4 Link Rear Suspension

 

   
 
 

4 Link Rear Suspension - Page 2

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The next decision was how to connect the links to the frame and axle.  The options include rubber bushings, Johnny Joints, heim joints or a combination of the these.  One factor to consider is whether or not your rig will see a lot of street driving.  Many people believe rubber bushings are better at reducing the shock loads the joints will see from street driving vs. heim joints.  If any of your links are going to see a lot of side load you'll want to avoid using a heim or Johnny Joint.  A good example of this is on a 3 link suspension where the upper links converge into one joint at the top of the axle.  That single joint will all that keeps your axle centered and it will see enormous side load.   Since I was going to triangulate all four links thereby spreading the stresses amongst the links I decided to go with 3/4" heim joints.  Under lightweight buggies, these joints have a good track record and appear to hold up well.  If I were building a heavier rig I'd go a combination of rubber bushings and Johnny Joints.  

Suspension Materials and Sources

  • Links -  1.50", .250 wall DOM tubing.  All the links are 40" long.  After running these for a few months I would recommend either using .50 wall  or bracing the links with a rib along the top.  The .250 wall lower links both had a slight bow in them from dragging them over obstacles, I added a stiffening rib made from a piece of 1 x .375 steel to both links.

  • Threaded tube Insert - #15589 from Coleman Racing (get enough for one spare link while your at it).

  • Heim Joints - #AUR-XM10 from Spidertrax, ask for teflon race (don't forget to order the jam nuts). 

  • Misalignment Washers - #MSW-58 from Spidertrax.

  • Link Tabs - Flexy Flyer (local race car shop).

  • Gussets - #950 from Coleman Racing (1 bag of 24).

  • Coilover Shocks - Sway-Away 2" Coilovers, 16" travel (I recommend going 14") w/ dual rate kit.

  • Springs - Initially Eibach 2.5" dia, 14" long 90lb and 100lb coils from Polyperformance. I also needed a helper spring since I couldn't get 16" coils in a low enough spring rate.  This is why I recommend going with a 14" travel coilover for a light rig, it'll save you having to buy the helper springs.  As of 6-03 I am running a 150lb spring under a 120lb spring.  The 150lb QA1 springs are from Summit Racing, the 120lb set is borrowed, I will eventually order a 130lb spring from summit (the closest to 120lb they have).

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Now it was time to get my hands dirty.  First, with the frame sitting on jack stands at the desired final ride height,  the old suspension was removed along with the back half of the frame.  Next I positioned the rear axle to the desired wheelbase and squared it with the frame.

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From here on out I had my friend Erik lending a hand.  First we built a small outrigger and braced it in all three planes.  the outrigger was located as close to the plane of the t-case output flange as I could get it.  Next, we attached the link brackets to the heim joints on the link. Holding the link roughly in place we moved it as close as possible to the CV joint on the passenger side, this put the two links about 21" apart.  We marked that mounting point.  With Erik still holding the link I positioned it roughly where I wanted it on the axle tube.  I made a mark roughly on the centerline of the tube and 17.5" from the center of the differential.  The link brackets can now be tack welded onto the outrigger and axle.  We eyeballed the other side and made a few measurements after the link brackets were tack welded to confirm the second link's placement.

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As you can see the link angle isn't very steep, I measured about 10 degrees.  We moved on to the upper links.

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I cut some access holes into the floor so could get to the top of the frame.  Since all my links were the same length, we just needed to locate the upper mounts on the differential housing and that would determine where the front mounts would go.  The mounts on the housing were spaced 10" higher than the lower mounts and as close as possible to each still allowing me to get a bolt and nut thru the brackets (4.5" apart).   I utilized my previous traction bar mount, bracing it with an axle truss.  This upper mount needs to be very strong so don't be afraid to go overkill on the gussets. 

Holding the upper links so they were parallel to the lowers showed us where the front link brackets needed to be.  I made a cross member and welded it to the top of the frame.  Next we tack welded the front link brackets to the cross member.  They ended up being about 6" above the lower link brackets and roughly 22" apart.

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Here's a side view showing the angle of the links in relation to each other.

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A view looking from the axle forward.  Notice how crowded it got with all the linkages?  I checked each link for chassis clearance and drive shaft clearance before welding the tabs on for good.

Here's a good diagram drawn up by Nate Moore.  It clearly shows why my links ended up the way they did due to the loaction of the transfer case driveshaft flange.

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With the link mounts figured out I set about bracing the link brackets and cross member.  Each link bracket is braced by one of the small roll bar gussets mention above in the materials list.  The cross member was braced via a tube from the frame seen in the second picture above.  Next, I ran a 1" dia tube from the cross member to the outrigger.

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Locating the coilovers proved challenging.  I wasn't sure how much the springs would compress so I had to mount them and put the weight of the rig on them to get a good idea on where to place the upper mounting points.  The lower mounts were initially mounted perpendicular to the axle tube as shown in the first picture but the ball joint would bind before full droop or stuff was reached.  The final configuration looks like the second picture with the mounting brackets inline with the axle tube.  The coilovers are angled inward appoximately 20 degrees and forward  roughly 25 degrees to cut their dampening rate a bit and to allow a little extra travel. 

Here's the dimensions for the final coilover mounting location.

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