Make bikes on roof rack more aero?
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
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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Recess (aka John)
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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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06-23-19, 08:56 AM
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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 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
06-23-19, 08:59 AM
#5
Banned
Lay them down sideways (less frontal area) .. Slow down , ( less air resistance , proportional to speed ) ..
06-23-19, 09:00 AM
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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 ± 2 mpg on the highway, whether or not we had bikes on top of the car.
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 ± 2 mpg on the highway, whether or not we had bikes on top of the car.
Last edited by Cyclist0108; 06-23-19 at 10:57 AM. Reason: spelling
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06-23-19, 09:01 AM
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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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06-23-19, 09:15 AM
#9
Non omnino gravis
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 ± 2 mpg on the highway, whether or not we had bikes on top of the car.
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 ± 2 mpg on the highway, whether or not we had bikes on top of the car.
Even with the astronomical price of California gas, this worked out to about an extra $13 to take the bikes. 1.3¢ cents a mile.
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06-23-19, 10:14 AM
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Power spent on drag and therefore fuel consumption scales with speed cubed
06-23-19, 10:38 AM
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06-23-19, 03:56 PM
#12
It'll be fine...
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Wow! Thanks for all the input here. I’ll just leave them as they are then, and suck up the cost. Slowing down is always a good idea...
Thanks
John
Thanks
John
06-23-19, 08:55 PM
#13
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It seems obvious but maybe you should get an aero bike...
06-24-19, 06:50 AM
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06-24-19, 08:30 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 ± 2 mpg on the highway, whether or not we had bikes on top of the car.
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 ± 2 mpg on the highway, whether or not we had bikes on top of the car.
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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06-24-19, 11:37 PM
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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.
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.
06-25-19, 12:04 AM
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06-25-19, 07:43 AM
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With petrol over twice the price in Scotland, the hitch rack installation is paid for in theoretically half the trips..... ;-)
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06-25-19, 09:24 AM
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I was gonna say the same thing
You know he is in Scotland, right? We Americans have be spoiled by cheap gas/petrol for years now...
You know he is in Scotland, right? We Americans have be spoiled by cheap gas/petrol for years now...
06-25-19, 03:28 PM
#21
Non omnino gravis
My 1.3¢ 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, 1.7¢ per mile? My heavens!
But then again, my car gets about 28mpg on the highway, which isn't stellar.
But then again, my car gets about 28mpg on the highway, which isn't stellar.
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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.
06-25-19, 06:22 PM
#24
Non omnino gravis
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.
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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06-26-19, 09:17 AM
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I would wrap the bars/stem/head tube in saran wrap.
