Ferrofluid is an amazing substance if you own a large Direct Drive hubmotor. It does NOT benefit geared hubs or mid drives, but…if you like the double D’s, you need to read this article.
Ferrofluid, what is it?
If you own a large Direct Drive (DD) hubmotor, you will likely find this information interesting and helpful. FF is a liquid with ferrous (iron oxide) particles suspended in it, that bridges the magnetic air-gap between the stator and the rotor in a DD hubmotor, and it allows the heat in the hot stator coils to move to the aluminum side-plates much faster.
Why should you care? The answer is HEAT…the “real” limiting factor in just about every ebike power system component is how much heat it can survive. Forget about what the factories ads rate their components at (the controller, battery, and motor). Some companies purposefully rate them for more amps of heat than they can ever possibly survive (in order to impress new customers), and other companies purposefully under-rate their components, so that they can avoid as many warranty failure claims as possible.
The actual power limit of controllers, battery packs, and motors is how much HEAT they can survive.
If you live where the uphills are long and steep, your hubmotor will just keep getting hotter and hotter all the way up. Hopefully, your motor or controller will not fry before it reaches the top. However…for those ebikers that live on relatively flat land? Large DD hubmotors have been holding on and surviving because they work quite well in that demographic. If a rider applies the full max amps when the light turns green, they are only applying high heat to the motor for a few seconds, and then…during the majority of the cruise-phase, the motor and controller are both cooling off while they are drawing the minimum amps needed to maintain speed.
This is where FF steps in and might even allow you to double the power of your system.
Inside DD hubmotors, there is an air layer between the stator (connected to the axle) and the rotor (connected to the spokes and rim). This air-layer acts as an insulator between the heat of the stator coils, and the part of the motor that is connected to the outside air.
I have been a vocal advocate for the hubmotor variations that have a cast aluminum stator support (shown below, on the right side), to act as a heat sponge (instead of the common thin stamped-steel stator support, shown on the left). This single feature allows builders to spec a higher-amp controller, because the aluminum mass will absorb heat-spikes during acceleration, and then it will shed it over time.
However, the motors that have this aluminum stator-support still have to shed their accumulated heat through that layer of air-insulation…eventually.
To be clear, an aluminum stator-support does not stop the interior of the motor from getting hot, it simply absorbs the temporary heat spike peaks, and then spreads that heat out, over time. On an extra-long uphill, a DD hubmotor with a cast aluminum stator support can still eventually overheat if you keep hammering it, but…the interior aluminum mass will slow the heat build up. What we need is a way to help the DD hub shed the interior heat faster.
There is only about one millimeters worth of air-space between the swept face of the stator electro-magnets, and the thin permanent magnets on the rotor. FF is a fluid that is drawn to magnets, where the magnetism of the motors’ rotor-magnets hold it in place (no leaks or drips), and…when you add about 10ml’s worth of it?…the heat in the stator electro-magnets is much more quickly moved to the aluminum side-plates so it can be shed to the outside air, instead of building up. The thin viscosity of the fluid still allows the rotor to spin easily, but the liquid FF fills-in the air-gap, allowing heat from the stator to flow quickly to the aluminum motor shell.
Where is FF from?
Back in the 1960’s, NASA was pondering several ways about how to feed fuel to space-ship thrusters for maneuvering, without any gravity to pull the fuel towards the fuel-pump inlet. One engineer came up with the idea of suspending tiny ferrous particles in the fuel, and placing a magnet next to the fuel-pump inlet.
Later, in the 1970’s…the music speaker industry was looking for a method to draw heat away from the voice-coils in the high-wattage speakers that customers wanted. The speaker-cone has a large permanent magnet attached to its center, and around it was a large electromagnetic coil. By varying the current to the outer electromagnet, the free-floating magnet on the center of the speaker-cone would be induced to vibrate back and forth, which then vibrates the cone, making the sound.
The problem was that…the magnet at the center of the speaker cone would get hot when a high wattage magnetic field was applied often (using high-watts, when the music is extra-loud), and its magnetism could be permanently damaged. They needed a way to get the heat in the free-floating magnet to be more easily shed, while not interfering with the speaker cone magnets’ ability to freely move back and forth.
The answer, was to take a very thin oil, and suspend ferrous particles in it. That “ferrofluid” would fill the air-gap between the stationary electromagnets, and the permanent speaker-cone magnet. Doing this transferred heat from the central speaker-cone magnet to the stationary voice-coil housing…where various other methods could be used to shed that heat to the air. Since FF is attracted to magnets, it sticks to the magnets and will not drip.
Ferrofluids are made up of tiny ferrous particles (often iron oxide), which are suspended in a thin oil (often a synthetic low-VOC kerosene), with a surfacant added to prevent clumping or settling (usually some type of oleic acid). Depending on the application, there are many minor variations to the various recipes of FF.
Here is a youtube video showing how the FF aligns itself with the invisible magnetic field of the large and strong magnet under it.
FF in hubmotors
FF does not help geared hubmotors, and it does not help mid drives. It is arguable that it may help non-hub outrunners, but…they already enjoy great success with air-fan cooling, due to their open framework, plus…the oil in the FF would accumulate dust and grit in any motor that is open to the air. Also, outrunners have a small area of aluminum baseplate, which is only attached to the stator on one side. DD hubmotors have two large-area aluminum side-plates. the best strategy for adding cooling to a common outrunner is to use a shaft-mounted impellor-style of air-fan, coupled with an aluminum stator-baseplate that has a pass-through channel that fluid can be pumped through, like many RC boat outrunners.
We wrote about cooling methods to help builders hot rod their hubmotors, and…back in 2012 there were two methods. First, you could cut holes in the sideplates of the hubmotor (ventilation) to simply let the heat out. This is what the Pikes Peak 111V racer did. However, this allows road-grit and rain into the inside of the motor-shell, so…it is important to coat the stator with a high-temperature water-proof coating, and use sealed bearings.
The second method was that you could add a coffee cups’ worth of synthetic automobile transmission fluid (ATF) to the inside of your hubmotor, which works in much the same way as FF, because when the motor starts spinning, the ATF spreads out across the interior rim and connects the hot stator coils to the aluminum sideplates. ATF is cheap and available, but…it can sometimes drip and leak out, in spite of a builders best efforts at sealing the side-plate edges.
FF has created a whole new world of options for large DD hubmotor owners. Now, it allows you to raise the amps on your system to a higher level (you should still add a temp sensor to the inside of the motor to avoid damaging it), and you can keep raising the amps until you reach our recommended max of 200F / 93C. Without FF, it wouldn’t take many amps to reach 200F, but…with FF helping the motor shed heat to the outside air, you can use MUCH higher temporary peak amps during the acceleration phase.
Conversely, it can also allow you to get the same power from a lighter and less expensive hubmotor.
[side note: if you add ATF to the inside of a hubmotor, you MUST also add a small pressure-equalizing vent to one of the sideplates, and…if the vent is a threaded removable version, it can also be used for a fill-port. Without the pressure equalization vent, the heated interior will actually push the ATF out of every possible joint. Any humidity that is drawn in when it is cooling will be vaporised the next time the motor gets above 120F / 48C in normal use]
Adding FF to a DD Hubmotor
First of all, this stuff is MESSY. Wear old clothes, because they WILL get stained. Wear snug surgical gloves, because this stuff is supposedly a skin irritant, and did I mention…it will stain everything it touches, including your fingers. Also…stain.
The sideplate bolts (8 per side) are typically M4 (4mm X 0.70mm) on hubmotors of this style and size, and I bought some thin stainless steel #8 SAE washers to put on them when I re-assemble the sideplate. I know adding washers decreases the penetration of the stock bolt threads, but I believe they will still be strong enough.
In the pic above, I have separated the hubmotor components by pulling out the cable side and stator as a unit. You “could” remove the sideplate on the other side without pulling out the stator, and then just squirt the FF in, but…for me? I find that this method is actually easier. If you are going to drill a hole to epoxy-in a vent to the side-plate, now is the time to do it, and de-burr both sides of the hole. Here is one example of a sideplate vent. And also, here are some more examples.
In the pic above, you can Google “oral medication syringes” to find these for less than $0.25 each.
There is a formula listed below about how to calculate the precise amount needed to get the heat bridging effect, without adding so much FF that the rotor-drag is significantly increased. 10ml should be plenty enough for a large QS 205/H50. A MXUS 3000W will likely be fine with about 9ml, and for the 35mm wide stator hubs (Like the Leafbike 1500W and the Edge 1500W that I like) will likely work well with about 7ml. The common Yescom and 9C hubmotors with 28mm-30mm wide stators will likely work well with 6ml. I’ve been told you can actually just dump it in, and the magnetic fields of the rotor magnets will automatically spread it out evenly (if you have a vent hole already).
WHEN RE-ASSEMBLING THE MOTOR, DO NOT HOLD THE SIDEPLATE EDGES WITH YOUR FINGERS. The very strong magnets will snap together with the steel stator core when they get close to each other, and it will crush your fingertips. Hold the central axle when reassembling the hubmotor.
Of course, if you do re-assemble the hub by holding the sideplate edges with your fingers, the good news is that you won’t leave any fingerprints at the scene of your next crime. Then, your electric bike will allow you to have a silent getaway, but…make sure to install a PAS (pedal sensor), because you won’t be able to reliably operate a hand-throttle with your mangled stubs…
This volume of FF needed was determined by builders measuring the very small “no load” amp draw of the motor (with the throttle on, and the wheel spinning in the air), and then they added more FF until there was a small bump in the no load draw, due to added drag.
Some builders have expressed concerns about centripetal forces sending some of the FF in the hubmotor out through the joint between the motor rim and the sideplates, so they have gooped that joint with a high-temp sealant. However, unless you are going faster than 40-MPH, the RPMs should be low enough that this “probably” wouldn’t be a problem. But…feel free to add some side-plate edge sealant if this concerns you.
After some builders began enjoying measurable success using FF, a few builders pondered the possibility of adding some type of aluminum fins to the outer motor shell in some way, in order to help the heat-shedding process. Some suggested that the amount of extra heat-shedding that such fins would provide would likely be very small. However…temperature gauge readings and hands-on rider results have indicated that adding aluminum fins to the outside of the motor shell are a big help.
To be clear, aluminum fins added to the motor shell by themselves have a small benefit (with no FF or ATF), but they do help some. However, if you are using FF or ATF to move heat from the stator to the motor-shell, adding fins has a much more dramatic benefit.
Using FF + fins is like 3 + 1 = 5, when it comes to heat-shedding.
There must be several shapes and locations of aluminum fins that would help, but one developer has created a kit of fin-sections designed to be clamped to the outside of the motor-shell rim. These are sold in six sections so they are small enough to be added to a popular and common size of hubmotor (with a 205mm diameter stator) without needing to remove half of the spokes. If your new hubmotor is already fully spoked-up to a rim, the fins can be added right away. The fins I bought for this article are called “hubsinks” by the retailer, and they are 15mm wide.
The side-plates of these large DD hubmotors are made from aluminum, and they were made from that material because aluminum is light, it’s strong enough, and it absorbs and sheds heat well. The outer rim of these hubmotors is steel, because it provides the magnetic-focusing backing material for the rotor magnets. It also provides the strong flanges that the spokes are attached to, and it is vital for those to be strong because the weight of a DD hubmotor can put a huge strain on the spoke-rim attachment points.
It is the steel material in the motor rim that these aluminum fins will help. The amount of area where the steel rim is bolted to the edges of the aluminum sideplates is not very big, and…that creates a bottle-neck to moving heat from the stator to the sideplates (and then to the outside air).
Since the common hubmotor rims are a steel component…anything that can be done to shed heat directly from the steel motor-rim to the air will help. The main reason I feel the rim-fins / hubsinks are important is because…they directly help to keep the temperature of the rotor-magnets lower. If you over-heat the magnets, the power level of their magnetism can be permanently damaged. The symptom of overheated magnets is that the top speed is now higher, and the power of the motor is lower (at the same input watts that you used before).
FF and hubmotor rim-fins go together in a very mutually beneficial way. However, if you don’t want to use both, definitely get the FF first.
[side-note: if you use motor shell rim-fins, you should spread out a thin smear of thermal paste between the motor-shell and the aluminum fins, google a thermal paste comparison review and choose one in the middle of the pack. reasonably low price, but still has reasonably good performance.]
High Performance Heat-Shedding Options in 2017
If your motor is only getting warm, you don’t need to add any extra cooling mods. However…if you are raising the amps of your system to get harder acceleration? Here are some options. [warning: if you raise the amps too much, you might break something in your system…even if it doesn’t overheat the motor]
Geared hubmotors: two options…
- First is adding ATF and a tiny side-plate vent (and maybe hubsinks later?).
- Second is cutting ventilation holes into the sideplates, with the interior being coated with a high-temp motor epoxy to reduce rusting.
Direct Drive hubmotors:
- First is adding ATF and a tiny side-plate vent (and maybe hubsinks later?).
- Second is cutting ventilation holes in the sideplates, with the interior being coated with high-temp motor epoxy to reduce rusting.
- Third is to add about 10ml of Ferrofluid, and after that…maybe even aluminum rim-fins.
- Fourth, if none of these work well enough…you need a bigger motor and higher volts.
The third option listed here is now my first choice. I always liked the idea of using ATF, but it seemed that no matter how well you sealed the sideplates to the rim with goop, there were still red oily drips on the floor, every where that you parked. And…I never liked the idea of a spinning motor occasionally flinging a drop of oil around near the brakes. Once the pads and wheel-rim (or disc) gets oily, its annoying to fix.
Builders who used the sideplate ventilation holes (with anti-corrosion coating over the entire interior), said that any moisture that got in was immediately vaporised by the motor heat on the next ride (like an automobiles’ open-framed alternator). They also stated that getting road grit and dust into the motor and bearing hasn’t been a problem so far. Sideplate ventilation remains a fast, cheap, and easy way to let the heat out of a race hubmotor, but…I am still uncomfortable with the idea of using that method on a long-term commuter build, with an expensive motor that I want to last.
Any direct drive hubmotor will run cooler with FF, but…the motor that I think will immediately benefit the most from FF + fins is the Leafbike 1500W. It does not have the aluminum stator support that I have mentioned that I like very much (which acts as a heat sponge). It had previously been my favorite middle-weight street-fighter to recommend, until the similar Edge 1500W came along.
Both the Edge and Leafbike have thin laminations (to reduce eddy-current waste heat, and also to reduce cogging), and both have a 35mm wide stator, which is about the most power-potential a DD hub can have (in copper mass) while still leaving enough room to fit a 6-speed freewheel, and remain narrow enough to drop right into the common 135mm wide frame drop-outs (which allows them to still fit a wide variety of common bicycles). The major difference is that the Edge has the cast aluminum stator-support (So I immediately bought one as soon as they were available).
To compare the two hubmotors fairly, I would have to get a Leafbike 1500W and add a temp probe to it, then data-log identical runs against the Edge 1500W, with both using the FF and fins. That way?…the only difference would be the Edge’s aluminum stator-support. If the temp-limiting results were the same for both, then…the Leafbike would actually be the better choice, since it is a little lighter and slightly less expensive.
[side note: I have not forgotten about the MXUS 3000W with a 45mm wide stator, or the QS 205 with its 50mm wide stator. Both of these will benefit greatly from adding FF and rim-fin / hubsinks]
I am currently happy running 52V X 50A = 2600W on my Edge 1500W hubmotor. However, I am now looking forward to getting a new controller so I can raise that to 52V X 80A = 4200W, so I can see for myself (be aware, I live where it’s flat, so…if you live in the mountains and fry your DD hubbie at 4200W, remember…I warned you). If this ends up the way that I think it will…this mid-sized DD hub with FF combo will be a total “sleeper” (a Superman, who looks like Clark Kent all the time)
More FF info
Karl at electricbike-blog.com has also written about his experiences with FF. Don’t take just my word for it, check out what Karl says about FF in his article here.
I want to thank Richard Fechter for bringing up using FF for heat transfer in this July 2015 discussion thread. There was an earlier 2010 mention that had been ignored, found here. FF has been mentioned before that, but never for heat transfer in a DD hubmotor, until then.
“If memory serves, the magic amount of ferrofluid is 1ml per 50cm^2 of stator surface area. Based on my own observations, as you increase the amount of ferrofluid [over that amount] you get slightly better heat-conducting at the expense of noticeably greater drag. Assuming the MXUS 3000W motor stator has a 20cm diameter, the sweet spot is about 5.6 ml. I’d probably err by overfilling it slightly at 6 ml to make sure I get most of the heat transfer benefit” -Bill Bushnell
A Quote from Grin Tech
“The original use of ferrofluid in hub motors stems from a research project by Justin at Grin Technologies to quantify the various cooling strategies used in ebike hubs, and they published their test results in this endless-sphere thread:
During the course of their analysis, they ended up building an entire wind tunnel test environment to quantify how well finds, holes, oil fill etc. does at cooling the motors. It was in the discussion of moving heat from the stator to the shell that Ferrofluids came up as a suggestion from Fechter.
What Justin discovered was that while filling the air gap with ferrofluids did work to cool the motor, it also nearly doubled the wheel drag from viscous resistance. However, when they experimented adding FF in very small amounts at a time they found it was possible to get near full thermal benefits in quantities that only filled a small portion of the airgap volume. This was the real breakthrough, a fluid that conducted heat out of the stator without noticeably increasing the drag on the wheel.
Grin has worked in direct partnership with Ferrotec to produce a product line called Statorade that is optimally suited for this motor use with minimal drag, high temperature resistance, and broad chemical compatibility to hold up in this application.
They also took the detailed wind tunnel test results and built a comprehensive online modeling tool that lets you see exactly how a given direct drive motor will hold up both with and without Statorade. Just click points in the top graph to create an elevation profile for a trip, and compare the resulting motor temperatures over the journey with the different hub options”
Written by Ron/spinningmagnets, April 2017