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


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#1 G-Duax

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Posted 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....
 
Na....
 
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:
 
DSC08374_1.jpg
 
DSC08381.jpg
 
DSC08376.jpg
 
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.
 
DSC08379.jpg
 
'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.

 



#2 ]2eDeYe

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Posted 07 April 2017 - 12:35 PM

Nicely done :thumbup:


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Whenever I think that I'm out of my mind for putting this much time and money into an old Datsun, all I have to do is look some of your threads to know that I'm not nearly as whack as you are.

You're going to skin a few knuckles, stand up too fast and bump your head, hunt around the floor for the nuts and bolts you dropped, invent a few new cuss words and when you're finished you'll say "That wasn't so hard after all!"


#3 flatcat19

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Posted 07 April 2017 - 01:04 PM

Hm.

You can't hang if your nuts haven't dropped...it's physics


#4 Lockleaf

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Posted 07 April 2017 - 02:28 PM

Where are the watts links connected? Interesting design. I've not seen one layed out that way before. Do you have any clearance issues with that setup?

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#5 G-Duax

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Posted 07 April 2017 - 10:14 PM

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.

 

DSC08380.jpg



#6 datzenmike

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Posted 08 April 2017 - 09:45 AM

Hm.

 

 

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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#7 G-Duax

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Posted 08 April 2017 - 01:00 PM

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.



#8 distributorguy

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Posted 25 April 2017 - 12:55 PM

A panhard bar can also induce massive bump steer, something that's far less likely with a Watts-link due to the center attachment point.  



#9 G-Duax

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Posted 01 May 2017 - 02:02 PM

Update:

 

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.

 

DSC08382.jpg

 

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.



#10 2wheel-lee

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Posted 01 May 2017 - 02:51 PM

Aren't those 5/8" (ID) tubes usually used for steering links? 



#11 willz

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Posted 01 May 2017 - 04:49 PM

All the ones I've seen are around a 1.25" ID & .156"

#12 G-Duax

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Posted 02 May 2017 - 12:46 AM

Most likely to take the abuse of a pro-stock car.

A little L4 powered truck with street tires doesn't need that type of strength.



#13 willz

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Posted 02 May 2017 - 02:35 AM

I have a terrible habit of overbuilding everything

#14 ]2eDeYe

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Posted 02 May 2017 - 06:41 AM

Most likely to take the abuse of a pro-stock car.

A little L4 powered truck with street tires doesn't need that type of strength.

 

 

Apparently it does :lol: 


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ratsun.net rising sun stickers                     ratsun business cards order thread

620 KA24 Motor Mount Swap Brackets

1977 620 KingCab Deluxe KA24DE Swap - Refresh and cleanup
1978 620 kingcab 4x4 - KA24E SAS w/Dana 44 front and Toyota rear

1967 RL411 Station Wagon

 

Whenever I think that I'm out of my mind for putting this much time and money into an old Datsun, all I have to do is look some of your threads to know that I'm not nearly as whack as you are.

You're going to skin a few knuckles, stand up too fast and bump your head, hunt around the floor for the nuts and bolts you dropped, invent a few new cuss words and when you're finished you'll say "That wasn't so hard after all!"


#15 G-Duax

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Posted 02 May 2017 - 11:31 AM

Apparently it needs something stronger than the cheapest steel tubes that I could buy.

Confident that the 1" x .095" wall dom will take care of the issue.



#16 Lockleaf

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Posted 06 May 2017 - 11:55 AM

So you're telling me that building a 4link out of the old trampoline frame in my backyard is a no go? Damn....

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#17 willz

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Posted 06 May 2017 - 01:22 PM

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.

#18 BEEBANI

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Posted 09 May 2017 - 05:52 AM

My lowers are 1.25 x .120 wall DOM.  Pretty common size for link bars.