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Kimjon

Kimjon - builds a 49cc goped

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After much thought, I'm going to put a CAG 49cc motor on it. 

My main reasoning for this was, I'd have to destroy a perfectly good pocket bike to put a C1 polini (replica) water cooled motor on it. I just dont have the heart to ruin a working bike just to get the motor - when I have two CAG motors sitting there in bits.

So, I made a start...

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Here's the goped. Purchased as is, missing the motor.

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CAG motor getting a 6mm thick steel plate attached to the bottom of it. This will enable me to weld a strong pivot point to it.

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Close up of the OEM engine mount. This pivot held the motor, there was no clutch as in a centrifugal clutch. However the wire cable tensioner literally pulls the motor directly onto the tyre and transmits power via a friction drive spindle. To start, you push it to about a walking pace and then tension the motor down to contact the tyre...then you can stop pushing and the motor takes over. Pretty clever low tech solution.

I'm going to do away with the friction drive system and transmit the power to the rear wheel using a chain drive via a centrifugal clutch. This has a number of advantages over the friction drive, and also solves the main problem with my motor spinning the opposite way to the OEM motor.

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I'm going to weld my steel mounting plate to this OEM pivot. I'll cut it to a suitable size to lower the motor down to provide a strong engine mount. I'll replace the cable with a spring and this will provide a constant chain tension.

So that's the plan...

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Got a little bit done. Motor is now mounted, tacked together and sitting well.

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This Goped was never designed to use a chain, so I'm offsetting the centerline of the wheels by about 10mm to allow room for it to fit. Hopefully this won't be an issue, but theres only one way to find out?

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Expansion chamber in the making. I want exaggerated proportions on this Goped. Trying to bias the bulk out one side to over inflate its looks. I took several (as in I've been fucking around for 2hrs) attempts just trying to find the best line for the exhaust to flow. I'm settling on this look, as I personally think it compliments the motor and look I'm going for.

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Hell yes! Exactly the look I was trying to achieve, like a 90's MX bike (yz, cr, kx, rm125 etc...)

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So next up on the list:

1) Cut the top of the cylinder head off.

2) mill/lathe cylinder head to get right deck and compression ratio.

3) port the fuck out of it

4) fit s domed head kit to replace the top of the cylinder that's getting cut off

5) fit a much larger diaphragm carburetor 

Then pretty it up and ride!

 

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Added a silencer, mainly for the restriction as my "stinger" (outlet side of expansion chamber) was too large in diameter. The additional benefits of a silencer is ummmm....added silence;)

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pretty happy so far with what I've made. 

 

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Carburetor test fit, looks sweet!

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And a dead sexy bracket to hold the expansion chamber. Mmmmmm sexy bracket...mmmmm.....

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Playing with these full circle camshafts. They're renowned for not being balanced. Bit of a black art, but basically a single cylinder engine can't be truly balanced...so lots of people have come up with trial and error rule of thumb like a "balance factor". Something between 55% - 60% seems to be what the I interweb says...so that will do.

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After (well actually more during)

Holes are drilled and each time the balance factor is checked to see if more material needs removal. It should  be close according to what I've read online that others have done, but likely to need a bit more of a tweak later when I've got better information from actually results, but I'll leave it there as I may window my piston and that would throw my results out the other way. So call it done for now.

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Okay, not quite turning out how I had hoped. 

 

I'm trying to get a balance factor in the ballpark of 55%. However I got 0% first try, as in neutral with no reciprocating mass added...cunt!!!!

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so I enlarged the holes to the maximum possible it could take which was 11.5mm...checked it...still no good...fuck!!! So added additional 6mm holes, which was once again the biggest I could go given the situation.

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mother fucker!!!  Only achieved 22% balance factor...double cunt!!!

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Did more math...

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Not looking good is the short answer...

But I'm not giving up on using the FCC (full circle crankshaft). Just have to approach this problem from a different angle. I'll give it some absorption time to think it over...but likely to involve drilling out the bottom side, then adding lead to it to increase the weight on the counterweight side.

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This photo shows the issue I have trying to find places for bigger holes:

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The con rod boss/webb the pin is pressed into can't be drilled into, or it would weaken it too much. Those holes I've drilled (in the photo above) have maxed out that location already.

So pretty limited space where I can put holes. 

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if I drill it where the vivid circle is, it would be counterproductive, as it's on the side I need to keep weight on, or possibly even add weight. Remove material here and it would have a negative effect.

There's just enough room to put another set of 6mm holes, but it's not ideal and won't achieve what I need...but I guess every little bit helps.

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Years ago I was involved with balancing a fancy piston compressor, went as far as writing a python script to work it all out, simple maths, confusing to apply!

Check the gains from lead carefully, went down that road but as the center of mass was close to the rotation axis, it didn't add as much as we hoped. We did however have the luxury of increasing the counterweight OD which was the eventual fix.

 

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1 hour ago, NickJ said:

Years ago I was involved with balancing a fancy piston compressor, went as far as writing a python script to work it all out, simple maths, confusing to apply!

Check the gains from lead carefully, went down that road but as the center of mass was close to the rotation axis, it didn't add as much as we hoped. We did however have the luxury of increasing the counterweight OD which was the eventual fix.

 

Thanks, yes lead is 11.342g/cm3 so only an increase of 3.332g/cm3 over steel. I looked online and tungsten is much heavier, which was a surprise. Tungsten is 19.25g/cm3 so more than double the weight of steel.

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I ran the numbers again a few different ways and if I drill a hole right through both sides of the counterweight at the same radius of the big end pin, I need a 10mm hole to add exactly the amount of counterweight I need. So...I'm now on the lookout for about 24mm (2x 12mm long bits) of diameter 10mm tungsten.

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☆funny, wrote below at same time you wrote above.

 

 

Had an idea. Tungsten carbide is about 15g/cm3 and readily available in the form of drill bits or milling cutters.

I'll run the numbers again and it may need to be 10-12mm diameter, but at least it's going to be cheap and easy to get.

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