Topic: Do windshields have a negative effect on MPG?

[manjisann]

I put a small windshield on my 1980 CB650c and my fuel economy went from 38mpg down to 28mpg. Maybe I've been romping on the throttle more just to see how she would handle at higher speeds with the windshield, and I've only done one tank with the shield on, but it made me wonder if a windshield would have a negative effect on MPG. I always thought if anything it could improve it due to making the bike slightly more aerodynamic. Thoughts?

Thanks,

Brandon

p.s. the bike has different carbs and pods on it than stock, so the 38mpg is probably to be expected.

[dave500]

if its a near vertical one itll be bad for fuel economy and performance,if its laid back itll be more slipstreamy.

[Jerry Rxman Griffin aka MuthaF'er]

I have a rather large removable upright windshield I use on my Fat Boy when I do my yearly excursions. I can normally get 45mpg without it running 80-85mph. When I hit the road I usually load it down with full saddlebags and a filled pack strapped to my backrest and a cooler with about 8 cold beers. The weight doesn't help the mpg either. Kinda like having a big broad on the back  With this configuration I can drop down to perhaps 35-38mpg. Bucking a strong headwind at that load and speed my mpg has gone down to 30mpg.

I think the advantages of my removable windshield on a 500+ mile day outweigh the downside of the mileage. I'm protected from the constant wind, rain, cold, snow, grit, gravel, bugs, birds, cigarette butts, airborn blown tire schrapnel (yes, all that happened the last 2 years). When I get off I can hear better and my neck isn't sore and I'm not beat to death. Now, my ass, that's a whole different ball game  but those cold beers help that!

[manjisann]

Thanks guys, that does answer my question and make me feel a little better about it all 

Brandon

[TwoTired]

A windshield is going to have an effect.  But, it can be positive or negative depending on how much turbulence it adds or subtracts from what the bike has before adding the windshield.

The ones I put on my 550s didn't effect MPG negatively.  If anything, they improved the MPG.
The first was a Vetter Windjammer 5.
I was commuting 30 minutes at 5 AM in winter.  I was quite worried that the bulk and air displacement would make the MPG so much closer to the the car's that I would go back to it for the regular commute.
Didn't happen.  One of the things I did notice was not only did air pressure on my chest go away, there was air pressure on my back, pushing me forward, when at cruising speed.  That never happened without the fairing.  Usually, the back of my jacket balloons rearward due to the negative pressure behind me as I move forward with the bike, sans fairing.  This bike was used almost exclusively for commuting as my wife hated the thing at the time.  My fill up intervals (after switching to reserve) extended by about 2-4 days, depending on time of year and how much choke I needed daily.

Next was the National cycle windshield on my CB550F.  I think this one didn't help the mileage.  But, it still gets about 45 MPG.  Which is OK for the 550.  It does remove the air pressure off my chest and face shield.

Then I put the Vetter Quicksilver fairing on a CB550F.    This bike gets 45-50MPG, and some air pressure on the rider's back during cruise.

I don't doubt there are other fairings/windshields that actually produce anti-drag benefits.  The small ones over the headlight and gauges don't do much of anything and are mostly cosmetic.  Same is true with many add-on windshields.

But, there is a reason why racers have faired in motorcycles, and why modern bikes have all that plastic surround.

When you push air out of the way it creates negative pressure.  This can occur in waves behind the disturbance as the air rushes in to fill a void.  What if your windshield was creating a negative pressure where your carb intake is?  Such a condition would make the carbs draw more fuel.

What if you had a windshield and you still felt air pressure on your chest?  Wouldn't that indicate you now have two sources of wind resistance instead of just the one you had before adding the windshield?

Your results may vary. 

Cheers,

[Gaither]

A Slipstreamer Sport S08 didn't effect the mileage on my '77 CB550F. Considered a "full-size" windshield. I can just see over it by raising my head bit which is what I wanted. If it ever fogs up, I can see where I'm going.

I'm still getting 46 mpg - all stock, 12,550 mile bike. IMO it made for a much more pleasant ride and catches all the bugs.

Caught it on sale earlier for $99 delivered - OldBikeBarn [with no problems like I've seen posted].