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Old October 21st, 2005, 02:41 PM
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The problem with the MN12/FN10 chassis (other then the Uni-body) is the fact that the rear window area is prone to flexing.

An article Jerry wrote that will shed some more light on the subject....

Part 1 - History

Before I can even go into all this stuff, I need to give some history and some term definition.

The name for the Mark VIII chassis, within Ford, is called an FN-10 and a T-Bird is a MN-12 (from 1989-1997). As I'm sure you know the chassis are almost identical. If I use one term, like FN-10, I mean both chassis unless I specify otherwise that my comment is just for one of them.

The FN-10/MN-12 chassis is very, very prone to driveline vibration due to its body structure. Body structure is the rigidity of the body and chassis. This lack of body structure is one of the reasons why these cars no longer exist today. There are some other reasons that I'll get into one day in the future.

Now, by lack of body structure I mean that the body itself is not stiff enough. This means that it is very suspectible to flexing. This poor body structure leads to NVH problems. NVH is Noise, Vibration, and Harshness. Noise and vibration should be easy to understand, harshness can be things like impact harshness going over bumps and things like that.

Now, in typical Ford fashion, the first year of new car, no expense is spared and all the good stuff is put in there to make the car right. Then the &^$* bean counters come along and say the car is not profitable enough and start to take out the good stuff that's in there. What I mean by this are things that are subtle that they can do to impove the body structure. Things like thicker mastic on the floor pans, structural foam in the rockers to stiffen up the car. While these are all band-aids for bad body structure, they are effective band-aids. The original Marks in '93 had this stuff, but it was quickly thrifted out by bean counters and complaining by the assembly plants that it was too hard to build.

So, with that foundation, I have decided to make a seperate post on the actual cause of the vibration. This is just a history.

Part 2 - Causes

Now to the meat.

Basically the driveline vibration can come down to a few subsystems, the wheels and tires, the driveshaft, the trans or the axle. There are few extreme cases that have torque converters that are out of balance, but these are rare. I'll cover each one seperatly and then tie them all together.

Wheels and Tires
Since these chassis are very sensitive to vibration input the wheels and tires must be balanced very, very well. This is pretty basic, I don't need to go over what to do, just get them balanced. A wheel vibration usually comes in slow, has a peak amplitude and then goes away or gets much better. If it's a front wheel you'll feel it/see it in the steering wheel. If it's a rear wheel, you'll feel it coming from the corner of the car that has the issue. I'll tell you how sensitive these chassis are to tires. The Mark VIII was supposed to have Goodyear tires at the start of production. All the preproduction cars had Goodyears. When Wixom starting making cars, the cars has more vibration than normal and it was traced to tires. Do you what brand the cars were made with? Michillen (I can't spell really well, it's the sign of an engineer). To throw away all those Goodyears cost a lot of money, that's how important it was.

The output shaft on the transmission appears straight. It also must be capable of handling a lot of torque. For example, let's say a Mark VIII motor makes 250 ft-lbs at a full brake stall. The torque converter has a 2.40 stall multiplication and low gear in the trans is 2.83999. So, the output shaft could see 250*2.40*2.83999 or 1704 ft-lbs. Now, you can probably add 10-15% for good cold weather.

So, to get a shaft to handle that much torque -- and remember, at it's narrowest point it's only about 1.25" in diameter -- you have to do one of two things. You either need really good material or you need to make it really hard. Initially, until 1997, the shaft was made from a 1020 steel that was carborized and then DOUBLE induction hardened. Now, if you know anything about metals, you know when you induction harden a shaft, it warps and twists. So, after the shaft is made from the raw materials, it's turned to rough finish surfaces. Then it's caborized and double induction hardened. This turns the shaft to a pretzel. It is then put into a huge press and is beaten back straight. I am very serious here. Then the shaft was put into a centerless grinder and finished. There are a few issues with this. You turn all the bearing surfaces in a lathe, and then finish grind it on a grinder that does not hold it to spin on the same centerline that it did when you initially turned it. Bottom line, you end up with the very likelihood of two centers on the shaft. Then after it's beat back into shape, it may be straight on a macro level, but if you look at it more closely, you'll see that there a bunch of little ups and downs all the way down it.

So, what's all this mean? The shaft is not straight, has no center and can make a vehicle that has a sensitive chassis have vibration.

In 1997ish, they improved the material of the shaft to 1045 steel. This results in a stronger shaft with little induction hardening so it is a lot straighter. You can see this shaft by its color.

There is also a ring gear in the transmission that the output shaft splines to. It's about 6-7" in diameter and can also be out of balance. I'll touch on this a little later when I get to the whole system.

The axle itself can only affect driveline vibration in a few ways. Picture the pinion spinning with the companion flange attached. The companion flange is a round flange that splines/bolts to the pinion gear and has 8 threaded holes that the driveshaft bolts to.

The vibration here can come from the center of the pinion and companion flange, not spinning in a true centerline. It can turn in an ellipse. Again this can set off vibration in a vehicle with a sensitive chassis.

The driveshaft has a few things that affect the vibration. The first is obvious, the shaft needs to be balanced well. The second is the straightness of the shaft or runnout. If you spin the shaft on it's centerline how true is the outside of the shaft spinning? This is similar to the runout of the pinion and companion flange.

Part 3 - Driveline Critical Speed

So you have a sensitive chassis and all these parts that can lead to vibration in a sensitive chassis, so what?

The car basically responds to what inputs it receives. Basically, the input is all these subsystems that can cause vibration, or the hammer, and the car is the tuning fork.

A car has what's called a first order driveline bending point. To explain it simply, if you are holding both ends of a shaft fixed and rigid, and spin the shaft, there is centrifigual force, from the weight of the shaft, that, as you spin the shaft faster and faster, the middle of the shaft wants to whip or spin in a much larger circle than the ends of the shaft. Make sense?

Now, from an engineering standpoint, the first order resonance point is when the displacement of the middle is it's greatest. This is NOT at the fastest speed you can spin the shaft. Ever have a tire out of balance and it's really bad at 60 mph but better at 80 mph? Well, what happens to a shaft is at a certain speed (based on stiffness of the shaft, length and weight) you have one speed with the highest displacement in only one point. If you spin the shaft faster, the displacement of the shaft now happens in two points, but the amplitude of each "node" is less. If you spin it even faster, then you get 3 nodes, and etc.

So, every car has a critical speed like I just described.

Now, in reality, the critical speed is controled by the driveshaft material, stiffness of the shaft, stiffness of the fixed ends. This means that the trans end is not mounted infinitly stiff nor is the axle. As the driveshaft spins and wants to spin in a bigger circle, it can start to move the transmission extension housing around as well. A stiffer trans mount will make it move less. Same goes for axle mounting. How much does this matter? If you calculate the critial speed of a steel shaft with fixed ends, it's actually pretty high, somewhere in the 7000 RPM range or higher. What's the critical speed of the Mark driveline -- 5320 RPM. What's the difference -- the stiffness of the whole system.

Now, what can you do? A stiffer trans mount and axle mounts will help to fix the ends of the shaft, but you do not want solid steel mounts -- you will break things.

The next best thing to stiffen the shaft would be to make it a larger diameter. Unfortunatly, there is not enough real estate to do this in a Mark. So, your other choices are materal. Steel is very stiff which is good, but weighs a lot, which is bad. Aluminum is not as stiff as steel but much lighter than steel, so its rotating weight is low. This low inertia of aluminum offsets the added stiffness of the steel, so aluminum is better than steel. There are two other alternatives. The best overall choice is a metal matrix composite shaft. This shaft is stiffer than steel and lighter than aluminum, so it's a win-win, other than cost. This is the shaft that Dennis is selling. I gave him the Reader's Digest of this explanation and he made it happen for these cars.

So, by stiffening up the driveshaft, you reduce the blow the hammer makes to the tuning fork.

So, why does a car vibrate when you are closed throttle deceling? On an IRS car this is caused by bad U-Joints or the pinion drooping. If the pinion is drooping then you need to fix the bushings. If the pinion is not drooping, then put in new U-Joints or better yet, get a good driveshaft. This drooping pinion is called a bad pinion angle. On a live axle this can be caused by alignment of the links that hold the axle in place. On an IRS car negative pinion angle is from bad bushings.

I think this explains the subject pretty much. a


I dunno who Jerry is, but I found it posted at TCCOA's forums.
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Old October 21st, 2005, 04:58 PM
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If that was on the TCCOA forums, I wonder if that might be Jerry Wroblewski? The transmission guy? I'm pretty sure he's a member there.
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