Tire pressure Zipp 404's - Page 2 - Triathlon Week

05-11-2005, 06:56 PM

Thank you again Phil

My experience with clinchers, going past 120 psi caused the rim to weaken
and eventually fail. I even had a rear wheel literally blow-up on me after a
short ride to work, I locked it up and started to walk away- then
Ka-BLAM!!!! pieces everywhere. I don't recall what size tires I was using
but it was probably a folding 23 clincher, mavic reflex rim, usually at 110

Laz

"Philip Holman" <[Only registered and activated users can see links. ]> wrote in message
news:[Only registered and activated users can see links. ]...
>
> "Laz" <[Only registered and activated users can see links. ]> wrote in message
> news:[Only registered and activated users can see links. ]...
> >
> > "Philip Holman" <[Only registered and activated users can see links. ]> wrote in message
> > news:[Only registered and activated users can see links. ]...
> >>> >> The only two reasons I can think of for Zipp to limit tire
> >>> >> pressure
> >> >> are:
> >> >> 1/ due to the constriction force which reduces spoke tension. 2/
> >> >> The
> >> >> harsh ride above 120 psi.
> >> >> For 1/ , a 20 at 150 psi is equivalent to a 23 at 113 psi. For 2/
> >> >> your
> > Like I said,
> >> 150psi with a 20 puts less stress on the rim than a 23 at 120psi.
> >> Phil H
> >>
> >>
> > Hey, I missed that.
> > Could you take a moment and explain how that is for me please ?
> > Always one for science.
> >
> > Thanks in advance
>
> The constriction force of a tire with a 45 degree bias ply is pressure
> times cross sectional area.
> The actual constricting component is P*A*2*tan^2(a) and the expanding
> component is P*A so the resultant force for a 45 degree tire is P*A.
> The bias ply acts like a scissor jack where the hoop tension is double
> the longitudinal tension so the tire actually shortens in circumference
> when inflated.
> A 20mm tire at 150 psi exerts a 73 lb constriction force and a 23mm tire
> at 120 psi exerts an 80lb constriction force. These forces are so low
> that it is obviously not a reason for concern.
>
> I'm thinking that Zipp has limited tire pressure on clinchers with
> aluminum braking surfaces because of the reduced heat dissipation on
> long descents (mostly carbon rims) resulting in possible overheating and
> tire blowoff.
>
> Phil H
>
>