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we were slow at work the other day so i decided to go ahead and put all polly bushings in the rear of my car, but heres what i did. i remember reading that pollyurethane bushings in the upper and lowers can cause bind, and lead to snap over steer. since i auto X and open track this is obviously bad, so what i did was replace both on the lower control arms, and just the chassis side on the uppers. i left the rubber bushings in the rear to give just enough to prevent that situation from happening. any opinion on this?

a side note about the bushings, the rear does feel a little more firm, and NVH went up a little bit, but not bad. cant wait to see how it does on the track.
 

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I did the same thing on mine. It seems better than stock but I never had it at the track with the stockers; so I have no direct comparison on track.
 

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Oldie but goodie, the Roll Bind Study done by a Maximum Motorsports engineer. The last case is the closest to yours IMO

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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 removes the 6lb/in from the UCAs, but adds 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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Hmm. I guess I missunderstood binding.

I thought it meant that the suspension moved in a nice "linear" way until some degree of roll, at which point it experienced a big increase in resistance and that's when you had sudden and unpredictable oversteer.

If the binding is manifest as a linear increase in spring rate, how is that different than just adding stiffer springs or a stiffer roll bar? Sure it will change the handling, but sometimes that can be good.
 

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As he said, the study only accounts for the added rate on a free movement, no-load situation. It does not account for additional issues brought to bear by lateral loading.

Here's some quotes from MM's Jack Hidley that might help explain further:

A little background on the Mustang 4-link snap oversteer.

Once you have turned into a corner and have 1g or so of lateral acceleration going, all eight bushings in the rear suspension, particularly the upper four, have either been compressed or stretched into an unnatural shape. The bushings behave as springs and have stored up a lot of energy. Once the rear tires start to slide a little bit, there is nothing really holding the axle in the lateral position that it was in. The bushings all push the axle back to the other side, which causes a large change in rear toe and side to side weight distribution. This is the snap part of snap oversteer. Everyone who has been bitten by this is amazed that they can't countersteer fast enough to catch the car.
That's essentially correct. Once the PHB is in place it takes all of the lateral loads in compression and tension of the PHB rod. There is virtually zero lateral load on the suspension bushings. With no lateral load on them, they can't compress, spring back and then cause snap oversteer.

When you replace all of the rear suspension bushings with rod ends, the rear roll stiffness goes up quite a bit. The car will oversteer all of the time from this. There will also be a lot of added friction, which makes the ride and traction over rough surfaces worse. It causes the rear axle to move through some strange geometry under roll and bump. Too difficult to explain with text. You won't have snap oversteer since there are no bushings to compress and release.
 
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