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  1. #1
    RC poster
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    May 2020
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    cv shaft problem

    Hi all I broke my right front a-arm and replaced both sides with rpm ones. my problem is on the right front which is the one that broke. the cv shaft keeps coming out of the drive cup. I replaced the cup and shaft but it still is happening. anyone have any ideas what could be the issue? Thanks

  2. #2
    RC Racer
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    Mar 2015
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    208
    Something else broke in that crash that you haven't found yet. Whatever that something is, it's allowing the wheel to flex away from the differential giving the axle shaft room to slide out.

    Unfortunately, you're going to have to go piece by piece through the right side suspension looking for cracks or other damage until you find what it is.

    My guess, since you broke the lower control arm is that it may be the upper control arm, which now allows the suspension to droop too much when the suspension is unloaded, and the axle shaft can then fall out of the drive cup. That's where I'd start poking my nose.

    Let us know what you find.

  3. #3
    RC Racer
    Join Date
    Mar 2015
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    Oh, some helpful troubleshooting questions to answer which can help you narrow down which parts to examine:

    Does the shaft fall out when the suspension is compressed or extended?

    Does the shaft fall out when steering?

    What is the vehicle usually doing when the shaft falls out? Landing a jump? Accelerating forward? Turning? Those can help you figure out what parts of the suspension or steering are moving when the failure is happening.

    Happy troubleshooting!

  4. #4
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    May 2020
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    The RPM arms seem to be the problem. Had the exact same situation. Broke a stock arm and thought RPM's would be an upgrade. But not so! Fully tore down the whole front end and carefully inspected everything couldn't find out why they kept falling out. Got tired of it and went back to all stock and CV's never popped out again. In fact I had problems with both CV's falling out with the RPM arms.

  5. #5
    RC Racer
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    Mar 2015
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    Dang, that's a bummer to hear that about the RPM arms. Their stuff is usually on point.

  6. #6
    RC Racer
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    Mar 2015
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    Have you contacted RPM regarding this? Or can anyone else confirm similar findings?

    I'm interested in a set of the RPMs since they won't trap debris like the stock ones. But this info certainly dampens that interest.

  7. #7
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    Jun 2020
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    The fact that the OP only has it happening on one side, and it's the side that broke, I'm leaning toward what Fallen originally suggested and that something else may be out of whack. I just installed both the lower and upper RPM control arms today. I haven't run it yet but I just took a careful look and everything seems just fine. I don't see how they could fall out.

  8. #8
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    Jun 2020
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    I have an update for my last post... Out of an abundance of curiosity and time, I pulled the new RPMs from one side and installed the old a-arms to compare. HOLY COW! Sure enough, the RPM arm allows for far more droop and nearly expels the dogbone pin at the 12 o’clock position. With enough flex or maybe crash I could absolutely see it popping out. The stock side doesn’t even come close...in fact the pin stays at least halfway into the drive cup. One of the most notable differences between the two lower arms is the lack of a travel stop on the RPM. The stock arms have a slightly longer forward pivot that incorporates an integrated stop for both full compression and full droop....this may be what’s going on. I’ll tell you what though, I put my stock arms back on, bought two spares and have some RPMs I wasted my money on IMHO.
    Last edited by b-mack; 06-05-2020 at 01:52 PM.

  9. #9
    RC Racer
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    Mar 2015
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    208
    Thanks for the update, and that is quite a bummer to hear.

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