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.
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.
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Jack Hidley
Maximum Motorsports Tech Support
So would it be best IF you were going to do a PM3L to use a rod-ed UCA like these adjustable UCAs?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.
In a stock system;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.
Read the whole thing until you understand it.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.
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).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.
Maybe I'm just lucky, but ...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. ...
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.This coming from my Jeep/rock crawling other hobby, would something like a retro-fitted Johnny Joint be of any use in either(or both) ends in a Mustang upper control arm?
Poly Performance JEEP/TRUCK & BUGGY#::#Suspension Joints#::#Johnny Joints®#::#Johnny Joints® - 4X4 Off-Road Parts, JK Synergy Suspension Systems, Fox Racing Shox, Beard, CNC Brakes, Johnny Joints®, Currie Enterprises, Edelbrock, Pit Bull Tires, Ramse
or
Poly Performance JEEP/TRUCK & BUGGY#::#Suspension Joints#::#Summit Machine Flex Joint#::#Flex Joint - 4X4 Off-Road Parts, JK Synergy Suspension Systems, Fox Racing Shox, Beard, CNC Brakes, Johnny Joints®, Currie Enterprises, Edelbrock, Pit Bull Tires
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.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.