ENGINE BMX 70% done

[CrimsonPrince]

Special thanks to @MrKitty for the absolutely beautiful 3D printed engine mount (in orange). After fitting it on, I realized I need to make some modifications to the bore holes and will have that addressed shortly. If you all are interested, I am keeping a total compendium of images on the main post.

Here is a bid question for anyone willing to answer: I am using neoprene rubber as a "shock absorber as a layer underneath the orange 3d printed mount. I think it might melt, so ill get silicon lining instead. If you guys recommend against it, I can just go bare contact between top tube and mount. What do yall think

Some things I still need work on:

1. Readjust the bore holes on the mount (and buy sized bolts to fit the engine mount unit more precisely)
2. Fix up the fuel line properly (I only have the basic polyurethane lining, need a downsized tube for fixing it to the carb valve
3. Develop a way to return the overflow valve back to tank
4. Bore the reduction hub sprockets
5. Model a new bottom bracket clamp
6. Configure the clutch assembly. I think I'll only have room for one brake

Thank you all so much for your help! I'll release more pictures as I progress forward with the build. Cheers.

 

[5-7HEAVEN]

Why do you only have room for one brake?

 

[CrimsonPrince]

Why do you only have room for one brake?

Hi there! Hope you've been well. I tried out both throttle and clutch drive, and I like the stability of clutch drive. I think I can get around this by actually using a thumb throttle and them allowing myself two brakes. My biggest concern is that the heat from the engine might compromise the integrity of the rear brake line, making it more probable to snap....

 

[5-7HEAVEN]

Hi!

You don’t need a lever for clutch drive; it’s automatic.

You can insulate the brake line with neoprene fuel line, if needed.

They also sell brake lever which can actuate both brakes simultaneously.

You could also use a twist throttle, like on pocket bikes.

Always use two brakes.

 

[5-7HEAVEN]

I noticed in one of your pictures that you have the gas tank reversed.

Due to the bike’s top tube angle, ya might have fuel delivery issues.

 

[5-7HEAVEN]

To return overflow, drill an undersized hole in the lower section of the tank’s filler neck.

Squeeze the overflow fuel line through, and secure it in the neck with a clamp.

 

[CrimsonPrince]

To return overflow, drill an undersized hole in the lower section of the tank’s filler neck.

Squeeze the overflow fuel line through, and secure it in the neck with a clamp.

Thanks so much, Sir!

 

[5-7HEAVEN]

Make sure u drill below the cap.
The cap is very long.

 

[5-7HEAVEN]

Take your carb to a small engine repair shop.

They should have the correct fuel iine.

 

[CrimsonPrince]

@5-7HEAVEN

I actually thought about purchasing the transmission reducer for this setup (the gearbox one, not the open gear one). This way, I automatically get 3:1 gearing, and I can use smaller sprockets on the spindle "jackshaft" (which is better because the 54T is huge!). Oddly enough, I thought of getting a sprocket for the rear hub as well, and simply drilling the holes. If that were to be the case, then I could have One 11 Tooth T8 transmission sprocket going to a 44 Tooth T8 outbound sprocket on the jackshaft. This would be bolted to another, 44 Tooth T8 sprocket, which would then be chained off to a 56 or 64 Tooth rear sprocket.

The gearing ration is thus (3) * 4 * (56/44) = 15.27, for a 56 T rear sprocket. For a 64 T sprocket, its around 17.45

I know we are trying to hit 18:1 gear ratio, but do you think I might have any trouble going up hills and such? I would like a little bit of *oomph* on the acceleration as well, lol!

At any rate, thanks for your time, Sir

 

[5-7HEAVEN]

What size rear sprocket do you have now?

Try to fit the largest rear wheel sprocket and the smallest inboard at the bottom bracket(BB).

Consequently, this allows you to use a smaller outboard sprocket at the BB jackshaft.

If you use #25 engine chain and 54t outboard sprocket,
the outer diameter of the outboard sprocket reduces from 5.4" to 4.44".

However, the smallest #25 engine sprocket might be 9 tooth.

Using #25 outboard sprocket instead of T8F sprocket would
reduce gear ratio from 9:1(54t/6t) to 6:1(54t/9t).

That'd make it harder to lower gear ratio closer to 18.75:1.

FWIW, don't worry about more top-end speed.

You'll have MORE than enough, even with 20:1 gears or lower(higher numerically).

Good luck.

 

[CrimsonPrince]

What size rear sprocket do you have now?

Try to fit the largest rear wheel sprocket and the smallest inboard at the bottom bracket(BB).

Consequentially, this allows you to use a smaller outboard sprocket at the BB jackshaft.

If you use #25 engine chain and 54t outboard sprocket,
the outer diameter of the outboard sprocket reduces from 5.4" to 4.44".

However, the smallest #25 engine sprocket might be 9 tooth.

Using #25 outboard sprocket instead of T8F sprocket would
reduce gear ratio from 9:1(54t/6t) to 6:1(54t/9t).

That'd make it harder to lower gear ratio closer to 18.75:1.

FWIW, don't worry about more top-end speed.

You'll have MORE than enough, even with 20:1 gears or lower(higher numerically).

Good luck.

Sounds great! Just a quick question, Can I leave the overflow valve from the pumper carb unattached from the gas tank? I was thinking about putting a little bag there and collecting the overflow. I don't know the mechanisms of action for the fuel flow through the carb so Im unsure if it is released with high flow or simply effuses only when the primer bulb is being pumped. Would you mind explaining how that overflow valve should be managed if I don't choose to drill the gas tank? (I may not need to drill as I've come across some siphoning caps that might do the trick!)

 

[5-7HEAVEN]

You should be able to have a fuel return reservoir, if that’s what you want.

It’d be simpler to rout excessive gas to the tank.

Managing fuel overflow?
Empty the return reservoir into the gas tank, as needed.

 

[CrimsonPrince]

You should be able to have a fuel return reservoir, if that’s what you want.

It’d be simpler to rout excessive gas to the tank.

Managing fuel overflow?
Empty the return reservoir into the gas tank, as needed.

Sounds good!

 

[5-7HEAVEN]

It'd be much simpler to connect the return line to the tank.

The carb is literally right next to the tank.

Make sure the tank's valve is mounted towards the rear,
or you'll have fuel delivery issues, as the fuel level drops.

 

[5-7HEAVEN]

I just realized that because you're using 20" wheels(not 26" ones),
your gear ratio can effectively be raised to 14.4:1

Another mistake I made is that there ARE 6-tooth #25 engine sprockets available.

Sprocket - #25H Pinion, 6 Tooth - 10mm / Pocket Bike, ATV, Dirt Bike, 47cc/49cc | eBay

Using #25 sprockets of 54t/6t from engine to jackshaft, that's 9:1.....
Switching from T8F to #25 drivetrain will shrink your outboard 54t sprocket's diameter from 5.4" to 4.44".

You just need 1.6: additional gearing from inboard jackshaft sprocket to rear wheel sprocket,
because 14.4/9 = 1.6.

You need an inboard sprocket with a large enough diameter to
bolt to the outboard sprocket.

Using your 25t BMX inboard sprocket and a 40-tooth rear wheel sprocket gets you 1.6:1 ratio.

40 Tooth 415 Chain Wheel Sprocket For 50cc 60cc 80cc Motorized Bike | eBay

9 x 1.6 = 14.4:1.

With a 20" rear wheel, that's equivalent to 18.75:1 and a 26" wheel.

I sold you 54t sprockets and chains in both #25 and T8F configurations.

You just need to buy the #25/6t engine sprocket and the 40t sprocket, then use smaller #25 sprockets.

Your pocket bike engine and the #25 AND the T8F pocket bike chains and sprockets were literally made for each other,
so should be very compatible.

If you have a larger rear wheel sprocket, that's fine.

It'll help you motor out of your steep driveway, and that first traffic intersection by your home.
(I visited the OP twice, and dropped off engines and sprockets at his home.)

Good luck, and Happy New Year.

And let me know when the "longan" in your neighbor's yard are in season.

 

[5-7HEAVEN]

Did you ever lengthen your BB jackshaft?

 

[CrimsonPrince]

Did you ever lengthen your BB jackshaft?

Nope, didnt lengthen it, and I may not have to! The pedals clear the engine with about 3-4mm clearance on each side if I seat them correctly! Unfortunately, not the safest thing to have the crank hanging off at the far end, lol

 

[5-7HEAVEN]

Nope, didnt lengthen it, and I may not have to! The pedals clear the engine with about 3-4mm clearance on each side if I seat them correctly! Unfortunately, not the safest thing to have the crank hanging off at the far end, lol

Maybe shorter crank arms would clear better.

When you motorize a bike, pedals may become embellishments.

 

[5-7HEAVEN]

There are offset crank arms on the web, which give substantial clearance to both sides.