Recommended tire pressure calculation...

Discussion in 'Shop Talk' started by Torrent77, Aug 7, 2017.



  1. Torrent77

    Torrent77 Well-Known Member

    Name:
    Dave
    I was listening to the Path podcast and they are using an interesting method to calculate the baseline tire pressure.

    Basically you take your weight and divide by 7. Next you add 2 psi for the rear tire and subtract 1 psi for the front.

    So for me its 32psi in the rear and 29 psi for the front which is pretty close to what I run now. I've ruined a few tubes running too low psi.
     
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  2. UPSed

    UPSed MTB Addict

    Location:
    Simi Valley
    Name:
    Edward Bottorff
    Current Bike:
    Niner Jet 9 RDO Plus
    I think that formula has been around for a while. I also believe it only works with tubed tires. It shows me at 30-32 but I run 20 - 22.
     
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  3. dustyyoungblood

    dustyyoungblood Well-Known Member

    Location:
    Ladera Ranch
    Name:
    Dustan Baker
    Current Bike:
    Foes Mixer Trail
    -10 of what that formula gave me. whats a tube?
     
  4. LLPoolJ

    LLPoolJ Well-Known Member

    Location:
    Moreno Valley
    Name:
    James Johnson
    Current Bike:
    Cannondale Scalpel
    Ha ha he said tubes!
     
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  5. Mikie

    Mikie MTB Addict

    Location:
    Lebec, California
    Name:
    Mikie Watson
    Current Bike:
    2012 Yeti ASR 5c
    Hey @Torrent77 I had not heard of that before. Those numbers are pretty close if I was using tubes. I assume you are? Did they happen to say this was for tubeless or not?
     
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  6. Sidewalk

    Sidewalk Well-Known Member

    Location:
    HorsetownUSA
    Name:
    Josh
    Current Bike:
    N+1
    It actually comes out to exactly what I am running right now (20/24).

    I've heard that formula before, only used it as a starting point when I first started riding.
     
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  7. Faust29

    Faust29 MTB Addict

    Location:
    Rancho Santa Margarita
    Name:
    Steve Macko
    Current Bike:
    Highball SS and Chameleon
    Yep... I learned that formula when I first started lurking on STR. I get upper 20s, but I run 19-21 in the front and 24-26 in the rear with 29ers.
     
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  8. Runs with Scissors

    Runs with Scissors MTB Addict

    Location:
    West Anaheim
    Name:
    Mark Whitaker
    Current Bike:
    2015 Giant XTC Advanced 1 29er
    This would put me at 22 psi front and 25 psi rear, which is way more than the 17F/20R I actually run...o_O

    I don't think I'll be using that formula.
     
    Mikie likes this.
  9. Rumpled

    Rumpled Well-Known Member

    Location:
    OC
    Name:
    Jim Martin
    Current Bike:
    2003 Giant Rainier
    Pretty close for me; on 26! I run tubeless.
    Maybe the formula needs updating?
     
  10. mtnbikej

    mtnbikej J-Zilla

    Location:
    Orange
    Name:
    J
    Current Bike:
    SC Highball Al SS
    Formula was Stan's No Tubes idea.....and it was for their tires on their rims.

    Doesn't really work for other tires....as tire technology has advanced.

    Based on my weight, I should be running 29 R / 25 F. No way in hell I'm running anything that high. 20-22 R / 18-20 F is where I am comfortable.
     
  11. herzalot

    herzalot MTB Addict

    Location:
    Laguna Beach
    Name:
    Chris
    Current Bike:
    2015 Intense Tracer 275c DVO
    I think better tire design has rendered that formula obsolete.
     
  12. UPSed

    UPSed MTB Addict

    Location:
    Simi Valley
    Name:
    Edward Bottorff
    Current Bike:
    Niner Jet 9 RDO Plus
    And wider rims. :thumbsup:
     
  13. herzalot

    herzalot MTB Addict

    Location:
    Laguna Beach
    Name:
    Chris
    Current Bike:
    2015 Intense Tracer 275c DVO
    Yeah, maybe. I was running 23 mm interior width and am now on 26mm. Not wide by modern standards.
     
  14. Varaxis

    Varaxis Well-Known Member

    Location:
    Perris
    Name:
    "Dan" Vu
    Current Bike:
    Yeti SB5c ('16 Yellow v1)
    The formula that divides weight by 7 and adds whatever, definitely sounds like the Stan's NoTube estimation for tubeless conversion. I think the main point it accomplished at the time, was to break the ice regarding how low of a pressure would be acceptable at that time. People were still used to ~35 psi with 26 tires with tubes. ~20 psi was a point of controversy, with people adamantly biased believing that such low pressures were unridable.

    Figuring out much pressure you put into your tire is simple to understand, but complicated to calculate.
    - the more weight being placed on the tire (inc. rider & cargo), the higher the pressure
    - the bigger the tire (29er vs 26, 2.8 vs 2.2), the lower the pressure
    - the flimsier the tire, the higher the pressure
    - the faster you're going & impacting unmoving obstacles, the higher the pressure
    - the more comfort/compliance you want, the lower pressure
    - the more grip you want, the lower the pressure
    ^^ within reasonable limits, not going too low nor too high.

    Some elaboration on the first point: weight is split between the two tires and if you are more rear biased, considering very short chainstays/RC (rear center) and defensive rearward stance, you will have more weight on the rear. Especially so if you have a full pack of water and tools (or textbooks) and don't unweight the rear for impacts (perhaps from exhaustion). This is a good reason why most run significantly more pressure in the rear (also considering more XC oriented tires in the rear).

    A fatbike 26x4.0 tire will feel similarly firm at 8 psi as a 29x2.2 tire at 25 psi. PSI = pounds per square inch. The 26x4.0 tire has about the same diameter as the 29x2.2 tire, but its carcass is about twice as wide (bead to bead measurement, with the tire flat). The lower the pressure, the bigger the contact patch is, between the tire and ground. A larger contact patch will spread your weight and torque forces over a broader area, decreasing the chance for the tire to break traction from "shear" forces (ex. skidding, sliding, spinning-out).

    What's it gonna take, someone to create a phone app that measures the sag of a tire, to help determine starting pressure? One that measures the difference in rim or axle height (with parallax error corrected), relative to the ground, once a rider mounts the bike? *shrug* Just like how people tune suspension to use full travel with minimal harsh bottom outs, people can tune tires similarly.

    On this subject, I'd like to note that the myths/misconceptions spread by tests by the likes of Schwalbe, regarding lower pressures resulting in less rolling resistance is highly misleading. On reasonably packed dirt, as opposed to "meadow" or "gravel", lower pressures result in increased rolling resistance. This is caused by energy going into the casing & rubber and causing it to deform, without the energy being returned in any useful manner. Too much pressure can slow you down, due to obstacles being treated as small inclines (energy being used to lift the axle and rider, with the force going into some old guy's sensitive back), or being deflected rearward upon impact. In general though, lower pressures improve grip as a trade-off to the wasted energy through rolling resistance, so it's not as bad for riding performance/capability as having too much pressure, other than the increased risk of damaging the rim and flatting.

    As far as rim width goes, I don't believe the marginal amount of increased volume really does much. The real benefit is from reducing the amount of lateral play, felt as squirm, that is inherent in the "light bulb" shaped tire cross-section. People ran higher pressures to provide support against tire squirm, which may have been way more than needed to prevent rim damage. Wide rims allowed people to run lower pressures, who were more sensitive to squirm.

    Anyone remember seeing the potential technology that I posted here in the patent thread? Bead-lock (huck norris/band clamp hybrid thingee) & full width rim strip that goes beyond the bead hooks: http://www.imtbtrails.com/forum/threads/patents-in-the-bicycle-industry.2265/#post-39701
     
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  15. bvader

    bvader Well-Known Member

    Location:
    HB
    Name:
    Stephen
    More like ÷ 8 nowadays +/-
     
  16. Runs with Scissors

    Runs with Scissors MTB Addict

    Location:
    West Anaheim
    Name:
    Mark Whitaker
    Current Bike:
    2015 Giant XTC Advanced 1 29er
    That's what I said....:whistling:
     
  17. Cyclotourist

    Cyclotourist MTB Addict

    Location:
    Redlands
    Name:
    David
    Current Bike:
    Gunnar Rockhound 29
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  18. Varaxis

    Varaxis Well-Known Member

    Location:
    Perris
    Name:
    "Dan" Vu
    Current Bike:
    Yeti SB5c ('16 Yellow v1)
    Unless you're still on 26" ;)

    "stiffer tire rolled slower at lower pressures than at higher ones". Sounds like a typical 60 tpi mtb tire.

    "For supple tires, pressure makes little difference in performance." Supple tire sounds like 120 tpi eXC casing (different from EXO, but not mutually exclusive). Nino Schurter is riding on pro-only 180 tpi casing Maxxis tires.

    I also notice how people sense/interpret increased ground feedback as feeling "faster" "sportier" "stiffer". It's like the sense that being out-of-control feels faster than being smooth, which is often heard when comparing times between a 29er and a smaller wheeled bike back-to-back. People also use a similar sense when describing how one wild looking pro DH racer looks faster than a smoother one (ex. Blenki/Ratboy vs Minnaar/Gwin).

    I'll note that 60 tpi casings are far tougher, holding up to abuse better than 120 tpi casings, if this weren't already obvious. I know I personally discovered this riding the higher end Ikon tires. I never really managed to wear one out fully, while I'm close to wearing out a HR2 used exclusively on the front. I know some of my rear tires look a bit thread-bare or have cracked sidewalls from running too low pressure. Now they're in a state of looking constantly wet from sealant seeping through (and needing more frequent refills).

    I have no arguments with the article's take-away. It's all info that reinforces what I've discovered over time.
     
    Cyclotourist likes this.
  19. john_hovard

    john_hovard Member

    Location:
    Las Palmas
    Name:
    john
    Current Bike:
    Ibis Mojo 2009
    I thought it was 19
     
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