neptronix said:
My bad on this. But even distilled water does not fit the bill for an environment where you need thermal dissipation and lubrication all in one.
Marine grease would be displaced by the cooling medium in the motor, wouldn't it?
Similar, so addressing together:
I'm actually not sure that distilled water would be worse for lubrication. Bearings and gears must have an appropriate lubrication film thickness -- in our applications, supplied by grease, which includes appropriate anti-wear additives. Tacky (especially marine) grease is quite resilient to and protective against water -- the nonpolar grease and polar water don't mix, so I wouldn't say there is a risk of water displacing grease. Instead, you'd want to carefully pack the bearings full of marine grease to displace the water.
On the other hand, exposing nonpolar grease to another nonpolar oil, like a mineral transformer oil,
would wash out the bearing grease. Of course, you'd then have oil in the bearing, which is better than running the bearing dry, but the lighter oil wouldn't provide a sufficient lubrication film thickness.
It's kind of a known compatibility/risk (and thus bearing maintenance) with water, versus an unknown compatibility/risk with proprietary dielectric fluids. I'd want to see a lubricant compatibility list from the company.
So I'm definitely not saying water is better, but I don't have the data to say it's worse, either.
neptronix said:
Opticool H is $100 a gallon and the amount you need for a single motor is around $5 worth of product. Not necessarily out of reach. Some people on this forum spend that kind of money for a big tube of exotic lubricant they'll only use an ounce or two of.
Ah. I checked Amazon and eBay, but didn't find any. $100 shipped direct from the company?
$100 isn't bad, but 6oz sounds more like Statorade quantity, not filling to the axle. The published figure was
650mL for a 9C = 22oz = $17?
neptronix said:
In overdriven applications, liquid cooling induces some drag but allows more power output but ends up as a net benefit by increasing power density when a more powerful option is not available... which is universally the case with 2 stage geared motors and to a lesser degree, mid drives.
The current limit of a geared motor is it's ability to shed heat. The same dimensioned stator which is rated for 1000W in the MAC is rated for 2000W when sold by MAC as a lawn mower motor where the stator has a dramatically better path for stator heat to exit. Since you cannot buy a 2000W rated geared motor, you only option is to alter the heat dispersion characteristics of an existing one.
You could instead use a 2000w rated direct drive motor that weighs 2x more, but the additional unspring weight will hurt your rear suspension's ability to do it's job, and also, changing a tire on a 20lb wheel versus a 10lb one is a pain in the ass.
There are situations where a tradeoff between power density and efficiency makes sense.
Yeah, these packaging constraints (geared hubmotor and bottom bracket drive) make sense, and I don't mean to discount them.
Liquid cooling definitely has a place when pushing these packaging envelopes, and I'll be interested to see future research, products, and testing. I just wouldn't dismiss water out of hand, at least until there's actual testing data to support doing so.