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[Andy]

Hi, not always that easy, depends on where your engine is mounted in relation to the rear sprocket, and chassis type etc

I'll give you an example, I run a 12 front and 78 rear gearing at one track with a 102 chain, the two guys I go with both run the same gearing but a 104 chain as they find the 102 to short, but I find the 104 to long.

Swings and roundabouts

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[M.Smith]

we found, for our chassis with a 12 front sprocket:

88-90 is a 108
86-88 is a 106
84-86 is a 104 etc
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[Road_Rebel]

some times you may want your engine in different posistions. like further forward 2 try and keep over steer to a minimal (well try) and to the reer for oposite effect. its not major adjustments (it could be if ur chassis has 2 really long rails) but these expert racers SUPER FINELY tune ther karts right down to the floor
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[jacksi]

what about a front 11 with a rear 86 87?
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jacksi racing
malta

[Aussie Bruce]

With you guys using sprockets of upwards of 80 teeth, where are you racing ? With an 11 front pinion I sometimes use a 78 sprocket if there are one or more good straights, but the max for acceration I've used so far with 11 up front is 80 teeth and that was on a fairly tight circuit at Muret, Toulouse. Lavelanet, France, 1,4km, with 1 really good straight and one shorter one, the winner of the Rotax semis for the Bridgestone Cup used a sprocket with 76 teeth.

86, 88, 90, ........ are you doing hill-climbs or something ?
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C'mon Aussie, c'mon

[Andy]

Hi, Most people run a 12 at the front rather than an 11, the engines seem to prefer the 12 and also I don't think you can get the 11 anymore (although I could be wrong)

[Aussie Bruce]

OK guys, I've figured it out ..........

with a 12 tooth front pinion and an 86 tooth rear sprocket you will get a maximum speed of 109.26 kmh.

With an 11 front pinion and a 78 rear sprocket you will get a maximum speed of 110 kmh.

So what's a bit less than 1 kmh here or there over a tight course ??!!

With that in mind, and thinking about the acceleration out of a corner so the guy trying to pass gets left in the slip-stream ............................ ?

Let's here it guys ..................

Bruce
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C'mon Aussie, c'mon

[Nick]

[Aussie Bruce]

Hi Jasper,

With a 12 front and 78 rear at 15,500 rpm and tyres with a circumference of 87cms (normal), the max speed would be 124.48 kmh or about 77 mph - enough to get you booked if you miss the end of the playground and end up on the street.

I think it's sometimes better to stay with the 11 tooth pinion in front as it could let the motor run a little more freely and thus give it a bit better acceleration.

But ......... I may be wrong on that 'cos even basic calculations on this sort of effort are way beyond my scope of capabilities - I'm having enough trouble trying to remember how many laps there are left in the race so I can plan better as to which end of the result list I'm going to get to first.

Cheers

Bruce
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C'mon Aussie, c'mon

[Nick]

Cheers for that Bruce.

[Aussie Bruce]

Hey Jasper,

Had a few words with some "experts" over here today (Gotta be careful of that word "expert" - x represents the unknown and a spurt is a drip under pressure .... !) .....

Best Rotax gearing : 12 pinion up front and minimum 80 rear. We race on a fairly technical track of almost 1 km here in Carcassonne and the top Rotax pilots all use either 80 or 81 sprockets on 12 pinions. The middle of the fleet guys are using 82 and 83 sprockets.

Word has it that if you use any bigger you will sacrifice speed for acceration on no matter what track ........ unless it's got a 1 in 3 uphill gradiant for at least 50% of the course.

I checked at Lavelanet where they always have the French semi finals of the Rotax bridgestone Cup - 1.4 km, 1 quite long straight plus another not so long leading into a very fast corner - best Rotax guys use 79 here and most still use 80.

New calculations : 12 pinion, 80 rear sprocket, 13,800 rpm (tops for Rotax) = 108.05 kmh max.

Please also take into account windspeed, shape of the helmet (or head), hand-signals (or gestures), type and number of spectators, .........

As my X-30 is in several thousand bits at the moment getting ready for next season, I had my very first run in my wife's Rotax with some reasonably worn Vega greens on an old SR3 Sodi chassis. It also had a 78 sprocket with a 12 tooth pinion which (refer to above) is way too big to be fast. However, I did manage to get the time down to 43.89 seconds on the 1km track which is 9/10ths of a second slower than my best time in the X-30, so I'm quite chuffed. I'm changing the sprocket for an 81 first thing tomorrow morning (8am damn) because we have club races all morning.

Cheers

Bruce
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C'mon Aussie, c'mon

[jacksi]

hey aussie i am not exact on the distance but i think it is a little bit less than a km, but the track is verry tight and there are some verry twisty fast corners so it is a quite low track and you have one particular u turn that you dont even have 4 metes on each side to turn so practically you have to turn360 degrees in a flash. i am now using a 11 front and a 86 rear revving up 13900. can you find out for me the best gearing and chain lenght please once again the track is verry fast and tight so acceleration is vital.
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jacksi racing
malta