Triangulated 4 link question/help

Quick question for anybody thats willing to help. Ive been working on this triangulated 4 link for my 53 chevy 3100 and I have come to a stop with mounting the links to the frame rail. I ran the number in the excel sheet at came up with level lower links and 5-7 degrees down towards the frame upper links. Tack welding the axle tubes I went to set lower and I noticed that its going to take quite a bit of fabrication to make the lower links level. I can mount them to the bottom of the frame rail and have a 4 degree uphill to the bottom of the frame with ease. My question is how badly will this effect my anti squat and roll steer? I put the numbers in and it only seemed to change by 1 degree roll steer and 2% anti squat. I also tried moving the upper link up by a couple degrees and it didnt seem to change anything ether.

Thanks for any help!

 

For an update.... I was able to fabricate the spacer needed to get the lower links level at ride height. I then put a 7 degree downslope to frame for the top links. It gives me 0 percent roll steer and 60% anti-squat. The antisquat isnt ideal but hopefully it will work ok.

 

What is your instant center height and length (from rear axle centerline)?

 

Good question, you didnt say what youre doing with the truck, drag racing, road racing, or a street truck. Also need to know the weight and horsepower. For the street, parallel links are great. Drag racing instant center depends on a lot of factors, but for a low horsepower truck I'd go with something like 50 inches out, 7 to 10 inches up. Our 7 second door car runs a 44 inch out, 7 inches up four link as a base. This gives a hard hit to the tires, but its a 1500 hp, 3000lbs car.Did you ever think about a three link? Really a better street set up. Rear end rotates freer, less bind. Look at factory fives web site, they offer a three link option for road racing. Note how they mount the top link. Good luck.

 

I explained the geometry of triangulated four-links diagramatically here.

Four-links with a Panhard bar/Watts linkage/etc. should ideally be parallel in side view for street use. This is not necessary with a triangulated 4-link: it is entirely feasible to have the upper and lower bars converge to the front in side view on a street application. That means that all that launch-oriented geometry and stuff is there to be exploited without inducing binding.

What software are you using for the roll steer and such? I spent a few days looking for something free that would do that before realising that I could get fair approximations using trig and Pythagoras. If you consider the lower arms as determining roll steer completely on their own - they don't but the difference is minimal - it is quite simple to calculate the fore-aft displacement of the axle ends of the lower arms with roll.

 

Nice job, Dawie. I missed that the first time around. To the OP, it sounds like you already have this part of it figured, but worrying about perfect anti-squat isn't the most important factor on a street vehicle. More anti-squat means less braking stability, which is equally (if not more) important on a street project.

 

Ned, thats a lot of big words you just spoke there.. lol. I used Dan Barcroft excel spreadsheet to determine the number.

My instant center is 8 inches up and 57 inches out from rear.

Its a street truck that may see some strip time, 450 ponies / 550 torque stroker, 3400 pounds, center of gravity is approx 24 inches from ground 72 inches from rear.

 

Here are some pictures to help you visualize the setup and to give any suggestions. Im open to them, its only tack welded in place.









Below is the box tubing I use as a spacer to make the lower link level.

 

looks decent but I'm gonna suggest that make arrangments for a panhard or watts link as there isnt enough angle on your upper bars to keep the rear centered.

 

I found this one. It's truck oriented, but it works.http://mysite.verizon.net/triaged/4linkcalcv15html/index.html

 

yep, what tinman said.

or....

My free advice, $2.50.....Take the upper links and arrange them parallel in plan view (from the top) to the lower links. Adjust the front and rear height of your upper links to get the AS numbers you want. The cool side effect of having the shorter upper bar is you will gain antisquat in bump and loose it in jounce. So the more it squats the harder it will bite, in theory. Run a panhard (or watts of you want to fight with it). Those trucks do not lend themselves well to a tri link. The upper links can be inboard of the frame, check out DSE's fourlinks for the musclecars.

We have fixed a couple of tri links this way and they have worked out quite well. You will also like the exhaust clearance you get by not having that triangulated link on the top.

 

The angles on the upper links are 50 degrees separated.. I have read that 30 degree separation is minimum... maybe I read wrong?

 

After researching 40 degrees is the minimum separation. I have 50 so my axle should stay located. I will try and make provisions for a panhard bar if my setup doesnt work. Initially I am going to try it without a panhard bar.

 

It will kinda work and may never be a problem depending how you drive and how tight your clearances are at the tires. It will just work better with a dedicated lateral location device.

I look at it like this. You can use a pair of pliers as a hammer and get the job done but the best hammer is still a hammer. Tri angulated upper bars will center the axle but a panhard (or watts) will do a much better job. I have never been a fan of a tri link. Look at the size of the bushings used in factory setups (a body and fox body) and compare to your urethane rod ends. The tri link does all things a rear suspension should do but doesn't do any of them really well. I guess that's my issue.

Donny

 

I defiantly see your point dontlift. I went with a four link because I heard such good things about them. But after actually setting one up I almost wish I would have went with a 4 bar with panhard.

After thinking about it and doing some research, you cant use a panhard on a 4 link due to bind? The only thing I could use is a watts link. Tell me if I am wrong here?

 

You are corect. You do not want to use a panhard bar with a triangulated 4 link. A panhard bar will pull the rearend in a sideways arc and put high side loads on the 4 bar.

I built triangulated 4-bars for my '31 Model A Tudor and '27 Model T RPU. For street use I see nothing wrong with your set-up.

Only difference between your set-up and mine is I made sure the vertical length of the upper and lower bars were the same. This ensures the pinion angle doesn't change as the rear axle articulates.

 

40 degrees is an absolute minimum and should only be used on off-road applications with Heim joint ends. We use 60deg included angle (30 each side) as a minimum for street use. Heims would help stiffen it up, as they won't flex like the bushings will. FYI, most factory tri 4 links use 80-90deg included angle.

DO NOT use a panhard or watts link with the triangulated uppers. If you look at Ned Ludd's diagram, you will see how far behind the axle the roll center is on a tri 4 link. A panhard or watts will force the axle into 2 different rotation paths and bind the suspension.

 

It doesn't look like they mount to the frame in the same location in this photo... Did you move the axle mounts for the lower arms forward to compensate?

 

Thanks for that advice. I am going to work in the shop in the morning and before I tack up the drivers side I will look at maybe gaining a few extra degrees of separation, I just dont know where I can gain any. Wish I already didnt have so much invested in this GM housing or I would have went with a Ford 9.

 

About the only way to do it with those cast center axles is to move the frame mount back and give up some effective length on the upper links. I'd take that (and safe handling) over a little pinion movement any day.

 

If you draw a line through the front mount and the rear mount, for the upper bars, and the lower bars, and measure them parallel to the vehicle c/l they are the same length.

Therefore the pinion angle will remain constant..... the basic principle of a 4-bar is equal length bars as viewed from the side of the vehicle (not the top pf the vehicle).

 

Got that, just couldn't see how you did it from the photos, as the lower frame mounts appear to be further forward than the uppers.

 

This seemed to be an interesting post and some surprise to note that some thought is being put into what actually happens with tracking rods and the effect of geometry on the positioning of the bracketry and the actions imparted to the axle..

Doesn't appear that you have any major issues you should be good to go, make sure your upper axle brackets are evenly spaced to your lower axle brackets To check your work just, tack your work in place, knock off the ride height brackets and run the axle thru maximum movement as well as single side bump and drop. I say tack because if you have any issues with bind the tacks will break or bend and you will know you have a design issue. But DO NOT follow the advise other the guys that want you to install a panard rod, you will have a bind.

 

heres the deal,, get everything tacked up and give a side to side shake on the rear end housing, if it moves very much, you need some sort of track locator.

your pushing is nothing compared to whats gonna happen when you put lateral load forces on it. You pushing may get 100 lbs of force if your a big guy, once its on the road you will be putting several hundred pounds or more of force when cornering.

and just for shits and giggles,,,, you can use a panhard bar on a loose triangluated four link. you just have to be carefull that the arc of the panhard is equal too or less than the amount of slop in the four link. Compliant bushings will also allow you to cheat that a bit too.

For the record I have never done it that way but the roadster that I am rebuilding now has a poorly designed four link and had a panhard bar added later. It has 50,000 miles on it with no real binding issues.

I have driven the car a lot and have been a bit hard on it at times and have noticed no ill effects of it being there. Your mileage may vary.

(regardless of the results of your push test I still say your gonna need a track locator with that setup.)

 

Update on my progress. I got everything tacked into place, and the coil over cross member. After everything was setup I went to putting the rear end thru full travel. No binds.. great! Next instead of a push test I decided to use a c clamp against a fixed point on the axle and a frame rail. I raised the axle at the pumpkin the the casters facing side to side. I put a dial gauge on said fixed point to measure side to side. I clamped the c clamp as hard as I could even with a cheater bar and the travel was measured at .050 + and -. I personally feel like that is acceptable, but what do you think?

Here are some more pictures... I am going to post them on my build forum too.

 

Sure, it's fine.....

But I don't get it. You took the time and did the research to get A/S and roll steer optimized but your willing to settle for .050" of movement with a c clamp?

Try this, loosen all eight mounting bolts at the ends of all your links and conduct the c clamp test again. What you will see is the links aren't holding your center section in place, the bushing's lack of compliance is.

Oh and the no bind thing. At ride hieght remove one upper bolt at the axle housing end. adjust that bar until the bolt slides in and out with no hammering or threading in and out, just slides in and out like a pin. Then load the rear suspension to fully compressed and try to slide that same bolt in and out. Let me know what you find, I'm curious.

Is it okay? Like I said before, depends on how you drive.

 

I have a new-guy question:

The rear end will be moving up and down perpendicular to the frame. The pivot points at both ends of the upper mounts are at a 50 degree angle to that movement. Shouldn't they also be perpendicular to the frame in order to follow the movement of the axle?

The bars could still be angled at 50 degrees, but it seems to me that the pivot points should be perpendicular (90 degrees).

I've seen many rear ends done like this and not many done as I've described, so I have to at least consider the possibility that I'm wrong (again). But it seems to me that the rear end would bind if the bushings were solid. The only reason it works so well is because the rubber compresses and is overcome by a long lever (the upper links).

Can someone explain this to me?

 

Nothing to explain.... You are correct. The factory bushings are very large in diameter for that reason.

 

When I mocked up the rear set-up you see in the photo I placed my floor jack under the rearend housing and lifted and lowered the rear axle. It doesn't bind at all as shown. Every triangulated four bar I have ever seen has rod ends mounted like the ones on the photo.

The bushings in the rod ends aren't rubber they are hard poly bushings.

I can't recall ever seeing a triangulated 4 bar set-up the way you are describing it. Got any photos?

 

I have learned alot from this thread. wish I had this info 2 years ago.