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Lets discuss the solid motor/tranny mount issue again

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The stock frame is engineered to flex

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You seem to say this a lot, any evidence to back it up cause it seems to me that its not designed to flex, it just does because of its design.
 
Tim, you can keep a stock frame from flexing without damage by stabilizing the suspension mounting points using the cage as a space frame. In this way you remove the stress from the frame almost completely.

BB62, it's one in the same. The engineers designed this frame to be inexpensive and hold up. They chose a ladder frame. Ladder frames flex, period. It answers all the design criteria, and it flexes by design...
 
But when the engineers designed it, did they want it to flex? Why are most trucks now made from box frames? If flex was so good in the frame then why did they get away from that concept?
 
Flex wasn't, "So good". It's just kinda the nature of the beast due to the design as pointed out above. If I had the time and knew how to do it correctly, I'd build an entirely new frame for my truck.

You've got to remember what our trucks looked like when they were stock. Negative arch springs, tension shackles in back, minimal wheel travel, tiny tires.....

We've added these soft long traveling suspensions, big tires, etc. That's the only reason we're talking about too much frame flex. What we once didn't notice or was not important has become a potential issue because we're using our trucks well beyond what any factory engineer ever imagined.

Trucks are probably made from box frames now for longevity. Engineers now want a truck that will go for a long time with its rated load and still have doors that open, body panels that line up, etc.
 
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But when the engineers designed it, did they want it to flex? Why are most trucks now made from box frames? If flex was so good in the frame then why did they get away from that concept?

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It's not that they *wanted* it to flex, or that it's good. It's that they had to work within the design parameters, and those requirements resulted in a ladder frame chassis being the logical choice. They knew that it would flex, so it was built to deal with it. You can see evidence of that throughout the chassis. Do you really think the flex was a surprise to them? No, they new it would flex and exactly how much and where. With the exception of the steering box issue, this was pretty much taken care of in the design and material selection. And FWIW, even the newer boxed frames flex, just not as much.

And even without rock crawling mods/use, stock trucks flexed like mad when carrying a load across uneven ground. GM, Ford, Dodge, they all used the same basic design for trucks up until recently when the tendency to box increased, big rigs use the same basic design, so the bottom line is it works and it's not a bad design even for our purposes. The vast majority of people screaming that "FRAME FLEX IS EVIL! WHAT WAS GM THINKING!" can't even articulate a decent answer for why they feel that way. Mostly it comes down to "It just doesn't *look* right and I don't like it". Even those few who do have real reasons will find that there are actually more effective and easier solutions to their problems that trying to stop the flex "the right way"..

So anyway, the point is that ladder frames flex, the engineers designed it to flex without damage, and people who simply tie the cage to the frame or box part of it to "eliminate that evil flex" are breaking the "designed to deal with the flex" part of the design. This causes significant troubles for long term durability. I don't care if so-n-so ran a cage tied to the frame for however long, it's a bad idea IMO and it will lead to failure. How long it will take to fail depends on MANY things. If a person does not like the flex for whatever reason, IMO, there is only one way to address the issue, and I've mentioned it many times already. You *must* use the cage to lock down all suspension mounting points just like Beck and Stephen have done. Otherwise, the suspension is still going to flex the frame and work it against the much more rigid cage mounts resulting in a failure much like that seen around the steering box. Just like the steering box, it may be years away in surfacing, but rest assured, it IS fatiguing and weakening the frame.

Dear god, I've been sucked into one of these debates again. I've got to start ignoring these.
 
Thanks for your reply, very well thought out. I was not trying to debate or be argumentative with you, just wondering why you guys always say the frame is designed to flex. /forums/images/graemlins/thumb.gif
 
LOL, sorry, forgot the smilie. /forums/images/graemlins/doah.gif That was just a reference to something I've said a few times in more involved versions of this. It's one of those discussions that comes up rather frequently and it gets old rehashing the same thing. I didn't mind discussing it with you at all, if I had, I would have let it slide again…
 
I think it is good for you to discuss this issue, Russ. You have a far better understanding of it than I do at this time. I'll provide the explantions 4 years from now when I graduate from engineeering school. /forums/images/graemlins/thumb.gif
 
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4 years from now when I graduate from engineeering school.

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Oh boy. I feel sorry for your professors. And I am sure we will hear more about this type of conversation before you might graduate.
/forums/images/graemlins/dunno.gif/forums/images/graemlins/1zhelp.gif /forums/images/graemlins/thinking.gif
 
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4 years from now when I graduate from engineeering school.

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Oh boy. I feel sorry for your professors. And I am sure we will hear more about this type of conversation before you might graduate.
/forums/images/graemlins/dunno.gif/forums/images/graemlins/1zhelp.gif /forums/images/graemlins/thinking.gif

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Wanting to understand and learn is the beginning of being a professional at something, is it not?
 
The high range example here is just one of the situations that the drivetrain sees. It's probably not the most severe but it illustrates the fact that it's bad to have the t-case mounted really solid and the motor mounts set up really loose.
The other situation shows that it's bad to have the motor mounted really solid and the t-case mounts loose.

So far all we've been really talking about is the torque reactions but you bring up a really good point about the drivetrain bouncing around. I know a lot of desert vehicles will actually use motor plates front and rear or use some fabricated brackets to grab the motor/bellhousing intersection to help stabilize the bounce. The motor mounts are actually pretty close to the front of the motor so it makes sense to try to grab the drivetrain somewhere else. My choice was to put a stock rubber transmission mount on the mounting foot of my TH700 to help keep it under control in a bounce situation but still leave it mostly ineffective for torque control.

Maybe more of our breakage is due to external forces from the vehicle? Given that stress can add up in combinations of torque and bounce (in this situation), it probably is a factor, the question is, how big?
 
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Maybe more of our breakage is due to external forces from the vehicle? Given that stress can add up in combinations of torque and bounce (in this situation), it probably is a factor, the question is, how big?

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That's what I said about 4 posts ago up there.

I'm not exactly sure how you could calculate those forces though. Any ideas? I'd say that the dynamic load of bouncing is going to be so high I doubt you can calculate it.
 
Can any engineers on the site run some FEA on our frames? I dont know how difficult that would be since I have never done it, but if one could get the exact dimensions of the frame and any other pertinent info I wouldn't think it would be too hard. Correct me if I am wrong.
 
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Maybe more of our breakage is due to external forces from the vehicle? Given that stress can add up in combinations of torque and bounce (in this situation), it probably is a factor, the question is, how big?

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That's what I said about 4 posts ago up there.



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Ok, let me clarify here, I'm talking about drivetrain bounce as a situation where we assume the chassis is rigid and the mounts are matched stiffness but with the drivetrain mounted near the front of the motor and between the T-cases the trans is the center of a long beam that can have gravity pulling around on it. Basically if you just dropped the truck really hard from a few feet up and landed on a rock (that sounds familiar) does the tranny have enough inertia to break itself?

This is independent of chassis flex which is when the mounting points move different directions or a torque input from the powertrain trying to twist the cases apart.

The additive stress could be calculated but it's not going to be by me, I think that requires software I don't have anymore. Just the variables involved make this a fun problem, torque input, various gear ratios, different mounting bolt patterns, part dimensions, how much inertia, stiffness of the different parts, blah blah blah.....
 
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This is independent of chassis flex which is when the mounting points move different directions or a torque input from the powertrain trying to twist the cases apart.

The additive stress could be calculated but it's not going to be by me, I think that requires software I don't have anymore. Just the variables involved make this a fun problem, torque input, various gear ratios, different mounting bolt patterns, part dimensions, how much inertia, stiffness of the different parts, blah blah blah.....

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I didn't realize you were talking about blunt force to the case of the unit. I don't really think it matters how it is mounted if you're talking about that kind of force, it's probably going to break if that happens, I would say more likley to crack than fail catastrophically just because you would attempt to "flex" that long lever far more than it's going to want to flex without failure. I suppose I'm going out on a limb to say that, but every time I've seen an adapter crack due to blunt force on the transfer case, that's generally how it fails.

The 2nd paragraph in my quote looks like it would be really fun stuff to know, but that's one problem you're not figuring out with pencil and paper. /forums/images/graemlins/eek.gif
 
Tim, I think that Stephen means that the drivetrain would not hit anything. Just the force of the whole vehicle falling then stopping but the drivetrain keeps moving for a very short period of time. Kind of like going over a hill in a car fast and you get that weird feeling in your stomach. Your stomach is the drivetrain. Could that movement make a part of the drivetrain break. /forums/images/graemlins/dunno.gif /forums/images/graemlins/screwy.gif
 
I think you're right in that. Hell any movement can create breakage. That stuff isn't supposed to move at all in theory. Of course it can move some, but I doubt any of us really want to find out how ductie whatever our trans/case/motor is made from really is.
 
Stephen, thanks for the reply. The reason I’m pushing to put numbers on the forces is because I feel exactly like you said, [ QUOTE ]
Maybe more of our breakage is due to external forces from the vehicle? Given that stress can add up in combinations of torque and bounce (in this situation), it probably is a factor, the question is, how big?

[/ QUOTE ] If it was just reaction torque alone, then why could Russ and others get away with the three point mount for so long? That does bring up the issue of metal fatigue though and, if I remember correctly concerning fatigue, aluminum is considered to have a finite life whereas steel can last almost indefinitely.

Still it would be nice to get some idea of the relative strength of the forces compared to each other. That way any design can work better to the control the strongest forces, but you have to know what they are first! I would have thought all this has been worked out by the manufacturers and in one form of racing or another, but am having no luck finding anything.

Anyway, I think it has basically come down to re-enforcing the cases externally. Either by using multiple mounts with a stiff frame, like the four point setup you describe, or putting bracing on the cases themselves and using a three point mount setup, something like a sub frame or a torsionally rigid skid plate which connects to all the cases and the block.

A thought on topic, if reaction torque is by far and away the largest force and most breakage occurs when spinning tires get a bite on a high traction surface, then won’t solid motor mounts make the problem somewhat worse by not allowing the motor mounts to compress and absorb some of the initial energy?
 
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then won’t solid motor mounts make the problem somewhat worse by not allowing the motor mounts to compress and absorb some of the initial energy?

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Most tend to think that although those mounts will absorb the energy they also create enough movement in the assembly to break something.

I really don't think it's reaction torque, otherwise guys would be breaking street driven rigs on the road. It has to be a factor that has something to do with the frame flexing, or all the bouncing off road. MANY street driven rigs are beat pretty hard without breakage. Off road, people are breaking them regularly.
 
Guys are breaking trans/t-cases regularly? You see it happen to one guy and now its a regular thing? I rarely hear of anyone breaking trans and tcases, and I would bet that usually its due to a worn or loose mount, loose bolts either in the trans to tcase adapter, or a driveshaft bottoming out. Just MHO.
 
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