At what point is "longer" long enough? I mean 1"....why not 2"?

But to your point, how much more cooler would the engine get if they were 1" longer?

 

Fluid mechanics and aerodynamics. Complicated math. Heat may never get pulled all the way to outside edge of the fins enough to justify the extra material is my educated guess.

 

I noticed the King of the Baggers bikes had "welded" additions to the fins....that's sort of what prompted my query.

 

interesting..... but a very specific bike with a specific purpose.
You did see that after a test ride, they cooled the bike with a leaf blower?

As with many race machines, what do you think was sacrificed to make the bike lighter?

 

Thats a very good question, and also good replies. I can see where all of yall are coming from.

 

Huh ..... I thought they were that way so that your tentacles didn't boil before your thighs were medium-rare.

 

I believe the strength of the fins comes in to play also.
On vintage dirt bikes it is very common to see broken fins because they are long and thin compared to what we have today, so this is part of the decision making process also for the engineers.

 

Not my words, but these paragraphs put things in layman's terms pretty well.

Qualitatively speaking the distance between two fins depend on the air flow velocity and the pressure drop you can afford. If the fins are to near to each other and velocity too small boundary layers will be too thick and heat transfer not very efficient. Thickness of fins will depend on manufacturing approach as well their height. It is an optimal ratio depending on the conductivity of the fin and convection coefficients.

if for instance the fins are too long the boundary layer can grow and reduce average heat transfer intensity. S that it is better for instance to have shorter fins (in flow direction) and if possible not in same plane in order to increase turbulence.

Fins improve heat transfer in two ways. One way is by creating turbulent flow through fin geometry, which reduces the thermal resistance (the inverse of the heat transfer coefficient) through the nearly stagnant film that forms when a fluid flows parallel to a solid surface. A second way is by increasing the fin density, which increases the heat transfer area that comes in contact with the fluid. Fin geometries and densities that create turbulent flow and improve performance also increase pressure drop, which is a critical requirement in most high performance applications. The optimum fin geometry and fin density combination is then a compromise of performance, pressure drop, weight, and size.

If you want to get technical look up the Nusselt number, and Laminar Air Flow in Fluid Dynamics.

 

Not my words, but these paragraphs put things in layman's terms pretty well.

Qualitatively speaking the distance between two fins depend on the air flow velocity and the pressure drop you can afford. If the fins are to near to each other and velocity too small boundary layers will be too thick and heat transfer not very efficient. Thickness of fins will depend on manufacturing approach as well their height. It is an optimal ratio depending on the conductivity of the fin and convection coefficients.

if for instance the fins are too long the boundary layer can grow and reduce average heat transfer intensity. S that it is better for instance to have shorter fins (in flow direction) and if possible not in same plane in order to increase turbulence.

Fins improve heat transfer in two ways. One way is by creating turbulent flow through fin geometry, which reduces the thermal resistance (the inverse of the heat transfer coefficient) through the nearly stagnant film that forms when a fluid flows parallel to a solid surface. A second way is by increasing the fin density, which increases the heat transfer area that comes in contact with the fluid. Fin geometries and densities that create turbulent flow and improve performance also increase pressure drop, which is a critical requirement in most high performance applications. The optimum fin geometry and fin density combination is then a compromise of performance, pressure drop, weight, and size.

If you want to get technical look up the Nusselt number, and Laminar Air Flow in Fluid Dynamics.

Yeah - talk nerdy to me.

 

I've been reading quite a bit about heat challenges (I'm going to a 124 cu in). It caused me to go look at my existing cylinder sleeves.

On the rider's left, the fins could EASILY be another 1'' inch (longer?) outward. On the rider's right, the same thing.....you would just need to re-work the aircleaner.

I'm sure HD engineers have a VERY good answer - just seeking some education please.

here's a pic of kyle wyman's king of the baggers bike. it's prolly not the best example to put here because he DNF'd due to what looked like heat problems. the factory enginerds had a hand in developing this so ima thinkin it's a process under refinement lol.

photo credit motorcycle.com

 

Notice the addition of forced air directed at those fins. Also looks to be where a gasket is located.

 

They could make them deeper. And they could add more fins. But there are a few other considerations. Deeper or more less-thick fins will make the jugs more expensive to manufacture and more expensive to handle. And it will make the jugs make more noise. Probably requiring dampeners, costing more money. The Harley management team is all about cost cutting, even at the expense of durability. When the M8 came out, the only concern on the jug was that it be the same height as the TC and be manufactured with the same tooling. And there was more than a little pressure to not change the look of the motor when viewed by the casual rider.

 

The thinner fins would probably make it a lot easier for breakage which would end up costing them money for Warranty


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