The Official Tom Thumb and Micro Bike Thread

Mini Bike & Go-Kart Parts | Parts from Hent on eBay

They’re Cool! There’s a nice set of 4” Go Powers on E Bay right now that would look good on a Micro.
You mean were... Hahahaha, Got'em, Karen can't be the only one in town with a nice set of GP wheels!. I also snagged up those 4" Hands wheels too, thought about the 4" Fox wheels but one had a chip in the bead. I love small, the smaller the better, I don't know what it is, I'm just big on small is all.

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I just ordered (2) 5/8" dies for my JD2 bender, $440 bucks!, OUCH!. I got the 2.25" CLR 180 Deg and the 4.50" CLR 180 Deg, the 2.25" will be for the side frame rails and forks, the 4.50" will be for the top and bottom hoops. I plan to start making some frames as soon as I am done with my RCF Micro. The plan for the Tom Thumb bikes will be to use the 8" caster wheels from Northern Tool and either one of those Subaru/Robin 47cc clone pocket bike engines or a GoPed engine seeing how they have mounting holes on the crankcase. I like the 47cc clone, it reminds me of the old Clinton Panther engines but the GoPed engine kind of has a Tecumseh look to it, I ordered both so we'll see!. I don't like the idea of the tuned pipe, to me it makes it look to much like a pocket bike so I ordered a box muffler and an R/C truck pipe to experiment with.
 
Does anybody make a Frijole style micro frame?
Not yet... but keep an eye out, I'm toying with the idea of making some micro mini's that will use GoPed parts, that way, if you have a GoPed, you can just transfer your parts over and have a cool mini bike. I bought a variety of engines, exhausts and wheels to experiment with. My goal is to make it the most cost effective as possible while being easy to acquire parts for. I also want to make them just a tad smaller that the original Tom Thumb while still being rideable. It may seem like an easy task but as I get one thing figured out, something else pops up. I want to keep my builds as scale as possible, finding wheels that aren't too small or too wide has been a challenge. Goped wheels aren't very wide, if I go that route, I can't make the frames too tall or too long or they will look out of scale with skinnier wheels. The largest tire size I want to use is 2.80/2.50-4, just finding the right wheel and the ability to use "off the shelf" parts that can be used throughout my series is my goal... and challenge.
 
I'd be interested in one of the Tom Thumb frames if yer making some for sale!!
From a potential customers perspective, what do you think would be better, traditional size with jack-shaft drive with a newer more reliable power plant, or more towards the plug-n-play idea using GoPed parts?. The only thing I don't like about the Goped idea is that the GoPed has solid tires and only one front bicycle style caliper for stopping. It's worked for GoPed forever so I figured other that seeing a bicycle caliper on the forks it should work just fine. GoPed's can go over 20Mph without a gearbox reduction so I thought about using the shortest gearbox and angling the engine forward just a bit to allow me to keep it as short as possible while still keeping it rideable, my target speed is around 15Mph. Another problem is that if I use GoPed wheels, the frame will have to be around 4" wide to maintain scale appearance, the GoPed wheel hubs are 2.66" wide, I'll use spacers so the actual mountable width will be 3.5" between the axle mounts. With a 4" wide frame, I don't think there's a way that I will be able to fit the whole engine under the frame even with the gearbox and this is the only thing that has been making me re-think the GoPed idea. I have thought about using 1/5 scale R/C engines but they have a 54mm clutch and by the time all the parts are installed to make it mountable for mini-bike use will make it just as wide or wider than the engines with a 78mm clutch which is way more user friendly. Let me know what you guys think, any and all input will be greatly appreciated!.
 
So here's my idea, I want to make a series of classic mini's but smaller, about 1/2 scale. Yes, to include a Taco Fijole style frame and a few other of my favorites. The goal is to have them all use the same driveline, wheels and related hardware. This will keep parts count down throughout the series so if you already have one bike, just buy another frame and transfer the parts from one to the other. I have been doing a lot of research to find parts that will look good on all the bikes within the series without it looking like a pocket bike. I also want to paint the fan shrouds to match what color the engine was originally, along with smaller decals to reflect the brand and era. So far, I will probably be going with a Chung Yang R460 engine, same as what the new GoPed's are using. It has 4.2 HP @ 11,000 Rpm. and is readily available anywhere you can buy GoPed parts. The gearbox is from a pocket bike but I will be going with the shorter one if I can, I think it looks like a clutch cover/chain guard anyhow and doesn't detract from the vintage look I'm going for. The gearbox has a final out ratio of 3.25:1 with an 11T-T8F sprocket. If I use a 44T rear sprocket, the final ratio will be 13:1 and have a realistic top speed of 15-20Mph. with an engine RPM in the range of 10,000 and an 8" tire. I plan on using 2.80x2.50-4 tires on the generic 3 spoke scooter rims. I will be using 2 front rims as the ones with sprocket holes are too wide for what I want to do, I don't know why they have to put the sprocket and brake another inch off each side but they do. I will just re-drill the sprocket to match the wheel pattern and use aluminum spacers for the correct sprocket to wheel distance. I also plan on making seats that look like the originals but smaller, it's going to be a while before I get started as I have to finish up my build-off bike, draw up plans and build jigs, below are some pictures of the parts I'll be using.

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This is my take on the Tom Thumb minibike, (somewhat updated).

I did this not because I needed a minibike, but because I was interested in TIG welding and fabrication, and I wanted to really "old school" it and make as much as possible myself. . I can MIG weld to some degree, but for TIG welding, I was only somewhat successful. I can actually do a stack of dimes on lap welds or but welds of sheets of metal on a table but three dimensions of shapes in the air will still take a lot of experience. THEN I read that one of the hardest things for even an experienced TIG welder to do is small-diameter pipe..."a grinder and paint make me the welder I ain't". The results may not look that good but I'm fairly certain that they are strong.

I bent the pipe (yes, pipe, 5/8" ID) with a Harbor Freight pipe bender. Since this has a lot of trouble going >90 degrees, the pipes were joined to produce >90 bends except the two at the back which I BARELY did and that was after about a 100% waste factor on the pipe and a lot of finnese. Without a jig, making these parts accurate and symmetrical is a LOT of work without the proper ($$$$-$$$$$) equipment!

The wheels/tires are HF also, cheap, and I bought four to come up with the two bearings on each side per wheel. Then I discovered like everyone else that the sheet metal bearings are no good and I replaced them with real bearings. The tires are supposed to be lousy also - so, buying these was probably a false economy. I made the hub/gear adapters (those, I MIG'ed).

I took some liberties with the design; the wheels are larger and bigger (5"); the frame is at least 1.5" wider because that's how the 49CC engine worked out. I made the handlebars taller in the hopes that it would be easier for me to ride (I'm ~6' tall). ALso, I'm (somewhat) wider than the very spindly kid who originally built this in 1970!

I polished the front cover of the engine to some degree. I'm running it at 16:1 for break-in and it's pretty smoky, I'll probably go to 25:1 later (with modern 2-stroke oil, this is supposed to be safe - comments, anyone?

I really wanted to mount the brake (yes, I wanted a brake) INSIDE the frame perimeter but this worked out to be about 1/4" beyond possible, so it wound up on the back. I really wanted to make sure it would stop and this was probably overkill.

The gas tank is made of PVC fittings. I made the seat and had a car shop upholster it. That cost too much but I was too far into it at that point...
The throttle cable and brake cables were shortened. Lots of good YouTube videos on how to do this with silver solder. I couldn't believe that this could be done but it did work, first crack. I even measured correctly. Good thing, I had only one set and exactly three ferrules!

Getting a muffler for this is tricky. I am using a box-type muffler now with a shield to deflect the exhaust from my foot! This may not be my final take on the muffler.

I am paranoid about getting the chain wrapped up in something so there is a chain guard made of aluminum on the left side, with mounting tabs for it on the frame (not shown here).

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I BARELY fit on this, I don't know what to do with my feet; i can barely make the foot pegs work for me. My initial ride showed me that the acceleration is pretty brutal (it's geared at maybe 20:1). I'm glad it has a good brake. It is scary to start it because it will do an instant wheelie with no one on it and any throttle; starting it while you are sitting on it is basically impossible, at least with my 60+-year-old physique!
 
No, I really worked from the original magazine article and the very nice early 21'st century build at the top of this thread, and used parts that were presently available to me on eBay, Harbor Freight, etc. Later on I saw minibike parts websites but I didn't want to invest too much into this and "buying" off the shelf stuff kind of ruins DIY for me. i'm still amazed that with fairly crude tools the brake disk has maybe 1/32" runout (with no shimming involved). I didn't think that this engine/clutch/transmission would need a jackshaft and I just HAD to have a brake. making it wider with larger wheels/tires and the handlebars taller didn't seem to hurt the spirit of it much (although it definitely made it bigger/heavier).

As it is, with the new tools (even a tube notcher) and so on that I bought it would have been much cheaper to just go buy a minibike. But it was still worthwhile and again, I don't NEED a minibike and this thing is so dangerous I could never possibly sell it to anyone..

I'm thinking about welding in "foot plates" between the foot pegs and front of the lower frame for my large/clunky feet that will just clear the turning front wheel.
 

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