What a drag!

[willowstreetguzziguy]

took my 1200Sport for a 140 trip in 75 degree weather with a (new to me) HB 40 L top case attached. On the interstates, I normally can get 41-43 mpg doing 75 mph. When I looked at the computer mpg, it was a constant 34 mpg! I can't believe that rear case makes that much difference but it must catch the air behind me. Once on the back roads at slower speeds, I was getting 44 mpg.

[HardAspie]

https://www.youtube.com/watch?v=Q9kqoD1t4lw

[willowstreetguzziguy]

Nice song.....so a 20% drop in gas mpg because of a rear trunk is normal?

[HardAspie]

I really don't know. I have had a top trunk but I did not do a before / after MPG test. I would expect a change possible, but that is a huge drop.

[Moto]

Nice song.....so a 20% drop in gas mpg because of a rear trunk is normal?

I remember reading a study that involved actual measurement of drag in which it was found that a squared-off tail
like a rear trunk can reduce drag, and improve mileage. It depends on the details, I think. A smooth airflow before
the squared-off rear is no doubt important.

[HardAspie]

And if riding solo there could be a great local turbulence between rider's back and the trunk. Thee combination could be shedding Karman Vortex Sheets.

[rocker59]

I think there were other factors. Headwind, mathematical error, etc. I've never noticed that kind of impact from adding luggage.

[kingoffleece]

Nor have I from several bikes with top cases.

[Kev m]

I say you're victim of a small sample.

Try again.

Yeah, I've used an HB top case on every Guzzi I've ever owned (and frequently on the Sporty too).

I can't say I religiously log fuel mileage anymore but I at least check it at each fillup and have for decades.

I've never noticed that kind of impact attributable JUST to that one factor.

So yeah, I say try again... and again... or forgedaboudit.

[kidsmoke]

slight veer...but relevant. This app is called Fuelly. You can track as many vehicles as you like. I've always remained vaguely aware of mileage etc but with this being so simple, just input the stats on your phone while you're filling up, it makes it stunningly apparent what's impacting consumption.

This is a screen shot of my phone this morning. Cool app, and would serve to research this type of issue on your bike. Not sure if it was free or not, but if it cost anything it wasn't more than a buck or two or I wouldn't own it (Guzzi content)

[oldbike54]

I think there were other factors. Headwind, mathematical error, etc. I've never noticed that kind of impact from adding luggage.

 Motorcycles are so short that the aero is all screwed up . Unless the bags are sticking way out , the turbulence created by the front edge of the bike and by the rider probably hasn't returned to hit the bags . The worst shoe box cars are much slicker through the air than any street bike .

 Dusty

[twowings]

The simple physics of pushing ANYTHING through the air faster than about 55-60mph force a MARKED upward spike in fuel consumption...a slippery shape helps, certainly, but not as much as slowing down...

[ITSec]

The trunk (and panniers, for that matter) makes a difference, but not nearly as much as:

• going 70 instead of 55mph
• headwinds or heavy gusts
• bad (or seasonally adjusted) gas
• taller or wider windshield than previous
• altitude (my Norge gets 5  mpg better at 5000 ft than at 1000 ft)
• enthusiasm (variations in speed vs steady throttle)
• and other factors...

[willowstreetguzziguy]

I was measuring the avg. mpg with the 1200 Sport's computer readout. I can consistently average 42 mpg on the bike's computer, when traveling on a flat interstate without the tail trunk rather than 34 mpg with the trunk.

[Vagrant]

couple of examples. my buddy put a lot of miles on a Pacific coast without a rear rack. when he put a rack on with a 45L bag he got 2000 miles less on a rear tire. he doesn't check gas #'s.
my Ev with HB 40L side bags got 2 MPG higher than it did with Givi 45L E45 bags.
I have a 2010 Tundra 4X4 with a 10,500 towing package. on the highway it runs 16-17 empty at 80MPH.
I ran to Colorado and back with a 2015 Kawasaki Versys without bags in the back. averaged 15.7 for 3000 miles.
I put the Gold wing in the back and did 1800 miles, average 11.0 I think it was that massive tour trunk catching the wind.