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my current bike is a hardtail GF Paragon. it's the only 29er that i have ridden on trails. i am approx 225 pounds and i've been setting the air pressure to about 31psi front / 33psi rear. i notice that the Bontrager 29-3 2.25 tires shape does not change near as much as the tires on my last FS 26" bike. On the 26" bike, the WTB Velocoraptor tires contact patch would "flatten" noticably when i mounted up and rode. and the bike's handling on pavement was squirrely when running lower pressures. the 29er doesn't have either of those traits. i cannot notice any flattening, and it moves along and handles just fine on pavement with the 31/33psi. i have only been on a few rides with the new bike. but the tires have not ever hinted at bottoming out to the rim or washing out. so i realize i can drop the psi down more. but i am wondering why there is the difference between the 26" and 29" tires at approx same psi? is it due to size of tires, tire compound, or what? or maybe this is all in my head:confused:? all of these are tubed tires. thanks in advance for any insights.
 

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Two things: The larger tire diameter makes for a longer contact patch at a given width and pressure - the tire bows out less to the sides at a given load, as the contact patch has more length to distribute the weight. So - for example, you have half your 225 plus (for simplicity) 25 lb bike. 250 lbs, with 45% of your weight on the front tire, makes a load of 112.5 lbs on the tire. At 31 psi, that means you'll have a contact patch of 112.5 lbs/31 psi or roughly 3.6 sq inches. The tire will compress down and deform to make a patch that is that many square inches. Since the contact patch is longer with the larger diameter tire, it tends to bulge out to the side less than a 26" tire with the same pressure will (assuming same casing size) to achieve this contact patch size.... This is a simplistic explanation, making the assumption that the surface is flat, etc... Also it's late and I'm tired! But the same-section tire in a larger diameter and pressure will need to "squash down" less to put that amount of tread on the ground because as it does compress, the area is longer and has to move out less to the side.

You will find as you play with it that you can run noticeably lower pressure on your 29" tires. The longer contact patch that they have helps in climbing and braking traction and also lets the tires' side knobs grip better in turns when leaned. I'd start off by lowering your pressure by about 2 psi per tire and trying the bike on a trail or course you are familiar with. Perhaps something with some rock ledges and/or roots to run over. As soon as you start to feel any tire wallowing or a tendency to bottom on obstacles - put a couple of psi back in and you will likely be right about at your best pressure. At 225 with a 2.25" tire I would bet you'll find somewhere in the mid-20's will be where you set your front, with 2 -3 psi higher rear. (Terrain and riding style can make this pressure vary). Your traction will improve and comfort will be better too. Have fun!
 

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my current bike is a hardtail GF Paragon. it's the only 29er that i have ridden on trails. i am approx 225 pounds and i've been setting the air pressure to about 31psi front / 33psi rear. i notice that the Bontrager 29-3 2.25 tires shape does not change near as much as the tires on my last FS 26" bike. On the 26" bike, the WTB Velocoraptor tires contact patch would "flatten" noticably when i mounted up and rode. and the bike's handling on pavement was squirrely when running lower pressures. the 29er doesn't have either of those traits. i cannot notice any flattening, and it moves along and handles just fine on pavement with the 31/33psi. i have only been on a few rides with the new bike. but the tires have not ever hinted at bottoming out to the rim or washing out. so i realize i can drop the psi down more. but i am wondering why there is the difference between the 26" and 29" tires at approx same psi? is it due to size of tires, tire compound, or what? or maybe this is all in my head:confused:? all of these are tubed tires. thanks in advance for any insights.
I use about the same pressures in 26 and 29 versions of the same tire
 

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What ATBScott said, and...I've found the same to be true with tires of the same diameter and claimed width but different volumes. As an example my Kenda Nevegal 2,1s bulge less at a given pressure than my Fire XC Pros 2.1s. It's not a huge amount, but definitely noticeable. Of course the difference in volume between the two tires is quite noticeable as well. Probably not the greatest example though. The Pannies set up tubeless at 35psi clock in at 2.0 at their widest point. The Nevegals tubeless at the same pressure measure out at a true 2.1 and are noticeably taller from tread to bead hook as well. So yeah, tire diameter, volume, width, etc. all have effect on how much a tire deforms under load.

Good Dirt
 
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