Crf450r broken jackshaft

C

Compu-kart

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Ran the crf450r in a roadrace until this happened. Any ideas on what caused it, other than I shouldn't have been doing it?
 

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It was used too much when it was new.
Lol

Was it new when you got it?
Shaft always loaded the same direction?

Look like a fatigue crack , maybe from stress riser.
It was the next weakest link.

You just might be hard on equipment

Lol

If making a new one, use a stress proof steel
 
End view should show the issue . Or how long its been bad .Stress riser do to stress at bearing edge .
 
I bought it used. I have another post "I Bought a Jack' that tells the story. I'm thinking a lot of heat went thru that part. When I received the engine from Andy Foley the head gasket was blown and the water pump seal was blown, surprise!. The thing was baked. Since getting it running again I burned up 3-4 clutches, more heat. Finally got the clutch figured out and the jackshaft snapped. Machining another shaft and will be back on the track soon.
 
I don't run a 450 but I have seen a several drivers on dirt with 450s with broken jackshafts, so.e even more than once.
 
Can confirm, I know several as well that have broken jack shafts in 450's. We've had several issues with breaking them on a 250 two stroke as well. And, still haven't found a solution. Still aren't even sure why.
 
We consider the shaft and bearings a wear item and replace at fairly regular intervals. Like ABR #69's experience, we can't find an obvious reason they break other than general fatigue.
 
I would say it's a maintenance item. replace every 10 races? 5 races? get the new one and keep track. Another support on the ends would probably help but i know space is tight on these deals.
 
racinnut15xm said:
I would say it's a maintenance item. replace every 10 races? 5 races? get the new one and keep track. Another support on the ends would probably help but i know space is tight on these deals.
Click to expand...
Will be building the new one with a third bearing support. On a quest to find the next weakest link 😀
 
I think it's partially shock load, we noticed if you spin, or get hit hard, there tends to be a higher chance that our tradition style jackshaft with the 250 would end up breaking.

So adjusting how you deal with the kart in a spin to keep it revved since under UAS you can't crank it again. It takes a considerable load on the jackshaft if you stay in it.
It got to the point we were absolutely monitoring it weekly. Always had a spare and a spare clutch. Because when it let's go it almost always destroys the clutch.

We started replacing them I think after 10 races. Then we got Danny York to make some out of M100 I think and it lasted a season, but had lots of wear and had to replace it. We ran it at the GN at GKK and survived a fairly hard hit that eventually took us out of the race. Using the second one made of M100 now with slightly different tolerances, we only have one race on it.
 
Hydrogen embrittelment
Though unlikely a possibility.
 

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M100 is what we have ours made out of with a local machine shop. They seem to last longer than the Buller. We still replace ours every 3-5 races, bearings get replaced every 1-2 races. We take it all apart after each race and make a decision on if it gets replaced sooner than our normal interval.

ABR makes a great point. UAS has a no start rule so you can't let the engine go dead. I tell my driver, if you have to whack the throttle to keep it running it "might" break something, if you let it go dead we are done for the night.......
 
Would dye penatrent testing help any with replacment decisions ?
At least you would know its starting too crack .
Routine replacment probably as simple and cheap .
 
flattop1 said:
Would dye penatrent testing help any with replacment decisions ?
At least you would know its starting too crack .
Routine replacment probably as simple and cheap .
Click to expand...
I was thinking you could put 5-6 spot welds at the weak point where it usually breaks and once you see your weld broke time to replace. Or is that not legal? Sorry I am learning the uas stuff as I read as I am preparing to get my son a uas kart in the next year or 2.
 
Well, the bearings and shaft are wear items… they should be replaced often? How often? Well I can list off plenty of races lost by heaps of fellas due to a $20 or less junked part.

Excessive heat in the clutch or running your drive belt too tight will speed up the process. Running the belt tight will dog the engine out quicker also.

Wheel hop will also destroy a jackshaft, clutch, or rear sprocket.
 
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