Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Project: ECOfamily Civic and its Aero Hitch Box / Boat Tail

christofoo · 04-23-2014, 02:42 PM

First off, our Southern Utah trip had great mileage - 48, 43, and 45 MPG, for a trip average of 45.6 MPG over 1111 miles. Not bad! This trip always has lower speeds than my other long-hauls, and a lot less air conditioning. The big question is whether the winds were favorable. The Arches/Moab/Hanksville area can be very windy, but there's a real shortage of wind-socks, so I usually don't have much idea what the balance of wind direction was. It's tempting to assume it always hurts more than helps because it's not a conservative force (unlike gravity), and in fact wind is very nonlinear (V^2 for aerodynamics), and even side-winds are expected to hurt.

=====================

The Kammback was destroyed at the beginning of the 3rd tank.

I'm really glad it didn't hit another vehicle or do any real damage to my vehicle. (I'm also really glad there were no witnesses, for the sake of my ego, and also really glad this is not the scenario where all our belongings get strewn across the road by a failed cargo box.)

On I-70 W I had a really horrible side-wind, I'm estimating it was well north of 30MPH steady, plus gusts. I saw there was a problem and I made two stops to attempt to strap it down. However, I thought that the only hazard was coming from the Kamm-to-box window section lifting off, so that's where I duct-taped. Right after the second stop, as I was accelerating, the side-wind got under the Kamm right-hand side-window and there was enough leverage to break the right-hand magnet loose. Once that happened I saw the whole structure break loose in the mirror and blow away - just like that. The hinge and aluminum brace holding the left hand magnet twisted and took a lot of damage when it was pried off.

These are beefed up magnets, they were strong enough I could open the trunk all the way and over-center the Kammback plus Kamm-to-box-window without any trouble. However, in order to accommodate the roofline curvature there was a hinge between the two magnet sections, so although it was a lot stronger against slide-off, it wasn't really stronger against a side-wind.

I made a snap decision to salvage some components (the camera, magnets, and trunk linkage), and leave the rest on the road side. I've had some thoughts about going back to retrieve what's left, but I'm not sure.

Lesson learned: this type of structure needs enough downward force at the trunk to secure it against side-winds. I didn't imagine this failure mode until I saw it happen. If I could also aerodynamically fish-scale the leading edges (EDIT- at the c-pillar) against a side-wind that would be really effective. I think I prefer attaching to the roof-rack mount channel rather than magnets, but neither is good enough unless the side-window is secure against lifting up in a side-wind. Correction: I'm pretty sure it's the lateral forces that caused it to break free. Enough lateral force from the right will cause the right-hand magnet to slide down the roof, then the curved roofline and turbulence on the side break the magnet's grip. One approach to correcting the problem is lateral tie-down webs hooked onto the trunk.

================

I also have some really interesting grille ducting I want to report. I have a real shortage of posting time.

04-23-2014, 02:42 PM	#202 (permalink)
christofoo Master EcoModder Join Date: Mar 2012 Location: Salt Lake City Posts: 292 00C - '00 Toyota Corolla 90 day: 43.54 mpg (US) Thanks: 147 Thanked 190 Times in 73 Posts	Kammback disaster First off, our Southern Utah trip had great mileage - 48, 43, and 45 MPG, for a trip average of 45.6 MPG over 1111 miles. Not bad! This trip always has lower speeds than my other long-hauls, and a lot less air conditioning. The big question is whether the winds were favorable. The Arches/Moab/Hanksville area can be very windy, but there's a real shortage of wind-socks, so I usually don't have much idea what the balance of wind direction was. It's tempting to assume it always hurts more than helps because it's not a conservative force (unlike gravity), and in fact wind is very nonlinear (V^2 for aerodynamics), and even side-winds are expected to hurt. ===================== The Kammback was destroyed at the beginning of the 3rd tank. I'm really glad it didn't hit another vehicle or do any real damage to my vehicle. (I'm also really glad there were no witnesses, for the sake of my ego, and also really glad this is not the scenario where all our belongings get strewn across the road by a failed cargo box.) On I-70 W I had a really horrible side-wind, I'm estimating it was well north of 30MPH steady, plus gusts. I saw there was a problem and I made two stops to attempt to strap it down. However, I thought that the only hazard was coming from the Kamm-to-box window section lifting off, so that's where I duct-taped. Right after the second stop, as I was accelerating, the side-wind got under the Kamm right-hand side-window and there was enough leverage to break the right-hand magnet loose. Once that happened I saw the whole structure break loose in the mirror and blow away - just like that. The hinge and aluminum brace holding the left hand magnet twisted and took a lot of damage when it was pried off. These are beefed up magnets, they were strong enough I could open the trunk all the way and over-center the Kammback plus Kamm-to-box-window without any trouble. However, in order to accommodate the roofline curvature there was a hinge between the two magnet sections, so although it was a lot stronger against slide-off, it wasn't really stronger against a side-wind. I made a snap decision to salvage some components (the camera, magnets, and trunk linkage), and leave the rest on the road side. I've had some thoughts about going back to retrieve what's left, but I'm not sure. Lesson learned: this type of structure needs enough downward force at the trunk to secure it against side-winds. I didn't imagine this failure mode until I saw it happen. If I could also aerodynamically fish-scale the leading edges (EDIT- at the c-pillar) against a side-wind that would be really effective. I think I prefer attaching to the roof-rack mount channel rather than magnets, but neither is good enough unless the side-window is secure against lifting up in a side-wind. Correction: I'm pretty sure it's the lateral forces that caused it to break free. Enough lateral force from the right will cause the right-hand magnet to slide down the roof, then the curved roofline and turbulence on the side break the magnet's grip. One approach to correcting the problem is lateral tie-down webs hooked onto the trunk. ================ I also have some really interesting grille ducting I want to report. I have a real shortage of posting time. __________________ ECOrolla89 (modding sixth gen Corolla sedan on a budget) ECOfamily Civic and its Aero Hitch Box I owe some posts about coast-down. My sweet blog: http://cannonblast.net/christofoo Last edited by christofoo; 08-12-2014 at 01:00 PM..