Riding RPM

[jbruney]

John Deere parts, eh?

 

[maylar]

So, what rpm is a 19T doing at 70mph?

I suspect you are agreeing with me aren’t you? Cruising at 4000 rpm (as a starting point) is what I was advocating.

Yes, I'm agreeing with you that 4000 is a good cruising rpm. Extrapolated from the chart above, that's 65 mph with my 20 tooth. A 19 tooth at 70 mph would be about 4565. The OP has a combat motor and they like to rev. 19T was standard on them.

I wouldn't want to go any bigger than 20T on my bike. I don't do high speed cruising, as the road conditions and traffic here make that scary. And with my 850 I can ride secondary roads in top gear with the isolastics doing their job.

 

[1up3down]

Original Poster Scout63,

You say you are under break in and are revving, and presumably holding rpm up to 6000, my opinion that is too high
nothing wrong with shifting to third and dropping the rpms

 

[oldmikew]

The important thing when running in a new or rebuiltengine is not to let it labour.. If you have a 19t I would leave it for a 1000 miles of running in , would progressively increase revs as mileage builds up .. I nitially about 4 max through the gears..so long as motor doesnt feel tight would cruise at 50-60..
After it has run in then would change for a 21t

 

[Fast Eddie]

Yes, I'm agreeing with you that 4000 is a good cruising rpm. Extrapolated from the chart above, that's 65 mph with my 20 tooth. A 19 tooth at 70 mph would be about 4565. The OP has a combat motor and they like to rev. 19T was standard on them.

I wouldn't want to go any bigger than 20T on my bike. I don't do high speed cruising, as the road conditions and traffic here make that scary. And with my 850 I can ride secondary roads in top gear with the isolastics doing their job.

Yes Combats like to rev, so does mine (Maney cam, stage 3 head etc) and so do I !

Revving out for acceleration and fun is one thing, holding your motor at a steady state high revs for no apparent advantage is another.

Its just my personal choice, but I wouldn’t be happy with a 4565rpm at 70mph gearing.

 

[Brooking 850]

With my gearing it seems to be 70mph at 3850 rpm

 

[Onder]

Out my way on the dual carriage 75mph is common. If you keep it under 78 you won't get done. So you need to keep up with traffic which means you will need to be able to keep the revs at 4k or less. Lower speed roads limit is similar unless you live in a built up area. A 850 will be fine at lower
rpm. Again remember I have a decided weight advantage though.

 

[gortnipper]

With my gearing it seems to be 70mph at 3850 rpm

That is where I got pinged coming back from Akaroa. 115k

 

[gortnipper]

There are some good charts with the gear ratios and speeds for quite a few Commando gearboxes at https://www.nortonownersclub.org/support/technical-support-commando/gearbox-ratios

 

[nattres]

Back in the 70s I ran my 850 with combat cam on a 23 tooth. Seemed fine at the time and I remember that 4000 RPM gave about 80 mph.
My 750 combat came with a 19 tooth and I have since changed it to a 21 – and that now seems just fine.

 

[concours]

Back in the 70s I ran my 850 with combat cam on a 23 tooth. Seemed fine at the time and I remember that 4000 RPM gave about 80 mph.
My 750 combat came with a 19 tooth and I have since changed it to a 21 – and that now seems just fine.

+1 on 22T on 850

 

[o0norton0o]

A good resource is: https://www.gearingcommander.com/

If you plug in the motorcycle data: brand (norton), model (commando 750) and then hit the "load gearing" button, it sets up the standard Norton ratios to figure out the associated relationships between speeds, gears, rpms, you name it...

I suggest taking a something flexible that does not stretch and measuring your rear wheel circumference, then adjust the inputs in the "custom" line to arrive at the measured circumference of your rear tire to increase the accuracy of your calculations. This way you'll get results more tailored to your bike, rather than just the average commando. In my case, I have a 21 front sprocket, a 42 rear sprocket, and a standard primary ratio. My rear wheel is a 110/90/18, and my measured tire circumference is 81.675" (a slightly smaller circumference than a stock commando 100/90/19 tire)

At 70mph, in 4th gear I'm doing ~ 3968 RPM's. The speed limit on the hiway in my area is 70mph, and everyone drives closer to 75 -80 mph. At 80mph, I'm doing 4,535 RPM's. The 21 sprocket seems perfect to me. Plenty of "balls", even a lower RPM's, and I'm not doing 5,000 RPM's going down the hiway.

I originally had a 19 tooth sprocket which I did not like on the hiway. It's RPM's calculates to 4,386 @ 70mph on my bike. At 80 mph, I was doing 5,012 RPM's.

 

[DogT]

All I can say is that with a 19 rear, 2nd gear will get you to 90 mph pretty quick. Since I drive around at 45-55 mph, I stick to the 19 tooth. No highway for me. Depends on how you use it. I've spent a lot of time in 2nd up and down Rt 211 over Skyline Drive (Thornton Gap), the thing loves 2nd around the curves, but now they've put speed bumps so that's over. It's been years though. I used to love hearing that engine purr in 2nd and it pulls strong there.

 

[Scout63]

Great feedback. I am sort of addicted to the rev rush now. I rarely break 60 so the 19T feels right. I also like the idea of running the bike as built, with ei of course.

 

[worntorn]

All I can say is that with a 19 rear, 2nd gear will get you to 90 mph pretty quick. Since I drive around at 45-55 mph, I stick to the 19 tooth. No highway for me. Depends on how you use it. I've spent a lot of time in 2nd up and down Rt 211 over Skyline Drive (Thornton Gap), the thing loves 2nd around the curves, but now they've put speed bumps so that's over. It's been years though. I used to love hearing that engine purr in 2nd and it pulls strong there.

Dave, I might be misreading your info.
Are you saying that the bike does 90mph in second gear with a 19 tooth gearbox sprocket?
With redline at 6500, the 19 tooth is topped out at 58 mph in second.
(1.7 second gear ratio)

 

[WZ507]

Below are a couple plots showing how countershaft sprocket selection affects the entire high gear speed vs rpm curve. In the 1st plot the primary drive is a stock set-up (26/57, engine/clutch, primary ratio 2.19) and in the 2nd plot the primary drive is an RGM belt drive (28/55, engine/clutch, primary ratio 1.96). The belt drive increases the primary gearing by ~ 12% which has quite a profound effect on bike speed, e.g., at 7000 rpm the stock and belt drive primary provide speeds of ~ 128 mph and 142 respectively.

 

[acotrel]

Might depend on what your friends are like - high overall gearing with close internal ratios is good. Changing down half a gear gives much quicker acceleration than a big step..

 

[robs ss]

Changing down half a gear gives much quicker acceleration than a big step..

That depends on how broad and flat your torque curve is.

 

[Onder]

As I said it depends where you live where you go and what sort of roads and traffic you have. I have to use the fast roads to get to the slow roads at times.
And you can see why "Adventure" bikes are becoming so popular.

 

[concours]

I reduced the overall gearing of my bikes a while ago.
With lower speed limits and ever stricter enforcement, I am more and more looking for the smaller roads.
For me, the ideal road is one where you cannot go over the speed limit without risking your life.

No need for a tall gearing here:

With a little GOOD luck, I’ll get old too.