Ford Mustang Forums banner

1 - 20 of 38 Posts

·
Registered
Joined
·
1,382 Posts
Discussion Starter #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.
 

·
Super Moderator
Joined
·
27,657 Posts
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.
 

·
Registered
Joined
·
217 Posts
...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..
 

·
Super Moderator
Joined
·
27,657 Posts
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.
 

·
Registered
Joined
·
136 Posts
Is a PM3L setup "safe" to run on street tires without reinforcing the chassis mounting point for the UCA you leave in place? Seems kind of iffy from what I've read...
 

·
Registered
Joined
·
5,521 Posts
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.
 

·
Registered
Joined
·
506 Posts
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.
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
 

·
Registered
Joined
·
135 Posts
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.
 

·
Registered
Joined
·
1,382 Posts
Discussion Starter #9
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.
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.
 

·
Registered
Joined
·
1,688 Posts
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.
 

·
Registered
Joined
·
1,382 Posts
Discussion Starter #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?
 

·
Registered
Joined
·
2,007 Posts
i guess the obvious answer would be to get a TA
 

·
Registered
Joined
·
1,614 Posts
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).
 

·
Registered
Joined
·
49 Posts
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.
 

·
Registered
Joined
·
1,430 Posts
Put a tip jar in your car when you take students out. Then get a Torque Arm and enjoy the traction
 

·
Registered
Joined
·
1,382 Posts
Discussion Starter #18

·
Registered
Joined
·
354 Posts
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?
 

·
Registered
Joined
·
5,521 Posts
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?
The stock arm will still deflect quite a bit with those big, soft rubber bushings. Hince, why the rubber bushings need replacement often and/or permit pinion issues and side movement. Mine may deflect too but if so it's pretty small.

In another thread, I beleive the consenus was not to run a PM3L like those in the link. FWIW, mine is not like the one in the link. Rather, it has a solid machined steel bushing at the axle with a .75" bolt through a rod end. At the chassis end, another rod end using the stock bolt and poly spacers between the ears rather than the set-up in the link. There is a steel tube between the two which is treaded to accept the rod ends. One rod end uses right hand and the other left hand threads. If you are familiar with the Mathis P2 book, the design is identical to his adjustable uppers (on page 111 I believe) except for the poly spacers.

As a result of this design, I do not have the pinion problem or side to side rubbing discussed above. I do have a bit of cabin noise due to the transfer of differential noises throught the steel bushings and arm, but not as much as might be expected. Ride comfort, suspension articulation, forward traction are so much better, there is no way I'm going back. The down-side is some cabin noise and potential upper torque box failure due to the extra loads. This potential failure point wold probably be resolved with reinforcement pieces. I have not done that because I do not intend to continue running this set-up. I plan on a true 3-link similar to the s197 cars or the now discontinued EvO Motorsports 3-link. Or I may just do a T/A.

The point is, the PM3L, however it is configured, is so much better than the stock four link, even with a PHB added, that it is worth trying just to see how a non-binding rear suspension feels. Be aware that if class limits are an issue, you may not be able to run just one arm or even one designed like mine.
 
1 - 20 of 38 Posts
Top