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how to estimate top speed


Paulwhite
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right there has been a few posts lately askin what gains ppl would expect from changing sprocket sizes so im going to show you how to roughly work it out. (and it is only theoretical estimated top speed) :D

count the number of teeth on each sprocket, so for an example the rear sprocket is 59 teeth and the front sprocket is 16 teeth (original sizes on a xvs 125)

divide 59 by 16 = 3.6875 (round it up to 3.69)

3.69 is the number of times the front sprocket turns to 1 turn of the rear sprocket (called a ratio).

now work out what ratio you would get by changing the sprocket size. so say i want 2 extra teeth on the front.

59/18 = 3.2777777777777778 (round it up to 3.28)

now here comes the tricky bit!

figure out the top speed you have had on the bike (on a flat road) mine was 60mph

so the 1st ratio (3.69) we know gets us to 60mph

now minus the 2nd ratio from the 1st ratio so 3.69 - 3.28 = 0.41

now add that to the original ratio (3.69 + 0.41 = 4.1) we will need this figure in a min.

divide the original ratio (3.69) by the top speed (60mph)

3.69/60 = 0.0615 (dont round it up)

now you need to times that number by how many mph you think you will get with the different ratio till you get that number that i said we would need in a min (4.1)

so 0.0615 X 75 mph = 4.6125 too high

0.0615 X 70 mph = 4.305 still to high

0.0615 X 65 mph = 3.9975 too low but we are close!

0.0615 X 68 mph = 4.182 too high but nearly there

0.0615 X 67 mph = 4.1205

0.0615 X 66 mph = 4.059

right so what we have figured out is that with 2 extra teeth on the front sprocket your top speed would be about 66 to 67 mph which is at least 6mph more than the original! (60mph)

anyone lost? lol :P

and please note this is just an educated guess! the actual top speed depends on allot of factors like wind resistance, how big the tiers are, how much torque and so on.

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now you need to times that number by how many mph you think you will get with the different ratio till you get that number that i said we would need in a min (4.1)

so 0.0615 X 75 mph = 4.6125 too high

0.0615 X 70 mph = 4.305 still to high

0.0615 X 65 mph = 3.9975 too low but we are close!

0.0615 X 68 mph = 4.182 too high but nearly there

0.0615 X 67 mph = 4.1205

0.0615 X 66 mph = 4.059

.

Rather than all the equations to estimate the speed do it this way instead:

MPH = 4.1/0.0615

This will give you the actual speed of 66.666r MPH

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The bike is restricted electronically. I was speaking to a bloke who'se daughter had the bike, and he said that you CAN bypass it by fiddling around with the tronics... but most people settle for a small increase by changing the sprockets and confusing the bike...

I'm changing my sprokets soon and I'll let ya know how it gets on...

Chris

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Rather than all the equations to estimate the speed do it this way instead:

MPH = 4.1/0.0615

This will give you the actual speed of 66.666r MPH

ahh didnt think of that :D cheers

in short hand the equation works ot like this

r/f = h1...............................r = original rear sprocket (number of teeth)

r2/f2 = h2............................f = original front sprocket (number of teeth)

.........................................r2 = new rear sprocket (number of teeth)

h1-h2 = n...........................f2 = new front sprocket (number of teeth)

h1+n = p............................h1 = original ratio

.........................................h2 = new ratio

.........................................n = number 1

h1/top speed = u................p = number 2

.........................................u = number 3

p/u = estimated top speed

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if the top speed of your xvs125 was 60mph paul something is wrong it should be at least 68mph mine and taskmasters when we had one was over 70mph,that was with orignal sprockets and well serviced.

christofesor as far as the bike being electronically restricted thats utter bollox the only restriction the bike has is its weight and the fact its a 125cc.

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christofesor as far as the bike being electronically restricted thats utter bollox the only restriction the bike has is its weight and the fact its a 125cc.

...and the weight of the rider.... :D

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if the top speed of your xvs125 was 60mph paul something is wrong it should be at least 68mph mine and taskmasters when we had one was over 70mph,that was with original sprockets and well serviced.

christofesor as far as the bike being electronically restricted thats utter bollox the only restriction the bike has is its weight and the fact its a 125cc.

when i had mine i didnt keep it up to date with the service as it cost too much i was 17 when i bought it and moved out of my parents when i was 18. i could get it up to 65 at a push (had to rev the bollocks off it) but kept it at 60 max to be on the safe side lol. but it was just an example for the equation :D

and christoff im afraid ur friend is a bit confused! the only possible restriction on a icu or ecu or cdi is a rev limiter and the advance curve but that is only on bikes like the aprillia rs 125, nsr 125, cbr 125 (mainly sport 125 bikes and dirt bikes and 2 strokes) on a cruiser 125 4 stroke there wont be any reason for a slower advance curve, possibly a rev limiter (not sure but it makes sense) so the cheapest and best option is the sprocket sizes :D

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if the top speed of your xvs125 was 60mph paul something is wrong it should be at least 68mph mine and taskmasters when we had one was over 70mph,that was with orignal sprockets and well serviced.

christofesor as far as the bike being electronically restricted thats utter bollox the only restriction the bike has is its weight and the fact its a 125cc.

Had mine a tad over 70 mph with original sprockets, tried up 2 on the front some time ago, lower revs for cruising, but out-accelerated by electric milk floats/horse and cart etc, plus additional loading on engine. Settled on 1 upon the front, nice balance between acceleration and top end speed which is now a tad lower at approx 65-68. Arseing around with jets etc aint worth the effort for the possible benefit, when weighed against possible damage.

Electronics on the 125? Only when the phone is in my pocket.

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  • 3 weeks later...

Was bored so wrote a quick Java Application for this. Not sure how to add an attachment on this forum, so here's the source code.


// Sprocket calculator

class Sprockets
{
public static void main (String[] args)
{

// Check correct syntax at runtime
if (args.length != 5)
{
System.out.println("Syntax: java Sprockets OrigRear OrigFront NewRear NewFront TopSpeed");
return;
}

//Set Variables and calculations
double OrigRear = Double.parseDouble(args [0]);
double OrigFront = Double.parseDouble(args [1]);
double NewRear = Double.parseDouble(args [2]);
double NewFront = Double.parseDouble(args [3]);
double TopSpeed = Double.parseDouble(args [4]);
double OrigRatio = OrigRear / OrigFront;
double NewRatio = NewRear / NewFront;
double n = OrigRatio - NewRatio;
double u = OrigRatio / TopSpeed;
double p = OrigRatio + n;
double NewTopSpeed = p / u;
int RoundedTopSpeed = (int)NewTopSpeed;

//Print Result
System.out.println ("New top speed is " + RoundedTopSpeed + " MPH.");
}
}
[/code]

You'll need to copy and paste to a text editor save it as a *.java file and then compile and run it. It will work on all formats.

If anybody wants the actual executable file for this, let me know and I'll email it over, Java Runtime Enviroment is needed for it though.

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Worked all this out, With my H2, standard 15x47,= [ 120mph] - i"v got 16x47,= [135mph]

But " it pulls 4th best, to about 110. into 5th = cluch slips , and with the wind drag. [ sittup & beg position] hardley pulls any better than 4th,, :huh: The other side of physics, Aero-dynamics.

Conclusion " :rolleyes:

Bigger sprockets for fuel consumption and cruising, B) smaller sprockets for performance and Wheelies :D

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Worked all this out, With my H2, standard 15x47, [ 120mph] - i"v got 16x47, [135]

But " it pulls 4th best, to about 110. into 5th = cluch slips , and with the wind drag. [ sittup & beg position] hardley pulls any better than 4th,, :huh: The other side of physics, Aero-dynamics.

Bigger sprockets for fuel consumption and cruising, B) smaller sprockets for performance and Wheelies :D

WTF,

jeez Blackie, if you take the co-efficient of the hoon-ratio then vector this figure by the 'tangent 3.1415927 it still comes round to a figure of 360 (rounded-up), in a roundabout way - the displacement of the arc effected by missing teeth is negligible, as a propreitery measure you could calculate the circumference of the tangible arc - then divide this by the time spent exceeding 115 mph (in your local area+ co-incidental altitude above sea level), so to sum up , , , , TOKNABOOT ? :huh:

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WTF,

jeez Blackie, if you take the co-efficient of the hoon-ratio then vector this figure by the 'tangent 3.1415927 it still comes round to a figure of 360 (rounded-up), in a roundabout way - the displacement of the arc effected by missing teeth is negligible, as a propreitery measure you could calculate the circumference of the tangible arc - then divide this by the time spent exceeding 115 mph (in your local area+ co-incidental altitude above sea level), so to sum up , , , , TOKNABOOT ? :huh:

You are absalutley spot on Jim thats the way i would do it to !

Im glad ya sorted all that out :D

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  • Moderator

WTF,

jeez Blackie, if you take the co-efficient of the hoon-ratio then vector this figure by the 'tangent 3.1415927 it still comes round to a figure of 360 (rounded-up), in a roundabout way - the displacement of the arc effected by missing teeth is negligible, as a propreitery measure you could calculate the circumference of the tangible arc - then divide this by the time spent exceeding 115 mph (in your local area+ co-incidental altitude above sea level), so to sum up , , , , TOKNABOOT ? :huh:

Oh no no no. That motor of his can take you to hell in a heatbeat just listening to i.t WTF does it matter WHAT gear its in. I rode for miles just listening to it.

If lucifer rides a bike then it sounds like that..... :D

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On a more serious note try this.

Using details from your manual.

Find out the primary drive ratio, this is the connection between the crank and the clutch. Then use the ratio you need from the gear cluster, in this instance top gear. Then from there use the final drive ratio and ultimately figure the rolling radius of the rear tyre.

From this you can work out, with an accurate taco the no bulshit top speed of your bike and see how final drive changes may affect the middle gears too as you can see the speed/rpm overlaps and decide if its a step too far.

;)

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.....

From this you can work out, with an accurate taco the no bulshit top speed of your bike and see how final drive changes may affect the middle gears too as you can see the speed/rpm overlaps and decide if its a step too far.

;)

Mmmmmmm ..... tacos .....

taco.jpg

However, this introduces the 'Chili Factor':

1. Speed increase due to additional tail wind power from the jalapenos

b. Speed increase to get to 'relief area' before the effects of the jalapeno overcome the stitching in your leathers

iii. Speed decrease due to watering eyes causing lack of visibility.

See? 'snot just about gear ratios .....

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and please note this is just an educated guess! the actual top speed depends on allot of factors like wind resistance, how big the tiers are, how much torque and so on.

hmmm i love jalapenos :D:P

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  • Moderator

Mmmmmmm ..... tacos .....

taco.jpg

However, this introduces the 'Chili Factor':

1. Speed increase due to additional tail wind power from the jalapenos

b. Speed increase to get to 'relief area' before the effects of the jalapeno overcome the stitching in your leathers

iii. Speed decrease due to watering eyes causing lack of visibility.

See? 'snot just about gear ratios .....

No i don't think you have that right. Surely the eyewatering would be either during the eating, or more likely during the relief area :o so there would be minimal decrease in speed from the eyewatering.

Did you know it was the pioneering jet designer at lockheed working late after too many taco's farted on a candle and discoverd the afterburner :P

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No i don't think you have that right. Surely the eyewatering would be either during the eating, or more likely during the relief area :o so there would be minimal decrease in speed from the eyewatering.

Beg to differ old boy, although eye watering does take place at both of those occasions, there is also the eye-watering stress of keeping certain sphincter muscles clenched to avoid an 'incident'. In fact, this 'clenching' may also negate the tailwind assist, owing to uncertainty over the gas/liquid/solid state of the pending emissions .....

Just saying, is all.

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i could get it up to 65 at a push (had to rev the bollocks off it) but kept it at 60 max to be on the safe side lol.

I did indeed get 74mph out of my 125 Draggie.

Little bikes have a trick to them - Go up the powerband steadily, but without a massive acceleration.

When you hit the top gear, ease the throttle up and increase your speed gradually.

You should already be at a decent cruising speed, perhaps 60-65. Just slowly add more throttle and allow the bike speed to catch up.

Before long you'll be at motorway speeds and happy as Larry*

Then comes the fun of forward planning - You have to see everything WELL in advance and plan your overtakes accordingly. If you have to brake, it takes a while to get back up to speed. The habits learned here do you massive favours when you come to ride a big bike later on.

*I have a friend called Larry, by the way and he is indeed a very happy guy!!

;)

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I did indeed get 74mph out of my 125 Draggie.

Little bikes have a trick to them - Go up the powerband steadily, but without a massive acceleration.

When you hit the top gear, ease the throttle up and increase your speed gradually.

You should already be at a decent cruising speed, perhaps 60-65. Just slowly add more throttle and allow the bike speed to catch up.

Before long you'll be at motorway speeds and happy as Larry*

Then comes the fun of forward planning - You have to see everything WELL in advance and plan your overtakes accordingly. If you have to brake, it takes a while to get back up to speed. The habits learned here do you massive favours when you come to ride a big bike later on.

I've had mine over 70, and that was with the standard sprockets fitted. I've read all thats been posted regarding sprocket ratios and found that the standards give the best top speed (albeit sounding like a wailing banshee). The ratio mods do indeed bring down the engine revs, but there is a definite additional loading which causes loss of potential max speed. Having said that, I have settled on one up on the front sprocket as I reckon that gives the best all round performance balance (for me).

*I have a friend called Larry, by the way and he is indeed a very happy guy!!

;)

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