Pinion Angle?

[russ in VA]

It just occured to me that, having lowered the rear of my MM, I have changed the pinion angle. I know just enough about this to be dangerous. I do know that having too much OR TOO LITTLE pinion angle can cause driveline vibrations. Question is, did I increase or decrease my pinion angle by lowering the car about 1.25 inches in back? Next question is, did I help or hurt the inherent high speed driveline vibration our cars have (with the stock chip, I have never felt it myself)? Could it be that the reason for the driveline vibe is that the exagerated rake dialed in by the factory yeilds too much pinion angle (like a high jacked 4x4)? In this month's SCCA national magazine (Sports Car), they have an article on driveline durability. They give just enough info to confuse me ... sure wish I could find the right answer here! I'm sure I could ask the helpful folks at my local LM dealer, not! ;)

[nomad]

Good qustions, let's ask the new guy 4DRLT1.
He's an auto tech.:D
Dennis could probably answer, E-mail him.

[WolfeBros]

I am thinking that you didn't change the pinion angle. The height of the rear end to the ground did not change. You basically lowered the body onto the frame. If it moved it wouldn't have been much. Anyone else ?

Now my head hurts from all this thinking again......I'm gonna lie down. RF.....you get the next question.
:lol:

[Pantherman]

Russ, don't get too caught up in the urban legend about the Marauder having some sort of driveline vibration. Having driven your car as delivered, you inherently know that isn't really true. Everything in the universe, however, vibrates at some frequency. For driveshafts that frequency is primarily determined by their length and diameter. Somewhere north of Ford's conservative speed limit, the Marauders shaft will hit it's vibratory frequency (critical speed) and, with little warning, suddenly try to shake itself out of the car. High exciting forces from a poorly angled u-joint can cause noticeable vibration even far below critical speed and will logically cause failure at a lower rpm. Because the upper suspension arm is shorter than the lower arm, as you lower the suspension, the nose of the pinion tips down. The rear joint angle is supposed to be carefully matched to the front joint angle so their accelerations cancel out. Actually, I would be a bit more concerned by the fact that you are now riding around on your jounce bumpers. To quote Autoweek's Marauder S test "it feels as though you're riding on the bump stops" (they were). "Go over a pothole and it's--BAM!--a teeth-rattling buckboard." Fortunately, you don't have to contend with Michigan potholes. The higher initial rear wheel rate has to also affect extreme handling, although since you have a lot of track time, this may be a minor concern

[hd2805]

I'm certainly not a mechanic but let me share what I had happen... I had the 4:10 gears installed last fall and on he same visit I had them lower the rear of my car about 1-2 inches by "fooling" the auto level gizmo... My rear end had one hell of a howl on decel right out of the gate, they checked and re-checked the backlash or whatever it is you call the setting with no good results, they finally put the leveling deal back to stock and not a sound from the rear end since.... Don't know if this helps or not...

[Heavy351]

As Wolfebros said..

The motor, transmission, and rear suspension are all mounted to the same frame. When you changed the height of the body all you did was increase the distance from the rear Axel (always the same distance from the ground) from the frame (where the body is mounted too). The angle at the rear diff did not change, but the angle at the transmision U-joint was decreased ever so slightly. There are no issues with pinion angle here but in some cases as noted above the new angle may change the resonant frequencies of the drivetrain..

get it?

[russ in VA]

Thanks for the insight guys! I think I get it. Let me say it back a different way and tell me if this sounds right. The rear suspension is designed to keep the pinion pointed down to maintain the angle as the suspension moves through it's travel so the pinion angle stays relatively unchanged. The drive shaft angle at the tranny is now slightly flatter since the driveshaft has moved up into the body at the rear of the car. I don't have any noise or vibration since making the change a couple of months ago so I should have nothing to worry about. Great!

As far as riding on the bump stops .... I need to crawl back under there and look to see how much travel is left ... but I dont think I've ever bottomed out on the stops. I lowered it all the way down as my first step in messing with the ride height adjustment and that was a good two or three inches lower than where I set it. The factory setup has the back end WAY up in the air. It looked really silly to me, like the Starsky and Hutch Gran Torino, all it needed was a set of Cragar Mags (no offense Sarge) and fuzzy dice. Now it still has some rake, but not as exagerated. At any rate, I have ridden in plenty of cars that DO bounce of the bump stops (yes I have friends that drive ricers ... and I'm not ashamed to admit it ;) ) and I am well aquainted with the loss of traction that comes from infinite spring rates (aka bottoming) arising mid corner and I don't have that problem with the MM. As long as I'm not hurting the driveline, I have no concerns about having dropped it.

Thanks for the help!

[Vince Gortner]

Russ,

Did you do the front springs too? What kind did you use? How bad is the install? Did you have to get the front re-alligned?

Most importantly.... Does it look cool?!?

[Pantherman]

Heavy351, I have to disagree with you about the side view diff angle not changing as the car is lowered. That would only be true if the trailing arms were parallel and equal length. Because the upper arm is shorter, for a given drop it swings through a greater arc, pulling the top of the diff forward and tipping the nose down. A lot of computer time goes into picking the all around best arm lengths and angles.

[russ in VA]

Vince, no I have not done the front springs, though I would like to. There is too much fender gap for my taste, especially compared to the rear which now has a very small amount of fender gap. I'll even this out when the tires wear out by putting 245 or 255/45's all the way around, but ulitimately it still needs springs. Check out the pics of Mensra's car to see what it looks like with tires almost the same size front and rear and lowered all the way around. VERY COOL.

[Heavy351]

The marauder does not have a trailing arm rear suspension not independent).

The pic that comes to mind when I posted is located here:

http://www.miracerros.com/mustang/t_suspension.htm

and there is an image of the rear suspension.

"Another improved rear suspension that Ford could have put on the Mustang is found on the 2000 Grand Marquis, Town Car, and Crown Victoria (illustrated above). It consists of straight-back upper and lower control arms (no bind) and a watts link located on the front of the differential. "

The new design allows for even less pinion ange deviation than the earlier CV angled upper control arm arrangement. In any rear suspension design there is an effort to reduce rear pinion windup (drag racers are real familiar with this) in newer cars with some sort of upper control arm(s).

My point is that there has been much engineering effort dedicated to removing as much driveline ange change as possible during suspension travel. An inch or 2 of height change is well within the rear suspensions' design limits to control pinion angle. Now if you were slamming it in the weeds with '14s or putting giant swampers on it there would be a concern here.