Chin or Belly Fairing


Information and questions on GL1100 Goldwings (1980-1983)
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lake_harley
Posts: 54
Joined: Mon Jan 25, 2010 7:06 pm
Location: Uniontown, MO
Motorcycle: GL1100

Chin or Belly Fairing

Postby lake_harley » Tue Jun 10, 2014 8:57 am



I'm just curious if Honda or the aftermarket ever produced what I'd call a "Chin" or "Belly" fairing to smooth the airflow on the underside of a GL1100I? The reason I wonder is that any speed above 70 MPH my fuel mileage goes down really quickly. Below 70 MPH I generally get +/- 40 MPG, but above 70 or in any significant cross-wind or head-wind I'm down to 35 MPG or less. I just experienced this again on a trip last week from Missouri to Colorado and back.

Since drag increases at the square of the speed (At least that's what I've understood to be mostly true) it seems that any efforts to reduce sources of drag would provide some benefits. Anyone made any efforts to improve the "Aero Package" on a GL1100I?

Thanks, in advance for any input.

Lynn



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WingAdmin
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1982 GL1100A Aspencade (sold)
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Re: Chin or Belly Fairing

Postby WingAdmin » Tue Jun 10, 2014 9:05 am

Motorcycles as a whole are generally not very aerodynamic, and this is more the case for older bikes. That's why higher speeds cause mileage to plummet - and it's not just your bike, it's all bikes.

lake_harley
Posts: 54
Joined: Mon Jan 25, 2010 7:06 pm
Location: Uniontown, MO
Motorcycle: GL1100

Re: Chin or Belly Fairing

Postby lake_harley » Tue Jun 10, 2014 10:17 am

I completely understand that, there's just a lot of "stuff" in the airstream creating drag. But, as my post questioned, has there ever been a fairing available for the underside of a GL1100?

I've considered building a belly fairing, but have always wondered how much airflow Honda relied on to take heat away from the things in the front of the engine and on the underside....ie: oil filter, oil sump? There are a lot of "bumps" and "warts" underneath to create drag and it seems an effort to reduce contact with the "direct" airflow could result in some improvement. I wouldn't envision sealing the area off from airflow by any means, but, for the lack of a better or proper term, make it more of a "passive" rather than "active" flow of air.

Lynn

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WingAdmin
Site Admin
Posts: 17045
Joined: Fri Oct 03, 2008 4:16 pm
Location: Strongsville, OH
Motorcycle: 2000 GL1500 SE
1982 GL1100A Aspencade (sold)
1989 PC800 (wife's!)
1998 XV250 Virago (sold)
2007 Aspen Sentry Trailer

Re: Chin or Belly Fairing

Postby WingAdmin » Wed Jun 11, 2014 12:20 pm

I haven't seen a chin fairing like you describe, no. Believe it or not, the most amount of drag is created BEHIND the bike. As you travel, the bike pushes air out of the way. The back end of the bike is not remotely streamlined (and neither are you), so this air is not allowed to gently come back together, like you see on a slipstreamer. Instead, the air gets pushed out of the way by the front of the bike, and then the bike is gone, causing large vortices and a vacuum behind it. This is the reason your passenger's helmet gets hit with wind from BEHIND them - it's a vortice coming around to hit them from behind. You see the same thing when crotch rocket riders go out riding wearing a T-shirt. The air is pushed up over them, then suddenly is pulled back by the vacuum left as the bike passes. This vacuum causes a vortice, which curls around and lifts the rider's shirt up from the bottom.

In any case, short of adding a large fairing on the back of the bike to allow the air to gently rejoin itself, you're not going to do much that will have a large effect on mileage.

Incidentally, you'll see some tractor trailers now have rear fairings on the trailers for exactly the same reason - to help rejoin the air, instead of leaving a huge vacuum at the back, causing vortices and poor mileage:

Tractor Trailer Rear Fairing
Tractor Trailer Rear Fairing

lake_harley
Posts: 54
Joined: Mon Jan 25, 2010 7:06 pm
Location: Uniontown, MO
Motorcycle: GL1100

Re: Chin or Belly Fairing

Postby lake_harley » Mon Jun 16, 2014 9:26 pm

Again, nothing but agreement. I've been interested in aircraft and am currently building an ultralight, so the drag created at the back of a moving object is quite familiar. I can't see hanging some sort of tail cone off the back of the my Gold Wing :o but figure any improvement/smoothing would be a step in the right direction.

The effect of low pressure area(s) seems apparent, because when my wife and I ride 2-up I get about the same, or maybe even better MPG than I do solo. I always figure that when my wife is behind me she is actually filling in the gap between my back and the top box reducing the low pressure area that would be behind me. The low pressure area behind the top box and saddlebags is still there, but at least the one immediately behind me is reduced. That's why I always tell my wife that "she completes me" :D

Lynn




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