porqui Posted June 20, 2016 Posted June 20, 2016 Hi thanks for all the replys. Got hold of dt swiss tech that confirmed there should be no problem. Their pressure chart shows for the particular rim and tyre size i can go up to 7.2 Where did you read that the max is 4 bar. May be a silly question but were your wheels built for a tandem? Were they standard on the bike? Wheels have a max mass rating so please check.
Tandemonium 2 Posted June 20, 2016 Author Posted June 20, 2016 Hi the tandem comes out with the specific dt swiss rims. They are tandem spesific. The 4 bar is written on the side of the rim. According to their tech and their pressure chart that 4 bar is aimed at high volume tubeless.
GrahamS2 Posted June 20, 2016 Posted June 20, 2016 Force = Pressure x Area So 6 bar from a high volume mountain bike tyre is allot more force on the rim than 6 bar from a road tyre.Pressure is already a force per unit area so that formula doesn't really work. 6 bar is 6 bar. The volume of air means nothing.
Captain Fastbastard Mayhem Posted June 20, 2016 Posted June 20, 2016 Pressure is already a force per unit area so that formula doesn't really work. 6 bar is 6 bar. The volume of air means nothing.It actually does, when you calculate the total force exerted on the system as a whole. EG: If you've got 2 surface areas of 100 sq Inches and 50 sq inches, and a pressure of 50PSI on both surface areas, the 100 sq inch platform will have a force twice that of on the 50 sq inch platform. So in essence the wider tyre / rim combo will have a higher overall force on it than a narrower tyre / rim combo for the same pressure.
Showtime Posted June 20, 2016 Posted June 20, 2016 Pressure is already a force per unit area so that formula doesn't really work. 6 bar is 6 bar. The volume of air means nothing.The formula is right. If you move the area to the left side of the equation it shows pressure is force per unit area. If pressure is force per unit area as you correctly say then force is pressure times area. Sent from my iPhone using Tapatalk
raptor-22 Posted June 20, 2016 Posted June 20, 2016 It actually does, when you calculate the total force exerted on the system as a whole. EG: If you've got 2 surface areas of 100 sq Inches and 50 sq inches, and a pressure of 50PSI on both surface areas, the 100 sq inch platform will have a force twice that of on the 50 sq inch platform. So in essence the wider tyre / rim combo will have a higher overall force on it than a narrower tyre / rim combo for the same pressure. If you're considering a system with a compressible fluid then some sort of gas constant is required. Its not as simple as p=F/A but for purpose of illustrating the concept its fine. There will be more force available hence why a larger volume can support the same weight at a lower pressure
Long Wheel Base Posted June 20, 2016 Posted June 20, 2016 How much pressure would you say it takes to lift a passenger jet up off the ground?When we go do aircraft recovery(I am rated to do just about any passenger jet except the Airbus 380) we have massive "balloons" that get placed under the aircraft and all we need to pump that balloon to is half a bar or about 7psi. But its so much air it takes about half an hour to fill the balloon. This is obviously for an aircraft that has done a wheels up landing and is still pretty much intact.
GrahamS2 Posted June 20, 2016 Posted June 20, 2016 The formula is right. If you move the area to the left side of the equation it shows pressure is force per unit area. If pressure is force per unit area as you correctly say then force is pressure times area. Sent from my iPhone using TapatalkSorry, didn't mean that the formula was incorrect, but it just doesn't help prove your ascertain that a mtb tyre will exert more force on the rim than a road tyre at the same given pressure.
Erroli8a8 Posted June 20, 2016 Posted June 20, 2016 It actually does, when you calculate the total force exerted on the system as a whole. EG: If you've got 2 surface areas of 100 sq Inches and 50 sq inches, and a pressure of 50PSI on both surface areas, the 100 sq inch platform will have a force twice that of on the 50 sq inch platform. So in essence the wider tyre / rim combo will have a higher overall force on it than a narrower tyre / rim combo for the same pressure. No you got it wrong. 50psi is 50 psi (Pounds per square inch) if you have more area then you have more volume but not more pressure. psi is the pressure regardless of size. small tire or big tire if the pressure is 50psi then for every inch in the tire you have 50 pounds of force pushing against it regardless of the size of the tire. if you had 100sqin as you put it and applied 50psi on the entire area then you would have half the pressure and not double and you can only apply 50psi, the bigger the surface the lower the pressure per inch. a lecturer showed us like this; a Lady of 50Kg walks in high heels. the pressure at the heel is 50Kg per inch as the size of the heal is 1square inch. if her heels double to 2 square inch her weight does not double but the effective weight per inch halves to 25Kg per inch. So psi is psi no matter the volume
madbradd Posted June 20, 2016 Posted June 20, 2016 I always thought the pressure rating of the rim was a function of the sidewall/bead strength. In a static system (your bike hanging on the wall) the force exerted on the rim would be constant regardless of the tyre you had on the bike for a constant internal tyre pressure and ourside air pressure. The dymanic force (when you climb on your bike and ride) would then be a function of the static components along with tyre volume, tyre type, compression constant of the gas, temperature, possibly riding speed etc. the pressure/force/area equation is only relevant when calculating the area of the tyre which would be in contact with the road under static conditions.
Showtime Posted June 20, 2016 Posted June 20, 2016 Sorry, didn't mean that the formula was incorrect, but it just doesn't help prove your ascertain that a mtb tyre will exert more force on the rim than a road tyre at the same given pressure. Think of it like this. The force needed to keep the tyre on the rim is total internal surface area of the tyre x air pressure The bead hooks are responsible for holding the tyre on the rim. If the volume of the tyre doubles the bead hooks have to contain twice the force for the same pressure.
Captain Fastbastard Mayhem Posted June 20, 2016 Posted June 20, 2016 No you got it wrong. 50psi is 50 psi (Pounds per square inch) if you have more area then you have more volume but not more pressure. psi is the pressure regardless of size. small tire or big tire if the pressure is 50psi then for every inch in the tire you have 50 pounds of force pushing against it regardless of the size of the tire. if you had 100sqin as you put it and applied 50psi on the entire area then you would have half the pressure and not double and you can only apply 50psi, the bigger the surface the lower the pressure per inch. no no - the pressure stays the same - 50 pounds per square inch. To get the force, you multiply the pressure by the area. So the overall force in newtons / pounds, kilos or whatever unit of measurement you want to use, will increase in line with the area the constant pressure is being exerted on. a lecturer showed us like this; a Lady of 50Kg walks in high heels. the pressure at the heel is 50Kg per inch as the size of the heal is 1square inch. if her heels double to 2 square inch her weight does not double but the effective weight per inch halves to 25Kg per inch. So psi is psi no matter the volumeyou're getting confused, Errol, but your assertions above are correct. And that's exactly what I've said On a 100sq in platform, with a PRESSURE of 50psi, you have a FORCE of 50*100 = 5,000 POUNDS acting upon that platform. On a 50sq in platform, with a uniform pressure of 50psi, you have a FORCE of 50*50 = 2,500 POUNDS acting upon that platform. I know that pressure is pressure regardless of size or volume of the platform / tyre etc. But what changes with the area or volume is the overall force in Newtons / Pounds / Kilograms / grams etc on the system. With your high heel example - a 100lb woman balancing on JUST the heel of the shoe... If it's a 1sq in heel, pressure = 100/1 = 100psi2 sq in heel = 100/2 = 50psi. Force remains the same, as it's a 100lb woman. Taking your example exactly though, we're trying to work out the overall force. You've got the calculation correct, you're just going about it the wrong way. Force = Pressure * Area 50kg=x * 2 inch therefore x=50/2 = 25kg/inch = pressure50kg=x * 1 inch therefore x=50/1 = 50kg/inch = pressure If the pressure remains constant with a larger area, the weight / force DOES increase Force = Pressure * AreaForce = 50kg/inch * 1 inch = 50kg FORCE being appliedForce = 50kg/inch * 2 inch = 100kg FORCE being applied. Essentially because the pressure remains the same (50kg/i) but the area over which that pressure is applied has increased, the overall force exerted on that area as a whole has increased as well
madbradd Posted June 20, 2016 Posted June 20, 2016 No you got it wrong. 50psi is 50 psi (Pounds per square inch) if you have more area then you have more volume but not more pressure. psi is the pressure regardless of size. small tire or big tire if the pressure is 50psi then for every inch in the tire you have 50 pounds of force pushing against it regardless of the size of the tire. if you had 100sqin as you put it and applied 50psi on the entire area then you would have half the pressure and not double and you can only apply 50psi, the bigger the surface the lower the pressure per inch. a lecturer showed us like this; a Lady of 50Kg walks in high heels. the pressure at the heel is 50Kg per inch as the size of the heal is 1square inch. if her heels double to 2 square inch her weight does not double but the effective weight per inch halves to 25Kg per inch. So psi is psi no matter the volume Nope, Myles is right. If you had 100 square inches and applied 50 psi (pounds per square inch), you would in fact have a force of 5000 pounds. (yes I know pounds is not a unit of force, but I didn't decide to talk in pounds an psi instead of metric units) If you had 50 square inches and applied 50 psi you would have 2500 pounds. Apply this to a real situation - I weigh 100kgs, and my bike weighs 10 kgs. In total we weigh 110 kgs. This is 242 pounds. When I sit on my bike, REGARDLESS of the tyre pressure, I (the bottom surfaces of my wheel/tyre system) will put 242 pounds into the ground and according to Isaac the ground will push back with the same "force". So ignoring all the complex bits of compresion and temperature and speed and all, and assuming that my front and back wheel take the same load I need a "force" of 121 pounds on each wheel. Let's assume a 1 inch wide tyre with a rectangular contact area: If the tyre pressure is 10 psi, I need 12.1 square inches of tyre to be in contact with the road (impossible, so I will sit on the rim which is now supporting me instead of the air inside the tyre). If the tyre pressure is 50psi, I only need 2.42 square inches of tyre contact area. So for the OPs case, 160kgs plus 15kgs for the bike (approx) is 175 kgs. distributed over 2 wheels, 87.5 kgs (192.5 lb) per wheel. With a 32mm tyre (1.26"), and a tyre pressure of 4bar (58 psi) the tyre contact patch will be approximately 2.63" long (at 1.26" wide) in order to support their weight. From my extremely scientific calculation, this seems to be quite a bit longer than the measurement I got on my bike while looking between my legs at the bulge in my rear tyre when I thought I had a flat.
eddy Posted June 20, 2016 Posted June 20, 2016 Newton, Pascal and Einstein are playing hide and seek and Einstein is on. Newton draws a square in the sand and stands in it. Einstein spots him and says, "I see you Newton " Newton says, "No you don't. You see 1 newton per square meter. You have spotted Pascal"
Erroli8a8 Posted June 20, 2016 Posted June 20, 2016 Agreed with above that if the pressure is the same per square inch then the larger the area the larger the pressure. but if the initial pressure remains constant then the greater the area the smaller the pressure per inch. "So 6 bar from a high volume mountain bike tyre is allot more force on the rim than 6 bar from a road tyre." my argument is with the comment above that if both tires are pumped to the same pressure then the pressure on the rim will be the same.
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