New to downhill grades

[Westinghouse]

On a long steep hill I let it coast until I reach a speed I think is getting to be too high. Then, I alternately pump and ride the brakes until the speed is not far from a standstill, and let it coast again. I do not allow attaining a downhill speed that I cannot reduce to a standstill in a reasonably short space of time should anything unexpected come into my path. A bit overly cautious perhaps, but I have been over countless hills and mountains and haven't that the first problem with it except only once when the pads and rims got wet and some debris of some kind got between the pads and rims. I hit the brake and discovered my braking power was about 95 % gone. Talk about sheer terror. There I was going down a hill with an intersection at the bottom with plenty of traffic, and me with almost no brakes. Luckily I had started braking far enough before the bottom of the hill that I was still able to come to a halt before entering the intersection. That happened only once, and I learned a lesson from it. Maintain your braking power. As long as you have normally good brakes you should not have to worry about it on the ST route, but read up on the subject, study safety practices, and take those safety pratices seriously. An ounce of prevention is worth a pound of cure.

[DaveB]

Consider a manikin, which would experience no wind chill. It would, however, stir up the air, warming it and the air in the process. That's where almost all the energy in a no-brakes descent goes, and it can be a significant part of our own descents.

Fred

The energy dissipation from air resistance is non-linear and varies with the square of the speed so, to a good approximation, decending at 30 mph dissipates about 4 times the energy that decending at 15 mph does.  That's why letting the bike run faster on straight road sections and braking hard before entering a turn requires less overall heat absorbtion by the brakes than using the brakes continously and maintaining a lower speed everywhere.   

[paddleboy17]

In 2000, I rode the Cabbot Trail on Cape Breton Island.  I still remember vividly riding down from the crest of French Mountain, in a light rain, on a fully loaded touring bike.  Yes being concerned about staying in control, glazing brake pads, and overheating rims was very stressfull. I cannot tell you how steep the grade was, but out east, they do not believe in switchbacks, and Canada engineers roads to a lower set of standards than the US does.  Plus, the maritime provinces are the poorest part of Canada.  So I am comfortable with believing the grade was steep.

When I built up my Waterford, I added a disc drag brake.  I have concerns about disc brakes in general shearing spokes on touring bikes during panic stops.  Not every one agrees with me, so let us leave it at that.  In a drag brake capacity, disc brakes work really well.

In 2009, on a tour in southern PA, we came off a ridge and descended into a valley to find a hotel for the night.  It was a very steep descent.  I used my drag brake, and had a nice ride in.  My buddy has V brakes, and cracked his rear rim on that descent.  Now granted it was not a particularly good rim, and we managed to finish our tour on the rim.

[lilfilly]

Thank you all for all the wonderful information that you all gave me.  Being rather new to touring (I just started last summer), I've got a lot to learn, and I admit that with no sence of embarassment what-so-ever.  Other people's experience is my best teacher.  I had an accident on my bike last year causing me to lose some of the mobility in my left hand and almost to lose a finger.  The words "hey, look at that!" and "LOOK OUT!" do not go very well together, needless to say I got my fingers caught between my handlebars and my husbands rear tire.... to after spending a couple hours in the ER and a week in serious pain... I take bicycle safety very seriously.... and I will put 100% into preparing for our ride out west.  Thanks everyone, you taught me a lot!

[FredHiltz]

... The problem is that you're thinking in linear terms, and heat and fluid mechanics are anything but.  When you ride the brakes, or lock them up, you're not going to reduce the rims' temperature -- you keep dumping heat in, so some is lost to air cooling, and the rest goes into the rubber of the tube, which eventually flows into the air in the tube.  If you pump the brakes, you'll generate a heat spike, but when you let off, MORE heat is lost to the air flowing around the rims than if you kept dragging the brake, because the temperature spike is higher, heat flow is nonlinear, and you give the rims a few seconds to cool off.  Also, this cooling is more efficient while you're riding than when you stop, because the air velocity around the rim is much greater while you're rolling than you're stationary, and convection is much more efficient at heat transfer than radiation (which is why the color of your rim doesn't matter).

No question, convective heat loss varies strongly with speed, and is the least when stopped. The question is whether pumping the brakes keeps the rims cooler at the same speed.

Now I warn non-techies to skip ahead while the engineers go at some of the smaller parts of this issue: the second-order effects <grin>. There are two: heat flow within the metal and radiative heat loss to the air.

I maintain that heat flow within the metal is linear. Double the difference in temperature between two points and twice the heat will flow between them. That is, the thermal conductivity is constant. Yeah, if it gets hot enough to shift crystal structure or to melt, the conductivity changes, but we won't go close to such phase changes.

Yes, there is a temperature spike, and radiative heat loss varies as the fourth power of temperature Kelvin. Cycling the temperature a few degrees above and below the average temp of steady braking will indeed lose more heat, but how much more? That is, how big is the spike? My experience (call it gut feeling if you like) is that the "spike" is just a few degrees, making a tiny difference.

This is amenable to calculation. We know the mass of a typical touring rim. We can look up the heat capacity of aluminum. And we know how much energy a five-second application of the brake on a 100-Kg vehicle at 25 kph will dump into that rim. How much will the temperature rise? It's late and I could not compute this confidently. Maybe someone will jump in with the numbers before I take a crack at it Saturday. No time tomorrow.

Fred

[cotterg3]

I am too tired to think about this but I am not too tired to do the calcs (with a little help from google).

Equations:

-Translational Kenetic energy: KE = 1/2*m*v^2
-Heat Equation: Q = m*Cp*delta_T

Assumptions:

-100% of kinetic energy loss of the bike is converted to heat generation in the rims. In other words, delta_KE = Q (no additional factors or losses considered).
-Rotational Kenetic Energy loss is considered negligible.
-That 5 seconds of braking slows a rider from 25-20 kph.
-This basic calc only applies to overall heat gain of the rim, and does not take into account that the heat will be concentrated on the rim's surface.

Data:

-Mass of rims = 620 g * 2 = 1240 g (alex adventurer)
-Mass of bike/rider/pack = 100 kg
-Specific heat of alumnium = 0.902 J/g-°C

Calcs:

25 kph = 6.95 m/s
20 kph = 5.56 m/s

KE[25] = 0.5*100 kg * (6.95 m/s)^2 = 2415 J
KE[20] = 0.5*100 kg * (5.56 m/s)^2 = 1545 J

Q = 2415 J - 1545 J = 870 J

870 J = 0.902 J/g-°C * 1240 g * delta_T
delta_C = (870 J) / (1118 J/°C) = 0.778 °C

Overall temperature rise of both rims = 0.778 °C = 1.401 °F

EDIT: This is the OVERALL temperature rise of the entire set of rims, which potentially gives some insight as to what kind of braking it will take to potentially heat the rim enough to pop a tube. Of course, the temperature rise will be initially concentrated on the surface of the rims and not immediately distributed evenly throughout the metal as my calculations suggest. Hence, pdlamb's comments regarding temperature spikes, cooling, rims feeling hotter, etc. are totally valid since they specifically address the rims surface where the temperature rise is concentrated.

[Tandem4Rider]

Whoa, hold it right there!  Some of us read this forum like the morning paper, and to see all that math before the sun rises...  have some compassion, please!  On behalf of the others here that were history majors and now history teachers - type all those calculations a little slower so we can keep up, thank you. 

[Pat Lamb]

... The problem is that you're thinking in linear terms, and heat and fluid mechanics are anything but.  When you ride the brakes, or lock them up, you're not going to reduce the rims' temperature -- you keep dumping heat in, so some is lost to air cooling, and the rest goes into the rubber of the tube, which eventually flows into the air in the tube.  If you pump the brakes, you'll generate a heat spike, but when you let off, MORE heat is lost to the air flowing around the rims than if you kept dragging the brake, because the temperature spike is higher, heat flow is nonlinear, and you give the rims a few seconds to cool off.  Also, this cooling is more efficient while you're riding than when you stop, because the air velocity around the rim is much greater while you're rolling than you're stationary, and convection is much more efficient at heat transfer than radiation (which is why the color of your rim doesn't matter).

No question, convective heat loss varies strongly with speed, and is the least when stopped. The question is whether pumping the brakes keeps the rims cooler at the same speed.

Now I warn non-techies to skip ahead while the engineers go at some of the smaller parts of this issue: the second-order effects <grin>. There are two: heat flow within the metal and radiative heat loss to the air.

How's that old saw go, a 15 minute experiement in a lab can save a week at the blackboard?  I'm going to suggest finding a good steep hill, 1/2 mile at 15% to 5 miles long at 6%, and try it both ways -- drag both brakes down, then alternate brakes, and check the temperature at the bottom.

And forgive me, I'm a techie; when you said "radiative heat loss to the air" I think you really meant "convective heat loss."  There is no significant radiative heat loss near ambient temperature -- that's why a vacuum bottle (Thermos) works so well.

 -- Pat

[cotterg3]

pdlamb,

My calc only addressed the overall mean temperature rise of both rims, not the concentrated temperature rise on the surface which appears to be most relevant to your points (touching the rim, and delta_T with the air for cooling purposes). I've edited my post to make this more clear.

[FredHiltz]

...
and forgive me, I'm a techie; when you said "radiative heat loss to the air" I think you really meant "convective heat loss."  There is no significant radiative heat loss near ambient temperature -- that's why a vacuum bottle (Thermos) works so well.

No, after writing about convection, I did mean radiative heat loss, and should have said "to the surroundings," not "to the air." We do agree that it is minor relative to convection. But when I stop for a hot rim, I can feel it on the back of my hand placed near the rim--perhaps half an inch away.

Cotterg's calculation is close, but not quite what I had in mind. I will try a similar one tomorrow for a constant-speed descent with just one rim taking the energy during a five-second braking.

Fred

[litespeed]

The worst grade I ever descended was down - westward -off the Bighorn Mountains on 14A in Wyoming. 10 degrees for 13 miles. It was hair raising. I had to brake a lot to keep from overtaking two motorcycles and a car. The road was too winding to pass them. Towards the bottom it finally straightened out some, they took off and I could coast. I figure that if that didn't melt anything I wouldn't worry about braking anymore.

[whittierider]

How's that old saw go, a 15 minute experiement in a lab can save a week at the blackboard?  I'm going to suggest finding a good steep hill, 1/2 mile at 15% to 5 miles long at 6%, and try it both ways -- drag both brakes down, then alternate brakes, and check the temperature at the bottom.

It won't be a valid test, because you won't be able to measure the destructive temperature and pressure peaks that way since it will be too late.  I have the equipment to set up an experiment to do this with sensors on the wheel and tire and transmitting data to a computer on the bike, but it would be a lot of work to set up.  If these discussions continue and I ever find myself out of a job, I just might do it.  I would log the temperature and maybe the air pressure every second or so.

And forgive me, I'm a techie; when you said "radiative heat loss to the air" I think you really meant "convective heat loss."  There is no significant radiative heat loss near ambient temperature -- that's why a vacuum bottle (Thermos) works so well.

Thermos vacuum bottles have the surfaces silvered.  Polished silver has an emissivity that is only 1/50th of that of a black body, making it ideal for the Thermos.  The emissivity rules hold down to absolute zero though.

My calc only addressed the overall mean temperature rise of both rims, not the concentrated temperature rise on the surface which appears to be most relevant to your points (touching the rim, and delta_T with the air for cooling purposes).

Aluminum is an oustanding conductor of heat, so unless you could pulse the brakes many times per second, I don't think this effect would matter.

10 degrees for 13 miles.

That's almost 18%, around 920 feet per odometer mile, meaning a 12,000-foot descent.

[John Nelson]

I agree with all those who have said that it's best to get comfortable with as high a speed as is safe for the conditions. The best way to avoid overheating due to braking is not to brake. I ride the mountains all the time and I have never yet had to stop to let my rims cool. That's not to say that there isn't a hill out there on which this would be necessary, but I haven't been there yet.

[litespeed]

Excuse me. It probably was 10%.

[paddleboy17]

Aluminum is an excellent conductor of heat.  I think you are all going about the math all wrong.  I would not expect any heat differential between the inside and the outside of the rim.  There is not that much metal there.

Suppose you number cruncher work out how hot things have to get glaze a pad or unclinch a rim.