Thenodynamics and Cooling.

[mx250]

I’ve been cogitating, and it hurts , for some time about the thermodynamics of air cooled motors and the wisdom of some weight savings methods I have seen over the years.

My current inferno of cognitive activity was sparked by this…

http://cgi.ebay.com/ebaymotors/1977-79-Montesa-250-VB-Big-Bore-Kit-Ahrma-Vintage-MX_W0QQcmdZViewItemQQcategoryZ35604QQihZ022QQitemZ350103672600QQrdZ1QQsspagenameZWDVW

Those drilled holes and cut fins; whilst beneficial to weight savings and thusly performance, what would the net benefit be over say a 5 lap moto ?

This balance between weight and effective, sustainable, protracted cooling has interested me for some time probably starting with the radical CCM solution of cutting out(breaking off) every second fin….

The universal switch to water cooling demonstrates the clear need for effective cooling on MX’ers. There are obvious advantages and disadvantages but obviously in the balance water cooling is the ‘go’. Water cooling brought closer tolerances (more power) and more reliable consistent cooling (equal power at end of moto).

These would have been issues in the period immediately before w/cooling and yet it was in this period that we saw weight saving trimming of fins. Counter productive?  Or is there a way to save weight and retain cooling efficiency, or at least minimize the loss of efficiency?

My limited understanding of thermodynamics of fins is that the two cooling properties are mass and surface area. My understanding of surface is that it includes the edging. The amount, the shape and the placement of the edging has an important effect on the cooling efficiency of the fins. Fins that come to a taper (sharp pointed edge) are more efficient than square edged fins. By what degree I’m not certain (anyone?).

Drilling circles has the advantage of increased perimeter for loss of mass but has the disadavantage that the edge is not necessarily presented to the cooling air.  'V'ing is the opposite.

Is reducing mass but increasing edge area by drilling or ‘v’ing fins an effective compromise? Of drilling and ‘v’ing  which is the more efficient?

Has anyone had experience, either now or ‘in the day’, of drilling and ‘v’ing  of fins? Cooments?

[mboddy]

The maximum surface area is when the diameter of the hole equals the thickness of the fin.

[mx250]

The maximum surface area is when the diameter of the hole equals the thickness of the fin.

Soooo, small holes rather than large holes?

[cyclegod]

The maximum surface area is when the diameter of the hole equals the thickness of the fin.

My math says the "surface area of the hole x 2 (pie x radius squared)" should equal the circumference (pie x diameter) x thickness of the fin in mm

[Lozza]

One of the biggest 'barriers' to thermal conductivity is in the junction between the iron liner and the aluminium cylinder.I have never seen anything written on the edge profile v's thermal conductivity.Those holes drilled in the cylinder would equal about 20g in weight and a huge loss of thermal flow.Dimpling with a large radius ball nose end mill adds a lot of surface area, then there is even some debate on whether that makes a boundry layer that keeps heat in(like a golf ball). Still the best proved aid to thermal flow is more oil.

[YSS]

That makes sense Lozza . 

[pancho]

i have somewhere a copy of Phil Irvings book 'motorcycle engineering' and it has a chapter on cooling , i remember reading about the difference in efficiency obtained by tapered cooling fins among other things.it is an interesting book, its old now but most engineering details dont change.... the book is probably available in public library lists. i will go and search for my copy.  cheers wally.

[Hoony]

is that a different book to Phil's classic "Tuning for speed" ?

[pancho]

yeah hoony it is but it was about twice the price for about the same size book     it must have been a limited edition or something . in a minute i'll initiate a search amongst my paraphenalia.[thats my big word for the week.]  cheers wally.