Definitive Tests on the Heating and Cooling of Hub Motors

[fechter]

I seriously considered doing a similar setup on my A2B motor. There is room inside for a small fan. Powering the fan was the problem, as I just don't have room in the axle for any more wires. One possible way around this was to make a little dc-dc converter circuit that picks up power from the phase wires. A 3 phase bridge rectifier made with ultrafast diodes should produce pack voltage at any speed due to the PWM being full voltage. I'd probably just let it run full speed all the time, but a temperature controlled speed adjustment is not too hard to implement.

 

[Offroader]

I'll let you know how effective using a fan in a sealed motor is. I'm going to also test it at various speed levels to see how much of a difference air flow makes. You may need a more powerful fan to get decent results then just smaller CPU fans.

Here is my setup all hooked up. Took a long time to do all this even though I knew what I was doing.

I'm running it from a 12 volt power supply, hooked to my ebike 20s battery.

I am looking at a max current draw of 8.5 amps at 12.5 volts, over 100 watts of cooling power on max using an 18 gram EDF. I'd never run it at full speed like that for any length of time but it could easily be turned up to run high like that during peak heat loads. I would normally run it at 1.5 to 3 amps, which is still alot of air flow.

I found the key to cooling a hub motor is turn on the fans at a slower speed right when you start riding even before the windings/stator heats up. Much better than waiting for the motor to get fully heat saturated and turning the fans on really high.

My goal here is to try and have the winding temp close to the hub case temp. At least or better than FF fluid.

I think Justin ran his tests on an MXUS so I can even run a similar test on my MXUS and compare the results.

[youtube]d2063GhDUhE[/youtube]

 

[Cowardlyduck]

Pretty sure fan speed/ air flow rate will have a sizable impact.
Although it wasn't in a sealed hub don't forget the testing I already did on this several years back:

I'm unlikely go to all that effort again though. Far easier/simpler to just run extra fans in series and put up with a push button on/off setup.

Cheers

 

[fechter]

With my other forced-air setups, I always just ran the fan continuously at the same speed (crude setups). I found it especially helpful when the motor was stopped.

 

[Cowardlyduck]

With my other forced-air setups, I always just ran the fan continuously at the same speed (crude setups). I found it especially helpful when the motor was stopped.

Yeah, agree 100%, and the data above also reflects that which the temp increase being almost identical during load, and the cool off time when stopped being the differentiating factor.

Cheers

 

[Quokka]

Just installed a set of hubsinks on a QS205 pushing 10kw on my Qulbix Q76R. I am back from my first run with them. The temperature here at the moment is stinking hot. 40 degrees C plus with the desert winds blowing in. Innitial feedback is that these do work. For me though, i was still able to go from 80 to 110 in a matter of a few minutes. I do still need to install my ferrofluid so i know that it will get even better. Will report back when i have liquid gypsy magic installed.
Cool down time is noticeably faster. At a stop it drops 10 degrees in 5 mins, and drops quickly back to 85-90 when just coasting along using 1000w or so.

 

[Emmett]

I've done 3 rides now with my new hubsink. Outlook seems very good.

I use 100ml of castor oil in a properly sealed H4040 hub motor pushed up to 6400W using 65-75V 18S2P LiPo. No temp sensor. I removed it. When I had the temp sensor the hub shell temp was always effectively the stator temp (oil inside) and my heat test became simple, if I cannot touch the hub for 3 seconds without burning my fingers then it's getting a bit hot. I don't race the bike or commute with it. I trail ride on it. On/off the power a lot.

Result = the hub never gets too hot to touch. The fins feel close to the same temp as the hub. I don't think I'll be thinking about the hub temp much in the future. Previously (without the hubsink) I would stop at some streams and use water to cool my motor down a bit then continue.

Twice I've ridden my standard loop with the hubsink and both times I thought I was riding the bike as hard and fast as usual or perhaps a bit quicker. To my surprise I used less battery energy to go the standard trail distance in a typical air temp around 30C. Quite a lot less. I'll monitor this more in the future to try get some comparative stats.

Sorry I don't have data figures. That probably disregards my feedback for some people here. But if this hubsink works for me then I confident I'll find out. With oil cooling, I cannot imagine how it could not work - really well. Seems great so far. BTW: My hub never drips oil. Even when stored for over a month.

 

[litespeed]

Just installed a set of hubsinks on a QS205 pushing 10kw on my Qulbix Q76R. I am back from my first run with them. The temperature here at the moment is stinking hot. 40 degrees C plus with the desert winds blowing in. Innitial feedback is that these do work. For me though, i was still able to go from 80 to 110 in a matter of a few minutes. I do still need to install my ferrofluid so i know that it will get even better. Will report back when i have liquid gypsy magic installed.
Cool down time is noticeably faster. At a stop it drops 10 degrees in 5 mins, and drops quickly back to 85-90 when just coasting along using 1000w or so.

Great looking bike....simply stunning!

Tom

 

[Rix]

Just installed a set of hubsinks on a QS205 pushing 10kw on my Qulbix Q76R. I am back from my first run with them. The temperature here at the moment is stinking hot. 40 degrees C plus with the desert winds blowing in. Innitial feedback is that these do work. For me though, i was still able to go from 80 to 110 in a matter of a few minutes. I do still need to install my ferrofluid so i know that it will get even better. Will report back when i have liquid gypsy magic installed.
Cool down time is noticeably faster. At a stop it drops 10 degrees in 5 mins, and drops quickly back to 85-90 when just coasting along using 1000w or so.

Great looking bike....simply stunning!

Tom

+1 Tom

 

[sketchism]

Hey guys! Every single pre order is now shipped out and on the way, they are now officially in stock as well as dealer stock going out at the moment


great to see the guys feedback who have already fitted them! Emmet Quokka (good to hear its going well so far but get the FF in there for a different world haha)

I honestly have a hard time believing this FF + hubsinks will work for me for the type of riding I do but maybe it will. I may try it and then if it doesn't work well enough I will try and clean all the FF out and then modify for fan air cooling.

I do most of my riding in the hot summer, so ambient is usually 85-90F and then I do stuff like riding up large dirt hills, riding down, then riding back up again. Wheelies which constantly has me pushing full throttle and not moving very fast. Stuff like that which really heats my motor up and I know the RC EDF fan works.

I just need to determine if I can somehow spray all that FF out of the stator if it doesn't work well enough and convert over to air cooling. I've heard the FF gets into everything and sits inside the stator, maybe gets stuck between the laminations. If this will constantly leak out into the windings and get dirt stuck inside the windings, especially where they pass through the stator, this will prevent the high air flow from cooling the windings. But maybe I can get all that FF out.

It is a hard decision because I know the air cooling will work, the only drawback is that there is fan noise, which honestly you don't hear it when you are moving and you don't really recognize it after a while. But it is still an open motor and you have to deal with the electronics to run the high speed fan and an open motor.

your terrain sounds Perfect, thats actually the type of terrain including some 'impossible' hill climbs that led to the development of the sinks (after venting and fan forcing didnt work), i cant wait for you to try them and report back

also to answer your other questions further down, the FF is an extreme order of magnitude better than air at transferring that heat away from the stator in this situation, almost a perfect solution when you consider the added simplicity vs fans etc

from my personal last rides, in 40-42C weather (nearly 110 for you guys across the pond) they have been working great but obviously the delta T is 20C higher than on a 20 C day, i dare say i wouldnt even be able to ride the trail at any level without them in this heat


for everyone whos just receiving theirs, post up some pics i love seeing them installed on your builds!

 

[Offroader]

Quokka, I'd like to hear your after FF report, any idea when you plan on getting some? I'm assuming you need to stop right now on your ride for a cooldown? Curious to know if adding the FF will eliminate all the stopping. With the power you are pushing, if the FF does eliminate all stopping, then it actually makes no sense to used forced air cooling.

 

[Quokka]

I have ff on order. As soon as i get it i will be putting it in- also have copied merlins gore valve for pressure equalisation. I need to get this stuff in and test it before i head overseas to live.. because it will take 3-4 months for my container to get there.
Perth is Flat... really flat, i had to head out a long way to find some decent hills but i could still hit the thermal cut-off easily. I hope the FF helps as sketch has described in his previous post. The heatsinks get hot to touch, from this its obvious they increase the surface area for heat exchange. The FF should increase the rate of heat transfer from the stator to the magnets and hub casing. Just need to confirm how effective it is. It is also clear that the sinks work better when moving. Ideally rolling down a hill for for a few minutes sheds more heat than a stationary bike. I also have a mxus v3 that i can test to compare the heat shedding comparison between mxusv3 and QS205 v3.

 

[Emmett]

BTW: My hub never drips oil. Even when stored for over a month.

Well after saying the above ... yesterday my leak-proof hub motor leaked!

Simple problem and fairly easy fix, so some people might be interested.

The castor oil leaked out past some of the countersunk bolt heads which hold the side plate on the disc brake side of my H40xx motor. The oil resistant silicone sealant held a good bond and the side cover seal is generally OK but some bolts loosened a little despite the loctite on the threads. My 7.5kg hub motor takes a good beating off-road, with 3" trials tire smashed into square edge rocks at moderate speeds. Often with substantial side loading. So the side covers, mating surfaces and bolt fasteners take a lot of load.

If anyone wants to seal their side covers on, to oil fill a hub then consider this:

0. Drill your breather hole and drill+thread tap your oil fill and drain holes in the cover on the chain side.
1. Thoroughly clean the mating surfaces using a high speed wire brush, then brake cleaner and compressed air.
2. Also clean out the thread holes for each bolt with brake cleaner jet/tube spray and compressed air.
3. Use oil resistant RTV silicone sealant. The auto gasket stuff. Put a very thin layer on both surfaces. Careful put it around each bolt hole and don't let any sealant contaminate the threads.
4. Use blue loctite on the bolt threads. Threads only. Keep the bolt head clean. Let no silicone sealant get on any bolt. I use phillips head bolts since they sit easily on the end of a screw driver.
5. This is my new step as of today ... Before torquing each bolt (about 15Nm is plenty) put a bead of JB-Weld epoxy (metal repair glue) around the neck of each bolt head. This will seal and seat each bolt.

I'm sure the bolts can be removed easily in the future with some heat on each bolt. Put a phillips driver tip into a large soldering iron and get each bolt up to temp and the epoxy will soften and let go under torque load. I've done this many times with no problems assuming the neck+head are the only part glued.

You could also skip the loctite and use epoxy on the entire thread length, however do that and you must first put a release agent on the bolt thread so the epoxy cannot strong bond to it.

Instead of ATF, I recommend castor oil, since it has no foul smell and is easy to wash off if it gets out. Just monitor how thick it gets at room temp and replace it occasionally.

 

[macribs]

Instead of ATF, I recommend castor oil, since it has no foul smell and is easy to wash off if it gets out. Just monitor how thick it gets at room temp and replace it occasionally.

From wikipedia:

Vegetable oils like castor oil are typically unattractive alternatives to petroleum-derived lubricants because of their poor oxidative stability.[30][31] Castor oil has better low-temperature viscosity properties and high-temperature lubrication than most vegetable oils, making it useful as a lubricant in jet, diesel, and racing engines.[32] The viscosity of castor oil at 10 °C is 2,420 centipoise.[33] However, castor oil tends to form gums in a short time, and therefore its usefulness is limited to engines that are regularly rebuilt, such as racing engines. The lubricants company Castrol took its name from castor oil.
Castor oil has been suggested as a lubricant for bicycle pumps because it does not degrade natural rubber seals

Does it gunk up on you? There was some coconut oil trail back when can't remember why it didn't work as planned.
I can see the part where castor oil is easily cleaned up if leakage, but high tech engine oil or AFT will give several other benefits and should not gunk up.

If you park your bike leaning hard toward the opposite side of the brake rotor maybe that will be enough not to leak when parked?

 

[Offroader]

Emmett: Couple of questions I was hoping you could answer.

Why did you decide to use 100ml of oil instead of FF? I believe FF has the advantage of less likely to leak. Especially out of the bearing and wicking through the wires.

Why do you only use 100ml of oil? I figure you can put a lot more oil into the hub.

 

[Emmett]

Castor oil has no significant odour and wipes off so easy with a rag, or when washing the bike. Both ATF and motor oil stink and are very difficult to clean away.

Yes the castor oil thickens slowly. How fast depends on your usage/temp. As I wrote in my last post, drain it occasionally and you'll see how it goes. This is the main disadvantage with castor oil. The cost is low under $20 for litre.

I use 100ml because I found by trial and error (when it had a temp gauge) that in my motor the stator's steady state temp for my usage reduced quickly as the oil vol increased from 0 to 100ml, but adding over 100ml the benefits were very minor. I took a syringe on the trail and stopped for a couple of minutes and added in 20ml increments then keep riding the same track and looked at the temp gauge to see where the stator temp levels out at.

At present I wipe the axle ends with a rag after about every 5th ride. After sealing the axle slot+wires with the oil resistant silicone so far (after 3 months) I've not detected any oil seeping along my phase or halls wires. Leaning the bike either way when stored makes no difference. The film of oil on the axle ends just comes out. But no smell and it wipes off.

With more than 90ml of oil and "no load" full power test, the RPMs come down a bit and the watts goes up. I don't recall the values, but the losses were minimal (a couple of km/h), and for my usage at mostly under 50 km/h I didn't care about the drag. My range and economy is mostly determined by my riding habits.

I don't understand why I'd consider FF, when I found that 90 or 100ml of relatively cheap castor oil is the best volume of oil for my hub. I didn't know FF existed when I converted to cooling about 2 years ago. Prior to oil cooling my bike was effectively useless. Today I've not yet found a reason to change anything. With liquid heat transfer I'm loving the new hubsink!

 

[macribs]

Castor oil has no significant odour and wipes off so easy with a rag, or when washing the bike. Both ATF and motor oil stink and are very difficult to clean away.

Yes the castor oil thickens slowly. How fast depends on your usage/temp. As I wrote in my last post, drain it occasionally and you'll see how it goes. This is the main disadvantage with castor oil. The cost is low under $20 for litre.

I use 100ml because I found by trial and error (when it had a temp gauge) that in my motor the stator's steady state temp for my usage reduced quickly as the oil vol increased from 0 to 100ml, but adding over 100ml the benefits were very minor. I took a syringe on the trail and stopped for a couple of minutes and added in 20ml increments then keep riding the same track and looked at the temp gauge to see where the stator temp levels out at.

At present I wipe the axle ends with a rag after about every 5th ride. After sealing the axle slot+wires with the oil resistant silicone so far (after 3 months) I've not detected any oil seeping along my phase or halls wires. Leaning the bike either way when stored makes no difference. The film of oil on the axle ends just comes out. But no smell and it wipes off.

With more than 90ml of oil and "no load" full power test, the RPMs come down a bit and the watts goes up. I don't recall the values, but the losses were minimal (a couple of km/h), and for my usage at mostly under 50 km/h I didn't care about the drag. My range and economy is mostly determined by my riding habits.

I don't understand why I'd consider FF, when I found that 90 or 100ml of relatively cheap castor oil is the best volume of oil for my hub. I didn't know FF existed when I converted to cooling about 2 years ago. Prior to oil cooling my bike was effectively useless. Today I've not yet found a reason to change anything. With liquid heat transfer I'm loving the new hubsink!

Never change a winning team, right? You found a method that works for you, perfect. I am looking into oil too, but because of the weather and temp changes where I live I will use petroleum based oil. For sidecovers I will try O-ring. I have not yet found a sure way to seal the axle and the wires. If I am still stuck at the same point in the spring I will just cave and use several layers of cardboards where I park my bike in the hallway. I guess oil drippings are after all a small price to pay to have adequate cooling. So if I can't solve the axle sealing, I will still go for oil filled hub.

Are you running additional cooling hardware, like heat sink or does the oil fill alone take care of all your cooling needs?

 

[Emmett]

Are you running additional cooling hardware, like heat sink or does the oil fill alone take care of all your cooling needs?

Yeah I'm using a HubSink. As I mentioned a few times. Must have item. For oil cooling at least.

If you have cardboard on the floor to catch oil dripping from your parked bike, then something is seriously wrong.

What model motor do you have? I didn't know that some use o-rings for the side covers. That could be great if designed right.

I have a H40xx and TC3065 and neither leak anything significant. I get upset if I feel oil on the brake rotor or the spokes look oily. Never does it end up on the ground. On rides in dry weather I'll often see a little oil from the breather hole which is very close to the rear sprocket. Just wipe it off.

To seal the axle slot aint too difficult. Use the oil resistant RTV silicone. Get some latex gloves. Clean out the slot of any oil, or sharp edges. Remove any extra sheathing over the phase wires to get a good seal. I have the fattest silicone type insulated highly flexible phase wires I could fit in the slot in each motor. Smother all the wires in the sealant and pump it into the slot. On my H40xx I used heat shrink tape to hold the wires in place while the sealant cures. On the T30 with hollow axle it's just easy.

One catch was sealing my hall wires. What I did was install new hall wires and I put the sealant inside the heat-shrink bundle sheath near the axle area, then heat shrunk it.

 

[Offroader]

Just wanted to update on my findings of using forced air EDF (electronic ducted fan) in a sealed hub motor. To give you an idea about how power a 35mm EDF is, if you hold it arm length away from your face and put it on full power it is like you stuck your head out of a car riding at highway speed, it is even hard to breathe. You can't compare this to a CPU fan, or even a large room fan in the power it has.

First I wanted to say my original thoughts were correct in that there must be a thermal bottleneck in the transfer of heat out of the hubmotor. Most likely through the metal case. I question the usefulness of hubsinks if you are moving at even moderate levels of speed.

With having my EDF installed I took my bike out today in a very cold 45F / 7C day, pushing about 6500-7000 watts on my MXUS turbo.

It takes a long time to heat the motor up even with the EDF OFF, the only way to heat my motor up without riding offroad (it was too wet and muddy to do that), what I need to do is full throttle the bike, then hit regen braking, full throttle it again, if I continuously do that it will heat the motor up fast.

After riding around long enough and finally being able to keep the windings at around 115C / 240F, with my cooling OFF, the outside case temp will only be slightly warm to the touch, and only reach 100F / 38C even after sitting for a long time. My MXUS motor has very bad thermal transfer.

When I turn my EDF ON after the motor is fully heated, the side cover on the exhaust side would get hot first and somewhat fast, maybe 1-3 minutes, so not super fast. This is because it is getting hit with super fast air speed.

My exhaust side cover would reach a max of 140 F / 60 C, it would take about a minute or so more time for the magnet ring and other side cover, to get to the same temp as the exhaust side cover. But eventually the whole hub will be 140 F / 60C, at 140F this is burning hot to the touch. This is with the bike stopped and indoors. After the bike sitting for 3-5 minutes to get the hub super hot at 60C, my winding temp stays at around 170F / 76C, and slowly fall to 150F / 65C, it takes a very long time to fall below 170F.

Here is where it gets interesting:

With my hub now at 140F / 60C, which is burning hot to the touch, and my EDF ON at almost medium speed, as long as I drive at 15-20MPH the whole hub will now only be slightly warm to the touch. As long as I am moving it seems that the hub motor will quickly cool itself and just be warm at around 90F / 32C . This is even after I turned my EDF at full speed using 100 watts of power, and also full throttle, stop, full throttle, reapeat.., up a hill to get my windings to stay at 115C / 240F .

Even when my side covers are at 140F / 60C, which is as hot as I can get them stopped, they will cool off in like 10 seconds while riding. This is even the exhaust side cover which gets Hot First and fast and hit directly with the high speed directly from the EDF.

This means that since my hub motor is basically just warm to the touch while riding, it tells me that hub sinks may be useless unless of course I want cooling when I am stopped.

This does not come as a surprise because on my cromotor I have found that after my hub shell was super hot to the touch, it only took 15 seconds of riding for it to cool off.


Now here is why I need to do more testing:

1) 45F / 7C temp is super cold ambient temps and it would be different at summer temps.
2) FF or oil, is a much better thermal transfer of heat than using super fast 100 watts of forced air cooling. The forced air cooling has the advantage of blowing through the hot windings.
3) Because I am using forced air, the side covers are acting as much better heat sinks (especially being aluminum for conductivity) and also allowing for much more surface area of thermal transfer inside the hub than just FF using the magnet ring. This is questionable because I could not get my hub hot while riding slowly with my EDF on MAX speed, even after I was maintaining my windings at 115C /240F. I need to stop for a couple of minutes before the hub heats up hot.

As it stands now, what I am finding is a thermal transfer bottleneck through the metal case. Hub sinks can not do anything to improve the cooling if the outside of the motor is only getting slightly warm to the touch when moving at only 15 MPH. This bottleneck is worse in how long it takes for the magnet ring to reach the temp of the exhaust aluminum side cover. The steel and magnets are not nearly a good conductor as aluminum.

I would need someone who is just using FF to check how hot his hub motor is getting while ridng at 15 MPH+ speeds, this is easily done by just touch your motor quickly and seeing if it is hot or warm. You would have to stop to see how hot your hub gets, and then ride and see how long it takes to cool back down.

It really could be that the FF gets the heat into the metal more quickly than forced air. However, I would think that when the outside of my hub is warm at 90F while riding, the inside is probably way hotter and closer to 140F. It just takes times for the heat to conduct through the metal.

That being all said, having the EDF fan circulate the air inside the hub does cool the windings down a lot. It takes a lot longer to reach 240F / 115C, and when I reach that temp it will cool much quicker down to 170F / 77C. What I question is the usefulness of hubsinks while riding at speed. It seems that the exterior of the hub cools very fast as long as you keep moving.

Unless you are actually stopping at red lights and doing a lot of stops, hub sinks probably won't do all that much if you are maintaining even a slight speed.

This may also show that you can't beat forced air where it is exhasuting the hot air out of the motor. No amount of external sinks are going to do anything unless you also add sinks to the inside of the motor.

 

[Cowardlyduck]

Offroader, thanks for the detailed post.

I think the FF + Hubsink combo is pretty solid from the reports we've been seeing. While I haven't tested the official Hubsinks, my own testing with thermally epoxied small heatsinks has proven it does improve cooling...even without FF.

As for your findings on the air circulation....there's a simple fix...vent your side covers. I'm unsure why your adverse to doing this anyway....what happened to cause you to shy away from side cover venting?

Cheers

 

[Offroader]

Offroader, thanks for the detailed post.

I think the FF + Hubsink combo is pretty solid from the reports we've been seeing. While I haven't tested the official Hubsinks, my own testing with thermally epoxied small heatsinks has proven it does improve cooling...even without FF.

As for your findings on the air circulation....there's a simple fix...vent your side covers. I'm unsure why your adverse to doing this anyway....what happened to cause you to shy away from side cover venting?

Cheers

I have no problems with venting, what I wanted to do is avoid having to use fans if FF+hub sinks work as well or close to as well.

I didn't want to put FF into my motor to then realize it doesn't work well enough and now it would be hard to remove from my motor windings to use air cooling.

I guess I should just add the FF and then wash it out with a hose if it is all stuck into the windings or something like that.

 

[Cowardlyduck]

Ok, fair enough.

It's not that hard to remove the FF. Remember I added it to my HS4080 together with venting + fans. I was able to get most of it off by just wiping it with wet wipes. I'm not that fussy though and there was still a bit in the nooks, but I don't care that much.

Are you going to try FF as well as the EDF running? That would be interesting.

Cheers

 

[Offroader]

Was doing some research to try and give me an idea about the thermal conduction / convection going on inside the motor to help figure out the bottleneck. This is just some stuff I came up with.

Different materials have different thermal conductivity values for example: W/mk

Aluminum 205
Iron or steel 40-60
Water .6
FF .1-.15
motor oil .15
Air .024

From a conductive standpoint Ferro Fluid offers about 4x-6x benefit over air. That is from conduction and not convection. This would basically mean that FF would help conduct the heat to the motor shell about 4 to 6 times faster than air/no FF.

I guess you can assume the FF is providing both conduction and forced convection as the oil will be flowing as the motor turns.

Because oil is such a bad conductor, better than air, but by no means as fast as metals, it would seem that FF could be the potential bottle neck. This is why they lap heatsinks to try and make them perfectly flat to lessen the need for thermal paste which is also a bad conductor. Air is an extremely bad conductor as we know and why FF works much better, but FF is really not that much better than air when you need a good thermal transfer of heat.


At first I thought maybe it was the steel magnet ring conduction causing the bottle neck, as it is thick, but from online calculators, the magnet ring would be able to shed something like 12,000 watts of heat from very rough thermal transfer calculations. Therefore this is not going to cause a thermal bottle neck.

It would seem that the FF is acting as a thermal bottleneck, being that it has to bridge a very large gap, using an oil which is about 400-600 times less conductive as steel when transferring heat. Bascially, oil or FF is a crappy thermal conductor, but still better than air.

What also is interesting is that you would think the bottle neck would arise in the windings to stator transfer. However, this does not seem to be the case because the stator seems to suck all the heat up until the stator gets too saturated with heat. This is because you can full throttle the ebike for a very long time on a cold motor before the windings get hot.

Justin also potted the windings to the stator, trying to fill in all the air gaps around the windings, and this offered almost no benefit with the FF/satoraide.

I would assume this is because the windings are wrapped tightly around the stator at enough points to transfer the heat to the stator fast enough, even though the windings are separated by plastic and fiberglass which are also bad conductors. I would still bet there is a thermal bottleneck here also, but not nearly as bad as the stator to magnet gap.

The questions still remain is how much forced convection is happening with the FF when the hub motor is spinning.

What would also be a very good test is if someone filled their hub motor with water if that is possible. Water offers about 6 times the thermal conductivity of FF. This would offer a huge increase in thermal transfer over FF and possibly show if FF is limiting the thermal transfer of heat to the exterior of the motor. However, I'm not sure if this would work because the FF sticks to the magnets all around the hub while the water would flow to the bottom? Basically, FF has the advantage of bridging the entire stator to the magnet ring, while water would only partially bridge the stator.

 

[Emmett]

However, I'm not sure if this would work because the FF sticks to the magnets all around the hub while the water would flow to the bottom? Basically, FF has the advantage of bridging the entire stator to the magnet ring, while water would only partially bridge the stator.

It depends how much fluid is in the motor and the speed it spins. Install enough fluid and spin the motor fast enough and there's no way it's all sitting at the bottom. Also I presume heat will move across the stator quite quickly.

Castor oil has 0.18 Thermal Conductivity (W/m K). In a H40xx motor, 100ml works a lot better than 80ml. For whatever reason.

 

[DasDouble]

Hey guys I am new to this so I wanted to ask some questions to get some know-how about ff. I have a QS V3 motor and also have ordered now 10ml ferrofluid 1110 from a german guy from this forum. He said it will reduce the heat about 40%. I have talked about this with some experienced guys in this and they have told me that it is unrealistic that 10 gram of liquit can cover 40% produced heat of a 45.000 gram heavy bike + 70.000 gram heavy rider.
Could you please explain me how the ff works / why it works?

I would really appreciate your help.

Cheers, Elias