Here’s a dyno plot of a modified 3,000cc Yamaha V-6 as used in Ford passenger vehicles fit with SHO engines. Do you think the acceleration and top end potential would be better near the torque peak (6800 rpm, 225 ft-lb, 295 HP) or near the HP peak (9900 rpm, 205 ft-lb, 390 HP)?acotrel said:
it started with "it pulls 100MPH at only 1/2 throttle"
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With a near flat torque curve like that, it won't make any difference no matter where you change gear !
Most engine makers would kill to have a flat torque curve like that
(even if it is over only a shortish portion of the rev range, which is somewhat misleading in some respects)
Not a good example. ?
You need the torque curve for YOUR engine, and a gear chart graph of your engine, and have a pow-wow with your race engineer to see which gear cluster is going to give the best results for the circuit you are at..
I mentioned earlier my old early dommie drops the revs by HALF when changing from 1st to 2nd.
That means you can rev the zinger out of it in 1st, and its still well under the torque peak when its in 2nd
(ie it bogs). Not that it has vast hp, that you'd go racing with it anyway.
For road use, this is acceptable, and it gives the clutch an easy time from standstill.
Most engine makers would kill to have a flat torque curve like that
(even if it is over only a shortish portion of the rev range, which is somewhat misleading in some respects)
Not a good example. ?
You need the torque curve for YOUR engine, and a gear chart graph of your engine, and have a pow-wow with your race engineer to see which gear cluster is going to give the best results for the circuit you are at..
I mentioned earlier my old early dommie drops the revs by HALF when changing from 1st to 2nd.
That means you can rev the zinger out of it in 1st, and its still well under the torque peak when its in 2nd
(ie it bogs). Not that it has vast hp, that you'd go racing with it anyway.
For road use, this is acceptable, and it gives the clutch an easy time from standstill.
lcrken
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acotrel said:
Not really.
It should be clear from simple physics that max top speed comes at max horsepower, but that doesn't seem to have worked for everyone, so I'm trying for a more intuitive explanation of why you don't gear for top speed at the engine torque max. What you are doing is gearing for max torque at the rear wheel with overall gearing that will still let you reach maximum top speed, and that doesn't turn out to be the max torque point on the engine curve. This is my best shot at explaining that in a more intuitive way.
On a given bike that is geared for maximum top speed, let's assume that it's a Commando with a torque peak at 5,000 rpm. Let's assume that we've tuned it up well enough that it will do 120 mph at 7,000 rpm for it's maximum top speed. Those are actually fairly realistic numbers. Now you think, great, I can just change the overall gearing so it's hitting 120 mph a little below the torque peak, and then it should be able to pull past that as it gets to the peak torque point. Let's say you think 4,900 rpm would be a good number to gear for. To do so, you have to change the overall gearing by a factor of 4,900/7,000, or 0.7. For example, if your overall gear reduction from crank to rear wheel was 4:1, you'd have to reduce it to 2.8, so you would be at 5,000 rpm at 120 mph. Unfortunately, you've also just reduced the torque multiplication from crankshaft to rear wheel by the same 0.7, and you no longer have enough force at the wheel contact patch to overcome drag, so you'd never get near 120 mph. If you just look at engine torque, you are leaving out the torque multiplication in the gear train. If you gear so that you get maximum speed at the horsepower peak instead of the torque peak, you are getting less torque at the crankshaft, but because of the gearing, you are still getting more torque at the rear wheel, where it counts. Because horsepower is a product of torque and rpm (and some constants), as long as the horsepower curve is still increasing, you can gain speed by gearing taller, until you reach the point where the torque curve drops off faster than rpm increases.
People get confused about the relationship between torque, horsepower, and top speed fairly often here. There's probably a much better explanation than mine somewhere in books on how to set up race car gearing, but I couldn't find one to copy. More graphs and examples would probably help, but it's more work to generate them than I'm willing to do. This is the best I could manage, so take it in that light.
Ken
Its maybe worth adding that max torque is merely the point where the cylinder filling is as good as it gets.
This can be pretty much any point in the rev range, depending on how the designer was instructed to design it.
But ALWAYS remember that engines make torque ALL THROUGH THE REV RANGE, so if an engine will still keep pulling whatever gearing you fit to it to max rpms, then so be it.
Commandos (750cc) make max torque about 5000 rpms, according to that tech page here, but obviously they will still be making more speed above that 5000 rpm. The Combat models being a little inclined to rev to 7000+ rpms (7000 rpm redline) on the rather low 19 tooth sprocket many of them were fitted with from the showroom, so if they were MUCH over the ton they'd be above redline. And well above max torque.
You really need to see a torque curve for YOUR engine, to see what might work best.
This can be pretty much any point in the rev range, depending on how the designer was instructed to design it.
But ALWAYS remember that engines make torque ALL THROUGH THE REV RANGE, so if an engine will still keep pulling whatever gearing you fit to it to max rpms, then so be it.
Commandos (750cc) make max torque about 5000 rpms, according to that tech page here, but obviously they will still be making more speed above that 5000 rpm. The Combat models being a little inclined to rev to 7000+ rpms (7000 rpm redline) on the rather low 19 tooth sprocket many of them were fitted with from the showroom, so if they were MUCH over the ton they'd be above redline. And well above max torque.
You really need to see a torque curve for YOUR engine, to see what might work best.
lcrken
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Fullauto said:
Good question. The advertising brochure for the 1972 models says that the 19 tooth countershaft sprocket was standard on all models, including the Combat, but notes that alternate 20 and 21 tooth sprockets were available for dealer installation. I'm not saying you should always believe the ad brochures, but it is at least interesting to see what they say.
Ken
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lcrken
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A little more on the countershaft sprockets. The advertising brochures I have show the following as the factory installed sizes for all models. They also all mention that alternative sprockets are available at the dealer.
1969 (Commando) - Standard 19T with optional 21T
1970 - Standard 19T with optional 21T
1971 - 19T
1972 - 19T
1973 - 21T
1974 - 20T
1975 - 20T
The 1975 service manual also mentions that the standard for the USA and Canada was 20T, but for the rest of the world it came with a 22T. Could also be that there were different standard configurations for the rest of the world in previous years.
Ken
1969 (Commando) - Standard 19T with optional 21T
1970 - Standard 19T with optional 21T
1971 - 19T
1972 - 19T
1973 - 21T
1974 - 20T
1975 - 20T
The 1975 service manual also mentions that the standard for the USA and Canada was 20T, but for the rest of the world it came with a 22T. Could also be that there were different standard configurations for the rest of the world in previous years.
Ken
Think of a gearbox and gearing in general as a torque multiplier.
Power is defined as the ability to get work done, an engine does the most work at max power.
Acellerating is work. Therefore at any given speed the bike will acellerate faster the closer it is to max power.
Power is defined as the ability to get work done, an engine does the most work at max power.
Acellerating is work. Therefore at any given speed the bike will acellerate faster the closer it is to max power.
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Multiple posts so replies duplicated.
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If you'd READ that Tuning for Speed graph, and the 3 curves, it does note that lightweight stripped bikes will need less than 30 hp = CURVE A. (I did mention this curve A, somewhere earlier)
(bikes at Brooklands were allowed to race like this. Not at the IoM though, it was a TT race)
It also notes that large or heavy or road equipped bikes will need more, possibly much more.
A fairing alters the picture too - Tuning for Speed and that graph was published before fairings were used !!.
Be interesting to see a modernised version of that graph. ?
Big wide tyres and full fairings being the norm....
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acotrel said:
If you'd READ that Tuning for Speed graph, and the 3 curves, it does note that lightweight stripped bikes will need less than 30 hp = CURVE A. (I did mention this curve A, somewhere earlier)
(bikes at Brooklands were allowed to race like this. Not at the IoM though, it was a TT race)
It also notes that large or heavy or road equipped bikes will need more, possibly much more.
A fairing alters the picture too - Tuning for Speed and that graph was published before fairings were used !!.
Be interesting to see a modernised version of that graph. ?
Big wide tyres and full fairings being the norm....
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Which is better on a road going commando - over-geared or under-geared ? My feeling is that due to it's mass, the crankshaft tends to spin up at a fixed rate. So if you are under-geared, the bike feels as though it is doing it's best and you only find out that it was not, when you raise the gearing. However my bike accelerated much faster coming out of corners when I fitted the close ratio 4 speed box instead of the standard box. With a road bike, you are probably intended to ride with more sanity.
I wonder whether the heavy crank gives a higher top speed and cruising speed ? When I ride my bike, the air always feels as though it has got bumps in it. Two strokes sometimes feel as though they have stalled if you turn into a head-wind. And the Triumph Saint 650 had a light crank - and poor performance compared with a Tiger 110.
I found this :
'In 2-stroke engines there is one power stroke in every revolution of the crankshaft but in 4-stroke engines there is one power stroke in 2 revolutions of the crankshaft. So the amount of energy stored in the flywheel in one revolution of crankshaft is more in 2-stroke engine than in 4-stroke engine. That is why the flywheel in 2-stroke engine is lighter.'
I wonder whether the heavy crank gives a higher top speed and cruising speed ? When I ride my bike, the air always feels as though it has got bumps in it. Two strokes sometimes feel as though they have stalled if you turn into a head-wind. And the Triumph Saint 650 had a light crank - and poor performance compared with a Tiger 110.
I found this :
'In 2-stroke engines there is one power stroke in every revolution of the crankshaft but in 4-stroke engines there is one power stroke in 2 revolutions of the crankshaft. So the amount of energy stored in the flywheel in one revolution of crankshaft is more in 2-stroke engine than in 4-stroke engine. That is why the flywheel in 2-stroke engine is lighter.'
acotrel said:
Undergeared or overgeared - for what though ??
A Commando with a 19t gearbox sprocket is very lively around the city.
Will keep ahead of all but the most determined tintop pedallers.
In the early 1970s, would have been serious sports performance.
Out in the country, 19t would feel like you are always in 3rd though.
22t gives more relaxed engine rpms (a bit), but even then you sometimes feel for that 5th gear....
Bit sluggish off the mark though, especially if you'd had a pillion and gear.
There is also a choice of 18, 19, 20, 21, 22, 23 and even 24 in the parts lists these days, aren't there. ??
I'd have thought 23 and 24 would be a tight fit, 22 is fairly large under that chaincase shield.
Each size sprocket change is only a few % different to the next sizes.
There are soo many speed limits these days, you barely even need a 4th gear, let alone max rpms..
Horses for courses.
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hobot said:RohandoB arrogant ignorance feeds on your attention so hobot quit it
Mighty proud of ya boy.
Dereck
If Steve chooses to not take part in the argy bargy here, thats his choice.
Getting too snooty for us ?
The secret to a good discussion is to check your facts before posting.
If the basic factoids are already wrong, it doesn't bode well...
And wear it if you don't.
The rest is just window dressing.
Yeah, I know, LAB.
Getting too snooty for us ?
The secret to a good discussion is to check your facts before posting.
If the basic factoids are already wrong, it doesn't bode well...
And wear it if you don't.
The rest is just window dressing.
Yeah, I know, LAB.
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Rohan said:
Rohan, sorry to be blunt, repeating here what I wrote on the other thread, but could you please possibly stop polluting these two threads with your endless repetitive armchair arguments?
Frankly at this stage we don't give a F***k anymore if you are right or wrong, we just want to exchange views on various subjects without having to navigate through pages of your boring prose. For a change, why don't you kickstart your Commando and take a ride? It would be a welcome relief for many of us to see you disappear in the sunset...
On behalf of the vast majority from the Access Norton community, thank you in advance.
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