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Member Since 15 Jan 2016
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620 4-link, or 'How to make your life difficult'

07 April 2017 - 12:06 PM

My Datsun 620 work truck decided to snap another main leaf spring.
Partly because it's a 44 years old, partly because of the 2" lowering blocks, without traction bars, that allows a lot of spring wind up.
Probably has nothing to do with the fact that I have no concerns about side stepping the OS Giken clutch at about 5k....
Anyway, leaf springs for the old dog are about impossible to find, and when a set pops up, they are rust pitted to hell.
So I decided to go 4-link :)
Now there are tons of kits to do this with good ol 'merican cars, but only one guy makes one for the 620, and the design wasn't something I wanted to go with.
So I bought some parts (meant for 'merican iron), and some steel, and made some measurements.
Came up with this:
Note the bottom mount Watts link, which give the lowest possible roll center.
The Watts link bell crank is form a PT Cruiser.
The 'difficult' part was all the little head scratchers that popped up along the way, like clearance issues.
Most solved by machining a bolt's head to half height, but one, the left side rear 4-link bolts, didn't even have room to do that, so I had to go to a FHCS, countersunk into the axle bracket.
'How did you hold the bolt in order to tighten it?'
Trickery, Magic, and Pure Luck !
Actually, I added serrations to both the underside of the bolt, and the tapered seat in the bracket.
Oh, and the real headache was that after doing research before starting the project, I never found anything about WHY you don't use a drag race 4-link on a street vehicle... 
The reason why (some things are better learned the hard way), is that the drag cars use non-parallel bars, that gives them better bite, but...
Does not allow the chassis to lean in the corners.
Boy, did I ever have an Oh S#it moment when I first tried to move the axle through it's normal path!
It would not allow one side to go up alone!
But after figuring out what was going on, I drilled new holes in the rear axle bracket to make the bars parallel, cut the bottom 4" off the axle brackets, and all was good.
How do I like it ?
The truck is worlds more stable !
It also make the rear end stick like glue in the corners.
I have one favorite highway interchange on the way home from work, that is a 270 degree clover leaf. I was getting into the habit of drifting it, just to unwind after work. Was nothing to enter the curve, jab the gas peddle, and initiate a drift with the old leaf springs.
Now, I can't get it to pop the rear end out at what I would consider a safe entry speed.
When I feed more of my limited L18 power to the rear wheels, the truck is very neutral, and pushes just a tad before the back end starts to slide.
Maybe when my 'L16' goes into the truck, and I have more power to deal with, that may change.
But I don't want a drift toy anyway, so for right now, it handles just the way I want it.
Not tail happy at all, like all my other Datsun trucks were.