Incorrect Information re: Tire Pressure in Efficiency Mods

[j12piprius]

Originally Posted by PaleMelanesian

All I'll say is I have mine into the "margin of safety" range.
I backed down a little when the back end started skittering sideways on speed bumps.

Would you consider 50 psi to be within that range for these tires?

[ktaylor]

Originally Posted by NeilBlanchard

My guess is that too low or too high pressure would each have different stresses, and neither would be good for the tire's integrity?

This is true. I learned about fatigue in 2nd year materials at UBC and while both instances of too high and low pressure have negative effects during operation, I believe higher pressure is much worse. Ductile materials (rubber) fail in tension, not compression. As you increase the internal pressure, you increase the tensile stress in the tire. The average pressure is what you would plot on an S-N diagram (refer to the wiki page reffered to) where S is average stress and N is cycles to failure. If you increase stress (pressure) by 2x, you greatly reduce the number of cycles to failure (much more than half) due to the nature of the curve in the low stress region.

Some tires when "properly" inflated have a bulge at the bottom. This adds another type of stress which propagates through the tire as it rotates. When inflating tires closer to sidewall max the bulge disappears. The reduction of those additional stresses may negate the additional stress added internally, although that bulge is material in compression and as stated, ductile materials fail in tension. Either way its not like you're living on the edge if you put your tires to sidewall max. Tires are tough and take a hell of a beating in their life.

On that note, everyone should check when their tires were made. Theres a 3 or 4 digit code written on the sidewall in an oval stating the week and year they were made. I belive the first 2 numbers are between 1 and 52, for each week of the year followed by 7 for 1997 or 05 for 2005, etc. for the year they were made. You should google it because after 8 years I believe, tires are pretty much garbage (Yes they have an expiry date, google it).

[Frank Lee]

Haw haw haw- throwing tires out cuzza the date of mfg! Run 'em til they pop.

[Xringer]

I keep a close eye on my bicycle tires. When the age, they get little cracks & splits in the side walls.
That's when I decided it's time to replace them.

[Frank Lee]

My understanding of it- especially as it relates to gumwall bicycle tires- is that the main purpose of the "rubber" is to protect the cords. I've had gumwalls so weatherchecked the gum came off in chunks but the tire still held air.

P.S.

Once the fabric has been woven between the beads, and the tire has its basic shape, it is coated with rubber. The rubber is mainly there to protect the fabric from damage, and has no structural importance.

http://sheldonbrown.com/tires.html

[Xringer]

Originally Posted by Frank Lee

My understanding of it- especially as it relates to gumwall bicycle tires- is that the main purpose of the "rubber" is to protect the cords. I've had gumwalls so weatherchecked the gum came off in chunks but the tire still held air.

P.S.


Bicycle Tires and Tubes

That's sounds true to my ears. But, if the walls are cracked, that means the
weather can get to the cords. So, it's only a matter of time.

I'm running tubes, loaded with that green goop. Keeps them from leaking down so fast.

[wdb]

Originally Posted by Xringer

That's sounds true to my ears. But, if the walls are cracked, that means the
weather can get to the cords. So, it's only a matter of time.

Agreed. Exposed cords = time for the trashcan.

[Arragonis]

You wait for the cords to appear ? Jeebus.

I just check these regularly. The law here is a minimum tread depth which these are set at...

[Frank Lee]

Aw, you're tossing them right when they're about to get the best fe!

[Arragonis]

Last time I checked LRR tyres, non were slicks.