16t to 17t Front Sprocket - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 17 (permalink) Old 05-19-2019, 12:25 AM Thread Starter
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16t to 17t Front Sprocket

Hey all, I just bought a Speed Twin and I’m considering swapping the front sprocket to a 17t. I’m posting here because the Speed share most in common with the Thruxton so I figured I’d get better info here. I went back about 15 pages and didn’t see much info.

I’m just curious about how people who have done the mod feel about it. It’s easy enough to go back but I figured I’d get some opinions on how it elongated the gears and how much if changed the ride.

Thanks!

Last edited by Hybrid81; 05-19-2019 at 08:35 AM.
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post #2 of 17 (permalink) Old 05-19-2019, 03:04 PM Thread Starter
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Ouch. Guess nobody made this swap on their Thruxton. Looks like I’ll be trailblazing.
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post #3 of 17 (permalink) Old 05-19-2019, 04:20 PM Thread Starter
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Found this thread:

https://www.triumphrat.net/water-coo...gearing-2.html

Wondering if everyone is still happy with their choice. I need to decide by Thursday.
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post #4 of 17 (permalink) Old 05-19-2019, 09:23 PM
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Ive not ridden the 900 but will assume it is smooth like the 1200.
We sometimes gear old bikes higher just to reduce vibration. That's not really necessary with the 1200, there is no appreciable vibration at any RPM. There is a power pulse when it's pulling, but we like that, wouldn't want to lose that!
So another reason to gear a bike up would be to increase top speed. It doesn't always work that way, sometimes they go slower. With my 1200 I've had it to 220 kmph on the speedo, lots of speed there.
The last reason I can think of is to increase top speed in the intermediate gears. Gearing it up 7% with the sprocket change will do that for you.
You will also lose 7% of the available torque at the rear wheel in each gear.
Gearing always is a compromise and it's also a bit specific to the individual rider's preferences.
I feel that the Triumph engineers got the gearing right with the bike, but you might like your 900 geared up a little.
Are you ok with 7% less torque in each gear?

7% less on the 1200 still leaves a good amount, but it won't pull as hard as with stock gearing.

Glen
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post #5 of 17 (permalink) Old 05-19-2019, 09:46 PM Thread Starter
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Originally Posted by worntorn View Post
Ive not ridden the 900 but will assume it is smooth like the 1200.
We sometimes gear old bikes higher just to reduce vibration. That's not really necessary with the 1200, there is no appreciable vibration at any RPM. There is a power pulse when it's pulling, but we like that, wouldn't want to lose that!
So another reason to gear a bike up would be to increase top speed. It doesn't always work that way, sometimes they go slower. With my 1200 I've had it to 220 kmph on the speedo, lots of speed there.
The last reason I can think of is to increase top speed in the intermediate gears. Gearing it up 7% with the sprocket change will do that for you.
You will also lose 7% of the available torque at the rear wheel in each gear.
Gearing always is a compromise and it's also a bit specific to the individual rider's preferences.
I feel that the Triumph engineers got the gearing right with the bike, but you might like your 900 geared up a little.
Are you ok with 7% less torque in each gear?

7% less on the 1200 still leaves a good amount, but it won't pull as hard as with stock gearing.

Glen
Thanks for the informative post. I actually have a Speed Twin so itís a 1200. Pretty much an upright Thruxton. I did the same mod when I had a Street Twin and liked the result so I assume Iíll like it on the Speed Twin as well, I was just trying to get some opinions.

Thanks again for your well thought out reply.
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post #6 of 17 (permalink) Old 05-19-2019, 09:53 PM Thread Starter
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Silly question: if it’s a 7% loss in torque per gear, is that cumulative? Meaning 36% less torque overall? That’d be a big loss...
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post #7 of 17 (permalink) Old 05-19-2019, 10:07 PM
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Quote:
Originally Posted by Hybrid81 View Post
Hey all, I just bought a Speed Twin and Iím considering swapping the front sprocket to a 17t. Iím posting here because the Speed share most in common with the Thruxton so I figured Iíd get better info here. I went back about 15 pages and didnít see much info.

Iím just curious about how people who have done the mod feel about it. Itís easy enough to go back but I figured Iíd get some opinions on how it elongated the gears and how much if changed the ride.

Thanks!
I've done the counter shaft and rear sprocket changes a dozen times are or so on the Thruxton R, same motor as the Speed Twin.


I've run 16,17 and 18 on the front as well as three other combinations on the back. The motor will handle them all from your current 16/42 to 18/37. The 17/42 your considering is a minor change that will make cruising a little more relaxed but you won't notice any decrease in acceleration.

The one tooth larger in the front might cause a problem if your chain is brand new and has not stretched enough. Check your chain adjusters. If the 17 won't fit because your chain is not long enough (not likely) you could drop two teeth in the back to a 16/40 combination.
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post #8 of 17 (permalink) Old 05-20-2019, 02:26 AM
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17t/42t will fit a brand new setup just fine.i just did it on my Thruxton about a month back. Third best mod I've done after a Quickshifter and the low clip-ons.

Sent from my SM-G955U using Tapatalk
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post #9 of 17 (permalink) Old 05-20-2019, 03:19 AM
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Quote:
Originally Posted by worntorn View Post
You will also lose 7% of the available torque at the rear wheel in each gear.
I'm thinking more like 3%, as torque is a function of radius, not diameter. Probably hardly notice. No it's not cumulative.
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I'll have you know I come from the best of convict stock.

Last edited by Flash Nick; 05-20-2019 at 03:40 AM.
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post #10 of 17 (permalink) Old 05-20-2019, 11:22 AM
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Not cumulative but Torque is a function of the gear ratio, in direct proportion.
If you gear a given motor to a load at 1 to 1and have 10 ft lbs available then lower gearing by 10% (1.1 motor rev to 1 load rev) , you will have 11 ft lbs available, or 10% more torque available.
Gear ratio is determined by dividing sprocket sizes.
You can use diameters or radii, or the most common, number of teeth, it doesn't matter which, as long as you are consistent ( r/r or d/d or t/t)
Changing the number of teeth of one sprocket 7% changes the diameter of one sprocket by 7% and also changes the radius by 7%, so it changes the gear ratio and torque at wheel in each gear by same.

To be precise, changing to a 17 tooth from a 16 tooth will give a 6.25% increase in gear ratio.
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Last edited by worntorn; 05-20-2019 at 02:39 PM.
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