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

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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Los Angeles, CA.
Maybe @Venice Motor Bikes can have a gander and let me know if there is something to look into.
What you're attempting is very complicated (& not for beginners)... I hope you have some serious skill at fabricating, because there isn't much out there that's 'off the shelf'.

What I suggest is looking at Sportsman Flyers pedal assembly... http://sportsmanflyer.com/shop/parts/sportsman-pedal-crank-assembly-with-reduction-hub/ ... this might work for you!

While you're there, look closely at all his motor mounts & maybe ask Pat (the owner) if he has any spare mounts laying around that might work for you

Good luck & post lots of pictures of your progress! (^)
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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So on paper, the 54t-24t reduction hub can be made at reasonable cost and skill, with optional
machining the teeth off and welding the idler sprocket to the 54t.

If your 19mm spindle cannot be lengthened,
this MIGHT fit:


 
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CrimsonPrince

Member
Mar 6, 2020
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What you're attempting is very complicated (& not for beginners)... I hope you have some serious skill at fabricating, because there isn't much out there that's 'off the shelf'.

What I suggest is looking at Sportsman Flyers pedal assembly... http://sportsmanflyer.com/shop/parts/sportsman-pedal-crank-assembly-with-reduction-hub/ ... this might work for you!

While you're there, look closely at all his motor mounts & maybe ask Pat (the owner) if he has any spare mounts laying around that might work for you

Good luck & post lots of pictures of your progress! (^)
This is what you're trying to replicate,
with the idler sprocket and PB sprockets, nuts, bolts and spacers.


This part is out of stock.


If you like, you can have that barrel-shaped spacer made at a machine shop.
Thank you so much, gentlemen! You guys are an absolute blessing! I'll look into this tonight when I'm off from work.
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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CrimsonPrince shared with me that he is an engineering student at the University of Hawaii.
 

CrimsonPrince

Member
Mar 6, 2020
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CrimsonPrince shared with me that he is an engineering student at the University of Hawaii.

Right, haha. This is a side project very near and dear to me. It will make me the object of attention amongst all of my exclusively bipedal classmates. I also have several girls to impress so the stakes are really high on this one, guys.
:cool:
 

CrimsonPrince

Member
Mar 6, 2020
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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.
Perfect, I am starting to get a better grasp of the design, Sir. Actually, and pardon the suggestions, as I know we've been over this several times, but would the gear ratio still be adequate if we use a 25T sprocket? The reason I ask is because most standard BMX sprockets are 25T. Now here is where it gets interesting: a lot of BMXers are using what's called a guard sprocket, essentially a sprocket with some sort of guard mounted on the outer face of the sprocket to protect the metal from the effects of grind tricks (a stunt involving jumping onto a rail, anchoring oneself with BMX pegs attached at the front and rear axle, and "grinding" down the rail on the axles. Sparks sometimes fly with this metal on metal contact).

I just purchased a guard sprocket online that actually has holes already drilled into it. Please refer to Figure 4 for reference. I could simply take the guard off and have it connect to the 54T that way!

Ok, but we still need a freewheel, correct? I believe you sent me an eBay listing for 3/4" in. bore idler sprocket earlier, so here's what I'm thinking:

On the left hand side of the spindle axle (the right hand side is exclusively for the pedal driven assembly), we have the 6T engine sprocket routed to the outbound 54T, which is then bolted (or welded) to the 3/4" idler, and then the 24T (or 25T, I hope) sprocket is connected to the 54T sprocket via a longbolt. I think that's the way to go about doing it.

I can't help but think, why go through the trouble in mating the idler sprocket, when its that freewheel component we really need. Is there a way to excise that freewheel assembly in the idler and put it into the 54T sprocket?
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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The idler components won't bolt onto the 54t.

The 54t is a thin sprocket with no hub. You need a thick hub to encase the bearing assembly.
It also has a 54mm bore. The idler sprocket has a 38mm bore. What we're trying to do is incorporate the idler sprocket's thick hub and bearing onto the thin 54t sprocket.
The teeth and part of the idler's hub could be machined off for a clean look.
Then it'd be bolted or welded onto the 54t.
 

CrimsonPrince

Member
Mar 6, 2020
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The idler components won't bolt onto the 54t.

The 54t is a thin sprocket with no hub. You need a thick hub to encase the bearing assembly.
It also has a 54mm bore. The idler sprocket has a 38mm bore. What we're trying to do is incorporate the idler sprocket's thick hub and bearing onto the thin 54t sprocket.
The teeth and part of the idler's hub could be machined off for a clean look.
Then it'd be bolted or welded onto the 54t.
Would it be feasible to do something like this:

STEP 1: Order a standard BMX 1.5" idler bearing (inherently has a 3/4" bore!)
1600786579665.png

STEP 2: Order a CNC milled 1.5" clamp adapter:
1600786718086.png

STEP 3: Place the idler bearing inside the adapter and clamp it down on the bearing (rubber/aluminum shim might be needed). My concern is the integrity of the housing around the bearing, but this is probably something that would have to be done with trial and error.
STEP 4: Machine off the 3 extruded bosses on the clamp, and drill out the back end so that bolts can pass through
STEP 5: Match up the bolt holes on the adapter to the 54T outbound and 25T inbound and bolt the whole assembly together

Admittedly, this whole design functions on the basis that the clamp is attached so tightly to the bearing that it has enough frictional force to drive the wheel and not slide. I don't know if I would trust a clamp to provide this kind of integrity, but at the same time, we aren't building for too much torque, so it should hold enough to drive a wheel, no?
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Yes, there will be enough torque to rip things apart, especially if they're shimmed and bolted..

I'd opt for bolting or welding the idler onto the 54t, since it's much simpler and cheaper to facilitate.

The aluminum adaptor doesn't have the option of being welded to ensure its stability.
 

CrimsonPrince

Member
Mar 6, 2020
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Yes, there will be enough torque to rip things apart, especially if they're shimmed and bolted..

I'd opt for bolting or welding the idler onto the 54t, since it's much simpler and cheaper to facilitate.

The aluminum adaptor doesn't have the option of being welded to ensure its stability.
(^)
 

CrimsonPrince

Member
Mar 6, 2020
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Yes, there will be enough torque to rip things apart, especially if they're shimmed and bolted..

I'd opt for bolting or welding the idler onto the 54t, since it's much simpler and cheaper to facilitate.

The aluminum adaptor doesn't have the option of being welded to ensure its stability.
How does this idler sprocket look:


It's big, but gives us a ton of room to work with
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Lol, I didn't realize it was over 8" diameter!

Anyway, any idler sprocket with a 3/4" bore and 80mm(3")or more face diameter would work, and be cheaper too.
 
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5-7HEAVEN

Well-Known Member
Aug 2, 2008
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This is the rear chain drive kit CP is considering.
It should install in an afternoon.

 

CrimsonPrince

Member
Mar 6, 2020
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Ok, so let's plan for the rear mount and then switch to the mid-frame.
This is the rear chain drive kit CP is considering.
It should install in an afternoon.

I've been doing a bit of thinking, and my only issue with the rear kit is that, since it doesn't ship to Hawaii, I'll be paying more for shipping, and ultimately, if I decide to get straight to working on the mid-frame assembly, I could actually be saving a hundred bucks or so. I think it might be financially optimal to actually go straight for the mid-frame, simply because that is my ultimate goal, and I won't have to wait for the rear rack kit, pay extra for an intermediary design, and have to uninstall the engine again to reinstall into the midframe.

Sorry for any trouble! I think I've settled on a 3/4" bore idler! I'll send you a parts shopping list soon!
 

CrimsonPrince

Member
Mar 6, 2020
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HI all!

So to update, I've received a few pieces and have started working on the reduction hub. The challenges that I anticipate are going to be chain alignment with simultaneous engine alignment, which could be a bit problematic, and then finding a tensioner to bolt on to the SEATSTAY (as opposed to the chainstay) to keep the 415 chain connecting the 25T and 50T rear sprockets from rubbing on the seatstay.

I might also need to go from 2.4" wheels to 2.25" because the chain is only like 3 mm away from the wheel, and if it doesn't ride 100% true, then it'll rip through the tire...
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Mount the rear sprocket and hub first.

Align the 25t chainring sprocket with it.

Then determine if you need a tensioner.
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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Do your chainring sprockets have enough "meat" to match the idler hub's 3 bolt holes?

The 25t's bore must be enlarged, so it never makes contact with the 19mm bottom bracket spindle.