GrahamS2 Posted June 20, 2016 Posted June 20, 2016 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.Don't agree - pressure is a measure of force, so the force required to contain the bead is completely relative to the the pressure in the tyre, not the volume. Pressure is simply force (pounds) measured across a unit area (square inch). The unit area remains the same no matter the volume of the tyre you're measuring pressure in. Great discussion, I must say!
madbradd 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. The larger the area, the larger the force. The rest is all good After all of this I'm still not sure what "rim pressure" actually is. Edit: clarification
Captain Fastbastard Mayhem 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 FORCE. but if the initial pressure remains constant then the greater the area the smaller the pressure per inch FORCE "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.Fixed. You're getting your terminology mixed up, that's where the confusion is coming in. Pressure is always referred to as a unit of weight / mass over an area. 1 Pascal = 1 N/m^21KPa = 1,000 N/m^2 = 10kg/m^21Psi = 1lb/In^2
madbradd Posted June 20, 2016 Posted June 20, 2016 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.What about hookless rims? And nope, the area of the bead is the same nomatter what the tyre volume is.
Captain Fastbastard Mayhem Posted June 20, 2016 Posted June 20, 2016 What about hookless rims? And nope, the area of the bead is the same nomatter what the tyre volume is.Rim seat, and the pressure of the tyre against the rim. Like car wheels & tires.
madbradd Posted June 20, 2016 Posted June 20, 2016 Rim seat, and the pressure of the tyre against the rim. Like car wheels & tires. Sorry that was rhetorical wrt the hookless rims. I learned about them before buying mine.
porqui Posted June 20, 2016 Posted June 20, 2016 Area has nothing to do with it at all. You are working with gas pressure in a closed system. To get to the same pressure in a larger tyre you need more air. The force exerted on the sides of the tyre wall and rim is exactly the same if it is in a fat bike tyre or tubby tyre or a bmx bike tyre no matter what at the same pressure. The bead on the rim of a race bike has to withstand a pressure up to 10 bar so does the tyre itself - by design it is made much stronger where a mtb could run at 2 bar. Try pump up a mtb tyre to 10 bar it will blow not because of more force but it is not as strong as a road tyre it will only contain say 5 bar. The forces at the same pressure is the same.
Underachiever Posted June 20, 2016 Posted June 20, 2016 Area has nothing to do with it at all. You are working with gas pressure in a closed system. To get to the same pressure in a larger tyre you need more air. The force exerted on the sides of the tyre wall and rim is exactly the same if it is in a fat bike tyre or tubby tyre or a bmx bike tyre no matter what at the same pressure. The bead on the rim of a race bike has to withstand a pressure up to 10 bar so does the tyre itself - by design it is made much stronger where a mtb could run at 2 bar. Try pump up a mtb tyre to 10 bar it will blow not because of more force but it is not as strong as a road tyre it will only contain say 5 bar. The forces at the same pressure is the same.This thread reminds me why I hated physics at varsity, and not chemistry!!! Back to your hypothesis....But then the tyre will fail (like a balloon fails), and this is not the case. The tyre normally pops off the rim.
Eugene Brown 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 7.2Hi Anton,I want to put road slicks on my DT Swiss 29"wheels too. What model of DT Swiss rim is fitted to your tandem?
porqui Posted June 20, 2016 Posted June 20, 2016 This thread reminds me why I hated physics at varsity, and not chemistry!!! Back to your hypothesis....But then the tyre will fail (like a balloon fails), and this is not the case. The tyre normally pops off the rim. You are right - the tyre usually pops off first.If the circumference of the bead was smaller and an indentation for the bead in the rim then the tyre blows and it does happen and it is a loverly sight when blowing up a tjoepless tyre with sealant. Some use a compressor to do so and forget set the max pressure - mtb tyre can't take 8 bar.
Captain Fastbastard Mayhem Posted June 20, 2016 Posted June 20, 2016 Area has nothing to do with it at all. You are working with gas pressure in a closed system. To get to the same pressure in a larger tyre you need more air. The force exerted on the sides of the tyre wall and rim is exactly the same if it is in a fat bike tyre or tubby tyre or a bmx bike tyre no matter what at the same pressure. The bead on the rim of a race bike has to withstand a pressure up to 10 bar so does the tyre itself - by design it is made much stronger where a mtb could run at 2 bar. Try pump up a mtb tyre to 10 bar it will blow not because of more force but it is not as strong as a road tyre it will only contain say 5 bar. The forces at the same pressure is the same.Force = pressure x area. That is all.
Capricorn Posted June 20, 2016 Posted June 20, 2016 The struggle by some in this thread, with the relationship between force, pressure and area, is almost alarming.
scotty Posted June 20, 2016 Posted June 20, 2016 You are right - the tyre usually pops off first.If the circumference of the bead was smaller and an indentation for the bead in the rim then the tyre blows and it does happen and it is a loverly sight when blowing up a tjoepless tyre with sealant. Some use a compressor to do so and forget set the max pressure - mtb tyre can't take 8 bar.Or leave your sealant tyre pumped near max in the sun for a while, sounds like a mini bomb. Been there done the messy clean up afterwards.
Rocket-Boy Posted June 20, 2016 Posted June 20, 2016 The struggle by some in this tread, with the relationship between force, pressure and area, is almost alarming.That is related to hydraulics though, this is related to gasses. I may have missed the connection though and just wanted to throw in a chirp while I eat this here popcorn.
porqui Posted June 20, 2016 Posted June 20, 2016 Force = pressure x area. That is all. You are dealing with forces in a gas which exerts a force in all directions equally and not a force in a very particular direction like a mass resting on a given area. Forces in a liquid also behave in a particular manner.
JXV Posted June 20, 2016 Posted June 20, 2016 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.More or less correct in principle but the stress on the bead hooks (rim flanges) is proportional to the internal height of the tyre casing (from rim bed to inside of tread) and not the volume. Think of it as a pipe. A certain wall thickness can withstand a certain pressure. A pipe with twice the diameter needs walls twice as thick to withstand the same pressure. So, back to OP's original query. If he fits a tyre with casing diameter 2/3 or less than the MTB tyre size for which the rim is rated 4 bar... it should be able to withstand 6 bar inflation pressure under static conditions. I still wouldn't ride it though...... Sent from my SM-G900F using Tapatalk
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