Grease is the word; a complicated word...

General cycling advice ( NOT technical ! )
meandros
Posts: 15
Joined: 1 Jan 2018, 7:34pm

Re: Grease is the word; a complicated word...

Postby meandros » 14 Aug 2018, 12:42am

As far as my experience with lithium based greases and bike maintenance goes, the additive MOS2 is not required. More so, MOS2 grease should only be applied where there is metal to metal contact and high temperatures occur (which is never the case with bikes, really - at least not the temperature required for the MOS2 to kick in - that is mostly applicable to petrol engines and extremes such as firearms), but it is overkill elsewhere and not to be applied on plastics as it acts as an abrasive. Even for IGH maintenance, MOS2 is not necessary - and may damage rubber/plastic seals. Moreover, MOS2 "enriched" greases may not seal as right as a regular lithium based grease would (in hubs and such), even if it has the same NLGI number, e.g. #2. For the most part (and except for the forks) marine grease should take care of all your cycling maintenance needs, for the long term.

Brucey
Posts: 30450
Joined: 4 Jan 2012, 6:25pm

Re: Grease is the word; a complicated word...

Postby Brucey » 14 Aug 2018, 1:02am

whether MoS2 (or indeed any solid lubricant additive) gets a chance to work as an abrasive on plastics is dependant on the particle size vs the film thickness of lubricant. If the latter is well in excess of the former, there is little or no scuffing. Since plastic and rubber parts are usually lightly loaded seals etc in bicycle components, with reatively thick lubricant film between it and the neighbouring parts, any worres about MoS2 particle abrasion are ill-founded.

Whilst MoS2 is widely used at high temperatures, it is an effective lubricant additive even at coolish temperatures; without it (or something similar) such things as CV joints simply would not work.

To assert that such additives 'are not necessary' is to assume that you know that the loading conditions in all bike parts are not severe enough to cause wear even when it is omitted. I can say with some certainty that this is simply not the case.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

meandros
Posts: 15
Joined: 1 Jan 2018, 7:34pm

Re: Grease is the word; a complicated word...

Postby meandros » 16 Sep 2018, 9:47pm

Hi Brucey,

Admittedly I am not a grease professional and am only basing my assertion on my own experience. I have thought long and hard about this but for the life of me I cannot think of any bike use scenario where temperature rises as dramatically as it does, say, in the barrel of gun going off (coaster/roller brakes excluded ;) ). Now, that particular scenario being one of the more extreme cases, MOS2 is best used in a scenario where the grease is worked closely or (in extreme cases) over it's dropping point. Within bike applications, extreme temperatures are not so much of an issue as, say, extreme pressure points (bearings and even cogs in the case of IGHs). I found out that a proper extreme pressure lithium grease will work in most cases, no other additives required. Moreover, it looks to me that a quality extreme pressure (~very high dropping point) grease will work better as it provides not only an additional seal but also improved vibration reduction characteristics. The MOS2 grease I have used (even if specified with quite high dropping points, did not provide the additional seal I was looking for and proved to be rather fluid when used for bearings and races). Admittedly it is not a scientific conclusion however, as far as I am concerned, I am done with MOS2 as far as bike maintenance goes.

Brucey
Posts: 30450
Joined: 4 Jan 2012, 6:25pm

Re: Grease is the word; a complicated word...

Postby Brucey » 17 Sep 2018, 1:09am

in order for the dropping point of a grease to be important the global temperature needs to be high, and as you say in most bicycle applications it isn't. However that doesn't mean that additives that 'work' at high temperatures are wasted in a lubricant that sees low global temperatures; far from it! You should not conflate the presence of things like MoS2 in a grease with that grease being specifically designed to work at high global temperatures only; the grease may well be designed to operate at high pressures and that alone is sufficient to justify its inclusion.

The reason is that whenever there are high pressures there are also high temperatures; it is just that these may be very localised and also transient. However they are high enough and they last long enough that all kinds of EP additives and solid lubricants have a chance to work in the way in which they are intended to, despite the low average temperatures.

For example if there is a particle of steel that is 'run over' by a ball, and crushed, it will reach temperatures of several hundred degrees and unless protected in some way there will be galling and welding between the surfaces. Chemical EP additives help but are not a complete solution; this exact mechanism is one that causes a lot of wear.

And local pressures are very high in bicycle bearings; they are moving very slowly (vs similar bearings etc in most industrial machines) so the metal surfaces are not separated by a film of lubricant as is found in full hydrodynamic lubrication. At best you get a mixed mode in most bicycle applications, and more often than that it is a boundary condition that applies; exactly the conditions that are most mitigated by having solid lubricants of various kinds in the grease.

As I explained (at some length) earlier some lubricants that work well at low speeds don't work well at very high speeds (speeds that are not seen in bicycles), such that rolling element bearings may not roll but can start to scuff instead, which causes all kinds of problems. Or the work rate in the grease is so high that it overheats, just from the viscosity of the solid lubricants in the oil, leave alone the effect of any thickeners. So greases that are designed to work well at low speeds only are not to be easily found in (say) a car spares shop; the applications for them don't exist in most modern cars. The greases that you will find easily that work OK at low speeds are usually ones with limited MoS2 content; just enough to make them work OK in CV joints etc (but they are far from the best greases for this) but not so much that they won't work in higher speed bearings. Otherwise you would get idiots putting high solids greases (for low speed applications only) into wheel bearings, which can cause the stub axles to shear at motorway speeds.... :roll: Arguably the closest thing you will find to a proper low speed lube is something like a MoS2 assembly paste, which can contain up to ~50% MoS2; probably this would be pretty good in bicycle bearings....

The other thing is cost; solid lubricants need to be manufactured and graded into particles of controlled size before they are added to lubricants; this can be expensive. Lubricant manufacturers don't bother making lubricants that are more than good enough for a given application; 'just good enough' is 'plenty good enough' in their book, and in most cases they are more fixated on lower running drag (i.e. lower friction through viscous losses) whilst retaining adequate life than they are in creating a lubricant that will completely eliminate wear per se.

Similar concerns exist in bicycles; for example if you run hub bearings in oil, there may be lower drag whilst the wear rate is low enough that the service life is acceptable. Or the wear debris gets flushed out by a 'total loss' regime, in which oil comes out of the bearing in service (carrying the wear debris with it) and needs to be renewed on a regular basis; this is how most bicycle bearings worked for about the first hundred years of there being bicycles with rolling element bearings in them. Ditto running bearings without full contact seals. These things may 'cost' less than 1W but when you are racing every little helps, or so it is believed.

[Witness those that spend a fortune on 'ceramic bearings' because 'they are faster'. Well the main 'benefit' of using them (which is actually tiny anyway BTW) comes from the thinner lubricant that can be used in them, whilst retaining an adequate service life: If the bearings were better designed in the first place, they would tolerate being run in oil for a race distance anyway; no need for ceramic bearings.... this is how Hinault's race mechanic prepped TT bikes BITD...]

if you choose your lubricant carefully, you can get extremely low wear rates in all kinds of bicycle components, such that their service lives are practically infinite in wear terms; the parts are then likely to fail by fatigue, eventually. However because of the low speeds and high pressures seen in bicycle bearings, solid lubricants are, I think, a vital ingredient in the best bicycle lubricants.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~