Gearing

Fast Eddie · Aug 16, 2014

Although I haven't put many miles on in yet, the rides I did have on my freshly rebuilt 'Blue un' gave the distinct impression that it would benefit from taller gearing.

It pulls away from standstill very easily, I'm changing up into second almost immediately, and when cruising it constantly feels like it wants 'another gear'.

I have a Norvil belt primary with 32 / 70 'teeth' which according to my calculation, give very close gearing to the stock 26 / 57 set up (that's what the book says it is anyway).

My gearbox sprocket is 23 and it has a stock rear drum.

According to what I read, this already seems relatively high gearing, but post rebuild, it feels low to me.

I could fit a 24 gearbox sprocket, but I am more in favour of fitting a bigger front primary pulley, Norvil do a number of different sizes, this would have the benefit of spinning the gearbox faster.

BUT... Am I missing something here? Why do others seem happy with gearing so much lower? Have I got the wrong end of the stick somewhere??

Bernhard · Aug 16, 2014

Gearing is relative to the engines power, mainly the torque figure, not the BHP figure.
If your engine produces max torque at, say, 4,500 rpm, which is what it will be happiest cruising at – it will do this all day long.

Some of you will not appreciate this; I once owned a 500 that I cruised at 80mph revving to 8,500 rpm which it was happiest at, as it was at that figure that it was “on the cam”

Onder · Aug 16, 2014

Is this a road bike or a track bike? If it is road going , what rpm are you turning at 60mph
in top gear?
Im at 3600. I could go to say 3200 for 60 but for local work it wouldnt be so happy. Not on
motorways hardly at all so I see what I have as about right.

acotrel · Aug 16, 2014

I found my 850 to be considerably different to other bikes that I have ridden and raced. I've built it to take full advantage of the torque in the interests of getting better acceleration. When I started racing it, it seemed quite quick, however it seemed to be over-revving. I raised the overall gearing to knock it's backside in, and the bike accelerated faster. I didn't expect that. It is very deceptive. The problem then came that with the close ratio box, first gear was too high. So clutch starts in races were a problem. I think with a four speed box, a low first gear and the rest close and high would be a good answer, as long as you could handle the gap between first and second gears.
If you don't over-port the head, use a two into one exhaust and tickle the cam timing a bit, the 850 puts out very high torque (mid-range power).

tomspro · Aug 16, 2014

There was a nice gearing chart (for standard chain primary) posted here a few months ago.
Cannot find it. Do not remember who posted it.

I did find this:

http://www.nocnsw.org.au/technical/speed-in-gears

SteveA · Aug 16, 2014

Fast Eddie said:
Although I haven't put many miles on in yet, the rides I did have on my freshly rebuilt 'Blue un' gave the distinct impression that it would benefit from taller gearing.

It pulls away from standstill very easily, I'm changing up into second almost immediately, and when cruising it constantly feels like it wants 'another gear'.

I have a Norvil belt primary with 32 / 70 'teeth' which according to my calculation, give very close gearing to the stock 26 / 57 set up (that's what the book says it is anyway).

My gearbox sprocket is 23 and it has a stock rear drum.

According to what I read, this already seems relatively high gearing, but post rebuild, it feels low to me.

I could fit a 24 gearbox sprocket, but I am more in favour of fitting a bigger front primary pulley, Norvil do a number of different sizes, this would have the benefit of spinning the gearbox faster.

BUT... Am I missing something here? Why do others seem happy with gearing so much lower? Have I got the wrong end of the stick somewhere??

Without an alternator to worry about that front pully would likey be 38 teeth. 32 seems pretty small, but as you say that is close to standard. A larger pulley would be the first move I reckon, for exactly the reasons you mention, but constrained by the Alternator, 34 or maybe 36. You might also need a slightly longer belt depeding how much movement you have available.

Matchless · Aug 16, 2014

Eddie,
My 920 is running a Norvil belt drive of 36t x 72t. The 36t engine pulley fits in fine with an alternator. The gearbox sprocket is 21t & the rear sprocket is 43t (Suzuki disc rear hub) This gives a final drive ratio of 4.19 : 1 giving an indicated 5,000 revs @ 90mph. This feels great & means 100 mph cruising is easily possible. I used to run 21t x 42t on the 850 giving much the same result ( 4.22 : 1 I think ) What dia. is your rear tyre as this can also make a difference as I am sure you know.

Gearing

Fast Eddie

Bernhard

Onder

acotrel

tomspro

SteveA

Matchless

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