Pinion gear/shaft. Also: Welding up a new PB frame.

The pinion gear/shaft on my air-cooled 49cc engine sheared off completely when a mate was riding the bike. This resulted in the chain wrapping itself around the rear wheel/frame- stopping the bike dead in its tracks and enticing my friend into an intimate encounter with a large rock.

Is this a common problem? Is it possible to get a stronger replacement shaft? All I have managed to find online is a standard replacement clutch housing (which I assume has the pinion gear already fitted- it does in the picture provided). I could get one engineered/machined from a billet, however it would be a lot simpler and easier if stronger parts are available.

It is possible that the chain may have been a little too tight (it wasn't excessively tight though, so I mostly put it down to the exceptionally high manufacuring standards held in chinese pb factories :P).

I'm really not too keen on the idea of the rear wheel locking up at 40kph+ when I eventually get to take it out on a proper track, so any info/experiences would be greatly appreciated.

Also, are there any external differences between the 49cc and 47cc motors? My manual states mine to be 49cc, though in actuality I have no idea (I'm also aware that these figures can be grossly inflated from the factory). Thanks and sorry for the epic post.

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The Chinese pinion and bearing are both pretty crappy and it may be that the bearing collapsed causing the whole thing to fail.

Best fix is to upgrade to a 4th Bearing Support assembly which is available from any of the sponsors on this site.

This unit uses an extended sprocket which is supported at the end by an extra bearing.

You can get it fitted with a US Hardened sprocket, or a cheap Chinese one. Unless you have heaps of HP the Chinese ones are pretty reliable as long as:

a. You ensure rear and front sprockets are dead in line with each other - put a ruler or straight edge against the side of the rear sprocket and sight it to make sure it is aiming straight at the same edge of the motor sprocket

b. Chain is kept well adjusted all the time. Not tight, but just a tiny bit of slack.

c. You lube the chain with a good chain lube regularly (We do about every 10 minutes of riding or after every race and have never had a chain or sprocket failure)

Hope this helps
Al

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i reckon your sprockets were slightly out of line over time it caused one of the parts to fail (front sprocket) once this comes off while riding the rear wheel is still turning causing the rear end damage did u hear at any time while pushing the bike a slight tap noise coming from the chain? also the chain should be oiled on the inside :wink:

Thanks guys, thats really helpful, appreciate it


Another quick question: Are the bearings just a sink-fit? I will be doing all the work myself so I thought I'd ask rather than just pulling it apart to find out lol. This is my first Pocket bike, so its a learning curve, though I've done a lot of work on cars/4WD's in my time, so basic tools and mechanical aptitude arent really an issue.

I didn't hear any unusual noises coming from the chain at all (when it was turned off), then again I have to wear earplugs around any moderate to loud noise (hearing probs), so I wouldn't have heard any noises while it was running unless they were nearly as loud as the engine (I'm amazed if a person with healthy hearing could hear much over the engine anyway!).

I'll have a look into the 4th bearing assembly and US sprocket (I'd rather fork out the cash and be safe than have another breakage). It is possible that the sprockets may have moved slightly out of alignment while we were riding it. When I put the bike back together I'll be sure to keep an eye on this.

Thanks again,

Aron

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your chain could have stretched and your addjusters that pull the wheel back would have worked there way loose as for the bearings they only seat into the wheel all u need to do is tap them out through the other side try to use something the size of the axle to push the bearings out so they come out strait not crooked

ive lost some of my hearing and im only 24 been fixing bikes since i was 12-13 years old i can hear most things wrong with a motor but i am very mechanical minded

Hey gazza, I was actually refering to the bearing in the clutch housing (for the front sprocket). Probably should have been clearer on that.

I would imagine that all the bearings would be sink-fit.



Also, I guess you would be able hear small rattles in the frame and whatnot over the engine if your hearing is not obstructed. I will say this though, I'm also 24 and I tell everyone to take care of their hearing, If you've already got hearing damage at our age, try not to make it worse- wear hearing protection around loud noise etc.

Do you have tinnitus? Can make your life hell sometimes, though I try not to let it get to me, in so much as that is possible (when its really loud/high pitched it can be very hard to ignore). My hearing is hyper-sensitive as well, hence having to wear earplugs to do just about anything that involves normal or loud noise levels.

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nah i dont have tinnitus just every time the missus says something i say what's that? she thinks i have selective hearing its a good thing :lol: yeah i know what u mean about hearing protection last job i was at good pay but didnt give me any ear protection so i quit two days of migrain's and working on a bench saw making over 1000 cuts a day with high pitched noise and no protection is just bs

That's pretty bad. I would have complained to Health and safety, then either taken my own hearing protection or quit (I'd say you made the right choice if they had a fairly lax attitude towards safety).


Have you tried welding up your own pb frame before?

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A the end of the day, it's not worth rebuilding the Chinese clutch/bearing/pinion assembly as it is cheaper and quicker to buy a new one, especially if you are using standard 6 tooth as most PB shops are over-run with them.

All the bearings on the bike/motor are easy to replace depending on how accurate the 7 year old kid was when machining the hole :cry: They either fall out or you just tap or pull them out.

There is no rules on frame origin in Aus, so anyone could build a frame I guess, but there is not a lot wrong with most frames for reasonable quality bikes bought of reputable distributors.

They all have areas that can be improved, but it would be far less work and cheaper to spec up and existing frame than build a new one. If you are going racing, then check all welds and clean up any you may not be happy with, square up the frame if it is not right, and make sure the area where the back axle clamps in, and the relevant adjusters are of adequate strength. Also check the top engine mount bracket and mount point is of reasonable quality as this can break or loosens and casues chains to go west :cry: if the vibration doesn't stop you first
Al

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as far as frames go, i would say have a look at other ppls bikes, and see
if they will let you sit on their bikes, there are a few different styles around
that have slightly different lenghts heights etc, you may find one you like better. Too much time and effort to build one, if you ask me, cause unless
you are going to copy another frame, you need to make sure all your mount points for motor etc are correct or you will never get the other bits to fit

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If it jams force it, if it breaks it was faulty anyway