top speed calculators

Clyde

New Member
#5
Another Jack-shaft gear ratio with tire size calculator = Rawge's Mini Chopper Information Site - Jackshafts

Tachometers?
GO TO ebay, or Amazon.com ... type in "tachometer hour meter" in their search. For about $15 USD there are cheap induction models flooding the market now (12/2010). They need NO other wiring hook-up (or power supply) other than its own single wirewrap to your spark plug.

A single wire from tach wraps 3-4 times around the spark plug wire and senses the electrical pulse. It will work on many 2 stroke and 4 strokes (it's programmable per how many spark plug firings per revolution). Has an Hour Meter on it too.
Cheap, and Chinese, it will only last as long as it's lithium battery works (they say about 4 years, probably less)
They look like this EXAMPLE:

This is just an example I pulled from some Amazon sellers page Amazon.com: Digital Hour Meter and Tachometer with Maintenance Settings: Automotive
 
#7
Since torque converters can do a 1:1 gear ratio, is that equivalent to having the same number of teeth on each sprocket for a non torque converter setup? Therefore, when you have a torque converter and 19 in tires gearing has no effect on top speed? The calculator said 203 mph... Or is that not how it works?
 
#8
Since torque converters can do a 1:1 gear ratio, is that equivalent to having the same number of teeth on each sprocket for a non torque converter setup? Therefore, when you have a torque converter and 19 in tires gearing has no effect on top speed? The calculator said 203 mph... Or is that not how it works?
sorry to bring this back from the dead

the 1:1 happens at the engine and jackshaft input

using a baja warrior for example, stock it has 10 tooth at the engine, jackshaft input is 20 tooth, jackshaft output is 10 tooth, and axle has 50 tooth (for every 1 rotation at the axle, the engine turns 10 times). a torque converter when at 1:1 will act as if the jackshaft sprockets are the same or non existant. (for every 1 rotation at the axle, the engine now turns 5 times)

pretty much can double your top speed but not lose any low end torque like you would if you removed the jackshaft or made the sprockets on it the same.

correct me if im wrong as im new to :scooter: and all their greatness

a stock warrior at 3600 rpm is 20.35mph, with a 1:1 torque converter it would be 40.7 mph, TAVs can get .90:1 making the top speed around 45.2 mph
 
#9
Seeing as though this thread was recently reserected, I'll add my 0.02 cents about buying a tach....
I recently purchased one of the cheap Chinese "knock off" tachs off of EBay for $ 20.00 that looks exactly like the one in Clyde's photo, My experience was that it is a piece of crap (You get what you pay for?) It would not program correctly as per the instructions and the rpm varied significantly (500+) even when holding the throttle constant. When I brought it to the attention of the distributor (T-Motorsports, Chino,CA) they immediately issued a full refund and also said "just keep the tach." I do not know if I got a lemon or if these are ALL junk. I do know that for the next tach I purchase I'll spring for the additional $$$ and buy either the (higher quality) "Tiny Tach" or a "Sendac".
 
#10
anyone ever use a $10 bell speedometer?

I programed my wheel in it with proportions (who knew i would actually USE those).. using the given dimensions (because its meant for bicycles not minibikes) and came out with like 1277 i think it was..

put the magnet on the wheel, had to use a fuel line between the sensor and the forks to get it to reach.

my bike is close- but not in running order right now so i cant test it yet. but i will post and let you know how accurate it is.

until then has anyone ever tried it?:shrug:
 
#11
I tried a bell bike speedo from wallmart didnt work for a motercycle with accelleration the readout was erratic so I returned it maby a mini bike accells slower it may work dont be suprised if it doesent.
 
#12
Here is the formula that I use for speed calculations. It is configured for minibikes with jackshafts but can be modified for just about any vehicle with tires and an engine.

(((RPM / S.R.) / J.R) X T.O.D) X .0029751 = MPH

RPM = engine revs. per min.

S.R. = sprocket ratio (big sprocket divided by little sprocket)

J.R. = jackshaft ratio (big sprocket divided by little sprocket)

T.O.D. = tire outside diameter in inches

MPH = miles per hour

.0029751 is the correction factor to get from RPMs to MPH.


Here is an example of how the formula can be modified for other vehicles. This is 1st gear in my GX200 powered MK8 Rokon:

(((((5000 / 8.0) / 2.0) / 3.0) / 1.23) X 25.0) X .0029751 = 6.3 MPH

RPM = engine revs. per min.

S.R. = sprocket ratio (72/9)

T.R. = transfer case ratio (64/32)

G.R. = 1st gear ratio (60/20)

C.R. = CVT ratio (Comet 20 w/ 7" driven)

T.O.D. = tire outside diameter (Titan 25X8-12)
 
#13
Thanks. Good stuff. I agree that having a calculator on some website is OK but few will go to the trouble of running to a computer and looking it up. It is far better to have a formula on paper in the same place where a calculator is kept in the tool box.

My formula gets about the same answer but is based on the one that drag races tend to use to roughly determine gearing. MPH=[rpm X tire dia.] divided by [Final ratio X 336]. That one tends to be close enough for my purposes.

Often someone will post some numbers about there bike going 45 mph.......then it comes out that the bike has 12.5 inch dia. tire and 11 tooth and 72 tooth sprockets. By flipping things around it is easy to see that the engine would have to be turning better that 7900 rpm. Yes, it is best to have a few notes in the tool box that deal with reality.
 
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