Author Topic: Disc Brakes or Rim Brakes?  (Read 9740 times)

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Offline HappyHamster

Disc Brakes or Rim Brakes?
« on: January 25, 2008, 09:06:32 pm »
For loaded (road) touring, how essential are disc brakes versus cantis?  I am considering getting a new frame and have option of discs or cantilevers.  The discs will most probably be cable actuated (Avid road models) but not sure if they are really necessary. Of course, if straight bars are used, hydraulic units could be used. Any comments would help.  Thanks.


Offline staehpj1

Disc Brakes or Rim Brakes?
« Reply #1 on: January 25, 2008, 09:14:11 pm »
Most dedicated touring bikes do not have disk brakes.  I would opt for cantis.  Easier to mount racks, less stress on spokes, easier to change flats. and so on...


cyclesafe

  • Guest
Disc Brakes or Rim Brakes?
« Reply #2 on: January 25, 2008, 10:41:04 pm »
Those are the downsides.  Upsides of mechanical disks include better stopping power especially under wet conditions and no blowouts from overheated rims while braking on descents.  Hydraulic brakes, however,  might be a maintanance concern. IMHO, for touring, mechanical disks, V's, or canti's all work fine.  Personal preference.


Offline whittierider

Disc Brakes or Rim Brakes?
« Reply #3 on: January 26, 2008, 12:03:09 am »
Our tandem with inexpensive Tektro mini-V rim brakes and STI levers can lock up even the front tire on dry pavement with one finger on the lever.  (I found out by accident as the 350 pounds of us riders plus bike hurried down a mountain grade.)  You won't find better braking than that.  I only rode it once in the rain, and the braking seemed to be just as good, even right after going through one to two inches of water, due, I suppose, to the machined rim braking surfaces.  While most rim brakes do poorly when wet, I saw some brake comparison results online that showed this particular kind of pad to not suffer in the wet.  The web page no longer exists or I'd post the URL.  The advantage to disc brakes would be that sand stuck to damp rims doesn't wear them out when you brake.  I've heard of rims wearing so badly that the force of the inflated tire broke them out.

We have the Arai drum brake on the tandem as a third brake for drag on long, twisty descents, but seldom need it.  Unless you're quite heavy, I wouldn't worry about overheated rims on a single (non-tandem) bike.  I was on the tandem@hobbes forum for several years and the subject of discs came up frequently.  It seemed that the greatest reason people wanted them was that they looked cool.  They weren't up to drag duty like the Arai drum brake (they would warp, sometimes melt caliper parts, or, if hydraulic, even boil the fluid) but the ones that did fine otherwise were the 8"-diameter ones, and, if I remember correctly, the favorite was the Avid ball-bearing mechanical ones.  A common complaint however was that they wore out pads extremely quickly, sometimes in less than a thousand miles.

This message was edited by whittierider on 1-25-08 @ 8:03 PM

FredHiltz

  • Guest
Disc Brakes or Rim Brakes?
« Reply #4 on: January 26, 2008, 09:16:04 am »
Unless you're quite heavy, I wouldn't worry about overheated rims on a single (non-tandem) bike.

It can happen in the mountains, of course, with average weight people and touring bikes. At 225 pounds bike plus rider, I recall stopping five or six times on the way down from Tioga Pass to Mono Lake to let the rims cool. (If they are too hot to squeeze firmly with bare fingers, they are too hot.)

Not a problem if you are not hurrying, though. The big descents invariably have great views to admire during the five-minute wait.

Fred


Offline whittierider

Disc Brakes or Rim Brakes?
« Reply #5 on: January 26, 2008, 11:54:41 pm »
Quote
If they are too hot to squeeze firmly with bare fingers, they are too hot

Not really.  I've heard of tandem rims getting to hundreds of degrees, and I've gotten them hot enough that touching them produced an instant sizzling sound on the skin.  I don't recommend getting them that hot-- all I'm saying is that you probably don't need to be so afraid if you're on a single bike.  One of the dumbest things I've ever done on the tandem was riding the rim brakes as we came down a grade of 8% or better in order to hold the speed down to 55mph (tandems tend to go much faster downhill than single bikes), then braking harder to stop, completely forgetting to use the drum brake.  Immediately after, I realized my stupidity, and touched the front shallow, silver-colored rim, and that's when I got the sizzling sound and crusty, dried-out thin outer layer of skin that took a week or two to go away.  There was no tire failure though, and the wheels stayed perfectly true.  Black anodizing and deep V rims would have been able to get rid of the heat a lot better than shallow silver-colored rims.

This message was edited by whittierider on 1-26-08 @ 7:58 PM

FredHiltz

  • Guest
Disc Brakes or Rim Brakes?
« Reply #6 on: January 27, 2008, 09:25:30 am »
I have seen three half-bike riders with blown tires on mountainous descents. None of them me! Of course we have no measured temps and no controlled experiments, but my rule of (literal) thumb says the rim is OK to resume riding when I can touch it, knowing that it will be much hotter just before my next stop.

I agree with Whittierider about the rim's design. More aluminum can absorb more heat, black radiates more heat, and more surface area dissipates more heat by radiation and convection.

Remember, folks, the rubber and the aluminum both get weaker when hot, just as the tire pressure spikes. And if you go down on a steep road you will slide a long way. Take it easy.

Fred


Offline whittierider

Disc Brakes or Rim Brakes?
« Reply #7 on: January 27, 2008, 04:55:47 pm »
Interesting.  I wonder if they just started with the pressure too high to allow for further increases from temperature.  The strength of the tire is not in the rubber, but in the strings underneath.  Five-time Tour de France winner Miguel Indurain wanted clincher tires when there were going to be twisty descents, because he was heavier than the other riders and the rim heating made the glue unable to hold tubular tires on.  This is what caused Beloki's crash in the 2003 Tour when Lance went off the road to avoid hitting him.  The hot glue let go, the tire came off the rim, and down he went.  I know this is getting partly O.T., but related background info helps make good buying decisions.


Offline WesternFlyer

Disc Brakes or Rim Brakes?
« Reply #8 on: January 27, 2008, 06:53:59 pm »
I have noticed my rims getting hot and have had tire failures that may have been related.  Also riding on hot summer pavement on tires inflated with cool  morning air have resulted in blowouts.  Reading these posts has me thinking about pressure relief valves.  In my past life with marine and aerospace designs I have used pressure relief valves in a number of situations at lower pressures 10 psi to .01 psf.  I did a quick search and found a source for relief valves for automobile racing tires.  There might be an opening for an entrepreneurial cyclist to adapt some existing technology.  http://www.diamondracingwheels.com/TireValves.htm

Western Flyer
Western Flyer

We must ride light and swift.  It is a long road ahead.

King Theoden

Offline biker_james

Disc Brakes or Rim Brakes?
« Reply #9 on: January 28, 2008, 08:10:11 am »

Quote

Our tandem with inexpensive Tektro mini-V rim brakes and STI levers can lock up even the front tire on dry pavement with one finger on the lever.  (I found out by accident as the 350 pounds of us riders plus bike hurried down a mountain grade.)  You won't find better braking than that

Is anyone else getting this good of braking with STI's? It sounds pretty impressive being able to lock it up with one finger, certainly better than what the people I ride with get on their single bikes, whether using V's or cantis.


Offline RussellSeaton

Disc Brakes or Rim Brakes?
« Reply #10 on: January 28, 2008, 10:23:02 am »
Beloki's crash in the Tour was due to his rear tire slipping on the oil on the asphalt.  He and Armstrong were going around a turn and he hit an oil spot on the asphalt.  Oil due to the day being hot and the oil bleeding out of the asphalt.  His rear tire slid sideways easily on the oil spot and then abruptly came to a stop when the sliding rear tire came to the edge of the oily spot and hit the non slick asphalt.  At this point the sudden stopping of the rear wheel sliding sideways caused the rear tire to come off the rim and the rear rim to break.  The rear tire came off the rim due to the sudden and forceful side pressure applied to the tire.

I've never had much trouble keeping rims cool going down mountains.  With a loaded touring bike you can just sit up and catch lots of wind to slow you down.  The panniers also catch considerable wind.  Roads are designed for cars to safely descend them at a speed faster than most bicyclists are comfortable with.  I've always figured I can go safely at least as fast as cars down the roads.  If I do need to brake, then I apply the brakes hard and come to an almost stop.  Then coast up to speed.  Applying the brakes for a brief time hard, and then letting them and the rims cool while coasting up to speed prevents any overheating problems.  Applying the brakes constantly at a light pressure is what causes rims and pads to heat up.


cyclesafe

  • Guest
Disc Brakes or Rim Brakes?
« Reply #11 on: January 28, 2008, 07:30:51 pm »
Anyway, you avoid the issue of hot rims with disk brakes.  I didn't know that STI brifters work with Tektro v brakes.....


Offline whittierider

Disc Brakes or Rim Brakes?
« Reply #12 on: January 28, 2008, 08:07:36 pm »
Quote
I didn't know that STI brifters work with Tektro v brakes.....

They have to be mini-V's, otherwise you'll need a cable-travel adapter.


FredHiltz

  • Guest
Disc Brakes or Rim Brakes?
« Reply #13 on: January 29, 2008, 08:55:59 am »

Quote

Applying the brakes for a brief time hard, and then letting them and the rims cool while coasting up to speed prevents any overheating problems.


A good point, Russell. I do this when the road is not too steep to prohibit it and think it helps a little, but doesn't prevent overheating entirely. There's a fixed amount of energy to dissipate, and most of it goes by radiation from the rims (or the disk). The physicists among us would say that the most radiation happens at the highest temperature you dare to use, but how well can we know that?

Fred


Offline DaveB

Disc Brakes or Rim Brakes?
« Reply #14 on: January 31, 2008, 11:02:50 am »
The technique Russell described for heavy-duty long-downhill braking is the one recommended by most experts I've read too.  Appling the brakes HARD near the corner and letting the bike run free as much as you can keeps the rims cooler and the braking more predictable than steady but lighter pressure.