On Monday, March 11, 2019 at 3:30:23 PM UTC-4, jbeattie wrote:
On Monday, March 11, 2019 at 10:36:06 AM UTC-7, Mark J. wrote:
On 3/10/2019 8:46 PM, Frank Krygowski wrote:
On 3/10/2019 10:10 PM, John B. Slocomb wrote:
On Sun, 10 Mar 2019 20:21:40 -0400, Frank Krygowski
wrote:

On 3/10/2019 5:52 PM, wrote:
On Sunday, March 10, 2019 at 8:34:24 AM UTC-5,
wrote:
I keep reading see all the bikes coming out and basically all disc
brakes. I cannot believe rim brakes are going to be gone but maybe
I am just kidding myself. I frankly hate the disc brake look and
certainly for a long time parts will be around but are these rim
brakes a dead deal.

Deacon Mark

One of my issues is that I realize we will have rim brake bikes for
awhile but I just hope to keep the nice looks and basic set up. If
it is not broke don't fix the puppy. The one item I have never
experience is the idea on a long descend you can blow a tube. In the
flatlands that to me seems impossible. To blow a tube on a long
descend does the speed have to be really fast like about 40mph or
say at 25mph for a long time. The biggest descend I have done is
about 7% grade total for about a mile and the last say 1/4 mile is
got to 9%. I could easily feather the brakes to avoid heat but maybe
my experience is really limited for true mountain riding. Can you
just pull the brakes up pretty good to get to a speed that is
comfortable. In my case this descend got me to about 43mph my top
speed for sure. Had the it been longer I don't know long I could
have continued before I got to damn scared.

Long, long ago I read a technical article in some bike magazine. (There
used to be real technical articles in bike magazines.) This one was
about brake energy (or really, power in the engineering sense of work
per unit time) and temperature rise during long descents.

The article explained that the braking power depended on brake force and
speed. For any given hill, you could always use the brakes to go super
slow. Brake force will be high, but speed will be low and power will be
low, leading to less temperature rise.

Alternately, you could descend very fast, braking only lightly or not at
all. Brake force will be low or zero. (There's also more aerodynamic
cooling.) This too will lead to less temperature rise.

The author claimed, using lots of calculations summarized in graphs,
that the greatest temperature rise occurred by using the brakes to keep
the speed about 30 mph or 50 kph.

Trouble is, this is exactly the speed lots of cyclists choose for long
descents. Any slower and they feel like slugs. Any faster and they get
scared.

I blew only one tire on a downhill, on the rear of our tandem, creeping
down a short ( 1/10 mile) steep grade well over 10%. We just rode the
bike to the bottomÂ* and I changed the tube. But I can see it would be a
problem if the front tire blew.

Phuket, Thailand has several extremely steep hills on the western side
of the island, steep enough that it is difficult to push a bike up
them.

Out of curiosity I did push the bike up one and coasted down the
eastern side.

Having read all the hoopalla about the rims getting hot and tires
blowing I stopped about half way down and felt the rims... they were,
perhaps, a bit warmer than ambient temperature.

But. As the east side of the hill is a series of "S" turns one can't
just coast down the mountain but must slow down every hundred yards or
so to make the next corner so my braking was a series of pretty hard
brake applications followed by, perhaps, an equal period of coasting..

I have since used that method when descending hills and an occasional
check shows that the rims do not get excessively hot.

Omega and others make temperature indicator dots. They turn black and
stay black when their rated temperature is reached.
https://www.omega.com/pptst/TL-C5_LABELS.html
They're single use products.

Back when Jobst was with us, there was talk of sticking these on some
rims and doing tests, but I don't recall if anyone actually did that, or
what the results were.

Yes, there were tests, I was a test subject. It was in about 1984. I
can't remember for certain, but my guess is the test was sponsored by
Buycycling magazine. I got sent the stick-on temperature indicators.
They were rectangular and had multiple, labeled temp. "windows". When
you reached the indicated temp, the window went (permanently) dark. At
some point I'm sure you sent in results, but I really can't recall that
part.

I'll look in my shop to see if any of the rims (and stickers) are still
around, but I doubt it. In those days we were running 36 spoke rims and
25mm (mis-labeled 1-1/8) Specialized Turbos, and rear rims lasted mostly
a year or two, mostly dying of spoke-hole cracks, and sometimes of
pothole-induced flat spots. Oh to be young and greyhound thin again!


I don't think the selling point for discs is that they prevent your tires from exploding. There has never been a tire-exploding epidemic from over-heating. Like Frank, I blew one tire on a tandem front descending Rocky Point on a hot day. The wet version: https://www.youtube.com/watch?v=-NPqQptjbF0 My wife was on the back yelling at me to slow down, so I complied. This was on a tandem with cantis and no drum or disk brake -- and the rims did get very hot. Discs get super hot on tandems and thus the mega giant 203mm rotors. Tandems are a special case.

Brake fade is a problem with all brakes -- and discs probably get less fade than rim brakes, so heating matters, but the likelihood of blowing a tire off the rim due to over-heating on a road single is pretty remote and not why one would or should buy discs. That's not even something I hear from zealous sales people. The usual pitch is better modulation and stopping in wet weather.

Rim heating was an issue in the tubular days because it didn't take tire-popping heat to soften tubular cement. Having squirming tubulars on a long descent was not uncommon, particularly in hot climates. It was one of the selling points of the new crop of light clinchers in the mid to late '70s. I think discs would be an easy sale to the mountain-climbing clydesdales using CF rims and tubulars.


I've never blown a tire from heat, but I did get hit on the back of the thigh with a small dot of hot glue going down flagstaff road in boulder once.