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  #1 (permalink)  
Old 03-23-2011, 08:53 PM
YerDugliness's Avatar
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Cobra Make, Engine: Cobra Auto Works body, Ron Godell Racecars chassis, 1989 Mustang GT 5.0 HO (converted to carb), W/C T-5, 3.73's in a Ford 9" Traction-Loc.
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Default Need advice on adjusting a parallel 4-bar rear suspension

I just crawled out from under the Cobra, having replaced all the 15 year old plastic bushings with graphite impregnated urethane. In the process I had to remove the bars and when I put it back together I went to great lengths to ensure that the longer lower bars were both the same length and the shorter upper bars were both the same length. When I put it back together I found that I had to lengthen one of the shorter upper bars in order for the bolts to fit through the brackets on the rear housing.

Is this going to be a problem? I worry that the differential housing might be in some kind of a bind, or that perhaps the wheelbase on the driver's side might be slightly greater than that of the passenger's side, or that the pinion angle might well have changed enough to put the U-joints into a bind.

These are nice pieces.....one merely needs to loosen the jam nut on each end of the bar, and then the length of the bar is quite easily adjusted, much like the way a tie rod adjusts.

So, here are a few questions:

1. Will a decent front end shop be able to make sure that the wheelbase is the same on both sides?

2. I do have an inclinometer, and I also have plans on lowering the engine (Mustang convertible motor mounts will lower the engine by 3/4", I'm told), so at some point the pinion angle will need to be adjusted to match that of the transmission yoke. Is it better to use the lower bars to adjust the pinion angle, the upper bars, or does it need to be some combination of the two?

3. How much of a problem is the manner in which the upper bars had to be different lengths? Logic tells me that as long as I had one side tightened up (the passenger's side), adjusting the bar on the other side so that the bolt slips through the bracket and the sleev smoothly didn't harm anything, and in fact might have eliminated any bind I might have induced had I attempted to move the rear housing around so that the bolt would fit without the bar needing adjustment....but a little knowlege is a dangerous thing in matters such as these !

I would also be grateful if you'd let me know if there were any "How To Adjust Your 4-Bar Suspension for Dummies" books out there. Magazine articles would also be welcomed....any "how to DIY" type info on getting these suspensions right.

Thanks!

*Note to moderators: I looked all over the place for an area where suspension issues would be placed, this seemed the best. If I missed something (a distinct possibility at my age), please feel free to move this thread to the appropriate location. TIA!!

Cheers from Dugly
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  #2 (permalink)  
Old 03-24-2011, 07:24 AM
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Dugly

You need to set the bars with the car at ride ht for your weight in the seat(sandbags)
The easist way is without the springs shocks hooked up (initaly)with one of the uppers disconnected, set the lowers to align the rear with the direction of travel(forward and centered) then adjust the connectted upper to set the pinion angle, and tighten those 3 bars end bolts plus jam nuts,and then adjust the other upper bar to fit the attach bolts and tighten those---leave the jam nuts loose--reinstall coiloversand set heights--then turn the link so as it is in a free turning area(by hand)Roll bar back and forth to determine this--then give it about 1/2 turn to preload the rear forwrd(pinion down) as the housing will want to rotateback under acceleration---go for a test driveand enjoy
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Old 03-24-2011, 07:45 AM
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I would follow Jerry's write up. Concise and well written.

Just to slightly clarify,preload is shortening that upper bar.
The upper bar I'd connect and adjust last would be the right (passenger) bar. This is the same bar you will go back to to adjust,1 or 2 flats at a time,to get the car to leave straight.Adding or subtracting preload.
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Old 03-26-2011, 10:51 PM
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Thanks for the info, guys!

I do have some documents relating to the Cobra from the period before I bought it. It seems that the previous owner had Keith Crafts's shop north of Dallas set the ride height--do you think I'll need to have the ride height reset since I didn't mess with any of the settings (only the one rod I had to extend about 1/8"). When I disassembled the rear suspension, it was with the idea of only replacing the old, hard, crumbling bushings with modern pieces. I was surprised to find when I got the bars off that some of them had one, and even a couple had both, of the jam-nuts loose. I don't know if that presented problems or not, but I could certainly see how it could.

I think the first thing I ought to do is to make sure that the wheelbase is the same length on both sides, that's why I asked if a good front end/alignment shop might have machinery to check that. I could see problems in that area causing "crabbing". If that happens, nothing I do will be right.

Jerry, do you mean that I ought to have installed all 4 bars with the end bolts, meaning the bolts that go through the sleeve in the bushing, loose? I certainly didn't do that. Initially I had difficulty getting the two bolts that hold the upper bars to the frame bracket to engage in the threads...the passenger side was the first to remedy itself with a well placed "whack" with a rubber mallet--I guess it was just slightly short of reaching the threaded area in the frame. On the driver's side I had installed the bolt into the wrong "hole" on the bracket...only one, the middle hole, is threaded. I felt pretty dumb when I realized what had happened!! Both of those bolts are "blind", meaning there is no nut on the other end, only a threaded portion of the frame (or, perhaps a captive nut welded on the inside of the frame). The other 6 ends have nuts as the bolts extend all the way through the brackets. I'll try to post photos in the middle of next week when I get back home, that might make it clearer.

To make things even more complicated, there is a panhard rod that extends from the driver's side of the frame to a bracket on the passenger's side of the area where the pumpkin mounts. That appears to use urethane bushings, too, but it is tight so I am not going to tempt fate by taking it loose right now.

Thanks to all who have given advice....this sounds much more complicated than I thought when I started to work on it if all this is going to be necessary!

Cheers from Dugly
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Last edited by YerDugliness; 03-26-2011 at 10:53 PM..
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  #5 (permalink)  
Old 03-27-2011, 03:34 AM
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i would block the car at ride height without the shocks with your axle blocked to the same height as the wheels if they were on.

check the pinion angle to see if it is close to whatever you want, you can measure the angle of the engine/tranny by using the centerline of the tranny output shaft and center of the front damper and use the bottom of the frame for reference, that will get you close and give you an idea of the drivetrain angle.

find the centerline of the frame then mark with nail polish or marker so you don't have to do it again. drop a plumb bob off these marks to a string ran down the center of the car.

you need to drop a plumb bob off each side of the rear axle to triangulate to some point at the front of the car to put the axle at 90 degrees to the centerline, which will be your thrust angle. you can put the thrust line slightly to the right but never to the left for drag racing, centered for normal driving and road course. no more then 6-7 degrees to the right to give you an idea if you know what that looks like.

either before the above or after you need to set the panhard bar to center the axle also, probably have to do the above and the panhard bar a couple times. i like to put a level across two of the wheel studs then drop a plumb bob off the bottom stud, just make sure to either use the front or back of the stud on each side, then you can use this to triangulate off the front for the thrust line also. use a marker on the floor or taped piece of paper, plan on a few marks.

recheck the pinion angle.

the machine will make the alignment off the thrust line of the rear wheels so you want this close.

once you get everything where you want it you can turn each link bar so that it is centered in the threads, ie each bar should turn easily to the right then left then center between these two. do this on all the links & panhard bar and you should be as close as you are going to get, lock the joints with nuts.

on the front end make sure the steering arm joints coming out of the rack are centered right to left on the frame and not the rack when doing the alignment, some of the racks are offset.

Last edited by vector1; 03-27-2011 at 03:38 AM..
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