Subframe Crack. Worth Replacing?


JW884

Well-known member
Location
Connecticut
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So before I had the suspension revalved I went for a jump in mode 1 instead of 2 by accident and cased it:muutt: not my brightest move. Didn't even fall but I know it wouldn't have been that bad on my other bike or correct suspension. Is this crack worth replacing it? It doesn't go all the way through, only what you can see here. I have a spare subframe here but I was hoping to keep it as a spare.
 

F451

Well-known member
Location
Seattle
I would be tempted to try some 2 part epoxy on that area, being careful with the prep and curing (somewhere warm and dry). Then test ride it a few times and see how it goes. Maybe take it easy on the jumping and build up to your normal jumps carefully checking it as you go?

If mine ever fails, I'm going to look into having a KTM aluminum subframe welded on to the Alta factory subframe mounting brackets, switch over the rear plastics while I'm at it.
 

Judaslefourbe

Well-known member
Location
Brisbane, CA
As Snydes says, it should be fine if you stop the propagation.
When we designed this part, we tested it to failure and it took more than 2 tons of pressure to break it.
Unless you often do big jumps, I'd think you can use it and monitor.
 

privateer703

AOF Addict
Location
Altoona, PA
I would definitely try some epoxy on the crack above where you drill a hole to limit propagation. But you might want to pick up a spare. Not sure how many are left but I'm sure they won't be available forever.
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Looks like I’ve now joined the sub-frame crack club on my MXR. Cracks on both sides. I always stand during jumps and bumpy stuff, so it must just be my fat ass.
:cry:

Remember kids: Don’t do crack!

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JW884

Well-known member
Location
Connecticut
@C5tor Glad you noticed before it got any worse but welcome to the club! You should get your complimentary snack package of chips & crackers anytime now :ROFLMAO:
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Looks a little worse from underneath. Fortunately I do have a spare subframe.

On the downside, I also got to join the f&#%ing stripped brass-insert club as well. Most of them were in the subframe itself, so I just popped them out. But one was in the rear fender, and I was hoping to re-use that. Oh well, I will relegate it to the spares pile, and will just drill a hole through that fender if I need to use it someday.

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C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Well, taking a look at the actual failure point of the subframe, it makes perfect sense why it would fail at these exact points. The cylindrical section where the bolts attach are very solid plastic, even without the metal inserts. But where the subframe transitions to the ribbed structures, there is very little surface area at all. Those ribs needed to be quite a bit thicker, IMHO. Obviously, that would have cost weight.

I was literally able to twist these pieces off with my hand with almost no effort. I suspect that for the most part, hard up/down impacts are not the real problem here, as far as failure mode. I think side-to-side or twisting motion is the real culprit. The ribs are very sturdy for up/down strength, but side-to-side motion, not so much. If you get a lot of torsion on the back of the bike, it will start to twist this section.

Also, while the two bottom supports probably get compressed as you add weight to the rear of the bike, the top supports are actually pulled apart (rearward) as downward pressure gets applied farther back.

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JW884

Well-known member
Location
Connecticut
Good thing you have a spare and personally my fender and plates are held on entirely by zipties and self tapping screws. The inserts aren't even worth trying to save. I wonder if there is a way to add perpendicular ribs in that area to stiffen up better against lateral movement
 

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