Cycling UK Forum

Are these aerosol greases truly semi-fluid? From some of the descriptions it seems:

a) they are designed to be fluid/foam immediately after spray application so that they will run and cover all the moving parts/surfaces requiring lubrication,

but also

b) once the solvent that allows it to be thin enough to spray has evaporated, the residual grease coating is quite sticky.

In contrast the grease used in Landrover swivel housings (which has often been mentioned when SFG has been discussed in previous threads) is sufficiently fluid/thixotropic that it would tend to leak from components which do not have seals.

Therefore are these aerosol spray greases sufficiently fluid to be used on freehub pawls, or are they likely to cause the pawls to stick like a non-semi fluid grease would do?

slowster wrote:Are these aerosol greases truly semi-fluid? From some of the descriptions it seems:

a) they are designed to be fluid/foam immediately after spray application so that they will run and cover all the moving parts/surfaces requiring lubrication,

but also

b) once the solvent that allows it to be thin enough to spray has evaporated, the residual grease coating is quite sticky.

In contrast the grease used in Landrover swivel housings (which has often been mentioned when SFG has been discussed in previous threads) is sufficiently fluid/thixotropic that it would tend to leak from components which do not have seals.

Therefore are these aerosol spray greases sufficiently fluid to be used on freehub pawls, or are they likely to cause the pawls to stick like a non-semi fluid grease would do?

Define 'quite sticky'? Compared to thin oil or WD40 of course it will be 'sticky'.

Standard grease is far more 'sticky' than this stuff, it won't hold a bearing in place like solid grease does, I think you're worrying over something that simply isn't a thing or are misunderstanding the product.
I've used it on more than 20 freewheels, they've all been better afterwards.

'define sticky' is actually quite a good question in this case.

Most greases have very non-linear viscosity vs shear load characteristics. The usual idea is that the grease stays put until it is sheared, at which point the viscosity becomes much lower than you might expect. (Tomato ketchup does this, which is why it all comes out of the bottle at once.) The viscosity characteristic also can have variation with temperature, time etc, so grease has 'a memory' i.e. it responds differently if it has been recently sheared. Traditional greases can also separate into oil plus thickener. This subject is really complicated (and I don't claim a full understanding of it by any means) but in the case of greases it helps to determine how slippery the lube feels, if grease 'sticks' to a surface or not, or whether it all slides off a surface in response to gravity, or perhaps even shears off leaving a layer on the surface.

When a SFG is worked, the layer nearest a (non-contacting) surface often sees the least displacement, so sees the least shear-thinning. This means that (having been worked) a thicker layer of grease may sag or run away under its own weight, but still leave a layer behind on the surface.

Ideally a SFG for a shifter will adhere to surfaces, yet offer a low viscosity once sheared, so the shifter doesn't dry out and yet moves freely.
By contrast land rover swivel SFG has only not to run out of bearings at a high rate (at a standstill), and still shear easily in use; in typical bearing applications it is churned about and gets distributed around the required parts by their motion, and a slightly higher loss rate in use is tolerated.

In high speed applications it is claimed that the viscosity of some greases tends towards the viscosity of the base oils, i.e. the contribution of the thickener to the viscous drag is relatively small. In others, it isn't like this at all. Other additives can be used to make the lubricant 'stringy' for example, and this also affects how it is distributed in use etc.

So SFGs are all different from one another. This isn't very well captured by the NLGI rating system, which classifies the viscosity rating using a fairly crude system based on a penetration test on a freshly worked sample; the velocity profile of the penetrator might be quite different with different greases, even for the same penetration depth, and furthermore the loads the grease sees may not resemble service loads either.

cheers

When re-lubricating freehub pawls, DT Swiss ring ratchets, etc, I use a 50/50 mix of heavy oil (SAE 90) and ordinary grease.