out of nothing smaller than 2” tube, .250” wall. One good way to determine link size/thickness is to use the material tab in the 4 link calculator and look at the factor of safety for bending. Anything above a SF of 2 will probably keep your links from getting bent. The most influential design aspect to keep your lowers unbent is tubing diameter, shorter links and thicker links also help keep the links from bending to a lesser degree.
When it's finally time to start putting everything under the truck the first thing to do is mark the location of the rear axle on both sides of the frame (to set wheelbase) and mark the location of the axle based on the frame using a plumb bob (so you can keep the axle centered under the frame). I extended the wheelbase 4" at the same time so I made another mark 4" behind the first to give me a reference mark for axle location when I'm ready to weld the links in.
Leaf springs and shocks come off, then the old leaf spring (and bump stop) brackets need to come off. I just drilled out the rivets and knocked the brackets off with a hammer (one of my favorite things to do ).
Next, jack up the rear end and remove the wheels. Set jack stands to your FINAL DESIRED RIDE HEIGHT and slide them under the rear of the frame (now would be a good time to consider where you links might end up, don't put jack stands where the links need to go but make sure the truck is properly supported)
Don’t forget that if you're changing the wheelbase the driveshaft may be a concern.
Resident Offroad Design rep
Last edited by 38377k5; 05-31-2008 at 11:24 PM.
|05-31-2008, 06:47 PM||5|
Now is a good time to clean up the axle for its new brackets (and cut off the old ones).
Link construction :
My lowers are ~30” long and are made from 2” .250” wall DOM steel tubing.
My uppers are ~29” long and are made from 2” .120” wall DOM steel tubing.
I used 8 of Ballistic Fab's 3.0" flex joint, they use a 3/4" bolt (nearly a pound each ). I chose them because they are reasonably priced, they're stupid strong, and they're rebuildable. Bigger joints last longer, it doesn't seem like heims last very long IME. Plus they can be assembled with a lightly modified 6 prong hub socket so you don't need to buy a special tool.
These are the basic parts that fit into a machined and threaded piece of 3.0" .210" wall DOM tube.
The Ballistic Fab joints next to a more common 9/16" Rubicon Express flex joint.
I made a truss out of 2”x3” .250” wall rectangular tubing. Make sure to account for how your upper links will mount and build your truss so you can mount your uppers.
Here’s my truss:
And here are the pieces that tie the truss into the pinion support made from 1/4" plate:
If you’re using a differential that doesn’t have a pinion support, tie the truss into the diff cover.
Resident Offroad Design rep
Last edited by 38377k5; 05-31-2008 at 11:25 PM.
|05-31-2008, 06:48 PM||6|
Here are the rest of the pics:
Axle roughly at ride height, you can see how the links are positioned.
Making sure that nothing is binding:
A front view of the truss installed:
The beginnings of the lower link mounts, this required some extra fabrication because the joints and tabs are wider than the frame:
I gusseted the lower link mounts and made sort of a ramp so that it can slide over rocks rather than getting caught on them:
Uppers, I haven't finished reinforcing the frame yet:
From above, the exhaust is temporary:
I used coils that came from the front of a stock Jeep TJ. They are very soft (165 lbs/in) and because of that the rear now flexes more than the front. I’m in the process of adding a sway bar to stiffen the rear up a little bit and to help with stability (though it still felt very stable on my last trip).
Some preliminary testing on the trailer:
I only have a couple runs on this setup but so far it has worked well. Axle wrap has kept me from trying obstacles in the past and I'm glad that those days are behind me
|05-31-2008, 08:45 PM||9|
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