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Thread: BOBBLE STRUT on LeBaron LE

  1. #21
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    Re: BOBBLE STRUT on LeBaron LE

    Pics of Ray's setup.
    Click image for larger version. 

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    Looks pretty slick! Nice job! The way you designed it should work very well with most of the force directed upward and downward. Also no welds to get rusty is a plus. I see you added a drain plug to your A543.

  2. #22
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    Re: BOBBLE STRUT on LeBaron LE

    Joe, thanks for the comments. I finally found this on page two, hence the delay in my response. I gave this problem lots of thought over the six months it's taken me to change from automatic to manual transmission. The torque from driving the car forward rocks the engine/trans unit rearward. Looking at it from the right side of the car, the torque goes anti-clockwise, compressing the strut. As you noted the force is taken by the lip of the frame. The underside piece of my mount extends rearward beyond the lip. A downward force effectively pushes upward on the rear of the bottom piece, because it rocks at the hard edge of the lip where it meets the frame coming down. The relatively weak lip wants to bend downward from any forces applied in that direction. If you mentally extend the bottom piece of my mount rearward, you can picture the leverage effect. Pushing downward on this mount puts very little force on the grade 5 bolts. Seems to me this downward force is the greatest of the forces the mount has to take, because driving the car forward imposes the greatest torque on the engine/transmission unit.

    Reversing, or backing down from downshifting, the engine/trans. unit viewed again from the right side rotates clockwise, opening up the shock absorber (bobble strut). When the shock absorber reaches the end of its travel it tries to pull the mount upward, and the two central bolts take most of the strain. The other four bolts along the bottom plate tend to spread the forces along the lip of the frame, rather than concentrating them on the two central bolts. These forces ought never to be as high as those that come from hard acceleration going forward.

    There's another factor here. The front engine mount is original, still flexible and in good shape. It is not, and never will be, filled to make it stiffer. Nor will I ever put a non-shock-absorbing strut in the back in place of the original design. Doing so increases the forces that the mount has to take right from the get-go. There's no delay, or spreading the torque forces over time, onto the mount, which you do get from a "soft" shock absorber. Could it be the designers had thought about this?

    Yes, there's a safety-wired drain plug. It's a tapered plug only because I couldn't find, and didn't want to make, a straight-thread plug with a copper gasket, which was my first choice for the drain plug.

    By the way, the shock absorber (bobble strut) is pressed against the line for the power steering. There's a piece of rubber between the two. There ought not to be much vibration there because the strut is rubber mounted at top and bottom, but I wanted to make sure the line would not get damaged.

    One more item. When the mount gets pulled upward, the angled rear edges of my mount press against the nearly upright part of the car frame. If this shows any wear over time I can add a plate back there easily enough to offset it.

    ..........Ray

  3. #23
    turbo addict
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    Re: BOBBLE STRUT on LeBaron LE

    ok , reading your above post there's a couple of things you need to consider

    an automatic car lacks the bobble strut all together .. why?

    because the bobble strut is an anti rotational link intended to keep the rear of the diff from lifting when you let out the clutch in reverse
    that's it's ONLY intended purpose

    it is not and was not ever intended to limit reward rotation in forward gears - that is the job of the front mount

    yes people use the bobble strut to preload the drivetrain and also use a solid style adjustable bobble bracket to help limit the rotation rearwards but effectively this is only a patch over as it's very difficult to invent another method of preventing engine assembely rotation in the engine compartment

    and , yes you do want a stiffer front mount to help prevent the engine from rotating backwards
    also a dog bone style bobble bracket would be far superior to the worn out shock I know you have
    they are ALL worn out and have been since the 80's

    why do you want the stiffer front mount and bobble bracket ?
    because they will prevent , or rather help prevent rotation
    a soft mount and bobble bracket will not
    why do you want it stiffer?
    because when you use the soft parts , the impact at the end of travel will be much higher as the softer parts allow a, more movement and b , more movement more quickly thus allowing MORE energy to be transmitted to the mount you just put on your crossmember

    I understand your thinking but you have it backwards

    basically the same physics as running into the side of a bus , with or without your seatbelt on
    one's gonna hurt a lot more

    and yes there is a dog bone style one for the 520-568 trans it came in 87&^ K cars and mini vans with a 5 spd trans.. BUT it has a different bracket to mount it to the diff so you need that too
    end location on the crossmember stays the same so it's a simple swap in

  4. #24
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    Re: BOBBLE STRUT on LeBaron LE

    Johny, Take that bobble strut off and drive your 5spd car around. It does more in the fwd gears than you think. I'd love to have a bobble/dog bone on my Auto car. I rocks the motor so hard back that it squished the PB pass mount I had in there by over 1/4 inch.



    Ray, Keep an eye that. you might get away with it being you still have a shock back there and I hope you do, but I've seen people mount a stock Lbody dog bone like that and bend then lip all to hell and back.


    Here's my $10 no vibration at all "bobble strut" Love this thing with a Poly bushings front mount with relocated mounting hole and an MP pass side mount. Helps with throttle response and makes downshifts easier/quicker




    Relocated PB front mount. You can see the stock hole is filled.







    I run the above on all my 5spd cars except my newest one because it doesn't fit with the GT30R sitting there so I had to find one of these 88 Caravan 5spd bobble strut mounts.

    Stock Omni mount on the bottom, Caravan up top Yes those "Dog bones" are the same part #. See how torn up the bushings are. Those mounts move around a lot.


    Stock non-lbody bobble strut bracket for the 520/555/523/568 transmissions vs. the 520 88 caravan mount.





    Saves quite a bit of height don't ya think?






    I'm hoping this will have enough dampening "rubber" to act like the original solid bobble strut but we'll see.



    And installed





    Now as to the designers/engineers who thought about having soft mounts from the factory, yep. Grandma isn't going to like it if the car has too much vibration. And it's easy to just throw the motor into some big old soft rubber mounts and not worry about it at all. The expensive way to do it is to do it like Porsche does it and has adjustable hydraulic mounts that are soft at idle and cruise but firm up when you jump on it. I like to pick the heads of the engineers on the race teams though. If I can rock the motor fwd for a better roll center and better oil drain back into the pan and not get an excessive amount of vibration I'm gonna do it. So my motor sits at 10 degrees vs 12 stock and 14+ with other mounts.


    Anyway, just throwing out some other ways to do it. Hope you have success.

  5. #25
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    Re: BOBBLE STRUT on LeBaron LE

    Johny,

    Thanks for your comments, but I have to take issue with 'em. Elementary physics tells us as a mass rotates or tries to rotate about an axis (the axles) there will be motion both upward and downward at the "bobble strut." Who says it's there for reverse only? The upper rear-strut connection moves up and down every time torque is applied to the axles, no matter the direction of rotation. The front mount cannot act alone.

    When you make the rear strut rigid the engine/trans unit cannot rotate about the axle axis. It has to now rise, rotating about the lower rear-strut mount, which now takes the full weight of the engine/trans unit (minus what the front mount supports), which is why the weld-on (add-on) mounts break there when the car uses a solid strut. If the designer wants or needs to use a solid link the mounting point must be designed to handle that. The lower strut mount on cars designed originally for manual transmissions is indeed sturdy, and you can get away with your suggested modifications. I don't think using a solid link is a good idea for a lower mount casually added to the frame crossmember, especially if it's welded without suitable bracing. My design (which I stole) allows for some motion of the crossmember steel, i.e., bending. As GLHNSLHT2 pointed out, a rigid link makes bending of the lip worse.

    A thousand thanks to GLHNSLHT2 for his commentary and his excellent photos. That's the first time I've seen the alternate mount designed for the short, not-quite-rigid mount that Johny mentioned.

    Auto transmissions don't need a bobble strut because they have a thing that converts rough torque on one side to a smooth output on the other side. It's called the torque converter. You can't bang out the clutch and break things as easily as you can with a manual transmission.

    How does one "preload the drive train" when there's no force on it? All a solid strut and stiffer (or rigid) front motor mount do is cut the delay of power application to the axles. This is handy if not necessary for racing. An entirely rigid engine/trans mount as seen with some racing cars (Formula One) cuts the delay to zero.

    The "impact at the end of travel" can't be higher because a soft front mount and a shock-absorbing rear strut, even an old one, are constantly acting against the rotation of the engine unit. By the time the engine/trans unit arrives at the limits of travel permitted by the front mount and rear shock absorber it is rotating at a lower angular velocity than when it started. If there was no rubber, or no air, then yes, the engine unit could build momentum and hammer the mounts at the end of travel. But that is not the case.

    At any rate, thank you Johny and GLHNSLHT2 for your time and your thoughts. I much appreciate feedback from those with long experience with these old cars. I'll have to let y'all know how this works once I get the thing on the road. Meantime, stay happy.

    .........RayO

  6. #26
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    Re: BOBBLE STRUT on LeBaron LE

    The pre-load comes from Ed Peter's a Chrysler Failure analysis engineer. It mostly has to do with a failing front engine mount and lowered cars by keeping the rear strut from compressing. When you lower the car it hurts the axle alignment so they jacked the bobble strut up or preloaded it to keep the axles from pulling apart. A blown front mount or a front mount installed in correctly can also cause this. The engine's rotation point is not the axles. It's higher up and more centered because of where the 2 side mounts mount. This is also why the IMSA race cars went to a 6 degree engine rock-back from the factory 12. To keep the axles lined up properly.

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