Triumphrat

bgaz

Can anyone tell me the difference between the 17, 18 and 19 tooth front sprockets on engine rpm at 70 mph?

I did about 300 miles on the bike today, and while the 17 tooth sprocket makes it zippy, the motor is spinning a bit to fast for 70/80 mph long runs.

At one point, everyone seemed to be doing 80 or 85 mph, and I did bursts up to 100.

Bike is running well!

Brett
1969 T100R
2005 Bonneville

dazco

On my speedmaster i got 200 RPM's less at any given speed by going up one tooth, so do the math and that should give you a good estimate.

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2005 Speedmaster, Neon Blue, Thunderbike pipes, snorkel removed, UNI filter, drilled airbox, 130 mains, TBS needles.(2 shims) And speaking of Speedmasters, HERE'S MINE

Hawk

Put an 18 on, it's the first mod most people do, you won't regret it

[ This message was edited by: Hawk on 2006-07-16 17:31 ]

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SCCTrim

I had a 17 tooth like yours and went to a 18. Glad I did, but I would suggest a 19 tooth unless you plan on 2 up riding.

Dont think I would want a 19 for that.

I have read 1 tooth up on the countershaft is the same as 2 teeth down on the rear sprocket.

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bgaz

I do take the wife on the back sometimes, but I am not a road racer, and thats what downshifting is for anyway...
She loves the backrest the PO added (Triumph part).
I think it ruins the looks...but it gives me a place to strap on a can of fix a flat and so on...

The trans ratios are a bit wacky, 1st is on the tall side, then the gears are close.
5th should be a bit more of an overdrive for me, but the bike sure is fun to do sudden bursts of accleration for lane changes.

200 rpm is not much, but I better try the 18 tooth one first.

Thanks for the info!

Brett


[ This message was edited by: bgaz on 2006-07-16 19:06 ]

dazco

You might be suprised at the difference 200 RPM's makes. On the freeway it doesn't seem like a lot, but on the street it's very noticable. I was suprised when i realize it was only 200 because it sure felt a lot taller on the street. On the freeway tho it doesn't feel quite as much because you're usually at higher RPM's anyway, and the higher the RPM's the less difference 200 feels like because it becomes a smaller percentage if you get my drift. But i think you'll feel quite a nice difference overall with just one more tooth. I sure did. and remember, these bikes are made to run at higher RPM's. It's the nature of the design. I don't know which model you have, but my speedmaster actually gets much smoother as the RPM's climb up to and past 5000.

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2005 Speedmaster, Neon Blue, Thunderbike pipes, snorkel removed, UNI filter, drilled airbox, 130 mains, TBS needles.(2 shims) And speaking of Speedmasters, HERE'S MINE

Motolink

I have a 2001 Bonne, from all the calulations I have done each additional tooth on the front sproket will drop the engine RPM by 250.
I have not changed mine yet but would like to try a 19T, stock is 17T.

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A1 removed, R&N filter, Poloris bellmouth, Thruxton needles, Baffels removed, 19T, Bellacorse Western bars.

lunus

It has to be a ratio guys, not a fixed number of RPM.

This is my understanding

If you replace a 17 with an 18 then, for any given RPM in any given gear, you're gonna go 18/17 times as fast. For example with a 17 tooth on my 2003 Bonnie I'm doing around 83mph at 5000 rpm. If I fit an 18 then I'll be doing 83 * 18/17 = 88mph approx.

Conversely at a given speed I'll be doing less revs, so again if I'm doing 5000 rpm at 83mph wth a 17, I'll be doing 5000 * 17/18 = 4720rpm at 83mph with an 18. In other words around 280rpm less.

However at 41mph with a 17 I've got about 2500rpm and if I fitted an 18 I'd be spinning at 41 * 17/18 = 2360rpm, or about 140rpm less.

So very roughly speaking you "gain" 1mph per 1000 rpm if you move from a 17 to an 18, or putting it another way, you're doing about 50 rpm per thousand less at any given road speed.

Mind you, as I've said before elsewhere, these engines rev very happily and will cruise all day at 85-90mph with the standard gearing. 5000-6000 rpm may seem high if you've come from a push-rod twin or a 45deg V (I presume, never having ridden one) but not if you look at what the triples and fours are doing.

Depends on your style and location whether it's a good idea to change.

Cheers

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Shorty

bgaz...I have a 19-t sprocket on my Bonnie and at 4000 RPM in 5th I'm a hair shy of indicated 70mph. At the same RPM at 3rd it's about 50mph, in 4th about 60mph, and in 2nd about 40mph indicated.
All these values are a little to the negative indication by about 1/2 pointer width on the speedo.

Shorty

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2002 Red & Silver Bonneville
1972 Honda CL450 Scrambler
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Silverado57

Before I consider this, how hard is it to get to the sprocket?

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Kevin
Luceo Non Uro
NJ USA