it started with "it pulls 100MPH at only 1/2 throttle"

then again, the last words so far, i'm still looking for a main point, a clear position?

as far as "we moved on and covered a fair bit of ground there"

i see a few exchanges with another poster & another question that someone may consider relevant,

i kinda doubt anyone has followed what all you have said, leading to this,

my first thoughts are what does that question have to do with anything here?

Rohan said:

In case you folks didn't notice, we moved on and covered a fair bit of ground there.

So, shrapnel john, is 30 bhp for 100 mph correct, or not - yes or no ?

Click to expand...

As I said - I have a 4 to 1 overall gear ratio - that's nearly as tall as using a 23 sprocket on a Commando. Taller than most would use but it's basically a lightweight race bike with a mild cam, drag bars, aluminum fenders and small lights (no battery or kick stand), free breathing exhaust, 32mm flatslide carbs, raised compression, and it pulls the tall gearing just fine. A race Norton can top 130 to 135. Even at the elevation & thin air of Bonneville on the crunchy salt - the production 750 Nort of Randy Johnson topped 129mph (one way) with stock airbox and stock muffler styling. My ride below basically has the same internal components as the Randy Johnson speed record holder - including Axtell copy ports (but I use a milder cam - slightly hotter than stock).


The bore on my short mufflers is 1-3/4"+ The headers are now at 1-1/2" - This and the lack of air box plus the longer intake manifolds & built in carb velocity stacks are the advantages over the land speed record holder below.


Randy Johnstons speed record holder


It was never my intention to get everyone so riled up - I'm just happy with the way my bike runs is all. It has a lot of racing technology in it and it gives me a thrill everytime I take it on the road.

If you are riding a Commando with a 22 tooth counter-shaft sprocket at over 100 MPH and pulling 8000 RPM, are you near the top of the torque curve ?

Hard to tell. My instruments are magnetic speedo and tacho, they move about quite a bit..there seems to be a very definite surge in torque/power above 6000, by the time it hits 7000 on the tacho it just keeps on climbing...as I am not running Jim's light pistons and lenghtened conrods I try not to let it rev much over 7500-7700...this is my roadbike, after all! :mrgreen:

This is the beast's crank:





And this is an older picture from the same bike, without its fairing:

no issues with the lower fairing against the exhaust pipes?

is that a stock magneto? i think i saw start vid that was an easy one kick deal, repeated

jseng1 said:

As I said - I have a 4 to 1 overall gear ratio - that's nearly as tall as using a 23 sprocket on a Commando. Taller than most would use but it's basically a lightweight race bike with a mild cam, drag bars, aluminum fenders and small lights (no battery), free breathing exhaust, 32mm flatslide carbs, raised compression, and it pulls the tall gearing just fine. A race Norton can top 130 to 135. Even at the elevation & thin air of Bonneville on the crunchy salt - the production 750 Nort of Randy Johnson topped 129mph (one way) with stock airbox and stock muffler styling. My ride below basically has the same internal components as the Randy Johnson speed record holder - including Axtell copy ports (but I use a milder cam - slightly hotter than stock).

Click to expand...

acotrel said:

If you are riding a Commando with a 22 tooth counter-shaft sprocket at over 100 MPH and pulling 8000 RPM, are you near the top of the torque curve ?

Click to expand...

Not sure what you are saying here. You couldn't pull 8000 rpm in top gear in a stock Commando with 22 tooth countershaft, unless you had a good tail wind and long downhill run. That would put you right at 145 mph, not a realistic scenario. Of course, if you could do so, you would be WAY past the peak of the torque curve.

I'm pretty sure something got turned around in your question.

Edit: Just realized you are probably referring to jagbruno's featherbed bike, not a stock Commando. He's seeing 130 mph at something less than 8000 rpm, so probably not running the same drive ratio as a stock Commando.

Ken

Not sure to understand completely the above question so I will attempt to describe my set-up which is far from being a 'stock Commando'. For starters, the bike probably only weighs around 155kg.

850 tilted engine in a Slimline frame, lightened crank (see picture above...), PW3 cams, flowed head, lightened rockers, velocity stacks, splayed admission manifolds (à la Triumph), Amal 32mm with main jets 250-280 depending on the weather conditions, Pazon ignition, Dunstall exhausts, 22 teeth sprocket, box section swingarm about 4cm longer than stock, Borrani alloy wheels, Fournales oleo pneumatic shocks.

Very loud. :mrgreen:

Alan, in the Tech pages here,
http://www.jerrydoe.com/nortonCommandoTechnical.html

It says the max torque for an 850 is 56 ft/lbs.
Says @ 5000 rpms for the 750 ( 48 ft/lbs), but doesn't actually state that the 850 is at those same rpms ?

Its in the Workshop Manual somewhere too (?), can't readily find it. I thought it was slightly more rpms for the 850 (?).

Depends on your gear ratios whether you'd still be in the torque band in the lower gears.
I notice in the gearchart I made up for my old dommie , changing from 1st to 2nd gear lowers the rpms by half !
Std road gears of course.

Commandos are pretty torquey beasts, it must be said.
You're rarely out of the torque band - on a road bike.

Jagbruno said:

Not sure to understand completely the above question so I will attempt to describe my set-up which is far from being a 'stock Commando'. For starters, the bike probably only weighs around 155kg.

850 tilted engine in a Slimline frame, lightened crank (see picture above...), PW3 cams, flowed head, lightened rockers, velocity stacks, splayed admission manifolds (à la Triumph), Amal 32mm with main jets 250-280 depending on the weather conditions, Pazon ignition, Dunstall exhausts, 22 teeth sprocket, box section swingarm about 4cm longer than stock, Borrani alloy wheels, Fournales oleo pneumatic shocks.

Very loud. :mrgreen:

Click to expand...

How about your primary drive; is your's stock or aftermarket. Some if not all of the aftermarket primary drives have ratios different than stock. As mentioned earlier, with a 22T countershaft at 8,000 rpm with the rest of the drive line in the stock ratio you should be going much faster. Not disputing your clocked speed by the way.

As Rohan said, the torque peak for the stock 750 and 850 Commandos is at 5,000 rpm, according to Norton, and is pretty broad. All the dyno charts I've seen for standard 89 mm stroke Commando race engines, 750 - 920 cc, have their torque peak somewhere between 5,000 rpm and 6,000 rpm. But the torque only drops slowly from then to somewhere just past 7,000 rpm, where it starts to drop rapidly. That doesn't mean you couldn't build one that hit peak torque at a higher rpm, but it would have to be pretty heavily modified, and might not be very rideable. Different story for the short strokes and ultra short strokes, but even there, the ones I've seen charts for still hit their peak torque around 5500 rpm to 6,000 rpm.

I'm sure there are exceptions to the rule, but in general, a Commando engine is going to have it's torque peak somewhere between 4500 and 6000 rpm, depending on it's level of modification.

Torque is nice, but it's power that determines top speed.

Ken

Dances with Shrapnel said:

Jagbruno said:

Not sure to understand completely the above question so I will attempt to describe my set-up which is far from being a 'stock Commando'. For starters, the bike probably only weighs around 155kg.

850 tilted engine in a Slimline frame, lightened crank (see picture above...), PW3 cams, flowed head, lightened rockers, velocity stacks, splayed admission manifolds (à la Triumph), Amal 32mm with main jets 250-280 depending on the weather conditions, Pazon ignition, Dunstall exhausts, 22 teeth sprocket, box section swingarm about 4cm longer than stock, Borrani alloy wheels, Fournales oleo pneumatic shocks.

Very loud. :mrgreen:

Click to expand...

How about your primary drive; is your's stock or aftermarket. Some if not all of the aftermarket primary drives have ratios different than stock. As mentioned earlier, with a 22T countershaft at 8,000 rpm with the rest of the drive line in the stock ratio you should be going much faster. Not disputing your clocked speed by the way.

Click to expand...

My primary drive is stock BUT I have 18" wheels on the bike. The tech sheet tells us that a stock 850 running 22T is supposedly doing 121mph @7000rpm on 19" rims, so I would say it is pretty consistent with my reading of 130mph @8000rpm on smaller rims.
In any case, at those speed/rpm, the magnetic tacho and speedo are seriously wobbly to say the least and I don't think we should take them for more than an rough indication from the bike's actual performance. It is probably a good ballpark though, give or take 5mph.

Further to Ken's remarks on peak torque, my old Commando 750 racer (89mm stroke) in a moderate state of tune had peak torque at 5,200 rpm.

The Ultra Short Stroke 500 Norton twin (59.5 mm stroke) had a peak torque at around 8,500 rpm.
The Ultra Short Stroke 750 Norton twin (75mm stroke) had a peak torque for one version of the build at 6,500 rpm but remained more or less flat for some rpm range above and beyond that. When I first raced it I described it as an electric motor.

84ok said:

is that a stock magneto? i think i saw start vid that was an easy one kick deal, repeated

Click to expand...

Its a late model Joe Hunt magneto with neodymium magnets and yes its a one kick starter. Usually starts 1/2 way through the kick. Light years better than the original mag that I often had to push start.

here's the one kick vid:

https://youtu.be/3D4VIzMNNYM

lcrken said:

acotrel said:

If you are riding a Commando with a 22 tooth counter-shaft sprocket at over 100 MPH and pulling 8000 RPM, are you near the top of the torque curve ?

Click to expand...

Not sure what you are saying here. You couldn't pull 8000 rpm in top gear in a stock Commando with 22 tooth countershaft, unless you had a good tail wind and long downhill run. That would put you right at 145 mph, not a realistic scenario. Of course, if you could do so, you would be WAY past the peak of the torque curve.

I'm pretty sure something got turned around in your question.

Edit: Just realized you are probably referring to jagbruno's featherbed bike, not a stock Commando. He's seeing 130 mph at something less than 8000 rpm, so probably not running the same drive ratio as a stock Commando.

Ken

Click to expand...

Surely the fastest you can go on a long straight stretch of road is determined by getting the highest gear the motor will pull while having the revs at the point where maximum torque occurs - If that point is lower than the revs where maximum horsepower occurs, would the motor still rev further than the revs at max torque ?
Surely when the heavy crank is wound up and the motor is producing maximum torque and pulling the highest gear it can without dropping revs ... that is all the speed you will get ?

I understand that two years ago when Kenny Cummins and a few Brits were at Phillip Island, that turn one had them a bit worried. It also worries me. The problem is that you get into the front straight really quickly and accelerate slightly downhill into that extremely fast sweeper. So there is not much go left to keep you on the power as you go around it. It's a much braver man than me who can go around it without braking before it. I haven't ridden there for years and the last time I was there, I had a bad day. Any crash there is always too big.

https://www.youtube.com/watch?v=DbtcmCSxnsE

acotrel said:

lcrken said:

acotrel said:

If you are riding a Commando with a 22 tooth counter-shaft sprocket at over 100 MPH and pulling 8000 RPM, are you near the top of the torque curve ?

Click to expand...

Not sure what you are saying here. You couldn't pull 8000 rpm in top gear in a stock Commando with 22 tooth countershaft, unless you had a good tail wind and long downhill run. That would put you right at 145 mph, not a realistic scenario. Of course, if you could do so, you would be WAY past the peak of the torque curve.

I'm pretty sure something got turned around in your question.

Edit: Just realized you are probably referring to jagbruno's featherbed bike, not a stock Commando. He's seeing 130 mph at something less than 8000 rpm, so probably not running the same drive ratio as a stock Commando.

Ken

Click to expand...

Surely the fastest you can go on a long straight stretch of road is determined by getting the highest gear the motor will pull while having the revs at the point where maximum torque occurs - If that point is lower than the revs where maximum horsepower occurs, would the motor still rev further than the revs at max torque ?
Surely when the heavy crank is wound up and the motor is producing maximum torque and pulling the highest gear it can without dropping revs ... that is all the speed you will get ?

Click to expand...

Acotrel, with all due respect, my experience certainly does not agree with your last sentence...The fastest I ever went on this bike was on the long straight (1km +) of the Dijon circuit in France, where it happened lap after lap. I can assure you that the bike continued to accelerate quite strongly well above 7000rpm. As I said before, you can really feel the power kicking in above 6000rpm, thanks to the PW3 cams and the lightened crank:

What is the difference in revs between maximum horsepower and maximum torque ? I suggest they usually occur at slightly different revs, and best acceleration occurs when you change up at max torque, not max horsepower. If you can gauge your over-all gearing so that top speed to occurs at max torque ... ? What does the dyno tell you about torque ? Some of the dyno torque curves I have seen have been very flat and some people say that is good. What I wonder about is the dyno instrumentation response and precision when measuring torque.
What I have found is that the heavy commando crank only spins up slowly, so the close ratio box is necessary to get good acceleration (spin it up and keep it spinning ), also in order to get smooth down-changes. I don't think the heavy crank adversely affects top speed, unless the road is short and the gearbox is wide ratio. Of course, if your motor is short stroke, you are probably using very long duration cams with radical timings. So as the revs get higher, the bike gets stronger. My short stroke Triton would power on forever getting faster if the gearing was high, however it would not pull the skin off a rice-pudding. I used to choose where I would lose races - the starts of the straights or near the ends.
I never believed in my near-standard 850, however I now believe that for what it is, it is very good. I usually try to change-up just below 7000 RPM, and the revs don't seem to drop below 6,000 RPM. I suspect max torque is at about 6,500 RPM. I use very high overall gearing even on short circuits.
I know from experience the bike is fast enough. It is up with the leaders when I race it, and our guys are not slow - all on methanol and over-capacity.

acotrel said:

lcrken said:

acotrel said:

If you are riding a Commando with a 22 tooth counter-shaft sprocket at over 100 MPH and pulling 8000 RPM, are you near the top of the torque curve ?

Click to expand...

Not sure what you are saying here. You couldn't pull 8000 rpm in top gear in a stock Commando with 22 tooth countershaft, unless you had a good tail wind and long downhill run. That would put you right at 145 mph, not a realistic scenario. Of course, if you could do so, you would be WAY past the peak of the torque curve.

I'm pretty sure something got turned around in your question.

Edit: Just realized you are probably referring to jagbruno's featherbed bike, not a stock Commando. He's seeing 130 mph at something less than 8000 rpm, so probably not running the same drive ratio as a stock Commando.

Ken

Click to expand...

Surely the fastest you can go on a long straight stretch of road is determined by getting the highest gear the motor will pull while having the revs at the point where maximum torque occurs - If that point is lower than the revs where maximum horsepower occurs, would the motor still rev further than the revs at max torque ?

Surely when the heavy crank is wound up and the motor is producing maximum torque and pulling the highest gear it can without dropping revs ... that is all the speed you will get ?

Click to expand...

As the plot below shows for a modified Norton 850 (Brooking 850's race bike) the torque and HP peak can occur 2000 rpm apart, and you need gearing that allows you to reach the HP peak, not the torque peak, to reach maximum speed potential. HP is work/time and as the plot shows you're getting a lot more done at 7000 rpm than at 5000 rpm. This dyno curve, courtesy of Brooking850, is posted elsewhere on this site and was cut and pasted here for clarification.

If you are changing up after you reach peak torque, wouldn't the acceleration rate be less ? You are depending on the twisting power at the crank to accelerate the total mass. If you go past peak torque in revs, you get more horsepower, but it increases at a slower rate ? Surely the best option is to raise the overall gearing to the max tolerable, then use 6 gears to ride the top of the torque curve ? Usually when you fit a race cam to a road motor, you get more torque right across the rev range, however it becomes peakier, and the peak moves higher up the rev range. What you gain by doing that comes at the expense of the motor's internals such as cases, bearings and valve gear. Personally, I use the best cam I have and work on improving the torque in other ways - a good dose of nitro might be cheaper ?