4-link debate

The version with the link bushings parallel to each other on the frame and axle exhibited zero bind in a straight bump situation with no articulation of the suspension. As we started to articulate the suspension, the bind increased at an exponential rate putting unbelievable amounts of force on the bushings initially, then the bolts and eventually the mounts. Since bumps in the road are rarely equal on both wheels, this is not a good design and will create large amounts of rear roll stiffness while inducing undue stress. We believe the mounts will fail in this design if used in real world driving conditions as they are most likely the weakest link.

Wouldn't the forces he's describing hold true for the parallel bottom bushings, regardless if the top ones are angled or not?

 

My understanding is because of the width of the lower arms to the frame and axle, combined with the longer length and parallel position of the arm itself, the twisting force is greatly reduced compared to uppers that are triangulated.

 

instant center is your friend....

 

thanks hotroddon! i have been scratching my head for days not knowing how to figure this out. in the pictures it did not look right but i had no proof. i can sleep tonight with a clear head thanks haha

 

Hmm, when viewed say from the back, when one wheel goes over a bump it seems to me the angle of rotation between the rear axle and frame would be the same regardless of how far apart, how long or how parallel the arms are. Longer arms might have more torsional give to ease the stress on the bushings. This has probably been brought up but to repeat, the diagonal 4 link suspension I am familiar with ('60s GM A body) used U shaped arms with likely more torsional flex capability than solid tubing.

Not trying to beat this to death, just something interesting plus I happen to be in the middle of a rear suspension redo.

 

The fat sloppy bushings the OEM used helped a lot with that as well.

 

Triangulated, 4 bars, 3 Bars, Bags, Coil overs, Watts links... sometimes I think I'm on street rodders.con.

Scotty

 

I don't see any rods ends, i see bushings, but no rod ends.

 

Would that matter all that much since the ends wouldnt be able to twist out anyway?

 

Which is nice if you're damn sure you'll never want to change pinion angle, alignment etc.

 

Good discussion on the differences. I first went along with the OP's method, and simply assumed the body roll, or asymetric bump would be the bind, and absorbed by the bushings. When I read Don's OEM buddy's modeling of the loads, he's right, the exponential increase means that brackets and mount welds are getting hammered.
One difference mentioned was that OEM's used the flexing open-channel style arms,
when we use tubing, the loads are delivered in full to the mount region. The non parallel (bushing) mount option, means that binding occurs w/every move up-and-down. So bushings and flex still play a valuable part in the setup. Good coverage all.

 

Loosening of the rod ends leads to accelerated thread wear : when the threads are "locked" w/ the jam nut , there is no fore & aft slop in the threads , unlocked they would gall & get sloppy in pretty short order.

dave

 

As I have often said, the laws of physics are not negotiable, and they don't care one bit about anyone's opinions. Learn them, or trust the judgment of those who already have.

 

Except that is NOT a triangulated 4 link, It is a parallel 4 link with a diagonal locator

It will quickly destroy the thread and lead to failure.

It's really even simpler than that - just a case of Classical Mechanics (the study of bodies acted on by forces and bodies in motion) which is a basic elemental topic of Physics

 

OK I'll ask again..I like the idea of eliminating a rear panhard bar with this triangulated 4 link, does it function as well in cornering? Could the diagonal be on the bottom links instead?

The lower links are very close together on that setup, I don't see any reason they can't be at least as wide apart at the frame rails which should give more rigidity.

 

One of the major chassis builders, I think it's Morrison, builds drag race four links with a diagonal locator like that and has a warning that they do NOT recommend it for street use. I believe the reasoning is it is not ideal for cornering and holding the rear end positively enough and that the stresses can cause it to loosen the jam nuts at a minimum and break at a worst case. Someone recently told me they softened their stance on this, but it was still not a good idea for anything other than a ProStreet type car whose main purpose was straight line
This is going from memory from a few years ago tough ........

 

(SORRY DON, missed your post above...)

That is not a triangulated 4 link by the common definition. That is a parallel 4 link with a diagonal locator. Very common in drag racing (with the diagonal on the BOTTOM!), useless on the street for the exact reason you mentioned. Any articulation causes the end points of the upper links to want to separate (measurement-wise), but the solid diagonal will not allow this. BIND!

 

Each "4-Link" arm should have at least 1 "Ball/Pivot" connection or it WILL Bind when only 1 wheel goes up. Chevelles bind, they're all bushings & that's fine.

I see a "Bind" in the Diagonal brace here when 1 wheel, any side, goes up, the rear of the car will have a slight pull to the right, & this is fine too & probally not even noticeable. If you insist on finding a problem with it, it might crack/fracture the Diagonal Brace Connections, do to lack of bushings to absorb bind.

 

That's the problem people see it on RACING Cars where they're NOT making 3 & 5 point u-turns & going up driveways at an angle causing 1 wheel to articulate. Like Split Wishbones , that's another one. Bushings are meant for street use, solid Hiem Joints are not. But, it all works & it's all fine. BUT THIS IS A DEBATE. SO F YOU!!!

 

I just read most of this thread. I found myself flailing about, simulating links with my arms and triangulation points with my elbows. My co-workers thought I was inventing a new dance move. There's some clever SOB's on this board.

 

With the suspension design in this picture the RH upper [with the diagonal] acts like a A-arm.
The roll centre and instant centre are on the RH side of the rear end housing.
It would be very difficult to equalize the Roll couple when the Roll Axis is diagonal down the vehicle.
I would behave like a Dirt Car with "Wedge" jacked into it.

I suppose on a "Highway Queen" it wouldn't matter at all

 

Absolutely !

This was the preferred method of locating a live rear axle used by one of the greatest minds in suspension and handling [ Colin Chapman of “Lotus” fame ]

The Mark 1 Lotus Cortina used this method, but the Cortina bodyshell wasn’t a very good candidate for Coil-over suspension [ the bodies would “Coke Can” themselves ]
Nowadays racers run roll-cage diagonal braces to the upper towers and this has fixed the problem, The 50 year old Lotus Cortina is still one of the best handling racers out there.

When playing with suspension, all these traits [ both good and bad ] only become important when at the absolute limit of tyre adhesion.
If a car is driven around like a “Luxury Liner” all this become academic .

By triangulating the lower links inwards together at the rear , the instant centre is at a fixed distance from the ground regardless of suspension travel.
If the lower links are triangulated inwards at the rear but each side of the diff head, the instant centre is at a point behind the rear end. The instant centre raises as the suspension compresses [and lowers as it extents ]

The real downside to triangulating of the underside of the rear end is the ground clearance at the point that is most likely to bottom out

On a hotrod where there is limited room, I cannot understand why more people aren’t using this method [ especially with cars with kick-ups in the frame and seating further back.]
You can use a longer lower A-Frame underneath everything to the centre of the rear end, and 2 shorter parallel uppers on the outside to the kick-up in the frame [ only these would be visible ]

In fact this would also be a good method on the front end, you could eliminate any “Shackle Wobble” with an A-Frame [ it could be designed to look similar to split bones ]

 

Thanks Kerry, like the one on the left? I believe those are radius rods on the outsides, solid mount at the axle. Must have some flex built in. I'd like to run the wishbone the other way though, center pivot at the trans cross member.

 

Paul here’s a pic I found of a Lotus rear , you can see how the body-roll compliance is done in the centre A-frame.
With this method you could use solid bushings [on the A-frame only] on a Race-Car.

Notice the mounting point is further forward for the outer links!
Lotus mounted them in front of the axle tubes to get them closer to horizontal with the factory front spring perch.
Then they mounted the A-frame slightly further back on the centre [and the front pivots] to keep them equal length and parallel.

3 and 4 links don’t need to be exactly on top of each other.

If you run the triangle forward it makes no difference with engineering integrity [ Triangulation is triangulation ]
In fact I can’t understand why more hotrodders don’t do this when converting a Ford rear to open driveshaft.
All that would be needed is a method to “float” [or pivot] the existing wishbone at the axle and to mount 2 outer upper linkages.

From a handling perspective it wouldn’t be noticeable on a street driven vehicle, but on a race-car the roll-centre will change with ride height because it is frame mounted.
In a race car diving into a corner or squatting out of a corner this could effect the roll centre height which has some effect on maximum cornering ability.

Any driver not driving at the maximum limit will be getting passed by another driver anyway.
On a street car better tyres would remedy this, until you find a new limit to drive at

 

Just to add to that point Kerry, the Satchell link works fine on a lower horsepower application. Keep in mind that on ANY link type suspension, when under acceleration all the "push" force is going through the lower link bars. When you really get some torque and tire under it, you are much better off having the lower links parallel and in line with the frame.

As you said, none of this matters until we get very close to the limits of traction in any direction....

 

Something else that has been bugging me about the Lotus photo Kerry put up - the outer links do appear to pivot at the rear end. And they also appear to mount slightly below the axle level. It would seem that under hard acceleration the links would have a hard time controlling axle wrap. Not with Lotus power perhaps, but how about a V8 and a hole shot?

 

When the upper and lower links are further apart a greater torque reaction from the axle can be transfered via the links [ The torque is divided by the distance ]
One advantage of having the links directly in front of the axle is the direct pull [tension] on the linkages under severe braking forces, and by having the A-Frame underneath also puts tension on the A-Frame because of rotational brake torque on the axle

During acceleration both the upper and lower linkages are under compression , even though we think the outers are under tension .
[Acceleration of the wheels against the resistance of mass]
The forces of acceleration try to push forward on the linkages, and the torque reaction of the rear-end housing also puts compression on the A-Frame.

If there was no outer links [ eg: 1 upper and 1 lower in the centre ] the rear end would try to “Toe-in” during acceleration

 

Another fine thread where the OP doesn't get the answer he was looking for and then wants to argue the point. Then when facts and data are thrown in he's never heard from again. This place is a wealth of knowledge if your willing to ask a question, listen to a well educated answer and take that advice and move on. Sometimes pride is a roadblock though. Good information all the way around on this one.

 

I'm not looking for "answers", if you can read, (which I'm sure you can), you should go back and read my original post where I said "I would like to hear others opinions on this subject." "opinions" not "answers". Ive seen both set ups on bagged vehicles that have been ran for yrs with no problems from either. I just want to hear others opinions on the subject. No one is "arguing", I have my opinions about the subject, some share my opinions, and some don't, this is why it's a discussion. If you'd rather talk shit than contribute to the discussion, then just stay off the thread, if you'd like to contribute, then by all means do so. Also, I'm a full time tattooer and single dad, so I apologize if it bothers you that I'm not posting on here everyday (or at 3:45 in the morning). My little girl is more important to me than this forum.

 

Not talking shit. There were some important FACTS given to you by some very knowledgeable builders and engineers, not opinions. Run your set-up any way you want, it's your car. Being in charge of a machine shop on 3rd shift makes it easy for me to post at 3:45 a.m. The biggest issue with your set-up is how rigid that it is . When you hit a bump or go up a driveway on a big angle this is going to stress the heck out of your pivot points. There's just not enough "give" built into the pivot points. Again it's your car so do as you please.