Author Topic: New to downhill grades  (Read 4062 times)

0 Members and 1 Guest are viewing this topic.

Offline lilfilly

New to downhill grades
« on: February 09, 2011, 02:47:44 pm »
I'm new to downhill grades, in fact, all of my miles have been only on the hilly backroads of Michigan, but my best friend and I are thinking about going across the southern states of Texas, New Mexico, and Arizona.  I've only driven through the mountains in my car, and never on a bike.  I hope this doesn't sound like a stupid question, but how do you keep from losing your brakes while going down those steep grades that are very long windey?  I can't imagine going down mountain passes and not being able to do it in a lower gear like you would a vehicle.  I've had my experience with hot brakes on my semi... what happens with a bicycle?

Offline Westinghouse

Re: New to downhill grades
« Reply #1 on: February 09, 2011, 03:02:37 pm »
Don't worry about it. I went across the ST of states with only the front brakes. No back brakes at all. I am not recommending using only one set of brakes. Do have both sets fully functional. It might seem like braking might be a big problem, but as long as you are cautious, safety conscious, and make allowances for wet-weather cycling, you should be alright. You might want to read up on the subject. The only reason I used only the front brakes was that I am poor monetarily, had to be very careful about spending, and could not afford to buy new calipers.

Offline Fred Hiltz

Re: New to downhill grades
« Reply #2 on: February 09, 2011, 03:10:22 pm »
Wow, Westinghouse, you were courting trouble! I have seen rim heat grief several times, fortunately only to others: the rim gets hot and the tire pressure goes up just as the heat weakens the rubber, and it blows out. When using rim brakes, you need to stop often and put a hand on the rim. Wait until it is cool enough to keep your hand there before continuing. You will soon learn how far to go between stops, depending on load and grade. Putting the heat evenly into two rims helps.

Disc brakes, of course, do not have any problem. They get really hot, but they can take it. All serious tandems have a disc drag brake for just these occasions.

Fred

Offline Westinghouse

Re: New to downhill grades
« Reply #3 on: February 09, 2011, 03:13:42 pm »
I have heard of this, but I have never had that problem myself. I may have been embracing trouble, but I definitely was not courting it.

Offline whittierider

Re: New to downhill grades
« Reply #4 on: February 09, 2011, 03:43:38 pm »
Caliper brakes themselves are not a problem.  Do use Kool Stop pads though.  Regardless of what brand of brakes you get, just consider it standard practice to replace the stock pads with Kool Stop pads.  They give better braking, no fade as far as I have noticed, longer rim life, and they don't get the metal bits embedded in them like the Shimano pads do.

The bigger threat is tires, since extreme rim heating may increase the tire pressure enough to blow it.  Unless you're quite heavy, I doubt you will ever have any problem on a single bike.  The problem is much more common on tandems, not just because of the double weight, and the higher tire pressure used to prevent blowout-like pinch flats with all that weight, but because the arrangement means you can't depend on wind resistance to hold your speed down as much.  I've gotten a rim so hot on ours that when I touched it after stopping, I immediately got a sizzling sound and even a little smoke as it instantly burned my skin, and there was no tire trouble.  I had forgotten to use the drum brake.  We had been going almost 60mph on a grade of 8-9% with some tailwind, and could easily have done 70 or 75 if traffic had permitted.  I was riding the rim brakes to hold our speed down!

The rim-heating problem for a single bike (ie, non-tandem) will mainly be on very steep grades of 12% and more-- not the 6% stuff that highways are usually limited to.  One thing you can do to help the situation is to use black-anodized aluminum deep-V rims to get rid of the heat better by having higher emissivity and more surface area.  If the situation looks bad enough, you can reduce tire pressure before the descent, although then you have to re-inflate afterward, after the rims cool.  A common thing to do with tandems is to put an Arai drum brake on the back wheel with a separate control, usually a shift lever of some kind so you can set it and leave it alone for long periods as a drag brake, kind of like reducing the grade of the downhill while leaving the rim brakes for normal control.  Bill McCready (sp?) of Santana has done testing on steep downhills with various brakes on tandems, and found that such hills will warp most disc brakes or even boil the fluid in the case of hydraulic ones, rendering them useless.  Sometimes they would melt parts in the disc caliper assembly.  IOW, don't think that disc brakes are the silver bullet.  The hill he uses for testing is about 20 miles from me and is definitely representative of some of the riding we do.

Offline Pat Lamb

Re: New to downhill grades
« Reply #5 on: February 09, 2011, 03:58:05 pm »
Most western grades are limited to 6% or so, although there was one (going down into Tonasket, WA), that was 8% with a stop sign at the bottom.  (Evil!)  We ran across one fellow in Virginia who had blown his loaded, single bike's front tire and bent the rim rather badly.  That was the 3-mile, 10% average grade coming off the west side of the Blue Ridge Parkway on the TransAm.  Steep and curvy!  Lots of other nasty grades (up to 18-25%) in the Appalachians, but not many that long and that steep.

I made it down that same grade by rotating brakes -- front brake for a six-count, rear brake for a six-count, repeat until the bottom.  That was apparently enough for the rims to cool enough that they didn't blow the tires.  My daughter stopped twice for (I'm guessing) 5 minutes each to let her rims cool down.  She also went downhill slower than I did.

What kills brakes (and rims and tires) is riding the brakes.  Either brake like you mean it, or let it run.  So you can try to let the bike run, then brake hard.  

And I second the recommendation for Kool Stop pads.  They don't hold grit like every other brand I've tried, so they don't turn your wheels down like a lathe.
« Last Edit: February 09, 2011, 04:00:10 pm by pdlamb »

Offline mucknort

Re: New to downhill grades
« Reply #6 on: February 09, 2011, 08:08:00 pm »
What pdlamb said, don't ride the brakes, use em hard every once in awhile and then if it is a long grade pull over and let the rims cool off for a bit. 6% downgrades can cause problems if they are like the 10 to 20 mile long ones I have experienced in places like the Cascades of Washington and you are heavy laden.
Erik
« Last Edit: February 10, 2011, 11:40:36 am by mucknort »

Offline whittierider

Re: New to downhill grades
« Reply #7 on: February 09, 2011, 10:14:32 pm »
Quote
I made it down that same grade by rotating brakes -- front brake for a six-count, rear brake for a six-count, repeat until the bottom.  That was apparently enough for the rims to cool enough that they didn't blow the tires.
Since each brake has to be used twice as hard to do the entire load by itself when used that way, it heats up much faster and to higher peak temperatures.

Offline Fred Hiltz

Re: New to downhill grades
« Reply #8 on: February 09, 2011, 10:18:53 pm »
This idea of intermittent rim braking has been around since Cycling Day 1, and I say it is a myth. Here's why:

In a nutshell: no matter how you brake, you are dumping the same amount of heat into your rims and they will get just as hot one way or another. The reasoning is certainly not original with me:

Physics 101: To get your weight from up here to down there, you need to get rid of a certain amount of energy, fixed and determined by the weight and the vertical descent. Wind resistance dissipates some of it, heating the air you pass through (and your body a little bit that wind chill overcomes). Rolling resistance converts a bit more into heating your tires. The rest goes into the brakes, heating the rims and the brake blocks.

Your style of braking does not change the energy dissipated by wind resistance and rolling resistance. Only your speed affects those.

Consider, then, two bikes of equal total weight descending side by side. One rider brakes steadily and one brakes intermittently. They both need to lose the same total energy. Because they go the same speed, they lose the same amounts to wind and rolling resistance. By subtraction, they also lose the same amounts to their rims and brakes. And because they go the same speed, they have the same time in which to absorb or dissipate all this energy as heat.

But we do not care much about energy or heat. We want to know how hot the rims get. Given a fixed amount of heat, what affects the temperature rise?

Certainly the heat capacity of the rims and brake blocks. A big, heavy rim can take more heat before its temperature rises--say 50 degrees--than a lightweight rim, the amount being roughly proportional to the amount of metal to be heated, that is, its weight.

Contact between brake block and rim assures that the blocks never get hotter than the rim. The blocks, being much lighter than the rims and made of rubber rather than aluminum, have such a tiny heat capacity that they absorb very little of the heat.

A black rim radiates more heat than a shiny rim. If they were red hot, the difference would be significant, but not much at all at our temperatures.

One might argue that letting some air between brake block and rim helps to cool the block. True, but note that neither rim nor block is absorbing energy then. When the block hits the rim again, it reaches rim temperature in milliseconds. Now they have less total braking time to absorb the same amount of heat; the temperature just has to rise faster.

Sheldon Brown writes, "On long, straight mountain descents, ... pumping the brakes, alternating between one and the other, will briefly heat the surface of each rim more and dissipate more heat before it spreads inwards to the tires." (http://www.sheldonbrown.com/brakturn.html) Also true, but a tiny effect. How long does it take heat to pass through the 1/16" of aluminum between brake block and tire bead? Less than a second, I'd think.

The two ways to really keep your rims cool: 1) Increase the wind resistance by sitting up, spreading your elbows, and descending faster. 2) Provide more time for the rims to lose their heat by radiation and convection, that is, descend slower.

Most of us cannot descend the hairy grades fast enough for option 1 to work. (Racers do it, though.) We hate to creep down the mountain, so we descend at a moderate pace, stop and wait for rims to cool, and get to the bottom at about the same time we would have made while creeping.

Fred


 

Offline cgarch

Re: New to downhill grades
« Reply #9 on: February 09, 2011, 10:55:06 pm »
Most western grades are limited to 6% or so . .snip

If only that were true. While that may be good for parts of WA and OR, it isn't so in California and it isn't true for the Cascades, Sierra, the Rockies, and a lot of other places. Some of the steepest grades on the Pac Coast route are up to 15% in CA. My recollection too is that the Three Capes route near Cape Lookout in OR has quite a bit of 12%.

Fred Hiltz's comments are spot on IMO. I tour with my SO on our tandem and bob. I have deep-V black rims with both caliper and disc brakes, and in our experience descending fully loaded (neighborhood of 100LBs in addition to us - 375 lbs plus bike) have not had any issues with tube or tire failures (in spite of some bad tire choices). Since I can alternate between disc and calipers on the rear some of the over-heating issues are significantly moderated. The only time I cooked a rear disc was an intentional test on a 20% grade in our area - but I still stopped. And unless I'm mistaken, most tandemnistas don't use a disc as a drag brake for that reason.

Perhaps more important is learning how to descend. I would strongly suggest reading the current article in Adventure Cyclist (Feb 2011) about cornering. It will help you learn to descend better. You're just going to have get used to the idea of going 45-55 mph on a downhill and learning how to moderate the rig into a corner. Sean Yates (of the 90's Motorola team) used to talk about sticking your elbow into a corner - try it, you'll find you go right around. Learning how to accept the speed and how to corner will instill you with the confidence you need to handle those wicked descents. After all, after the top you've earned that descent - learn to enjoy it.

Craig
« Last Edit: February 10, 2011, 09:23:30 am by cgarch »

Offline cotterg3

Re: New to downhill grades
« Reply #10 on: February 10, 2011, 12:22:01 am »
Fred,

I found your post very interesting. I also tend to agree with your overall assessment that pumping the breaks vs constant pressure will not significantly impact heat generation on the rims surface over time (assuming average velocity in both cases remains). Perhaps using one method over another causes riders to inadvertantly adjust their overall speed?

However, your analysis makes the assumption that the ratio of energy loss when the brake is applied (ratio between kenetic energy loss and energy lost as heat dissipation into the rims) is constant regardless of brake pressure applied. It is possible that braking at higher or low pressure may result in a greater or less "efficient" energy transfer when it comes to heat production vs wheel slowing.

This would not violate the energy balance that you set up in your post. Given your example of constant velocity over time, a difference in heat generation between the two methods would result in a difference energy required to brake (i.e. less energy drained from your muscles to achieve a given speed) and conservation of energy is maintained.

Hopefully that made sense.

Also just to nitpick:

"Physics 101: To get your weight from up here to down there, you need to get rid of a certain amount of energy, fixed and determined by the weight and the vertical descent. Wind resistance dissipates some of it, heating the air you pass through (and your body a little bit that wind chill overcomes). Rolling resistance converts a bit more into heating your tires. The rest goes into the brakes, heating the rims and the brake blocks."

I do not believe wind resistance affects air temperature. When biking through wind you are displacing air molecules and affecting their momentum (which in turn slows you down), but you are not changing the internal energy of the air molecules (aka temperature). The heat transfer between air and rider is the result of convective heat transfer. Wind chill just the name we use to describe the increase in convective heat transfer coefficient (due to increase in current flow). It is not its own form of energy transfer, assuming I understood you correctly.





Offline Galloper

Re: New to downhill grades
« Reply #11 on: February 10, 2011, 04:39:13 am »
As regards checking rims for heat, I was taught to use the back of your hand and not finger tips.   If the surface is sufficiently hot to burn, it's better to lose a bit of skin on the back of your hand rather than a finger tip!   Ideally you should try and sense the heat before touching.   Spit is good - it sizzles :)

Oh, and by the way, this was on vehicle hubs not bikes :)

The descent from the In-Ko-Pah pass on the ST to Ocatillo is interesting.   I recall passing semis on that descent :)

Offline Fred Hiltz

Re: New to downhill grades
« Reply #12 on: February 10, 2011, 04:54:53 am »
... It is possible that braking at higher or low pressure may result in a greater or less "efficient" energy transfer when it comes to heat production vs wheel slowing.

This would not violate the energy balance that you set up in your post. Given your example of constant velocity over time, a difference in heat generation between the two methods would result in a difference energy required to brake (i.e. less energy drained from your muscles to achieve a given speed) and conservation of energy is maintained...

I do not believe wind resistance affects air temperature. When biking through wind you are displacing air molecules and affecting their momentum (which in turn slows you down), but you are not changing the internal energy of the air molecules (aka temperature). The heat transfer between air and rider is the result of convective heat transfer. Wind chill just the name we use to describe the increase in convective heat transfer coefficient (due to increase in current flow). It is not its own form of energy transfer, assuming I understood you correctly.

I'm not sure what you mean by "efficient energy transfer." You are right that I ignored the muscular effort of squeezing the levers, which we all know is real. Letting up on the lever probably helps the muscles, but does not change the energy transferred to the brakes and rims. We still convert all the potential energy of the descent into heat, almost all of it in the rims.

My attempt to separate convective loss of body heat (wind chill) from the braking energy of wind resistance was rather clumsy, wasn't it? 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

Offline Westinghouse

Re: New to downhill grades
« Reply #13 on: February 10, 2011, 05:02:57 am »
There is this one road on the ST that you get on when you have to leave I-8 west of Ocotillo, CA. It goes into Jacumba, past a gambling casino, and to Pine Valley, a small and very nice little town. PV has a cool restaurant with lots of memorabilia from decades gone by. Anyway, west of Jacumba and before you get to the immigration check point east of the town of PV the road gets very hilly and rolling and steep in places. I had to use the brakes constantly going downhill mainly because it was night, I couldn't see well, and the speeds downhill were not safe in the dark. I would definitely check brakes before tackling that stretch of road. If your rims and brakes get very wet, caliper brakes, the stopping power can become seriously reduced. You must become aware of this problem. Even if you have to spray your brakes with a hose and and test them over and over again to understand this, do it.

Offline Pat Lamb

Re: New to downhill grades
« Reply #14 on: February 10, 2011, 06:09:37 am »
Fred, I must respectfully disagree with you.  I agree that the same amount of heat is generated, but I disagree on how it is dissipated.  I think you misunderstand Sheldon's point, below:

Sheldon Brown writes, "On long, straight mountain descents, ... pumping the brakes, alternating between one and the other, will briefly heat the surface of each rim more and dissipate more heat before it spreads inwards to the tires." (http://www.sheldonbrown.com/brakturn.html) Also true, but a tiny effect. How long does it take heat to pass through the 1/16" of aluminum between brake block and tire bead? Less than a second, I'd think.

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).

The net result is that the air temperature inside the tubes doesn't get as high, so the pressure stays lower.

If you want to try this out, you can make a couple of runs on a short, steep hill, and compare rim temperatures between riding the brakes and alternately pumping them.  Or, if you just want a demonstration, brake most of the way down to a point where you can safely let the bike roll out the rest of the way.  Stop and feel how hot the rims are.  Wait a minute, or three or five minutes, and feel again.  Go back up part way, stop and feel the rims; then let it roll the rest of the way down, which will take 15-30 seconds, stop, and feel how cool the rim is now!