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rockstarsomeday 08-04-2019 07:47 AM

Mini bike throwing kids
Picked up a monster Moto 80cc minibike for the boys last christmas. Recently while the boys are riding it will lock up and throw them over the handlebars. Yesterday I saw it happen with our 10yr old. He was trucking along and we heard a loud clunk noise (engine stopped). The bike immediately stopped and he didn’t, went flying. Glad we make them wear helmets and only ride in the yard.

Any idea why this would happen? I’m thinking its the engine causing this? The bike started and ran just fine afterwards. Engine has been leaking oil pretty much since we got it, but I keep oil levels where they should be. I’ve seen this happen twice now, so we’re parking it until I can figure this out.

itsid 08-04-2019 08:09 AM

usually if the engine locks up on a bike,
the rear wheel locks up and you don't get ejected over the handlebar..

for you to go flying you need to lock the front wheel..
as long as it's rolling there's little chance you go over the handlebar.

So my first suspect is indeed the front wheel and it's brakes
a wobbly brake disk or worse a loose brake pad can lock up the wheel for good;
if that's fine rigid and tested to be in perfect working order (always a good plan ;))
the next thing to check is the front wheel bearings.

Cannot imagine why the engine died in the process tbh, but such things happen.

it's hard to identify why an engine locks up for no reason without disassembling it completely.
especially if it's running fine just after it locked up.
My first guess is: it didn't ;)
Instead check the killswitch, a loose contact and it might ground out (killing the enigne) on a bump ..
run the engine (neutral and idle) and whack the handlebar with a rubber mallet.
if the engine dies, you have a good suspect for interrogation ;)
If not, check all wiring and connectors more closely, wiggle, the wires, clean the contacts etc...
And if you still haven't found any issue yet,
it'd be time to see if fuel and spark delivery is sound and reliable.
And while being at it, check chain, rear brake and rear wheel (nothing's binding, chain is well maintained, brakes working well etc.)

And now (and ONLY now IMHO) is the time to pop your engine open to see if it's indeed the culprit;
Take a look at the cylinder/piston first (ideally with an endoscopic camera) to see if the cylinderwalls show any markings from a piston lock happening before)
then crank and finally gearbox since it's the hardest to reassemble properly ;)


rockstarsomeday 08-04-2019 08:47 AM

Thanks Sid. I tested the killswitch and the bike rolls after hitting it, which is what I would expect. There are no front brakes on the 80cc moto, only a rear.

mckutzy 08-04-2019 10:04 AM

How often are you oiling the clutch???
It should be oiled every 2hr of use???

mrdeadman1 08-04-2019 11:40 AM

also make sure the chain isnt binding, some times if there is side to side play the masterlink can bind up and stop forward motion pretty quickly

itsid 08-04-2019 12:30 PM

oh mini bike 80cc.. my fault..
I was thinking pitbike for some reason (maybe the 80cc confused me..)

Anywhoo, clutch needs to be oiled occasionally, sure, gearbox there is none
(neither are front brakes obviously ;))

but that still keeps me wondering:
How on earth can you fly over the handlebars if the frontwheel is NOT locking up?
(if by a faulty bearing or a pithole it dives into doesn't matter)

But that might explain why the engine is shutting off when it's falling over..
the low oil sensor is kicking in in case it's still active :D
(such things do not exist on pit bikes)

No matter how sudden you lock the rear wheel, as long as the drivers hips are behind and/or below the handlebar it'd be hard to go flying...

I don't want to ask to videotape your kid crashing again,
since I do not want your kid crashing again for obvious reasons :D
But I'm somehow unable to imagine what's happening there.

The good news is:
if it's in fact caused by the low oil sensor it'll be gone once you disabled that thing,
(I think it's the result not the cause tbh)
but it's easy to check and resolve.


PS tell your kids to keep their center of mass below and behind the handlebar especially on rough terrain.

madprofessor 09-12-2019 07:48 PM

You didn't say what the kids were doing when they went over the bars. It's an off-road machine. It's kid-sized, like a regular motorcycle would be to grownup me. They are riding it off-road. Whenever I was riding my bikes in the dirt, my behind was seldom on the seat. Unless I was crotching the gas tank to get my inside foot out beside the front axle (Nike 3rd wheel) in a powerslide, I was standing on the pegs. Locking the rear wheel up would definitely throw me over the handlebars if I wasn't planning it myself, so something like a chain jumping, binding for a second, and then getting snatched back out by the force could easily toss a 75 lb. kid over the bars if they're not sitting down.

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