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  1. #1
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    Lower vs Higher 29'' tire pressure issue...

    So...given the bigger tire volume/wheel perimeter + bigger contact patch of a 29er

    vs

    the lower equivalent of a 26er...

    ..when riding a 29er,trying to be in the same-or better-circumstances (traction,control,everything) as with a 26er,isn't it unreasonable that most big wheeled riders use lower pressures?

    Bigger contact patch/wheel perimeter means 'better',isn't it?

    Please excuse the simple minded way of putting it,maybe it is a rhetorical question of the moment,maybe ignorance,i really don't know what i'm missing and if,and i understand that now on a 29er we can have very low pressure as a possibility,but should we for some reason?

    ....when i was riding my 26er i had low pressure too,and it was traction etc that i had to achieve cause of the smaller perimeter/patch,and at that time there wasn't any 29er to compare.....

    so....Why lower and not higher????

    ...



    I appreciate your input,
    Kosmas.
    Last edited by Zaratustra; 06-19-2011 at 09:16 PM.

  2. #2
    Plays with tools
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    I like the stans rule of thumb, body weight + bike wight divided by 7. Anything lower than that is far past the point of diminishing returns.

    According to the rolling resistance article that was published by cyclingnews, 29'ers have a very comparable contact patch to a 26" tire. It's just longer than and narrower than the 26" counterpart.

  3. #3
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by Zaratustra View Post
    So...given the bigger tire volume/wheel perimeter + bigger contact patch of a 29er

    vs

    the lower equivalent of a 26er...

    ..when riding a 29er,trying to be in the same-or better-circumstances (traction,control,everything) as with a 26er,isn't it unreasonable that most big wheeled riders use lower pressures?

    Bigger contact patch/wheel perimeter means 'better',isn't it?

    Please excuse the simple minded way of putting it,maybe it is a rhetorical question of the moment,maybe ignorance,i really don't know what i'm missing and if,and i understand that now on a 29er we can have very low pressure as a possibility,but should we for some reason?

    ....when i was riding my 26er i had low pressure too,and it was traction etc that i had to achieve cause of the smaller perimeter/patch,and at that time there wasn't any 29er to compare.....

    so....Why lower and not higher????

    ...



    I appreciate your input,
    Kosmas.
    I generally use the same pressure for the same model tire, same width in 26" or 29".
    mtbtires.com
    The trouble with common sense is it is no longer common

  4. #4
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    Same here.

  5. #5
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    Lower tire pressures will result in a short and wide contact patch, resulting in less rolling resistance (to a point of course). Higher pressures will give you a long and narrow contact patch, which has higher resistance. There was a thesis produced by a gentleman in Germany I believe where he tested this theory on grass, pavement, gravel/single track... Not sure if it's still able to be found, but it has a lot of good info and is what convinced me to go lower tire pressures when I was on 26" wheels.

  6. #6
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    That is a decision better made by your butt than your brain. Soooooo many things are a factor, trail, soil, bike, tires, rider experience and technique, personal preference.

    Find a length of trail, two miles or so long, that has as much variety as you can find, and represents the kind of trail you like. Set tire pressure and ride it out and back. Drop pressure a couple psi and repeat. Drop pressure a couple psi and repeat. Repeat. Repeat.

    Now try it with a couple psi higher and repeat, repeat, repeat. The answer will come clear. It will change with different tires, so you may have to do it again and again. As you gain more experience on different tires on different trails, you will find the sweet spot that works for you, based on each tire.
    Last edited by slocaus; 06-20-2011 at 09:37 AM.
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  7. #7
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    Quote Originally Posted by PHRANQUY View Post
    Lower tire pressures will result in a short and wide contact patch, resulting in less rolling resistance (to a point of course). Higher pressures will give you a long and narrow contact patch, which has higher resistance.
    False. Lower pressure makes the contact patch longer and wider. Higer is the opposite. Take your tire at high pressure and it may have 1 square inch hypothetically, and air it down and now its longer front to back and wider side to side. Now your contact patch is like 1"x3". Try it for yourself and look at the difference.
    Now rolling resistance, regarding low vs. high tire pressure, will be different on different types of terrain. Higher press on smooth path with be faster than low on smooth. Switch it around for terrain. Lower pressure will have less resistance than high pressure bouncing you all over the objects rather than conforming and flowing over.

  8. #8
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    Quote Originally Posted by hal0tw0 View Post
    False. Lower pressure makes the contact patch longer and wider. Higer is the opposite. Take your tire at high pressure and it may have 1 square inch hypothetically, and air it down and now its longer front to back and wider side to side. Now your contact patch is like 1"x3". Try it for yourself and look at the difference.
    Now rolling resistance, regarding low vs. high tire pressure, will be different on different types of terrain. Higher press on smooth path with be faster than low on smooth. Switch it around for terrain. Lower pressure will have less resistance than high pressure bouncing you all over the objects rather than conforming and flowing over.
    Tire width plays a huge factor as well. If you want to PM me your email address I'd be happy to send you the paper.

  9. #9
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    send me that paper...THX

    Quote Originally Posted by PHRANQUY View Post
    Tire width plays a huge factor as well. If you want to PM me your email address I'd be happy to send you the paper.
    PM me or just put it in this thread....THX

  10. #10
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    It's a PDF, so I'm not able to attach it.

  11. #11
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    Quote Originally Posted by PHRANQUY View Post
    It's a PDF, so I'm not able to attach it.
    Yes you are.

    Ronnie.
    Attached Files Attached Files
    The trouble with having an open mind is that people will insist on trying to put things in it.

  12. #12
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  13. #13
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    Tube or tubeless? One man's high may be another man's low, what air pressures are we talking? FS, hardtail, rigid?
    Good thread.

    I ride FS and read that means a little more air pressure.
    Was not aware of the Stan's rule, interesingly, that's about where my tire pressure has evolved.
    dirtaoist

  14. #14
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    Quote Originally Posted by Zaratustra View Post
    no discussion of pressure.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
    29er Tire Weight Database

  15. #15
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    Now we start adding all the variables and its not apples to apples anymore.
    Do this test with the same wheel/tire set and play with your pressures on rocky vs. pavement. When you take your MTB on the road and ride to the trail head youre not airing down first. You air up to high pressure on pavement and then air down when you get to the trail. Why? because of the higher rolling resistance of that knobby tire with low pressure on pavement. It makes no sense if you just think about.

  16. #16
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    Fwiu...

    Quote Originally Posted by hal0tw0 View Post
    Now we start adding all the variables and its not apples to apples anymore.
    Do this test with the same wheel/tire set and play with your pressures on rocky vs. pavement. When you take your MTB on the road and ride to the trail head youre not airing down first. You air up to high pressure on pavement and then air down when you get to the trail. Why? because of the higher rolling resistance of that knobby tire with low pressure on pavement. It makes no sense if you just think about.
    If the surface is smooth, then higher pressure yields lower rolling resistance. If the surface is soft and uneven, the lower pressure yields lower rolling resistance. It has to do with the tire being able to 'envelope' the bumps rather than being deflected by them, and bouncing the whole bike up.

    Me, I found I could run lower pressure on my 29er in general than I could with my 26er. I tend to run as little as I can get away with.... without pinch flatting or with the tire feeling squirmy. For my 215 pound self, that is around 25/27 psi, with a few pounds more if I'm going to ride some place with lots of pointy rocks.

  17. #17
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    Quote Originally Posted by Maheoway View Post
    Tube or tubeless? One man's high may be another man's low, what air pressures are we talking? FS, hardtail, rigid?
    Good thread.

    I ride FS and read that means a little more air pressure.
    Was not aware of the Stan's rule, interesingly, that's about where my tire pressure has evolved.
    I've found I can get away with a little less tire pressure in my case, running a 29x2.4 tire on a 4" travel bike I've foudn the bike even felt comfortable, but on the edge of squirly, at 20lbs... 22-24lbs seems to work well. This is running tubeless though, so the tires react a little differently. On that note, on my previous HT with tubes I was still running 27lbs in a 26x2.0 last year, which is still pretty low. Then again, I also weigh 170lbs pluss the weight of a camelback w/ 100oz. of water and misc. spares/tools.

  18. #18
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    My initial thought was more theoretical,trying to have as objective answers as possible,out of what i or someone else does,which means more difficulty to explain all the personal factors etc...all caused because very often people are promoting low pressure (or it is my idea)...

    I mean..of course i'm experimenting with my pressure and i know,or i think i know what is better,for the present,for me,for my conditions.(HT,tubeless,similar to my ex 26'' pressure too.Nothing too different.27psi rear/23 front)

    But even this-my-personal situation constantly changes since our bicycling story is like travelling,it seems impossible to have a conclusion from time to time,from place to place,from person to person..

    On the other hand,my current 2.10 Pythons have the low limit at 29psi and the Maxxis over 30 if i'm not wrong.This again???Isn't too high?
    If i inflate over thirty my bike is dancing and nor rolling (i'm 70k/154lbs) + my LB mechanic insists that my pressure is unacceptable.He proposes what is written on the tire,even in the case of my gf = 50k/110lbs....i don't get it.

    ..but the conversation helped me to clarify some things.


    Thanks for the pdf and everything!!!

  19. #19
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    Quote Originally Posted by hal0tw0 View Post
    Now we start adding all the variables and its not apples to apples anymore.
    Do this test with the same wheel/tire set and play with your pressures on rocky vs. pavement. When you take your MTB on the road and ride to the trail head youre not airing down first. You air up to high pressure on pavement and then air down when you get to the trail. Why? because of the higher rolling resistance of that knobby tire with low pressure on pavement. It makes no sense if you just think about.
    One could argue it would never be apples to apples though. Ones riding style, weight, bike setup (rim width and tire width paly huge factors), terrain, environmental conditions (heat, rain) can all make a difference.

  20. #20
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    Quote Originally Posted by PHRANQUY View Post
    One could argue it would never be apples to apples though. Ones riding style, weight, bike setup (rim width and tire width paly huge factors), terrain, environmental conditions (heat, rain) can all make a difference.
    The original question proposed one variable: the wheel diameter. As close to apples-to-apples as you are going to get.
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  21. #21
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    Maybe the better answer then would be that the larger volume of the 29" tire allows the same amount of air to fill up the larger space resulting in a lower pressure then? Much like the fact that I run the tires on my truck @ 25psi compared to the 35psi that the factory tires were at.

  22. #22
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    Quote Originally Posted by PHRANQUY View Post
    Maybe the better answer then would be that the larger volume of the 29" tire allows the same amount of air to fill up the larger space resulting in a lower pressure then? Much like the fact that I run the tires on my truck @ 25psi compared to the 35psi that the factory tires were at.
    i can't make sense of this at all.

    for off-road use, the lowest pressure you can comfortably and safely run seems to be the best for traction, comfort & efficiency. 29ers seem to allow a lower pressure floor than 26" bikes, perhaps due to the angle of attack.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
    29er Tire Weight Database

  23. #23
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    I've played with tire pressure at different places I ride. One place I can get away with 26 psi, others I need a little more to prevent rim hits. I normally ride 28 psi.

    The above equation works well to get you started - rider weight + bike weight divided by 7. This equation gets me to 29 psi.

  24. #24
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    Quote Originally Posted by frdfandc View Post
    I've played with tire pressure at different places I ride. One place I can get away with 26 psi, others I need a little more to prevent rim hits. I normally ride 28 psi.

    The above equation works well to get you started - rider weight + bike weight divided by 7. This equation gets me to 29 psi.
    Thanks,it gives me exactly the average that i use = 25 psi.

  25. #25
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    Holy crap. Does everything have to become some sort of quantified, scientific equation? Optimum volume ... ideal contact patch ... perfect rolling resistance ... maximum efficiency ... jeebus. I thought you were supposed to ride your mountain bike in the woods, not a physics lab.

    Just put some air in your damn tires. If they bounce you around, take some out. If your rims get banged up, put more in. Simple.

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