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Hiya. My replies below in purple in good faith (Oh no: oil thread descends into typical oil thread thread... play nicely children lol.) Good to have the brain cells stretched, cheers 😊
I will not go into arguing that an exponential decline curve is a simply a special case of a hyperbolic curve.
I thought that too bernie, but not clever enough to go into the mathematics that have long since been erased from my aging brain
I finally looked it up and the empirical formula for liquid viscosity is indeed a two parameter exponential. I admit some laziness here, as I saw the curve and knowing that oil will freeze into a solid given low enough temperatures (Ie: infinite viscosity) I assumed it was asymptotic - a hyperbola. The point is that it's a non linear curve. ( I will not go into arguing that an exponential decline curve is a simply a special case of a hyperbolic curve. )As for engine temperature being linearly dependant on ambient temperature, yes, it is somewhat more complicated than that. For example, a water cooled engine with a thermostat and switchable cooling fan will not necessarily get as hot as ambient plus a fixed number. But for something like a rear drive (Or an air cooled engine) , I believe it's essentially correct that it's operating temperature will be ambient plus X. Here's why:I come from an electronics background where device junction temperature is calculated as : Tj = Ta + Pd x Rja (Pretend the lower case letters are subscripts.) Tj is junction (ie; device) temperatureTa is ambient temperature Pd is power dissipatedRja is thermal resistanceThe formula tells us that the device temperature is equal to the amount of power (times a factor) PLUS the ambient temperature. In other words, with everything else being equal, the device temperature is ambient temperature plus X. My reasoning is that it matters not whether the heat is generated electrically or mechanically nor whether we are talking about a chunk of silicon vs a chunk of steel.The rear drive is going to see huge variations in operating temperatures (And therefore viscosity) with different climates and for that matter different riding styles. I agree that putting sewing machine oil or grease in the rear drive is a bad idea. But otherwise, put something labelled as gear oil in there and be done with it.There is a far more important factor for rear drive longevity. EP oils have support additives that get depleted over time. That can lead to gear pitting and worse. To extend the life of your rear drive, do regular oil changes. If you live in a hot climate or ride like (Insert favorite Moto GP racer here) , replace it more often! BTW, some may be surprised to read that SAE 90 gear oil has the same viscosity as SAE 50 motor oil. It's all about the additives.
Yup RPN fanboy here (sorry) HPs were always my favourate but expensive here in the UK.
Gearbox (and engine) still slightly different case though:
Forgot to add. I’m under the impression that gear oil differs from engine oil in that gear oil does not have viscosity improvers. So a motor oil may act like a 10W when cold, but the VI makes it act like a 40 weight when hot. In contrast, gear oil has to fall within the viscosity specs “naturally.” I think that is because VI additives are easily sheared, and gear boxes are shear machines.
Shall we get into the importance of shear strength? Viscosity is only part of it!
You do realize I can never speak to you again.
It wouldn't have even occurred to me to use grease in a gearbox ! Was this a thing for older low powered machines?
This is an interesting topic, even though I suspect it ultimately makes no practical difference.
This is an interesting topic, even though I suspect it ultimately makes no practical difference. I disagree lol I’m assuming there is some kind of engineer’s formula that is used to spec things like gear oil grade and viscosity. You plug in important parameters like the size of gears, type of gear, type of bearings, rotational speed, load, expected heat, etc. Then the formula gives you a recommended range of outputs. Yes there are various charts used that deal with gear surface finish, clearance, load, speed, etcDoes anyone know that formula? Tried to apply it to a Guzzi? Impossible without engineering drawings My hunch is the output of that standard formula probably translates to “75 to 90 weight is fine. 80 is optimal absent special circumstances.” As Margate Thatcher would say: no, no, no ! Please see my first post as to why
Question to PhilnewbikeDid you use an expression like this to draw your graphs?V=e^(b1+b2T)Where V is dynamic viscosity (mm^2/s or Cst) T is temperature (K)b1 & b2 are constantse is Euler's constant (aka Napier's constant or base of natural logarithm)Just wondering as I wrote a short program for my calculator for both the above and the Walther formula I posted earlier so as I can compare results
As a mechanical engineer, and former mechanic on all sorts of old junk...... I'd be very comfortable betting the farm that you are right SmithSwede.The two concerns that are probably much more a factor than actual gear face wear are 1. Noise. and 2. Specifying a viscosity that makes the differential work properly (N/A for Guzzi's).Until someone wants to perform two ride till gear failure, or ride 100k miles on two identical bikes with different oils and measure wear, it's all just fun campfire banter. Yes and no. It is objectively fact that using 75W90 instead of 85W90 or 90 will mean you're running on an oil much thinner than the oil engineers at Agip (and BMW/Getrag in my example) recommended. As for using 75W140 etc expensive stuff then yes, more subjective Also it is fun to spoil our babies with special oil treats.Don't run it dry, don't fill it with cheap vegetable oil from Piggly Wiggly, don't use genuine Moto Guzzi Invisiable Bearing Grease (tm), and enjoy the ride.
No nothing that complicated. I had two data points and know that the function of viscosity they follow is fairly exponential. I therefore plotted the variable as a log base 10 so I could read off in-between readings. Your way sounds better
Now I'm just confused, although that doesn't take much. Disclaimer: I an an engineer but not an oil or gearbox expert (but I know a couple and have a read a lot), so though I do think I know more than the average oil threader (the reason I posted) I am by no means 'gospel' !Are you saying you used log10^x and not e^x to plot the viscosity, I thought earlier you stated that you used the exponential function, which I understand as "e", if I'm labouring under a misapprehension please tell meYes. See here:https://www.engineeringtoolbox.com/dynamic-viscosity-motor-oils-d_1759.htmlThe plot of viscosity is approximately a straight line when done on a log10-linear graph. OK not perfect but not far off. So, given I have 2 data points (visc at 40 and 100 C is always given in data sheets) and the knowledge that log10-linear is approx straight, I fitted a line of best fit through those data points using a log10 function in Excel. I then read off the in-between numbers at eg 65 C (gearbox temp)I don't know if it's a better solution or not given Walther's equation uses log10, albeit twice. Pretty easy to do as a little bit of high school algebra will give you 2 equations with 2 unknowns.So b1=lnV1-T1*(lnV2-lnV1)/(T2-T1)b2=(lnV2-lnV1)/(T2-T1)WhereT (K)V (mm^2/s or CSt)I'm not familiar with the Walther's equation sorryI reckon overall Walther's equation will probably be the more accurate of the 2 methods (see my earlier post), but even then it will be ballpark (at best).I'm a bit off topic here as what I'm looking at is a method to calulate engine oil viscosity at different tempsWhat I'm trying to do is calculate what the change in viscosity will be for a given grade and manufacturer at a specific temp, percentage and absolute. Good idea. I think the main thing missing from my estimate is the viscosity index (VI) variable - i.e. each oil will have it's own VI number and so will actually need a slightly different line of best fit. This goes back to what the chap above, Smithswede, was saying about gear oils and VI polymers. Actually some gear oils of wide range do use VI polymers (like 10W60 engine oil does) but in the gearbox are in a environment perfect for chopping up polymer chains and thinning the oil towards its 'W' base viscosity. E.g., if you use an 75W140 then this (according to my rough plotting) will be OK as the viscosity at the operating temp is approx same as 85W90. But, after a period of time the polymers might be chopped up and you slowly end up with a 75 weight. It is hard to say which gear oils have VI improvers and which don;t though... Using that I can determine the equivalent temp of the new calculated viscosity would be with my existing oil. I already know what the pressures will be with my existing oil at various temps so I can determine if changing the oil weight would make any difference and what that the pressure difference might beAs an example I currently run 15/50 and I know that when the sump temp gets above 110C the pressure starts to nose dive and by 120C the pressure is around or a little below 10psi at idle (oil light on and confirmed with a gauge). So if I go to a 10/60 for example I could get an idea if at 120C the additional viscosity would be as high as my exisitng 15/50 at 110C (I'm just throwing out numbers here).I'd suggest 20W60I know the 10 part is irrelevant but there's another reason that a thinner grade would help when cold, which I can also now roughly calculateHowever theoretical and experimental are 2 quite different animals
EP additives and Brass synchros don't play well with each other. Paul B
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