6400 rpm in top

pommie john said:

So long as you change into top at reasonably high rpm, only the top gear determines the top speed.

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This was going well, until you threw in this line. (?).
One of the joys of riding a Commando is the very torquey engine, one of the best in British motorcycling I'd suggest.
Put it in top at 30 or 35 mph, and it will amble along, and go to max speed at a twist of the wrist.
Vincent owners get about the same performance, from an earlier era.

Frankly I'm surprised Aco didn't know this 1:1 stuff about these gearboxes.
Very few brit bikes have indirect gearing or an overdrive top gear ratio.

Rohan said:

pommie john said:

So long as you change into top at reasonably high rpm, only the top gear determines the top speed.

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This was going well, until you threw in this line. (?).
One of the joys of riding a Commando is the very torquey engine, one of the best in British motorcycling I'd suggest.
Put it in top at 30 or 35 mph, and it will amble along, and go to max speed at a twist of the wrist.
Vincent owners get about the same performance, from an earlier era.

Frankly I'm surprised Aco didn't know this 1:1 stuff about these gearboxes.
Very few brit bikes have indirect gearing or an overdrive top gear ratio.

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Yeah, you could probably change into top at 20 mph and still get all the way to 7000 in top eventually. Revving it out through the gears just gets you there a bit quicker.

Rohan said:

ashman said:

850 Commando
with standard gearing (19 tooth sprocket) my bike at flat out got to 116 mph and 6,500 rpms

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There is a (fair) bit of speedo or tacho error there. ?
The workshop manual quotes a stock 850 with 19t as ~100mph @ ~6500 rpm.
You'd have needed 7500+ rpms to do 116 - or a 22 t .

Some of the older magazine tests quotes the shown and actual speedo mph,
and that would be at the upper end of speedo errors allowable ?
Cheers.

Incidentally, redline with a 19t is 107 mph.

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Speedometer/Tachometer optimism plus "the older I get the faster I was". When quoting speed on Smiths magnetics, the term should be "indicated speed".

illf8ed said:

Speedometer/Tachometer optimism plus "the older I get the faster I was".

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Good comment !

pommie john said:

you could probably change into top at 20 mph and still get all the way to 7000 in top eventually. Revving it out through the gears just gets you there a bit quicker.

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That is a racers comment.
Where the lovely torquey engine low down in the rev range enjoyed for road riding is lost on you.

Magazine Road Tests of old used to quote a 'minimum non snatch speed'.
(Where the throttle could be opened low down in the rev range with impunity, without engine roughness intruding).
Vincent Rapides were oft quoted as 15 to 115 mph in top gear, a 100 mph revband without needing to change gear.
Not seen a Commando quoted thusly, but with the right sprocket, 20 to 120 mph in top is probably possible...

My factory Trixie spends lots of time just above 2000 rpm in 4th with 19T going ~30-33 mph and can smoothy throttle up too WOT w/o lugging. This is basic Gravel Travel sane speed for miles in order to avoid lower gears sleeve bush wear. A well functioning Smiths speedo is more accurate to exact mph than most if not all digial speedos - off wheel or GPS. If on level and holding speed a few seconds a GPS, or stop watch and Smiths should match. An mostly factory 850 pulling mid 6000 in 4th is geared pretty good for best top speed.

Surely a lot depends on the shape of the torque curve ? If it is extremely flat it doesn't matter much what gear you are in, as long as you don't over-rev, you will eventually reach maximum speed if the road is long enough - the speed determined by the wind and rolling resistance. If the torque curve is slightly peaked, it is better run high overall gearing with at least the top three gears close together. Then once you have the motor revving well up near peak torque and race change as you go up through the gears, you stay near peak torque. The first 80 MPH is easy on a bike, the difficult part is going from 80MPH to 120MPH plus. I suggest that if you get to 100 MPH and you are not well up the near the top of the torque curve, you have buckley's hope of reaching the potential maximum. The wind resistance defeats you.
When I first started using the commando engine, one of the 'experts' told me that if you have a torquey motor you don't need a close ratio gearbox. That showed me how much he didn't know. In a racer, the standard box is revolting.
My redline is 7000RPM, and on changes I often see 7,500 RPM. The motor spins up extremely quickly with the close box once the bike is mobile. If you don't try to ride the bike slowly, it is really great. - Suits an idiot.

acotrel said:

If you don't try to ride the bike slowly, it is really great. - Suits an idiot.

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You.
???

We are wandering off road bikes here again though....
Yes, we know you have never ridden one.

comnoz and other have shown that 850 bore size x 89 mm stroke develop friction rather faster than hp to exceed mid 6000's. Only horse power matters for top outs, more hp = more speed wether naked class or sleeker. My seriously confusing life changing factor was how quick Peel was to top out but could only pull 6000 with my face on the clocks and wind at my back but then saw a few more mph maybe from rear tire expanding but rpm the same. Btw don't matter if Smiths are dead on accurate stable while topping out naked the wind buffetting will blur eyes a mph.5+ both ways, if face held stable enough on the bezels just long enough. In real life its very rare for seasoned saned pilots to exceed 120 and most hold back to 90's then slow down a fair sane bit to corner so any proper Commando can hold its own if willing to strain the snot out of it to accelerate in similar or even better manner up to the ton or so. Any Cdo should be a low 13s 1/4m'r. In digitial age we should have firm GPS & Smiths to 'objectivitly' report what was observed while being a stupid squid on obsolete cycle making for life time replays.

Smith gauges are pretty accurate when they are new and when I hit 116 mph my bike was just run in and less than 6 months old from new so what ever anyone says I got in trouble with speed radar trap over 112 mph not long after owning my Norton FROM new and run in with less than 2000 miles on the clock, no vibrations at all (new bike) and just serviced from the dealer I brought my Norton NEW and yes when they get older they aren't as accurate but pretty close, mine you it did take a few miles to wind it up to that speed, but it did get there, a lot of poeple on the site can say it can't happen but how many of them had a brand new 850 Commanbo, without worn carbies and anything else that could slow them down, I know what my Norton did and I will stick by it.

Ashley

How can your bike be doing 20 mph more at the same rpm than everyone elses,
AND what the factory workshop manual says it should be doing. ???.

Either you didn't have a 19t fitted,
or your speedo was telling porkies.

And we seem to recall some discussion that 850's didn't come out of the showroom with 19t fitted,
Nortons already learned that lesson with the 750 models.
??

Your middle name isn't hobot, is it ?
His bike does 150 mph, in 2nd gear.
But only if you feed it magic mushrooms...

I prolly should apologise for that comment,
been quite sensible lately.
Thoughtful even...

I don't give a hoots what you say Rohan I am still running the same size sprocket my new Norton came out with , so you can say what ever you want, you can insult me as much as you want I don't care to much about what you say any more. you seem to make a habit of insulting folks or disbelive what poeple say on this site and I don't give two shits about your imput no more. If you want to carry on more about what my bike has done over 40 years of ownership then thats up to you.

Ashley

Steve, your comment :
' In real life its very rare for seasoned saned pilots to exceed 120 and most hold back to 90's then slow down a fair sane bit to corner so any proper Commando can hold its own if willing to strain the snot out of it to accelerate in similar or even better manner up to the ton or so. Any Cdo should be a low 13s 1/4m'r.'
You are talking about two different things when you are talking about top speed and the time over a 1/4 mile. The two seem to contradict each other.
I would think that 6,400 RPM would be where max torque occurs with a standard 850 motor ? I seem to remember that the best time at one of our club sprints in the mid-60s was about 13.8 seconds over the 1/4 mile for a new Super Rocket BSA. I doubt that either that or a standard 850 commando would do over 120 MPH without pulling a higher gear and working up to it.

There is another thing too. If your piece of road is of limited length, the speed you come around the bend onto it has a large bearing on top speed as does gradient, and wind direction. If the road is relatively smooth, the suspension damping works to pull the bike down and become more stable and that helps you go faster.

This dyno chart from Mick Duckworth is interesting. Looking at where the peak horsepower occurs for the 850 and what happens afterward, 6400 rpm is about it, no use holding on for more because the power drops off from there. It is possible that a higher top speed might be reached with a 22 tooth sprocket rather than the 21 I have on there, provided the bike has enough power to still pull to 6400. Peak torque occurs way down there at about 32-3300 rpm.
A Combat that has not been detuned should give a little higher top speed than an 850 with the same gearing, since it makes about 3 or 4 more horsepower than the 850, which makes a couple of horsepower more than a standard 750. I should state that we tried a top speed run with my cousin's new Combat after a 1500 mile break in and broke the crank which destroyed the engine, nearly killing my Cousin, so maybe it's best to keep that top speed theoretical only!

Once again, I am in awe of the low and midrange torque/horsepower by the 850. At about 3000- 3300 rpm , where the standard 750 still has a fairly anemic level of power and the Combat has even less, the 850 is putting out Max torque. In this area it produces about 40 % more power than the standard 750 and over 50% more power than the Combat. It keeps the upper hand all the way to 5k vs a standard 750 and 5500 rpm vs the Combat. I still don't know how this is possible when the 850 is essentially the same engine as the 750, just a 12% bigger capacity and slightly lower compression. Same cams, same combustion chamber ( or is the 850 squish band bigger?) Etc. Where does all that torque come from? Perhaps the porting improvements referred to by Dyno Dave in his info on Norton heavy twin heads comes into play.

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Edit: Added the reversed 850 torque curve, to easily compare to the original.
See posts a page later.



You'll have to use your imagination for the rpm scale along the baseline.

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Glen, we hope you noted that LAB deleted the recent discussion about that dyno chart from Mick Duckworth.
(Unfortunately without leaving a note as to why, but that is a separate issue)

The torque curve shown there for the 850 is clearly complete rubbish - we have speculated it was drawn as a mirror image,
but a few calculations show it cannot simply be reversed - or an 850 would outpower a Combat.
As drawn, an 850 could pull a diesel backwards at idle - which just isn't right.
The 850 power curve seems to be correct though, so they have done the calcs from the torque curve correctly,
but then misplaced/misdrawn the torque curve somehow.
Cheers.

Ashley, I'm sorry, but when you are claiming your stock bike is ~20 mph faster on a 19t than anyone elses Commando at the same rpm,
then clearly something is wrong somewhere. Until you acknowledge this, and WHY this might be, you are in a league of your own...

Rohan, I missed that deletion and apologize if the info from Mick Duckworth's book is incorrect. Other than your comment, I have not read anything to indicate that his numbers are wrong. As far as an 850 outpowering a Combat, I'm quite sure they do up to fairly highrpm , then the Combat has the advantage, if it holds together. This is pretty typical of a smaller version motor with hotter cam etc- power lost down low and in the middle in order to produce more up top.

Also, I intend to hook the MK3 the F350 diesel here shortly, will let you know how that goes.

Perhaps Jim or Dynodave will comment as to whether they have found results similar to the Mick Duckworth chart.

Glen

That torque curve error was previously discussed here - I note that is my scan,
which you have taken a screenprint from.

Yay, I noted in the pub to LAB that I didn't agree with his just deleting posts,
rather than explaining the problem with that chart.

An 850 makes max torque (of 52 ft lbs ?) at somewhere just above 5000 rpm, doesn't it ?
That torque curve sure doesn't show that....

P.S. It should be perfectly possible to sit down and reverse-calc the 850 torque curve from the shown power curve.
I started doing this a while back, but put the envelope down, and haven't sighted it since.
Will work on it.

The big white book says max torque for the MK3 850 is 56 ft. Lbs at 5000 rpm. This would be at crank whereas presumably the Duckworth chart is at rear wheel.


Glen