Author Topic: Suggested MTB air pressures by GEAX (full suspension vs front suspension  (Read 1044 times)

jim-ratliff

  • 6+ Year Member
  • 1000 Posts
  • ******
  • Posts: 2739
See the link below that Svend sent me in email.  I find it very interesting that they recommend different air pressures for full suspension bikes than for front suspension bikes (though I can manufacture a reason why).  Equally interesting that they don't differentiate between tubed and tubeless tires.

http://www.geax.com/en/technology/?cat=15&prod=79

Examples for Lynn and I.  I weigh 183 with a 27 lb full suspension bike. She weighs 120 with a 26 lb full suspension bike.

J. Full suspension bike, 2.1 tire, 210 lbs combined weight.  Front = 42, rear = 50.  (Front susp:  Front=39, Rear=46)
L. Full suspension bike, 2.1 tire, 150 lbs combined weight.  Front = 33, rear = 41.  (Front susp:  Front=30, Rear=38)
L. Full suspension bike, 1.9 tire, 150 lbs combined weight.  Front = 36, rear = 44.  (Front susp:  Front=33, Rear=41)


B. Full suspension bike, 2.3 tire, 190 lbs combined weight.  Front = 36, rear = 44.  (Front Susp:  Front=33, Rear=40)
« Last Edit: August 12, 2011, 03:30:48 pm by jim-ratliff »
"If you're gonna play the game boy, ya gotta learn to play it right."


bushwacka

  • Instructor
  • 400 Posts
  • **
  • Posts: 471
they are right that you can run lower pressure on the rear of a FS bike. The suspension make it less likely to pinch the tire(or tube if you still insist on running those things).

With that said I never run much less than these guys, I do run tubeless but noone runs pressures that high even with tubes.

I weigh 166lb and run 24 psi front and 28 psi rear on my FS bike and run 30 psi rear on my SS hardtail.

jim-ratliff

  • 6+ Year Member
  • 1000 Posts
  • ******
  • Posts: 2739
Bushwacka:

They are saying the opposite, that it's more efficient to run higher pressures front and rear on a full suspension bike.

I'll edit the first entry and post their recommendations for your weight and full suspension. (cause I'm 20 pounds heavier than you.
« Last Edit: August 12, 2011, 03:55:08 pm by jim-ratliff »
"If you're gonna play the game boy, ya gotta learn to play it right."

bushwacka

  • Instructor
  • 400 Posts
  • **
  • Posts: 471
I have no idea then seems strange.

I would ask their pros before blindly following some website. ask any legit pro XC race guy there pressure and I be surprised to ever hear over 35 most are closer to 20-25. if it wasnt faster they wouldnt do it.

jim-ratliff

  • 6+ Year Member
  • 1000 Posts
  • ******
  • Posts: 2739

I remember the excellent link you provided of the guys using power meters across uneven terrain. 
But this is interestingly different.  Equally interesting that they broke it down by type of suspension and not for tubeless vs. tubed.
"If you're gonna play the game boy, ya gotta learn to play it right."

bushwacka

  • Instructor
  • 400 Posts
  • **
  • Posts: 471
yeah I do not find it interesting I find it incredibly flawed and missed leading. I have spent the last half hour trying to find a pro racer who rides Geax tires to no avail.

jim-ratliff

  • 6+ Year Member
  • 1000 Posts
  • ******
  • Posts: 2739

Here's the logic.  I don't think it's flawed, may be incomplete.

Sheldon Brown's website (a road biker) was the first place I read about the advantages of lowering pressure to let the tire absorb some of the terrain.  Obviously in road bikes, its just minor irregularities.  Every time a bike goes up and over an irregularity, it bleeds enough energy to lift the entire weight of the bike by that amount.  Energy has to come from somewhere, and it comes from pedaling.  And so, while most people ride their bikes with 100+ pounds of air, he recommended 90 or even less and also pointed out that pro riders use considerably less.  This is consistent with the New Zealand power meter tests that you provided the link for.

However, it's like race cars where you reduce the unsprung weight as much as possible (partly for different reasons).  On a hardtail, all of the rear weight (including the rider) is unsprung.  But, if all that has to move to absorb the bump is the suspension, then it's not nearly as much work to lift the rear suspension as to lift the whole bike.

MAYBE.  But GEAX also didn't support their recommendations with power meters or any evidence at all.  And none of this even talks about relative grip. My son is a off-road Jeep fanatic, and they do the same thing.  They air the tires down to 5-10 psi for grip off-road and over rocks even if it requires more horsepower to turn them, and then air them back up for the ride home.  Most hard core jeeps have an aftermarket air compressor under the hood just to air the tires back up for the road.


I would guess that long MTB endurance races run higher pressures than shorter races???
« Last Edit: August 12, 2011, 04:23:21 pm by jim-ratliff »
"If you're gonna play the game boy, ya gotta learn to play it right."

Svend

  • 4-6 Year Member
  • 1000 Posts
  • ****
  • Posts: 1107
Jim, it sounds like you are talking yourself into running lower pressure than you are at present  8)  I know that to a veteran road rider like yourself, 30 or 35 psi must sound like a near-flat tire, but it actually works better.  Even with tubes (sorry Josh). 

When I started lowering the pressures on all our bikes, everyone noticed an improvement in grip on the single track.  Rolling resistance on the hard gravel rail trail is much worse, so I pump 'em all back up to 50psi for those rides.  But for rides in the woods, the girls (being slightly lighter than me) are all down to 30 psi, and I run 35 psi on my 29er.  Luckily there are few sharp rocks here to shred sidewalls (mostly glacial moriane sand and gravel deposits here -- nice round rocks) and we don't do big hits to cause pinch flats, so we can get away with that with tubed tires.  At 45 to 50 psi on single track, we were bouncing and slipping all over the place, esp. the three of us riding hardtails.  Not fun.

FWIW, I have not had a pinch flat in about 10 years, since I used Panaracer Smoke which had really thin sidewalls.

So.....why not try it for a ride? Nothing to lose, eh? If you don't like it, take out your pump and puff 'em back up again.  (Er....you did buy a mini-pump to carry along....didn't you?)  :D

« Last Edit: August 12, 2011, 09:39:02 pm by Svend »

jim-ratliff

  • 6+ Year Member
  • 1000 Posts
  • ******
  • Posts: 2739
 :D


I've been up and down the range of pressures some (pinch flatted with front tire at 35).
Rolling resistance at lower pressures with my tires and bike is noticeably more work.
But "a pressure" is also not apples and apples. A 26" takes more pressure than a 29".  A 2.1 width takes more pressure than a 2.3.  And Jim at 180 takes more pressure than Josh at 165, and Lynn at 120 takes a lot less pressure than Jim.
Tires with stiffer sidewalls require less pressure than tires with really flexible sidewalls.



"If you're gonna play the game boy, ya gotta learn to play it right."

bushwacka

  • Instructor
  • 400 Posts
  • **
  • Posts: 471
a 26er does not take more pressure than a 29er..... I was able to run even lower on my 26er because I was able to use a wider stiffer tire on a 26er than I can find for a 29er.

remember wider tires roll faster but only at lower pressure, and only with out tubes. If you lower pressures with a tube in it the tube is now flexing with the tire flexing, its also causing friction as they rub on each other. The friction is compounded by such a skinny tire as the contact patch is much longer on the ground than a wide tire.

a ghetto tubeless setup will take a 30 minutes and 30 dollars to set up and can be done on nearly any bike and any wheel combination. Judging by how much time both of you guys are posting on here discussing you slightly lower but still way to high pressure, why not look up how to do it, and just do it. Ill even give my number out so I cna walk you though it.

http://www.tetongravity.com/forums/showthread.php/98811-Ghetto-Tubeless-A-how-to.?highlight=Ghetto%20Tubeless

sorry the pictures are down but really I would just try it, you have nothing to lose.


jim-ratliff

  • 6+ Year Member
  • 1000 Posts
  • ******
  • Posts: 2739
BW:

Ok, you make my point. For the same tire the 29 will require less pressire than the 26 because the 29er has a larger contact patch (longer). The same is true of a 2.3 tire as opposed to a 2.0 (15% wider contact patch).

I'm 20+ lbs heavier than you (and 35 years older), running a  15% narrower tire than you, and a tire with much less stiff sidewalls than you. It's inconceivable to me that we should be running the same tire pressure.

My tires, with me on the bike, do NOT roll easier at 35 pounds than at 45.

Maybe the problems you had with sidewalls on the SB8 were because you were locked in to a given pressure and they needed 7 pounds more?
« Last Edit: August 13, 2011, 09:09:29 pm by jim-ratliff »
"If you're gonna play the game boy, ya gotta learn to play it right."

jim-ratliff

  • 6+ Year Member
  • 1000 Posts
  • ******
  • Posts: 2739
BW (and everyone):

The fact that I may not choose to totally follow anyone's advice doesn't mean that I don't totally enjoy all of the information that you have brought to the forum.

People like me that are in their 60's are entitled to be a bit obstinate?   ::)   Who knows, I may "discover" the advantages of lower tire pressures in a year or two.

Lynn and I were both wanting to ride 50 miles today.  She got hers, I got a flat after 33 and stopped.
« Last Edit: August 13, 2011, 08:03:03 pm by jim-ratliff »
"If you're gonna play the game boy, ya gotta learn to play it right."

bushwacka

  • Instructor
  • 400 Posts
  • **
  • Posts: 471
BW:

Ok, you make my point. For the same tire the 29 will require less pressire than the 26 because the 29er has a larger contact patch (longer). The same is true of a 2.3 tire as opposed to a 2.0 (15% wider contact patch).

I'm 20+ lbs heavier than you (and 35 years older), running a  15% narrower tire than you, and a tire with much less stiff sidewalls than you. It's inconceivable to me that we should be running the same tire pressure.

My tires, with me on the bike, do NOT roll easier at 35 pounds than at 45.

Maybe the problems you had with sidewalls on the SB8 were because you were locked in to a given pressure and they needed 7 pounds more?

the 29er does not have a larger contact patch it does have longer one though.  The deal was to get the SB8 from stop flatting I had to run them at a pressure that exceeded my skill level of bike handling. So being that I like to make things as easy as possible I got rid of the SB8. The Pirannha is just as fast with sidewalls that can withstand 28 psi alot better.

like I have been saying you CAN NOT run a low pressure on a tubed tire and expect it to roll better. It will grip more but it will not roll better. There is a TON of friction between the flexing tire and tube that becomes rolling resistence. You can not understand low pressure and why it rolls faster and grips more untill you ride a tubeless tire.


jim-ratliff

  • 6+ Year Member
  • 1000 Posts
  • ******
  • Posts: 2739

the 29er does not have a larger contact patch it does have longer one though. 


Contact patch means "area" of the tire (bike or car) in touch with the surface.  Area = length X width.  A "longer" contact patch or a "wider" contact patch both result in a "larger" contact patch.
"If you're gonna play the game boy, ya gotta learn to play it right."

jim-ratliff

  • 6+ Year Member
  • 1000 Posts
  • ******
  • Posts: 2739

Josh:  So give me a summary by answering the following questions.

1. Your new bike.  What size wheel and tire, and what pressures do you run front and rear?

2. Your single speed. What size wheel and tire and what pressures do you run front and rear?


And a counter-point article that maybe hits at the real point, you and I are different riders.
I especially liked his summary:

http://www.downcycles.com/blogs/techtips/2009/05/24/title_1



I recommend tubeless tires to riders who are competitive. Many a race has been lost due to a tube pinch flat. Riders who have a fat wallet and a lot of patience are also good candidates for tubeless. Weekend warriors should stay clear of tubeless. I feel that most riders will suffer with problems with tubeless. Tube systems work pretty well and are pretty easy to work on trail side.
In the long run, most riders will end up running tubes in the tubeless tires. I?ve battled the tubeless system tooth and nail. As of now, I have a leaky tubeless Igniter on the front of my bike and a Cross Mark on the rear with a tube since the tires bead is weak, not enough to bulge the tube, but just enough to leak all the air if set up tubeless.
In other words, I?ve been beaten by tubeless tires. I have surrendered, forced to run tubes inside this sophisticated, brilliantly engineered, tubeless work of art.
« Last Edit: August 15, 2011, 07:30:38 am by jim-ratliff »
"If you're gonna play the game boy, ya gotta learn to play it right."