How does a 3/4-link work?

[ethan67]

I am not suspension expert by any stretch. I have looked at 4-link rear setups for pro-touring and I have a pretty simple question. I may be missing something totally obvious, let me know.



The links are usually attached to the frame and axle via bolt-in bushings. As one end of the axle rises and the other falls in cornering or bumps, I don't see how the suspension travels without binding at the bushing/bolt. Any lift or fall at either end of the axle would create a diagonal, and looking at a 3/4-link, I don't see where the point of bind free travel would come from.

A leaf spring/rubber bushing combo, as I understand it, flexes at both the spring and rubber point.

Anyone care to fill in my gap in knowlege? I have been considering a leaf alternative in my 67 Camaro.

Thanks,

Ethan 1967 327

[baz67]

bushing compliance or rod ends

[Mean 69]

I'd encourage you to look at the archive section on this forum, there are several really outstanding threads there that can help explain things in pretty specific terms. DSE is basically ready to release a four link setup, and there is this company on the West Coast that has a design too, but they are a bit later to the party. You can likely see one or two of their setups at SEMA, they will probably be a bit more involved in the installation, but "better" in terms of overall compromises from a suspension standpoint.

Nothing is free, just keep that in mind, most especially in terms of the ease of installation, etc. Race cars don't have the same constraints as most street cars, a cut here or there isn't a big deal for them. You should really ask yourself early on what you can and will live with, and what you won't.

(Sidebar: The link setup I am running in the prototype has all solid rod ends, and you can feel it. Not a "new" car feel. Sorry).
M

[Norm Peterson]

I've heard about a hard rubber rod end style bushing that supposedly takes a LOT of the sting out of an all rod end arrangement, and may also extend the life of the remaining rod ends. They're a good bit firmer than OE rubber, but I'm given to understand that they're not quite as stiff as poly. I think Jon A has a write-up somewhere about using one hard rubber and one rod end per link, but offhand I don't recall where I saw it (corner-carvers or FRRAX).

Anyway, keeping one rod end per link holds the unintended side effects to a minimum. That's as in zero induced torsional loading and maybe half of the induced lateral bending loads that using hard rubber at both ends would attract. Like any compliant bushing, they're going to be progressive in their effects, so there are probably some caveats about how much angularity (think LCA lateral bending and plan view here) that these bushings will permit before they get really stiff. Hence it might be more difficult to justify their use on cars with relatively soft springs and small bars.

Norm

[Mean 69]

I can tell you, using all rod ends on a linked suspension on the street would probably get old for most folks. I am doing just that on the 69/three link car, and it really transmits a lot of the higher frequency type "hits," like the little reflectors, etc. I know Katz has a source for a compliant rod end alternative, and I am also going to Currie today, I'll look at the Johnny Joints.

M

[MrQuick]

Hi Mark, I went with Currie on the 84. The problem is the JJ's aren't availible in a 3/4" LHT. I had to get some made. They were $19 each and 25 cost around $400. Let me know if you need a source, they were the only ones that would do it it low numbers. Other venders wanted around $22 each with a 100 minimum.

[parsonsj]

Hey guys. Count me as interested in a rod end alternative. Katz's design on my 3 link was all done with rod ends and will be fine to start, I'm sure. But an alternative would sure be welcome if I get tired of it.

thanks,
jp

[68protouring454]

let meknow also, i am going after a 3 link also
jake

[Camaro Zach]

whats the problem if there is one for using poly. bushings in 3/4 links?

[Norm Peterson]

That's an issue that's been discussed more times than you can possibly imagine. A search on the term 'bind' on the various fora that deal with stick axle cars would probably turn up a week's worth of reading, had it all been saved.[/rant]

But if you look at the picture, you'll see that the poly bushing is completely free to rotate about only one axis (around the outer diameter of the bolt sleeve). Motion in any other direction requires that the poly be squished/compressed/whatever.

If suspension arm motions really were about only that one axis, it wouldn't matter. The problem is that rotations about both of the axes that are perpendicular are required to some extent when locating a stick axle. And compressing the poly to accommodate these motions takes force, which translates into additional (and unintended) wheel rate. And there's another side effect - needless harshness over certain types of bumps.

Norm

[Norm Peterson]

At one point in time, there was a single thread over on camaroz28.com on this very topic - that ran to about 200 replies.

[Travis B]

I just ordered some rubber adjustable links for my parrallel 4 and panhard in my Chevelle! They are a OEM type rubber should be very quite and last for as long as I need them here is the link
http://www.ridetech.com/productinfo/brackets_2.asp

[Camaro Zach]

That's an issue that's been discussed more times than you can possibly imagine. A search on the term 'bind' on the various fora that deal with stick axle cars would probably turn up a week's worth of reading, had it all been saved.[/rant]

But if you look at the picture, you'll see that the poly bushing is completely free to rotate about only one axis (around the outer diameter of the bolt sleeve). Motion in any other direction requires that the poly be squished/compressed/whatever.

If suspension arm motions really were about only that one axis, it wouldn't matter. The problem is that rotations about both of the axes that are perpendicular are required to some extent when locating a stick axle. And compressing the poly to accommodate these motions takes force, which translates into additional (and unintended) wheel rate. And there's another side effect - needless harshness over certain types of bumps.

Norm

Yes i understand that a bushing like that needs to compress for any other range of motion. Does it really take that much force to compress the bushing just a little bit? But so far i havent found anyone that has tried it to see how much extra rigidity they add over an axles 3" range of motion per side.

I dont know maybe its just me but i didnt see why having a little extra force needed would affect the car as much as everyone says. Does that mean that sway bars should never be used because they would cause the rear axle to "bind" unless its range of motion was only up and down. Not trying to sound like a smarta$$ i just would like to know.
Anyways thanks, ill do some more reading

[Norm Peterson]

Here's some more reading. Trust me when I tell you that the difference can be clearly felt from the driver's seat, and that it can adversely affect handling as you approach the limit if you haven't tuned the rest of your combination with these "added stiffness" effects specifically in mind.

This is a summary of a test sequence that was run by the folks at Maximum Motorsports (a Mustang outfit), which has been passed around and across several fora that I am personally aware of. I'm not sure if it's been posted on this site before or not, but since I have it handy as a file it's easier just to paste it all here. I'm sure that the results will surprise you.

"While bind is only one of many parameters determining the handling characteristics of a suspension system, it is useful information, and has been a subject of great debate on these message boards. As part of the research we did in developing our rear suspension system, MM has actually done quite a bit of roll-bind testing. I can offer some hard numbers for everyone to consider. I will define ‘bind’ to be any resistance to wheel movement in a roll situation that is not from the spring or sway bar.
Let me say that this information is not intended to be negative toward any particular system, but should be used to gain understanding of the way cars with different setups feel/handle. This information can help everyone to optimize whatever setup they may have.
Of the tests we have done, following are the tests relating to the rear suspension systems most often discussed. All tests are with the sway bar disconnected, cycling one wheel through 3” bump/droop as if in a roll situation. The results are organized in order from least bind to the most bind.

1) 4 Link - LCA with spherical bearings or rod ends at both ends / Stock UCA’s: 6lb/in Linear This shows the stock upper arms introduce 6 lb/in of wheel rate.

2) 4 Link – MM LCA 3 piece poly, spherical bearing / Stock UCAs: 9lb/in Linear This shows an additional 3 lb/in resistance from our 3 piece urethane compared to a rod end.

3) MM TA/PB – LCA with spherical bearings or rod ends at both ends: 10lb/in Linear Here we removes the 6lb/in from the UCAs, but adds 10lb/in due to lateral deflection of the TA during roll (see above post).

4) 4 Link – Stock LCA / Stock UCAs: 11lb/in Linear This shows that the stock LCA adds 5 lb/in of wheel rate, which is actually more than our LCA of case 2.

5) MM TA/PB – MM LCA 3 piece poly, spherical bearing: 13lb/in Linear Again illustrating an additional 3lb/in additional resistance of our 3 piece urethane compared to the rod ends in case 3.

6) 4 Link – LCA with 3 Piece Urethane at both ends / Stock UCAs: 26lb/in Linear Case 6 shows that the 3 piece poly (or any LCA) works best with a spherical bearing at one end. 17lb/in is added over case 2. Note that the effect of adding a 3 piece urethane at only one end adds 3lb/in. Add it at BOTH ends and the increase is 17lb/in… NOT 6 lb/in as one might expect.

7) 4 Link - LCA with delrin, spherical bearing / Stock UCAs: 30lb/in Linear This shows that delrin does not allow necessary angular deflection resulting in an additional 21lb/in over case 2.

8) 4 Link With PB - Stock LCA / Stock UCA: In the first 1” travel 47lb/in Between 2-3” of travel 30lb/in Decreasing Rate
In case 8 & 9 the Panhard bar defining a new lower roll center is forcing control arms to travel a new path of higher resistance.

9) 4 Link With PB – MM LCA / Stock UCA: In the first 1” travel 50lb/in Between 2-3” of travel 30lb/in Decreasing Rate

10) 4 Link – Stock LCA / UCA with rod end at chassis, stock rubber at axle: In the first 1” travel 63lb/in Between 1-2” travel 39lb/in Between 2-3” travel 20lb/in Decreasing Rate
Case 10 represents trying to locate the axle with a stiffer bushing configuration on the upper control arms. Since the upper arms need to have an effective length change, the rod end in this case actually creates MORE bind.

11) 4 Link – LCA with urethane at both ends / Stock UCA’s: 67lb/in Linear Case 11 is similar to case 6, but shows that a standard poly/poly control arm does not allow much angular change.


Keep in mind that the above information is with no cornering force on the axle. Therefore, there is a huge gap in this information if you are trying to correlate this data to how these systems would feel in use. I would say that the Torque-arm in case 3 & 5 outperforms any other case shown, although it does not have the least amount of bind in this test. We have begun to build a fixture that loads the axle laterally, as if in a corner, to THEN see how the bind behaves. Any system with a Panhard bar should have no significant increase in bind over what is already shown here. This predictability that a PB provides is why we recommend it on a 4 link (with the correct control arms) for people on a budget, or Solo II Street Prepared cars (not allowed to remove uppers). True, you are inducing bind in this situation, but that bind should not significantly change as you load the suspension laterally. When driving the car, the effective added spring rate (from bind) balances well with the new lower RC, and the improved stability and predictability. YES this is a compromise, but I feel it beats trying to locate the axle laterally with stiffer bushings. Obviously, if the pocket book or rulebook permits, the best thing to do is add a Torque-arm and remove the upper control arms. All this binding is also why you are able to add at least 50lb/in wheel rate to the rear when you add a Torque-Arm and remove the UCAs.
Jon- the barrel shaped bushing was designed to maximize contact area and minimize resistance to angular motion. When cycling the suspension in straight bump and droop, its effect is immeasurable.
Ehren VanSchmus
MM Design Engineer"

I think you can take the TA/PHB data as being reasonably close to what the same bushing configuration in a 3-link/PHB combination would provide. Ditto for the various 4 link combinations being only approximately what you'd see with a GM car, as the 30-something degree UCA angle in the Mustang differs somewhat from the 45* of typical GM practice.

I'll further note that the above test as described is actually that of combined bump or droop plus roll. But I doubt very strongly that this represents enough difference from pure roll to make 60-ish lb/in of added wheel rate drop by an order of magnitude, or even by only 80% or so such that you might reasonably neglect it. And certainly the various combinations of cornering and either braking or acceleration that would be encountered in actual (hard) driving will tend toward some approximation of the actual test procedure anyway.

Norm