Hey there guys- this is my 1st post about my 1st ever NSR250 MC21.

I just got a Rothmans replica a few weeks back and have only just had time to 'sort' it out.
It needs new sprokets and and I wouldn't mind if it pulled a bit harder (not concerned about loss of top speed) .Surely between everyone out there just about every combo has been tried- whats a good choice for quicker acceleration not top speed?

Also just say I am cruzing at 80km/h or even 100km/h for a long time (sticking to the limit..) it there a good or a bad constant rev range for NSR's to sit on?

PS bike is bone stock

15/42 or 14/40 would be a good start. If you want pretty wacky accelleration go for 14/42... it's a right laugh!

15/42 will give you 111mph (179kmh) @ 12,000rpm
14/40 will give you 109mph (175kmh) @ 12,000rpm
14/42 will give you 103mph (167kmh) @ 12,000rpm

No reason to keep it bone stock! Wire splice it (and preferably drill 2 x 20mm holes in the airbox lid) and release 50% more power!

I currently run 15/40 with an HRC transmission with a long 1st gear, but 14/40 is still fairly lively. Funniest was 16/38 which gives me a theoretical top speed of almost 142mph! 1st gear is a chuckle round town and away from traffic lights as it will do 67mph!

I think StephenRC45 tried 16/35 on his MC21F3 once for a laugh!

Welcome to the forum Brett!
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Last edited by Andy on Mon Jan 05, 2009 2:23 am; edited 1 time in total

The highest I went was 16/36. It was more scary than funny as its like doing 150mph odd on a bicycle.

I run the same as Andy most of the time 15/40 through a HRC 'box. 15/42 though I believe is the best fun gearing.
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Welcome back Stephen!!!
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Thanks for the suggestions so far-
Andy- I am tempted to splice it but want to get to know the bike 1st , do the gears now thru necessity then still have some power up my sleve for later

I thourght that now that I have an NSR I would stop looking but find myself still trolling the net for another......

You can't get "more torque" by changing gearing, that's not the way the laws of physics work.
Once you're up in the powerband not slipping the clutch there will be zero difference in acceleration.
The only way to make your bike faster once it's on the move, the clutch is fully released and the engine it in the powerband is to increase either the spread of power or power itself.

Acceleration will always be determined by the torque the motor makes at the revs it's running at.
So if we got two bikes runing along side each other, both with identical engines, one running 16/36 thus doing 127kph at 10,000rpm in 3rd gear and another one running 14/44 doing 127kph at 10,000rpm in 5th gear.
When both riders snap their throttles wide open, they'll accelerate at exactly the same rate, click into 4th and 6th respectively at exactly the same time and (presuming the difference in ratio step between 3rd and 4th is the same as the one between 5th and 6th) they'll continue that way until either the lower geared bike is topped out in 6th or neither will accelerate anymore.
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Hey Andy, your MPH to KPH calculation is off a little.

Fontyyy. Your example is correct, as both bikes are running the same ratio. How ever, if you were to dyno both bikes in 4th gear, the bike with shorter gearing, would make more torque.
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You're right, the KMH was based on @10000rpm as in the HRC manuals, but the MPH was calculated at 12000rpm to give the theoretical top speed.

Fixed now.
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80XAR wrote:Fontyyy. Your example is correct, as both bikes are running the same ratio. How ever, if you were to dyno both bikes in 4th gear, the bike with shorter gearing, would make more torque.

Only if it was Torque vs Speed. They would both produce the same when viewed as Torque vs RPM.
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My original question was asked with the knowledge that you can't change the power or torque of the engine with gears, but I can change the way it is put to use via shorter gearing ( just putting my experience with V8 cars to use- I run 4.11 diff gears to make them 'pull' very hard and just wanted something similar with my bike....more fun

Sorry Brett, we all got side tracked.
You you are right.
Shorter ratio will make it accelerate harder with less top end available. Same thing as running a short diff ratio in a car, or low range in a 4wd, mehcanical reduction results in more torque produced at the wheels. The engine however, is still only making the same power and torque.
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I think what Brett wants to know like all other MC21 NSR250 owners is what is the best compromise for sprockets between acceleration and speed.

Is the 14/40 'lively' setup a faster accelerating but generally slightly slower NSR setup than stock?

I am racing my NSR's in interclub in VIC, Australia and I use stock gearing everywhere at broadford, winton and philip island but I know that for some tracks like PI you need more top end speed to get a better lap time??

Is the 14/40 setup quite noticeably harder accelerating than the stock setup??

Doesn't my initial reply answer the question?

Yes, the figures give the theoretical top speed at 12000rpm, but the point of the post was to demonstrate the reduction in overall top speed for the given gearing options, and for someone to make an educated decision based on what the bike will currently pull and/or runs.

I'm not going to sit down and work out what the speed is in every gear for a given ratio - someone else can do that!

You have 3 realistic options, each noted in my first post.
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A long time ago I had a 43t rear made. Now that coupled with a 14 was a bit of a laugh!
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evilhermit wrote:I am racing my NSR's in interclub in VIC, Australia and I use stock gearing everywhere at broadford, winton and philip island but I know that for some tracks like PI you need more top end speed to get a better lap time??

Is the 14/40 setup quite noticeably harder accelerating than the stock setup??

For the final time, changing sprocket ratios DOES NOT CHANGE ACCELERATION once the bike is out on the track in the middle gears. The speed your bike goes from (say) 120-170kph is limited by the power of your engine.

For racing start by gearing so you're completely flat out in top at the fastest part of the track. Quite clearly if you're sat there at max revs for hundreds of yards you're giving time away and risking trouble.
Then if you're stuck of a gear at a problem corner (and don't have other gearbox options) try going a tooth smaller on the rear. See if that makes for a better lap time.
No? Try going down one more, and again and so on.

Or take a little tip from some smart guy I spoke to;

Me "I've got no gear for Park Corner, it's either 3rd and go too slow or 4th, be out of the power and slip the clutch"
Freddy Pett "How about 4th and go faster?"

Genius eh?
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