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SUVs Volvo XC90 / XC60 Death of the body-on-frame SUV?
106 messages, Last post on Sep 12, 2009 at 1:50 PM
You are in the Volvo XC90 / XC60 Forum. Your Hosts are steve_ & tidester
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Replying to: bammbamm65 (May 22, 2007 9:52 pm) But here's what is not sometimes understood: the less stiff vehicle protects both its own occupants and the occupants of any vehicle it crashes into. In a head-on crash between a BOF and a unibody, the crumple zone in the unibody reduces the peak crash forces experienced by the occupants of both vehicles. This is the case because in a two-vehicle crash the vehicles exert equal but opposite forces on each other. This is one of Newton's laws relating force and motion. The forces are always equal in magnitude, but if one of the vehicles is more massive than the other, the effect of the forces on the vehicles will be different. Remember that F = m a, so a = F/m. That is, the crash acceleration of the lighter vehicle will be greater than that of the heavier vehicle because the F is the same for both vehicles, but m is different, and this is in the denominator. The injury to the occupants of a vehicle is strongly related to the peak crash acceleration that the vehicle experiences. Consider a head-on crash between two Suburbans, identical except that one is red and one blue. Each is travelling at 30 mph relative to the ground. High and identical peak crash forces are transmitted to the occupants of the two vehicles Now consider the same crash except that a serious but lightweight engineered crush zone 4 ft in depth has been attached to the front of the red Suburban. That is, the two Sururbans are still the same mass. The result will be lower peak crash accelerations to both vehicles. The crumple zone strapped to the red Suburban has protected equal benefit to the occupants of the blue one. If both vehicles had had the same 4-ft crumple zone, then the peak crash accelerations would be even less because there would be a total of 8-ft of crumple zone. If each vehicle had had the same design 2-ft crumple zone it would be the same as either one having a 4-ft crumple zone.
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Replying to: jim314 (Jun 13, 2007 6:05 am)
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Replying to: sagaliba (Jun 13, 2007 12:56 pm) But it also seems to me that a unibody would be less stiff than a BOF design outside the passenger compartment and so would protect from the other source of crash injury to vehicle occupants--the high peak g-forces of the passenger compartment due to the sudden change in velocity (the "delta-V") that occurs in a collision. I personally am attracted to the idea of a body-on-frame design because I drive conservatively and I "don't think" I'll be in a crash. I am sure that I must underestimate my risk. I value strength and durability in a vehicle. At my age, early 60s, I enjoy imagining that the car I'm driving (2004 base V70) will be the last car I'll buy. My ideal vehicle would be a BOF "minivanoid" with relatively high seating position (comfortable leg angle), 3000-lb curb weight, powered by a small gas or diesel engine (120 hp max) and a 6-spd manual. It would be rated to tow up to 2000 lb. It would have a variable height suspension to get best fuel economy on pavement, but be able to go on rough roads when necessary. But when I awaken from my dreaming I find that in the real world a unibody design will best protect me from crash injury at a price I can afford.
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Replying to: jim314 (Jun 14, 2007 7:51 am) Less stiff does not necessarily translate into better crumple zones. On BOF vehicles, crumple zones are strategically placed in front and rear portion of the frame (often appear as tiny waves), so that when enough force is applied (i.e., when vehicle is hit hard), the frame will bend at certain points, and in certain directions. In other words, the way it "crumples" is controlled, and not by relying on the random way the metal may fold.
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Replying to: sagaliba (Jun 14, 2007 12:05 pm) First, note how in these pictures BOF Example -- click on this..., the frame is low, and is still extending out, and has actually penetrated the barrier, representing another car. This is because the frame, with engine/suspension mounts is very stiff and tough, and will skewer the occupants of other, lower vehicles if the BOF hits on a door. Very dangerous. What Ford has done on the above late-model F-150 is to secure the cab to the very tough frame to keep it from collapsing, although the protruding frame doesn't make a great "crumple" zone at all. Now compare that to Unit body crash click here... where the front end is much flatter, with no protruding tough frame to stick out and through the barrier. Conclusions: The frames on pickup trucks and SUVs protrude and can enter a unibody structure. The frames are very stiff and don't collapse much when hitting a unibody vehicle. |
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MY wife and i were in a 6 car pile up about 2 years ago, our doge was in the front and only took the rear bumper out the chevy taho be hind us and ours were the only ones to drive away and the only ones not having to go to hospital, the other 4 were totaled, we heat with wood and haul it in the durango we camp, and haul frinds around in car pool would not change even if some one gave me something els for free!!
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Replying to: whiskey62 (Feb 21, 2009 12:54 pm) What will you do if we ever get to the point where no one is making them anymore and your Durango is on its last legs? tidester, host SUVs and Smart Shopper
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Replying to: whiskey62 (Feb 21, 2009 12:54 pm) |
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Replying to: tidester (Feb 22, 2009 12:14 am) |
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