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Tranny xcase crossmember ?'s A study in torsional strength?

Discussion in 'Center Of Gravity' started by Kyle89K5, Mar 16, 2004.

  1. Kyle89K5

    Kyle89K5 1/2 ton status

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    Tranny xcase crossmember ?\'s A study in torsional strength?

    The loads applied down wind to my tranny/xcase are torsional in nature, NO? So here's where things get fuzzy. I know (at least I think that I know) that marvin is running bushings on his crossmember as opposed to the factory style tranny mount. Is this enough give to control the "twist" of the drivetrain? Is there TOO much give?

    The factory style mount controls the load by twisting down onto a flat surface. Would it not be safe to assume that the pass side of your tranny mount would be under a semi contstant bit of compression while the drivers side is being pulled? Only when the truck is in reverse would this change. To that end, would bushings mounted at the framerails be able to live under the constant one way, but not the other pull? How about the problem of have the extended leverage of the load insulator being so far from the centerline of the load?

    Thoughts?

    I've got some crossmember designs that are NOT simple fab items, that will be ran through a stress analysis package first chance I get to see how they look, but I thought I would see what others thought as well.

    KP
     
  2. BILLY RAY

    BILLY RAY 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    I just used spring bushings.
    [​IMG]
    [​IMG]
     
  3. Kyle89K5

    Kyle89K5 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    How long have you ran it like that? Have you ran it like that? Nice pass side exhaust /forums/images/graemlins/grin.gif /forums/images/graemlins/thumb.gif Have you noticed any probs?

    ANSWERS MAN, I NEED ANSWERS /forums/images/graemlins/thumb.gif /forums/images/graemlins/grin.gif
     
  4. RoknRollr

    RoknRollr Registered Member

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    You've got the right idea.
    Its hard to tell if the bushings will fail under the strain with out knowing more about how you're using them - Billy's design won't destroy the trans if the bushing fails.
    Remember: don't strain the front mount more than the rear. The factory mount will send too much Tcase induced torsional stress through the transmission-belhousing-block up to the forward motor mount -- you do not want this! Especially if your running 4:1 low or a doubler (Klune-V, Offroad Designs, etc). Carry as much of the torsion at the transfercase as possible without being rigid. Keep some safety (hardstops) incase a bushing fails - that way when/if it does break you don't crack/damage the tranny.
    If you have a 250ft-Lb stocker * 3:1 low first gear, the the belhousing transmits the majority of the resulting 750ft-Lb to the front m-mounts. When you multiply that by a mild 2:1 Tcase, you get about 1500ft-lb through the small adapter-trans section -- a 4:1 Tcase or doubler will kill the factory mount and tranny without some additional support. Yet, what drive line will hold up under 3000ft-Lb full throttle? ...not many!
    Don't underestimate the need for flexibility in the mount - it maybe the only thing dampening out driveline shock for crawlers with lots of gearing. I say all this because most people only change the mount when putting in doublers, a dual tcase, etc and I assume that's what you're doing.
    Good luck.
    One more thing - /forums/images/graemlins/thinking.gif its tough to FE model the front m-mount stiffness which has a major contribution to your stress analysis. Its best to model the Tcase mounts as though it were divorced from the transmission.
     
  5. BILLY RAY

    BILLY RAY 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    I have run this set up for a couple of years now it has worked real well for me you just need to put a stiffer motor mount due to the fact you are going to a stiff t-case mount. I do not have alot of vib. in the body with stiffer mounts well maybe just a little. Others may not like this set up but anyone that has done any real wheeling with me can tell you I beat the hell out of the truck and damm sure not scared of the gas pedel free to ask any of my buddys (butch, txbluethunder, nonesuch, thelakerat, just a couple of other sick buddys). /forums/images/graemlins/yikes.gif
     
  6. Butch

    Butch 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    Billy Ray is a WUSS that only wheels in his driveway and some flat parking lots. /forums/images/graemlins/doah.gif







    Oh yeah and also all the hardcore trails of Moab, Katemcy, Montrose, etc. I was suspect of his crossmember at first, but after seeing him get air under all 4 more than once I have decided it will work just fine. /forums/images/graemlins/waytogo.gif
     
  7. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    Yeah, we had some long discussions not too long ago about outboard bushings on case/tranny mounts vs. inboard. I'm in the inboard camp.

    I don't want the case mounts stiffer, or even close to as stiff as the engine mounts. With an outboard setup, especially using spring bushings, the deflection allowed basically results in a solid mount as far as drive train torque is concerned. Due to the long moment arm, even a 1/8” deflection (which is extreme with poly spring bushings) translates to only a very small angular deflection. All it really does is isolate vibration. There is certainly some Newtonian “reaction” torque to deal with from the drive shafts, but the engine torque seems to be more problematic for the adapter and trans housing. I’ve seen quite a few people dealing with problems and breakage caused (apparently) by simply replacing the stock trans mounts with poly while still running rubber engine mounts. Assuming this “cause and effect” are accurate, out-boarding poly spring bushings only amplifies the problem dramatically. It’s certainly easier to build an outboard mount cross member, but I would highly recommend VERY rigid engine mounts if you go that route. Lots of people seem to “get away with it”, so it’s not cut and dried, but based on all the accounts of busted trans and adapters, I shy away from outboard setups. Of course, if the assumptions supporting my prove opinion are found to be incorrect, I could easily change my mind.

    Inboard is what the factory used, and it seems there would be no cost influence in this case that would lead them to do so in opposition to longevity (which is often the case). However, due to space constraints, it’s much more difficult to get maximum clearance if you try to provide a flexible mount in the center. So far I’ve been running stock rubber mounts with great success. The only damage so far is hanging the 205 on a rock shelf and ripping the range box adapter from the back of the trans. I also run stock motor mounts.

    That said, I’ve got some sketches for a revision on a cross member that I posted some time ago that I may well recycle in the near future. Basically, it will be solid mounted to the frame rails with no bushing. If concerned about vibration isolation (like a daily driver) then poly bushings could be placed at the outboard mounts. Then the mount would be a single point in the center with a bolt run parallel to the drive train center line forming a sort of hinge. In this way the trans/case mount provides no tortional restraint at all and lets the engine mounts handle all tortional forces. Still working out the details though...
     
  8. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    I'm sorry, I got so wrapped up in that last post, I forgot to answer your specific questions. /forums/images/graemlins/rolleyes.gif That's why I stayed away earlier, I didn't have time to get embroiled in this discussion yet again. /forums/images/graemlins/laugh.gif

    Anyway...

    [ QUOTE ]
    Is this enough give to control the "twist" of the drivetrain? Is there TOO much give?

    [/ QUOTE ]
    As previously stated, I feel it will not give enough.

    [ QUOTE ]
    To that end, would bushings mounted at the framerails be able to live under the constant one way, but not the other pull?

    [/ QUOTE ]
    The spring bushings will be fine with a constant load. Think about the original application. Generally they are always supporting the body/frame to greater or lesser degree. That's variable force in (more or less, ignoring friction, decel/acceleration (in any direction) and shackles) one direction which is similar to what we're talking about here.

    [ QUOTE ]

    The factory style mount controls the load by twisting down onto a flat surface. Would it not be safe to assume that the pass side of your tranny mount would be under a semi contstant bit of compression while the drivers side is being pulled? Only when the truck is in reverse would this change.


    [/ QUOTE ]
    Generally correct with the small exception of engine braking and lateral acceleration (inertia).

    [ QUOTE ]
    How about the problem of have the extended leverage of the load insulator being so far from the centerline of the load?

    [/ QUOTE ]
    As stated, I feel this is a huge problem. Others do not, and many seem to have no problems… <shrug>
     
  9. BILLY RAY

    BILLY RAY 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    That is what I get for the post about you wearing a pink dress.
    /forums/images/graemlins/histerical.gif
     
  10. Stephen

    Stephen 1/2 ton status Moderator Vendor

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    I'm with Russ, I've always tried to mount the T-case using similar bushings with similar mounting widths to the motor mounts. So far the back of the TH700 has stayed attached to the front of the TH700.
    Russ, Ford used a similar pivot style mount on some of their 205 applications but they also used the side mount tied to the frame to control torque. There is some torque reaction between the trans and T-case and I don't think you want all that reaction taken up by the back of the trans housing.
     
  11. yunit

    yunit 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    So do you guy's think I will have issues since my latest crossmember consist's of 2wd drive mounts, one mounted under each adapter?

    Idea of what I am talking about.



    [​IMG]
     
  12. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    Hmm, didn't know about the Ford joints.

    I had considered the effect of reaction torque, and also the effect of inertia loads from the big heavy case hanging off the side. But in my mind, those seem small compared to the loads due to the engine torque. Small enough that I don't think it would break the case at least. I've seen trucks with the stock narrow set rear rubber mounts completely turned to mush by leaking oil and being beat on without mercy, but with no broken adapter or trans (other than impact damage). In fact, with a factory style mount at the back, I’ve rarely seen a broken case other than from impact. At the same time, I've seen broken trans housing or adapters with very nice, clean, well implemented but tortionally stiff rear mounts and no sign of impact. And some of these were running very stiff engine mounts too. <shrug>

    Of course these observations are a bit skewed due to the fact that most of the more “extreme” guys are running the nice clean and well maintained non-stock rear mounts while the more stockfish setups (especially the poorly maintained ones) are more often on more intermediate rigs (even though they are sometime bashed hard). I don’t know, there are probably more issues that play into this than what I see. Otherwise, I can’t account for why some (like Billy Ray who I’ve had this conversation with before) have no problem at all with the almost solid mounts.

    I should probably just break down and buy some of your supper zoot motor mounts and remove the engine torque issue all together. But I just really dislike the engine vibes that I’ve seen in the past. Of course I've never run stiff mounts with a stock engine before, it was always on “built” drag motors that tended to shred stock mounts. Might not be such an issue here…
     
  13. Kyle89K5

    Kyle89K5 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    There's no perfect world explanation, it becomes a matter of "I was just driving it and it broke" at that point. What I have in mind is a setup that will move the bushing closer to the centerline of the vehicle, not mounted at the frame. Think of it like a 3 piece crossmember design. The middle piece mounts to the adapter, the other two move the mount points from the frame into a much closer relationship with the adapter. Make any kind of sense? Stephens motor mounts are on the list to add at the same time. I've got seriously weak motor mounts that let the motor toss and turn all over the place, no way I'm going to ridgid mount the tranny and leave it to snap things off.

    If worse comes to worse, then I could an additional mount on the skidplate, similar to yunit, but I really don't want to , that would know my skidplate down farther than I would like.

    This is good info folks, keep it coming.

    BTW, on the FE running of the crossmember, I was going to remove the MM fudge factor and start with worse case numbers. When I go to the new MM, there won't be near as much slush in the MM's anyway, therefore helping to drop my numbers into the reality range.

    Fuzzy math? Yes! Worse case scenerio? Yep!
     
  14. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    If I understand you correctly, watch out with that 3 piece cross member. The leverage of the side arms on the frame rails will likely cause them to twist. Those rails are not designed to handle any significant torque load. In fact, the only way they are relatively strong is with vertical loads. Structural and load bearing cross members should be pretty much rigid to avoid loading the frame rails incorrectly. The normal outboard bushing designs work because the moment arm is not long enough to give the drive train weight enough leverage to twist the rails.

    FWIW, this is similar to another design I've sketched out. Hehe, there have been at least 8-10 since I did the doubler, only one was partially implemented before Stephen and Beck shot it down due to failure of the same basic design on Beck's truggy. /forums/images/graemlins/crazy.gif Anyway, take your basic premis (bushings just outboard of the adapter, but beside it to reduce the belly clearance issues) but extend a piece of angle (or better yet, shallow channel or rectangle tube) underneath to make a full width cross member. In this case, the piece bridging the center is there only to eliminate the torque loading on the frame rails. It would not need to be nearly as strong as the outer arms (which will then transfer vertical load to the rails) but may need some vertical triangulation at the bushing location.
     
  15. Kyle89K5

    Kyle89K5 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    [ QUOTE ]
    If I understand you correctly, watch out with that 3 piece cross member. The leverage of the side arms on the frame rails will likely cause them to twist. Those rails are not designed to handle any significant torque load. In fact, the only way they are relatively strong is with vertical loads. Structural and load bearing cross members should be pretty much rigid to avoid loading the frame rails incorrectly. The normal outboard bushing designs work because the moment arm is not long enough to give the drive train weight enough leverage to twist the rails.

    FWIW, this is similar to another design I've sketched out. Hehe, there have been at least 8-10 since I did the doubler, only one was partially implemented before Stephen and Beck shot it down due to failure of the same basic design on Beck's truggy. /forums/images/graemlins/crazy.gif Anyway, take your basic premis (bushings just outboard of the adapter, but beside it to reduce the belly clearance issues) but extend a piece of angle (or better yet, shallow channel or rectangle tube) underneath to make a full width cross member. In this case, the piece bridging the center is there only to eliminate the torque loading on the frame rails. It would not need to be nearly as strong as the outer arms (which will then transfer vertical load to the rails) but may need some vertical triangulation at the bushing location.

    [/ QUOTE ]

    Didn't really want to roll this out just yet, but this is where I'm heading on my crossmember. This is not a final design, and the dims are way off for my applicatin, but this is a nice general idea of where I'm looking to go. This is the first creative steps in doing my new xmember. There is bracing that's not shown as well as a yet to be designed skid plate.

    [​IMG]
     
  16. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    Drool! /forums/images/graemlins/laugh.gif

    Assuming that bracing you mentioned is on what appears to be strap coming in on the sides, that is very sweet indeed... /forums/images/graemlins/waytogo.gif
     
  17. Kyle89K5

    Kyle89K5 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    [ QUOTE ]
    Drool! /forums/images/graemlins/laugh.gif

    Assuming that bracing you mentioned is on what appears to be strap coming in on the sides, that is very sweet indeed... /forums/images/graemlins/waytogo.gif

    [/ QUOTE ]

    The "straps" are actually 5/16" plate and yep, thats the bracing I was talking about /forums/images/graemlins/grin.gif

    Glad you like it. Does that help make more sense as to where I'm heading /forums/images/graemlins/grin.gif /forums/images/graemlins/thumb.gif
     
  18. Depdog

    Depdog 1/2 ton status Author

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    Long time no see dude. You still get on ICQ? Whats been going on? Parts look cool, shoot me an email.

    Glenn
     
  19. jimmy88

    jimmy88 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    Good stuff. One thing not discussed (unless I missed it) it torque from frame flex, like when all crossed up in the rocks. It would seem the cross member with the bushings at the frame rails would less tolerant of frame twisting, especially with all poly mounts. Unless, of course, the amount of twist between the motor mounts and the tcase cross member is so small as not to matter.
     
  20. jimmy88

    jimmy88 1/2 ton status

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    Re: Tranny xcase crossmember ?\'s A study in torsional strength?

    Russ, I have to disagree with you as to where the most torque on the drive train comes from. Engine torque is multiplied at each gear reduction and each case containing the gears resists the increase in torque in the opposite direction. To visualize what I mean think of holding up a trans by the input and output shaft. When in any gear besides 1:1 twisting the input shaft and holding the output shaft still will cause the case to turn. Basically its a bunch of levers with the fulcrums supported by the cases. Push on a lever and there is force applied to the fulcrum. So engine torque is multiplied through the gear train and the resulting reaction forces are taken up by the cases which are bolted together as a unit so it has nothing to do with the mounts.

    The major player here is the equal and opposite reaction to the driveshaft torque. With a single flexible tcase mount all that drive shaft reaction force is sent back through the cases to the motor mounts. So the cases have to carry the driveshaft reaction force in addition to the reaction forces due to torque multiplication.

    This should not be too hard to calculate for regular conditions but what safety factor do you use for when the rig hops on a high traction surface, where drive train inertia and sudden shock loading come into play? A factor of 10 ? Which tends to be more damaging, the sudden loading upon landing or the backlash from sudden unloading?

    In a pure theoretical sense, assuming a frame that doesn't flex or twist, it seems the best arrangement would be a stiff tcase mount that takes all the reaction force from the drive shafts and doesn't allow the case to twist and load the adaptor. Back in the real world all frames flex and using a stiff tcase mount will mean the motor/trans/adaptor/tcase unit will fight any frame twist when all crossed up. I can imagine those loads could become quite large.

    That brings up the more important question of why the adaptors fail in the first place. Is it because of loose bolts, side loading from front driveshaft ujoint bind, slip yoke bind, bottoming a driveshaft or plain smashing the tcase into something? Its hard to picture them failing from pure reaction torque acting perpendicular to the centerline.

    Why not follow the factory's lead and strengthen the connection between engine and tcase using struts or something along those lines, especially to the side of the tcase where the front driveshaft output is?


    Ok I'm prol full of crap, but, for the moment anyway, thats how my pea brain sees it /forums/images/graemlins/doah.gif
     

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