Originally Posted by Pete Hamer
Since you are going to know more about grease than me, I'd like to ask you a few questions.
Everything I've read states that he most important part of a grease is the viscosity of the oil used to make it, high viscosity oil for high speed applications, low viscisotiy oil for low speed applications. Is it safe to assume that automotive greases are made from high viscosity oils? What would the theoretical problems be if you used a grease made of high viscosity oil in a low speed application and visa versa?
I've also heard that aoutomotive greases are becoming increasingy acceptable for bicycle bearings. Is this a function of adding paraffinic oils after cooking the grease? My understanding is that the paraffinic oils added, to back off the apparent viscosity, aren't "trapped" in the thickener like the nepthanic cooking oils, therefore the paraffinic oils are more readily available to lubricate. Or is this because of EP additives that allow the automotive greases to release the oil at a lower load/temp? Or is it because this calcium sulfonate complex is becoming more widely used?
Edit: Is there any merit to the belief that some types of automotive greases don't perform well at the relatively low teperatures seen in bicycle bearings? If so, what greases would those be?
Thanks.
You are so kind, to give me so much credit! Sadly, I'm still trying to learn most of these answers myself. I hope someone else can shed more light. Jobst Brandt talks a lot about the importance of viscosity (I'll try to find a link for this) in regards to how well a grease can refill a space in a moving bearing. Especially in a very low-movement bearing like a headset, it's vital that the grease be able to re-coat the balls with very little movement side to side. That is what prevents them from making micro-welds with the races upon impact, and pulling loose material from the race, which (in Jobst's opinion) is indeed what causes "brinnelling". Intuitively we might always think a thicker, stickier grease is better--and I used to use aluminum-based wheelbearing grease for that reason, it's stiff and very sticky. But what happens with these thicker greases is that they just get shoved to the side, as the balls roll around. That grease doesn't re-fill and re-coat, so the net result is a big loss of protection. This is most likely to happen in a low-speed application like a bicycle bearing.
The Lubrimatic is the same consistency as Poly-Lube. Buttery, tacky enough to stay put within the races very well, but not so thick that it gets pushed aside in a glob. I think that's the most important criterion, not necessarily why a certain grease is the proper viscosity for bicycle use--just that it is.
The whole issue of temperature has a lot more to do with drop point, i.e. whether a grease will liquify at a certain temperature. As you know, when that happens, the grease does not re-constitute as it cools. It's broken down and essentially useless. But that's not a concern with bicycles, thankfully. I don't know if there are indeed greases that *require* a high temperature in order to be most effective. I can understand why that could be the case, for the reasons that you alluded to before in terms of how readily the matrix will release the oil.
The more I learn about this, the more I want to know. If you have any links related to the differences between the paraffinic vs. nepthanic oils, I'd love to read more. Thanks again for your extremely interesting and valuable work on this!