Motorcycle Airbox Physics

  • Thread starter HandSolo
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  • #1
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Main Question or Discussion Point

So I'm trying to work out why the inlet port of my 200cc four stroke motorbike looks like this.
DSC_0513.jpg

The box itself is approximately 1.5 litres and encloses a cylindrical foam based air filter. That then feeds into a 25mm diameter carb.

I've know car inlet systems be tuned for helmholtz resonance at a frequency that constructively adds to the pressure conditions at the inlet port at time of opening (for a desired frequency - usually quite low down rev range for road going cars). I can't see how this arrangement would either affect or improve resonant frequency in the air box so don't think this is the reason.

It could also be possible that the arrangement could assist in airflow order, perhaps creating laminar flow conditions, but again, this is feeding onto a cylindrical filter which draws from all parts of the box - not just around the inlet area.

So, whats the deal here? I haven't seen it on other motorbikes - or cars for that matter.
 

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  • #2
ChemAir
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Which way is this oriented? Is the camera looking down?
 
  • #3
Baluncore
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So, whats the deal here? I haven't seen it on other motorbikes - or cars for that matter.
It may be designed to utilise the surface area of the air filter in a more even way, to keep small birds from nesting, or just to sound good. What covers the holes to keep rocks out ?
I assume engine is single cylinder. It would help to know make, model and year.
 
  • #4
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Camera is looking down yes.

The box is covered by the seat which gaps where it meets the frame.

The engine is indeed single cylinder - a Honda TLR 200 trials bike from the early 80's. It sounds incredibly quiet and civilised so guess it isn't for the sound!
 
  • #5
ChemAir
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The reason I was asking if it was facing down, was I expect the holes with a lip on top right before entering a plenum box might be for disengaging moisture droplets that get pulled in the intake, to prevent them from being ingested by the carburetor.

Air Box resonance information.

Changes in intake tract cross section and length have an effect on torque. The exhaust and intake are usually optimized for best driveability at lower and mid RPM ranges. I would expect the multiple port inlet to be the (hopefully) optimized reasonable use of the available area.
 
  • #6
berkeman
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Changes in intake tract cross section and length have an effect on torque.
That was my first thought when the OP mentioned that it's a trials bike. But that old 200cc model is not really a trials bike, IMO (sure is made to look like one though :smile:). It's based on a a very fun dirt bike model that a buddy of mine in high school rode. Very light and nimble on the trails. The airbox design could still have some purpose for increasing the torque of that small motor, though.

It would be helpful to see the shop manual's exploded view of the airbox, air filter and carburetor. I'll see if I can find something online, unless @HandSolo can find and post something first.

https://i.pinimg.com/originals/0a/ba/25/0aba25688cb44d6117ca6a3f3dc1707b.jpg

0aba25688cb44d6117ca6a3f3dc1707b.jpg
 

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  • #7
ChemAir
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It would be helpful to see the shop manual's exploded view of the airbox, air filter and carburetor. I'll see if I can find something online, unless @HandSolo can find and post something first.
The ductwork arrangement might also give some insight why there are small ports on one side and large on the other.
 
  • #8
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I wonder if the box also provides some structural support for other pieces. Is anything else bolted onto the box, or does the seat bear on the box? Or maybe the tubes prevent the box from collapsing due to the intake vacuum.
 
  • #9
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Sorry not been able to respond as had a bereavement.
 
  • #10
berkeman
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No worries @HandSolo -- sorry for your loss. Take all the time you need.
 
  • #11
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Sorry for the delay, and apologies if someone has already alluded to this, but it occurred to me that the Honda engineers may have applied the same principles applied to microwave doors in attenuating waves - in this case sound waves. The bike was engineered roughly at the same time microwaves were becoming popular.

So, the air-box is of a fixed volume and the engine has a spectrum of frequencies it typically operates within. If the engine frequency coincides with the resonant frequency of the air-box (or harmonics to a lesser extent) then there may be a non-linear/sudden amplification of the engine noise. Having the hole sizes tuned to attenuate these amplified frequencies will dampen the effect.

When I get a chance I will measure the size of the holes and see if this helps or hinders the theory.
 

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