Maximum torque from a Commando.

acotrel said:

If the bike is under-geared, the motor does not pull as hard, so the major factor which determines acceleration becomes the rate at which the heavy crank spins up when you gas the motor. When the heavy crank is spinning above 6,500 RPM and you race-change with a close ratio box, if the gearing is right the drop in revs is negligible and acceleration is the max. With a light crank, the drop in revs is greater but the ability to spin the crank back up to speed with the throttle is better. With a light crank you would be using lower overall gearing, Neither acelerates faster than the other if you have got the overall gearing wrong. The simple fact is that spinning the Commando crank too high is asking for trouble. A light short-stroke crank in a Commando engine revving to 8,500 RPM has to be faster, but for how long ?

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The drop in revs when changing gear is greater with a light crank?
I beg to differ Alan, the drop in revs when changing gear is dictated by the speed at which the bike is traveling and the gear ratios.

As I understand it torque of a motor is only horsepower relative to the revs. If your motor has a lot of crank twisting power at the middle of it's rev range, it is torquey. If the motor has to rev it's tit's off to make power, it is usually not torquey. There is no point in changing gear past the point of maximum torque if you want maximum acceleration. However if you hold on to top gear for long enough past the point of maximum torque, you can eventually reach a higher top speed. Might be OK if the circuit is big enough, you might out-run the opposition - but on a short circuit acceleration is the go. Most true race cams give an increase in power right through the whole usable rev range even below the 'cam spot', however the usable rev range is usually narrower and higher. Is the usable rev range different when a Commando engine is compared with the same size Nourish engine ? To my mind a Harley is a completely different kettle of fish. You can do a lot more with a V-twin.

Fast Eddie said:

The drop in revs when changing gear is greater with a light crank?
I beg to differ Alan, the drop in revs when changing gear is dictated by the speed at which the bike is traveling and the gear ratios.

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On Winton Raceway, there is one second gear corner almost regardless of what bike you are riding. In most of our races first gear is reserved for the clutch start. If you accelerate out of that corner on a bike with a heavy crank, and you compare it with a similar bike with a light crank, when you select third gear the revs will drop further with the bike with the light crank, but the bike with the light crank will respond better to the increasing throttle opening. The determining factor is the crank inertia when you apply the load by changing up. The heavy crank has more guts, but the light crank recovers quicker from the drop in revs in response to the throttle. Overall there is little difference in the acceleration of the two bikes, unless you get the gearing wrong. Then you are stuffed anyway.
There is also another aspect - the rate at which you open the throttle. Some guys get their bike upright then whack it open. If your bike is tuned right you need to feed the throttle on as you would with a two-stroke. The final thing is how early you can get on the gas coming out of the corner. I usually knock the speed off to about a third of the way into the corner, then get straight back onto the gas - GENTLY but strongly - a nice firm squirt with the rear tyre slipping slightly. The bike takes care of the rest.

One thing I would say, is that I have never ridden another bike which was so responsive to tuning. With most you end up beating your head against a brick wall. I love the heavy crank because you don't have to be so careful in the way you race, you can be extremely aggressive and you don't get bitten. A very easy bike to ride fast.

acotrel said:

when you select third gear the revs will drop further with the bike with the light crank.

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Al, the crank, irrespective of its weight, is connected directly to the rear wheel. Unless the clutch is slipping, there is no variable. Thus at a given speed, in a given gear, the crank is rotating at a given speed. Period.

What I am saying is that if you compare a Commando engine bike with other bikes, there is a noticeable difference which is related to crank inertia. If you try to race a Commando with the standard gearbox, the bike accelerates much more slowly when race-changing up through the box and it is impossible to do smooth down-changes. No 650 Triumph was ever like that. With a close box behind the Commando engine, the crank keeps spinning higher and it does not have to go so far to recover after up-changes. The crank inertia has less adverse effect
I actually tried racing with the standard box, that is when I bought the 4-speed close cluster. The standard box was disgusting, - the close box was perfect, except off the clutch starts. With the Commando engine, you need the low first gear, then at least 4 more gears up high and close. Then you have a weapon. There are very few race circuits where you have to use first gear once the bike is rolling.

Fast Eddie said:

Al, the crank, irrespective of its weight, is connected directly to the rear wheel. Unless the clutch is slipping, there is no variable. Thus at a given speed, in a given gear, the crank is rotating at a given speed. Period.

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Eddie, Get on your bike and do race-changes up through the gearbox, by easing the throttle slightly and standing on the gear- change when the motor reaches near max revs. You will see the needle on the tacho drop. With a close box, that drop is less than with a wide ratio box. With the heavy crank it needs to be less because the inertia makes the response to the throttle less. If you are depending on throttle response rather than using the crank inertia to advantage, the bike will accelerate slower. With a lighter crank, you can lose revs and the motor will recover better -so the bike is easier to ride fast. Once you lose revs in any gear with the near standard Commando engine, you are buggered. Not good if you are balked in a corner. Personally, I like the heavy crank -if you keep it spinning high, nothing stops the bike - not wind gusts, rises in the circuit - nothing.

acotrel said:

the bike with the light crank will respond better to the increasing throttle opening.

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Yep, what I have been saying all along - “respond better” = greater acceleration due to greater torque. And as you have said in the past, torque wins races!

Are we talking about the power the engine creates or the effect of the heavy crank in delivering that power, compared with a light crank ? I suggest each type of crank is as good as the other if you have the gearbox to suit. In each case the best gearbox and overall gearing are different. The motor with the light crank recovers better after an up-change, so wider ratios matter less. A guy once said to me 'if you have a torquey motor, you don't need a close ratio gearbox'. I don't think he had ever used a close box. With the usual Commando motor, the close box gives much better acceleration than with the standard box. With a 650 Triumph motor, the close box does not make such a marked difference, but it is still better than wide ratio.

I have often wondered why the 60s' Triumph Saint with the light crank, was inferior in performance to the 650 Triumph Thunderbird with the heavier crank. I think it might have had a lot to do with our bumpy roads.

acotrel said:

The motor with the light crank recovers better after an up-change

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Yep, agree with that assertion. “Recovers better” = better acceleration. Better acceleration is in part due to greater torque and as you have said, torque wins races.

Dances with Shrapnel said:

Yep, agree with that assertion. “Recovers better” = better acceleration. Better acceleration is in part due to greater torque and as you have said, torque wins races.

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I've been watching this thread and I feel a point needs to be clarified.
My (limited) understanding is that the amount of torque is generated by how well the combustion process works.
If two engines were identical in every respect other than crankshaft mass the they both produce the same torque.
The engine with the lighter crank (same torque!) will accelerate faster due to the lower rotational inertia of the crankshaft - not due to more torque.
A lighter bike will accelerate faster with the same engine due to lower inertia - again, not because of more torque.
Cheers
Rob

A matter of semantics and battery limits. Useful torque to accelerate the bike versus torque potential based on BMEP alone. Does one want to waste some of the torque on accelerating a flywheel or a lighter flywheel as well as whole motorcycle and rider? I suppose if you are looking at some regenerative braking system, a heavier flywheel might come into play.

When you have the heavy crank spinning at 7000 RPM and race-change up one gear, you use the crank inertia to recover the potential loss of revs, more than you would if the crank was lighter. However the lighter crank is easier for the combustion process to spin up to recover the loss of revs caused by the up-change. By a race-change upwards, I mean taking the pressure off the gear train slightly with the throttle and standing on the gear-lever then forging ahead. With the heavy crank, doing down-changes with a standard box is horrible - takes a heap of revs and is never smooth.

There are two things at play - BMEP and crank inertia. On an up-change, when you stand on the gear change and select the next gear, there is a mismatch in revs - the heavy crank tends to keep going where the lighter crank slows down a bit more. However the BMEP can spin the lighter crank back up more easily. If you race-change up through a standard commando box with the heavy crank, you seem to wait forever for the revs to rise again. The effect is the acceleration rate of the bike is too slow. With the heavy crank, I always try to keep it spinning high because if you lose revs, it takes ages to recover. But with a close box, it is not all bad.

acotrel said:

However the BMEP can spin the lighter crank back up more easily.

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“acotrel said:

With the heavy crank, I always try to keep it spinning high because if you lose revs, it takes ages to recover.

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So who wants a steam boat anchor tied to their race bike?

Think of it this way. If you have a grappling hook on a rope and chuck it up to hook onto the top of a wall, when you start to climb the rope, the wall moves slightly in the opposite direction. If you grab onto a sand dune with the hook, the hook moves more in the opposite direction as you climb, to the one in which you want to go. When you changeup with the heavy crank, the load comes on and more inertia means the crank slows down less. With the lighter crank - when the load comes on, it slows a bit more than the heavy crank and the combustion process has to help it recover. The difference in energy means more acceleration with the heavier crank because it is recommencing from higher revs. The advantage of the lighter crank is it will spin up more easily when throttle is applied. But the main difference lies in the nature of race circuits, most are not entirely level or smooth, and the air above them often has bumps. The heavy crank keeps the bike going better, when the bike encounters things which tend to slow it. Otherwise there is not much difference.

Your fixating on the act of upshifting (akin to energy recovery from the flywheel by “dumping” the clutch) which is all of maybe a handful of milliseconds. I am talking about the additional available torque to the rear wheel with a lighter crank over several tens of seconds.

Fast Eddie said:

Al, the crank, irrespective of its weight, is connected directly to the rear wheel. Unless the clutch is slipping, there is no variable. Thus at a given speed, in a given gear, the crank is rotating at a given speed. Period.

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Eddie, When you race-change up from 3rd to top, the crank is spinning high in 3rd, but when the load is reapplied the crank loses revs. On my bike, the box was an American close ratio 4 speeder. It was perfect everywhere except off a clutch start. On an up-change, it would lose about 1000 RPM which the combustion process must recover or the acceleration rate drops. With a light crank, that drop might be 1500 RPM. With the heavy crank, you don't have to go so far to regain the lost revs because of the inertia. However the light crank is easier for the combustion process to spin back up. With a Commando, the combustion process limits the ability respond. So it is better to use crank inertia in the best possible way. The disadvantage comes on the down-changes. With the wide ratio box, it takes a heap of revs to get a smooth change - that problem does not exist with the close box - you can actually ride the bike fast, without the potential for a crash..

Dances with Shrapnel said:

Your fixating on the act of upshifting (akin to energy recovery from the flywheel by “dumping” the clutch) which is all of maybe a handful of milliseconds. I am talking about the additional available torque to the rear wheel with a lighter crank over several tens of seconds.

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I never use the clutch on an up-change and only a tweak on the down-change. You can ride the bike without using the clutch, if you are careful on the down-change. Up-changes are a soda. The only time I go a decent race start with the Seeley, I revved it to about 6 and dumped the clutch. I thought it would destroy the box, but it actually jumped very well. I would not make a habit of doing that.
When you are riding your bike and you have completed an up-change, as the load comes back on in the next gear, the revs must drop unless you have got the most powerful motor in the world. It is how fast the revs rise again which determines the acceleration rate. The heavy crank regains it's revs from a higher level. With either type of crank, the problem is the same but different. A light crank drops more revs when the load comes back on, but is easier to spin back up than a heavy crank. The lighter crank is probably easier to work with - if you lose too many revs with the heavy crank, the acceleration rate drops off a lot.
I don't think there would be many guys in America racing commandos with the standard gear box. It is a horrible experience.

When I first built my Seeley 850, I did not believe in it. However I am amazed that so many things are so right about the motor. It is fast enough without going stupid. I don't have a problem with the heavy crank - not what I expected. It looks ten times worse than any Triumph crank.