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I've heard that coaster brakes can reach high temperatures, so it desirable to use grease designed for high temperature applications. My questions are:

  1. Would a grease labeled max 220 °C be good enough? Or it can get much hotter?
  2. What happens if the grease is not good enough? Will it liquefy and drip out of the hub?
  3. How should you use the brake when going downhill to avoid overheating? Is it better to use it in short bursts, or continuous but not very hard braking?
  4. How hot can it get exactly? A chart showing the heating and cool down rates would be interesting, but I don't think anybody has done the experiment, it's hard to stick a thermometer in the hub (maybe doable with a temperature camera sensor?).
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    There's a reason one of the first mountain bike race courses in California was named "The Repack" - they had to repack their coaster brake with grease after each run. Jul 13, 2018 at 23:28
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    This question should really be of academic interest; if you have a bike with coaster brakes and you think they may be near their limit in the riding you have planned, you really should use a different bike.
    – Batman
    Jul 14, 2018 at 0:58
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    Ordinary bearing grease is not, as many think, a thick oil. Rather it is relatively ordinary oil combined with "soap". The soap is basically just that, something resembling ordinary bar soap that has been worked into a lather and then mixed with fairly thin oil to form an "emulsion". Heat breaks down the soap emulsion and releases the oil, which then runs out of the bearings it was supposed to protect. Jul 14, 2018 at 2:38

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  1. Most people who aren't using a manufacturer grease are using a high temp automotive grease. I'd suggest looking at the manufacturer's recommendations and following them; brakes aren't a place to save a few bucks. 220 C seems low for the type of grease; people have failures with high temp automotive greases which often rated for 100 C+ higher than that on tough descents.

  2. Brake fade, loss of braking. Brake might be smoking and ruined. Loss of grease and dropping of the grease is the least of your problems at that point given the fact that you're probably in for a not so nice crash.

  3. You need to give the brake adequate time to cool off. This means controlling your speed by modulating your brakes (use in controlled bursts). You should also have 2+ independent brake systems which you can alternate with. If you use continuous braking over a long descent, you're just going to build up heat and have brake fade. Unfortunately, coaster brakes are prone to this in long descents making them not ideal; my advice is not to take a coaster brake bike on long descents. All brake systems will fail if overused.

  4. Pretty damn hot ; an old article but probably still quite relevant. You normally only see coaster brakes on kids bikes and bikes which are used in low load leisure systems (e.g. beach cruisers), where this would not be a problem (not going very fast, not braking very hard for prolonged periods of time). See (1). Measuring this on the surface of the hub wouldn't be too hard (thermal camera), but internally, you'd need some work.

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  • Does a high temp grease also help dissipate the heat? Or "high temp" just means "able to withstand high temperatures"?
    – Robert
    Jul 13, 2018 at 23:58
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    Greases have a variety of different properties depending on use. Lubrication, viscosity and heat transfer abilities are a few of them (and they aren't independent). Certainly the grease in a coaster brake helps keep the temps under control and without it you'll cook the brakes. It's moreso a question of what grease will work though with high temp vs low temp; if the low temp causes failure, no point in using it.
    – Batman
    Jul 14, 2018 at 0:55
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    The grease in a coaster brake has essentially nothing to do with braking ability. The grease is used to lube the bearings and cogs. It's just that the grease is in the same container with the brakes and hence will get hot when the brakes heat up. If it can't withstand that heat it will break down and fail to do its job. Jul 14, 2018 at 2:33
  • @DanielRHicks The grease in a coaster brake is essential for proper brake operation. Coaster brakes do not use rubber break pads. Both the rotating part and the part at rest are metal. If there were no grease, there is a high chance of the brake seizing and blocking the wheel even if you apply it only with little force. Having a thin layer of grease is required for controllable braking force. If the grease gets too hot, it looses viscosity, the grease film tears apart, and the brake gets hard to control. You might hear it squeal before the holes in the grease film get too large... Sep 5, 2022 at 19:30
  • ... but a squealing coaster brake is a sure sign of an overheated brake and requires you to take immediate action for continued safety. Sep 5, 2022 at 19:31

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