I couldn't find this answer nowhere, but i'm very curious about why some motorcycles have a carburetor intake connected to their chassis.

In the image below we can see how the chassis is designed with holes in the lateral/front part of the chassis, to channel the intake air that come through the air filters.

This is a Yamaha V-star 250/Virago 250 chassis

Just for context, this is the whole motorcycle:

Yamaha Virago 250

Any idea why a design like this exists at all?

2 Answers 2


It puts the filter up high and the use of the already existing frame tubes reduces the need for extra pipes that would be a large diameter.

Frame tubes have also been used for storage of oil.

  • Essentially the air box, which would need to be a separate component otherwise, is built into the frame. It saves space and a certain amount of weight, but calls for a design compromise in the frame - obviously the designers thought that it was worthwhile.
    – Frog
    Commented Jul 20, 2021 at 9:21
  • I believe that at least one manufacturer, Buell, used the frame for a fuel tank.
    – jwh20
    Commented Jul 20, 2021 at 13:06

It’s done because air cooled engines heat the air and fuel (reducing density)in the “breathing zone” of the carbs during no, and low speed operation. Higher velocity air is more dense when compared to slower moving air. Really just a way to designate and deliver a consistent air charge. Potentially less effected by the heat radiating off the cooling fins. At least in the short term. I’ve even seen designs that include directed intake air flow over fuel petcocks enroute to the throttle bodies. If you look at the carb placement between the two hottest components on the engine. Fuel is just as susceptible to density loss as temps rise. If you note the majority of the gas tank sits above this air cooled section of the frame rails. Also in an attempt to reduce heat rising and conduction through components.

  • Do you have any credible sources for your assertion "higher velocity air is more dense when compared to slower moving air"? Commented Nov 17, 2021 at 11:04
  • 1
    You're talking about a ram air effect (ie: mass in motion stays in motion), not about density. Again, please quote your source. Just because it's at a higher velocity doesn't mean it's more dense. It has to be in motion for a reason. The reason on a naturally aspirated engine is vacuum. A vacuum is less dense than normal air pressure. Commented Nov 19, 2021 at 2:15

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