Maximum torque from a Commando.

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I did say “akin to energy recovery from the flywheel by dumping the clutch”. A few milliseconds versus tens of seconds of acceleration with a lighter flywheel.
 
I sure enjoy my 1200 Triumph with its lightweight crank. That engine is a wall of torque from 2500-7500.
But then again, those racebikes with big heavy Nourish cranks seem to do pretty well.

Maybe it's a bit like rod ratio. One guy swears by short rodding, the other says long rodding makes all the difference.
Ed Iskanderian said its not really much of a factor...
 
worntorn said:
I sure enjoy my 1200 Triumph with its lightweight crank. That engine is a wall of torque from 2500-7500.
But then again, those racebikes with big heavy Nourish cranks seem to do pretty well.

Maybe it's a bit like rod ratio. One guy swears by short rodding, the other says long rodding makes all the difference.
Ed Iskanderian said its not really much of a factor...
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The optimum rod length varies with the stroke and the power characteristics of the motor. Shorter rods give more leverage, but the rock-over time is quicker. The 70s Triumph 750 twin has short rods, but the concept is different to the previous 650s - more torque ? There are similarities in considering the difference between a light and heavy crank. With Triumphs, they made a mistake of using a light crank in the Saint 650. It never performed as well as a Tiger 110. It amazes me just how good the Commando motor is as standard. I think the designers at Norton must have been more experienced.
 
acotrel said:
It is more like the difference between being ahead of somebody else and not.
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Well that’s exactly my point, you are promoting some sort of point and squirt strategy with a clutch dump like bump in position on the track whereas I am talking about useful torque over several seconds. When was the last time anyone heard of a race win on a shift point? I’ve never realized a track position with a shift but have realized many track positions through better overall acceleration. I’ve run Commando engines with heavy and light flywheels and know the real difference.
 
Didn’t Triumph only use short rods in order to make the engine fit the ‘new’ OIF chassis?

If so, it kinda makes any reference here rather invalid.
 
I would have thought the frame would be made to fit the engine, not the engine made to fit the frame. Good handling takes priority over power output ? - Do 750 Triumphs with the short rods perform better than 650 Triumphs with the long rods ?
 
Dances with Shrapnel said:
Well that’s exactly my point, you are promoting some sort of point and squirt strategy with a clutch dump like bump in position on the track whereas I am talking about useful torque over several seconds. When was the last time anyone heard of a race win on a shift point? I’ve never realized a track position with a shift but have realized many track positions through better overall acceleration. I’ve run Commando engines with heavy and light flywheels and know the real difference.
Click to expand...

If you have run the two types of crank with the same gearbox, do you get the exact same loss of revs after an up-change when the load comes back on ? If you had a 100 pound flywheel powered by a one horsepower motor and spun it up to 5000 RPM then dumped the clutch on a direct drive to the rear wheel, would you get the same acceleration as you would if the flywheel weighed one pound ? Flywheels have mass for a reason. In a commando engine, the stroke rod length, reciprocating weight, flywheel mass and combustion characteristics are inter-related. If you change the stroke and flywheel mass, you are creating something different, but not necessarily better. In my experience, going short stroke and light crank makes the right type of gearbox even more critical.
 
acotrel said:
I would have thought the frame would be made to fit the engine, not the engine made to fit the frame. Good handling takes priority over power output ? - Do 750 Triumphs with the short rods perform better than 650 Triumphs with the long rods ?
Click to expand...

It’s a sad part of BSA / Triumph history. When the new bikes were finally put into production, after long delays, they discovered on the Production line that the fine wouldn’t fit!

I know the rocker boxes were modified at that late stage, the short rod format had already been changed. I don’t know for sure if it was because of the frame, maybe someone does?

As to performance difference, too many other things changed to be able to compare as you suggest Al, cams and compression, as well as air filter arrangement, exhaust pipes, silencers, etc.
 
worntorn said:
Maybe it's a bit like rod ratio. One guy swears by short rodding, the other says long rodding makes all the difference. Ed Iskanderian said its not really much of a factor...
Click to expand...
Rod to stroke ratio is certainly a design consideration. The question is what are you designing to? Say a big lumbering cruiser, a heavier flywheel would be appropriate
acotrel said:
If you have run the two types of crank with the same gearbox, do you get the exact same loss of revs after an up-change when the load comes back on ? If you had a 100 pound flywheel powered by a one horsepower motor and spun it up to 5000 RPM then dumped the clutch on a direct drive to the rear wheel, would you get the same acceleration as you would if the flywheel weighed one pound ? Flywheels have mass for a reason. In a commando engine, the stroke rod length, reciprocating weight, flywheel mass and combustion characteristics are inter-related. If you change the stroke and flywheel mass, you are creating something different, but not necessarily better. In my experience, going short stroke and light crank makes the right type of gearbox even more critical.
Click to expand...
My experience is faster lap times with a lighter crankshaft. Most modern after market roadrace cranks (vintage and otherwise) are lighter rotational mass. Have you seen a Formula 1 engine cank and flywheel from over maybe the last 50 years? Generally speaking, heavier cranks are apparently reported as more desirable for dirt riding/racing applications. What I am reading here is you feel the point and squirt thrill of a flywheel bangshift gives you an edge - ok.
With the rod to stroke ratio of a Commando, it is considered a rather angry motor with a relatively long stroke and short rods. This and the excellent valve flow coefficients and combustion chamber swirl all contribute to the phenomenal off the line grunt the Commando engine offers.
As for R/S ratio, the longer rods offer more dwell time in and around TDC, generally good for higher speed engines where more time for the combustion process to play out is a good thing.
 
acotrel said:
If you have run the two types of crank with the same gearbox, do you get the exact same loss of revs after an up-change when the load comes back on ?
Click to expand...

Yes, pretty much the same loss of revs. The bike does not lunge ahead, it does not spin the rear wheel and the front wheel does not come leaping off the ground. It does continue down the track at a higher rate of acceleration.
 
Dances with Shrapnel said:
Rod to stroke ratio is certainly a design consideration. The question is what are you designing to? Say a big lumbering cruiser, a heavier flywheel would be appropriate ...
Click to expand...

I hope we may soon end the discussion about heavy vs. light cranks. The math undeniably proves the facts re. acceleration, but if someone feels he/she has a benefit of using the inertia of the spinning flywheel, then good for him/her.

With the rod to stroke ratio of a Commando, it is considered a rather angry motor with a relatively long stroke and short rods. This and the excellent valve flow coefficients and combustion chamber swirl all contribute to the phenomenal off the line grunt the Commando engine offers.
Click to expand...

I am not that familiar with racing longstroke Nortons, but "the phenomenal off the line grunt" seems questionable when viewing the dyno graph above. The torque drop between 2K and 3.5K doesn't make the engine phenomenal in my eyes compared to contemporary production engines like the Ducati 750, which is superior according to the recorded dyno graph. The Ducati 750GT features oversquare engine parameters (80x74.4mm) yet it breathes better from low to medium revs than the Norton, no doubt thanks to the bigger bore permitting larger valves to be fitted. CR of the Duc engine was a modest 8.5:1.

Maybe you could call it "grunt" if starting with a slipping clutch and rev it past 3.5K rpm. Now, most racing bikes will sport bigger valves and higher CR, which alters the characteristics of the engine. I have yet to see the graph of a big-valve short-stroke (or even ultra-short stroke) 750 engine.

-Knut
 
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My god Knut, spend some time at or on the track and see when bikes launch. Yes, it is a clutch slipping situation and yes they should be well past peak torque and maintaining it there. Typically, the throttle is pinned wide open and one modulates power through the clutch lever. Starting at 2k to 3.5k is for light commuter traffic!
As a great example, my last race of the season three bikes were closely leading into turn 1, all Norton twins, one of which was even a large valves 650! The balance of the pack was comprised of well sorted out Nortons, Triumphs and BMWs. This is typically a replayed scenario, weekend after weekend. There’s something about a Norton big twin that allows it to launch exceptionally well. There is one exotic Ducati that occasionally shows up but it is something like a 900cc bore with a short stroke crank - really impressive.
 
One of the more pleasant modern bikes to ride is Moto Guzzi. So much torque you damn near don’t need a gear box.
 
Interesting.

I have a hypothesis here:

Norton’s, and especially Cdos have enjoyed a reputation for their ability to launch well from the very beginning, aided by those US magazine drag start group tests.

And in clutch start classic racing they continue to enjoy this reputation today.

But I think that some ‘normal’ riders (ie not racers or magazine testers) might have miss represented this and believe it’s down to the Cdos massively low end torque. Whereas in reality, those testers and racers are dropping the clutch with the motor doing 6,000rpm (give or take) and it has, basically, nothing to do with the low end torque!
 
Agree with Dances, my race bike with standard clutch (although running a 40mm belt) and tall 1st gear launches extremely well and I can often make up places into turn 1 as I dont normally qualify well.
It will wheelie if I dump the clutch from 3500 rpm and above.
Standard crank weight, 11.5 kg (24lbs), longer rods than standard , shorter piston, standard stroke length.
Regards Mike
 
'Off the line grunt' ? The last time I raced, I was using the 4 speed close box with the high first gear. I only got one good start out of five. That was when I revved the tits off the motor and dumped the clutch. Every other time the bike was as slow off the line as a wet week. The weak part in a Norton is usually the gearbox - easy to blow it to smithereens. Getting the motor to be fast enough is the easy part. I use methanol fuel but I don't think that makes much difference, for short circuit road racing a near standard 850 motor is fast enough for anybody. All you need to do is learn how to ride the bike. I was brought up riding bad shit-heaps so I probably learn more quickly than most people.
Brook, if your bike wheelies the weight distribution is probably wrong. Mine always spins the back wheel before it lifts the front.
 
JimC said:
One of the more pleasant modern bikes to ride is Moto Guzzi. So much torque you damn near don’t need a gear box.
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A friend of mine once said ' if you've got a torquey motor, you don't need a close ratio gearbox'. That is probably correct if you are happy riding slower. I don't think he has ever ridden a bike with a close box, the acceleration rate is much higher when you keep the motor spinning high and race-change up through the gears of a close box. A close ratio box in a road bike is a waste unless it is a two-stroke.
 
Brook, I have never raced with a standard Commando frame, so I don't know how they handle. However with most old British bikes, it is important to get the weight well forward. Nothing will turn you off quicker than to be doing 70 MPH in a corner and get that airy loss of purpose feeling. If your bike feels positive, it inspires confidence and you will ride faster. probably the ideal weight distribution is when the front just starts to lift when the rear wheel is losing traction. When I built the Seeley, I made sure I kept the weight well forward. It always spins the rear wheel before it lifts the front. This is the reason that a 500cc Manx is always faster than a 650cc Triton. With a Triton, you can never get the weight distribution and handling right, so in races it is always slower. I think the Domiracer was probably a pig, but it had Tom Phillis riding it.
 
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