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Maximum torque from a Commando.

Discussion in 'Norton Commando Motorcycles (Classic)' started by acotrel, Dec 14, 2017.

  1. Dances with Shrapnel

    Dances with Shrapnel VIP MEMBER

    Joined:
    Jul 8, 2011
    A bit off topic but it has my interest. Yes, a Norton vertical twin will probably have more vibration than an XR750 vee twin.

    Do you have a handle (estimate) on the deflection of a standard Norton crank at say 7,000 rpm expressed as displacement and/or displaced mass. My hunch is the deflection and resulting offset of mass would amount to a fart in a windstorm when compared the vibration induced by the "static" out of balance condition of a Commando crank and reciprocating components. I vaguely recall seeing calcs done and the forces are in the thousand or thousands of pounds.

    I think the stock Norton crank was decent design for the intended purpose and with an improvement in the materials of construction, is suitable for significantly higher outputs. An excellent example is what Steve Maney offers; three piece bolt together but better materials - billet steel cheeks and billet steel flywheel as opposed to the factory cast steel cheeks and cast iron flywheel.
     
  2. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    The XR750TT in one form is a road race machine. All vertical twins and singles when road raced have cranks which are balanced to give smooth running through a narrow usable rev-range. With a street bike, it gets used over a much wider rev-range. The balance factor in a street bike cannot be set to cover both commuting and high speed running. That is the reason Commandos have isolatics.
    The XR750TT which Don Emde rode at Calder in about 1973 had a failure in the timing side bearing, so did not compete against our fast guys. I believe that failure mode was fixed in later Sportster motors. A late 1000cc Sportster motor in a MK3 or Mk4 Seeley frame could be a good thing if, fitted with a 5 speed close ratio box.
     
  3. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    I am amazed that the Commando engine is as good as it is. There is a lot about it which is excellent. Rigidly mounted with the correct balance factor, it is a real rocket. It is just a pity that the Atlas could never compete with the CB750 Honda for smoothness. I have a friend who has a road race Atlas which he used to race back in the 1970s in A grade races. The meeting organisers used to get upset that a road bike did so well against production road race bikes. He used to race it with the registration number plate on the back.
     
  4. acotrel

    acotrel

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    Jun 30, 2012
  5. Dances with Shrapnel

    Dances with Shrapnel VIP MEMBER

    Joined:
    Jul 8, 2011
    Last edited: May 10, 2018
  6. mdt-son

    mdt-son

    Joined:
    Jan 19, 2012
    Hi Dances,

    Out of curiosity, which dyno test of a *stock* Norton 750 are you referring to? I am curious to learn the actual shaft torque compared to the factory quote of 48 lb-ft.
    Was the Norton Commando ever tested on CycleWorld's dynomometer under identical conditions as the HD Revolution X 750 ?

    Interesting observation, the measured max. torque was 43.2 lb-ft @ 3790 rpm in the referenced dyno run. The official HD figure for the 2018 model is 44.5 lb-ft @ 4000 rpm (HD refined the injection system in the meantime). Not bad, just 1.3 lb_ft off and within the expected deviation by testing conditions and some tweaking. The factory would measure directly off the crankshaft of course. Certification of engine power and torque by HD is according to SAE J1349.

    Factory figures should use the minimum of measured results but I am not sure they always do.

    The dyno values are of course crankshaft torque figures calculated from rear wheel readings.

    -Knut
     
  7. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    One of the things which make me wonder about using a dyno for tuning purposes, is the accuracy and precision of torque measurements. In my own situation, it is always seat of the pants - I know how long it takes me to get down a certain length of track. However I never seem to know if I've improved torque output, until I raise the overall gearing and make the bike pull harder. It doesn't seem to matter what I do, the motor always spins up at the same rate if the gearing is too low - i.e lowering the gearing does not make the bike accelerate faster. With a Commando engine, it is all about improving torque because revs kill them.
     
  8. Dances with Shrapnel

    Dances with Shrapnel VIP MEMBER

    Joined:
    Jul 8, 2011
    I searched this thread and cannot find where I was referring to a dyno test of a *stock* Norton 750. What I was referring to was the peak power of an HD Revolution X 750 to a Stock Norton from my base of knowledge and experience. The stock HD 883 Iron is probably the closest match to a 750 Commando both in peak torque and peak power and it achieves it with more capacity than a 750 Commando. Your factory quote of 48 lb-ft is about right for a stock 750 Commando.
     
    Last edited: May 11, 2018
  9. o0norton0o

    o0norton0o

    Joined:
    Apr 27, 2015
    Lowering the gearing DOES increases acceleration rate. I'm not going to explain it to you, because it's complicated, but just go to youtube and watch a few video explanations of torque, HP, and acceleration.
     
  10. worntorn

    worntorn

    Joined:
    Dec 22, 2006
    Sure does, just ask the front wheel on a Sportbike after dropping a tooth on the CS and adding a couple of teeth at the rear.
    All of a sudden the front wheel won't stay planted 1st three gears.

    Glen
     
  11. mdt-son

    mdt-son

    Joined:
    Jan 19, 2012
    Actually, CycleWorld tested a 750 MkV in 1973. Cylinder head would have been RH6 giving a CR 9.5:1 (Cycleworld quoted 10:1) and carburettors were 32mm. Camshaft was the standard SS one. Still, the results fell short of the factory quoted figures. The drop in torque with revs above 6500 rpm indicates insufficient breathing. Particularly surprising is the drop in torque between 3K and 5K. Deducting the effect of the higher compression (std. CR was 9:1), the measured torque curve would have been even lower.

    Fig. 1
    Dyno test Norton Commando 750 Mk V Cycle World 1973.png

    In 1974 Cycleworld tested the Commando again, this time the JPN which as basically an 850 Mk2A. As can be seen, the mid-range torque between 3500 and 6000 rpm has been bolstered significantly at the espense of high rpm torque where the drop off is mor pronounced than readings from the 750. The torque dip at 3-3.5 K rpm is still rather pronounced which is disappointing.

    Fig. 2
    screenshot.png

    https://www.accessnorton.com/NortonCommando/6400-rpm-in-top.18481/page-3
    http://www.classicbike.biz/Norton/Mags/1970s/74JohnPlayerNorton-Cyc.pdf


    Diagrams have different scaling obviously.

    I'd like to see the dyno graph of a shortstroke 750 (F1 engine or similar). I think the shortstroke engine has a greater potential for producing a consistently flat torque curve (say, at 45 lb-ft) than the longstroke motors, which would be useful for a road bike.

    -Knut
     
    Last edited by a moderator: May 11, 2018
  12. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Sportbikes don't usually have massively heavy cranks. The reluctance to spin up on the throttle because of the heavy crank, was one of the first things I noticed about the 850 Commando engine. With a close box, the heavy crank keeps spinning at high revs, so a racing up-change gives max acceleration. When you lower the gearing, the crank does not tend to spin up any faster - because of the inertia. So what you do effectively is ride the top of the torque curve. If the motor is not torquey, the revs will fall off the top of the curve, then you depend on throttle response to recover. When I ride my bike, the revs usually stay well over 5,500 RPM, both going up and coming down through the box. I usually up-change at 6,500 RPM and often see about 7,300 RPM. The drop in revs on an up-change is virtually nothing. With a big drop, you will wait forever for the crank to spin up. A light crank is far more responsive, but the drop in revs is usually larger due to lack of crank inertia. Pulling a wheelie with a Commando is not easy, it will usually spin the back wheel first, unless you risk shagging the gearbox.
    The argument about light and heavy cranks is not very productive, you win and lose with both types - the gearing is different depending on which one you choose. If you get your gearing wrong with either type, you go backwards. I have always raced with light cranks, however I really like what the heavy crank in the 850 does. The motor has real guts - nothing stops it. I found it really strange when I raised the gearing and it accelerated faster.
     
  13. Dances with Shrapnel

    Dances with Shrapnel VIP MEMBER

    Joined:
    Jul 8, 2011
    Generally, for an identical engine with a lighter crankshaft, it will yield greater acceleration. No argument here. Draw your own conclusions.
     
  14. o0norton0o

    o0norton0o

    Joined:
    Apr 27, 2015
    NOPE, the only way a raising the gearing could make you accelerate faster, is if you raised the engine's HP as well. This isn't my opinion. It's a factual mechanical principle which I've said twice in this thread, and you obviously don't believe.
     
  15. worntorn

    worntorn

    Joined:
    Dec 22, 2006
    Bob is right. Its pretty straight forward

    Newton's Second Law of Motion is F= ma.

    Lower gearing increases F. M stays the same so A is greater.
    That's why Norton geared the Combat down with a 19 tooth CS.
    Much more relaxed with a 20 or 21 and possibly higher top speed, but quicker in acceleration with a 19.
    It was important to have an impressive quarter mile et. It helped sales

    Glen
     
    Last edited: May 11, 2018
  16. Fast Eddie

    Fast Eddie VIP MEMBER

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    Oct 4, 2013
    Raising an lowering gearing is a relative thing though isn’t it?

    What I mean is, maybe ALs bike had stupidly low gearing in the first place, and by ‘raising’ it, he has simply corrected it?
     
  17. o0norton0o

    o0norton0o

    Joined:
    Apr 27, 2015
    YES,... but acceleration isn't everything in a road race. Top speed can be more important depending on the road course configuration. I don't know what Al means. I'm only talking about physics. He may misuse the term acceleration by assuming that since he's doing faster laps with the taller gear that he has greater acceleration. Greater acceleration doesn't mean faster top speed...
     
  18. acotrel

    acotrel

    Joined:
    Jun 30, 2012

    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 ?
     
  19. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    If you gave a standard Commando a dose of nitro, would it accelerate faster if you did not raise the gearing ? I don't think it would by much. Nitro improves torque.
     
  20. o0norton0o

    o0norton0o

    Joined:
    Apr 27, 2015
    Alan,.... You're right... What was I thinking trying to teach you physics...
     
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