Too much air cooling is bad

A cautionary tale. .. I bought a second hand 9C rear hub motor which had been drilled for air cooling. I wasn't totally happy about the location of the holes. ... especially a number of large ones near the axle on the freewheel side.

It actually lasted quite well- a few thousand kilometers- but I like to pedal and with the chain on the smallest gear you get the maximum flex and leverage on the cover. The other day it started creaking whilst pedaling and this is what I found:



the air cooling did work well- it never got hot even in the Australian summer, but there were too many and too large holes in the freewheel cover.

I've replaced it with a motor that has intake and exhaust holes on the brake side and outer exhaust holes only on the freewheel

That's really interesting and serves as a good cautionary tale. We have to remember about engineering stresses when we modify load-bearing parts.

Rather than saying 'too much air cooling is bad', perhaps it would be better to say, "Removing load bearing material from an object can have negative consequences."

To be honest, it's a surprise you caught it when you did and that this didn't end in a far more interesting way.

I understand the rationale behind ventilating a hub motor, but poking holes willy-nilly in a Chinese made pot metal part that has to carry your weight, shocks and vibration, plus torque, seems foolish to me. That's apart from the water and road grime and foreign objects you invite into the motor that way. I won't be venting my motors, and I even have a machine shop and a lot of manufacturing experience to work with.

But if I had to do it for some reason, I would take note that the ribs on the inside of the cover are probably reinforcements, and I'd work around them. I'd put vents nearer to the axle on the non-drive side and out at the perimeter on the drive side, so the motor's rotation would serve as a crude impeller to draw in air from the left and exhaust it on the right. And I'd hope than none of the many steel shavings and small fasteners that inhabit my world would find their way in there.

I would take note that the ribs on the inside of the cover are probably reinforcements, and I'd work around them. I'd put vents nearer to the axle on the non-drive side and out at the perimeter on the drive side

Click to expand...

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I worked nights at McDonnel Douglas, and we had a lot of time to kill just chatting. One example is the landing gear of a new C-17 cargo plane. It was fitted to a cube-like framework, and it began raising and lowering 24 hrs a day. If the hydraulics sprang a leak, I stopped the cycling, fixed the leak, started the cycling back up, and cleaned up the mess. I was allowed to read and talk, but not sleep.

I am not an engineer, but I've had a lifelong fascination with mechanical devices of all types, and many long conversations with mechanical engineers at McD's and also at Sargent Hydraulics (submarine hydraulic components).

I agree, Chalo. Don't cut through the reinforcing ribs, I see three of them that are cut. I can stand my 200-lbs of body on a single aluminum soda can, if I carefully climb aboard and apply my weight gently and evenly across the top. Then, if I pick up that same soda can, I can collapse the sides quite easily with two fingers. The aluminum can will survive 200-lb of load stress in one direction, but is very weak from another direction.

Also, I'd suggest that the holes are too large. Add a temp sensor, and start with smaller holes...you can always make them larger. I also don't like the squared-off corners of the holes next to the rim. They should also be round to eliminate stress-risers. Just a thought...

I think you'd be surprised at how much flow you can get (even with smaller holes) using a centrifugal fan, such as this (the common model for this is a ventilated front brake disc for an automobile...it works):

http://endless-sphere.com/forums/viewtopic.php?f=30&t=62658

Some good points there. I agree about never cutting reinforcing ribs! That chinese cheese metal is weak enough to start with. Yes it was lucky it didn't fail suddenly- I had to do an emergency temporary fix by dribbling some superglue into the cracks, which worked suprprisingly well.

I always add a temp sensor and for quite a while I've been using a vacuum cleaner impellor as a fan on one of the covers- works quite well.

I think that a big part of the problem (other than excessive weakening) is me wanting to pedal at 50kmh- I should just put a single gear on the freewheel so at least it won't have the levering effect that it does right now with a 7 gear cluster. I managed to break the freewheel off my mac motor and a buddy of mine here in Perth did the same on one of his C'lyte motors during a group ride (but at least with his the bearing area was intact so he could still ride just not pedal.

Or just ghost pedal. :wink:

bowlofsalad said:

Rather than saying 'too much air cooling is bad', perhaps it would be better to say, "Removing load bearing material from an object can have negative consequences."

To be honest, it's a surprise you caught it when you did and that this didn't end in a far more interesting way.

Click to expand...

Yeah, I did take some time to think of the subject title but I like to be a bit controversial and thought the way it is might stir some people up? :wink:

Air cooling (apart from getting stuff,rust etc isnide) is very effective on the whole.... and some people get very passionate about the subject.

As for catching it in time- I noticed the creaking noise whilst pedalling and didn't take much to suss out what it meant- but it was a fairly tentative ride home!

Some of these things also come with pre-installed cracks where the materials either weren't consistent or weren't heated sufficiently (or cooled wrong) when cast, because I have seen voids and cracks (or things that look like cracks) in some of the covers I have here that are unmodified.


There's a couple of "interesting" problems with the covers of my HSR3548 that would lead one to not want to further weaken them.
http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=12500&p=819086

That's some pretty hitech race lightening there- honeycomb aluminium. Don't they use that on F1 cars?

Has anyone measure the thickness of 9c clyte etc

My buddy said my clyte covers are thicker than the cros at least.

I've run a bafang 750w on my clyte 4065, so far no problems and you'd think there would be compared to your pedalling blues mentioned above.

touch wood I have no problems when I go to single speed hd freewheel granny sprocket at back...

One thing for sure is that I would never be able to ride the terrain I do now with out going to a 20" wheel and both venting and running two motors for as long as I can.

lol.. very poor choice of hole location. Drilled right through important structural parts of the hub's casing. ( those ridges ). And besides, the heat is at the magnets and copper, not in the center of the stator..

Very immoral :lol:

Yeah, cutting the ribs was not a great choice. Might have weakened it less, to have smaller holes near the hub, then some more smaller holes further out on the part.

All the motors I vented used smaller holes, located as close as possible to the coils. They were not effective in letting the motor run longer, but helped with controlling the resulting heat spike when you stop. Stopped, the air would suck in the lower holes, and vent out the holes that were higher at that moment.

I had less problems with toasting the halls when I stopped riding that way.