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Make bikes on roof rack more aero?

Old 06-23-19, 04:03 AM
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Make bikes on roof rack more aero?

Hi folks,

I hope this is the right sub-forum to post in.

I'm about to drive 800 miles with three bikes on the roof of my Mazda 5. I've noticed quite a large drop in fuel consumption when the bikes are up there and was wondering how to reduce the air resistance. It's a Thule 591 mount for all three, so both wheels need to be on.

My thoughts:

1) Turn handle bars 90 degrees (all mountain bikes)
2) Remove pedals - zip tie a crank to chain stay to stop the bikes free-wheeling
3) Remove saddle

My questions then are 1) Are any of these bad ideas in themselves. 2) Will this make much difference?

It'll be interesting to see just how bad the MPG gets though when I'm at 70mph on the motorway....

John
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Old 06-23-19, 04:43 AM
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If you want to make a real mileage difference, install a hitch mounted rear rack and clean up your roof
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Old 06-23-19, 06:43 AM
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Originally Posted by Archwhorides View Post
If you want to make a real mileage difference, install a hitch mounted rear rack and clean up your roof
Of course. However Iíve spent the money now!
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Old 06-23-19, 08:56 AM
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Originally Posted by Recess View Post
Hi folks,

I hope this is the right sub-forum to post in.

I'm about to drive 800 miles with three bikes on the roof of my Mazda 5. I've noticed quite a large drop in fuel consumption when the bikes are up there and was wondering how to reduce the air resistance. It's a Thule 591 mount for all three, so both wheels need to be on.

My thoughts:

1) Turn handle bars 90 degrees (all mountain bikes)
2) Remove pedals - zip tie a crank to chain stay to stop the bikes free-wheeling
3) Remove saddle

My questions then are 1) Are any of these bad ideas in themselves. 2) Will this make much difference?

It'll be interesting to see just how bad the MPG gets though when I'm at 70mph on the motorway....

John
I think those three things will have a miniscule effect and not be worth the effort. The three frames and wheelsets alone are not aero at all at car speeds. The most aerodynamic thing I've ever hauled on the roof of one of my vehicles was a my old canoe which was made to be efficient in another fluid (water).
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Old 06-23-19, 08:59 AM
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Lay them down sideways (less frontal area) .. Slow down , ( less air resistance , proportional to speed ) ..
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Old 06-23-19, 09:00 AM
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Originally Posted by Recess View Post
Of course. However Iíve spent the money now!
If you can't modify the rack, reduce your road speed... air resistance (drag) is proportional to the square of velocity.
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Old 06-23-19, 09:00 AM
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Even the roof-rack itself gives you a mileage hit.

On my Rav4 hybrid, having a single mountain bike on the top of the car dropped the mileage from 32mpg to about 21 mpg, on a drive from the West Coast to Moab. I put the bike inside the car for the trip back, so I have a very good control.

Interestingly, on my old 2004 Sienna, it would get about 20 Ī 2 mpg on the highway, whether or not we had bikes on top of the car.

Last edited by wgscott; 06-23-19 at 10:57 AM. Reason: spelling
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Old 06-23-19, 09:01 AM
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Originally Posted by sweeks View Post
If you can't modify the rack, reduce your road speed... air resistance (drag) is proportional to the square of velocity.
This also is true. When driving around (slowly) in and around Moab, the mpg went up to about 34, even with a bike on the top. But it isn't terribly practical on an interstate highway.
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Old 06-23-19, 09:15 AM
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Originally Posted by wgscott View Post
Even the roof-rack itself gives you a milage hit.

On my Rav4 hybrid, having a single mountain bike on the top of the car dropped the milage from 32mpg to about 21 mpg, on a drive from the West Coast to Moab. I put the bike inside the car for the trip back, so I have a very good control.

Interestingly, on my old 2004 Sienna, it would get about 20 Ī 2 mpg on the highway, whether or not we had bikes on top of the car.
^^^ This right here. We did ~1000 miles round trip to Mammoth and back a few years ago with two bikes on the roof of my car. I lose ~2mpg from the unloaded rack, and the very same ~2mpg from the loaded rack.

Even with the astronomical price of California gas, this worked out to about an extra $13 to take the bikes. 1.3Ę cents a mile.
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Old 06-23-19, 10:14 AM
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Power spent on drag and therefore fuel consumption scales with speed cubed
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Old 06-23-19, 10:38 AM
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Originally Posted by DrIsotope View Post
this worked out to about an extra $13 to take the bikes. 1.3Ę cents a mile.
This too. Not expensive enough extra to worry about
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Old 06-23-19, 03:56 PM
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Wow! Thanks for all the input here. Iíll just leave them as they are then, and suck up the cost. Slowing down is always a good idea...

Thanks

John
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Old 06-23-19, 08:55 PM
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It seems obvious but maybe you should get an aero bike...
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Old 06-24-19, 06:50 AM
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Originally Posted by Recess View Post
Wow! Thanks for all the input here. Iíll just leave them as they are then, and suck up the cost. Slowing down is always a good idea...

Thanks

John
Also look for opportunities to draft tall trucks.
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Old 06-24-19, 08:30 AM
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Originally Posted by wgscott View Post
Even the roof-rack itself gives you a mileage hit.

On my Rav4 hybrid, having a single mountain bike on the top of the car dropped the mileage from 32mpg to about 21 mpg, on a drive from the West Coast to Moab. I put the bike inside the car for the trip back, so I have a very good control.

Interestingly, on my old 2004 Sienna, it would get about 20 Ī 2 mpg on the highway, whether or not we had bikes on top of the car.
When I had a VW TDI, the rooftop Thule box was a 5 mpg hit on mileage (40mpg down to 35 mpg), agreed that bikes on the roof would be worse.

In terms of cost, let's say for the sake of discussion that the bare Mazda gets 30 mpg over 800 miles and burns about 27 gallons of fuel at $3/g = $80. A 10 mpg hit with bikes on top would result in 40 gallons being consumed for $120, a premium of $40 for the extra drag.

If modifying the bikes reduced the hit to 8 mpg, the expenditure would be 36 gallons and $109 (a savings of $11 compared to worst case). So perhaps the small mods could save a few bucks, but is it worth the time?

Also, let's say the rear hitch rack setup cost $400 and resulted in zero extra drag (somewhat unlikely) after cleaning up the roof. With $3/gallon fuel cost, it would still take 10 trips @ 800 miles/trip with the bikes on board for the fuel savings to pay off the more efficient setup.
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Old 06-24-19, 11:37 PM
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Originally Posted by Archwhorides View Post
When I had a VW TDI, the rooftop Thule box was a 5 mpg hit on mileage (40mpg down to 35 mpg), agreed that bikes on the roof would be worse.

In terms of cost, let's say for the sake of discussion that the bare Mazda gets 30 mpg over 800 miles and burns about 27 gallons of fuel at $3/g = $80. A 10 mpg hit with bikes on top would result in 40 gallons being consumed for $120, a premium of $40 for the extra drag.

If modifying the bikes reduced the hit to 8 mpg, the expenditure would be 36 gallons and $109 (a savings of $11 compared to worst case). So perhaps the small mods could save a few bucks, but is it worth the time?

Also, let's say the rear hitch rack setup cost $400 and resulted in zero extra drag (somewhat unlikely) after cleaning up the roof. With $3/gallon fuel cost, it would still take 10 trips @ 800 miles/trip with the bikes on board for the fuel savings to pay off the more efficient setup.
Great maths example! Here in Scotland, itís about £5.80 a gallon. Just over $7 a gallon. Iím moving!!
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Old 06-25-19, 12:04 AM
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Originally Posted by Recess View Post
Great maths example! Here in Scotland, itís about £5.80 a gallon. Just over $7 a gallon. Iím moving!!
Think twice, the gallons are smaller here
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Old 06-25-19, 07:43 AM
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Originally Posted by Recess View Post
Great maths example! Here in Scotland, itís about £5.80 a gallon. Just over $7 a gallon. Iím moving!!
With petrol over twice the price in Scotland, the hitch rack installation is paid for in theoretically half the trips..... ;-)
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Old 06-25-19, 09:24 AM
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Originally Posted by Mista Sparkle View Post
It seems obvious but maybe you should get an aero bike...
I was gonna say the same thing

Originally Posted by DrIsotope View Post
Even with the astronomical price of California gas...
You know he is in Scotland, right? We Americans have be spoiled by cheap gas/petrol for years now...
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Old 06-25-19, 10:49 AM
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Originally Posted by Darth Lefty View Post
Think twice, the gallons are smaller here
Forgot about that... dividing by 1.2 means itís still $5.80 per gallon...
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Old 06-25-19, 03:28 PM
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My 1.3Ę per mile was based on unleaded going for about $4.50 a gallon at the time. So if the OP suffers a similar loss in fuel economy, he'd be looking at like what, 1.7Ę per mile? My heavens!

But then again, my car gets about 28mpg on the highway, which isn't stellar.
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Old 06-25-19, 03:49 PM
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I wonder if you were to wrap the front of each bike with a tarp to create fairings of sorts. They'd have to be on there tight enough to not flap.
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Old 06-25-19, 04:13 PM
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Originally Posted by DrIsotope View Post
Even with the astronomical price of California gas, this worked out to about an extra $13 to take the bikes. 1.3Ę cents a mile.
Misplace one pedal trying to squeeze out better aerodynamics and the whole business model blows up.
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Old 06-25-19, 06:22 PM
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The cost was essentially negligible. Dinner the first night cost more than I spent on gas during the whole trip.

The worst part of bikes on the roof is the bugs. I was scrubbing dead bugs off of the hoods and bar tape with a kitchen sponge.
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Old 06-26-19, 09:17 AM
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I would wrap the bars/stem/head tube in saran wrap.
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