Triumphrat

RobKennedy · 09-21-2005

I was thinking of changing my front sprocket and was wondering how it would effect performance around town? Also, will it effect mpg? Rob K.

Shorty · 09-21-2005

Rob...it will give you better gas mileage and decrease the RPM. I have a 19-t installed on my Bonnie and like it. Lost a little on the bottom end but made up for it in other modifications like jetting, intake and exhaust.
You will find that if you research a little in this forum you will find about as many pros as cons on the subject. One good thing is that this is an inexpensive modification which can be easily reversed if you don't like it. Some of my data if your interested is that I average 47 riding 2-up to about 56 mpg riding single. At 4000 RPM in 3rd I'll be doing about 50 mph, in 4th about 60 mph and in 5th about 70 mph with a 19-t sprocket and plenty of throttle left.

Shorty

dazco · 09-21-2005

Quote:

At 4000 RPM in 3rd I'll be doing about 50 mph, in 4th about 60 mph and in 5th about 70 mph with a 19-t sprocket and plenty of throttle left

Are you sure about that 70 MPH RPM? My speedy with the stock 16 does about 4100 RPM at 70 ! You should be at probably 3500-3700 at 70 i would think unless the gearbox on the boniie is shorter. But i always heard the speedy was shorter internally. But even if they're the same and the speedy is only shorter due to the sprocket, you should be well under 4k. Or maybe it's tire size?

Shorty · 09-21-2005

Positive daz...that's indicated of course. I don't know what the actual is. Also, I haven't red-lined it yet either to see how fast it can really go. It's hard around here to find a straight section of pavement so that I can let it rip (or not have to worry about getting a ticket). The pointer might actually hover a bit over 70 but not by much (maybe a hair).

Shorty

dazco · 09-21-2005

I can't figure out why that is. But maybe it accounts for why a lot of people go to bigger sprokets while i don't feel the need to on my speedy. Must be the internal gearing. Wheels can't be it i don't think because speedy's rear is 15" while bonnie's is 17". Of course the speedy's tire is bigger so they're pobably equal or close. But wheel size would indicate the bonnie should go even LOWER in RPM's if tires were equal in size as far as height. This is odd because i always thought the overall gearing on the speedy was shorter than all the twins.

Anyone know whats going on here?

RobKennedy · 09-21-2005

The reason I am thinking of switching my front sprocket is when I was on the highway the other day I was looking for that "6th" gear at about 70mph. I don't do a lot of highway driving so I don't want to mess anything up for around town. I have re jet and installed Togas if that makes any difference. I was thinking of an 18 tooth but the 19 sounds pretty good. If I switch is there any other adjustments that need to be done? Rob K.

dowop · 09-21-2005

I have a 2004 Speed master.
I went from the stock 16t to the 17t.
The tach reads 4000rpm at 70mph indicated on the optimistic speedometer

Shorty · 09-21-2005

Rob...no adjustment of the chain is required if that's what your asking (lengthening). Someone in here has step by step instructions with pics documented on how to do it. I don't remember who it was, but I'm sure someone will enlighten us. You will need a torque wrench and 36 mm socket (Autozone about $13) and a jack or center stand. The torque specs are on your left under downloads. Takes about 30 minutes max...no kidding.

Shorty

PS Found some info from Sept last year under "18 & 19 tooth sprocket" thread from Thumper723 with some mathematical input on how and why. It's in the advance search area under "sprocket". Thumper723 estimates a 5.5% RPM drop with a 18-t and 10% RPM drop with a 19-t assuming 4000 RPM equals to 70 mph.

[ This message was edited by: Shorty on 2005-09-21 13:25 ]

SpeedyPete · 09-21-2005

Rob,

I've got an 03 T100 as well and had exactly the same experience looking for 6th at highway speed. I went to 18T and it effectively gave me the 6th gear I was looking for. I'd never go back to 17T.

I can also confirm Shorty's estimate of 70mph at 4000rpm with19T. I do about 65mph at 4000 rpm with the 18T. Its also dirt cheap. If you have the socket for 1 7/16" sprocket nut you'll only spend $20 plus shipping for the part from newbonnie.com. The socket and 3/4" drive adapter to go with will run another $15 or so. Nothing else you do to your bike will deliver the additional performance for the cost & time involved. Just do it babe.

Folks on this site have step by step instructions for how to do the mod (Captdon?). You can't go wrong.

FYI the 865cc bonnies now come with the 18T stock. Maybe the 17T sprocket is good for UK riders doing a lot of city or low speed riding, but for Americans on the 75-85 mph interstates and parkways, we need the 18T at least.

Pete

dazco · 09-21-2005

Quote:

I do about 65mph at 4000 rpm with the 18T

Wow ....looks like it's confirmed then. Everyone with a bonnie seems to run higher RPM's at a given speed than the cruisers. That would make sense, but i was always led to believe it was just the opposite, but i guess i was thinking America vs Speedy which IS a case of the speedy running higher RPM's do to the sprocket if not the gearing too. This relieves the confusion....bonnies are internally geared lower than the cruisers.