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The "NLGI consistency number (sometimes called “NLGI grade”) expresses a measure of the relative hardness of a grease used for lubrication".

How much grease should I use to pack hubs? Should I use just a tiny bit, as in the left of the figure? Here I'd use just a minimal amount to coat the surfaces.

minimal or maximal grease

Or should I pack in as much as I can, as on the right of the figure?

I suspect that racers and those who really care about getting an extra 1% of performance should, before a race, though not necessarily to practice:

  • use a minimal coating of grease, and
  • use a grease of low NLGI, perhaps even "000", one similar to cooking oil.

While the rest of us, who:

  • do not race,
  • do not care about a 1% reduction in performance due to viscosity, but do care about a 10% reduction, and
  • who do not want to overhaul the hubs all that often

should:

  • use a maximal amount of grease, especially to make it possible to wash the bike with just the mildest of water jets and not fear water ingress, and
  • use grease with NLGI between 2 and 3, such as marine grease, one that will last the longest.

Is this about right?

Update

  • If you're curious how the ball bearings can last, given the point-to-point contact, which would mean that the stress at the point would in theory be an infinitely large value, head on over to engineering.SE.
  • When riding a bike, we are constantly and unconsciously doing micro-steering adjustments to stay upright. These micro adjustments make the ball bearings rotate inside the hubs, which means that the ball bearings get to distribute the load over the entire surface area, whereas the axle hubs and the cones get the stress in just one continuous band of limited area. Moral of the story is this: you are likely to need replacing the axle and the cones before you need to replace the ball bearings.
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  • 2
    Point of info: minimal grease or 100% fill aren’t the only two options. Many cartridge bearings come partly filled, e.g. 50-75% filled.
    – Weiwen Ng
    Commented Dec 22, 2021 at 13:51
  • 1
    Didn't you answer your question already in the question? You show two extremes and point out when it's needed. What's the point of asking?
    – gschenk
    Commented Dec 22, 2021 at 16:07
  • 2
    @gschenk I wonder how often I'd end up being right if everytime I had an educated guess I went ahead and applied it without first confirming with folks who likely know better.
    – Sam7919
    Commented Dec 22, 2021 at 17:13
  • 2
    @Sam Bearings are not infinitely stiff, and I do believe balls contact at a line, not at a point.
    – MaplePanda
    Commented Dec 22, 2021 at 19:22
  • 1
    Well, it's a good exercise to ask questions without suggesting the answer one likes to hear from the very beginning. I appreciate the effort you spend researching your questions. But leading replies that way reduces the chance to get a new, eye opening, answer.
    – gschenk
    Commented Dec 22, 2021 at 20:05

3 Answers 3

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I don't know much about the actual science of race day watt-saving as it applies here, and neither do most racers or mechanics. When people use low quantities of grease, or just run oil or lower-viscosity grease such as suspension greases in hub applications, it's not simply a given that you will actually have less friction under load. Maybe it's great, but you would need to test to know.

What's not a major concern is too much grease slowing things down or causing heat issues. Applications with much higher rotation speed than anything on a bike need to worry about that.

As for how much to add: the main consequence of adding too much is some can get pushed out the sides of the hub. That doesn't matter if it's your own bike, just wipe it way, but it's good to avoid more as a matter of decorum if you're doing it for money. When new mechanics take literally the idea that there's no upper limit and it will be better the more you can jam in, the hub can puke out a lot of it. So in a shop context there's a need to know how much is a good, generous amount but will stay looking neat.

Here is what I do: using a grease gun, put a ring of grease down in the cup where the thickness of the globs that make the ring is about equal to the size of the bearings. Push the bearings into place, then do another thin line on top (one more ring from a Dualco gun). In my observation that is a generous amount that will stay looking neat. It's reasonably near the point of diminishing returns, if you were to identify that point as the one where any more greases escapes out the sides. As far as I can tell, the only way to really know you're hitting that point is to go past it, which again is fine if it's your own hub.

Bike bearing greases are mostly all NLGI 2, even the ones that are unusually webby and viscous. My understanding is the kind of heat and load that NLGI 3 greases are made to endure are pretty alien to bike applications, although I don't know if the mainstreaming of cargo e-bikes will have anything to say about that.

Putting in a generous or maximal amount of grease does not give you license to fear pressure washers any less. Yes it may give some protection against a moment of bad aim or technique, but plan A is always keep the pressurized stream well away from all bearing seal areas.

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I assume these are cup and cone hubs. The easy answer is that you can’t go wrong with a 100% fill. In a video on their YouTube channel, Calvin Jones of Park Tools recommended “lots” of grease, by which he meant put a bed of grease down, put the bearings on top of that, and put some more grease on top. In a cup and cone hub, excess grease would just get squeezed out of the system. If you are servicing a cartridge bearing, you likely won’t go wrong with a 100% fill either.

I'm unfamiliar with the NGLI system. It does include viscosity as one of the properties that can affect the classification, but there are others. I am also not sure how marine grease differs from the lithium greases typically used on bikes. I believe I've heard that automotive greases can be used on bikes, as they have similar properties.

As a longer answer: I don’t know what fill rates are specified for typical bicycle cartridge bearings. They may not all be 100%. Phil Wood’s general use bearings come with 100% fill rates. Ceramicspeed, which focuses on performance bearings, recommends a 60-80% fill for its general purpose grease and 70-100% for its long life grease. It does have a longer white paper discussing friction differences between some greases and oils. That paper also mentions friction mechanisms, e.g. greases generally have higher viscous drag than oils, greases can be designed with various levels of viscous drag, and higher fill rates will produce more viscous drag. My understanding is that bearing engineers may be able to calculate optimal fill rates given the properties of the bearings (e.g. expected load, size of the bearings) and the lubricants, but I know nothing of this process.

In any case, the friction differences between 25% and 75% fills reported in the Ceramicspeed test are small, so I'd anticipate that a 100% fill shouldn't meaningfully penalize you in normal operation.

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  • I saw this video from Calvin Jones @ Park Tool. It even partially inspired my question. I saw that he put a "smudge" of grease, not so much to fill, but not so little to create just a layer either. So is the general guideline then the following? Put just about as much worry into how much grease you add as you put into choosing the type of grease you add, just so long as you do overhaul regularly and you do clean any grit carefully.
    – Sam7919
    Commented Dec 22, 2021 at 17:18
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You want a "goldilocks" amount.

Not too much: that can stop the bearing balls from turning and encourage skidding,

Not too little: then there's more metal-on-metal contact which is bad.

Just right: The range is quite wide - you need enough to keep the metals slightly apart.


Bicycles are low load, low speed, and low temperature bearings compared to many other places that bearings are needed in machinery.

Bike bearings don't get hot enough to melt or significantly effect the viscosity of average greases, so the lube doesn't really move around that much - it will get pushed around the races by the balls if you have too much. This slightly saps power from the drive train but we're talking about fractions of a watt - insignificant to most riders.

On a car/trailer the bearings get significantly hotter, carry more load, spin faster, and may be exposed to salty road water. In this case adding more grease makes sense. (it is also a lot of work to repack car wheel bearings compared to bike bearings, and adding more means longer service intervals.)

For a race bike on race day, one might prioritise power savings over part lifespan, and use a lightweight oil in the bearings over grease, but that's a specialist case.


Greasing and adjusting bearings is one of those "feel" things, its very hard to explain in words, and requires practice to get right.

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