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  #1  
Old 03-28-2013, 11:37 AM
ProDigit ProDigit is offline
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Default direct drive an 18T fixie sprocket?

Wallmart sells these $89 bikes with fixie sprocket.
It has 18T.
Anyone tried direct driving this yet?
How's the performance/top speed/mpg?

Thanks!
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  #2  
Old 03-28-2013, 12:01 PM
Nashville Kat's Avatar
Nashville Kat Nashville Kat is offline
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Default Re: direct drive an 18T fixie sprocket?

Probably too much gear for the motor. That small a rear sprocket is a really really big gear.

I've often thought a track cog and lockring would be the best solution- particularly for the kind of riding I do onm the motorized bicycle

but I searched the net a few years back and the largest track cog I could find was I think a 22-

there's also a problem with the lockring then- which is designed to lock on the PEDAL side- and the force is pulling it from the opposite direction then on the motor side.
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Old 03-28-2013, 12:05 PM
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bairdco bairdco is offline
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Default Re: direct drive an 18T fixie sprocket?

the china 2 stroke won't be able to pull an 18t.

i mean, you'll have to pedal it up to 15-20mph for the engine to take over. they don't have enough torque. and it'll never reach it's top speed (which calculates out to 80mph or something.)

on a heavily modified engine with a 28 tooth, my bike is still rideable, but it struggles on big hills and against the wind.
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Old 03-28-2013, 12:12 PM
ProDigit ProDigit is offline
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Default Re: direct drive an 18T fixie sprocket?

actually it's 72MPH, and the 66cc kit should be able to have enough torque to bring it at least to 40MPH.
But I agree, that acceleration will probably be lousy (unless the 66cc kit has improved acceleration over the 49cc).
and if a 36T sprocket gets an engine running at 5k rpm @35MPH, then an 18T sprocket gets it running at 2.5k RPM, with probably a minimum speed of 17MPH @~1250RPM without stalling the engine.

It is indeed too high...

I'm mainly concerned about flat land, no wind, and I never mean to take the engine to top RPM. Just the ability to go fast would be amazing, but great gas mileage, and an engine that can go fast should I need it, would be amazing too.
Though most of the time, I probably will be going 30-35MPH with it...

Are there any sprockets interchangeable with the fixie sprocket mount (or are fixie gears really fixed (like welded) to the rim?

Last edited by ProDigit; 03-28-2013 at 12:17 PM.
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  #5  
Old 03-28-2013, 01:01 PM
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Default Re: direct drive an 18T fixie sprocket?

you might get a cruising speed of 40, but everywhere else, the ride-ability will suffer.

might work good at bonneville, though...

you'll probably need a flip flop hub, but as kat pointed out, the lockring will be going in the wrong direction and would probably thread itself off.

for a lightweight rider ( i'm 135) and a hopped up motor, 28 is about the lowest you can go and still be able to ride comfortably at any speed.

the track cog, flip flop, and left side drive hub ideas have been kicked around the forum for a while now, but no one's been able to make it work, as far as i know.
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Old 03-28-2013, 03:37 PM
ProDigit ProDigit is offline
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Default Re: direct drive an 18T fixie sprocket?

Thanks for the answer! I was desperately looking for the smallest rear sprocket.
I probably go with a 32; here's why:

I did some non-engineering, quick calculations and found the following:
(this is all pretty theoretical, only to have an average 'feel' of what to expect when equipping a bike with the kit)

The max speed I am willing to go on a bike would be 40-45MPH.
Since the 49cc engine, even on scooters, tops ~40MPH, I think I'll be taking the 66c engine.

The average cruising speed I would want to have would be between 25 and 32MPH
In this 25-32 MPH range, I want the engine not to rev too high, so round about 2500 RPM.

Most people have said that their 44T sprocket on a 26" tire gives them 32MPH.
This concludes that a 32T sprocket gives 44MPH; as according to the calculation there are only constants (Pi, Radius, front and rear sprocket teeth, RPMs).
So a 32 sprocket should give ~44MPH.
Since the bike is bigger, and at higher speeds there's more wind resistance, I am expecting to get ~37-38MPH out of the 49cc engine (since the engine just simply can't keep up).

To top 40MPH, I will equip it with a 66cc kit, and I'm still contemplating on the bike (which I will do in another topic).
I presume the 66cc kit should top ~40-45 MPH.

Presuming that the engine does 32MPH @5500RPM with a 44T sprocket, and that the minimum RPM not to bog out would be ~1000RPM; At that speed the bike will go 32/5.5*1 = 5.8MPH.
Converting that to a 32T sprocket, I would be going 5.8*44/32 = 8MPH.

at ~2000RPM the bike should be doing ~16MPH.
~2500RPM it should be doing ~20MPH, which is not the best, but still a great cruising speed.

If I would equip it with any smaller rear sprocket, I might lose top end speed, by over-gearing; so some compromise has to be made.

This ~20MPH speed is about the max speed you're comfortable with pedaling on a beach cruiser.
Beach cruisers have low gearing, good for fast acceleration, low speeds; thus it has the pedal power to go to ~10MPH, after which the engine easily takes over.

In the beginning, the engine does not have sufficient torque to accelerate fast, at which it is possible to further aid the engine with the pedals until ~20MPH (at which the engine RPM is doubled).
After this, every RPM that gets added to the engine, and every moment of time, where the bike is accelerating, less and less the pedals can keep up, until the engine fully takes over.
It is that situation I would want to see in a cruiser bike.

I think the math is pretty solid (with some variables taken from people's experiences (like top speed) etc...

So based upon this calculation a 32T sprocket is about the best you can get for a 66cc engine on a 26 in wheeled bike.

Last edited by ProDigit; 03-28-2013 at 03:40 PM.
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  #7  
Old 03-28-2013, 05:37 PM
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16v4nrbrgr 16v4nrbrgr is offline
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Default Re: direct drive an 18T fixie sprocket?

Bolt the kit sprocket to the 18t. 44t will still be way overgeared on a 700c.
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  #8  
Old 03-28-2013, 07:21 PM
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Nashville Kat Nashville Kat is offline
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Default Re: direct drive an 18T fixie sprocket?

I have this 34 alloy sprocket- very light- on my 66 build

http://www.ebay.com/itm/34-T-Sprocke...-/121074414707

It works well here in flat Florida- and I can easily cruise 25 to 30 with little effort or strain on the motor- I've gotten it up to 35- and that's on an analog speedo that may be reading a bit low since I have 27 x 1 1/4 alloy wheels and not 26".

I've had a 32 in the drawyer for some time- I didn't put that one on originally because I was still riding some in hilly Indiana and hadn't then arrived at 27" on both wheels of the Micargi Panther cruiser frame- and the 32 is steel and heavy-

this 34 alloy sproke has all sizes of drillings and I have one directly bolted to a flat sided flip/flop hub I drilled for the small circle of six holes- the "Disk Brake" pattern-

which would also bolt directly to this spin on freewheel hub:
http://www.choppersus.com/store/prod...W-Disc-BLK-36/

Haven't put it on yet while I get more life out of the fussy 27 rear.
But I started 4 years ago with the kit 44, then got a steel 36, and finally this 34- smooth as silk-
the 50 build has an alloy 39

Last edited by Nashville Kat; 03-28-2013 at 07:25 PM.
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  #9  
Old 03-28-2013, 09:29 PM
ProDigit ProDigit is offline
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Default Re: direct drive an 18T fixie sprocket?

From the responses on the forum it seems that the 34 seems a good option for 26"-ers.
32 might be a little on the low side (slow acceleration, and not the potential to get to top speed.
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  #10  
Old 03-29-2013, 12:01 AM
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bairdco bairdco is offline
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Default Re: direct drive an 18T fixie sprocket?

depends on how fat you are, too. i had a 26" with a 30t that worked great, but also had a lot of mods on the motor.
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