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Old 11-12-2010, 06:44 PM   #1
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Default Breaking MBX5R Lower Front Arms

I'm having an issue with my MBX5R breaking front lower arms where the pivot ball screws in on both left and right sides. I have broken two on the left side and one on the right in the same exact manner. I originally broke the lower left front arm at the inboard mounting point when I had a very bad hit, this seems normal. I purchased a new set of arms and replaced both sides at the same time since I purchased this buggy used and it had some slop on the right side. I'm breaking these new arms while just landing large jumps at my local track.

Anybody got any ideas, have I gotten a bad batch of arms? The only thing I can figure is that I am stressing the arm when I screw in the pivot ball. My next step is to buy a tap and pre-thread the arms before installing the pivot ball.

Need some help guys. Thanks.

Working on getting some pics.
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Old 11-12-2010, 10:20 PM   #2
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I have a kyosho DBX. I think I broke 4 left and 1 right. All snapped the lower arm where it joins with the pivot. Once it also snapped the joint on the bottom of the shock. I believe this is either due to the fact that its a cheap China made car, or the fact that I bash it around places with thin hard objects.

Examples of how I broke them:
*First: friend drove into a tree root
*drove into a METAL chair so hard the chair made a backflip
*drove into a tree
*friend drove into a kind of like place where pavement lifts over the grass
*doing epic doughnuts on sandgravel when I drove into a metal picnic table arm
(doughnuts were so epic that apart from the wheels flattening out, the centrifugal forces made the screw that holding the front shock fall out)

Oh and I also once had the engine mount screws fall out on the bumpy sand lol.
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Old 11-13-2010, 02:56 AM   #3
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I had the same issue years back with the mbx5r. I broke 5 front lower arms in 2 races, on a track with some good sized jumps, but it was nothing I hadn't done before with other cars.

Bad design? I dont' think so. But they did change the plastic from the mbx5, in which I dont' think I broke anything on in 8 gallons of fuel.
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Old 11-13-2010, 04:16 AM   #4
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the plastic issue is mugens nightmare.. you can boil the arms in hot water for several minutes to make them a little more flexible..
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Old 11-13-2010, 06:07 AM   #5
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Originally Posted by D8 View Post
the plastic issue is mugens nightmare.. you can boil the arms in hot water for several minutes to make them a little more flexible..
Point blank, %100 TRUE

Youre a fan boy in denial if u think the Mugen arms are on par with other brands!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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Old 11-13-2010, 06:29 AM   #6
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Thanks for the help guys. I'll try boiling the arms as well as tapping the holes before installing the pivot balls. Here are some pics.
Attached Thumbnails
Breaking MBX5R Lower Front Arms-img_0235.jpg   Breaking MBX5R Lower Front Arms-img_0236.jpg   Breaking MBX5R Lower Front Arms-img_0237.jpg  
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Old 11-13-2010, 06:34 AM   #7
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Looks like air bubbles formed in the mold, which would make them weaker.
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Last edited by mgtmadness2007; 11-13-2010 at 06:44 AM. Reason: spelling.
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Old 11-13-2010, 06:42 AM   #8
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they have updated arms. i had the same problem a few years back. i had gotten 3 packages from ebay and they were the old design broke every one in that same spot as you. updated arm = never broke another.
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Old 11-13-2010, 07:58 AM   #9
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Yous a drop off oil ( i yous aftherrun oil) on the pivotball treads and don't force the pivotball in.
Take your time screwing in the pivotball, let the treads do there work so you have no stress on the plastic.

The only time i break my arms is wen i hit something and as they break its normally around the area were the swaybarmount attach to the arm or, around the inner hingpin area.
Never hat it around the pivotball area your showing.
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Old 11-13-2010, 10:09 AM   #10
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Yeah these new ones I'm breaking at the pivot ball are all the updated design as far as I know, P/N MUGE0107-B, I purchased one set from Amain and two other sets from my local hobby store all within the last month.


The original break I had on the left arm was exactly as you described at the inner hinge pin, it should have broken since I hit a 2x4 on the pit lane coming in a little hot.

I'll try oiling the threads as you described before installing the next set, they are very hard to thread the pivot ball in though. Wonder if since the pivot balls are old they have lost some of the sharpness to the threads so instead of "cutting" into the plastic they are just expanding the hole and stessing the plastic?

Just an FYI, i ran the one that is still on the buggy cracked for another 2 tanks after i discovered it was broken and it held together.

Anyone know what size thread the pivot balls are?
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Old 11-14-2010, 09:49 AM   #11
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I've been racing my 5r for over three years. I race every weekend at different
tracks year round (about 150 mains plus about 450 5mins quals) never broke
an arm or to tell you the truth anything. I just bought a X6, but still like and race my 5r. Always grease the pivot ball threads before installing.
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Old 11-14-2010, 12:55 PM   #12
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You could try these....


Do not try the ZX-0021 since they are the same as the Mugen ones.
I mass produced the first electric 1/8 scale buggy and truggy.
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