What are the physics advantages of putting spare in trunk vs on rear mount?

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JohnJ78

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I remember hearing Elon Musk saying how porsches made a huge mistake putting engines in the back, because by putting the weight behind the rear axle, it messes up the handling big time. Anyone moved their spare to inside the jeep and noticed a difference?

Thinking of doing this.

This video is about trailer weight load distribution, but kind of demonstrates the effect.

 
Leave your spare at home get at flat and cry! lol TJ has the aerodynamics of a house brick, stick another 2 spares on the roll bars and you wont notice any difference. lol
 
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Vtx531 said:
Maybe he was high when he criticized Porche handling?

Is that really what he said?
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In theory he is correct. There is more to the story though. Sanctioning bodies have different rules for different cars. The use of intake restrictors and weight breaks sometimes determine the winners.

Back to the matter at hand. Mine is usually on the back. I am going to start putting it in the tub. Mine is a daily and my spare is lower to help rear visibility. There are times when the spare hits the ground though.
 
During a steep climb, the added weight behind the rear axle will teeter totter the front wheels off the ground. I noticed a decrease in this after I got rid of the heavy swing out bumper and swing out carrier. Moving the tire to the tub or not having the weight of the spare at all will further improve traction and stability.
 
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it's not going to matter with the little pizza cutters on the OEM mount you are running
 
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Brianj5600 said:
Musk is a brilliant man, but he missed on the Porsche.
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Musk wasn't wrong. Without all kinds of chicanery, the 911 would have been disco'd in the '70s, which is why the 944/928 were developed-both are front mid-engine cars. Porsche engineers understand the physics, and I guarantee that Musk knows a thing or two about physics-BA in physics from U of Penn.
 
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JohnJ78 said:
I remember hearing Elon Musk saying how porsches made a huge mistake putting engines in the back, because by putting the weight behind the rear axle, it messes up the handling big time. Anyone moved their spare to inside the jeep and noticed a difference?

Thinking of doing this.

This video is about trailer weight load distribution, but kind of demonstrates the effect.

Click to expand...
Without getting into what Musk knows and what he pays people that know, the physics are simple and basic. If we assume a tire weighs 100 lbs., then the effective weight is multiplied by the distance from the center of the rear spring perches to the tire. That is far enough to have the tire act upon the chassis with a force around double the weight of the tire. Also for every pound that is added behind the axle, a portion of that acts to lessen the weight on the front axle.

If you move the tire into the cargo area laying flat, then only the weight of the tire past the rear spring perches contributes to how the chassis reacts negatively and any of the weight forward of the rear axle acts to increase weight on the front axle.

The effects are highly diminished in the cargo area.

Also why we giggle and point at them that put the tire, big ass swingset tire carrier, cooler rack, jack, and fuel jugs behind the tailgate. Nothing says I do not understand how weight works against me like seeing all that crap.
 
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Now I sort of want to move my tire into the rear cargo area when I remove my shittybilt carrier. And then install the Moryde gate. And then I wonder why I bought the MOryde and could do a CJ style drop down gate......
 
all based on moment calculations, Force x Distance. I was never a fan of the tire on the tailgate but this reason makes sense too. That and watching someone go up for a steep climb, not make it, then get stuck because of their spare/bumper getting hung up.
 
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@mrblaine, can you please shed some light on why there are so many ultra 4 rigs with their tire mounted way out back? I assume since they are shells of Jeeps they are trying to balance the weight? But I hate to assume. Thanks
 
Superjay5 said:
@mrblaine, can you please shed some light on why there are so many ultra 4 rigs with their tire mounted way out back? I assume since they are shells of Jeeps they are trying to balance the weight? But I hate to assume. Thanks
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They put the weight there to give the shocks something to work against.
 
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SecondChanceTJ said:
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A great vid if you are comparing a 911 to, say, a Honda Civic or a VW Golf. All of the sports cars in the same market category as the lower-end 911s are rear mid-engine, which he studiously ignores in this vid, and which would be the more relevant comparison.

Still, it was a good show of basic handling and weight movement. I would love to see him try this snow business in a pre-nannified 911. It would be like trying to drive a twirling baton.
 
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