Project BMP

Page 27


Suspension Fabrication

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I didn't get a whole lot done on the axle swap during the holiday weekend, I was on baby detail.  I was able to play around with the suspension geometry in the 4 link calculator spreadsheet and learned quite a few things about link placement.  One of the biggest lessons learned is you shouldn't compensate for tire height by adding more nitrogen to the airshocks.  Doing this has a pretty big impact on the anti-squat numbers and roll axis.  What this means is you really have to design your suspension around a specific tire size and stick close to that size of tire.  I designed my first suspension around 38" tall tires but tried to compensate for the short height of my well worm 37's which were closer to 35.5" tall at trail pressure.  I believe this caused issues with climbing steep obstacles and induced extra axle steer when the axle articulated.

I also didn't know just how quickly the anti-squat numbers climbed for the drooping tire. In my particular case if one of the rear tires droops 10", the anti-squat percentage on that tire can quickly go well above 200%, limit the droop to 8" and it is only around %150.   Really high ant-squat numbers on a drooped tire can cause it to want to drive under the rig which can lead to wheel hop.  Armed with this new knowledge and knowing my tires will be 40" tall at 5psi I made some adjustments to the suspension.  I am aiming for a neutral anti-squat number (90-100%, 100% being neutral, no squat or anti-squat) with more up travel than my first effort.  With the new bigger tires I won't need the suspension for much lift and shouldn't need as much down travel since 42's should help fill bigger holes than my 37's did.

Up front I fabricated an axle bride to mount the upper links to. I welded a flat steel plate spring perch using some hi-nickel welding rod and a stick welder.  The bridge was then welded to the plate using my mig welder. I also fabricated a plate that bolts to the diff housing using the existing 5/8 spring plate bolt holes,, the axle bridge was then welded to this plate using my mig welder.   On top of the differential housing the bridge is braced via some tabs that were already welded to the housing, I just cut the tabs down and welded the axle bridge directly to them.

Since I will need to control the up travel of the suspension when articulating due to some very close fits I mounted the bump stops further outboard on the frame.  I will need to make a small plate to weld to the axle bridge to give the bump stop a nice big target to hit.  I have two spots I need to watch for interferences when the suspension cycles, one is between the driveshaft and the clutch cable lever on the transmission. The other is between the upper link on the passenger side and the alternator.  I will definitely be compressing the suspension all the way to check clearances on these two points before final welding.  Mounting the steering ram is next on the list along with beefing up the steering arms.

Back view of the front axle showing the upper link mount placement.

The upper link mounts on the frame end were extended down almost two inches for a flatter roll axis and to bring the anti-squat percentage down when the suspension is at full rest on 42's.

To get more up travel I had to rotate the upper shock mounting tabs upward.  It was easier to cut and re-weld new tabs which netted about 3" of up travel.  I am also going to try running some 10w shock oil in the front shocks (factory is 5w).  A few people have told me the increased oil weight provides a better controlled ride and less stiction.  Since the shocks are out of the rig, now is the time to make changes.  I also increased the oil in shocks to 435cc's from 425cc's.

Out back things are nearly complete, the lower tabs and shock mounts are all welded and painted.   The upper shock mounting tabs were also rotated upward  about 2".  With the weight of the chassis sitting on the coilovers I have approximately 9" of up travel and plenty of adjustment left to adjust the chassis up or down.  I am using a 130lb spring over a 175lb spring on the coilover. I also changed the oil on the coilovers while I was at it since I had a lot of new 5w from the airshocks.  A quick call to Swayaway and I knew how much oil they needed: 620cc's. The oil I got out of the coilovers looked pretty dark and murky like it was well used.

The upper link brackets on the axle end out back were plated along the back side.  Boxing a link mount like this adds some strength but also makes it look nicer.  I still have to figure out some bump stop mounts and a tab to mount the rear limit strap to the axle housing.

Update 6-4-05 - Most of Saturday's effort was concentrated on the front end.  The previous night I had mocked up the steering cylinder location and on Saturday morning I got right to work mounting the hoop and then cutting a plate that would serve as the cylinder base.  I also put an extra strip of material along the front edge of the hoop since it will get up close and personal to the rocks.

Next I cycled the suspension to figure out where I needed to build my bump stop targets.  I built the targets from a bunch of plasma cut pieces of 11 gauge steel.

Oddly enough the spacing between the steering arm holes is exactly the same as my Toyota axles.  I drilled out one of the tapered holes to 3/4" and will modify the steering linkages to use rod ends on both ends.  I plan on modifying the steering arm so I can get the bolt for the rod end in double sheer.  I would also like to somehow tie the upper arm into the knuckle with some sort of bolt on gusset.

Out back I boxed in the rear coilover mounts.

Update 6-5-05 - I managed to hit my goal of having the rig resting on the suspension by Sunday afternoon.  I have a few more items to brace but the bulk of the suspension work is done.

The front axle is at ride height but the shocks are actually sitting a little high.  I added some extra nitrogen to account for the settling that occurs when the oil mixes with the nitrogen.  The shocks are filled with 430cc's of 10w oil and I filled them with 230psi of nitrogen.  belly clearance should be right around 25". 

The rear axle is sitting 2" lower than ride height so the links will be flatter when the tires are mounted.  I still need to come up with some bump stop mounts and then decide where I want to mount the swaybar mounting tabs on the axle.  The goal for this coming week will be to complete the steering, rear bump stops and brake lines.

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