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Rear UCAs and Panhard Bar

10K views 37 replies 17 participants last post by  Ciotti 
#1 ·
About every 5-6 weekends I have to replace one or both diff side bushings and annually the UCAs. Does this sound normal? It seems to me that with a PHB there should be less stress on the UCAs.
 
#2 ·
It's not less stress on the UCA's when you still have both of them attached, because the roll center they define is much different than the roll center defined by the panhard bar, and the panhard bar's roll center wins every time. That's forcing the UCA's outside their limits, and it's why the PM3L (Poor Man's 3-Link) was invented. Put new bushings in ONE of the UCA's and remove the other one. The remaining one is no longer trying to define a roll center but it is free to control axle windup like intended.
 
#7 ·
Here we go again. We had this who discussion a few months back.

http://forums.corral.net/forums/road-racing-auto-x/1276349-effects-movement-rear-end-fox.html

If I may quote Jack Hidley (skip to the last paragraph if you're short on time):

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
Maximum Motorsports Tech Support
 
#3 ·
...this might be a stupid question, but, if you install a panhard bar must at least one uca be removed always in every situation? My car came with both uca's and a panhard bar installed as a dealer option or sorts.. no ones ever mentioned removing the control arms..
 
#4 ·
Lots of people drive them around that way just fine, but if you start wailing them around corners on sticky tires, you're going to run into this.
 
#6 ·
Been running a PM3L on the street now for more than a year. Mine has rod-ends on both ends (i.e., no rubber bushings), so there is not the typical bind found with a stock arm. As of now, the upper boxes are fine. I suppose the stress with some sticky rubber and more power to the ground the upper torque box might be an issue, but as of now the only thing I need to watch are the rod-ends. By the way, this is a great way to determine whether a T/A or true 3-link is for you. The suspension works so much better that I'll not go back to the stock 4-link.
 
#19 ·
Been running a PM3L on the street now for more than a year. Mine has rod-ends on both ends (i.e., no rubber bushings), so there is not the typical bind found with a stock arm.
So would it be best IF you were going to do a PM3L to use a rod-ed UCA like these adjustable UCAs?

Do you think these help control the pinion angle and reduce the issues That89GTGuy is having?
 
#8 ·
That changing rate seems excessive to me. My 97 Cobra is probably similar in weight to your Mach. I've had to change the UCA bushings twice over 5 years and about 80 HPDE days. Most of that time was PHB and 4-Link.
 
#9 ·
Yeah I don't get it.According to J Hidley the PHB should increase the life of the UCA bushings.I tried to call MM yesterday but couldn't get thru so I will try again today.
 
#10 · (Edited)
He's talking about a five link there.

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.
In a stock system;

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.
Read the whole thing until you understand it.
 
#11 ·
I spoke to MM yesterday and they said that a lot of the CMC guys change bushings every weekend,of course they don't have a PHB. They said for what I am doing changing diff bushings every 5-6 events and CAs every 10-12 sounds about right.

Which brings up another point. Acc to MM FRPP no longer sells UCAs and MM only has a few left and they are going to gouge us for $190. Now what do we do?
 
#15 ·
I spoke to MM yesterday and they said that a lot of the CMC guys change bushings every weekend,of course they don't have a PHB.
CMC cars can run a PHB; I doubt there are that many guys a) not running PHBs, and b) changing out UCA bushings every weekend. In Texas, the CMC guys run pseudo-PM3Ls (both UCAs are there, but one is essentially removed by having soft foam rubber bushings).
 
#16 ·
Here is my experience with the whole 2 uppers and a PHB/going to PM3L thing:

With both uppers and a PHB, the ride was awful, and very stiff. The rear felt far too stiff for the car, much more so than when I did not have the PHB. Even on smooth highway the rear would buck around anytime the axle needed to move around. It felt like it wanted to oversteer more than before, but I never was able to autox/track it.

When I removed the drivers side upper, the ride was instantly better. It rode much better than before, no bucking or nonsense. HOWEVER, I have had issues with crazy pinion angles causing the drivers side shock to hit the disc brake hose bracket screw (I have converted to Thunderbird rear discs). No failures, but lots of noise, and it's eroding the shock body (rubs back and forth as the PHB moves the axle), and probably crazy pinion angles on hard acceleration and braking. My diff side bushing is very, very old (possibly 22 year old original... I hope not), so I will be replacing that soon and seeing if it helps, along with a lower profile screw.

Another note. If you go to the PM3L, you will probably want to get some torque arm springs. The rear will be way too soft without them.

My opinion is it's way better, but the annoying pinion issues I've been having make me want to say otherwise. Before I noticed that issue, I was happy as a clam. Sounds like a good number of people use it with no issues though, so it's a crap shoot.

TL, DR : It's a free mod, so it's worth a shot.
 
#23 ·
Here is my experience with the whole 2 uppers and a PHB/going to PM3L thing:

With both uppers and a PHB, the ride was awful, and very stiff. The rear felt far too stiff for the car, much more so than when I did not have the PHB. Even on smooth highway the rear would buck around anytime the axle needed to move around. ...
Maybe I'm just lucky, but ...
To me, a MM panhard bar was the second-best mod I've ever done (shifter is No. 1) for making the car pleasant to drive, especially on the street.
Just feels much more solid and predictable.
I've had the PHB now for more than 5 years and about 25,000 miles of autocrossing and daily driving. Just recently, I finally replaced the ORIGINAL upper control arms.
Knock on wood, maybe it'll be another 145,000 miles before I replace them again. :)
 
#24 ·
I too ran the MM PHB with the FRPP UCAs for well over two years, both on the street and many autocross driving schools (20-30 runs a school). If you believe the PHB was the second best upgrade to your car from a comfort and handling perspective, then you are in for a treat if you just removed the D/S upper and went with a PM3L. Until I switched to the PM3L, I had no idea how much better the car rides and handles without the bind and uppers fighting with the PHB. Of course, I do have the rod-ended PM3L and the MM sta-bar, so YMMV.
 
#25 · (Edited)
As an update if anyone was interested, after replacing the upper diff side control arm bushing and getting a lower profile screw, the shock still hits the bracket for the brake softline. For reference, my upper control arm is an FRPP arm. I'll be cutting the bracket portion that it's hitting in all likelyhood in the next week or so, and see what happens from there.
 
#27 ·
On the axle end, I'd be a little concerned with adiquate room to fully articulate. With my set-up, I can see where the rod-end begins to hit the .75" bolt head. I need more spacers to improve the clearance at those angles. The Jonny Joint/Summit Machine design at the axle, one would have to run an axle end mount on the PM3L similar to the one dpspeed linked above. I don't think that would allow enough clearance for the angles needed.
 
#28 ·
Those joints are used in extreme articulation occurrences. I can only see an actual WORKING rear suspension "working" within a MAXIMUM of 4 or so inches up/down/left/right. I could be completely wrong on this and if I am by all means correct my information.

I'm using them on the chassis side upper and lower control arms on my jeep.

 
#29 · (Edited)
Those joints are used in extreme articulation occurrences. I can only see an actual WORKING rear suspension "working" within a MAXIMUM of 4 or so inches up/down/left/right. I could be completely wrong on this and if I am by all means correct my information.

I'm using them on the chassis side upper and lower control arms on my jeep.
I have no idea how long your Jeep's arms are, but on the Mustang the PM3L is pretty short so the joint limits and clearances are used up fast. Moreoever, on my PM3L the alignment is not "square" since the axle end is mounted to the outboard side of the axle ear which puts the rodend at an slight angle as it runs up to the chassis mount at static ride height (yes, the poly bushings were designed to account for this but it's not perfect). Hence, my comments. If the arm's ears are designed with a fairly wide opening (unlike those in the Strano Parts link), then there may be enough clearance. One could go ahead and try the Jonny Joint/Summit Machine pieces, but would have to monitor them carefuly to make certain they do not cause the arm's ears to contact the axle's ears. But just using the bushings alone without all of these other considerations would be folly IMHO.
 
#30 ·
I understand your point(s), they make sense. I guess until someone gets bored enough and tries something with that joint, we're stuck at just some what if's and might be's. ;)

And FWIW, the Jeep's arms are the same if not maybe an inch or so longer than a Mustang LCA.
 
#33 ·
No no no. That's just the picture. It's installed with 1 of those solid metal spacer pieces on each side of the axle pickup point. Not sure why they chose to show the picture that way, perhaps its just because it ships that way?
 
#38 ·
I'm not sure if this is a dumb thought or not but after re-reading thread I'm wondering if the FAYS2 watts link has a roll center that is closer to the factory 4 link's and would reduce the amount of bind between the 2.

Any thoughts?
 
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