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Discussion Starter #1
I find the Speed Twin pretty grunty down low, so much that I never use full throttle in the lower gears. I wouldn't mind going a bit taller in the gearing and spreading out the gearing a bit. However I don't know if it's even possible, some bikes will detect a change in gearing and throw a CEL and/or disable ABS/TC.

Has anyone tried it on the Speed Twin? I'm thinking a 16/37, just replacing the stock rear sprocket with one of a T120.
 

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sorry double post
 

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There's no issues with ABS/TC or a CEL.
Not sure if the Speed twin has the same gearing as the Thruxton (42/16), I found it really short especially in the lower gears and changed to a T120 17t front sprocket but going from a 42t to a 37t rear will make a bigger difference.
Hopefully you'll have enough backward adjustment and won't need to shorten the chain.
 

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Discussion Starter #4
Yep, it's the same 16/42 gearing. I thought of the 17t front sprocket, but with the 102 chain it gets a pretty awful hunting ratio and it will eat chains and sprockets. 16/40 is closer to what you have in your bike, but the 37t would be more "convenient" as I can get it at the stealership. I've noticed it doesn't really get past 205km/h, where power peaks, making the gearing 16/39t would put peak power right at 220km/h (where the speedo tops out), but I'm more concerned about dropping the RPMs a bit on the highway.

How do you like the taller gearing, did it improve the mpgs? I wouldn't mind stretching the 14.5 liter tank a bit further.
 

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I don't pretend to fully understand it but I used GearingCommander.com and their calculations have the 17t far better than the standard 16t for same tooth/same link contacts, 4X better.
According to gearingcommander and triumph parts its a 100 link chain not 102, maybe that's why your calculations are different.
I don't know the difference in fuel consumption but it can't hurt.
Mines currently averaging 65mpg, according to the readout anyway.
I changed mine soon after buying it as I found the lower gears especially really short, although I hadn't adapted my riding style from my Daytona and was hitting the rev limiter instead of using the midrange.

With the decat and remap especially it can definitely handle taller gearing and I may still go down at the rear, maybe halfway to a 39t as it looks good on the wear figures, way better than a 40t.
 

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I went to a 39T on the rear of my 2017 T100, and it brought a useful improvement. The bike is a good few hundred RPM lower at open road speeds, and each gear is usable for longer before needing to shift up.

There's no way I could have stuck with the original chain though, and took out a couple of links and adjusted it almost all the way forward.

Kevin
 

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Discussion Starter #7
I don't pretend to fully understand it but I used GearingCommander.com and their calculations have the 17t far better than the standard 16t for same tooth/same link contacts, 4X better.
According to gearingcommander and triumph parts its a 100 link chain not 102, maybe that's why your calculations are different.
I don't know the difference in fuel consumption but it can't hurt.
Mines currently averaging 65mpg, according to the readout anyway.
I changed mine soon after buying it as I found the lower gears especially really short, although I hadn't adapted my riding style from my Daytona and was hitting the rev limiter instead of using the midrange.

With the decat and remap especially it can definitely handle taller gearing and I may still go down at the rear, maybe halfway to a 39t as it looks good on the wear figures, way better than a 40t.
The Thruxton has a 100 link chain, Speed Twin has a 102 link, thus the Speed Twin has 15mm longer wheelbase. Unfortunately that means that a 17t with the 102 link chain will have a hunting ratio of 1 (17 / 102 = 6, every 6 turns of the front sprocket equals one full chain revolution, as it is an integer that means a link will always come in contact with the same tooth on the sprocket). I'm in the same boat as you, I keep running into the rev limiter as I'm used to engines that rev past 10k, and it kind of unnerves me to see the tach past half way while riding on the interstate.
 

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Sorry, I didn't realise the swingarm & chain length was different on the speed twin, I thought you were saying the 17t was a poor choice for MY bike, and I was sure I'd checked it on gearing commander.

It does take a bit of getting used to if like me its your first twin, especially a low revving one. My Daytona revs to 14.5k and I still ride them both so its a big change for me, first few rides on the TTR I was hitting the limiter regularly but now i'm used to were the bulk of the power/torque is I rarely go over 5.5/6k.
Whereas my Daytona feels like its screaming now just pottering around and I've searched for 7th gear a few times.:)

I think I'd go aftermarket if I was changing the rear on mine rather than one off a T120, the steel sprocket looks nasty after a few of our winters although I'm sure its different in Mexico.
If you change the front though definitely stick to OEM for the rubber damper.
 

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Discussion Starter #9
I haven't got used that much to the low revving twin, but when I get on the Daytona after riding the Speed Twin I feel like the clutch is slipping... Lots of revs but it doesn't pull as hard! I'm also never going for the redline on the Daytona, I usually shift at 10 or 11k, which leaves me a lot of room when I go into a corner so I don't have to shift mid corner; on the Speed Twin if I do that at 5k rpms I'm hitting the rev limiter mid corner. I've noticed that there is a vibration at around 6k, getting used to watch out for the rev limiter once I get past the vibration.

That being said, I'm enjoying the big twin a lot. The power is instant and you don't have to be breaking the speed limit to get into the powerband, it's much more usable and fun.
 

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Hi Arawn, I put Bonneville T120 sprockets on my Thruxton R for the same reason and I'm ecstatic about the result. The process is identical for the Speed Twin. I put a video together on it which you might find useful:
 

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Hi Arawn, I put Bonneville T120 sprockets on my Thruxton R for the same reason and I'm ecstatic about the result. The process is identical for the Speed Twin. I put a video together on it which you might find useful:
Must admit i'm curious how my thruxton R would feel with the full on Bonneville gearing, i'm already running the 17t front which I prefer over the 16t but just wondering if going all the way with another 5 teeth off the rear would change the character of the bike too much or blunt the acceleration. A bike with so much torque shouldn't need such short gearing though and mines been getting more vibey lately over 4k so this would help.
I think i'll definitely try it next year for the sake of a £20 sprocket, and if its too tall I can go back to my standard 16t front for a sort of halfway house, although that will be pushing the limits of the adjustment.
 

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I've been running 17/40 for a few thousand miles. It works great, and feels like the way it should have been from the beginning. No downside on back roads, and makes it far better on the highway.
 

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Be wary of changing the wheelbase with change of sprocket size, it might adversely affect the handling. I went 2 teeth up on the rear on my last bike and the cornering suffered - when it came time to change the chain, I added a link and the improvement in handling was instantly noticeable
 

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I've been running 17/40 for a few thousand miles. It works great, and feels like the way it should have been from the beginning. No downside on back roads, and makes it far better on the highway.
Sounds like a good "halfway" measure but the same tooth/same link number is really bad with a 40t rear, if I find the 37t gearing too tall I'll put the 16t back on the front, which is almost identical to 17/40 gearing.
Be wary of changing the wheelbase with change of sprocket size, it might adversely affect the handling. I went 2 teeth up on the rear on my last bike and the cornering suffered - when it came time to change the chain, I added a link and the improvement in handling was instantly noticeable
Not sure I'm skilled or sensitive enough to notice the difference to be honest, its nearly fully forward at the moment with the 17t front, so shorter wheelbase, does that make it less stable and turn quicker?
Can't say I noticed any difference, how did your cornering suffer?

The 37t will lengthen it back again, about 20mm longer than the stock gearing, not a massive amount on a 1415mm wheelbase, again I doubt I'll notice, even when I tweak the suspension a rarely notice much difference in cornering, usually just the ride quality.
 

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Discussion Starter #15 (Edited)
According to gearincommander it's about 3.7mm difference from going to the 17t sprocket, not worth worrying about. Changes in wheel base are much more pronounced when messing with the rear sprocket, for example, if I were to change to a 37t on the rear the rear wheel would have to be moved back 21mm.

Edit, let me rephrase that:

The Speed Twin has a 102 link chain, with a slightly longer wheelbase than the Thruxton. 17/37 gives you a perfect hunting ratio with a 100 link chain, with the 17/37t set the Speed Twin can run a 100 link chain with an almost identical wheel base to stock.

Doing the 17/37 swap on a Thruxton, which already has a 100 link chain, would give you about the same wheel base as a stock Speed Twin.

I'm thinking of waiting until the factory set wears down to replace it with a 17/37 kit from a T120.
 

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I'm pretty sure that, with 17/40, my wheelbase is about 1/2" shorter (probably 5/8, as that would be a link shorter) I changed the chain too, and I think it's a couple links shorter. I can't recall in great detail, as I work on a lot of bikes. I do know the wheelbase is shorter, and at 58" stock, that's not a bad thing.
 

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Can't say I noticed any difference, how did your cornering suffer?
Just didn't feel as planted in the corner - bike was an XSR900 (with suspension upgrades). Not a dramatic difference, but noticeable - similar scale of effect to changing a worn front tyre for a new one
 

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Discussion Starter #18
Well, I guess that can of worms has been opened... Wheelbase is not the only thing that is changing, you're also changing the rear ride height as the swingarm is not parallel to the ground (it will be at some point of the suspension travel) and weight distribution. But there is no universal right or wrong setup, different riders and riding styles will prefer different setups. As with any change on the bike, you've got to test it to find if you like it or not, take it easy after making a change as you've got to re-learn how to ride that bike.
 

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Well, I guess that can of worms has been opened... Wheelbase is not the only thing that is changing, you're also changing the rear ride height as the swingarm is not parallel to the ground (it will be at some point of the suspension travel) and weight distribution. But there is no universal right or wrong setup, different riders and riding styles will prefer different setups. As with any change on the bike, you've got to test it to find if you like it or not, take it easy after making a change as you've got to re-learn how to ride that bike.
I change front and rear sprockets like some people change socks.

On my Thruxton R I've run 16, 17, 18T and 42,40, 37T in all combinations. Some combinations needed a 2nd chain depending on chain stretch. I've run the suspension full soft and full hard and in between including strapping the front end. I've run the forks up an inch in the triple trees.

The bike has been run at full speed over 3 miles on salt and speeds over 140 mph.. The bike has hit 119 mph in the quarter mile. The bike has been run naked, Triumph fairing and with a full fairing. Run 145 mph on asphalt.

I've run the chain at front, middle, and end of the chain adjuster and never had one handling problem.

Adjust your chain with proper tension and wheel Alignment, tires balanced and correct air pressure you unlikely to have any handling problems in my opinion.
 

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I've run the chain at front, middle, and end of the chain adjuster and never had one handling problem.

Adjust your chain with proper tension and wheel Alignment, tires balanced and correct air pressure you unlikely to have any handling problems in my opinion.
That's exactly what I think, its highly unlikely Triumph would give you a standard chain adjustment range that would cause handling problems, maybe Marc Marquez would notice the change in wheelbase but I doubt I will.

I've ordered the 37t anyway and i'll give it a try after the winter.
At least with a drastic change from 42t to 37t It'll make a big difference and I guess I'll either really like it or hate it.
Although its weird that many Bonneville owners want lower, more Thruxton like, gearing and we are going back the other way, maybe somewhere in the middle is the best compromise?
I really like how easy it is to change the overall gearing on a chain driven bike though to suit your own riding and change the character of your bike.
 
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