First Mini Bike

Cpl2442

Cpl2442

New Member
#1
#1
Never owned a mini bike before. Rode one a little as a kid. Rebuilt an old vertical shaft Briggs on my lawn mower, and got kind of hooked on small engines.

Ended up getting some free pipe from work and an Briggs Intek from Craigslist from a broken power washer and slowly the dream is becoming a reality. I've learned so much from this website that I felt obligated to share what I've been tinkering on in the garage. Glad to finally join the forum and excited to keep you guys in the loop with this...

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Cpl2442

Cpl2442

New Member
#7
#7
Got mail from OldMiniBikes Warehouse today. Some bushings for the rear axle, tougher valve springs, taper bearings for front fork, and some extra animal side gaskets.

Worked on the rear axle last night... First mistake: when I welded the frame together, I wanted it to be close around the rear tire assembly (tire, sprocket, and band brake drum). Well, now when I mount the tire, it was very off center. The solution could be one of two: Install jackshaft and put band brake drum on that. Or make a new frame.

Obviously I'm adding a jackshaft. Have my eye on the 3/4" adjustable jack shaft assembly from OldMiniBikes warehouse.

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So my next issue is, I believe I have a 4" Manco band brake drum... and for the life of me I cannot find a 3/4" bore hub that fits this thing to get it on the jackshaft. Any ideas?
 
danford1

danford1

#8
#8
Looking good so far. I don't know where to get your brake adapter, sorry.
Please read up on rake and trail before for making the front end. This is the time to get it right not latter in a redo.
Keep us posted.
Danford1
 
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FOMOGO

FOMOGO

#9
#9
Cpl2442 said:
Got mail from OldMiniBikes Warehouse today. Some bushings for the rear axle, tougher valve springs, taper bearings for front fork, and some extra animal side gaskets.

Worked on the rear axle last night... First mistake: when I welded the frame together, I wanted it to be close around the rear tire assembly (tire, sprocket, and band brake drum). Well, now when I mount the tire, it was very off center. The solution could be one of two: Install jackshaft and put band brake drum on that. Or make a new frame.

Obviously I'm adding a jackshaft. Have my eye on the 3/4" adjustable jack shaft assembly from OldMiniBikes warehouse.

So my next issue is, I believe I have a 4" Manco band brake drum... and for the life of me I cannot find a 3/4" bore hub that fits this thing to get it on the jackshaft. Any ideas?
Click to expand...
just a thought..... have you thought about using a DB30 rear wheel with the disc brake? Could be easier than the jackshaft brake. :shrug:

Doug
 
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Cpl2442

Cpl2442

New Member
#10
#10
I feel kind of stuck using the tires I have, spent a pretty penny on them and most likely couldn't justify buying a different one to SWMBO. Thanks for the idea tho, I believe using a jackshaft will be mostly a benefit. Will allow me to center up the engine better on the frame and get a good stable feeling bike.

Danford, thanks for rake and trail reminder. I did skim over an article the other day someone posted here about it. I roughly cut my frame where the forks will be around 25-30 degrees. I will research that more when I get to it, right now want to focus on the seat and seat springs.

I mocked up what I have in mind with some clamps. Will have to drop the seat a little to accommodate the cushion height, so I have a nice line from the gas tank...

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Cpl2442

Cpl2442

New Member
#11
#11
Was able to work on the bike a little last night. Got the seat "hinge" all welded in, and also the seat shocks tacked in. I was very happy to find that the shocks do give when I bounce on the seat. I was concerned with how stiff they felt when I first got them... I believe they are the 450 lbs ones you find a lot of online. Looks like they are going to turn out fine.

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Cpl2442

Cpl2442

New Member
#12
#12
Was able to get the jack shaft welded on. I found that a 3/4" bore Azusa hub fits the 4" Manco brake.

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Overcame a little issue of not being able to have the drum "inside" the two bearing mounts on the jackshaft. I'm just gonna place it on the outside. Welded a tab on the jackshaft bracket to hold the brake band so it will move with the jackshaft when I go to tighten up the rear chain.

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Now I want to pick your brains again. I have a 30 series torque converter with a 10 tooth sprocket and my rear wheel has a 60 tooth sprocket. The jackshaft has a 10 tooth and a 14 tooth. I have no idea what side I want those sprockets on....should I gear it for higher top end since the TAV should give me a good low end? Thoughts please...
 
danford1

danford1

#13
#13
If you are riding on flat land or on streets etc, I would gear it for speed. If you are off roading or climbing hills, gear it for low end.
JMO Danford1
 
Cpl2442

Cpl2442

New Member
#14
#14
The project continues! Sorry for the leave of absence, the "fun money" was low for a while...

Anyways, The motor is mounted. RVL exhaust and silencer installed. Exhaust Wrap. Walbro PZ carb installed. Now i can finally move onto getting the front forks installed. Will update my plan of attack on those here in a little bit. Got some ideas...

For now, enjoy some pictures ....
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S

SAT

#16
#16
Nice work. Are you considering a ventilated cover for the band brake assembly? If you are, please come-up with something cool...
SAT
 
Fearless

Fearless

New Member
#17
#17
Good job on an interesting project. Love the looks of the pressure washer engine. Good choice of the gas tank as well. I await further progress.
 
danford1

danford1

#18
#18
Cpl2442,
I was looking at this thread again and wondered what you ended up with for the gear ratios. Did you go 12 tooth clutch to 12tooth jack shaft hooked to a 14 tooth to the 60 tooth sprocket? Or did you reverse the 12 and 14 tooth sprockets?
One way gives a final drive ratio of 4.28 and the other way is 5.83. That is a big difference. Now when you multiply those with the 2.7 ish torque converter ratio at engagement, you get A LOT of torque. That Briggs has some good low end torque also.
You may find that is will wheelie quite easily, especially since it looks like your butt will be right over or behind the rear axle. The beauty of the jack shaft is you can change the gear ratios easily. You might have to get a bigger sprocket for the jack shaft that goes to the 60 tooth. That would give more top end but will also lessen the instant torque available on take off. Either way, this minibike is going to be a hand full :) I see you haven't added the front suspension yet. You mentioned 25-30 degrees rake which is good. The trail is going to be important.
Make sure you figure that out before you make your triple tree. If you put a lot of offset in them, the trail goes bye bye and the handling gets sketchy. My suggestion would be to read A LOT about it and get the trail at least 1" for a stable ride especially at the speeds your minibike is going to go.
JMO Danford1
 
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Cpl2442

Cpl2442

New Member
#19
#19
First, Thanks everyone for your feedback and especially Danford1 for the gearing info. Right now my set up it this: Crankshaft to 30 series TAV, to 10 tooth sprocket, to 14 tooth on jackshaft to 10 tooth on jackshaft to 60 tooth on rear wheel sprocket. I would like to be able to lift the front a little bit, but I don't want to scare the living daylights outta myself.

Finally got some work done on the front forks, just tacking everything right now until I get it all assembled...
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The Rake is 25 degrees... IMG_0222.JPG

Trail... IMG_0223.JPG

Front view... IMG_0224.JPG
 
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Cpl2442

Cpl2442

New Member
#20
#20
Got more work on the front forks done.The "outside" tube had to be taken to a local machine shop to get bored out to a big 7/8" to allow the "inside" tube to slide snuggly. I think I will drill and tap in some grease zerks into the top of the "outside" tube to get grease inside the homemade shock. I still want to add support to the bottom spring cup, because the way it is now, when pressure is applied will fold the cup. It needs a flat support so it takes the weight evenly all the way around the cup.

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Also, I haven't cut the "plates" on the triple tree to final shape yet, as I don't know how much I want the forks to steer. Is there a common or general amount of degree turn to left and right a bike uses? I'm guessing around 30° each way?
 
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