Is spare donut tire needed as part of crumple zone? - Subaru Outback - Subaru Outback Forums

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Old 02-11-2013, 12:10 AM   #1 (permalink)
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Default Is spare donut tire needed as part of crumple zone?

I have full size spare tire in my OB trunk and I am going to Arizona from Canada next week towing a small camper in the winter.
So, I was thinking about leaving behind the donut spare tire.
However, it occurred to me that this donut tire might play some additional role in the overall safety of the car by design. I read about the crumple zones built in into the modern cars and I wonder whether the spare donut tire (with considerable amount of rubber on the steel rim) play any role in the overall safety of the car structure by its ability to absorb extra energy on impact?
Should I leave behind the donut spare tire?
Thanks.
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Old 02-11-2013, 01:27 AM   #2 (permalink)
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You would have to think not, as I wouldn't think they would want the car to become less safe after you got a flat. Right?
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Old 02-11-2013, 08:20 AM   #3 (permalink)
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As far as I know "no". More and more car manufacturers offer brand new cars without a spare tire - or, only as an option. They are all trying to cut down on weight and improve the mileage.
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Old 02-11-2013, 09:00 AM   #4 (permalink)
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Originally Posted by jogosub View Post
As far as I know "no". More and more car manufacturers offer brand new cars without a spare tire - or, only as an option. They are all trying to cut down on weight and improve the mileage.
Yes but those cars are designed and crash tested without a spare tire in the trunk. In contrast, the Outback and any other vehicle with a spare tire would have been designed and tested with the spare on-board. I never really thought about this but jackw49 may be on to something. These tires are stored at bumper level and would likely be crushed during a rear end impact. That crushing requires energy which is good because that means it won't be available as kinetic energy to cause harm to the occupants. The more I think about this the more I have to think that not including the spare tire would change crash results.
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Old 02-11-2013, 09:18 AM   #5 (permalink)
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Originally Posted by vtmecheng View Post
Yes but those cars are designed and crash tested without a spare tire in the trunk. In contrast, the Outback and any other vehicle with a spare tire would have been designed and tested with the spare on-board. I never really thought about this but jackw49 may be on to something. These tires are stored at bumper level and would likely be crushed during a rear end impact. That crushing requires energy which is good because that means it won't be available as kinetic energy to cause harm to the occupants. The more I think about this the more I have to think that not including the spare tire would change crash results.
How do you know how OB was crash tested? I kind of agree with you but being a trend to sell cars without spare tires nowadays, I would think Subaru crash tested them with and without spare tires. Also, if he is going to tow a small camper that would be an extra crash safety zone.
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Old 02-11-2013, 09:29 AM   #6 (permalink)
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How do you know how OB was crash tested? I kind of agree with you but being a trend to sell cars without spare tires nowadays, I would think Subaru crash tested them with and without spare tires. Also, if he is going to tow a small camper that would be an extra crash safety zone.
A coworker of mine worked for Chrysler conducting crash tests on cars until a couple years ago (he was laid off). They do the absolute minimum required by law and the car must only be in the configuration sold. If Subaru sells it with a spare tire under the truck that's how it is tested.
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Old 02-11-2013, 09:32 AM   #7 (permalink)
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Originally Posted by vtmecheng View Post
These tires are stored at bumper level and would likely be crushed during a rear end impact. That crushing requires energy which is good because that means it won't be available as kinetic energy to cause harm to the occupants. The more I think about this the more I have to think that not including the spare tire would change crash results.
While it may change the results, I would hope it is a negligible if the engineers did their jobs right. Also remember that the tire is flexible, but the rim is not and after a certain amount of crumple it could be a rigid body in part of a crumple zone which is the opposite of what is desired there. I have not studied the crumple zones of my vehicle at any length apart from the knowledge that our engines are supposed to partially drop out of the way in the event of a front-end collision. Given that, I would think they would try to design the crumple zone so the spare tends to move a little up/down and out of the way as well rather than having full lateral travel as part of the crumple zone so that it is more-so removed as a variable.
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Old 02-11-2013, 09:37 AM   #8 (permalink)
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I'd think it would be safer without the spare, especially if it's interior mounted. One less thing to be pushed/propelled forward into the passenger space. It is certainly not designed into the "crumple zone". If it were, it would be mounted a lot more securely than a bolt with a plastic wingnut to hold it down.

Under mounted spares are probably less of a concern, although their proximity to the fuel tank could mean that you're safer without a spare under the vehicle, too.
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Old 02-11-2013, 09:45 AM   #9 (permalink)
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Default Is spare donut tire needed as part of crumple zone?

Just fill it with Portland cement and get a AAA card.
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Old 02-11-2013, 10:01 AM   #10 (permalink)
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they have no idea if you're hauling dogs in the back or a 12 cylinder engine block either.

It isn't worth worrying about one way or the other. I suspect they test the cars in an 'as delivered' configuration for NHTSA but, so what? they have no idea how the car will be used and couldn't be expected to do extensive testing with any weird deviations from the 'as delivered' condition. With/without spare MAY be something they test - but, wouldn't most people put the flat in the wheel well? even if the cover didn't quite fit flat?

I wouldn't lose any sleep over it one way or the other.
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