Engine BMXs Challenge: Series Premier

What Engine Kit should I try working on? (Reference the 21SEP20 update in the Idea List)

  • Rear-Engine [Easier | Reduced Learning Curve | Cheaper]

    Votes: 1 33.3%
  • Mid-Frame [More stable | More Compact | Advanced Experience level]

    Votes: 2 66.7%

  • Total voters
    3

CrimsonPrince

Member
Mar 6, 2020
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Hi everyone, I'd like to introduce myself here by posting this thread concerning my goal to build a motorized BMX bike. You're probably thinking, "motorized BMX", is this guy crazy? I would agree and say yes, but I'm not that crazy. Like, think of all the other crazy things I could be doing, like building an 80cc engine powered unicycle, or riding a non-engine powered BMX. Now that's crazy. I'll update this post with new ideas, objectives, help requests, and pictures as I go along. Please feel free to throw some insight my way! It would be greatly appreciated.

NOTE: This is a collaborative build with @5-7HEAVEN! He is my sensei and I am the novice bike builder in training :cool:


Objective List:
1. BMX is a standard 20" top tube bike. NO modifications will be made to the frame itself.
2., A standard pocket-bike engine will be placed in the mid-frame of the bike. Yes, this means I'll have to get several custom parts made, including a custom exhaust, a custom freewheel crank-arm set, and perhaps even custom mounting hardware.
3. I will try to maintain an 18:1 gear ratio if possible.
4. I would like this build to be helmet worthy

Idea list:
[21SEP20]: Pocket-bike engines can run upside down. This means that I can run the engine upside down and have a left hand driven sprocket. This also means that I can use the bolt holes already machined into the engine for bracket mounts on the flat top-tube as opposed to the bottom down-tube.
[21SEP20]: We are going to have an idler sprocket mated to a larger sprocket to allow for the engine to run without forcing pedal spin (the idler is like a loos axle that, when mated to the new sprocket, will ultimately allow the engine to drive the left hub rear sprocket without spinning the spindle). We have 2 conditions to address now: Gearing Ratio with Engine: Idler : Mated Sprocket: Rear Sprocket ratio and the fact that it might be optimal to use a 25T or smaller sprocket in this assembly due to the design of the BMX frame. For those who know what the standard BMX frame looks like, the chain stay flares outward, which prevents larger sprockets from being integrated into the space that would align directly with a rear sprocket. Using a standard BMX sprocket fixes the issue, but now the gearing ratio might be a bit more difficult to achieve. I haven't even gone and though about the issues concerning matching a chain across all these gears o_O. Check out Figure 3 for reference.
[21SEP20] [POLL]: Thanks to @5-7HEAVEN, I've been provided with an option to set up a rear engine mounting arrangement for the build. There are several advantages to doing this: its going to be cheaper, it's got a reduced learning curve, and is a bit more forgiving in terms of tolerances. However, I fear that stability may be an issue with regards to this, and ultimately, I would want to "upgrade" to a mid-frame mounting style. Essentially, the rear engine design experience would be a bit more comfortable for a novice, but would take time away from focusing on my objective goal, which is to build a mid-frame engine bike. I set up a poll to see what you guys think, and please feel free to leave comments in the thread!
[22SEP20]: This idea came to me just now, but it could be possible to sandwich an idler bearing (no teeth, same as the bearings you find in the bottom bracket of standard BMXs) and "clamp" the 1.5" adapter onto it, machining off the extruded bolt bosses, and sandwiching the outbound and inbound sprockets together, via bolt connection.

Help Requests:
[21SEP20]: Do any of you guys know where I can get brackets for the mounting arrangement as shown in Figure 1? I went to my local hardware store and couldn't even find the right diameter long-bolts to screw into the engine holes itself, so I'll have to travel far and wide to get the appropriate bolt diameter and length to fit the mounting goal shown. Also, another thing is that I want the engine to be slightly offset to the left end of the bike to allow for exhaust pipe clearance and I would like it to be pushed up as close as it can to the front of the bike (in that case, that carburetor will touch the top and down tubes). Tentatively, I'm thinking of using a two-hole strap clamp extended with a mending brace to hold the engine to the top tube. So, as of right now, can anyone point me toward a place where I can find bracket mounts that allow me to maintain this offset AND securely keep the engine in this position, or will I have to get it machined out instead? Also, does anyone know of any anodizing services?

Pictures
Figure 1: Target engine arrangement. Notice that the engine itself is upside down. The end result will have the engine pushed up further towards the bike a few more centimeters to provide clearance for the pedals
IMG_20200920_181726_4.jpg
Figure 2: Top Down view of the engine mount. The brackets would secure the engine to the top-tube. There may be need to secure the bottom half of the engine with another set of brackets to the down-tube.
IMG_20200920_181733_3.jpg
Figure 3: Potential Mounting arrangement for the idler sprocket assembly. This is just a standard 25T that hasn't been mated to an idler (I haven't bought it yet). Notice the native 25T sprocket on the right hand side. Pedals for the win, baby.
IMG_20200921_122013_2.jpg
Figure 4: 25T sprocket with pre-drilled holes that were previously designed to affix a guard attachment, but can be repurposed to accept the bolts that would connect the 25T to the 54T in the fabricated "reduction" hub.
IMG_20200922_015836_3.jpg
 
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5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,542
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City Mill is a few blocks from your home, on Waialae Ave.

Your engine is very light. Put it and the motor mount in your backpack and go there.
Take a couple q-tips and measuring tape with you, to find the proper bolts with correct depth.

I believe you need M5 engine mounting bolts; Google to be sure.

You really need to get a caliper for working on your bike.

Note: I dropped by CrimsomPrince's home on Saturday to check out his build.
I sold him a few boxes of parts cheap to help his build, like 3 pocketbike engines, 5:1 trannies, sprockets, motor mounts, rear racks, exhaust, etc.

CrimsomPrince is off to a good start.
 
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5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Your motor mount is 1/8" steel plate.
You could extend the motor mount by welding on a couple inches of 1/8" steel plate on the right side.
Then use 3-4 pipe clamps to bolt onto that plate.

Because of the extended plate, there will be a LOT of strain on the mounting clamps, so the bottom mounts to the down tube must be strong and triangulated.
 
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5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Be very careful bolting in the mounting holes in the engine.
They are very delicate, and don't have enough meat to repair.

The q-tips help determine bolt lengths, until u get the caliper.

Short bolts strip the engine's aluminum threads.

Bolts that are too long will crack the engine bosses.
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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You'll have to buy or fabricate a wider spindle, to accommodate the 3 extra chainring sprockets on the left side.

Because of the lack of space for a jack shaft, your entire mid frame install depends on it.

That, and the idler bearing/sprocket to rotate on your spindle.
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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148
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19mm? Great!

Then this 3/4" idler sprocket MIGHT work:


You'd have to weld or bolt it onto your 54t sprocket and drill it to conjoin the inboard 24t chainring sprocket.

The advantage of using an idler sprocket is that there should be zero chain/sprocket drag from the driveline while pedalling.
 
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CrimsonPrince

Member
Mar 6, 2020
30
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19mm? Great!

Then this 3/4" idler sprocket MIGHT work:


You'd have to weld or bolt it onto your 54t sprocket and drill it to conjoin the inboard 24t chain-ring sprocket.
Awesome, but we run into a slight issue now with the dimensions of the BMX (if this is even an issue, I won't know up until closer measurements are made). As shown in Figure 3, we are going to have a sprocket attached to the left portion of the axle. I realized (recently) that the idler is actually designed so that our spindle doesn't move when the engine is running, and is only designed so that, once mated with another sprocket, we get a driven sprocket that doesn't move the spindle.
Ok, so now this is where we run into a problem if we are trying to achieve optimal compactness: the BMX chain stay has a bit of an outward rise that might prevent a 54T sprocket from being squeezed all the way down the spindle, This can be mitigated if we use spacers up to the point where the 54T sprocket comes just outside of the chainstay but there is an advantage of using the smaller 25T (or even 24T or smaller sprockets): the chain alignment is near perfect, as in, once we get the proper gearing ratio, mate the idler to whichever sprocket we decide to use (≤ 25T), we wont have to worry too much about chain alignment! I updated the pictures to show what I'm vying for, and also put the consideration in the Ideas list! Thanks again for your time!
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Because you're extending the engine to the left, the outboard 54t chainring should clear.

The left side spindle might grow by 2-3".
 

CrimsonPrince

Member
Mar 6, 2020
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Because you're extending the engine to the left, the outboard 54t chainring should clear.

The left side spindle might grow by 2-3".
Exactly! My question at this point is, how are we going to route a chain from the 54T to the 48T rear wheel sprocket? Won't the chain be a little wonky? If so, is there a way to adjust that?

Another predicament that I foresee in the future is getting matching sprocket pitches so that the mated spindle sprocket and the rear hub sprocket have a chain across them. I'm not opposed to getting a custom built rear hub or mated spindle sprocket, but I also don't want to drop $400 on a sprocket haha
 
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5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Nah, no worries.
The chains will not collide or cross over each other, when spaced properly.

I've run this setup for years.

Use this exercise as a comparison:

Stand up, reach straight forward with your left arm.
Now reach to the rear with your right arm.

Your arms are the chains; one runs forward, the other runs to the rear.

Your shoulders are the chainstays.
Properly spaced by your shoulder width, your left arms/shoulder wlll never touch your right arm/shoulder.



The 6t on the engine connects to the 54t as the outboard chainring.

The 24t inboard chainring is set to perfect alignment with the rear wheel sprocket.

THEN the 54t and the 24t chainrings are bolted to each other, with distancing.

Basically, the 24t is not directly connected to the spindle.
It is suspended in mid-air by the 1" long connecting bolts.

It's like the outrigger("ama") on a Hawaiian canoe.
 
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5-7HEAVEN

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Aug 2, 2008
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The 24t chainring will have the same pitch as the China Girl rear wheel sprocket, and use the same type chain.

Like this one:


Notice that this sprocket has a 38mm(1.5") bore, which easily passes thru your 19mm(3/4") spindle.

It needs to be drilled to accept five M5 or M6 bolts in a 65mm bolt circle diameter(BCD).

A few of us motorized bikers use pocket bike(PB) rear wheel sprockets as chainstays.

Most PB rear wheel sprockets have this BCD, as well as a 54mm inside diameter(ID).

These sprockets accept freewheels.
However, i your case, you don't need these PB freewheels.
 
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