6400 rpm in top

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Rohan said:
So how can a 1973 bike/test have Mk3 pipes ??
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Presumably the Mk3 model was already under development at that time but in mid '73 it probably wasn't known that the Mk3 would only be available with what was at that time known as the Mk1A black cap.

Note that the graph came from the factory development department, not from a "1973 bike test" article.
 
Rohan said:
And what does the ftlbs @ rpm actually work out to be ?
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Highest BHP graph curve figure at 3,000 RPM (as best as I can tell) is approximately 24 BHP, so = 42 ft.lb. @ 3,000 RPM
And approx. 47 BHP @ 5,000 RPM, = 49.4 ft.lb.
 
All this argument about top speed. All I can say is the older I get the faster I was.

Cheers
 
No argument, just an armchair discussion. If the sun comes out I might get out of my armchair and do another run with the GPS mounted this time.
And find a dyno! :D

Glen
 
Is there something important about the top speed of a bike or it's time over the 1/4 mile ? I would have thought the weight, handling and the speed the bike gets around corners into the next straight piece of road is more important. I've never used my bike for a sprint or a land speed record, and I would never attempt either activity. What do they prove about the rider's skill ? If you are trying to prove the owner is good at getting performance from the engine, you can do that with a dyno.
 
acotrel said:
Is there something important about the top speed of a bike or it's time over the 1/4 mile ? I would have thought the weight, handling and the speed the bike gets around corners into the next straight piece of road is more important. I've never used my bike for a sprint or a land speed record, and I would never attempt either activity. What do they prove about the rider's skill ? If you are trying to prove the owner is good at getting performance from the engine, you can do that with a dyno.
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Marketing.

It still happens today. You can't sell a new bike that's slower than last year's model. People won't pay more money for less performance.
 
acotrel said:
Is there something important about the top speed of a bike or it's time over the 1/4 mile ? I would have thought the weight, handling and the speed the bike gets around corners into the next straight piece of road is more important. I've never used my bike for a sprint or a land speed record, and I would never attempt either activity. What do they prove about the rider's skill ? If you are trying to prove the owner is good at getting performance from the engine, you can do that with a dyno.
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Just because it doesn't interest you then it must be a worthless endeavour?
All of the people involved with Bonneville or those interested in drag racing have wasted their time on the incorrect hobby because you feel it proves nothing? Legends like Burt Munro ,TC Christenson or Don Garlitts should have tried something else, perhaps skeet shooting.

Sorry, I can't agree Allan, to each his own.

Glen
 
pommie john said:
Marketing.
It still happens today. You can't sell a new bike that's slower than last year's model. People won't pay more money for less performance.
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Marketing indeed - thats why Marketing Depts were invented.
They just make up numbers that show more hp and 1/4 mile times than last years model.

How do you think Commandos are claimed at 60 hp, when no-one has ever seen that on a dyno.
(Yes we know here about rear wheel horsepower and engine hp...).
The Atlas was just as fast with 52 hp.
Or was that the Manxmans claimed output - they were just as fast too, on a good day. ?

But didn't the Mk 3 have slower numbers (and more weight), even though the claimed hp and performance didn't alter. ??
 
L.A.B. said:
What it says is Mk3 pipes, STD silencer, as this graph is dated May 1973 "standard" obviously refers to peashooters, not what eventually became the standard black cap 850 MkIII silencer referred to on the graph as the "Mk1A" silencer.
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Thanks LAB.

Interesting that they used Mk 3 pipes (for all ? these tests ) that early.
Wonder if there is some hp advantage over using the pre Mk3 pipes ??
(the Mk 3 pipe going into the head has those part spherical seats thingies, for those what didn't know)
(if I have that right ?).

Shame those dyno results didn't use stock bikes as they were sold...
 
Rohan said:
pommie john said:
But didn't the Mk 3 have slower numbers (and more weight), even though the claimed hp and performance didn't alter. ??
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Testers of the day noted how quiet the MK3 with black caps was and also how it ran out of puff up higher in the rpm range. Generally speaking, quiet exhausts mean restriction and lost horsepower, especially up in the rev range.
Take off the black caps, put on some open peashooters and the engine pulls quickly to the red line in intermediate gears if desired. Skinny up the head gasket for a couple more free or cheap horsepower and you have a good performer. After that it gets expensive, the law diminishing returns kicks in.
Glen
 
Competing on the salt or in a 1/4 mile sprint is still competition, however how are these relevant to a road motorcycles ? My friend bought a Hyabusa to find out what 168 BHP was like. It was a useless heap of shit. If he went out riding with the group of other old road racing idiots he could only keep up by giving it a good squirt between the corners. Then he bought the KTM V-twin and was stoked by it's massive torque.
 
How is top speed relevant to road motorcycles?
Pulling a high rpm in top gear for me is an easy affirmation (without a dyno) that the engine is running as it should at the top end. I also have a very steep hill which I use as a sort of dyno for midrange, tho it is probably more useful than a dyno for a midrange check as it is constant.
I know what speed each of my old bikes should hold or accelerate to going up the hill in top gear when tuned well. When they drop a couple of MPH on dyno hill there is always a tuning reason.

I just like to keep them in the best tune possible the easiest way possible. Seat of the pants is not so accurate.

Glen
 
How is top speed relevant to road motorcycles?
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Glen don't be silly as same as asking what relevance does motorcycling have in routine life instead of more sense-able econo car, sheeeze. Motorcyclists are known as thrill seekers with skull and bones icon very appropriate. Quess which rides will be remembered first-most when ya can't do it no more.
 
If Allan has never ridden a Commando on the road, nor ever ridden even a faintly stock Commando,
he may not be the best authority to be preaching about these things....
 
Rohan, You are quite right. I don't ride a motorcycle on public roads for very good reasons. There is nothing nicer in life then to get on a high powered motorcycle and really fang it. If I owned a road going commando I'd probably keep my licence for about two weeks. If I owned a modern bike I'd probably have my next heart attack very quickly. Where I live it is 30 miles of boring straight road to the first winding interesting road and the police sometimes hide in the bushes with the speed camera and radio. There is no way you can use most motorcycles as intended on public roads in Victoria. I mentioned that group of old road racers who get out along the back roads of the hills around Melbourne. If you are the one that gets caught by the police, you become the subject of mirth. It is 100 MPH around the bends and much faster down the straight bits. We have bugger-all public transport.
If your commando will pull 120MPH at 6,400 RPM, I think that is lovely. Been there, done that and now I'm older and wiser. Those police really piss me off with their 'speed kills' bullshit. 'Speed plus incompetence kills' and our government licences a lot of incompetent drivers. I've driven in the UK and the drivers there are much faster and safer. Also their education system seems better.
 
acotrel said:
If your commando will pull 120MPH at 6,400 RPM, I think that is lovely. .
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We have already established that with a 19t sprocket, it won't.
The factory published this data in the workshop manual, back when they were new....

For road riding, top speed is not terribly important.
But its the underlying speed potential and acceleration that makes a good bike fun to ride.
No need to break the sound barrier every time out, its the ability to twist the wrist and just
nip past obstructions on the road as required....
 
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.
 
In light of the discussion of motorcycle speed vs engine rpm, thought a picture would be worthwhile having, so am reposting a graph of Norton 4th gear speed vs rpm. Values given in the factory manual for sprocket tooth counts as well as a tire rolling radius of 12.57" (Dunlop 4.10" x 19" rear tire) were employed in the calculations. The plot allows the observer to view a large range of ground speed over a continuum of engine speed, i.e., ground speed at cruising rpm, top rpm and every rpm in between.

6400 rpm in top
 
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