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  1. #1
    RC Racer
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    Broken suspension screw pin p/n 5161 3x63mm

    So, a couple short backyard runs ago, I started noticing some slop in my steering. So I investigated further and:



    SURPRISE!!!

    This break is shocking to me and based on the side it's on, I know exactly what caused it. It was last week when I was out doing speed runs and towards the end I was coming in and was calculating a stop just before a tree that was near me. I barely bumped the tree. The left front tire was the only portion of the vehicle that hit the tree. I feel like I need bumpers to go in front of the front tires, lol. I must have been going max 5 mph, no faster. It was super slow but obviously the truck isn't designed to make that kind of dead stop through a frontal impact on the front tire.

    Judging from the break, the material seems more on the brittle side and less on the ductile side because of the small reduction of cross sectional area at the break, which is fine, because you probably don't want much flex in something like this. The thing I'm surprised about is the location of the break, it wasn't at a joint where the arm meats the bulkhead brace, it was "inside" the bulkhead brace. Anyways, bottom line, and like I always say when it comes to durability, don't crash, even if it's a tiny bump/head on collision and you should be fine.

    Now, here's the thing I hate, and it troubles me because I'm uncertain what my next steps are. Because of the breakage, there was some slop in the lower front left arm. Because of the slop AND because of the fact that I ran the truck for probably a combined 40 minutes of runtime since the break, the hole in the bulkhead brace was hollowed/wallowed out even bigger (which you can see in the picture). So, if I get a new replacement suspension screw pin, the slop will still be there... which sucks and means I need to replace the bulkhead brace. I've already bought a set of RPM bulkhead braces and was waiting for them to break before replacing them, but I NEVER expected this, lol. I wonder if the plastic that RPM uses will be any different should the next suspension screw pin ever break. The crazy thing is, I feel like there's nothing wrong with the Traxxas bulk head brace if the suspension screw pin had never broken in the first place.

    Is there an upgrade suspension screw pin out there that's made from material with higher yield and ultimate tensile strengths?

  2. #2
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    There are some threads floating around with ppl using ejector pins which can be ordered from mcmaster. I'd have to find it tho, I used to have the dimensions on the pins. But you simply cut to length a and from other experience had yielded better results then the stock pins.

    I use lungsford titanium pins but with the weight of truck, I still bend them.

  3. #3
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    You know, without doing a bunch of calculations or analyzing the components using a 3D environment analysis tool in 3D CAD, I'd say the suspension screw pin broke because a brittle component (screw pin) was mated with a flexible component (bulkhead brace). It's hard to imagine the bulkhead brace as being flexible when you consider the girth of the component, but the only way the screw pin could break (when you consider how brittle it is) is if it was allowed to flex. Considering my particular break occurred "within" the bulkhead brace itself, I'd have to say the bulkhead brace "flexed" while the pin was within it and this caused the screw pin to break. Solution of course could be a stronger pin as Buggybasher is suggesting, OR a stronger bulkhead brace. I don't know if the RPM bulkhead braces are stronger, but I'm going to install that and go with the stock screw pin (mainly because I don't have to go out and buy something since I have it already, also I figure I might as well part out my 3906).

    Also, titanium is well known for its properties when heated. There are much stronger options out there.

  4. #4
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    Yes and one other thing to consider is RPM's plastic is very flexable. So that maybe something to consider if you wanting to add more flex or strength

  5. #5
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    i went to ejector pins and haven't looked back. they are VERY durable even with running aluminum bulks and RPM arms.
    I have an RC problem...

  6. #6
    RC Turnbuckle Jr. Wildman4910's Avatar
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    Stock bulks are cheap, go either the ejector pin route or Lunsford pins, forget the stock cheapo screw pins, they are buttery soft.

    Ejector pins, 1/8x6inches, $3.51ea.
    http://www.mcmaster.com/#headed-precision-pins/=xp42me

    You will also need some 1/8 pin collars for the ejector pins.
    Look out for the tree/crunch!

    LiPo? Naw NITRO!!

  7. #7
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    Yea, if the RPM bulkheads have a lower yield strength than the stock bulkheads, then they won't go well with the stock screw pins at all.

    I'm trying to learn more about the ejector pins, 6" long? How does that work, how do you cut them? How do the collars attach and secure to the ejector pins? Does anyone have any pictures?

    Just from my experience with material science, if by durable you are saying the ejector pins have a higher yield strength than that of the stock screw pins, then honestly they could go with any bulkhead brace and it would be an improvement. On a different note, the stock screw pins would work great if a bulkhead with a higher yield strength is used.

    I would definitely not use the description buttery soft on the stock screw pins. They are most definitely hard. This is evidenced by the large necking at the break (the neck at the break is nearly the same diameter as the pin itself). I know what you mean though, they break easy. The bottom line is it was a bad union between the screw pins and the bulkhead brace to begin with.
    Last edited by jmwman; 06-19-2015 at 08:01 PM.

  8. #8
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    I did just see a post somewhere just now about the use of ejector pins and they simply cut them with light duty bolt cutters. I'm still trying to find the collars though.

  9. #9
    RC Turnbuckle Jr. Wildman4910's Avatar
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    Made by DuBro #597, 1/8 12pk.
    http://www3.towerhobbies.com/cgi-bin...?&I=LXE106&P=7

    you can cut the pins with bolt cutters or a cut off wheel on a Dremel, just leave enough for the collars.

    The ejector pins are a hardened steel, they break before bending, but the breaking force needed is high, SS/Ti pins will bend and can jam in the bulk.

    I would much rather have the pin break then bend & jam in the expensive bulks I'm using and chance trashing my HRC bulkheads.
    Look out for the tree/crunch!

    LiPo? Naw NITRO!!

  10. #10
    RC Turnbuckle Jr. ReglarGuy's Avatar
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    Those pins are like little fuses. I break them if I have a bad crash and I cart wheel for ever. What I do is I get a package of them. Take out the ones that fit, and cut the ones that are longer to the size I need. Then I put them all together in my parts bag. I usually break a couple a season...no big deal, and pennies to fix.
    The ReglarGuy is kid tested and father approved.

  11. #11
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    Yea, I'm seeing that they are cheap to replace, which is good. I may just stick with stock and try to avoid trees next time, lol.

    Oh, I was thinking back and I do recall I had a "bent" screw pin on my 3906 after a dead stop crash. I wonder if the materials used for the screw pins are different from the 3906 to the 3908.

  12. #12
    RC Turnbuckle Jr. ReglarGuy's Avatar
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    Couldn't tell you on that...I'm just a ReglarGuy. But, I do know Wildman is right. It's better to break those pins when they get to a certain point then bend them to much. Also, if your truck gets a tad sloppy don't worry about it. If you try to keep your truck at a perfect 100% all of the time, you won't have as much fun with it as if you just play the poop out of it. I say run it, run it hard, and if/when it won't run right (or, at all) fix it...then hammer it again. That's what an Emaxx is for. Plus, when you buy parts it helps Traxxas employees send their kids to college. (lol)
    Last edited by ReglarGuy; 06-20-2015 at 05:31 AM.
    The ReglarGuy is kid tested and father approved.

  13. #13
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    Quote Originally Posted by jmwman View Post
    This break is shocking to me and based on the side it's on, I know exactly what caused it. It was last week when I was out doing speed runs and towards the end I was coming in and was calculating a stop just before a tree that was near me. I barely bumped the tree. The left front tire was the only portion of the vehicle that hit the tree. I feel like I need bumpers to go in front of the front tires, lol. I must have been going max 5 mph, no faster. It was super slow but obviously the truck isn't designed to make that kind of dead stop through a frontal impact on the front tire.

    Judging from the break, the material seems more on the brittle side and less on the ductile side because of the small reduction of cross sectional area at the break, which is fine, because you probably don't want much flex in something like this. The thing I'm surprised about is the location of the break, it wasn't at a joint where the arm meats the bulkhead brace, it was "inside" the bulkhead brace. Anyways, bottom line, and like I always say when it comes to durability, don't crash, even if it's a tiny bump/head on collision and you should be fine.
    Keep in mind that metal fatigue is a cumulative process. It is quite possible that your tree impact was merely the proverbial straw that broke the camel's back.

    When upgrading it is tempting to replace parts with stronger counterparts, but sometimes that could actually lead to more headaches down the road. Personally, I would prefer to remove a snapped pin rather than have to cut out a bent pin.
    Last edited by grumpy42; 06-20-2015 at 09:29 AM.
    Proud member of the red card club.

  14. #14
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    Yea, the screw pin is very easy to replace, the problem though is that the way the screw pin broke, it wasn't immediately noticeable and I kept going with it for only 2 runs or so. This resulted in a ruined bulkhead brace, a part that's much harder to replace. The short portion of the screw pin basically gouged the hole through the bulkhead brace even bigger. I feel like the pins almost need to be somewhat elastic to work with the stock or RPM bulkhead braces. Either that, or have a yield strength 4 times that of the stock screw pins. ... I wish they advertised the physical properties of the material used in the stock pins.

    To continue using all stock, I would almost have to do a thorough inspection after every semi hard impact rather than just drive like I didn't have a care in the world, lol.
    Last edited by jmwman; 06-20-2015 at 11:14 AM.

  15. #15
    RC Turnbuckle Jr. ReglarGuy's Avatar
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    Once you get use to your truck you'll know what/what not to look out for. By that I mean, you won't need to spend a lot of time inspecting your truck when you're playing with it...you'll get a feel of when you need to take a closer look see.
    The ReglarGuy is kid tested and father approved.

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