6400 rpm in top

ewgoforth said:

I've never dynoed my bike, but I've seen the dyno charts from the Duckworth book and have had my 1973 up to 110 a few times with a 21 T sprocket, which is somewhere around 6500 rpm. I've more recently been using a belt drive, roughly equivalent to a 22T sprocket. With that setup I haven't gone much over 100, but I have more throttle left at that point, it feels like it'd probably run to around 115. which makes sense since 6500 is over the power peak, but below redline on an 850. Basically my point is that an 850 is better suited for taller gearing if your objective is top speed. 750s make peak power at more like 7000 and would probably like 1 tooth or so smaller sprocket. Period road tests had then running to 7300 or so with a 19T sprocket.

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With Commando gearing it's a case of finding a good compromise between speed , engine rpm and vibration, but in exactly the opposite way to every other motorcycle I have messed with.
Though I have read here that some stock bikes are smooth running as low as 2000 rpm, mine sure isn't. It shakes like an old Harley in the 2000 rpm range, but goes quite smooth at 3,000 rpm and is glass smooth from 3500 to the red line.
So you need to have gearing that keeps you in the smooth range most of the time, or run in intermediate gears a lot.
Having tried 20,21 and 22 tooth cs sprockets I settled on the 21 as best compromise for the type of riding most frequently encountered here.
If I was using this Interstate only on the Interstate, then a 22 tooth would be best. These bikes have plenty of low down torque and can pack the high gearing easily, however aside from the small reverse vibe problem this brings, you also lose a bit of acceleration with the 22 tooth.

Glen

Kurt & crew I've reved up a few cycles to see the rear tire expands at hi speed so that may need to be accounted for in ploting speed raios. i don't know how much but have seen like 3-4 more mph show up yet rpm stayed at 6000 for a sense of it. Peel had 21T sprocket plus higher ratio belt drive but still don't compute to agree with what the digital speedo's on catching up sport cycles in longish straights told me they were doing. Its stupid to ride cycles and more so to top out but boy howdy what a memory eh.

Steve, the tire expansion factor probably isn't much on these tyres since they are small in width and overall profile, however a modern bike rear tire such as a 180 or 190 width really does expand a lot with speed. Michelin lists the static and at speed diameter for their Power Pilot sportbike tires and the difference has a substantial effect on gear ratio.
Glen

i've no 2 wheel things that run but would be educational to watch rear on center stand - fastened down in front and on thick pads of course. Tire expansion if does occur would only happen near top out and not that much extra over drive as you say. I can't imagine anyone ain't imagined hanging on till terminal velocity reached. Doing it takes way more energy. Two things I wasn't brave enough to top out yet still eager to go, '69 P!! and my ole Blue '78 Chevy van, awed passengers nicked named 'Millennium Falcon'. I could die happy with just past ride thrills

hobot said:

i've no 2 wheel things that run but would be educational to watch rear on center stand

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Watch that it doesn't leap off the stand full throttle and head for the wild blue yonder. !!
Or one more thing to add to list of thrills......

Norton 650 prior to Commando was measured 119 mph, so why not later Nortons, here's Leno's review.
https://www.youtube.com/watch?v=lICP7S-jpvA

hobot said:

i've no 2 wheel things that run but would be educational to watch rear on center stand - fastened down in front and on thick pads of course. Tire expansion if does occur would only happen near top out and not that much extra over drive as you say. I can't imagine anyone ain't imagined hanging on till terminal velocity reached. Doing it takes way more energy. Two things I wasn't brave enough to top out yet still eager to go, '69 P!! and my ole Blue '78 Chevy van, awed passengers nicked named 'Millennium Falcon'. I could die happy with just past ride thrills

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on my commando i had a close fitting custom front mudguard every time i went above 90mph i could hear a droning noise that i thought was coming from the engine finally i noticed the paint starting to bubble off the top centre of the front mudguard !! ,,,,,,,,,,,,,,,,baz

hobot said:

Norton 650 prior to Commando was measured 119 mph, so why not later Nortons,

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If you read the tests and reviews of Dommies etc, that 119 mph was a one-off with a tailwind.
The 2 way average was more like 112 from memory, without getting out the booklet in question.

baz said:

on my commando i had a close fitting custom front mudguard every time i went above 90mph i could hear a droning noise that i thought was coming from the engine finally i noticed the paint starting to bubble off the top centre of the front mudguard !! ,,,,,,,,,,,,,,,,baz

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What tires were these, and were they fully pumped up ?
If the pressure is a bit low, the sidewalls aren't as solid as they could be, and the tread might deform a bit more, and grow... ?

the tyre was a bridgestone battlax as far as i can remember and as for tyre pressure i really dont know it could have been soft but i doubt it as i usually run a higher tyre pressure than the book says ,,,,,,,,,,,baz