Roof rack or trunk rack? (solution: none of the above... modded trunk!) - Fuel Economy, Hypermiling, EcoModding News and Forum

Jakins · 09-17-2012, 02:30 PM

Which is better? I drove with a trunk rack with my bike on it for about 400 miles and instantly notice like a 4 or 5 mpg decreas. And when I'm with my friends like the only way to fit our bikes in is to have them hanging out of the trunk with the trunk open. I also notice about the same loss. But what about roof racks? I'm thinking about mounting our bikes on the roof upside down with the wheels off. They would be facing in a straight line with the car as opposed to being sideways on the back. What do you guys think?

2000neon · 09-17-2012, 02:36 PM

The roofrack may be worse than the trunk, although I am not positive about that. Do you have a trailer hitch? If so, looking into a hitch mounted bike rack to get as much of the bikes out of the airflow as possible. Although the cost of a new bike rack may offset the fuel savings.

Daox · 09-17-2012, 02:53 PM

Roof rack will be way worse than trunk mount.

If you can adjust the trunk mount so that the bikes don't stick up as far you'll be a little better off.

MetroMPG · 09-17-2012, 03:57 PM

By "trunk mount", do you mean mounted on the rear of the car, behind it? (I'm pretty sure you don't mean on top of the trunk lid itself.)

Your goal should be: (1) inside the car, if possible with the trunk closed; (2) rear mounted behind the trunk (not on top), with as many wheels/seats removed and put in the trunk as possible (3) roof mounted.

The problem with roof racks is they increase both frontal area (A) and drag coefficient (Cd), where a rear rack probably only increases Cd (unless the bikes stick out the sides too). So roof racks are a double whammy.

As Daox says, for a rear mount, you want the bikes riding as low as possible behind the car.

Saskwatchian · 09-17-2012, 03:59 PM

If you are willing to take the wheels off for a roof rack how about taking the wheels off for the trunk rack? That should get the remaining parts of the bike mostly in the car's wake.

Jakins · 09-17-2012, 04:19 PM

Well here is the thing. When it is just me I put it inside the trunk or back seat no problem. But when I'm with my friends and we have 3 or 4 bikes in my like 2door it is really hard to fit all of that in the trunk (though it is possible) but we won't be able to fit other stuff in. We are going on a week long road trip and im Thor sure what the best idea is. A regular trunk rack like I have is kind of out of te question. They are very weak and with 3 bikes, it is one good sized bump away from scattering them all over the freeway in traffic. Really my only options are putting them on the roof upside down and like ratchet strap them on (staying a stream lined as possible) so that we can put our stuff in the trunk OR hanging them out of the trunk with the trunk open. Or of course I can completely disassemble them and fit them
In the trunk the best we can but we may not have enough trunk space for like camping gear and stuff. Here is a pic of how we normally do it.

Is that causing to much drag with the trunk open?

PaleMelanesian · 09-17-2012, 04:25 PM

It's causing drag, for sure, but it's MUCH better than the whole bike exposed on top of the roof.

Look at the car from the front. How much extra is sticking out? Not much the way you have it now. That's what the air sees, expressed as Frontal Area. How cleanly the air departs the back end is expressed as Coefficient of Drag, or Cd. Multiply those to for total drag. What you have now makes Cd worse, but doesn't change FA. Mounting them on the roof makes both a lot worse. When you multiply those, you get hit twice.

Saskwatchian · 09-17-2012, 04:31 PM

Your current setup actually looks pretty good, I don't think you will get much better with a car like that unless you want to take the trunk lid off (or build a custom one) for when you are using your car as a bike hauler.

I am so used to SUVs and trucks where 5mpg is a big deal, how much extra fuel are you burning with your 5mpg loss? is it actually significant for an occasional trip with friends?

MetroMPG · 09-17-2012, 04:33 PM

I'd say it explains the decrease you're seeing. I would still bet the roof mount approach would be worse though. You could pretty easily test which is better by doing a coast down test to compare them.

My thoughts are:

1) Make/modify/get a cargo rack for your camping gear to go behind the car instead of the bikes. Cram the bikes in the trunk and close it. The gear could be packed more densely and closer to the rear of the car, which is good.

2) Much harder, but more fun: fabricate some kind of aerodynamic shape that will fill/sit in the void between the rear of the roof arching down to the high part of the trunk lid (when it's in that partially open position, with bikes sticking out). Such a mod may even be favourable for approaching an ideal streamled shape depending on the height of that open trunk. Dealing with side taper from the C-pillar back to the open trunk lid will be trickier though.

ChazInMT · 09-17-2012, 04:35 PM

That's probably the best you can do there. Nice of you to include a few more facts like 3 friends and the car loaded to the max. Also a Honda Civic huh? So yer mileage drops from 35MPG to 30MPG.

I'd think your SUCC is doing great. (Sport Utility Civic Coupe)

As others indicate, Just Say No to Roof Racks.

If it's that bigga deal, buy a little cargo trailer and modify it to fit in the template area behind your car and put the bikes there. But the expense of the trailer and hitch would probably translate to a 400,000 mile break even financially.