best tire preasure for DH

PB Forum :: Downhill
best tire preasure for DH
Author Message
flag erbissell
Posted: Feb 3, 2010 at 19:52 Quote
t3rr1 wrote:
la115 wrote:
t3rr1 wrote:
I tend to run around 18 or 19 in the front and between 18 and 30 in the rear depending on the course. Seems to work.
Why so low? Don't you flat or put dents in your wheels like crazy?
Nope.. haven't dented a rim in, and really no flat spots either.... 4 years? Rarely flats either and I tend to ride xc tubes. Low tire pressure ='s increased traction.
Rims are pretty much the only things I don't kill on bikes however..
how do you not fold the bead off in hard cornering? im gonna have to say that your pressure gage is off, otherwise, i call bs. What kind of terrain are you riding/racing? I can possibly see running 18 or 19 in the front, if its just super deep loam, or maybe sand, with no roots or rocks, but i still think the tire would just fold around like a limp...
flag la115
Posted: Feb 3, 2010 at 19:55 Quote
erbissell wrote:
t3rr1 wrote:
la115 wrote:

Why so low? Don't you flat or put dents in your wheels like crazy?
Nope.. haven't dented a rim in, and really no flat spots either.... 4 years? Rarely flats either and I tend to ride xc tubes. Low tire pressure ='s increased traction.
Rims are pretty much the only things I don't kill on bikes however..
how do you not fold the bead off in hard cornering? im gonna have to say that your pressure gage is off, otherwise, i call bs. What kind of terrain are you riding/racing? I can possibly see running 18 or 19 in the front, if its just super deep loam, or maybe sand, with no roots or rocks, but i still think the tire would just fold around like a limp...
Don't forget that t3rr1 is Female and not to be sexist or anything but they weigh alot less then most males. I'm guessing she weighs around 130 pounds when someone 200 pounds using 25psi.


Edit: That came out kind of bad. I mean that she weighs alot less which is the equivalent of someone 200 pounds using 25psi
flag erbissell
Posted: Feb 3, 2010 at 20:05 Quote
la115 wrote:
erbissell wrote:
t3rr1 wrote:
Nope.. haven't dented a rim in, and really no flat spots either.... 4 years? Rarely flats either and I tend to ride xc tubes. Low tire pressure ='s increased traction.
Rims are pretty much the only things I don't kill on bikes however..
how do you not fold the bead off in hard cornering? im gonna have to say that your pressure gage is off, otherwise, i call bs. What kind of terrain are you riding/racing? I can possibly see running 18 or 19 in the front, if its just super deep loam, or maybe sand, with no roots or rocks, but i still think the tire would just fold around like a limp...
Don't forget that t3rr1 is Female and not to be sexist or anything but they weigh alot less then most males. I'm guessing she weighs around 130 pounds when someone 200 pounds using 25psi.


Edit: That came out kind of bad. I mean that she weighs alot less which is the equivalent of someone 200 pounds using 25psi
Oh that makes a lot more sense than, i can see 19 for someone who weighs 130. I weigh 155 and run around 25, but i am also a fairly aggressive rider.
flag t3rry
Mod
Posted: Feb 4, 2010 at 9:22 Quote
erbissell wrote:
la115 wrote:
erbissell wrote:

how do you not fold the bead off in hard cornering? im gonna have to say that your pressure gage is off, otherwise, i call bs. What kind of terrain are you riding/racing? I can possibly see running 18 or 19 in the front, if its just super deep loam, or maybe sand, with no roots or rocks, but i still think the tire would just fold around like a limp...
Don't forget that t3rr1 is Female and not to be sexist or anything but they weigh alot less then most males. I'm guessing she weighs around 130 pounds when someone 200 pounds using 25psi.


Edit: That came out kind of bad. I mean that she weighs alot less which is the equivalent of someone 200 pounds using 25psi
Oh that makes a lot more sense than, i can see 19 for someone who weighs 130. I weigh 155 and run around 25, but i am also a fairly aggressive rider.
All terrain.. and I'm 175 pounds.. without gear. 19 is for front tire - which isn't so heavily weighted as the rear tire. Rear tire is most often at the 30 mark, rarely down to the 20ish range.
I've known guys bigger then me that have run as low as 9 psi in the wet and loamy conditions. 18 to 25 range is pretty common for alot of racers around here. Don't forget it also depends on tire width and stuff.
Tire gauge isn't off, as it's been set up that way, by myself and various mechanics. I doubt all of our gauges are off. I don't know.. it seems to work.
flag jonbikes
Posted: Feb 4, 2010 at 9:27 Quote
[Quote="t3rr1"]
erbissell wrote:
la115 wrote:
I've known guys bigger then me that have run as low as 9 psi. Don't forget it also depends on tire width and stuff.

I wouldn't say that to loud, rideonjon will have another hissy fit.
flag t3rry
Mod
Posted: Feb 4, 2010 at 9:28 Quote
[Quote="jonbikes"]
t3rr1 wrote:
erbissell wrote:

I've known guys bigger then me that have run as low as 9 psi. Don't forget it also depends on tire width and stuff.

I wouldn't say that to loud, rideonjon will have another hissy fit.
I'm most definately running road bike tires at 5 psi. I win!
flag mtnbyker
Posted: Feb 4, 2010 at 9:28 Quote
i weigh one nintey, but i prefer to have my tires at like 35-40psi, i tried less and i had 4 flats in 2 weeks, so I went back to normal...
flag erbissell
Posted: Feb 4, 2010 at 12:17 Quote
On my 15 inch travel custom freeride/dh bike with a moto fork on it, i run 6 inch tires at 0.04 psi, for rock crawling.
flag brigand
Posted: Feb 4, 2010 at 12:30 Quote
lol

lowest i run front is 20psi, that was maxxis mobster 2.5 2-ply with dh innertube
flag TomBo-The-One
Posted: Feb 4, 2010 at 13:17 Quote
Dose sub 25psi not feel squirmy in the turns? I found the while 25ish feels great for roots and trail junk. Any aggresive cornering leads to the tire starting to fold. Also charging rocks becomes far too risky. As any line or timing error, leads to pinch flat. I still feel the rim bottom / flat spot out the odd time at 30psi.

I used to try to run low twenties in the front and hight twenties in the rear. Once I got used the slighty more ping pong feel of 30+ it really is worth it. Less flats, more precise feel and more confidence, in the end. May be worth trying for a day if you haven't.
flag brigand
Posted: Feb 4, 2010 at 13:20 Quote
i'm 155 pounds

i'm running around 25f - 30r on my am bike now - single plies
flag t3rry
Mod
Posted: Feb 4, 2010 at 13:41 Quote
I've run higher pressure.. it's okay... I don't get flats.. well rarely. 3 or 4 a year typically. Doesn't feel squirmy in the corners at all, might be worth a try running lower pressure Razz
flag TwistedBench
Posted: Feb 5, 2010 at 20:16 Quote
So it is Friday night again and I came back to check on this thread.

Clearly the PV=nRT thing went over a few heads. Some of you understand, and some slept through their high school physics classes, or just day-dreamed about biking. (that’s OK)… In any case, I will try to offer up a little insight.

PV=nRT is Pressure x Volume = n (number of moles) x R (universal gas constant) x T (absolute temperature).

Well – google that if you want, but to avoid boredom, lets try to just see how this applies to the discussion here. What is the relation between volume and PSI and who cares if it is Oxygen or Argon.

First off, remember that gas is compressible. If you take a balloon and fill it with 1 cubic foot of air at sea level and then dive down to 33 ft below water with it, it will compress to ½ cubic feet, the pressure will be 2 atmospheres or 2 X 14.696 psi. PSI, by the way, is pounds per square inch. The balloon still contains the same number of gas molecules (moles). However, as the pressure has doubled, the volume is now half. If you were to measure the volume of air in the balloon, even 33 feet below sea level, it would contain 1 standard cubic foot of air. That is to say that the air is measured adjusted to standard temperature and pressure conditions. So a 80 cubic foot oxygen cylinder (for welding) isn’t really 80 cubic feet, but contains 80 standard cubic feet of oxygen. (80ft3 *14.696psi/3000psi = 0.4 cubic feet.)

Oxygen, Argon – doesn’t matter. Both are gases and behave according to the gas law above. Air is actually about 79% nitrogen and 21% oxygen, and a few ppm CO2 but we will leave that to Al Gore to worry about. Note that technically, PV=ZnRT. Z is the compressibility of a gas and is a factor to account for the deviation from the perfect gas law for various gases at elevated pressures. …. Lets not get into that.

So how does PSI and tire width/volume effect pinch flats etc.

Say you have a 175 lb t3rr1 riding a 25 lb bike with even weight distribution front and back. So 100 lbs on each tire. If you visualize the effective tire width is 2 inches (the flat spot touching the ground) and there is 20 psi in the tire a downward force of 100 lbs would require an area of 5 square inches to balance that force. So the 2” wide tire would have a 2.5” long flat spot touching the road, or trail. If the tire were only 1” effective width. that flat spot on the bottom would be 5” long with only 20 psi in the tire. So a narrower tire must have more pressure to support the same load. When a tire hits a rock or other share edge the effect is over a short ‘length’ or short arc of the tire. If it is a narrow tire, the impact has to be absorbed by a very small area – lets say 1” of effective tire width and 1” of length. On a wider tire, the same impact would effect an area of 2” in width and 1” in length. So only half the pressure is required to resist this blow in a wide vs a narrow tire.

Hopefully I have been successful in stating this in an easy to understand fashion. If not, no matter, just Ride. Cheers,Smile
flag jonbikes
Posted: Feb 5, 2010 at 20:54 Quote
TwistedBench wrote:


So how does PSI and tire width/volume effect pinch flats etc.

Say you have a 175 lb t3rr1 riding a 25 lb bike with even weight distribution front and back. So 100 lbs on each tire. If you visualize the effective tire width is 2 inches (the flat spot touching the ground) and there is 20 psi in the tire a downward force of 100 lbs would require an area of 5 square inches to balance that force. So the 2” wide tire would have a 2.5” long flat spot touching the road, or trail. If the tire were only 1” effective width. that flat spot on the bottom would be 5” long with only 20 psi in the tire. So a narrower tire must have more pressure to support the same load. When a tire hits a rock or other share edge the effect is over a short ‘length’ or short arc of the tire. If it is a narrow tire, the impact has to be absorbed by a very small area – lets say 1” of effective tire width and 1” of length. On a wider tire, the same impact would effect an area of 2” in width and 1” in length. So only half the pressure is required to resist this blow in a wide vs a narrow tire.

Hopefully I have been successful in stating this in an easy to understand fashion. If not, no matter, just Ride. Cheers,Smile

PERFECT!

This is exactly what I was saying, you just have the science behind it which I could not provide.

So rideonjon, would you still care to try to make a fool of me?
flag rideonjon
Posted: Feb 7, 2010 at 9:47 Quote
jonbikes wrote:
TwistedBench wrote:


So how does PSI and tire width/volume effect pinch flats etc.

Say you have a 175 lb t3rr1 riding a 25 lb bike with even weight distribution front and back. So 100 lbs on each tire. If you visualize the effective tire width is 2 inches (the flat spot touching the ground) and there is 20 psi in the tire a downward force of 100 lbs would require an area of 5 square inches to balance that force. So the 2” wide tire would have a 2.5” long flat spot touching the road, or trail. If the tire were only 1” effective width. that flat spot on the bottom would be 5” long with only 20 psi in the tire. So a narrower tire must have more pressure to support the same load. When a tire hits a rock or other share edge the effect is over a short ‘length’ or short arc of the tire. If it is a narrow tire, the impact has to be absorbed by a very small area – lets say 1” of effective tire width and 1” of length. On a wider tire, the same impact would effect an area of 2” in width and 1” in length. So only half the pressure is required to resist this blow in a wide vs a narrow tire.

Hopefully I have been successful in stating this in an easy to understand fashion. If not, no matter, just Ride. Cheers,Smile

PERFECT!

This is exactly what I was saying, you just have the science behind it which I could not provide.

So rideonjon, would you still care to try to make a fool of me?


your doing fine on your own.

 


Copyright © 2000 - 2024. Pinkbike.com. All rights reserved.
dv56 0.030890
Mobile Version of Website