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Discussion Starter #1
It seems I don't understand the PSI and vehicle weight relationship because the PSI (4) x Contact patch (20") x Tires (4) = 320 lbs is not enough to support my atv's loaded vehicle weight of about 700 pounds. Why am I wrong in thinking the PSI times total contact patch should equal the total vehicle weight?

Thanks.
 

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Discussion Starter #2 (Edited)
I found my mistake. I measured the contact patch instead of guessing at it and the measured contact patch is about 8"x7" which is almost three times my original guess. 8x7" x 4psi x 4 tires= 896 pounds which is close enough. So, I think the formula is correct.
 

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Your wizard like equation is mystifying to me. What theory are you using to associate contact patch to a tires ability to support weight?
I'm a firm believer in proper tire pressure. Tire pressure according to my learning is directly proportional to the weight it supports. However a tires sidewall build dictates the tire pressure it's able to support.(not to be confused with MAX PSI or bead seat pressure). A thicker sidewall will support more weight with less air pressure. For instance a run flat tire can support the full weight of a car with no pressure at all. Contact patch size is irrelevant in this instance.
Tire build and air pressure setting will dictate the size of the contact patch. To me the contact patch is the by product. How do you use the contact patch to determine weight a tire can support?
If we squished a kids ballon into an 8x7 patch of contact, would it support 800lbs?


I'm eagerly awaiting an explanation, a chart or supporting theory. I love learning
Thank you in Advance.


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Your wizard like equation is mystifying to me. What theory are you using to associate contact patch to a tires ability to support weight?
I'm a firm believer in proper tire pressure. Tire pressure according to my learning is directly proportional to the weight it supports. However a tires sidewall build dictates the tire pressure it's able to support.(not to be confused with MAXPSI or seat head pressure). A thicker sidewall will support more weight with less air pressure. For instance a run flat tire can support the full weight of a car with no pressure at all. Contact patch size is irrelevant in this instance.
Very true... I have a Polaris sportsman diesel running 8 ply mudlites, the machine weighs ~800lbs, and at one point, I rode an entire weekend with a flat tire and never even realized it, because the tire never so much as squatted.
 

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I am a 310lb rider. Despite Yamahs recommendation of 5psi in the front and 4.4 in the rear I use 7lbs front and rear. It's a nice comfy ride. It doesn't allow dirt to enter between the rim and bead area of the tire. It doesn't roll on high speed corners. Over 4300 miles at 7psi on my 2016 Grizzly.


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It seems I don't understand the PSI and vehicle weight relationship because the PSI (4) x Contact patch (20") x Tires (4) = 320 lbs is not enough to support my atv's loaded vehicle weight of about 700 pounds. Why am I wrong in thinking the PSI times total contact patch should equal the total vehicle weight?

Thanks.
all your calculating is what the ground pressure your machine is exerting, and PSI doesn't really work into it, if you take the weight of the machine plus the weight of the rider and gear and divide it by your contact area you end up with the ground pressure. less air means bigger contact area and lower pressure. I do the same math to figure out if a excavator is gonna mess up a road or break an underground utility. he weight "loading" has more to do with the tire rating and allow PSI than PSI alone
 

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I do kind of see what he’s getting at. If air is pushing out on a tire with 10 pounds per square inch. And there are 8 in.² touching the ground, then the tire could support 800 pounds before it got squished into the rim. This is if the tire rubber had no affect on the tires ability to stay round. Then that would seem like the tires ability to hold up X amount of weight. If there were too much weight the tire would squish in and require more air pressure to push it back to round. You could then also make the connection between how much weight is on the machine and how much air pressure you would need.


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:shocked:

It appears that you folks are confusing the maximum design load capacity to point load (contact patch) completely different animals. Also the strength of the rims are a factor. Rims in general also have a max design load and PSI.

tirerack/tires/tiretech/techpage
 

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<img src="http://www.grizzlycentral.com/forum/images/smilies/shocked.gif" border="0" alt="" title=":shocked:" class="inlineimg" />

It appears that you folks are confusing the maximum design load capacity to point load (contact patch) completely different animals. Also the strength of the rims are a factor. Rims in general also have a max design load and PSI.

tirerack/tires/tiretech/techpage
All I know is this thread has my head spinning! Seriously. What the heck is wrong with Yamaha's recommendations? Could someone clear that up for me?
 

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Hum....
When I add more air to the grizz tires, there is less tire on the ground at any time.
I better load less refreshment in the morning and buy more in the afternoon when passing through town.
What happens during a wheelie?
 

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all your calculating is what the ground pressure your machine is exerting, and PSI doesn't really work into it, if you take the weight of the machine plus the weight of the rider and gear and divide it by your contact area you end up with the ground pressure. less air means bigger contact area and lower pressure. I do the same math to figure out if a excavator is gonna mess up a road or break an underground utility. he weight "loading" has more to do with the tire rating and allow PSI than PSI alone
That is exactly the right answer.
 

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Hum....
When I add more air to the grizz tires, there is less tire on the ground at any time.
I better load less refreshment in the morning and buy more in the afternoon when passing through town.
What happens during a wheelie?
Ha, Ha!!!
 

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All I know is this thread has my head spinning! Seriously. What the heck is wrong with Yamaha's recommendations? Could someone clear that up for me?
There is not a thing wrong with the manufacturer recommendations. Some folks just have curious minds. Good tires, air pressure around the recommended, don't overload the machine, ride on!
 

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There is not a thing wrong with the manufacturer recommendations. Some folks just have curious minds. Good tires, air pressure around the recommended, don't overload the machine, ride on!
I just got so confused. I was about to inquire about what I should run. Actually I just messaged you.... Anyway, I thought it would be pretty simple, then I saw this thread and all you engineering, ground pressure, contact patch bla bla bla, guru's have my head spinning! Lol.

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Hey Grizzcrazy...I hear ya...
The really batshit crazy off the wall stuff can be found in the CVT and tire discussions.
Once a guy said he was suffering from constipation. He claims that he put 2mm shims in and has been sheetting like a goose ever since.
 

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Hey Grizzcrazy...I hear ya...
The really batshit crazy off the wall stuff can be found in the CVT and tire discussions.
Once a guy said he was suffering from constipation. He claims that he put 2mm shims in and has been sheetting like a goose ever since.
Well that is just strange.... The things on the internet....
 

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Discussion Starter #17
Hum....
When I add more air to the grizz tires, there is less tire on the ground at any time.
I better load less refreshment in the morning and buy more in the afternoon when passing through town.
What happens during a wheelie?
When you wheelie, the PSI in your rear tires increases and your contact patch increases. The contact patch of your front tires becomes zero and the PSI drops.
 

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When you wheelie, the PSI in your rear tires increases and your contact patch increases. The contact patch of your front tires becomes zero and the PSI drops.


Contact patch increases but PSI does not change. Jack up your quad and set the tire pressure. Lower it to the ground and check you PSI. It doesn't change. The shape of the tire changes.


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I agree with cornfedinohio. Otherwise, you would be changing PSI in tires based on load which isn't feasible. Load of the vehicle does not impact tire pressure.
I've come to accept that a tire with air on the inside performs better than a tire with no air on the inside, (flat:). Find the PSI that works for you and ride on. If you find that you have a tire without any air on the inside, plug it, air it back up, and then ride on, ha.
Be safe
 

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Kind of wished I had read this forum this morning abet interesting, started doubting myself but now at peace.... best practice for me will be to keep the recommended pressure according to the Mfg.


Side note: We have two ATV's with the Fire Department, some frigging engineer with us decided to prevent flats in the future to fill with a solid material ( similar to raising the PSI to 40 lbs), they ride like crap now, no grip, tippy, etc....I used to take my Griz on some of those calls knowing it was a lot safer getting to the emergency, many times only the Griz got to the scene. Similar to airing down my Wrangler on ugly trails in the back country.
 
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