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How are transmission ratios calculated...

This is a discussion on How are transmission ratios calculated... within the Grom Performance forums, part of the Honda Grom Garage category; Hi.. im not completely in the dark on this one but some light would help.. So i have put a 14t front sprocket on the ...

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    How are transmission ratios calculated...

    Hi.. im not completely in the dark on this one but some light would help..

    So i have put a 14t front sprocket on the grom and it feels great for acceleration ! it goes through the gears and places the change inside the rev-range where the power is...
    have the speed healer and in 4th gear its revving hard to get to 90km/hr now.

    question... the standard ratio in first is 2.5.. which im guessing means 2.5 turns of the crank gives you 1 turn of the front sprocket... or does that mean 2.5 turns of the crank give 1 turn of the rear sprocket/ wheel on standard 15 f / 34r gearing?

    what i am liking is the low down 1st and 2nd pull as i live in a hilly area but would still like to get to 100km/h without having to sit on redline.. so id like to calculate the final drive of 1st with a 14t.

    so i guess how do i calculate a final drive for 1st , 2nd 3rd , 4th whilst having the 14t sprocket on??

    I am thinking about asking takegawa to custom make a 5 speed transmission .. which would just be the same as the standard 4 gears but a tall 5th which would get it to the original 0.923 final drive with a 14t front sprocket on.
    I love my grom

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    you are forgetting to take in the primary gear ratio being the crank gear to the clutch gear. I don't know what that is off the top of my head. best thing to do is rotate crank 10 times in 1st gear and count rotation of the rear wheel. subtract the 1st gear ratio and the final drive ratio and there you have the primary ratio, but im sure its listed in the specs somewhere.
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    Last edited by CiscoSanJose; 01-09-2017 at 10:21 AM.

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    simple example shown

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    changing final ratio's does not change any of the ratios inside the motor. The overall ratio has changed but the interal ratio split between gears is exactly the same.

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    my reply was in response to this part in particular:

    question... the standard ratio in first is 2.5.. which im guessing means 2.5 turns of the crank gives you 1 turn of the front sprocket... or does that mean 2.5 turns of the crank give 1 turn of the rear sprocket/ wheel on standard 15 f / 34r gearing?
    Too many toys? nah. 09 Ducati sport classic, 74 rd350 LC hybrid, 73 RD350, 73 Kawasaki S2a, 67 Kawasaki C2TR 120 Roadrunner, 71 Kawasaki G3SS powered by a CR85 liquid cooled lump, 74 Moto-Morini 3 1/2 strada, 84 Yamaha RZ350, 1962 Ford falcon, 76 Ford maverick, very early 1970 non-us Spec Yamaha R5, 89 Yamaha DT50 bought NEW in 2015, 2015 KTM 390 Duke, 72 yamaha CS5.. . oh.. and 2014 Honda Grom with lots of mods.. (some useful, most not)

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    Don't worry about figuring it out, someone else already done it for you.

    Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator

    select grom/msx125 from drop down menu.

    youll be disappointed when you work what gearing you need to do 100km/h at a reasonable rpm, stock the engine will never pull. With 15/34 stock gearing your doing 7639rpm to do 100km/h. With your 14t front sprocket 8184rpm. To get it down to 7000 you'll need 16/33 which the motor stock won't pull with any type of incline or wind. With a temp gauge I found the engine run cooler pulling easier gearing at high rpm over hard gearing at low rpm to achieve 100km/h.

    Juls
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    Quote Originally Posted by Juls View Post
    I found the engine run cooler pulling easier gearing at high rpm over hard gearing at low rpm to achieve 100km/h.
    This right here. Exactly what I figured but I didn't have the hardware to verify. Thanks for that comment.
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    Ok so i have done a load of calculations and figures out the takegawa gear box will be sweet with stock sprockets except for 1st gear where the fd is 2.333 which is taller than stock! (2.50) i would need a special cog made to give me a 2.676 final drive in first. I dont know much about teeeth mesh in a gear box but if i had a gear made.. Would it be as simple as putting it on? Or is there tolerances that would be thrown out?

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