Tandem Cycling - Living without a drag brake

First off we are not climbers (think rail-trail :love:), so we should not encounter many "serious" descents, but after reading about blow-outs from over heated rim brakes I've become a little paranoid about how much of a hill I need to worry about and what should I do if we find ourselves facing something steep and long. I know there are all kinds of factors (team weight, speed, grade, etc) but is there some rule of thumb I can apply to know if I should stop mid decent for a brake check/cool down?

Also any thoughts about the cooling cycle for rims? For example, is it better to alternate dragging the front and rear brakes, allowing one to cool while the other does double duty, or is it better to share the friction simultaneously on both wheels.

You ask about a few different issues. First, why do you need to slow down going down hills? Is there a stop sign at the bottom? If you're just riding the brakes because you don't want to go "too fast" then get a drum brake.

As to tires exploding, I've got to take these anecdotes with a grain of salt. My thought is that there was something else with the tire or rim to start with or that the tire was extremely over inflated to start. Air is a very poor conductor of heat. Even if a rim gets hot, it would take a long time to heat the air in the tire to the temperature of the rim.

After that, you got to figure out how much hotter the air in the tire would have to get to actually increase the pressure so much that it would actually blow the tire off the rim. We have the combined gas law equations for that.

Suppose you've got a tire at 120psi and it's 100F outside. Manufacturers test the tire to blow off at double the rated capacity or 240psi, but let's suppose it blows off at 180psi. In order for that to happen the air inside the tire would need to be 380F. Just like the exploding scuba tanks in the trunk fable, I suspect BS.

If you need to modulate braking, either front/rear or both on/off are fine and there are cases where one might be more suitable. For example, it's a bad idea to brake in a turn, but if you have to, I'd prefer light brakes or light brakes on the rear only over alternating front and rear.

On a straight downhill stretch, it doesn't matter so much.

Also any thoughts about the cooling cycle for rims? For example, is it better to alternate dragging the front and rear brakes, allowing one to cool while the other does double duty, or is it better to share the friction simultaneously on both wheels.


On long descents it can be helpful to alternate which brake you use, to allow the other to cool.

I would suggest however, that you not "drag" either rim brake. Use the brakes firmly to decrease your speed as needed, i.e. for a turn etc, then let off them.

"Dragging" the brakes will cause overheating and can cause blow outs.

I would say that unless climbing is a problem, you won't find descending a problem. That is, if you can make it up, you probably won't have a problem getting down. Drag brakes are for long descents with serious loads. If you are just riding around the rail trail, it should not be something you miss, or even want.

We are fairly new to tandeming and I too had read about the dangers of over heating rims. I live in fairly hilly country, since I have V brakes, overheating is a real concern. To get a feel for how quickly the rims heated up, we rode down a long hill with many sharp turns that I have done many times on a single bike (3 miles, 1200 ft drop). I stopped 3 times to check the rims. The first and second time, the rims were warm to very warm. The third time the front rim was very hot. We waited for the rim to cool before we completed the descent.

A few weeks later, we did a loaded tour (55 lbs with a BOB trailer) through Big Sur. There is a big descent from the Ventana Inn to the coast. About 3/4's of the way down we had to stop and let the rims cool.

I think you can ride fairly big hills without a drum, but you need to be very attentive and willing to stop to allow the rims to cool. A good sign for me was when my hands were getting tired, it was probably time to stop.

On big descents, I always alternate my brakes unless I need to slow down alot for a turn or to stop.

As to tires exploding, I've got to take these anecdotes with a grain of salt. My thought is that there was something else with the tire or rim to start with or that the tire was extremely over inflated to start. Air is a very poor conductor of heat. Even if a rim gets hot, it would take a long time to heat the air in the tire to the temperature of the rim.

After that, you got to figure out how much hotter the air in the tire would have to get to actually increase the pressure so much that it would actually blow the tire off the rim. We have the combined gas law equations for that.

Suppose you've got a tire at 120psi and it's 100F outside. Manufacturers test the tire to blow off at double the rated capacity or 240psi, but let's suppose it blows off at 180psi. In order for that to happen the air inside the tire would need to be 380F. Just like the exploding scuba tanks in the trunk fable, I suspect BS.

Bearing in mind that the OP is not into loaded touring or mountainous terrain, I would say a drum brake is dollars and pounds badly spent in that case.

What's missing in the argument about tire pressure and blowoff strength is that the manufacturer most likely tested it at something like 20C, maybe 30C. It is quite possible that when the temperature of the tire - the part in contact with the rim - rises to 60C (not unreasonable) the tire is substantially weaker due to the elevated temperature. So while the pressure has only increased about 14%, the hot tire is a LOT softer.

Team weight has an effect, and loaded touring changes things also.

In our experience, we've been able to do 6000 foot descents (Everest Challenge) and steep technical descents (Brasstown 21% grade) with just rim brakes.

As for a sign when it's time to stop, when the brake pads are noticeably hissing ,and it smells like a tractor trailer just lost their brakes, it may be time to give it a rest.

As to tires exploding, I've got to take these anecdotes with a grain of salt. My thought is that there was something else with the tire or rim to start with or that the tire was extremely over inflated to start.

I tend to agree that root cause may be misplaced; however, the end result is the same regardless of what precipitates a tire blow-off on a descent or even while just riding along (JRA). I'll elaborate on my theory in a minute.



I A good sign for me was when my hands were getting tired, it was probably time to stop.

+1. This is one of the other very compelling reasons to use a drag brake when you head to the switch back mountain descents in Europe and a few here in the states.



On big descents, I always alternate my brakes unless I need to slow down alot for a turn or to stop.

+1. I'll abuse the rear brake a bit, then switch to the front while the airflow over the rear rim and tire does it's thing to bleed off some of the heat build-up. After abusing the front brake for a bit, it gets a break to cool while the rear wheel gets abused.



It is quite possible that when the temperature of the tire - the part in contact with the rim - rises the tire is substantially weaker due to the elevated temperature.

You and andr0id are both knocking on the door of my own little theory. Again, I have a hard time believing that air expansion in the inner tube due to heat is the root cause of blow-offs. Instead, and as andr0id suggests, I personally believe a weak tire bead may instead be the culprit. Now, here's where my theory becomes a SWAG because I just haven't bothered to collect enough data to see if there is circumstantial evidence to support my theory. Anyway, here goes:

1. Folks who report blow-offs (and there are indeed blow-offs) seem to be riding on wire-bead tires more often than not. The latter could simply be a by-product of OEM fitment and/or buyers heeding certain manufacturer and dealer recommendations to avoid foldable clinchers but, again, more detailed data would be needed to establish what types of failures folks who use wired-on tires experienced vs. folks who use foldable tires with Kevlar beads.

2. When using heavy breaking on a long or steep descent there are two different sources of friction that cause the tire temperatures to rise: (a) rim heating from the brake pads passes directly into the tire at the bead seat, and (b) tire heating from road friction (remembering, it's the tires that stop a tandem's forward momentum, not the brakes).

3. Heat from both the rim brakes contacting the rim's brake track and from the tire's contact patch with the road would logically meet at the tire's rim bead... normally a metal wire that can clearly conduct heat or a pliable synthetic, kevlar reinforced chord for foldable tires that does not conduct heat like the steel wire, both attached to the tire's carcass by a cloth strip and adhesives.

4. My theory here is, the concentration of heat at tire bead -- and on wire-beaded tires in particular -- degrades the cloth and/or adhesives over multiple heat cycles that, when coupled with other things that break down the bead casing such as UV light, normal wear and tear or old age on tandems that aren't ridden all that much (remembering, tandems that are ridden a lot go through tires a couple times a year) invites a weakened bead.

5. Just like a small hole in a dike, once a weak spot develops the tire's inner tube will begin to push out and through the weakened section of tire bead and it's the tube that eventually ruptures with a loud rapport and concussive forces that shred the weakened tire sidewall and unseat the tire's rim bead.

Again, this is just my own little theory having watched older wire-beaded tires 'blow-off" on dead-flat but very hot roads where it was the well-used or heat-affected tire's bead casing that let go first, followed by the 'pow' and tire unseating / shredding that often times accompanies a blow-off.

I could, of course, be wrong. It's just a theory.



As for a sign when it's time to stop, when the brake pads are noticeably hissing...it may be time to give it a rest.

+1. This is the sound glazed brake pads... which is accompanied by brake fade. If you continue to ride your brakes once glazing and brake fade starts your stopping power will diminish and increased hand power at the brake lever won't help much. The same is true with a disc brake.

Blowouts from overheated rims?
Have they occurred? Possibly . . . but also likely due to some other factors.
In over 225,000 miles of riding tandems and up/down mountains we have NEVER experienced a blowout from 'overheated rims.'
Have we had blowouts? Yes.
Testing tires for a well known company years ago we had 2 of their brand new tires (wire bead) blow off the rims in only a couple days of riding. Rim and tire were not compatible.
Have had a weakened sidewall on tire blow out at 30+mph.
Blowouts can/do happen but are quite often blamed on 'overheated rims'.
Make sure tires are properly seated and inflated. Look for possible weakening/cuts/bulge/wavey look in tire/sidewall that could cause blowout. Even check to see that rimtape is covering those spokeholes properly.
As for heat buildup, we live in Arizona where air temps get over 100 degrees regularly; we have minimal shade and road surface temps are high enough to make cement sidewalks buckle every year.
Today, May 7, our air temp was 101 degrees . . . in the shade; and yes we rode this morning when temp was up to mid-90s and our temp guage on 'puter registered 104 in the sun.
You develop your own style and comfort level on braking. On long and twisty descents our method is to alternate on/off between front and rear brake. Scrub some speed on those descents so you can minimize full-on panic braking. Sit up straight in the saddle to catch the wind/create drag. No, we do not have a disc or drum on our tandem(s).
When the pilot's fingers are cramping from constant braking, then we know it's for a break!
Doubt that you have any hills in New Jersey that would warrant concerns; however each tandem team has their confort zone.
Just our input/experience.
Enjoy riding TWOgether!
Rudy and Kay/zonatandem

My tandem has a rear drum brake that I use for the majority of braking. I rented a Santana road tandem, without a disc or drum brake, in France for a week of cycling. I had no issues with braking until riding 100 km of mountain roads in the rain. The tires did not overheat, but I used up one set of brake pads. I can only think it was the combination of road grit and increased braking due to the wet road that resulted in extreme pad wear. My stoker wasn’t too pleased when I told her we were walking down the last steep hill of the day!

My new Calfee will have a rear disc. I’m getting ready to move to Sicily, and I know all about the steep and curvy roads on mount Etna.

I love our Arai drum brake. Our old Burley Duet came with one (we are the second owners); however, I've heard that the Arai is no longer manufactured. In any case, we are heavy riders, and with or without bags I enjoy long, steep descents in northern California with a touch of drag brake. Having ridden many years solo in Colorado (steep and long), I wish I had one on my solo bike. No hand fatigue, no worries about brake fade or heat.

My two cents worth on the "tires exploding" issue: I've not had a tire explode on steep descents with hard braking, but I have had tubes pop. I believe it was heat that caused the problem: on the same descent (Boulder folks will know the spot, heading down Flagstaff from Kossler Lake just past the flagpole/amphitheater turn-off) I hit about 45-50 mph and must brake hard before the hair-pins, which approach all too suddenly. At about the SAME LOCATION over ten years I have had three (I know, N=3 isn't much of a sample) abrupt-onset, FRONT flats, one causing me a small spill. In each case the tube had a strange, 1-2 cm, oval hole on the sidewall aspect. Intuition tells me it was heat. And of course, the rim was too hot to touch. In any case, after ten years I finally don't brake hard for long at those speeds.

My two cents worth on the "tires exploding" issue: I've not had a tire explode on steep descents with hard braking, but I have had tubes pop.

So, I gotta ask....

Boxers or briefs?

er, wait a minute.

Wire or kevlar... rim bead that is.

I'd definitely be willing to amend my theory to include heat-damaged / weakened tubes as well. Tubes are, after all, perhaps the weakest link in a bicycle wheel that that they don't like to be pinched or poked and perhaps we need to expand that to include poached.

So, I gotta ask....

Boxers or briefs?

er, wait a minute.

Wire or kevlar... rim bead that is.

I KNEW you would ask, which is why I left it out... making sure you are still in TandemGeek form (you did not disappoint). They were...

ALL WIRE BEAD!

I KNEW you would ask, which is why I left it out... making sure you are still in TandemGeek form .......

hehe! Really! Which is why I have a hard time believing that there is a single tandem related issue for which Tandemgeek:


just hasn't bothered to collect enough data to see if there is circumstantial evidence to support my theory.


:roflmao::roflmao::roflmao:

Manufacturers test the tire to blow off at double the rated capacity or 240psi.

Where does that statement come from? I hear it all the time but never from anybody who had anything to do with making bicycle tires. Is there a bicycle tire testing protocol somewhere?

On big descents, I always alternate my brakes unless I need to slow down alot for a turn or to stop.


+1. I'll abuse the rear brake a bit, then switch to the front while the airflow over the rear rim and tire does it's thing to bleed off some of the heat build-up. After abusing the front brake for a bit, it gets a break to cool while the rear wheel gets abused.

Let me begin with caveat that I am not a physicist nor an engineer. So if there is a long conversation about the topic, I'll get lost eventually.

I always wondered about the alternating brake strategy. Isn't it the case that the amount of energy dissipated through braking to heat and so on, will be the same regardless of whether you alternate or use each brake with half as much force?

That is, in order to slow the bike down to a particular velocity, your brakes/rims have to dissipate a certain amount of heat from the kinetic energy of the bicycle. Whether you alternate or not, the total amount of energy is going to be the same and so is the resulting heat and risk of tire failure. No? Allowing the tire to cool shouldn't matter since you are just heating up each wheel faster than by using two brakes; i.e., at some point down the mountain, the rims will be the same average temperature regardless of whether you alternate or use two brakes at a time.

Now one thing this doesn't take into account is the ability of the human machine to calibrate half the energy going to the front and rear since the front brake is so much more powerful. So is the strategy of alternating provides good feedback of the amount of energy being dissipated in each round whereas using two brakes it would be easy to be out of balance.

I always wondered about the alternating brake strategy. Isn't it the case that the amount of energy dissipated through braking to heat and so on, will be the same regardless of whether you alternate or use each brake with half as much force?

Yes, in theory and most likely in practice that would be correct. The rationale I use for alternating brake application is that I'm less likely to glaze or induce brake fade on both front & rear brakes at the same time by getting into the habit of riding both brakes on long descents.

Again, I tend to use my rear brake as my speed governor on long descents and use my front brake to slow the tandem ahead of a turn or when I truly need to stop. However, and as noted, when the descent is long or challenging and I find that I'm over-using the rear brake, I'll give it a rest and rely on the front brake for a little while. You'd be amazed at how fast your wheels and tires will cool once you remove the brake energy and road friction heat processes. In fact, I think I've probably seen about 6 tandem teams make the mistake of coming to a fast stop after prolonged heavy brake use and then watched as one of their tires blows... robbed of the cooling power of forward momentum and left to deal with all of that heat build-up.

Anyway, I adopted the alternating brake strategy back when we rode an off-road tandem with dual V-brakes early on in our tandem career and never thought much about what would happen if I ended up with front & rear brake fade at the same time. Unfortunately, I got to find out what that was like about 3/4 of the way down a dirt and gravel forest service access road and found myself with no front or rear brake power, trees to the right and a drop-off to the left ambling along at about 20 mph (which, if you haven't ridden off-road on a tandem feels a lot faster than it does on a paved road). Thankfully the grade of the road began to flatten out near the bottom and we were able to keep it upright until I could do a Flintstone stop. Upon inspecting the brakes, the rear brake pads had melted and deposited most of the brake compound on Debbie's seat post and stays and the fronts weren't much better having glazed over and been worn down to the nubs. I bought our next tandem a few weeks later and it was fitted with front & rear downhill racing discs and adopted my one brake at a time for speed control, reserving both brake application for stopping.

So far, it's worked out pretty well for the 9 years hence.

I'm a newbie tandem-wise and I can say that the ability of a tandem to pick up speed on any sort of a downhill is impressive. My theory on my 1/2-bike which I'm applying to the tandem is brake intermittently , slowing down below my target speed and allowing the bike to accelerate to somewhat greater than the target. If the downhill is twisty then this deceleration/acceleration occurs naturally as a function of the turns. Of course we're taking things real easy until I understand the handling of the tandem.

Let me begin with caveat that I am not a physicist nor an engineer. So if there is a long conversation about the topic, I'll get lost eventually.

I always wondered about the alternating brake strategy. Isn't it the case that the amount of energy dissipated through braking to heat and so on, will be the same regardless of whether you alternate or use each brake with half as much force?

That is, in order to slow the bike down to a particular velocity, your brakes/rims have to dissipate a certain amount of heat from the kinetic energy of the bicycle. Whether you alternate or not, the total amount of energy is going to be the same and so is the resulting heat and risk of tire failure. No? Allowing the tire to cool shouldn't matter since you are just heating up each wheel faster than by using two brakes; i.e., at some point down the mountain, the rims will be the same average temperature regardless of whether you alternate or use two brakes at a time.


Drawing on some old fuzzy memories from thermodynamics and appling some crude translation... The hotter something gets the faster it cools, therefore alternating between two energy sinks (rims/brake pads) would allow more heat to be disapated by alternating than by equal use, this would effectivly lower the average temprature.

2. When using heavy breaking on a long or steep descent there are two different sources of friction that cause the tire temperatures to rise: (a) rim heating from the brake pads passes directly into the tire at the bead seat, and (b) tire heating from road friction (remembering, it's the tires that stop a tandem's forward momentum, not the brakes).


I'll go with (a), but not (b). The only friction between the road and the tire, unless you are in a skid, is static friction. Static friction does no work and generates no heat. There is a little tiny amount of energy dissipated in the rubber molecules getting attracted to the molecules of the road surface and getting pulled away, but for all intents and purposes that one can be ignored. Where rolling resistance (and hence heat comes in is primarily in bending and unbending the tire. If you take a piece of rubber and flex it back and forth enough times you will be able to feel it getting warm. This is independent of whether you are braking. There might be a change in the amount of deflection as you brake (hard), partly as the wheels aren't weighted the same when braking as when riding at constant speed. But the main cause of heating from the road is happening all the time, not particularly when braking.

I like your point about the wire beads.

I'll go with (a), but not (b). The only friction between the road and the tire, unless you are in a skid, is static friction.

True... and perhaps I oversimplified my theory a bit too much in my posting.... leaving out hysteresis and not being more specific about friction.

Indulge me while I ramble on here and, as always, I could be wrong.

The rear tire on a tandem has a lot more traction than the average rear single bicycle tire given the increased weight and torque of the paired riders that, per my flakey theory, causes the rear tire on a tandem to spend a lot more time living at the tipping point where static friction is converted to kinetic friction during acceleration, climbing or braking. You can see the effects of this higher degree of tire slip and the increased rolling resistance by looking at the amount of increased tread wear you see on rear tandem tires. The latter is rather dramatic and takes many first time tandem teams by surprise when they discover a squared-off rear tire in 1/3 as many miles as they observe on their single bikes.

Now, while I didn't get into too much detail beyond adding the wire-bead aspect of my theory, some of my other assumptions about folks who have tire blow-offs is that those wire-beaded tires tend to be greater than or equal to 28mm and use the specified tire pressures on the sidewalls because, well, that's what a lot of people who ride tandems tend to so. Unfortunately, those tire pressures tend to be ones that were developed based on a single rider bike application and not a tandem which yields a tire that deforms a lot (hysteresis) even when just rolling along dealing with static friction and rolling resistance. Now, I'm pretty sure as you go faster tire heating from deformation (aka, that hysteresis thing) keeps pace and increases as well, although I'm not sure it's linear and will vary by tire construction, material, size and psi.

OK, so what do we have here when we put that tandem on a steep grade with 28mm- 32mm wire-beaded tires inflated to around 95 psi per the sidewall rating rolling along at 40 mph and the stoker says, slow this puppy down?

We have a tire that already has a fairly high amount of deflection with the associated tire heating that occurs just riding along at 40 mph that's now being put under what is, at least for the rear tire, threshold braking because tandem captain's tend to use a lot of rear wheel braking to slow down their tandems and under-use the front brake. Admittedly, the additional heat coming from rear tire hysteresis and threshold braking pales in comparison to that little heat machine... the rim brakes. However, having put my hands on some rear tires at the bottom of descents, those suckers do get warm and I can't believe their getting all of that warmth via radiation from the rim and brake track.

But, as noted earlier, I could be wrong.

Now, I'm pretty sure as you go faster tire heating from deformation (aka, that hysteresis thing) keeps pace and increases as well, although I'm not sure it's linear and will vary by tire construction, material, size and psi. ...

However, having put my hands on some rear tires at the bottom of descents, those suckers do get warm and I can't believe their getting all of that warmth via radiation from the rim and brake track.

But, as noted earlier, I could be wrong.
I don't know I'd use the word hysteresis to talk about the effect, but tires definitely do heat from being ridden, and given that the number of times the flex per minute increases with speed the heating is bound to increase with speed. Being trained as a physicist I'd say it is linear over at least a narrow range :) (but how narrow I'd hesitate to guess). I would expect that it does go non-linear fairly soon as the rubber warms up - likely reducing rolling resistance - and the amount of air going by increases, and the temperature difference between the tire and the surrounding air increases - so cooling becomes more efficient. My only quibble was with the use of the word friction. There's a reason car tire pressure ratings are "cold tire pressure". Besides flexing to generate heat, there are times when the asphalt itself is burning hot, just to make matters worse.

I don't know I'd use the word hysteresis to talk about the effect, but tires definitely do heat from being ridden...
Hysteresis is the correct term. Elastomeric hysteresis isn't as commonly taught as magnetic hysteresis.


...Admittedly, the additional heat coming from rear tire hysteresis and threshold braking pales in comparison to that little heat machine... the rim brakes. However, having put my hands on some rear tires at the bottom of descents, those suckers do get warm and I can't believe their getting all of that warmth via radiation from the rim and brake track...

Again, speaking without data, I think to generate significant heat through hysteresis effects a bicycle tire would have to be severely underinflated. Even then the deflection and frequency is pretty low. It would seem that the heat most likely transfered from the rims. On the other hand, significant heating can occur through frictional abrasion of the rubber. In our example this would assume that braking was close to the limits of the tire grip and the cohesive strength of the rubber compound. Perhaps enough abrasion is occuring to account for the heating. Again, just thinking out loud.

Does anyone with both a rim and disk brake on the rear of their tandem wan't to run a controlled experiment for us?;)

Just like the exploding scuba tanks in the trunk fable, I suspect BS.


Fable? I have a friend who is a certified commercial diver, who would argue with you.

I didn't find a scuba photo, but how about a nitrous tank photo?

http://forums.vwvortex.com/zerothread?cmd=print&id=4134291

I think to generate significant heat through hysteresis effects a bicycle tire would have to be severely underinflated. Even then the deflection and frequency is pretty low. It would seem that the heat most likely transfered from the rims. On the other hand, significant heating can occur through frictional abrasion of the rubber. In our example this would assume that braking was close to the limits of the tire grip and the cohesive strength of the rubber compound. Perhaps enough abrasion is occuring to account for the heating. Again, just thinking out loud.

Put a 350 - 400lb tandem team on a tandem with 32mm tires at 95 psi and you get a pretty fat bulge... But I digress....

The only role that tire heating has in my theory is to note that bicycle tires typically end up holding heat generated through the same things that cause a motorcycle or car tire to heat up, e.g., rolling resistance, road temperatures, 'friction' as already addressed. The point being, a tandem's tires and in particular that rear tire is rarely cold once it starts rolling and I'm pretty sure when you apply heavy braking for a prolonged period of time on a tandem descending a steep grade all of those things that can put heat into a tire do so in parallel with the heat being generated at the brake track by the rim brakes.

The place where the heat from the tire meets the heat on the rim's brake track is the back side of the brake track which pulls double-duty as the hook bead. That means the hook bead interface with the tire's wire bead is coincidentally what I've got to think is hottest spot on the rim and tire. It's been a while since I stuck on heat strips just to see how hot a rim gets under hard braking, but if memory serves it was pretty darned hot. I'm also pretty sure that the steel bead is a pretty good heat conductor and that butyl rubber inner tubes really don't like a lot of heat concentrated in a small thin line against their very thin-under-pressure walls.

Anyway, that's my theory. Perhaps I'll do a tire blow-off survey just to see if there are enough kevlar-bead tire users to 'bust' the theory.

As someone who has experienced a front tire blowout on a tandem I will throw in my two cents.

I would not worry about it unless you are on a steep, extended descent, with many turns (using your brakes a lot) on a hot day.

The first time our tire blew, it was a Continental Gatorskin, wire bead, 28mm. It was a hot day (90+) and we had been riding through the Santa Monica Mountains - up and down. We were not too far from the bottom after a descent of maybe 1,500 or 2,000 feet on a road that is know for its steep and twisty terrain. Grades range from a minimum 5% to some short sections of about 20%. Most I would guess is 7% to 8%. We were accelerating to over 45 mph on the straightaways and back down to about 20 mph for the curves. (There is a reason that many Pro Teams use this area for their winter training ground prior to the Tour of California - and that the side of the road is littered with motorcycle, Porsche and Ferrari parts.)

A couple of weeks later we had a much longer descent, about 7,000 ft over 30 miles from a mountain top finish, where we averaged well over 45 mph all the way down where we used the brakes sparingly. The road was a highway, did not have the steep sections and had corners that you could take at speed. I was never concerned about overheating as the road never got over 8% and generally stayed in the 5% zone.

We have also blown out our rear tire (Panaracer 32 wire bead), after descending about 4,000 feet at about 6% to 8% - on a roller going up hill! (What is up with that???) Again, it was a hot day maybe near 100 down in the valley.

I finished a 600k brevet yesterday, on my single bike, where I descended about 5,200 feet in 30 miles, usually in the 5% zone and on relatively straight roads and I do not think I hit the brakes once except for the stop sign when coming into town.

On rides that do not have steep (10%+), extended descents, on twisty roads, with temperatures above 90 degrees I doubt that you would ever have a problem. Frankly, riding rail trails with the easy grades and no sharp turns I would think it would be hard to put that much heat into a tire.

Good rule of thumb, when your hands are getting so fatigued from braking so much that you are having trouble squeezing the brake lever - your tires are probably getting hot, and it is a good time to pull over.

I always wondered about the alternating brake strategy. Isn't it the case that the amount of energy dissipated through braking to heat and so on, will be the same regardless of whether you alternate or use each brake with half as much force?
That is, in order to slow the bike down to a particular velocity, your brakes/rims have to dissipate a certain amount of heat from the kinetic energy of the bicycle. Whether you alternate or not, the total amount of energy is going to be the same and so is the resulting heat and risk of tire failure. No? Allowing the tire to cool shouldn't matter since you are just heating up each wheel faster than by using two brakes; i.e., at some point down the mountain, the rims will be the same average temperature regardless of whether you alternate or use two brakes at a time.

First, I will assume that we are only talking about reaching a temperature extreme that could theoretically blow a tire/tube, not to debate the other possible factors. I agree that the total amount of energy is going to be absorbed by either both wheels or one wheel - given a fixed amount of stopping or slowing, but remember that heating and cooling is not linear, but rather based on a curve, as DCwom pointed out. For example, when taking a hot meal out of the oven, the meal quickly cools from 300 degrees to 200 degrees, but takes much longer to go from 200 to 100 degrees due to the temperature difference between the meal and the ambient temperature being less extreme. Like-wise, the cooling effect when moving through the air, as TG reminded us, cools the rims from a very hot temperature (close to the danger point) fairly quickly since the ambient temperature is much cooler. This fairly short time allows the rim/tire to quickly cool down and be ready to absorb more heat.

The trick, in my opinion, is to let the brakes off long enough to get out of the high heat danger zone. Whether this is by letting both brakes cool for a few seconds or releasing/activating one at a time, we need to allow for some cooling. It is easier for us to only apply one brake at a time, thus allowing the other brake/rim/tire to start cooling, than to know when the rims are getting too hot and release both brakes to allow the cooling to take effect. It is also harder to maintain control when barreling down a steep slope while releasing the brakes.

It doesn't take too terribly long to get out of this danger zone of high temperature, but in my opinion, those are the reasons why it is usually recommended to alternate front/rear braking rather than applying and releasing both brakes on very long decents.

Or use a drag brake and don't think much about it anymore http://www.bikeforums.net/images/smilies/smile.gif

Fable? I have a friend who is a certified commercial diver, who would argue with you.

I didn't find a scuba photo, but how about a nitrous tank photo?

http://forums.vwvortex.com/zerothread?cmd=print&id=4134291

And if you read that thread, it is pointed out that it is bunk. There is a burst disk that will release long before a tank would explode.

Also the pics aren't consistent with the alleged mode of damage. The car got rear ended.

Also the pics aren't consistent with the alleged mode of damage. The car got rear ended.

Reminds me of a bunch of tandem folks standing around looking at someone's shredded tire, bent rim and displaced cantilever brake trying to figure out what caused the tire to explode while the couple was just riding along on a very hot day on a very flat road. Yeah, I was there and it wasn't me. But, as luck would have it, the couple had an older tandem with 27" tires and no spare. Uhhh, not a good idea in 700c world.

But I digress... onto the off-topic thread.

Merlin: Gotta disagree with you on that one. All of the damage looks consistent with what's alleged to have happened, e.g., a pressurized aluminum tank burst inside a closed car. The rear bumper is blown off, the hatch blew up and folded forward, the rear quarter panels buckled as the air expanded in the rear hatch area, almost all of the glass blew out of the car, etc... Better photos can be found here: http://www.theturboforums.com/smf/index.php?topic=117747.0

As to why the tank burst, who knows. All of the comments about pop-off valves, burst discs, etc. are correct and those devices if working properly should have prevented a catastrophic failure of the tank.