|CyclingSite > CO Collected Wisdom > Touring Info > Cycling Concerns > Downhills|
can I zoom safely down again?
|Thought #1: Cool!
Even though we will be riding in the latter part of the summer, a fast descent from a high altitude can produce some wicked wind-chill, especially if the day is on the cool side, or (perish the thought) wet. A word to the wise: be prepared with layerable clothing including leg and arm warmers or such.
Thought #2: Bonzai!
Specifically because of excessive speed, there were several riders who did not resist, and paid a painful price when coming to a sudden stop after a short flight through the air. One of the crashes involved a couple on a tandem which I heard blew a tire due to overheating from brake usage - presumably resulting from continuous versus intermittent application of the brake.
The word here is to know your limits, and those of your bike. Train on the hills - up AND down.
I am a basic chicken on the downhills...
Remember they usually come with lots of curves. These make it hard to see scattered gravel or potholes. These small roads that suffer severe weather (cold winters/hot summers) often have some of both. Also, there is often heavy shade around some curves and this can make things hard to see, too. Don't ride so fast you can't spot hazards and go around them. Cycle Oregon will mark many hazards, but nobody can find everything!
You can slow your descent by sitting as upright and wide as possible. Its amazing how much the wind acts as a brake. Of course, your wind chill also increases. Besides the arm and leg warmers we also include a lightweight nylon windbreaker. If you are going to encounter the downhill in the morning when it is cold also include a fleece vest and some gloves that cover your fingers (these don't necessarily need to be cycling gloves!) A polypropylene ski cap or skull cap or some ear warmers really help, too!
Remember when cornering to put your outside pedal down and push on it just as you would any other fast corner.
About your brakes - yes they will overheat. The more weight the bike is carrying the hotter they will get. After all, they are basically using friction to stop you. Put your brakes on for a short time and release them and let them cool. Do this over and over instead of just using steady pressure. Remember the front brake works stronger, but be careful not to hit it too hard, too fast. Also, wet brakes stop you slower. If you think your brakes are overheating you can always stop and admire the scenery while they cool... By the way, overheating brakes not only damage the brakes, they can overheat the rim and cause a blow-out. This is not a fun way to stop.
Brake pads have a limited life expectancy. You may want to consider new pads this August so you have good stopping power. Also, check your rims are smooth, clean, and not grooved from worn brake pads.
I always find it hard to remember the scenery much on the downhills, I am too busy watching the road. I can sure remember the scenery on the uphills, though!
"THE HEAT is on " – DOWN-HILL brake management by Capt.Dink ~
Alternate – <bikers definition> -- to use one brake, then the other independently; as in - front and rear separately of one another. Thus allowing the brakes to cool between uses. See - prevention of bodily harm.
1. While going straight, or " a slight curve" alternating the front and the rear brakes regularly/independently will prevent excessive heat build-up. Front brakes are more efficient than the rear while going relatively straight, therefore use them with equal firmness.
2. When coming to the corner, visually inspect the road surface for loose debris. If debris is present then slow as much as possible before entering the corner using 50/50 pressure on both brakes. NOTE - do not apply firm brakes on loose debris in a turn at speed. YOU WILL CRASH!
3. If the corner is clear of debris then use the rear brake more through the corner simultaneously with the front brake. Approximately 30/70 = front/rear.
4. As you exit the corner, start the alternate braking process once again to maintain a controllable decent speed.
Why you say is this important? Descending a long down hill on a hot day can be a disaster if "heat management" isn't considered.
The tire manufacturer's determine the proper inflation by inflating the tire until it blows off the rim. The blow-off psi (pounds per square inch) is then divided by 2 to determine the recommended consumer tire pressure.
EXAMPLE: 240 psi blow-off number divided by 2 = 120 psi. Therefore, 120psi is the recommended tire pressure you see labeled on the side of the tire. Touring/MTB tires are generally less psi, so read what the recommended pressure is for your tires. Add the hot surface temp of the asphalt and the heat generated by the brakes , and you have a possible over inflation occurrence.
I have had the unfortunate experience of watching rider's blow tires while descending off of Bear Camp pass in southern Oregon on a 90 degree day with switch backs. A cyclist went down about every 10-15 minutes just because they where riding the brakes too long.
Hope these tips will prevent a bad day.
Lonnie Morse ~ "Capt. Dink"
Old habits die hard.
The engineers may be right. BUT, my intuition leads me to stay with the on-off braking pattern for cycling, adding a healthy dose of common sense.
I am reminded of one time about 15 years ago, when I drove down W. Burnside, applying the "even, steady braking pressure" you refer to. When I got to the bottom of the hill, I had almost no brakes. I could smell them. Since then, when going down that hill in my car, I have slowly "pulsed" the brakes, keeping myself under control without stressing the brakes. As a graduate engineer myself, much of what we come up with in the lab is not a complete answer. I think in this case, being aware of what is happening and dealing with it in a sensible way is by far more superior than their suggestion.
Capt. Dink's technique will work for a wedgie bike also - with the provision that the rider must be more careful about front-wheel-braking-only. It is not nice to fly over one's handlebars - which BTW - rarely (note understatement) happens with a recumbent.
[Written in response to a bike shaking when coasting downhill...]
First, when on a downhill, pull your legs in so that they are pressing against the top tube. This will help absorb most vibration in the frame. Also move your butt off the back of the seat so that your quads are squeezing the saddle. This allows the frame to "float" while you dampen the harmonics with your legs. Another piece of advice is to not stop pedaling. Centrifical movement will help to squelch harmonics. Back in the days when my friends and I were being silly on motorcycles we'd sometimes experience "Tank Slap" when harmonics would cause the handlebars to wiggle so violently at high speeds that they would slap each side of the fuel tank. To hit the brakes was certain death. The solution was to apply power. Try it on the bicycle.
Steve "Mr. Badwrench" Heim
Weight distribution has a big impact on stability. If you were too far forward you might experience "whippy" handling such as you described. The downhill you speak of is sufficiently steep to set you into a strange relationship If such happens again ease your center of gravity rearward and relax your grip on the bars, don't fight the oscillations (dampen them but don't go white knuckle here), trail the rear brake lightly and see if things don't settle down.
If this continues, consider a refit to the bike and specifically talk to the fitter about the problem. My first road bike purchase had to have too long a stem added in order to fit me proper. I had to be extra careful transitioning on the bars and slide back (like an off roader) on downhills or under certain conditions I'd end up as you described.
Don "speed wobbles are NOT your friend" Bolton
You've got to get the weight off the front and off the pedals. It's better to put it on the top tube and over the rear wheel. Use your body as a counterbalance. Follow Bolton's advice about letting the handlebars do their thing. Don't allow the vibration to run through your body and into the frame.
Steve "I rode a Suzuki, my buddies rode Honda hinges" Heim
[Concerning a downhill shimmy problem…]
The shimmy. Aaaaah, the shimmy. I took the bike into my shop yesterday am and got two different diagnoses; the shop owner thought it probably was definitely a problem with the wheel, even though it seemed to spin true on cursory inspection. He suggested that a new tube (installed on Saturday before a ride with the Diablo Cyclists) might be inflated unevenly, or that the magnet was too near the tube valve (funny - that's where it's been since I took delivery of the bike and is where the shop placed it in the first place). What made more sense - and Mike had suggested this back when the problem occurred on Sunday - was a possible looseness in the headset. This was apparent when the repairman who checked my bike in at the shop demonstrated it to me. Since the bike now has nearly 1000 miles on it, he said such adjustments were a common occurrence with new bikes as they get broken in. Hopefully, I will have the bike back in a day or two.
On Saturday I had experienced no shimmy problem, even traveling well above 40 mph coming down Dublin Grade from Hayward to San Ramon. The difference is that Dublin grade is relatively straight with very long, gentle curves and very smooth surface and is not actually that steep, whereas Deer Park is a steep grade with sharp curves and a rougher surface. My sense is that the vibration from any looseness in the headset isn't activated on a road like the Dublin Grade, but does become apparent on a road like Deer Park.
Would appreciate any comments other riders may have about front wheel shimmy. I'm sure hoping the problem is fixed permanently; it's sheer terror when it occurs, trying to bring your bike under control before you spin off and crash at high speed.
For a headset to be loose enough to cause what you describe it should have been noticeable (creaking sounds) when climbing, feeling the bike "set" when stopping, and several other clues. I still suspect a weight transfer issue but..
I rode a frame for several years before discovering it had a cracked head tube. It devoured its OEM headset in the first two weeks, I had a cartridge style headset put in and rode it for several years before noting a clunking on climbs. Never could chase it out. Sometime later I had changed the stem and bars and was showing the bike to someone and stood on the opposite side of normal and saw the 1/4 inch long crack.
The point here is many mechanics worked on the bike with either the noises or the ground bearings as indicators of an issue, yet none spotted it.
Did you have a longer stem put on in order to fit the bike? Was the seat pulled forward to shorten your reach?
When you descend are you in the drops, on the hoods, etc? Are you seated, semi stand/crouch? If seated are you forward on the seat?
Does your bike have adjusters in the dropouts? When you transport the bike doe you remove one or more of its wheels? Is the front and rear wheel on the same plane of trajectory? (that’s where I'm going with that).
Can you "borrow" another wheelset and try that on a known shimmy zone?
Your statement regarding being stable while faster on shallower grade however keeps pointing me back to weight transfer.
Don "you aren't using Spinergy wheels are you?" Bolton
On the Bike Friday e-mail list, they discussed this very issue in great detail (downhill shimmy), and the overall consensus seemed to be the top tube. Putting more weight on the top-tube, i.e. bringing your knees together.
From personal experience, I just never allow myself to go that fast! ;)
It's hard for me to believe that the center of gravity changes much between knees together and knees apart. I would think the big difference is the damping effect of your legs. Think of it as adding a shock absorber. More verses less weight would primarily be controlled by seat position, and secondarily by upper body position (leaning over the bars to "get small" shifts weight forward over the front wheel.
But then, those ME classes were never my forte'.
Don "ELECTRONIC engineer" Gross
I'd bet in pinching the tube with their legs they are also unconsciously sliding backwards some. Too much weight either way creates interesting handling quirks.
There are times where one wants to use those quirks to bend thru crisis situations, but if your actions at that point aren't instinctive you be road pizza.
A shimmy is a terrible thing and must be isolated and corrected, or at least understood to the point of being able to control it.
Don "Spandex and asphalt don't play well together" Bolton
Oh yeah, I just remembered what it was that worked for one of the BF owners...He said that when he relaxed his grip, it helped his shimmy. He said that when he held on too tight, he would have a lot more movement. So maybe the answer is to lighten up that grip??
Two other things that can cause a front-end shimmy, both of which I have had experience with include:
2) Misalignment of the frame.
Finally, I would like to reiterate and augment two other causes that list subscribers have mentioned.
A) Tires (and rims) - At high speeds, even a slightly out of round tire can cause problems. The problem can be side-to-side or direction away from the axle.
B) Weight distribution on the bike can be very important, especially on the lightweight and/or higher clearance (bottom bracket to the ground) frames. Even a very slight shift in body weight relative to the front or back wheel can make a big difference in bike stability. In my case, I could easily fly down a hill at above 45 mph if my hands were kept in the drops or on the brake hoods. As soon as I put my hands on the top of the bars near the stem, I went into an "imminent crash" mode - more than 2 seconds like that and I would have been vulture fodder.
I promised a report on my "shimmy" problem and how it got worked out.
Bicycling Magazine's book "Bicycle Maintenance and Repair" (1999, p. 28) advises clamping your knees together on the top tube to stop the wobbling and states that the condition is "usually related to component adjustments" such as faulty or improperly installed tires, loose headset or warped wheels, or to "sizing mistakes" such as a too-high seat.
My bike shop did a warranty tune (no charge) since I have nearly 1,000 miles on it now since I bought it. Among things which they did were to reset the computer magnet opposite the valve on the front wheel (it had been placed at the valve by the shop in the first place), tension the front wheel (I hadn't noticed any slackness on manual inspection) and adjust the headset. They also adjusted the brakes, checked the wheel true, and adjusted the derailleurs and hubs. The "feel" of the bike when I picked it up was nice and firm and solid, so I hope it will not re-exhibit the wobble when I get out on Saturday and do at least one fast downhill to test it out.
The shop owner reiterated what I think is apparent from the statements quoted above from my repair book: it's hard to assign one particular cause to the problem, but it appears that all the possible mechanical sources have been dealt with. I don't think there's a "sizing" issue here, and I'm still not clear whether my body position (too far forward?) has had anything to do with the problem when it has appeared. As the shop owner pointed out, sitting too far back can also create a problem where the front end is too light and may tend to "buck up." It was his idea that on fast downhills (he is still an active rider himself; in his mid-40's I would guess) a good solid, driving pressure on the front end is necessary for optimum stability. The knee-clamp maneuver was not something I had heard of before checking my repair book, and no one I talked to mentioned it. I wonder if anyone has had this experience and found that to be a way to stop the shimmy when it happens. Braking seems to be a natural reaction, but it may be counter-productive. Any thoughts on that?
Best to all. On to CO!
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