Triumphrat

marly1971oif

Looking to drop rpm by 300-400 or so on ' 71 650 Bonneville (4 speed). Would be interested in hearing others experience with this. Cant do rear sprocket smaller. Loss of acceleration a good trade off for smoother running at highway speeds of 60 mph + ? Usually dont carry a passenger.

Blue09SE

I had my '69 650 converted from 19T to 20T. This 5% increase, from overall 4.84 top gear to 4.60 top gear, resulted in less vibration at 65-75mph, with little additional effort needed to start up and seemingly little additional need for downshifting. To fit the 20T without dismantling the gearbox, the mechanic filed 20 small notches in the inner primary to insert the sprocket.

The 21T sprocket would increase gearing by 11% over the stock 19T, from overall 4.84 top gear to 4.36. This gearing might smooth out the engine even more than the 20T, but might require noticeable additional effort to start up, might noticeably increase the need to downshift, and might increase clutch wear.

panda

I do not understand your references to starting up.

Gearing depends a lot on the kind of riding you want to do. I increased my 500 Triumphs gearbox from 19 to 20 teeth back in the sixties, but at the same time added more than another 35% power. I guess if you want to cruise at a speed which makes the bike vibrate badly then that is one way. This business about acceleration, I would not worry about, On my old bike the ability to hold the gears a bit longer actually improved acceleration. But you dont want to be so high geared in top it cant pull easy. It is easy to overgear to a situation where you use more fuel and run hotter because the engine is working too hard ( like going up an eternal hill). My old 500 vibrated worse at around 85mph +-5 mph so I normally rode below this speed sometimes above. Fortunately I could drive through this range quickly in third gear.

Currently On my new Enfield at low speed it is inclined to snatch the transmission because the power pulses are about 630 degrees apart unlike a twin with power pulses every 270 degrees or so. It runs very smooth and relaxing from 45 to 60 mph, after that vibration starts to creep in. To put it in perspective, at 45-60 it is as good as my bmw at its best, after 60 is starts to get more pronounced than on the bmw.
This model has one tooth more than the old models partly due to increased power, and partly to increase the comfortable cruising speed.
If I only used it for short journeys I would drop it by one tooth neaxt time I need to change the gearbox sprocket.

__________________
A wise man learns from his mistakes..
A genius learns from other peoples mistakes.

jimballard

There might be a small interference when you try to put the sprocket through the hole in the primary case. I was told this by a very knowledgeable dealer back when I first bought my '70, in 1979. I had the 21 tooth sprocket but never did use it. He said I might have to file a small slot in the case hole to allow a tooth to clear the edge of the hole when it was inserted at an angle.

Don't know for sure, just figured I'd mention it: Jim

__________________
Jim Ballard

The older I get, the faster I was....until lately!! (Speed IS the fountain of youth)!

marly1971oif

Thanks for the info !

Regarding cutting notches in the case to fit in the bigger sprocket have you experienced any subsequent stress cracking in the notches ?

With limited funds where would you start adding power - big bore kit, cams, or carbs/intake ?

Marly

pookybear

Marly,

The 20 tooth sprocket just fits through the trap door in the
primary. So a 21 will need notches. The thing to do is make
them rounded and then blend the edges back into the case to
reduce the stress risers.

As for getting off the line, I have a 20 in the front and a 43 in
the rear. Slow off the line now, but it never stalls, unless I am
being a meathead!

And remember one tooth on the front will act close to two teeth
on the rear of most stock triumph made for the street.

Pookybear

Blue09SE

Panda, I meant: starting in 1st from a stop, opening the throttle and slipping the clutch until a road speed is reached from which the engine can smoothly accelerate.

panda

OK, now I understand. thanks. never had abike i could not pull away easily in second, so it really should not be an issue.

__________________
A wise man learns from his mistakes..
A genius learns from other peoples mistakes.

TigerDave

If you ride two-up the 21 tooth might be a bit tall.

Taller gearing puts greater stress on transmission parts when taking off from a stop and makes it easier to lug the motor (which Triumph Twins don't like).

I once broke a layshaft first gear due, in part, to tall gearing/high load during a two-up fast start so now I would rather keep my RPMs, and my oil pressire up, with standard 19 tooth gearing.

The 20 tooth sprocket does not require case modification.

If you are looking for more speed at lower RPM, I recommend a 20 tooth as a good compromise.

Cheers,

Dave

farric1

Hi there

I run a 20T engine sprocket on my 70 Tiger. It originally had a 19T, but the vibration was too much at Freeway speeds (only 100kph or 62mph here in Australia) - I lost a couple of instrument bulbs on long runs! The vibration doesn't really come in now until you get to about 75mph.

Now it cruises nicely at that speed with power for overtaking in top gear. Acceleration was hardly affected, so I consider it a very good conversion for me. Also, fuel consumption improved as well.

The only downside is that the rear wheel will have to come forward a little to accommodate the new sprocket with the chain. This means that the clearance between the wheel and drive chain is lessened, you may find that the chainguard will need careful positioning to prevent the chain rubbing on the chaincase.

Regards

Craig.

__________________
Norton may have developed the oil leak, but it's Triumph who perfected it