Rubber vrs. Poly rear bushings

danond said:

I recommend a matched set of control arms, upper and lower, with polyurethane bushings in the lowers and uppers, but use spherical bushings on the axle-side bushing of the uppers.

Big +1 on the torque box reenforcement kit. Necessary stuff if you plan to drive it hard.

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srtthis said:

**** bushings...

SOLID FOR LIFE!

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tunedin302 said:

I would advise you stay away from poly on the UCAs because it will cause binding issues.

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Sharad said:

I've been into Mustangs for 25 years and I've owned literally dozens of them. I've never seen anyone have "binding issues" with poly UCAs. I'm beginning to think this is an urban legend, like sasquatch or something.

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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) MMTA/PB – LCA with spherical bearings or rod ends at both ends

10lb/in Linear

Here we removed the 6lb/in from the UCAs, but added 10lb/in due to lateral deflection of the TA during roll.

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) MMTA/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.

Ehren VanSchmus
MM Design Engineer

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