A math problem related to engine / tranny / xfer case mounts...

[Greg72]

So let's say you decided to fabricate all your own mounts.

You decide to build your own motor mounts, and everything else behind it. Some of the mounting points (to the frame) are going to be wider (or) narrower than others, and you don't want to mix solid mounts with rubber or polyurethane because the drivetrain should be allowed to move as "a system" right???

I'm visualizing an imaginary "centerline" through the entire driveline, and each set of crossmembers (and their soft mounts) are some distance from that centerline. Based on that distance, as engine torque is applied and the driveline begins to twist....the amount of force (and deflection) of those bushings will be a result of how far away from the centerline they are..... bushings mounted closest to the centerline would deflect the least, and bushings on a long crossmember would move the most due to the increased leverage??? (or perhaps, the longest crossmembers do the best job of resisting the torque loads because of a longer lever arm?)

Is there a way to determine what the "hardness" (which I believe is called Durometer) of the bushings needs to be, and whether it needs to be higher on some of the mounts so that the entire system of crossmembers and bushings is working together....and not just putting the loads on a few particular bushings???


.

 

[blk87K5]

I have always set mine up to where the tranny/t-case will flex more than the motor. By setting the motor and tranny/t-case up the same (same bushings, width on frame, etc.) I have found that under hard hits the tranny becomes the weak point and cracks the case. No, I dont have a mathematical formula for it though.

Some poly bushings well flex a lot more than others though, and strategical placement of a soft bushing verses a hard bushing can make a large difference.

 

[ntsqd]

If you know the torque at any given loading and the distance from the centerline of powertrain rotation to the mount, then you can calculate the compressive load on the mount. I would imagine that you could back into the Durometer of the bushing material knowing this, but I do not know what the eqn that relates load to Durometer might look like.

The distance from rotational CL to the mount is the lever that the mount has to resist rotation. So further out = less load in resisting same torque.

Absent a truly rigid chassis I'm in favor of the above mentioned 3 point mounting arrangement, BBR's success w/ a 4 point not withstanding. I suspect that his x-member is just flexy enough to make the system behave as a 3 point.

 

[sickquad]

Getting into calculating this might not be so easy. These mounts with a unknown durometer will have a non linear reaction. This would be a real interesting problem to look into.

-Chris

 

[bartamos]

Compression of the rubber or poly is what you want to know. If you know the compression % before becoming ridgid (of the bushing/mount material) and If you can estimate how much movement a component can take (twist or bend) you can judge the mounting method. In other words, if the 2" thick material will compress 50% before becoming ridgid then it will allow the tranny/xfer case or crossmember to move 1". depending on the "rubber" material's distance from C/L, you can calculate the movement of the metal mounting foot or tranny case or ? draw it out then trig it. Might give you a good feel. If you don't know the compression: Take a test piece, measure it then squeeze in a vice and re-measure. I would say a med hard/durometer material would give the flex you want to have. Too hard will put the load on the tranny case/etc, too soft it will flex too far, either will twist/crack things. Like you said it depends on the geometry of the design. Distance from CL. Follow the truck manufacterers ideas. rubber flexability and size of rubber and distance to CL. They know what they are doing. Look at foreign and domestic 4X4's. I use the word rubber to mean whatever polymer you end up using. Good old closed cell rubber is good though. Anybody try small "air ride" bellows on this type of mount? It's all adjustable you know. Doesn't have to be used on just suspension, does it?

 

[ntsqd]

Re-reading this and thinking some more about it; I think what you really need to start with is "How much NVH can I live with?" and "How far can I let things move b4 trouble starts?"

If you can somehow get equal sized cross sections of the bushings you're considering and a like-sized piece of the stock mount(s) then you could use something like a valve spring tester to find the relative durometer of each. Then you would know which bushing material to choose based on your NVH requirements.

Next would be to look at how far your choice moved and decide if it will self limit the allowable motion range within what is tolerable, or if you need to construct limiters.

One further thot, they're getting rare but GM used a damper on the engines in the X bodies. Always wondered what would happen if one were used in other applications.

 

[pvfjr]

Torque = Force x Distance

For a given torque (i.e. what your motor produces), the distance from your centerline will determine how much force is applied to your mounts. More distance means less force, less distance means more force; for the given torque.

 

[Greg72]

...you would know which bushing material to choose based on your NVH requirements....

NVH??

 

[Triaged]

NVH??

Forgive him...he's an engineer

NVH = Noise, Vibration, & Harshness.

 

[Greg72]

Forgive him...he's an engineer

NVH = Noise, Vibration, & Harshness.

And what is the unit-of-measure for this Engineering term known as "harshness"??

 

[Triaged]

And what is the unit-of-measure for this Engineering term known as "harshness"??

Units should be rate of change of acceleration so [m/s^3]

You thought there wouldn't be an answer...some engineers can't sleep at night because they are thinking about this stuff. Check this out:

What is the term used for the third derivative of position?

It is well known that the first derivative of position (symbol x) with respect to time is velocity (symbol v) and the second is acceleration (symbol a). It is a little less well known that the third derivative, i.e. the rate of change of acceleration, is technically known as jerk (symbol j). Jerk is a vector but may also be used loosely as a scalar quantity because there is not a separate term for the magnitude of jerk analogous to speed for magnitude of velocity.

In the UK jolt has sometimes been used instead of jerk and may be equally acceptable.

Many other terms have appeared in individual cases for the third derivative, including pulse, impulse, bounce, surge, shock and super acceleration. These are generally less appropriate than jerk and jolt, either because they are used in engineering to mean other things or because the common English use of the word does not fit the meaning so well. For example impulse is more commonly used in physics to mean a change of momentum imparted by a force of limited duration [Belanger 1847] and surge is used by electricians to mean something like rate of change of current or voltage. The terms jerk and jolt are therefore preferred for rate of change of acceleration. Jerk appears to be the more common of the two. It is also recognised in international standards:

In ISO 2041 (1990), Vibration and shock - Vocabulary, page 2:
"1.5 jerk: A vector that specifies the time-derivative of acceleration."
Note that the symbol j for jerk is not in the standard and is probably only one of many symbols used.

As its name suggests, jerk is important when evaluating the destructive effect of motion on a mechanism or the discomfort caused to passengers in a vehicle. The movement of delicate instruments needs to be kept within specified limits of jerk as well as acceleration to avoid damage. When designing a train the engineers will typically be required to keep the jerk less than 2 metres per second cubed for passenger comfort. In the aerospace industry they even have such a thing as a jerkmeter; an instrument for measuring jerk.

In the case of the Hubble space telescope, the engineers are said to have even gone as far as specifying limits on the magnitude of the fourth derivative. There is no universally accepted name for the fourth derivative, i.e. the rate of change of jerk, The term jounce has been used but it has the drawback of using the same initial letter as jerk so it is not clear which symbol to use. Another less serious suggestion is snap (symbol s), crackle (symbol c) and pop (symbol p) for the 4th, 5th and 6th derivatives respectively. Higher derivatives do not yet have names because they do not come up very often.

Since force (F = ma) is rate of change of momentum (p, symbol clashes with pop) it seems necessary to find terms for higher derivatives of force too. So far yank (symbol Y) has been suggested for rate of change of force, tug (symbol T) for rate of change of yank, snatch (symbol S) for rate of change of tug and shake (symbol Sh) for rate of change of snatch. Needless to say, none of these are in any kind of standards, yet. We just made them up on usenet.

Now class, repeat after me. . .

Momentum equals mass times velocity!
Force equals mass times acceleration!
Yank equals mass times jerk!
Tug equals mass times snap!
Snatch equals mass times crackle!
Shake equals mass times pop!!

 

[Stephen]

You could make this easy and listen to brian. Wally's drivetrain is set up with the same bushings in the motormounts and on the T-case mount between the 203 and 205. They are probably slightly narrower at the T-case than the motor although it was really laid out to where things fit, not on a specific split. Regardless, the result is a drivetrain that has stayed together through several years of thrashing around pretty good.
I do have a stock rubber 2wd mount bolted to the pad on the back of the TH700 but it does nothing in torque and very little in vertical. I like it there more to damp and support the span of drivetrain between the motor mounts and T-case mounts, it probably doesn't do much.

I think measuring stuff yourself is going to be the trick without becoming a urethane engineer. Apply a load, measure displacement, apply more load...you get the idea.

 

[Greg72]

It's settled......I'm going to go buy a jerk meter!!!

Could come in handy more often than you'd think. Especially for posts in "The Garage"!!!



Dan,

How's the new job treating you?


Stephen,

The tough part for most of us is knowing what we don't know, and then knowing if those things are important or unimportant to know.


Ex.

Bushings - Maybe turning out to be unimportant

Position and angle of links in a new suspension - Fairly important.


I figure my job around here is to ask questions about whatever I don't understand, and allow the answers dictate how "obsessive" I should get about the topic.

 

[ntsqd]

My personal plan for mounts of this type goes like this:

Pick rubber front leaf spring eye bushings with convenient OD & ID sizes & buy from Metro Molded Parts.

Buy tube for OD and bolt sleave of mounts.

Sparks, fire, & noise.

Next project.

 

[Triaged]

Dan,
How's the new job treating you?

My job right now is to actually go out and get a new job
I have been enjoying my semi-vacation. I have a bunch of prospects...just need to follow through on them. I might also have my ugly mug on "Top Gear" on the Discovery channel