670cc V-twin mini chopper beginning plans

xXNightRiderXx said:

The gearing will be the equivalent to having a 60t all the way down to a 32t. The chainrings will be 44t inside, driving the wheel, 38t inside, driven by the jackshaft. The sprockets on the shaft will both be 12t, the one on the engine will be a 15t. I doubt a torque converter will fit on this setup.

With most internal parts being swapped out for high performance parts, I will have a range of 1600 to 8500 rpms, with redline starting at 7500. Power band will be between 2000 and 7000, hopefully. I will need advice on how to achieve this power band. I will have an absorption muffler for noise control, as I would really like to ride this out of my apartment complex. The gearing on the cassette will be a 38t to a 15t. If there is anything you would change to optimize for all terrain, please, feel free to post here.

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The sprockets you're mentioning make no mathematical sense at all. This is junior-high math.
You must know that sprockets' relationships to each other have precise values. These values, when plugged into calculators arrive at specific results.

In other words, 3 x 2 is always 6.

Do you have any idea which sprockets should drive or be driven?

I call BS on your choice of sprockets, cassette, gear ratios and rpm calculations.

Let me work the numbers you provided.

Be back with the discrepancies:

A 60t wheel sprocket has a gear ratio of 24.6:1.

A 32t sprocket has a gear ratio of 13.12:1.

These two ratios should directly relate to your custom 7-speed cassette's 38t AND its 15t gears.

There is no 15t-38t 7-speed cassette available. Ok, yours will be custom-made, right?
FWIW, 14t-34t 7-speed cassettes are readily available.

You have a mechanical disadvantage of your engine sprocket(15t) to jackshaft sprocket (12t).
12t/15t = .8:1.....

Your second jackshaft sprocket(12t) and chainring(38t) = 38/12 = 3.16:1.....

Your intermediate gear ratio is .8 x 3.16 = 2.528:1.....

Your cassette(38t) to chainring(44t) = 38/44 = .86:1.....

So your 1st gear ratio is .8 x 3.16 x .86 = 2.17:1.....

Your clutch will burn out, without your race bike moving an inch.

And THAT'S in 1st gear!

You need 24.6:1.....

Your 7th speed is (15t/44t) x 2.17 = .74:1

It should have been 13.12:1.....

If 1st gear is equivalent to a 60t, then your gear ratio is arrived by multiplying the China Girl's 4.1 gear reduction by 6:1. That 6:1 value is arrived by dividing the 60t wheel sprocket by the 10t engine sprocket (60t/10t = 6.00:1).

Multiply that 6:1 by 4.1 = 24.6:1.....

Correspondingly, a 7th speed of 32t is arrived by 32t/10t = 3.2:1.....

Multiply 3.2 by 4.1 = 13.12:1.....

HOWEVER, if you have a 1st gear equivalent to a 60t wheel sprocket, your 7th gear will NOT be equivalent to a 32t sprocket, as you claim.

Using your 44t chainring and 38t 1st gear gives you .86:1.....
To reach 24.6:1, you get 24.6 divided by .86:1 = 28.6:1, the effective intermediate gear(EIG) ratio.

So 7th gear(15t) is divided by chainring(44t), then multiplied by EIG = (15/44) x 28.6 = 9.75:1.....

9.75 divided China Girl's 4.1 gear ratio = 9.75/4.1 = 2.378:1 or 24t sprocket.
24t wheel sprocket/10t engine sprocket = 2.4:1.....

So if your 38t 1st gear is equivalent to a 60t wheel sprocket, then the 15t 7th gear will be equivalent to a 24t wheel sprocket, not 32t.

Using this forum's gearing calculator at 7500rpm:

First gear (60t) will peak out at 23.6mph in first gear.

Seventh gear(24t) will peak at 60mph.

And that's with your custom 15t-38t 7-speed cassette.....

with an Effective Intermediate Gear Ratio(EIG) of 28.6:1.

With the sprockets you chose, your EIG is 2.53:1.....

Your choice of sprockets need to be jumbled more.

Make it happen!

xXNightRiderXx said:

Yes, this is custom. Where do the chainrings come in? How will they affect the ratios? A 44 driving the wheel, and a 38 being driven by the engine. I want to reduce torque stresses on any parts possible, as much as possible, so I can focus on strengthening the frame and reducing vibrations where necessary.

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Where do the chainrings come in? How will they affect the ratios?

You're kidding, right?

The sprockets on the bottom bracket (BB) are called chainrings.


Ok, look at the BB as a "jackshaft". The 38t sprocket on the BB is driven by the 12t sprocket on your first jackshaft.

The 44t sprocket on the BB drives the 38t on the cassette.

All of this will be clear when you reach high school.

xXNightRiderXx said:

The Night Prowler

Frame: custom made soft tail chopper bicycle with 7 speed freewheel, pneumatic shocks, and seat post suspension.


Engine: 20HP 670cc V-twin Predator

Gearing: pedals and crank arms removed; 38t square taper sprocket on the right, 20t Sq taper sprocket on the left; 38t sprocket is aligned with center of cassette, providing a total of 7 gears in this transmission


Wheels:[front] 700x38c 36 spoke, cross three laced w/ schwalbe CX tire
[rear] 26x3.8 36 spoke, cross four laced, 6-10 speed freewheel and 7 speed cassette and surly black floyd 120tpi 26x 3.8 tire


I've decided that I will build these big bikes for sale, but they must be pre-ordered. The Night Prowler will likely cost me about $2000 total to build, and I charge $10/hr labor.

SAFETY WARNING: This bike will go a top speed of 140 with a 7 speed drivetrain!

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Only a special kind of stupid would mate a 20HP 670cc V-twin Predator
with a bicycle chain derailleur drive train.

The number of miles this combo would theoretically survive would be in the
single digits.

I say theoretically survive because this bike will never be built.

Even Nashmoto's 9 HP KTM 50 cc race bike chews up his 3 speed shifter set up.

SAFETY WARNING: This bike will go a top speed of 140 with a 7 speed drivetrain!

With a 60t first gear and corresponding 24t, your engine must spin 17,700rpm to reach 140mph.

With a 60t and your hypothetical 32t, at 140mph, your engine must spin 23,800rpm.

Will you just stop throwing out BS predictions?

Can someone move this thread to "The Tavern"?

xXNightRiderXx said:

Guys, please keep in mind that I am a bicycle mechanic. I know what every single part is called. I simply need schooling in gearing, as 5-7 so clearly pointed out. There is no need to insult my intelligence, otherwise you are making yourself look like an *******. When I asked where do the chainrings come in, you did not include them in your gear ratios. So, evidently I need at most a 10t on the engine, and a 15t on the left side of the jackshaft, and keeping the 12t on the right side, and switching the chainrings, what ratios and speeds will that give me? I understand now that the speeds I threw out there were for a direct drive transmission, specifically designed to reach those speeds. But I'm also not using an engine with an internal gear reduction, as I'm sure you are aware, so the china girl reference is pointless. Simply let me know what I need to know and don't insult my lack of knowledge in gearing. This is very good to know for my next build, which will be a simple 212.

Insult a guy for not knowing one thing, and he can dis you for not knowing another.

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Feel free to dis me when I BS, seriously.

In my long life, I've only come across a handful of guys who spin big fat whoppers.....and you're one of them. This single post has so many of them, it's hilarious.

You seem like a nice, likeable kid. I'm sorry to disrespect you.

I just can't help myself.

Ok, I promise I'll do my best not to insult you.

FWIW, the China Girl is a reference point in gear reduction.
When you mention 60t and 32t sprockets, what are you referring it to, in perspective?

I always refer my overall gear ratios to what a China Girl rider could relate to.
Example: a 40t rear sprocket chained to a 10t engine sprocket has a gear ratio of 4:1. Combine that with a 4.1 built-in gear reduction of its engine gets you 4 x 4.1 = 16.4:1.....

So when someone who thinks he needs 18.75:1 gear reduction, it'd be like comparing a China Girl setup with (18.75 divided by 4.1) = 4.573 ~ 4.6 x 10 = a 46-toothed rear wheel sprocket.

Consequently, your 60t rear sprocket on a China Girl would be (60t/10t=6) x 4.1 = 24.6:1 gear ratio.

This is a little to much for me to follow, all these numbers and gear ratios just looks like a bunch of crazy chit. Is it all worth it? Ok let me go back to looking at Craigslist motorcycles for sale...

xXNightRiderXx said:

When mentioning the 60t to 32t sprockets, I was referring to the direct drive equivalent of a 7 speed cassette after the jackshaft. Maybe the best way to go is to keep a 1:1 ratio until the chainrings. 12t engine, and jackshaft sprockets, with a 52 outside chainring, and a 44 inside.

Using your math, that would be 52/12 which would give me 4.3333, then it would be 38/44, right? That would give me .863636 then its that times 4.3333 which would give me 3.74. That's first gear, right? I'm trying to understand this, and its hard.

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I have a small 47cc engine. It needs very low gearing for hills and flat ground. We're talking 40:1 first gear ratio with 8th gear of 15:1.

You have a huge engine. You probably need a higher gear, like 10:1 or so. You must calculate it by finding where your peak hp and TQ is.

You have lots of research to do.

You should do what other builders with giant engines do:

Use a single sprocket, one gear ratio with a super strong chain.

xXNightRiderXx said:

For me, being able to reach freeway speeds is worth all the hassle.

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Your bicycle chain will be a hassle with that big engine.......

Custom made means more expense.
Just use motorcycle chain and sprockets that fit.

Why spend $$ foolishly?
Your dreams are for a powerful bicycle with a not-so-powerful engine, when compared to real motorcycle engines.
JMO, it'd be cheaper and easier to buy a donor motorcycle, then transfer everything onto the bicycle.