TR-3/4 Front Suspension Upgrade


TR-3 and TR-4 front suspension conversion to TR-4a through TR-6 pieces

 Why would you want to convert from the earlier front suspension bits to the later bits?  Two reasons:

1. Main reason - the upright / vertical link is beefier in the later suspension.  The screw that goes into the trunnion is 0.125" larger in diameter although it also has a hole through the center connected to the grease zerk.  The upper portion of the vertical link is larger in cross section too.  The TR-3/4 screw that goes into the Trunnion is 13/16" diameter.  The TR-250 / TR-6 one is 15/16".  I calculate that this is a 33% increase in cross section area.

2. The Trunnion connects to the lower wishbones with a through-bolt rather than the studs used in the earlier trunnion.  This makes disassembly / reassembly / maintenance easier.

The later pieces are pretty much a direct bolt in replacement.  If you have the zero degree caster setup, you will be going to 3 degrees of caster so will need to replace the upper A-arms and upper ball joint.  If you have the 3 degrees of caster setup already, you will be replacing the upright, lower trunnion and lower wishbones or you can also use a pair of spacers and longer bolts and use the original lower wishbones.  Where the trunnion bolts to the lower wishbone is narrower in the later trunnion.

Installing the later parts and making no other changes will lower the front of the car by about an inch.  Richard Good sells spring spacers that are 1/2", 3/4" and 1" in height that can be used to compensate for this change in ride height.

Also, the lower wishbones for both the old and new suspension bits can be installed in two different orientations.  If you flip them upside down from the stock installation it will lower the car.  The inner stud hole may need to be tapped all the way through to allow you to move the stud from the top to the bottom of the wishbone when it's flipped upside down.  If you flip the lower wishbones for the lower ride height, you want to make sure the inner spring pan studs don't stick out too far or you'll have trouble getting the nut on them.  1/2 to 3/4 of an inch of stud protrusion is all you need.  Use medium strength threadlocker on the studs so they don't screw out the next time you disassemble the suspension.

In my case the original lower wishbones were already set to the "lower the car" orientation (upside down).  I used the TR-6 lower wishbones in the same orientation and a 1/2" spring spacer.  The result was that I still lowered my front end.  I ended up flipping the wishbones back to the "raise the car" orientation.  I was able to swap them in pairs (do both rear wishbones first then both front wishbones) to avoid the pain in the *ss of screwing around with the lower spring pan to get it lined up with the wishbone.

Tip: use one of the steering arm attachment bolts as a locating bolt to help line up the spring pan with the lower wishbone - it's nice and long and you can shove it through both the wishbone and spring pan while trying to compress the spring and spring pan up to the wishbone.

When you swap the lower wishbones, you must use the earlier inner bushing as the inner diameter is different between the earlier and later setups.  The later setup uses a bolt rather than the "stud" welded into the frame, and the bolt is smaller in OD than the piece welded into the frame.  So you use the TR-6 trunnion / outer bushings in the lower wishbone but the TR-3/4 inner bushing in the lower wishbone.


Tip: You can use upper ball joints from a Jaguar XJ6 to gain some caster adjustability.  These upper ball joints are 0.150" thinner than the stock ones.  You use 0.150" in shims to get it back to the right width, but you can choose which side of the ball joint to insert the shims.


Tip 2: unrelated to this parts swap, but from an excellent article regarding TR-3 front suspension in the Nov 2006 issue of Classic Motorsports...  To reduce bump steer when lowering the car, you can use "bent" tie rod ends from a late '70s / early 80's Chrysler product like the Cordoba, Carquest part numbers ES401R and ES401L.  Also use the steering arms from the later (TR-6) front suspension.  I looked up that parts mentioned in the article and it appears that the only way they would work is if you shortened the tie rod and threaded it further up the shaft - the Chrysler tie rod end has threads like a bolt rather than a threaded hole like the stock tie rod end.

Reader feedback: Terry states "TIP #2 does not make sense.  The threads are rolled onto a TR steering "rack end". this means the threads are proud of the shank and you cannot safely cut & re-thread one. There are 3 possible steering rack end lengths".  Tony comment: On my rack, the threads are not proud of the shank and I was able to thread them further up while shortening the rack end.  However, another rack I recently purchased has a larger OD on the non-threaded portion which would make it very difficult to modify.  It seems that the rack ends are manufactured in several different ways. 


Tip 3: I IS possible to use the TR-6 trunnion / upright with the stock TR-4 lower wishbones if you create spacers for each side of the trunnion and use a longer lower pivot bolt.  Pictures of Christian Marx's installation done in this manner are at the bottom of the pictorial section.


Pictorial section (click on any picture to expand it to full resolution - use back button to return to this page)

TR-4 vs TR-6 wishbone - darker one is the TR-4 wishbone - upper one in first pic, "underneath" one in the next two shots

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TR-4 (left pic) and TR-6 (right pic) wishbone showing offset between right side up and upside down

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Comparison of TR-4 and TR-6 trunnion showing that the pivot point hole in the TR-6 one is about an inch lower than the TR-4, lowering the car and increasing camber gain (the TR-4 trunnion has studs sticking out of it)

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Comparison of "screw" diameter and why it's important.  It's kind of a big deal when it breaks.  Wheel shoves up into fender, points in funny directions, and you basically lose steering.

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Diameters of base of screw thread (TR-4 on left, TR-6 on right)

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Tapping inner spring pan stud hole through the wishbone, installed stud with threadlocker

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1/2" spring spacers (should have used either 3/4" or 1", it turned out)

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Assembled suspension

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While messing with the front suspension I discovered I had odd radial cracks in both front rotors, so I replaced them with new cryo-treated rotors from Frozen Rotors.  They machined slots in the surface to vent the outgassing from the front pads as they are used.  The cryo-treated rotors don't warp like the non-treated ones do and seem to be easier on the pads for some reason.

Old rotors showing cracks (you'll need to click on the a pic to see the full sized one to really see the cracks)

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New rotors being installed

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Final assembled suspension pics

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Installation of TR-6 trunnion / upright in stock TR-4 lower wishbone (courtesy of Christian Marx).  The picture of the bolt is a comparison of the stock TR-6 lower pivot bolt and the longer bolt that Chris used.

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