Several years back I heard some cyclists talking about the need to stop a few times coming down one of the local hill climbs/descents to let the brakes cool so they didn't have to worry about having the tire blow off the rim. I'm talking aqbout a 3.7 mile long, 2200 feet climb/descent that is quite twisted with 2 or 3 hairpin turns. Constant braking is pretty much a necessity.

Talking with another cyclists today after finally climbing and descending the hill he was saying that with clincher tires their shouldn't be any issues with constant braking coming down the hill. The problem would only be a problem if you were riding on sew ups.

Is this notion correct. I know my rims were getting quite warm and I was puppy them and did get off the bike a couple of times on the way down and walked a short distance down as well while letting the rims cool down. Was any of that necessary?

Clincher rims have lips that kinda hooks tire beads, so there is some kind of mechanical attachment. Sew ups are dependent solely on your "glue", and this is where a problem can arise, if the adhesive were to fail. I have never heard of an alloy clincher rim failing on descents. It is definitely possible with full carbon clinchers, however.

Full carbon clinchers are another kettle of fish, because of the brake tracks getting overheated, which can result in "warping" or failures of other kind.

Yes, it is an issue with clinchers. It's worse for high-pressure, hot roads, heavy riders and tandems, and long descents.

If you sort of drag one brake all the time, the heat just keeps building up and if you're unlucky, you'll have a blowout - almost always on the rear... not a flat, an explosion. There is some disagreement about what actually causes the blowout (heat causes pressure increase but not enough for this result) but it does happen and your rim will be seriously hot when it does. The good news is, your rim will cool off pretty quickly once you let off the brake, so the proper technique is to feather the brakes on and off to control your speed rather than constant braking at less pressure.

It takes a decent amount of energy to cause enough heat buildup to be an issue with both clinchers and tubulars. But it's definitely possible for heavier riders, or tandems, on long steep grades.

Even cars have to deal with overheated brakes, so it isn't anything to totally ignore.

The first step in dealing with it is to manage speed without using the brakes. In a car you'd downshift so the engine drag helped control speed. Bikes don't have an engine, but one sure way to keep speed down is to raise your body and catch more wind on straight sections. The other is to allow the bike to gain more speed, where wind absorbs more energy, then brake to scrub off more speed, and repeat the process. Whatever drag you can get from the wind, relieves the load on your brakes.

If you watch professional racers, you'll note that none ever need to stop due to rim heat, including those on tubulars. That's because they have to safely ride at terminal velocity for most of a descent, and only need the brakes in short bursts before hard turns. Improving descending skill to handle more speed, knowing your limits in corners, and using air resistance to advantage is key to preventing rim heat.

If that isn't enough, you will may have to stop and rest from time to time, but until you're familiar with your rate of heat buildup, odds are that you have no reliable way of knowing how often or how long it's needed.

Bigger riders, don't forget that weight is a big factor here. Heavier riders don't get as much help from the wind (proportionately) and so have higher terminal velocities, plus scrubbing off speed involves more energy, so while your 160# friend is doing fine, you might have problems on the same descent.

A while back I did a ride that featured a 6 mile long, 4,000ft descent. Continuous 10% downhill, with stretches of 13-14%. Here's the route: https://ridewithgps.com/routes/1055578 . My tires held up fine, even though I was dragging them to keep my speed within my wife's comfort zone. I'm about 220 lbs., and we both ride faired recumbents- a recipe of lots of speed.

My only problem was that the heat soak melted the #%^*&$%!! plastic rim strip, causing an internal blowout- twice. I had to be sagged in while my wife rode the rest of the way. Oh, the shame. It's Velox for me, forever!

You rode the sag wagon, while your wife bombed down that hill?

That's what I heard several years back and hence why I took it easy. I did end up feathering the brakes most of the way trying to use only one brake at a time. Granted their were a few spots where I had to make up for the feathering though and apply both hard and I would always end up hearing a change in the pitch of the noise coming from the brakes. I have heard that once before as well on a MUCH shorter descent when I was braking hard.

The thing I really wish I knew was how much heat is too much heat. I do have metal rims versus carbon rims so that does help me a bit.

This is the funny thing. As I was at the top of the mountain I was thinking too darn bad I'm not like a car and I can't use the engine to help slow me down. I've driven down this particular road so many times when helping out hang glider pilots it's crazy. I love getting in all the different pilots vehicles and seeing which one has the best gearing. The most recent vehicle was incredible. I think I only put my foot on the brake twice all the way down and the engine barely ever made it over 3000 rpms. I hadn't seen any other vehicle even come close to that yet.

Here's the road I was on: https://ridewithgps.com/trips/706822

It happened to me ONCE. I just wasn't thinking and it was early in my mountain riding days and I knew it could happen but wasn't thinking that it actually might. As soon as it did - whoa... ringing ears and a welt on my calf... I immediately remembered that I was supposed to remember that it could happen and kicked myself. Went to take off the rear wheel to change the tube and ZING - burned my fingers on the rim (I'm pretty sure it would've been literally red hot if it weren't for the annodizing ). Triple reminder never to not think about it again.

I've heard other folks say that they've seen steam (real 212°F STEAM) coming off their wheels from when they were descending mountain passes at snow level with a little melted snow in the wheels. OK... water boils at a lower temp at higher elevations but boiling hot is still hot at 8000 feet.

Now that's the weird thing. I must fess I have never been the good boy biker but I have adjusted the way I do things so I've never had any trouble with in all the riding I have did. I've always been a front braker. I hardly ever use the rear brake in normal riding, only when I have to stop real fast or have to scrub a lot of speed fast. Yesterday when coming down, whenever I was stopping the first thing I was doing was reaching down to check and see how warm the rims were. The front rim was always much warmer than the rear rim...even when I swapped over to feathering while trying to favor using the rear brake more to save the front rim/brake for more powerful slowing. The front rim would always be warmer. It was was never too hot to touch and keep your hand there but it was hot.

Have descended Kitt Peak near Tucson, AZ.
About 8.000 ft elevation, 6 to 7% grade, twisting turns for 11 miles on our tandem bike back in 1978.
Utilizing front and rear Mafac canti brakes. No third brake. No tire issues.
Did have to stop halfway down as fingers were cramping from constant on/off braking. Felt the alloy rims; they were warm but by no means hot.
Continued descent without interruptions.
Justy our experience.

Yessir. A couple days later, we had a bear cross the road 100 yards in front of us. And then a couple raccoons starting fighting and bouncing off of tents that night. Heck of a ride.

IIRC: heat build-up is maximized when you hold your speed to 1/2 of the air-drag limited terminal velocity. That is, if you let the bike coast to its maximum speed, "feathering" the brakes to half that speed will put the most heat into the brakes.

This is a problem velomobiles (streamlined enclosed recumbents) encounter. They can descend so fast, they quickly overwhelm most bicycle brakes. They tend to pulse their brakes: slow almost to nothing, then let gravity accelerate them to maximum while the brakes cool. FWIW: descents at 70+ are not uncommon:
https://www.youtube.com/watch?v=pzL2ww-0QVg
https://www.youtube.com/watch?v=ncyt6Ofevv8

Seems to me, folks doing a lot of this should be running discs. Particularly recumbents and tandems...

197.5 degrees. Also, boiling is bubbling. You can have vapour rising from water at much lower temperatures; that's a result of vapour pressure which you get in any liquid, which increases with temperature.