Effects of movement in the rear end (fox) - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 53 Old 01-12-2011, 12:22 AM Thread Starter
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Effects of movement in the rear end (fox)

Hi All,

With AutoX starting in a few months, I'd like to get some more work on the mustang to help it handle better this coming year (year 2 of doing autoX).

Every event I went to, they do the car inspection and they go about yanking on all 4 tires. The front end is solid. I have updated to poly bushing from MM when I upgraded to 13inch / 5 lug setup.

The rear end is not the same, it is original rubber and it moves like crazy. The newer guys usually call over a more senior guy to have them explain that 20 year old mustangs 'do that'.

So my question is what effect does this movement / play cause in autoX and overall performance handling.

The rear end is already 'off' by about 1 1/2 inches to the passenger side, I was hoping that upgrading the upper / lower control arms to units with poly bushings would help straighten this out.

The car is mostly stock. I upgraded brakes last year and went to 5 lug with some 17x9 rims to fit those and get a bit more meat to the ground. This is not a daily driver except on Fridays and track days.

Thanks for an advice.


Charlie R
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post #2 of 53 Old 01-12-2011, 05:33 AM
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post #3 of 53 Old 01-12-2011, 09:57 AM Thread Starter
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I think I'll eventually get to that point (and I would like to), but considering that my rear suspension is stock, wouldn't it be more productive to address the other components first (springs / shocks / control arms)?

Would upgraded / poly control arms help in stiffening up the side to side play of the rear end, or is this really what the panhard bar is for?

Also from what I understand, having a panhard bar requires a TA? Or is that me mis reading MM website?

Thanks again!

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post #4 of 53 Old 01-12-2011, 10:43 AM
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Originally Posted by sprayin50 View Post
...or is this really what the panhard bar is for?
yes

And you might want to do a search(hint: search for the word 'bind')...poly bushings in the uppers is a definite no-no, all poly in the lowers is also a no-no.
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post #5 of 53 Old 01-12-2011, 10:52 AM
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A TA isn't required to run a panhard bar, however they work best together. I ran my panhard bar for nearly two years with the stock upper control arms before getting a TA.

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post #6 of 53 Old 01-12-2011, 12:37 PM
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Originally Posted by sprayin50 View Post
Hi All,

With AutoX starting in a few months, I'd like to get some more work on the mustang to help it handle better this coming year (year 2 of doing autoX).

Every event I went to, they do the car inspection and they go about yanking on all 4 tires. The front end is solid. I have updated to poly bushing from MM when I upgraded to 13inch / 5 lug setup.

The rear end is not the same, it is original rubber and it moves like crazy. The newer guys usually call over a more senior guy to have them explain that 20 year old mustangs 'do that'.

So my question is what effect does this movement / play cause in autoX and overall performance handling.

The rear end is already 'off' by about 1 1/2 inches to the passenger side, I was hoping that upgrading the upper / lower control arms to units with poly bushings would help straighten this out.

The car is mostly stock. I upgraded brakes last year and went to 5 lug with some 17x9 rims to fit those and get a bit more meat to the ground. This is not a daily driver except on Fridays and track days.

Thanks for an advice.
You better check the rules for your class before you start doing stuff. Having said that a PHB is a huge improvement even with stock CAs.

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post #7 of 53 Old 01-12-2011, 01:24 PM
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Originally Posted by sprayin50 View Post
I think I'll eventually get to that point (and I would like to), but considering that my rear suspension is stock, wouldn't it be more productive to address the other components first (springs / shocks / control arms)?

Would upgraded / poly control arms help in stiffening up the side to side play of the rear end, or is this really what the panhard bar is for?

Also from what I understand, having a panhard bar requires a TA? Or is that me mis reading MM website?

Thanks again!
As noted above a PHB can be used with the stock 4-link, but ...

The stock uppers are trying to do two jobs at the same time: (i) lateral axle control and (ii) preventing axle rotation. To do these two jobs, the upper arms and bushing must twist/deflect. Adding poly to this mix causes more bind than already exists. Leaving the stock-design uppers in place and adding a PHB to the mix creates a conflict between the uppers and PHB for lateral control of the axle. It's better but not optimal. The solution is removing one or both of the uppers completely. This would require a PHB/Watts Link for lateral control and the T/A, PM3L or true 3-link for rotational control. When this is done the rear suspension is transformed.
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post #8 of 53 Old 01-13-2011, 12:12 AM Thread Starter
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You better check the rules for your class before you start doing stuff. Having said that a PHB is a huge improvement even with stock CAs.
I don't really race for class. My brother has a 350z (stock) and we race on weekends for bragging rights, it's what keeps it more than fun for now.

He's been doing it for 3 years, last year was my first and I was able to get within 2 seconds of him by the end of the season (it started with a 10 second gap on my first runs).

So really I'm trying to beat him with a old mustang vs his newer 'expensive' car.

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poly bushings in the uppers is a definite no-no, all poly in the lowers is also a no-no
I see. I had always thought we wanted to stiffen up things from original factory rubber. But after this comment I started digging around MM and found out that isn't exactly the case. Thanks for the info.

@qtrracer - thanks for the detailed explanation that explains a lot in a short paragraph.

I'll get to searching for more info on the binding effects of the rear end. Thanks all!

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post #9 of 53 Old 01-14-2011, 05:58 PM
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I guess the "moving around alot" is a subjective measurment but I would thing you wouldn't get that much movement from rocking the car by hand unless the bushings were in bad shape.

I'd also wonder about an inch and a half of variance from side to side on the axle, I know some is allowable but that seems a bit much.

I'd do this:

Check all the bushings by removing the parts and inspecting the bushings close up.

Check your torque boxes for distortion.

If everything checks out, it's up to you, do a set of springs and shocks/struts and whatever else you want (LCAs- ones meant for the track, PHB, TA) or can afford.

I used my car with stock LCAs (good, stock bushings), stock UCAs (good, stock bushings), stock swaybars, stock front end with just lowering springs and Tokico shocks/struts for 4 years and it wasn't the fastest thing around, but it was safe and handled better than stock. Once my driving got better and money became available, I put a better suspension under it (and am still in process of finishing that job- TA going in this year).

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post #10 of 53 Old 01-14-2011, 07:18 PM
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Just one person's opinion, but the MM PHB was the best mod I made on the car, as far as feel on the street (a shifter was the best).

Really didn't help my times much in autocross, but it FEELS so much more solid and predictable.


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post #11 of 53 Old 01-15-2011, 02:40 PM
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Originally Posted by qtrracer View Post
Leaving the stock-design uppers in place and adding a PHB to the mix creates a conflict between the uppers and PHB for lateral control of the axle.
I've seen that before, but I don't understand.
I mean, on a four-link WITHOUT the PHB, the uppers must be working like a SOB to help keep the axle in place.
The PHB keeps the axle from moving side-to-side hardly at all, so how does that mean MORE work for the uppers or push them outside the range they were designed to handle?

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post #12 of 53 Old 01-15-2011, 02:55 PM
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I've seen that before, but I don't understand.
I mean, on a four-link WITHOUT the PHB, the uppers must be working like a SOB to help keep the axle in place.
The PHB keeps the axle from moving side-to-side hardly at all, so how does that mean MORE work for the uppers or push them outside the range they were designed to handle?
It has to do with roll centers. A roll center is the point in space about which the body tries to rotate when leaning in a corner. Both the front and rear suspension have roll centers, but we'll just talk about the rear.

The roll center is defined by the geometry of the device used to locate the rear suspension laterally. In a factory Mustang, this is the upper control arms. they define a roll center well above the axle centerline -- usually about 12-18" above the road surface.

A Panhard bar's job is to locate the axle laterally, too, so it defines a roll center too. It is, for the most part, where the bar intersects the centerline of the car -- generally 8-10" above the road surface.

Notice that the two roll centers are not the same. This means as the body rolls, the upper control arms and the PHB both tri to make the body roll around different points and they ending up fighting each other and binding up.

When the two bind, roll resistance goes way up, the same as if you had stiffer springs or a bigger sway bar. The stiffer a suspension is, the less grip it has, so when it binds, it looses grip. That results in snap-oversteer when the rear suspension binds up. Not a Good Thing.

Binding also puts ENORMOUS loads on the UCAs and the PHB. Generally the busings of the UCA's end up getting torn up over time.

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post #13 of 53 Old 01-15-2011, 08:33 PM Thread Starter
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@MJ91GT

My passenger side tire is dead even with the fender lip. The drivers side is off by about 1 1/2 inches. So I guess 3/4 inch is probably more accurate of a measurement.

@gt40mkII

So with what your saying, should I wait and to a TA and PHB at the same time to avoid that type of conflict? If we where to do the PHB, would we likely not see much improvement on handling?

I do only autoX at this point, would I be seeing snap over steer in that type of driving or would it be more prevalent in road course type of driving.

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post #14 of 53 Old 01-16-2011, 11:43 AM
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This is what u want:

MM XD LCA
MM Panhard bar
MM Torque Arm
UPR lower torque box reinforcements

(i hope u already have good subframe connectors)

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post #15 of 53 Old 01-16-2011, 11:54 AM Thread Starter
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@EscortSportage

Already have MM Full Lengths.

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post #16 of 53 Old 01-16-2011, 02:18 PM
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@EscortSportage

Already have MM Full Lengths.
very nice, i have the MM STB but it touches my distriutor and throws the engine off, so i dont know what im gonna do with it..kinda pissed actually

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post #17 of 53 Old 01-16-2011, 08:34 PM
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very nice, i have the MM STB but it touches my distriutor and throws the engine off, so i dont know what im gonna do with it..kinda pissed actually
Get some 'Vert motor mounts. I have lots of clearance for both the distributor and oil fill neck/cap. Check it out:
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post #18 of 53 Old 01-17-2011, 10:32 AM
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It has to do with roll centers. A roll center is the point in space about which the body tries to rotate when leaning in a corner. Both the front and rear suspension have roll centers, but we'll just talk about the rear.

The roll center is defined by the geometry of the device used to locate the rear suspension laterally. In a factory Mustang, this is the upper control arms. they define a roll center well above the axle centerline -- usually about 12-18" above the road surface.

A Panhard bar's job is to locate the axle laterally, too, so it defines a roll center too. It is, for the most part, where the bar intersects the centerline of the car -- generally 8-10" above the road surface.

Notice that the two roll centers are not the same. This means as the body rolls, the upper control arms and the PHB both tri to make the body roll around different points and they ending up fighting each other and binding up.

When the two bind, roll resistance goes way up, the same as if you had stiffer springs or a bigger sway bar. The stiffer a suspension is, the less grip it has, so when it binds, it looses grip. That results in snap-oversteer when the rear suspension binds up. Not a Good Thing.

Binding also puts ENORMOUS loads on the UCAs and the PHB. Generally the busings of the UCA's end up getting torn up over time.
I have to replace my diff side UCA bushings twice a year and the UCAs every other year.

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post #19 of 53 Old 01-17-2011, 09:22 PM
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When the two bind, roll resistance goes way up, the same as if you had stiffer springs or a bigger sway bar. The stiffer a suspension is, the less grip it has, so when it binds, it looses grip. That results in snap-oversteer when the rear suspension binds up. Not a Good Thing.

Binding also puts ENORMOUS loads on the UCAs and the PHB. Generally the busings of the UCA's end up getting torn up over time.
So remind me again why I installed a panhard bar?

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post #20 of 53 Old 01-17-2011, 10:41 PM
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So remind me again why I installed a panhard bar?
What bushings do you have in the upper control arms?

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post #21 of 53 Old 01-18-2011, 10:10 AM
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What bushings do you have in the upper control arms?

Actually, I was just half-kidding when asking "Why did I install one," because it has helped handling.
I still have the original police uppers (heck, it's only been 17 years and 140,000 miles) ... but there is a nice clunk now and then from the rear, so I assume that by now the uppers are in there for appearance purposes only.

Time to replace them with the ones MM sells.

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post #22 of 53 Old 01-18-2011, 01:51 PM
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Mark,

Don't get caught up with what they are saying as gospel. Theory does not apply to reality sometimes. I am still on the original UCA's and they still look good to me after over 8 years with a panhard bar. I don't know about you, but I have never experienced any snap oversteer with just the panhard bar. So it is working as designed. The panhard bar might be one factor among many that are causing the UCA's bushings to be ripped. How replacement UCAs are installed, and suspension setup and application can also play a roll.

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post #23 of 53 Old 01-18-2011, 04:02 PM
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Mark,

Don't get caught up with what they are saying as gospel. Theory does not apply to reality sometimes.
And sometimes it does.

Snap oversteer due to rear roll bind does exist. Ask Jack Hidley. he has published a study done by Maximum Motorsports examining rear roll bind in several common rear suspension configurations. This isn't something that just exists in theory.

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I am still on the original UCA's and they still look good to me after over 8 years with a panhard bar.
Do you road race the car with sticky race slicks or R-compound radials? Open track events? HPDE weekends?

If not, you may never have loaded up the rear end enough for the suspension to bind.

And even if you have, the increased roll resistance because of bind may not have been enough to make much of a difference in the rear grip. Depending on that bushings you have, what tires you are using, and your driving style, it may net be noticeable even then.


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How replacement UCAs are installed, and suspension setup and application can also play a roll.
Err, running a Panhard Bar is part of that suspension setup and application.

I can almost guarantee you, if a Mustang is being driven hard, especially with sticky tires and upgraded LCA bushings, that the UCA busings are going to eventually get torn up. In my case the chassis bushings were fine, but the diff bushings fell out as soon as the upper arms were removed.

No, I'm not claiming you're not seeing snap oversteer, but one incidence does not prove the point. Now if you can point to dozens of cases of track-driven Mustangs running two UCAs with a PHB and no snap oversteer, that'd be different. I CAN point to dozens of cases where they DID have snap oversteer.

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post #24 of 53 Old 01-18-2011, 04:28 PM
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I don't know about you, but I have never experienced any snap oversteer with just the panhard bar.
Yeah, I think my car is nicely balanced, and I've never experienced snap oversteer since I installed the PHB, either (well, nicely balanced for a 1993 Fox body, anyway).

I love the feel of the MM panhard bar.

Anyway, good explanation from gt40mkII on why the PHB would cause bind, which I'd never understood.

Thanks, all, and sorry to have sidetracked this.

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post #25 of 53 Old 01-18-2011, 07:21 PM
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Yeah, I think my car is nicely balanced, and I've never experienced snap oversteer since I installed the PHB, either (well, nicely balanced for a 1993 Fox body, anyway).

I love the feel of the MM panhard bar.

Anyway, good explanation from gt40mkII on why the PHB would cause bind, which I'd never understood.

Thanks, all, and sorry to have sidetracked this.
I don't think most have experienced snap oversteer issues either. Otherwise we would be hearing from others (which I have yet seen). I can only imagine it happening under extreme conditions. As long as you keep body roll under control, it shouldn't be a problem. At that point it is a suspension setup issue.

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post #26 of 53 Old 01-18-2011, 08:51 PM
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Anyway, good explanation from gt40mkII on why the PHB would cause bind, which I'd never understood.
The UCAs impose one roll center.

The PHB imposes a second.

The two fight each other.

The stiffer of the two mechanisms win, usually this is usually the PHB.

The stiffer the UCAs are, the harder they'll fight, and the bore abrupt the binding will be.

Worst case is if you're running UCAs with rod ends.

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post #27 of 53 Old 01-18-2011, 08:54 PM
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As long as you keep body roll under control, it shouldn't be a problem.
Yes. But you do that by stiffening the rear suspension with either springs or sway bars, both of which reduces rear grip...

...leading to oversteer.

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post #28 of 53 Old 01-19-2011, 07:40 AM
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I don't think most have experienced snap oversteer issues either. Otherwise we would be hearing from others (which I have yet seen). I can only imagine it happening under extreme conditions. As long as you keep body roll under control, it shouldn't be a problem. At that point it is a suspension setup issue.
I smushed in the ds of my car at VIR several years ago due to snap oversteer. Since I have installed the PHB I have not had another problem.

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post #29 of 53 Old 01-19-2011, 08:10 AM
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I smushed in the ds of my car at VIR several years ago due to snap oversteer. Since I have installed the PHB I have not had another problem.
Ouch that must not have been pleasant. But it sounds like the panhard bar solved your snap oversteer which is what it was designed to do. It did for me as well.

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post #30 of 53 Old 01-20-2011, 03:40 AM
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There are two different issues here that are being mixed up.

One is the location of the roll center in the suspension, any bind that results from that and the resulting load and subsequent destruction of the UCA bushings.

Two are the loads seen by the bushings from the cornering forces and the link stiffness.

The stock Mustang 4-link suspension has a fairly high roll center. When you add an MM PHB to it, you end up with a new roll center that is much closer to the ground. Properly speaking there is only one roll center now, even though the PHB has defined a new roll center that is in a different vertical location than the original 4-link roll center. There are not two roll centers.

Since the PHB has moved the roll center from the original location to a new lower location, the bushings in the control arms are now forced to deform more to allow this new motion. This is only true when there are no cornering loads and the axle is rolled in the chassis. This definitely does cause an increase in roll stiffness compared to the 5-link case where the UCAs are more parallel and have their angle adjusted to put the roll center of the 4-link in the same location as that of the PHB.

Now look at the two cases (4-link and 5-link) when cornering loads are applied.

In the 4-link case, nearly 100% of the cornering loads are resisted by the UCA bushings. Due to the direction the RLCA bushings are loaded in, they have very little stiffness. The direction the UCA bushings are loaded in gives them some stiffness. The percentage of the load that any component absorbs in a mechanical system is in direct relation to the stiffness percentage that component has in the system. If you removed the UCAs completely and applied a cornering load, the bushings in the left and right RLCAs would resist the load equally (50%) since they have exactly the same stiffness as each other. With the UCAs in place, they provide 90%+ of the stiffness, so they see 90%+ of the load. This is why they are destroyed so quickly when the car is cornered hard. In a case where there is 1,000lbs of cornering load, the UCA bushings are going to see 900lbs of it.

In the 5-link case things are very different. When the same cornering load is applied, the PHB sees almost all of the load since it has 100x the stiffness that the UCA bushings do. For every 1,000lbs of cornering load, the UCAs might see 10lbs of load from the cornering force. Even if they see an extra 100lbs of load from the added bind, their total load is still only 110lbs. Much less than in the 4-link case.

In our experience, adding a PHB greatly increases the life of the UCA bushings, because of this.

Jack Hidley
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post #31 of 53 Old 01-20-2011, 06:04 AM
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There are two different issues here that are being mixed up.

One is the location of the roll center in the suspension, any bind that results from that and the resulting load and subsequent destruction of the UCA bushings.

Two are the loads seen by the bushings from the cornering forces and the link stiffness.

The stock Mustang 4-link suspension has a fairly high roll center. When you add an MM PHB to it, you end up with a new roll center that is much closer to the ground. Properly speaking there is only one roll center now, even though the PHB has defined a new roll center that is in a different vertical location than the original 4-link roll center. There are not two roll centers.

Since the PHB has moved the roll center from the original location to a new lower location, the bushings in the control arms are now forced to deform more to allow this new motion. This is only true when there are no cornering loads and the axle is rolled in the chassis. This definitely does cause an increase in roll stiffness compared to the 5-link case where the UCAs are more parallel and have their angle adjusted to put the roll center of the 4-link in the same location as that of the PHB.

Now look at the two cases (4-link and 5-link) when cornering loads are applied.

In the 4-link case, nearly 100% of the cornering loads are resisted by the UCA bushings. Due to the direction the RLCA bushings are loaded in, they have very little stiffness. The direction the UCA bushings are loaded in gives them some stiffness. The percentage of the load that any component absorbs in a mechanical system is in direct relation to the stiffness percentage that component has in the system. If you removed the UCAs completely and applied a cornering load, the bushings in the left and right RLCAs would resist the load equally (50%) since they have exactly the same stiffness as each other. With the UCAs in place, they provide 90%+ of the stiffness, so they see 90%+ of the load. This is why they are destroyed so quickly when the car is cornered hard. In a case where there is 1,000lbs of cornering load, the UCA bushings are going to see 900lbs of it.

In the 5-link case things are very different. When the same cornering load is applied, the PHB sees almost all of the load since it has 100x the stiffness that the UCA bushings do. For every 1,000lbs of cornering load, the UCAs might see 10lbs of load from the cornering force. Even if they see an extra 100lbs of load from the added bind, their total load is still only 110lbs. Much less than in the 4-link case.

In our experience, adding a PHB greatly increases the life of the UCA bushings, because of this.
I wonder why I continue to have to replace diff side UCA bushings so frequently?

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post #32 of 53 Old 01-20-2011, 08:15 AM
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Thanks for adding some truth to this discussion, Jack. It explains why my UCAs have survived so long, and why I have not experienced any snap oversteer. Your panhard bar has been a great product.

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post #33 of 53 Old 01-20-2011, 06:03 PM
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Mr Kirby,

If you ran the car for a long period of time without the PHB, the ID of the ears in the differential may be damaged from having the bushing shells work themselves out. With new bushings in them, the holes may just be too loose.

Do the bushings themselves become destroyed or does the shell move inside the the differential ears or both? What size front/rear tires and model do you have? What front/rear spring rates?

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post #34 of 53 Old 01-20-2011, 10:44 PM
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In our experience, adding a PHB greatly increases the life of the UCA bushings, because of this.
Huh! Interesting.
I wouldn't know a roll center from a roll bar, but intuitively, that's what I thought when I posted the question earlier.
The uppers can't move MORE with a panhard bar, so it made no sense that they would wear out faster.
THANKS!

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Mark,

Don't get caught up with what they are saying as gospel. Theory does not apply to reality sometimes. I am still on the original UCA's and they still look good to me after over 8 years with a panhard bar. I don't know about you, but I have never experienced any snap oversteer with just the panhard bar. So it is working as designed. The panhard bar might be one factor among many that are causing the UCA's bushings to be ripped. How replacement UCAs are installed, and suspension setup and application can also play a roll.
Still running stock original uppers and lowers here. All the stuff I've done have made the car handle better but the single BEST mod was the watts link. Second was big fat sticky tires.

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