MTCFORUM
02-13-2002, 09:27 PM
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. Whats 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 realestate to do this in a Mark. So, your other choices are materal. Steel is very stiff which is good, but weighs alot, which is bad. Aluminum is not as stiff as steel but much lighter than steel so it's 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, it's a win win, other than cost. This is the shaft that Dennis is selling. I gave him the readers 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 shaft from Dennis. 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.
jerry
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. Whats 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 realestate to do this in a Mark. So, your other choices are materal. Steel is very stiff which is good, but weighs alot, which is bad. Aluminum is not as stiff as steel but much lighter than steel so it's 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, it's a win win, other than cost. This is the shaft that Dennis is selling. I gave him the readers 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 shaft from Dennis. 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.
jerry