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Old 06-02-12, 05:15 PM   #1
TomD77
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Bikes on the back of your car are a drag!

I travel back and forth between Pensacola and Atlanta a lot and recently had a chance to compare the gas mileage on a new car with two bikes on a rack behind Vs no bikes. The car is a Nissan Altima, one of the seemingly interchangeable lozenge shaped cars so common these days. The reason recent design cars are all so seemingly similar in exterior design is because fuel consumption is now a VERY important element to the popularity and sales of a vehicle and the "lozenge" shape works.

Last week I had occasion to drive to Atlanta with a "slick" car and come back with two bikes on a Saris rack. With cruse control set right at 77 mph both ways and the trips timed to keep me as clear of the Atlanta traffic as possible, I made 34 mpg slick and 29 mpg with the bikes hanging in the slipstream. I knew there would be a difference but was a little surprised that it was that much. The car that I moved out of (Infiniti I35) got pretty close to 25 mpg, bikes or not.

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Old 06-02-12, 07:21 PM   #2
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I bet the difference between the two conditions would've been closer if you had driven 55 or 60 without and with the bike. You have to overcome greater air resistance the faster you go and the impact of air resistance is going to be far greater at 77 than at 60 or even 65. BTW, what was the speed limit on the road?
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Old 06-02-12, 07:40 PM   #3
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Better on the back than on the roof.
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Old 06-02-12, 07:43 PM   #4
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Did you consider the prevailing winds in your calculation?
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Old 06-02-12, 07:44 PM   #5
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What WNC said and you knew this was coming. When you put the bikes on the back the airflow around the tail section was made turbulent instead of the smooth release the engineers had put into the car. Its shape is just more susceptible to disturbing the airflow. Roof would have been even worse though. Nice looking car though Tom!

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Old 06-02-12, 08:11 PM   #6
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Did you consider the prevailing winds in your calculation?
^ This.

To get an accurate comparison, you need to travel the same road in the same direction with identical weather conditions.

I normally carry my bike on a rear hitch rack. My only fear is getting rear-ended and I get a little nervous when cars pull up too close behind me at a light.
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Old 06-02-12, 08:54 PM   #7
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What WNC said and you knew this was coming. When you put the bikes on the back the airflow around the tail section was made turbulent instead of the smooth release the engineers had put into the car. Its shape is just more susceptible to disturbing the airflow. Roof would have been even worse though. Nice looking car though Tom!

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The effect on the drag was worse than just screwing up the air release, it was like adding a 18"x50" piece of plywood just above the spoiler.

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I bet the difference between the two conditions would've been closer if you had driven 55 or 60 without and with the bike. You have to overcome greater air resistance the faster you go and the impact of air resistance is going to be far greater at 77 than at 60 or even 65. BTW, what was the speed limit on the road?
Well yes. But if I had gone 25 mph the difference would have been closer yet but I'm sure you can see the absurdity there. . BTW: The roads were I65 and I85, limit 70 but keep it below 80 and you won't get ticketed, so that's what I do.

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Better on the back than on the roof.
Right about that. You can see the top of the car looks like an airfoil (wing section), the key to its aerodynamics. Screw the roof line aerodynamically and you have screwed the whole thing.

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Did you consider the prevailing winds in your calculation?
The trips were over 3 calendar days with the winds generally from the SW at aprox 7 mph which would tend to favor the northbound trip.

Last edited by TomD77; 06-02-12 at 09:11 PM.
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Old 06-02-12, 09:17 PM   #8
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That's another reason I transport my bike inside my mini-van or SUV. That way I don't worry about wind drag, or 34 mpg for that matter.
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Old 06-02-12, 09:17 PM   #9
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I've noticed about a 6mpg difference with my Corrolla.
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Old 06-02-12, 09:54 PM   #10
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That's why my husband and I just got folders! When we travel in the summer we like to take our bikes so we can keep up with a reasonable level of cycling fitness. But I can feel the difference even with just the rack on the car! So when we go to Texas next week, the bikes will be in the back seat :-)
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Old 06-03-12, 01:09 AM   #11
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Bike and car like this and who worries about MPG

[ATTACH=CONFIG]253945[/ATTACH

I try to keep the interior of my car clean and without tears so I mount on the back . Mine is flat backed and not so much Aerodynamics that would be affected on this type of car. However on one trip I noticed a lot of wind noise and took the seatpost off. Quietened the car. But 34 mpg. To me- that is bad. Cars have to be economical over here due to the cost of fuel. Cars have to be in the region of 45mpg+ to make them economic to run--Unless it is a company car with them paying the expenses or you do low milage.
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Old 06-03-12, 04:55 AM   #12
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This is bike forums, why were you driving a car in the first place? I would be ashamed to even admit I owned one.
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Old 06-03-12, 05:41 AM   #13
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I have to admit, the price of gas is 50% of why I started bicycling again. If I need to transport the bike any sort of distance, I'll likely just toodle down to the local Transit station, and stick the bike on the front rack of the bus...
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Old 06-03-12, 05:42 AM   #14
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This is bike forums, why were you driving a car in the first place? I would be ashamed to even admit I owned one.
OK, I own a car. It's a necessary evil. Not exactly the wisest financial decision a person makes. It gets me from point A to point B, in no time. However, I did ride my bike yesterday, to drop off this month's rent check. Besides, I need the car to get to bike trails. It's still a necessary evil.

I have the Nissan Altima, too. It was an economic decision. Too many dang redlights that just kills the gas mileage but on open roads, the mileage is fantastic. It just about doubles my range. So, I'm not gonna worry about a drop in my fuel economy. But yeah, I did notice I came home from VA on Monday and I had used a little more gas than I normally do. I didn't think about the bike on the back. Just figured it's because I drove from Chesapeake to Williamsburg and back with beach traffic. Come to think of it, there was alot of returning home beach traffic on Monday just driving back from Chesapeake to NC.

My new bike doesn't fit on the back of car like last week's bike did. I"m not quite sure if it's the bike, the bar or the trunk rack. This bike is longer. Not sure what I need to do about besides, making padded pedal cozies to prevent more scratching. I'm not sure if the trunk rack is the best for the Altima or if I need to have a hitch installed and buy that type of rack. I'm not even sure a hitch can be put on the Altima. That rear bumper is deep and it's loaded with foam which is a safety design for rear end collisions.
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Old 06-03-12, 06:00 AM   #15
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Tailwind?? Or did a possible first run with a headwind settle down for the 2nd trip?

Also, factor in weight. Fuel economy is directly related to this.
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Old 06-03-12, 07:46 AM   #16
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We go from Austin to West Texas to hang out in the Fort Davis/Marfa area to vacation and ride. We take my 2008 Honda Civic hybrid. Driving with fuel mileage in mind I can always maintain 40+ mpg per gallon commuting and etc. I don't fool myself, I calculate fuel consumed into miles traveled. Since I often commute by bike my fuel bill isn't squat. The drive out to West Texas is really boring and about 6 hours long. Speed limit is 80 much of the way. Two bikes on a hitch rack, and two on the trunk rack almost get the mileage down to 20. Wind resistance is proportional to speed squared, and it shows. Last trip out there, my wife was driving and told me the "low fuel" light had come on and, thinking about normal mileage, I told her we were fine to get to Fort Stockton. Then I remembered we were down to 20 mpg and did some calculations. We would likely not have made it so we made an emergency detour for fuel.

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Old 06-03-12, 07:50 AM   #17
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And then there is always this solution---

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Old 06-03-12, 07:52 AM   #18
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I'm not even sure a hitch can be put on the Altima. That rear bumper is deep and it's loaded with foam which is a safety design for rear end collisions.
There are hitches that will fit the Altima with no problems. Find a trailer hitch installation dealer or a UHaul where you live. The rear end collisions are a potential bike killer but a lot of people use them. Your call, a roof rack like I am getting for our Accord will dip into the mileage and pulling into a low overhang or garage door can ruin your whole day.

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Old 06-03-12, 08:07 AM   #19
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Now that the wife is retired, we just purchased a new Ford Transit XLT for the purpose of transporting our bikes for our rides across Florida and the U.S.. The gas mileage isn't the best but it's better than what I was getting on my pickup. Plus, the bikes are more secure and there is lots of room for everything we need to take with us. My wife rides a recumbent tadpole trike so a rear or roof rack is out of the question.
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Old 06-03-12, 11:32 AM   #20
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There are hitches that will fit the Altima with no problems. Find a trailer hitch installation dealer or a UHaul where you live. The rear end collisions are a potential bike killer but a lot of people use them. Your call, a roof rack like I am getting for our Accord will dip into the mileage and pulling into a low overhang or garage door can ruin your whole day.

Bill
I won't think it would matter if you were using a hitched carrier or a trunk rack. In fact, any type of rear-end collison is gonna cause major damage to something. I really got nervous when a state trooper call tailgated me when I switched from US-64 to I-95 yesterday. He was right on my bumper in the acceleration lane of I-95 and I don't know why. At first it looked like he took the I-95 ramp and was gonna swing back westbound on US-64 then suddenly, he switched over to the I-95 lane and tailgated me.
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Old 06-03-12, 11:36 AM   #21
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..Or, put the bike racks inside a streamlined shaped trailer,
then it is just the weight towed rather than aerodynamic turbulence
and frontal area drag.

they will stay cleaner.. too..
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Old 06-03-12, 11:44 AM   #22
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Now that the wife is retired, we just purchased a new Ford Transit XLT for the purpose of transporting our bikes for our rides across Florida and the U.S.. The gas mileage isn't the best but it's better than what I was getting on my pickup. ....
My 270 hp SUV gets better gas mileage than a Ford Transit, about 23 mpg at 75 mph. It doesn't quite have the same interior space, but no problems fitting two road bikes in the back with lot of other stuff. If I need more room, I put the bikes on a hitch rack. I haven't quantified the loss in mpg due to that but it's probably pretty small as the bikes are hidden behind the body of the SUV.
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Old 06-03-12, 11:49 AM   #23
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First, congratulations on knowing how to use the cruise. I do a section of I5 where the traffic flow is right about 80, and I set to 78 in the fast line so cars can go around, and I don't have to kick it out of cruise for slower cars in front of me. Inevitably some impatient jerk in a Beemer jams past gets in front, then proceeds to demonstrate that he hasn't figured out how to use the cruise that I know his mid-life crisis came equipped with.

That said, I am a "ride out the front door" kind of guy, but on longer trips I'm glad that the bike fits inside the back of my Pontiac Vibe (Toyota Matrix) fully assembled.

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Old 06-03-12, 12:42 PM   #24
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But 34 mpg. To me- that is bad. Cars have to be economical over here due to the cost of fuel. Cars have to be in the region of 45mpg+ to make them economic to run--Unless it is a company car with them paying the expenses or you do low milage.
So I guess that my Nissan Titan full sized pickup truck wouldn't be a big hit over there with 14/18 mpg?
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Old 06-03-12, 01:23 PM   #25
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I won't think it would matter if you were using a hitched carrier or a trunk rack. In fact, any type of rear-end collison is gonna cause major damage to something. I really got nervous when a state trooper call tailgated me when I switched from US-64 to I-95 yesterday. He was right on my bumper in the acceleration lane of I-95 and I don't know why. At first it looked like he took the I-95 ramp and was gonna swing back westbound on US-64 then suddenly, he switched over to the I-95 lane and tailgated me.
Hopefully he wasn't trying to goad you into speeding so he could write you a ticket. I'd be tempted to report him.
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