leaf spring modifications

[in progress]

This page details the modifications to the leaf springs on my truck.  My design goals were (1) a high-compliance, comfortable ride, (2) minimal resistance to movement, (3) minimum weight, and (4) minimum leaf twist.

I ordered the leafs from Flex-A-Form in 235 lbs./in., the lightest spring rate that F-A-F was comfortable providing.  I will add rate with the helper airbags.  Note that in practice, F-A-F states that their ratings should be reduced by about 20% to compare with steel.  if this is accurate, then a 235 lb. spring comes very close to the stock 180 lb. rate.  I ordered the springs with the eyes reversed for 1" more drop.

On the rear, I installed modified spring sliders both to provide smoother spring travel, and to allow for easy ride height adjustment (the sliders can have spacers underneath to raise ride height).

On the front, I installed modified Deaver Spring Baja Bushings.  These allow the leaf to articulate without twisting, and help to prevent static twist.  With no interleaf friction, and both ends of the springs riding on bearings, this is the most friction-free leaf spring system possible.

Flex-A-Form rework:

The fiberglass was not smooth before they were painted, and the paint was beat up. 

The bolts are installed without washers, and are torqued down so hard that they dug into the aluminum (which is some sort of super-soft alloy).  The adhesive used is Liquid Nails.

The center pins are skinny bolts, with the "pin" being a the bolt head with the edges ground off.  Note that F-A-F paints the white smooth-faced nylon that they use for the mounting gaskets.  There is about zero possibility of that paint sticking.  In this photo, I scratched it off with my fingernail.

The only good thing that I can say about the build quality is that the two springs are identical, except for varying thickness at the ends where the eyes slip over.   I have heard that F-A-F makes the leafs as one unit, then cuts the unit in half.  I could not verify this from a visual inspection.

I then stripped the leafs of paint, and spent several hours smoothing them down.  I then sprayed them with many thick coats of epoxy.

As long as they were off, I sanded and buffed the eyes.  The eyes are shown here with the front pivots and rear slider bushings installed (more on that below).

All of the hardware was replaced.  I bored out the eye mounting holes from the supplied 5/16" to 3/8".  Note that the fiberglass can't be drilled with a standard bit -- it will grab and crack the fiberglass.  A Unibit worked great.  I used 3/8-16 X 1.5" flange bolts, with flanged locknuts.

I replaced the center (axle locating) pins with much larger bolts.  Not because of strength, but to be able to mount my airbags directly to the center pins (I could no longer have them mounted on the leafs, as the steel airbag mounts would dig into the fiberglass).  I bored out the holes and used 1/2"-13 X 2.5" allen head bolts.  Those without airbags will want something in the 2" range.  The hole on the axle needs to be slightly bored out to accommodate the head of a 1/2" allen bolt, and the head needs to be ground down just a tad (it's a bit too high).

Note that with the eyes flipped, the top of the eye may make contact with the edges of the front hangers.  Check the eyes for contact, and clearance the hangers with a grinder if needed.

sliders:

Since RUSlow stopped making sliders, I copied his design.

I used Capital Motorsports roller bearing sliders from Speedway Motorsports.  The rollers may rattle a little, but they just seemed more substantial than the nylon-bearing sliders formerly used by RUSlow.

Here is a comparison of the bearing sliders from Afco (left) and CMS (right):

The Afco is beefier, but the CMS has better mounting options, and is cadmium plated.

They both use a 1/2" grade 5 bolt.  I upgraded to some "airframe" or "An" bolts (AN8-45).  I found a great deal on them at Aircraft Spruce.  If you want to use a full-size nut, you will need a longer bolt.  The only nuts that will fit these bolts are the short AN nuts ("jam nuts" in AN parlance).

The AN bolts are considerably stronger (and way more expensive) than Grade 8 bolts, but it's a small price for peace of mind.  AN bolts are specifically designed for maximum strength in shear applications like this.  Most importantly, SAE bolts are designed to be used in tension, and the shafts are slightly undersized.  An bolts are not undersized.  A -8 AN bolt will fit within .0001 into the 1/2" bore of the bearings and spacers, making a stronger assembly.

I also sourced wider bearings with a higher load rating (1,000-1,500 lbs versus the supplied 501-1,000) from McMaster Carr -- I essentially used the Afco bearings on the CMS sliders.

For the slider mounts, I ordered the following pieces of A36 hot rolled steel (precut) from onlinemetals.com:

(2) 9" x 8.5" X .25" (top plates)

(2) 9" x 4.25" X .1875" (side plates)

(1) 6" X 6" X .25" (gusset stock)

The sliders and mounts weigh 21 lbs, and the stock hangers and shackles weigh 12 lbs, for a net weight gain of 9 lbs.  But this weight is worth it.  The stock rear hangers are stupid flexible.  The slider plates are stiff as a board.  Note also that I could have saved a couple of pounds by making the plates smaller, but I wanted to be able to provide for adjustability for different leafs if further experimentation is needed in the future.

It's best to drill the frame mounting holes before the plates are welded.  It would be difficult to chuck these up on a drill press after they are welded.  The upper holes of the side plates are right up against the intersection of the top plate.  I mocked them up with c-clamps, and used a sharpie to trace the holes and mark the line of intersection between the plates.

Once removed, and the holes drilled (with my trusty Unibit), I used magnetic triangles to square everything up.  The braces can then be cut.  After everything was cut and mocked up with the magnets, it was easy for the welder to weld everything up without making any mistakes.  I just set the assemblies on the welding table and walked away.

I then mounted the sliders to the frame to mark the locations for the sliders.  It's important to note that the leafs are not perfectly parallel to the frame, so it's not possible to mount the sliders perfectly parallel to the edge of the plates without inducing bind.  Using a framer's square held against the leafs, you trace the line of the leafs onto the underside of the sliders.

Then hold the sliders roughly in place, and mark for the first hole.  The sliders should be installed to where the springs are at about 1/3 of their travel at rest.  In other words, the rear spring eyes have 1/3 of the slot to go forward on extension, and 2/3 of the slot to go backwards on compression.

To minimize bind, and ensure that the sliders are as parallel to the leafs as possible, the best practice is to then mount the sliders to the leafs, and the one hole, and put weight on the frame to force the leafs to compress.  Then check to make sure that the lines that you scribed before are accurate, and drill the remaining holes.  I used 1/2-13 flange bolts and flanged locking nuts to mount the sliders to the frame.

Since I used Flex-A-Form springs drilled for 1/2" bolts, they mated right up to the sliders.  The stock springs may need custom bushing work.  Energy Suspension and Prothane make just about every kind of bushing and sleeve you can imagine.  Also note that the F-A-F leafs are slightly longer than the stock leafs, so the sliders must be a little further back to keep with the 1/3-2/3 rule above.

For the Flex-A-Forms, I had to rework the rear bushings if I wanted to use the aluminum bushings supplied with the sliders.  I bored out the bushings to 15/16" (a snug fit for the aluminum pieces), and then used a small belt sander to remove the flange portion of the bushings.  Since the F-A-F bushings do not fit snugly into the eyes, I smeared a coating of black RTV in the bore, then inserted the bushings, then applied pressure with a 4" c-clamp to swell the urethane into contact with the inside of the bore, and left it overnight to dry.  Seems to be working okay.

front pivots:

The Deaver Baja Bushings are just spherical bearings that slip into the front eyes.  They use a common 5/8 bore COM10 spherical bearing, sleeved down to a 9/16" bolt.

They will not work as supplied.  The stock front eye bolts are 18mm (.719"), but the BBs take a 9/16" (.5625") bolts.  I found some 9/16" ID X 3/4" OD bronze bushings on eBay (the stems have to be trimmed way down to fit).  The front hanger holes have to be slightly bored out to 3/4".

I also bought some AN 9-42 bolts (9/16-18 thread, 4.4" L shank).  As with the sliders above, they make for a nice, tight fit in the spacers.  The fit is so tight that I had to use fine sandpaper to smooth the shaft of the bolts and the inside leading edge of the sleeves.

I also upgraded the spherical bearings with Aurora Bearing PWB-10T bearings from HRP World.  They are nearly three times as strong as a standard COM10 bearing.  In off-road racing applications, the supplied bearings will pound out, sometimes over the course of a single race.  On the street, or in mild off-road use, they will last much longer, maybe even the life of the truck.  But for $64, I figured it was worth it for the peace of mind, and reducing what would certainly be a PITA maintenance item.  Note: to replace the bearings, you will need some very long circlip pliers.

The housings allow a little slop in the fit between the bearing and the shell.  I actually returned one because the bearing was rotating in the shell, rather than the ball rotating in the bearing.  To help prevent further slop, I added some blue Loctite in the bore before I drove in the new bearings (use just a little, or it will get into the circlip groove).

Finally, the BBs are just a hair too short for the F-A-F eyes, so the eyes have to be ground down a little on the sides with a belt sander.  The OD of the BBs is 0.03" smaller than the ID of the Flex-A-Form eyes, so I epoxied them in place by coating the inside of the eyes with JB Weld.  The only way to get them out now would be to heat the eyes to somewhere near the JB Weld melting temperature of 600°.

spring mounting:

After investing all of that time and treasure into the leafs, I couldn't mount them up with crappy hardware.  So I found some nice 3/8" thick spring plates from Foothill Offroad (part # FO1791).  These are ideal for fiberglass leafs, as I found that the stock plates also create four distinct pressure points on the leafs -- not a good thing for fiberglass.  The only downside is that they are about twice as heavy as the stock plates.

Since these spring plates are  drilled oversize, I had some custom u-bolts made by Deaver Spring.  9/16" OD x 7" long were the closest Deaver could supply (stock is about 12mm).  I mocked them up, then cut them to size.  To protect the bolts from rust, and minimize the abrasion of the threads against the sides of the leafs, I covered the bolts in 3/4" shrink wrap.

Also note that the center hole needs to be enlarged slightly if you use 1/2" center pins.  The nut is just slightly larger than the ID of the center pin hole.

I sprayed the plates with a black epoxy to prevent rust.

Rear suspension before (Hotchkis leaves, 2" drop shackles; helper airbags and and panhard bar removed; frame partially stripped):

Rear suspension mockup being tested with high-tech methodology:

Finished product during final assembly:

More photos of rear suspension here.