Airbox "propellers".

[SteveD]

Does anyone have any function and importance info on these attachments within the airbox tubes? Mine have 1/3 of the "propellers" (yeah I know they're static and not actual propellers ) left.

What effect will they have on the tuning of the bike, airflow characteristic etc?

Screenshot 2024-09-27 at 21-39-41 1983 BMW R100RS Install Carburetors Cables And Air Box - YouTube.png (168.13 KiB) Viewed 707 times

[Rob Frankham]

My belief is that they are to centre the oil breather in the air duct in oirder to assist the dispersion of oil droplets into the airflow. I don't think they'll have any significant effect on the breathing of the engine...

Rob

[SteveD]

Thanks Rob. Mine might be a tad off centre.

[ME 109]

For centreing the tube as Rob says, for best practice least airflow restriction.
Of more interest is the angle cut end of the tube.
That concept only works for airflow across the tube, like the air tubes on the bottom of the carb slides.
I don't see the benefit of an angle cut with airflow along the tube?

[barryh]

Of more interest is the angle cut end of the tube.
That concept only works for airflow across the tube, like the air tubes on the bottom of the carb slides.
I don't see the benefit of an angle cut with airflow along the tube?

The angle cut might induce turbulence to help mix the crankcase vapours with the general air flow ?

[ME 109]

Of more interest is the angle cut end of the tube.
That concept only works for airflow across the tube, like the air tubes on the bottom of the carb slides.
I don't see the benefit of an angle cut with airflow along the tube?

The angle cut might induce turbulence to help mix the crankcase vapours with the general air flow ?

The angled end of the tube is at a position where the intake elbow is near vertical, so the air perhaps isn't flowing longitudinally right at that point, more across the tube, but, that might also mean that indexing the propeller has an effect? If that were the case, I'd think the longest part of the angled cut would be at the top, for best venturi effect.
I pulled all the breather hoses out of my 40mm bings, leaving just the crank vent hose entering the airbox.
It hasn't made a mess and cured the carbon build up problem on piston crowns. I think a life of higher revs sucks the guts out of the crank case somehow, with all the crank vent plumbing in place.

[Rob Frankham]

Having an angled end to the tube causes the oil droplets to flow together and be drawn off more quickly... al least, that's always been my assumption.

Rob

[ME 109]

Having an angled end to the tube causes the oil droplets to flow together and be drawn off more quickly... al least, that's always been my assumption.

Rob

I think that would be part, or all of the reason Rob.
Looking at Steve's pic I can't tell if it's right side or left of the airbox.
If left side, given the depth of protrusion into the intake tube, then I think air at the angled end of the tube is definitely travelling across the end of the tube. The angled cut in that case would work as a venturi.
I don't have my propellers anymore to check indexing, which I don't think can be adjusted, and would mean L+R orientation?
Memory fading, lol.

[SteveD]

The pic isn't from my bike. Mine has one or two of them broken off, each side. Hence the question. My seat of the pants state of tune monitor is likely insensitive to any effect I'd guess.