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Reciprocating weight

Discussion in 'Norton Commando Motorcycles (Classic)' started by acotrel, Sep 8, 2018.

  1. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    I am wondering - would the combined weight of Jim Schmidt's long rods and light 12:1 comp pistons for an 850 Commando weigh less in total, than the combined weight of standard comp. rods and pistons from an 850 Commando ? In my experience, it has always seemed that achieving higher compression ratios to get more power, has involved using heavier pistons because the crowns are higher and more domed. However fitting lighter pistons to a twin cylinder 360 degree motor allows it to spin up faster. Perhaps Jim Scmidt's rods and pistons are a way of achieving both higher comp and lower reciprocating mass ?
    Also I would question about whether anyone on this forum has priced or used Arrow titanium con-rods for their Commando..

    http://arrowprecision.com/pages/conrods
     
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  2. Triton Thrasher

    Triton Thrasher

    Joined:
    Feb 10, 2009
    Well weigh them.

    Debate won’t settle it.
     
  3. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    So I should buy them without knowing ?
     
  4. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    I think the JS weight info is all on his web site, if not, an email will soon clarify.
     
  5. p400

    p400 VIP MEMBER

    Joined:
    Jul 14, 2014
    What are your current reciprocating component weights?
     
  6. worntorn

    worntorn

    Joined:
    Dec 22, 2006
    Not all of the info needed is on the site.
    I have some complete stock 850 rods and pistons here. The combined weight of rods with bolts &shells plus pistons with rings, pin and clips is 824 grams.

    I get 761 grams for the same combo from the JS website, using his standard lightweight parts.

    Glen
     
  7. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Thanks for that - 63 grams lighter times 2 ? The JS products seem to be significantly lighter, so I would get a performance boost from that. I wonder how much extra weight is involved in the higher crowns of his 12 : 1 comp. pistons, compared with standard comp. pistons ? Going high comp. is good with the methanol fuel, but added reciprocating weight can be a bigger step backwards. If I can get higher comp. , even without a substantial reduction in reciprocating mass , the motor should be better. Every time the pistons and rods reach near top and bottom of the stroke, they reverse direction and the deceleration consumes energy. I once fitted much lighter pistons into a Triumph 650 and there was an improvement in performance. I'm thinking of my next upgrade.
     
  8. worntorn

    worntorn

    Joined:
    Dec 22, 2006
    Yes, 63 grams per cylinder.

    Glen
     
  9. Fast Eddie

    Fast Eddie VIP MEMBER

    Joined:
    Oct 4, 2013
    Al, you have to ensure you’re comparing apples with apples. Pic below shows two pistons I had, a ‘normal’ design of Omega 10.25:1 piston vs a JS piston that actually came out at 10.5:1. I forgot the specifics, but recall the JS being about 1/3 less weight.

    However, if you wanna compare apples with oranges, I’m pretty sure that I noted that my 10.5:1 850 JS pistons were lighter than stock 750 pistons.

    I’m sure Jim could tell you if you mail him, but I do not believe the domed pistons to be significantly heavier as he can machine under the crown, thus the crown thicknesses are the same, thus the extra metal involved is relatively insignificant.

    I’m sure you know all this already, but total weight is not so important. Weight at the big end as FAR less impact than weight at the piston end.

    In my limited experience of such things, setting up a flat top piston with a .030”-.040” squish will give around 10.5:1. I don’t know what JS domed piston would give.

    BTW & FWIW, my understanding from others is that a squish band has little or no effect when running methanol.


    3700FC07-A551-419A-B42F-7C6EAC47DAB6.jpeg
     
  10. jseng1

    jseng1

    Joined:
    Nov 26, 2009
    Here is the 850 lightweight domed piston. It weighs only 210 grams. It as a 3D undermilled dome (uniform thickness) so it costs a little more than my 850 flat top pistons but the weight is about the same.


    [​IMG]

    rings are 18g
    pin is 58g
    small end of rod is 106
    big end of rod is 328

    Reciprocating total is 392 one side
    Big end total with shell bearings is 396 one side

    The longer rods also help reduce the shaking forces and stress to smooth things out and increase reliability.
     
    Last edited: Sep 10, 2018
  11. lcrken

    lcrken VIP MEMBER

    Joined:
    Mar 15, 2009
    For a direct comparison, I measured a late stock 77 mm bore 850 reciprocating weight (piston, rings, circlips,pin, and rod small end) at 488 grams per side. That's a 19% reduction in reciprocating weight with Jim's kit, a significant amount in terms of vibration and stress.

    Ken
     
    Last edited: Sep 10, 2018
  12. worntorn

    worntorn

    Joined:
    Dec 22, 2006
    So from Jims post my numbers are a bit off for the JS total weight.
    I had 761 grams complete.
    Should be
    396 big end with shells
    106 small end
    210 piston
    18 rings
    58 pin ( with clips?)
    =788 grams vs 824 stock.

    That is for flat top /flat top.
    The advantage when using the JS domed vs Hepolite etc will be greater.

    And these are total weights, point already made re importance of reciprocating weight.

    Glen
     
  13. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Jim, is that one of the 12:1 comp. pistons you have listed ? And is it possible to buy Arrow titanium con-rods which would be similar in dimensions to the ones you normally supply - ex their stock ? I don't know about he effect of the squish-band with methanol - most guys run too rich - when that is happening nothing really matters because methanol hides up their tuning errors. To get the best out of methanol is as difficult or easy as it is with petrol. Phil Irving says that if it is run rich, you still get good power. That is correct, but you don't get BEST power that way. If you leanit off to just before you get destruction, you get the best performance. A slightly thick crown on he piston might be desirable. All things considered, low piston weight is probably more important than higher compression ratio.
     
    Last edited: Sep 10, 2018
  14. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Are jim Schmidt's long conrods steel Carillo ? For the same dimensions titanium rods would be lighter and stronger.
     
  15. lcrken

    lcrken VIP MEMBER

    Joined:
    Mar 15, 2009
    Lighter, but not necessarily stronger, for the same dimensions. The tensile strength of the steel Carrillo uses is higher than the typical 6Al4V titanium used for rods, with both being heat treated appropriately. To make them stronger, you would need to increase some of the dimesions, although they would still be significantly lighter than steel rods. And yes, Jim's rods are Carrillo steel, pretty much the industry standard for steel rods. Carrillo uses a proprietary 4330M steel, a high strength NiCrMoV alloy. A while back, Carrillo decided to offer custom titanium rods on request, but they were pretty pricey. Now that they've merged with CP, I don't know if they still offer to do custom titanium rods.

    Ken
     
  16. jseng1

    jseng1

    Joined:
    Nov 26, 2009
    The JS Carrillo rods are bushless and that saves about 30 grams each on the small end compared to bushed Carrillo rods. You must use DLC coated pins. The small end on my regular rod is about 106 grams. The small end on my ultralight rod is about 95 grams (up to 75 HP).

    A titanium rod must have a bronze bush so it probably won't be much lighter on the small end. It would be a lot more expensive and may end up with the same small end weight. Lets hear some gram weights of the small ends of titanium rods (with the same strength as steel) so we can make some accurate comparisons.
     
  17. mdt-son

    mdt-son

    Joined:
    Jan 19, 2012
    For conrods, not only tensile (and compressive!) strength is to be considered, but also buckling strength. Buckling strength is a function of the rod shank's cross-sectional area, it's static area moment of inertia (an engineering figure describing a section's bending stiffness), AND the material's modulus of elaticity as determined by Hooke's law. Now, for Ti-6Al-4V the E-modulus is 88--116 GPa in SI units, while for highly alloyed steel the E-modulus is about 200 GPa. That puts Titanium rods in a distinct disadvantage. The counteraction is an increase of the cross-section, which of course causes the weight to increase also. Another important issue is fatigue strength of Ti vs. Steel. For Ti, the strength is very high initially but drops decidedly as load cycles accumulate, aproaching an infinite life at 250-300 MPa cyclic load stress. NiCrMoV steels excels with a figure about 3 times higher, suggesting that steel rods should be the preferred rod material for all engines where a long life span is demanded.

    -Knut
     
  18. mdt-son

    mdt-son

    Joined:
    Jan 19, 2012
    This should be compared to the stock conrod small-end weight of 75-78 grams including the bush, according to a previous posting by Ken. The steel conrod used in Norton's shortstroke engine was heavier at the small end - 145 g according to Ken. Of course, looking at the conrod only is meaningless - the piston and gudgeon (wrist) pin also have to be taken into account. It would be hard to undercut the total reciprocating weight offered by Jim's conrods and pistons.

    -Knut
     
  19. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    I use a few aircraft titanium bolts on my bike. I've noticed that if they come loose when holding a bracket, the vibration causes a dramatic change in dimension. As a con- rod, the cycling might have less effect, but I take your point about fatigue. I'm pretty much convinced that for me, Jim Schmidt's 12:1 comp pistons and long rods are the way to go when I next rebuild my motor.
     
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