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620 4-link, or 'How to make your life difficult'


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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.


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Vertical posts made of 1 x 2 x 1/8 wall rectangular tube.

First picture, right side just behind the axle, but ahead of the stock chassis cross tube, and left side just ahead of the axle.


The left side is where I had to countersink the bolt heads into the axle bracket to get clearance, and the right side, required a half height head on the bolt that holds that Watts tube, to clear the axle.


On the left side, the top inner edge of the axle bracket for the 4-link tubes had to be tapered to clear the vertical post for the watts link tubes, or it would rub when the axle was up against the bump stop, and the right was extended all the way down.



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Panhard would have been good enough without being so complex. Do you think those two vertical bars the links are anchored to will support without flexing? Does it have to be on the bottom of the differential? Could you not mount on top and the side supporting ends would be shorter and less leverage on them for side to side flexing?


Better ground clearance also.

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A panhard bar's roll center is to one side of the center of the vehicle, where it attaches to the axle, not centered.

OK for a drag car, or an oval track car.

I didn't want that.


A Watt's link roll center is at it's pivot point. Mounting it at the top would have been much easier, but would have raised the roll center higher than it was stock (with leaf springs, is considered at the center of the diff). 

A high roll center leads to lifting the inside rear wheel, and can cause roll overs, like the old swinging axle VW bugs.


I didn't want to extend it way back from the axle to clear the shocks, and possibly cause clearance issues with the spare tire.

Then I stumbled on to a Lotus-7 build where the guy went to the bottom, and liked it, and it's benefits.

Ground clearance isn't an issue for me, as my oil pan will be lower, and I have no intentions of off-roading the thing.


Doubt that there will be any noticeable flex, more than what the stock frame already does.

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Seems that the 5/8" swedged tubes (7/8" dia, .062" wall) are a bit on the wimpy side, even for a sick L18.


Side stepped the clutch at about 5k, and this happened.




To be fair, I do run a twin plate OS Giken clutch, and 4.6 rear gears, so the shock load was probably a bit on the high side,

even though, my flywheel only weighs 7 lbs, and the engine is on its last legs.

And only the bottom tubes that were in compression collapsed, top ones that are in tension, are fine.


So I ordered in some 1" OD, .095" wall DOM tube, and weld on bungs from Steinjäger.

Will post pictures once I have welded them together.

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Id still recommend you go with the 1.25". The 1" is just borderline for you. It'll probably work just fine, but there's a lot to say about trusting your equipment. I ran 1.25" in my ae86 autox/drift car when I re-designed the rear end suspension and it didn't feel like it was too much. There's no major weight gain, the price difference isn't significant and you've gotta re-do them anyway so might as well.

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  • 8 months later...

Bit late to this but i wouldnt use anything smaller than what beebani suggsted.

You may have more load on them as well because the distance between your pivot points are quite close together at the diff end. Are larger spread would give them less of the axle twist to effect them.

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I've had no more issues, and it's been daily driven, abused.

Thats good to know. I am from the over engineering camp and end up making most thiings too heavy, it looks like you have hit the balance between light and strong enough to do the job. I spend to much time working on 500 ton machines! Every thing is made out of inch thick plate.

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And even the big stuff gets torn up now and then.


Many years ago when Triumph went after the AMA crown, they made every to break in the engine, then went slightly stronger on parts until they had no breakage, and Gene Romero took the crown for them.

(but that was back before CAD, and computer analysis of how things like rocker arms, and rods flexed) 

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