OzVMX Forum
Clubroom => Tech Talk => Topic started by: mx250 on September 02, 2011, 07:28:19 pm
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It's a subject that has had me a little intrigued for sometime - cutting off/out cooling fins in the pursuit of lighter weight (I presume). Here's a few Ossa's from the early '80's.
There are others including the KSI Honda featured here recently and, famously, CCM.
(http://i323.photobucket.com/albums/nn458/mx250syd/Fins-Ossa/ossa250copa1985.jpg)
(http://i323.photobucket.com/albums/nn458/mx250syd/Fins-Ossa/a1978OSSADeSERTTT250.jpg)
(http://i323.photobucket.com/albums/nn458/mx250syd/Fins-Ossa/1983_ossared303pic.jpg)
Despite bulk dollars and an all out assault on weight I can't recall any of the Japanese factories taking this option. So I wonder about the wisdom of the practice.
My limited understanding is the effectiveness of the cooling fins is that it comes down to mass and surface area. Therefore the best cooling for weight is many very thin densely spaced fins you would typically see on aircraft engines.
So it must be the original low powered BSA (and others) barrel must have been neither over cooled or the high performance, high power CCM barrel (and others) is dangerously compromised.
Your thoughts?
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The normal reason given for removing every second fin is not weight savings but to stop the fins getting plugged with mud causing the motor to overheat. Sometimes less is more
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The normal reason given for removing every second fin is not weight savings but to stop the fins getting plugged with mud causing the motor to overheat. Sometimes less is more
Sounds like BS to me ::) - it would only be applicable in very few situations on very particular tracks.
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3 very nice looking bikes there.
Dare I say it HOT looking. ;D
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3 very nice looking bikes there.
Dare I say it HOT looking. ;D
I would love to have any one or all three parked in my garage ;) 8).
I don't think any of them made it here to Australia did they.
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The normal reason given for removing every second fin is not weight savings but to stop the fins getting plugged with mud causing the motor to overheat. Sometimes less is more
Sounds like BS to me ::) - it would only be applicable in very few situations on very particular tracks.
Remember it was most often done in UK - with mud aplenty!
Generally speaking yr right about more surface area & mass. But (there's always an exception isn't there??) as I understand it, its not just conductive heat transfer, there's also radiative & convective, & if the fins are too close together they interfere w ea other's functioning, losing efficiency & being counter-productive.
Therefore sometimes less is more, eg the diff between early 9 fin Bul 'round' barrels cf to later 6 fin 'square' barrels. Likewise Maico square barrels cf to later radial fins.
There is an optimum spacing for heat transfer efficiency
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and of course the uk see a lot of cool weather so overheating is not as much as down here in aussie where we see lot"s of hot weather.
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Didn't the gp3 have seizure issues,which many put down to the removal of the fins on the cylinder, although my understanding is that the cylinder itself doesn't get all that hot,just the exhaust port and head ???
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Biggest barrier to heat transfer is betweem the liner and the cylinder. Yamaha were the first to overcome that somewhat with the use of cast-in liners
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They serve the same purpose as fins on a radiator on your car. Remove 10% of them and watch the temp rise.
Lozza. from Memory over 40% of combustable heat exits via the exhaust port and the rest is absorbed, hence the reason we need ( pre-mix )oil mixed @ 20-1 as this acts as a coolant also ;D
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In keeping with the Ossa theme of less fins here's another I lifted off facebook.. 8)
(http://img.villagephotos.com/p/2005-1/923869/340260_10150303047082042_246994827041_7819386_4106339_o.jpg)
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Biggest barrier to heat transfer is betweem the liner and the cylinder. Yamaha were the first to overcome that somewhat with the use of cast-in liners
are you referring to their effort to use high-silicon content aluminium, to allow them to not install the steel liner? I have to be cynical and think that heat transfer was not their primary objective.
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It was a big breakthrough for reliability when the "chrome" bore or whatever it gets called came into production. It may be more of a maintainance issue nowadays, but it is worth it for the much better heat transfer. Iron liners don't have enough contact surface area. Under a microscope the ridges in the machined barrel and liner show up as only point contact. If a manufacturer casts around the liner this also softens the liner and it doesn't wear as well. There is also the problem of different rates of expansion.
Cheers, Grahame
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Biggest barrier to heat transfer is betweem the liner and the cylinder. Yamaha were the first to overcome that somewhat with the use of cast-in liners
are you referring to their effort to use high-silicon content aluminium, to allow them to not install the steel liner? I have to be cynical and think that heat transfer was not their primary objective.
When a press in lner is used the boundry between one surface and the next only conducts something like 60-70% of that of a homogenous material. So 30% of the heat that would normaly transfer through a material is 'trapped' at this point. Yamaha started to cast in liners and they left the cast iron very 'rough' unmachined finish that gave more surface area for the heat transfer.
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Lozza has explained it.
The lack of heat transfer is well demonstrated when using an oxy-acetylene cutting torch . It's very easy to oxy cut a nut of a half inch bolt without damage to the thread on the bolt because of the poor heat transfer.
cheers pancho.
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In keeping with the Ossa theme of less fins here's another I lifted off facebook.. 8)
I know I'm biased towards dirt track Doc, but that Ossa is one hot looking bike.