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Airing down on a steep descent?(20 posts)

Airing down on a steep descent?BenLomond
Nov 3, 2003 10:46 AM
friend of mine got a blowout on a very steep descent(jamison creek)and thought it was caused by over heating his rim and tire. Does this sound plausable? Would it be wise to drain 10 or 20 psi before I descend? I could air up at the bottom.

The road is steep, rough and twisty, with speeds between 10-20 mph.
Hugh?KG 361
Nov 3, 2003 10:50 AM
10-20mph? Most of us here do that on the FLATS. A steep descent? Try 35-55 mph. The blowout was just a coincidence. I've NEVER had that happen at sustained speeds above 30mph w/ some sharp turns and having to grab the brakes, big-time.
Nov 3, 2003 11:31 AM
On most descents I go as fast as the next guy, but this road is very rough and very tight. I ride the brakes most of the way down. I may go a bit faster than stated, but not much. If you think it's poor technique(I'm not saying it isn't)I would be happy to follow you down the road.
Hugh?KG 361
Nov 3, 2003 11:44 AM
I wasn't criticizing you, it's just that 10-20 mph isn't that fast. No mater; that didn't cause the tire to blow.
Nov 3, 2003 1:02 PM
I don't feel criticized. I was asking this because, I had never heard of such a thing and wanted peoples ideas.
I was not trying to be defensive by saying I'm a competent descender, just giving more info.
might depend upon pressures to start withDougSloan
Nov 3, 2003 11:44 AM
If you were 20 psi over the recommended tire pressure to start, then a lot of braking could cause a tire bead to blow off a rim, I suppose. However, if you are not starting well over the limit, I doubt it would. I routinely ride in big mountains near here with long, twisty descents, and never had a problem. My guess it was from another cause.

I'd be worried about having too little air in a tire by bleeding off and having a pinch flat.

Get empirical about it. Take a tire pressure gauge and check the pressures at the top and bottom of the hill. That may tell you something.

I just did a quick calculation.boyd2
Nov 3, 2003 11:57 AM
If you start with 100 psi at 70F and you heat up the tire (and all the gas) to 160F by long braking the pressure in the tire would be approximately 119psi. This assumes that the whole tube and tire heat up evenly. It is a worse case assumption. It is probaly not enough a pressure increase for a total failure. Now this analysis is only for static pressure. Localized heating of the tire bead may be the primary failure mode.

For reference I am an engineer that designs aerospace pneumatic systems.
~650wt to dissipatecyclopathic
Nov 3, 2003 12:08 PM
assuming 200lbs bike/rider going down 10% grade at 20mph.
Don't you think it may end up more then 160F?
~650wt to dissipateBenLomond
Nov 3, 2003 1:15 PM
bike and I= 225lbs, the average grade is 10-11%, but there are sections of over 15%. My tire inflation is 110psi.
Just out of curiosity I'm going to do what Doug suggested and check psi at top and bottom. I will post the data in case anyone cares.
Nov 3, 2003 2:51 PM
put in negative grade ~850wt
~650wt to dissipateAndy M-S
Nov 3, 2003 1:20 PM
I think that the WCA above assumed that there *wasn't* any dissipation. Remember, you have a fair breeze blowing by the rim even at 15 there's some cooling going and it's unlikely that the rim would get much warmer than 160...
Nov 3, 2003 3:39 PM
All braking heat is dissipated through convection to the air ultimately, right? I would think a fair amount of it is dissipated during the descent. Also, some is going to be conducted to other bike components, like spokes, brake pads and caliper arms, etc. Big heavy rims are a better heat sink to begin with, and would require more braking before heating the tube air excessively, right?

I don't know how you could ever even begin to calculate all this, throwing in the pavement temp, air temp/direction, rate of slowing, etc.

Nov 4, 2003 9:45 AM
there're tables with physical constants (probably online), which would tell you how much heat can be dissipated through Al to air surface. You'd need to calculate surface of the rim (including internal for double wall rims), know temp of air, both inside and outside rim. However airflow, uneven heat distribution (heat generated by brake pads is confined to braking surface) makes things more complicated it would be probably easier to build test machine to test it.

I'd suspect rim/brake pad mfg test their products, you might have better luck writing them.

One thing obvious: deep aero rims would be better heat sinks.
might depend upon pressures to start withBenLomond
Nov 3, 2003 12:52 PM
I will do that.
re: Airing down on a steep descent?utxjohne
Nov 3, 2003 12:26 PM
I flated at over 30mph, on Sat, coming down bear mountain which is quite narrow and twisty. I actually was a little bit underinflated, hit a bump causing the tire to pinch, and flat.
when you say "blowout"PmbH
Nov 3, 2003 1:54 PM
do you mean a catastrophic failure? Or just a flat?

The reason I ask is because of a descending/flat-tire phenomonon I've had in the past:

I've had single-wall rims heat up enough on descents that they transfer a LOT of heat to the (brass) spoke nipples. The hot spoke nipples have actually melted a hole through the tube, straight through a layer of Velox rim tape!
Could be the rim tape shifting...Lon Norder
Nov 3, 2003 4:53 PM
The heat can soften the glue that holds the rim tape in place. Then the tube can push the tape aside and herniate into the nipple hole. Then, KA-BLOOIE!

I had a pair of FiR rims that did this a number of times. I had to put two layers of strapping tape over the rim tape to prevent this. Never had any problems with Open Pros, though.
Could be underinflationlyleseven
Nov 3, 2003 10:09 PM
This can be caused by using a tube too large and heat build up from the braking on the steep grade. Also, underinflation can contribute to problem. I was always told that you can use a smaller than normal tube, but not a larger than normal tube for any given tire.
Stay off the brakes? -nmfiltersweep
Nov 4, 2003 6:59 AM
yes, had this happenDougSloan
Nov 4, 2003 7:56 AM
I tried that blue Ritchey nylon rim tape, and at the bottom of a long, steep descent the tube blew. The tape had moved just barely enough to allow the tube to burst. Back to Velox for me.

The wide Velox is stiff enough and wide enough that, I suspect, even when it overheats it stays in place. That narrow thin stuff is risky.