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Question regarding tire pressure...(10 posts)

Question regarding tire pressure...DINOSAUR
Nov 10, 2001 9:27 AM
I know this subject has been covered many times in the past but it has been in the back of my mind for a couple of weeks. I've been using Conti GP3000's and Performance Forte Pro Kevlar's exclusively for the past three years. Both have a max psi rating of 130psi. I've been inflating my tires to the max psi after experimenting with different pressures. I purchased some Michelin Hi Lite Prestige's that were on sale this summer. My rear Conti is just about due for replacement. The max psi rating for the Hi Lite Prestige is 110 psi. My question is can I inflat the Michelin to 120 psi without risk of a blowout? Also what determines the psi rating? I understand how tires are testing, but is it the threads in the sidewall casing? The reason this bugs me is because I blew out a front tire once and crashed and I don't want to go through this again. Thanks in advance for any input.....
Additional info..DINOSAUR
Nov 10, 2001 9:36 AM
The Hi Lites are fold up's. I blew out a Conti Ultra 2000 wire bead because of over inflation once..
Re: Tire PressureJon
Nov 10, 2001 10:02 AM
Dino,

I use Michelin Axial Pros which are rated @ 110 psi. However, I routinely inflate them to 120 - 125 psi and have
never had a problem. Ditto with Conti Grand Prix 3000. However, when in doubt be conservative; inflate
to spec. I say this because of your previous crash history.
Additional info..peloton
Nov 10, 2001 12:08 PM
Dino,

I have been using the Hi-Lites for most of this season as well in a 700x20. I run the front and the rear at 110psi and have never had a problem. I have tried to inflate them up to 115-120 on occasion and found that the rolling resistance didn't feel any different or result in any difference in times for the local ITT series but the ride was slightly harsher, especially in the corners. I would imagine that there is some margin of safety in the reccomended psi, as I never experienced a blow out or do I know anyone who has by going a little over on the Michelins. Also, even at the 110psi (checking it before every ride) I have also only had one pinch flat in several thousand miles and it was when I hit a rather large, square pothole at about 25mph. I think it would have happened anyway, regardless of pressure. I weigh about 190 FWIW. My hardly scientific opinion is that you could get away with running a little more pressure safely enough, but I don't know if it really is needed for the Hi-Lites to get a good ride quality or better flat protection.
A curiosityJofa
Nov 11, 2001 11:47 AM
It appears that the bead doesn't bear the effect of inflation in tension. Wire and Kevlar beads have been cut clean through in a few places, mounted on the rim and inflated to high pressure, and this way have performed normally. I've never done it but the reports are reliable- maybe someone else has tried this?

Jofa
re: Question regarding tire pressure...zzz
Nov 10, 2001 2:05 PM
Sheldon Brown has a good article on this at http://www.sheldonbrown.com/tyres.html
scroll down to Pressure Recommendations. I have been using open corsa cx's for quite a while now. They have a reccomendation of 105 to 135, 7 to 9 bar. I usually run 130 to 145 on the rear and 120 to 130 on the front. At the higher pressures I have noticed less traction on corners with rough surfaces but on smoother surfaces they feel much better than at the lower pressure. I try to match my pressure to my intended ride surface.
re: Question regarding tire pressure...Jofa
Nov 11, 2001 2:55 AM
It is the clinch between tyre and rim that fails in a blowout. The casing if undamaged can maintain much higher pressures as we see from tubulars.

The problem is that the tyre is only half of the mechanism: the rim may be responsible for the failure, in which case it is difficult for the tyre manufacturer to be precise about the limits of his tyres. Therefore as Sheldon says it is probably an arbitrary compromise between legal and marketing departments ("Battle of the Titans" our modern fable). In this case all you can do is experiment.

There is one condition in which to apply particular caution, however: long mountainous descents. Your braking will heat the air in the tyre and cause its pressure to increase significantly. This may blow the tyre off the rim if it began near the pressure limit. This is not uncommon amongst cyclists in those mountainous regions- in these cases, start with a lower pressure. You will appreciate the ride improvement anyway.

Jofa
re: Question regarding tire pressure...peloton
Nov 11, 2001 8:45 AM
Good point on the rim interface. Mavic is starting to reccomend that one doesn't inflate their tires over 140psi on any Mavic product regardless of the tire's reccomended max.

I think that higher psi's reach the point of diminishing returns in terms of reducing rolling resistance. I notice no difference in my time trial times when runing Michelins at 110, or say a Htuchinson at 140. The lower pressures sure feel better in the corners, and are more comfortable to ride as well. Over some of the rougher sections of the course, I feel the lower psi might actually be faster because they don't get bounced around as much, and keep some of the shock from my body. It's worth playing with the lower pressures to see what works for you. Higher isn't always better.
AgreedJofa
Nov 11, 2001 12:02 PM
Very high pressures always produce better RR in perfect situations but the advantage is usually vanishingly small even for paid pro's who weigh their meals. The rest of us ride our bikes for fun and satisfaction and enjoy the luxury of a wage which isn't determined by our cycling successes. We, more than pro's, can afford to be comfortable.

I know where you're going with the bouncing thing- I've wondered it myself, during a particularly unpleasant TT- but I suspect it's wishful thinking. I've not seen any data (it would be a difficult experiment to do) on this topic though.

Jofa
Thanks, questions answered...DINOSAUR
Nov 11, 2001 1:04 PM
Thanks for all the input. The Sheldon Brown site has a lot of info I was looking for, and I added it to my favorite's list. When I slapped on some Permormance Forte Pro Kevlars this summer and inflated them to the 130 psi max, I rode through the whole summer with only a couple of flats. My main problems are with pinch flats as I weigh 210. On my next ride I'll try a lower pressure with my front tire. The high psi doesn't seem to bother me with my rides, if I go with too low a psi my rear tire will bottom out.
Lots of food for thought, experimenting is half of the game. I finally got my bike dialed in, now I'm looking for a good tire combo. When I install the Michelins I'l try 120 psi with the rear and 110 with the front and give it a go...