Seems like a favourite topic is what psi to run tyre X at or I puncture loads on tire Y.....
Also seems that in spite of many many wheel manufacturers moving to "wider rim" technology tire manufacturers are still adding guidelines based on older rim dimensions such as a Mavic Open Pro (no doubt for sensible legal reasons as those are still on sale).
Notice in the below article from ST in 2014 that internal rim widths vary from 13mm to 20.6mm across manufacturers!
http://www.slowtwitch.com/...idths_2014_4370.html
This has large implications for both what size your tire *actually* measures out at once mounted and inflated to reasonable psi.
See an earlier post of mine here for examples of how widely actual versus printed width can be:
http://forum.slowtwitch.com/...ost=5967064#p5967064
Now apologies engineers and Tom A. if my terminology and simple logic is a bit off but it occurred to me that if we desired a "constant" ride sensation then the surface force is more relevant.
Let me explain.....
1) We mount a 23mm tire on a Mavic Open Pro training wheel, we inflate it to our preferred, within manufacturer guidelines, psi e.g. 110psi and it measures exactly 23mm wide.... this provides us with our desired ride quality.
2) So applying some simple math the surface area (length x width) of that tyre under 110psi is simplistically the length (which = the radius of the tyre (e.g. 330mm) squared x pi) x the width (which = pi x (23mm/2) squared)
All in all and converting to square inches in this example we get a number of.... 1336
3) So at 110psi this means the force is 146,900
So if we desire that same surface force on the tire all the time (i.e. ride sensation) logic and math dictates our tire pressure should drop substantially as the tire widens substantially, not remain the same (unless puncturing more often is your preference)
I ran through all these calculations from a starting base point of 23mm actual width at 110psi and concluded an equation can be used to calculate psi for same ride sensation based on actual width.
tire pressure = 74069 x [(tire width) to the power of -2.079]
For those of you that use excel it would look something like "tire pressure" = 74069*power("tire width", -2.079)
Some examples (with rounding up)....
23mm 110psi
24.6mm 95psi
26.2mm 83psi
28mm 73psi
29.6mm 65psi
So the next time you are wondering why your 25mm Conti GP4000sii is jamming in your bike frame on your Zipp's or you think you need to run 25mm Gatorskins on your ENVE wheels because you always seem to puncture and you like your tires at 120psi remember...... the answer is almost invariably your psi is wildly off based the dimensions of your rim and the actual ensuing tire width.
Of course YMMV depending on your bodyweight but same principle applies.
Regards,
p.s. Brainiac's feel free to add/subtract wisdom :-)
David T-D
http://www.tilburydavis.com
Also seems that in spite of many many wheel manufacturers moving to "wider rim" technology tire manufacturers are still adding guidelines based on older rim dimensions such as a Mavic Open Pro (no doubt for sensible legal reasons as those are still on sale).
Notice in the below article from ST in 2014 that internal rim widths vary from 13mm to 20.6mm across manufacturers!
http://www.slowtwitch.com/...idths_2014_4370.html
This has large implications for both what size your tire *actually* measures out at once mounted and inflated to reasonable psi.
See an earlier post of mine here for examples of how widely actual versus printed width can be:
http://forum.slowtwitch.com/...ost=5967064#p5967064
Now apologies engineers and Tom A. if my terminology and simple logic is a bit off but it occurred to me that if we desired a "constant" ride sensation then the surface force is more relevant.
Let me explain.....
1) We mount a 23mm tire on a Mavic Open Pro training wheel, we inflate it to our preferred, within manufacturer guidelines, psi e.g. 110psi and it measures exactly 23mm wide.... this provides us with our desired ride quality.
2) So applying some simple math the surface area (length x width) of that tyre under 110psi is simplistically the length (which = the radius of the tyre (e.g. 330mm) squared x pi) x the width (which = pi x (23mm/2) squared)
All in all and converting to square inches in this example we get a number of.... 1336
3) So at 110psi this means the force is 146,900
So if we desire that same surface force on the tire all the time (i.e. ride sensation) logic and math dictates our tire pressure should drop substantially as the tire widens substantially, not remain the same (unless puncturing more often is your preference)
I ran through all these calculations from a starting base point of 23mm actual width at 110psi and concluded an equation can be used to calculate psi for same ride sensation based on actual width.
tire pressure = 74069 x [(tire width) to the power of -2.079]
For those of you that use excel it would look something like "tire pressure" = 74069*power("tire width", -2.079)
Some examples (with rounding up)....
23mm 110psi
24.6mm 95psi
26.2mm 83psi
28mm 73psi
29.6mm 65psi
So the next time you are wondering why your 25mm Conti GP4000sii is jamming in your bike frame on your Zipp's or you think you need to run 25mm Gatorskins on your ENVE wheels because you always seem to puncture and you like your tires at 120psi remember...... the answer is almost invariably your psi is wildly off based the dimensions of your rim and the actual ensuing tire width.
Of course YMMV depending on your bodyweight but same principle applies.
Regards,
p.s. Brainiac's feel free to add/subtract wisdom :-)
David T-D
http://www.tilburydavis.com