Ok I found this on one of those ask a question sites... yea it is most likely for a Honda CRF50 dirt bike, but it should still have some relevance. Tell me what you think and if this make any sense or is a bunch of hog wash...
What should the gear ratio be on my CRF50 with an 88cc big bore kit?
It has awesome acceleration very peppy low end. The top end is horrible though. What size gears should I get to even this out?
That may be Easiest Bike in the world to re-gear for more top end speed.
Simply "Gear it as High as You Can"
Stock Sprocket set is 14/37. That’s good for almost 30mph.
Biggest Front Sprocket commonly available is 17T, Smallest Rear Sprocket is 32T.
Use a 17T Front x 32T Rear. “That's as High as You Can" That gives you about 42 mph.
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The Stock CDI Boxes on those things are Rev-Limited at 8500 rpm.
Any CHEAP Un-Limited CDI will do fine, $20~30. You don’t NEED a $100+ CDI.
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CRF50 have a relatively LOW RPM power peak at around 6000rpm. That means they pull hard early, then the TORQUE drops off after 6000rpm. This is a BIG part of the reasons behind Your Description of your bike's performance mannerisms.
It Runs out of Breath at 20mph, then just weakly revs-out to 30mph Limit. The 20 to 30 Acceleration is almost none. Then believe it or Not, a Big Bore Kit makes this characteristic even "WORSE" in a way. It AMPLIFIES the impression.
The Bigger Motor gives it almost TWICE the Torque so Zero-to-20 gains a Huge Improvement.
But the Bigger Motor has Bigger Demand for Airflow Volume. So it Runs out of Breathe SOONER and Quicker. By 6000rpm the power peak, Power falls off a cliff.
Huge Rush of Early PULL from 0-20, Then a Rapid Decline in PULL from 20-30.
That Wider Spread in Acceleration Rate exaggerates the seat-of-pants impression and
makes it FEEL Slower than it actually is.
The HIGHER Gearing will put that Big Bore Kit to work.
With engine's 6000rpm Torque peak, Stock Gears quit PULLING around 20mph, and engine quits Revving at 30mph. Game Over
With the 17T / 32T gearing, It PULLS hard up to 30MPH and quits Revving at 42mph
If you want it to PULL harder past 30mph, You will have to install a "Race Cam",
Or RE-Index your Stock Cam, which is FREE and easy to do.
Simply re-indexing cam can extend power peak as much as 1000 rpm or so AND leave enough Post-Peak Power to pull hard Another 1000 revs. That's Pulling Hard to 7500 to 8000 range, which is 35mph to 40mph.
20mph Stock Peak & 30mph Top Speed vs. 35 to 40 mph Peak & 40 to 45mph Top Speed.
NOT Huge Numbers in absolute terms, No. But in Relative Terms compared to STOCK,
It's MORE than HUGE. Not only Actually a Lot Faster, it will FEEL a WHOLE LOT Faster.
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Here's some Numbers on Gearing / Speed.
Not Perfectly Accurate, but pretty Close.
14/37 = 2.64 = 29.75 MPH (STOCK)
15/37 = 2.47 = 31.8 mph
16/37 = 2.31 = 34 .0 mph
17/37 = 2.18 = 36.0 mph
That Range is as High / Fast as you can Go with 37T Stock Rear.
32T Rear is as 'bout High as you can get for Rear Sprocket.
Keeping your Current 30mph top Speed in mind, lets see what that 32T looks like
12/32 = 2.67 = 29.5 mph(Approx Equivalent of Stock speed)
13/32 = 2.46 = 32mph
14/32 = 2.29 = 34.3mph ( Stock Front size) 4mph Gain
15/32 = 2.13 = 36.9
16/32 = 2.00 = 39.3
17/32 = 1.88 = 41.8
Note that I included 2 SMALLER front sprockets in the combos, a 12T & 13T.
Mostly to illustrate that even going from Stock 37T down to a Higher Geared 32T
It's Still NOT so High that you cant reach "Stock Gearing" with simply a Front Sprocket Change.
The Point of that is, It establishes a Range of Gearing Options: From as Low as Stock---what You NOW have to "AS High as Possible" with commercially available gears.
And You can Cover that entire range with the Least expense & effort by simply changing the Front Sprocket.
Assuming You'll never want to go LOWER than what You have NOW.
Using a 32T REAR gives you The Widest range of Options between STOCK and MAX.
You'll Have your original 14.
14 x 32 = 34~35mph, up from Stock 30mph
You Buy a New 17T.
17 x 32 = 42 Mph, "Highest Possible”
Your Original 37T Rear, you already have.
With your new 17T.
17 x 37 = 36MPH
Buy 2 new Gears, a 17T & 32T,
You'll have 4 Combos
14/37 ---30mph(stock)
14/32--- 34mph
17/37----36mph
17/32----42mph
Addition of another Cheap Front Sprocket, You can chose a size that'll go LOWER than what You have NOW or Split the Range of your 4 Sprocket Set.
See the PRACTICALITY & Economy of Using the 32T Rear?
It's just a better Basis to build your Final Drive ratio on.
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Note in the Gear / Speed lists a 1 Tooth change on Front Sprocket is about 2 mph.
Not near enough, nor worth the trouble.
A 35T Rear is most commonly available "Higher Geared Rear".
Actually it's an Oddball, It cannot MAX-out Top Speed potential.
When paired with alternate Front Sprockets, it Drops LOWER GEARED than You Now have. That "Cheats" you out of some Gearing Options when you exclude the TOO LOW combos. Forget a 35T.
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List of "EVERYTHING" you need
1-17T Front
1-32T Rear
1-Unlimited CDI
