Speed and Sprockets

The KMX125 I've recently bought fires through the gears too quickly and has
a top speed of 55mph. I can go from nought to sixth gear in about 10 seconds
and when at top speed the bike feels like it needs another 6 gears.

It has been de-restricted but upon checking the sprockets I've found 12
teeth on the front and 45 on the rear. The proper spec is 14-48 and 16-50 on
the KMX200.

Can anyone tell me which sprocket to change and to what spec to get a better
spread of the power? Is there some maths that can work out the ideal number
of teeth? I'm looking for a better top speed and longer gear changes.

 

dave says...

This may seem like a strange suggestion, but have you considered
resorting to the standard 14-48 gearing? Kawasaki spend millions of yen
each year developing such things, sometimes I just can't help feeling
they know what they're doing.

 

Fair point, but I'm looking for a method to determine the optimal gearing
given that the bike is NOT standard. If it was giving out the restricted
power that it was designed to then the millions of yen solution would be the
logical choice. However, the engine is definately more powerful than the
original spec intended it to be.

 

In uk.rec.motorcycles, dave amazed us all with this pearl of wisdom:

I'd go for a 15 on the front (if it'll fit) and a standard on the back.
That'll give you less acceleration than you've got now but should be
good for 70-75mph top speed at a guess.

 

dave fumbled, fiddled and fingered:

Bigger front sprocket
Smaller rear sproket
either gives "taller/higher" gearing ie more mph for less rpm (slower
acceleration more top speed), and often better MPG

Smaller front sprocket
Bigger rear sproket
either gives "shorter/lower" gearing ie more rpm for less mph (slower
top speed more acceleration) and often poor MPG.

My start point if the bike is derestricted would be to look at the 200's
gearing.

It'll mess it up off road though, cos the low gearing is useful in the
dirt. Gearing, like many things, can be a complex subject to get right.

--
Steve Parry
K100RS SE & F650
and a 520i SE Touring for comfort

(not forgetting the SK90PY)

http://www.gwynfryn.co.uk

 

Thanks for your suggestion.

I've just been messing with some numbers:

Dividing the front teeth by the back teeth gives a ratio:

12/45=0.266 my bike
14/48=0.291 standard 125
16/50=0.320 standard 200
15/48=0.312 your suggestion
14/45=0.311 my suggestion

Looking at the numbers I think I can achieve similar to your suggested
gearing (and close to the 200's spec) just by replacing the front sprocket
with a standard one.

 

In uk.rec.motorcycles, dave amazed us all with this pearl of wisdom:

TBH, I'm just going by what I would've done when I had a DT125. I
changed the gearing round on that a lot and a one tooth up on the front
was the best compromise between acceleration and speed.

 

48/14 = 3.43
50/16 = 3.13
45/13 = 3.46
45/14 = 3.21

I'd try 45/14. Not as high as the 200, but higher than standard. Only
requires a front sprocket too, so saves some cash. If it turns out to be
gutless up hills, buy a 47 tooth rear for 3.35:1 ratio.

HTH

 

Gear it for maximum speed. If it turns out to be gutless up hills,
change down a gear.

 

You've got the general idea right. The only other variable to be
thrown into the equation is chain and sprocket wear. There's a thing called
the polygon effect, which basically depends on the difference to the centre
of a polygon from the vertices (the furthest distance) and the mid-points
of the sides (the closest). If you imagine a length of chain wrapped totally
around your sprocket, that's your polygon. The two different radii are
proportional to the speed the ends and the centres of the links move around
the sprocket. i.e. the greater this difference, the more stress -- *cyclic*
stress -- your chain gets. And, since for every action there is an equal
and opposite reaction, corresponding stress on the sprockets.

Having looked up the figures once, I came to the conclusion that
14T was marginal and 15T better; the incremental benefit of 16T having to
then be weighed against the probablility of needing to increase the rear
sprocket size as well.

This was in a road bike context of course. You may not be
concerned about chain and sprocket life at all. From your figures you are
not going to be able to tell the difference in performance between 14/45
and 15/48 (0.3%!).

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD, DT175MX "You Porsche. Me pass!" DoD #484 JKLO# 003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

There is no "right" gearing, it all depends on where and how who is
using it. That said, get a 14 teeth front sprocket right away, that
12 tooth thing will eat your chain in no time and there is a good
chance that the chain is going through the swing arm protector in
record time.

 

I was going to put my pennies worth in about wear, but you got to it
first. My CG125 comes with a 14 tooth sprocket on the front, combined
with the standard chain and my riding style I got just over 5000 miles
out of the chain / sprockets. With a 15 tooth front sprocket and a
slightly better chain, I'm still ok at over 12000 miles.

 

two points.

where
nF = number of teeth on front sprocket
nR = number of teeth on rear sprocket
nL = number of links in chain

point one.

nR/nF = your overall gearing reduction of the final drive.

there are more than one solution of nR/nF to give the same gearing, eg
15/30 = 17/34, exact same overall gearing ratio.

the bigger the sprockets the faster the chain speed and therefore chain
wear, and you balance that against the smaller the sprockets the faster
the sprocket wear, particularly if your front sprocket is a bastard to
change, go for a larger sprocket as it will last longer.

point two.

never ever ever specify sizes where

nL/nF = an integer (whole number)
or
nL/nR = an integer (whole number)

because if you do the same sprocket tooth will meet the same chain link
again and again and again, and you end up with a chain with uneven
wear, eg tight and slack sections.

generally speaking this means ALL even number of teeth sprockets are to
be avoided.

the above poster mentions 12 and 14 for nF and 48 or 50 for rear

bad

say chain length is 120 links

120 / 12 = 10

so any given front sprocket tooth will meet the same ten chain links
every full chain rotation, this will reduce chain life by as much as
80%

120 / 14 = 8.5714285714285714285714285714286

___much___ better

120 / 48 = 2.5

___really___ bad

120 / 50 = 2.4

even worse

remember, you are looking for a number as far away from an integer as
possible, it's not the absolute size of the number that matters, but
how much non recurring fluff there is to the right of the decimal point
when doing the sums above.
(sometimes the only solution is to use a "half" link to get the chain
length you desire)



note well, selecting larger sprockets will increase overall chain
length as you only have a given amount of adjustment to alter the
distance between the centres of the two sprockets.

HTH etc

 

"dave" wrote in message

Are you sure it has been de-restricted? After I de-restricted mine it filled
the clock on standard gearing. The only restriction is a reducer in the
exhaust.

Tullyhubbert

 

http://www.kaila.net/tl125/gearing.html

HTH

 

In the expansion pipe or the silencer?

I thought the restricted version didn't have the KIPS power valve rigged up?
And smaller jets in the carb?

 

On Thu, 25 Aug 2005 18:13:16 GMT, Whinging Courier

[ of KMX125 sprocket sizes ]

Which is fun on knobbly tyres. The front wobbles all over the place and
scares the children.

 

In uk.rec.motorcycles, Switters amazed us all with this pearl of wisdom:

I had loads of fun on mine, got caked in mud, used to go up the building
site on it, through the woods, over my girlfriend's Dad's garden...

 

"dave" wrote in message

In the neck of the exhaust where it bolts onto the barrel. Its tack welded
in and reduces the exhaust diameter from 35mm to 20mm approx. Its 5 years
ago since I did this but I remember it wasn't necessary to change jets. The
needle clip (in carb) has 3 positions and as standard its in the middle. I
moved it either up or down, can't remember which as the jetting wasn't just
right after removing the reducer.

 

I meant at 75mph on knobblies the front gets very very wobbly.