The science of going downhill?
 

Hey.

So we've all experienced this at one point, usually when we start fixed.

You go downhill and suddenly you're at an "oh ****" moment--that second where you begin to lose control (mentally, at least) and your bike takes YOU for a ride. For many of us, this is perhaps the most exhillarating part of everyday riding. My friends like to bomb downhills with 30+ degree declines and an equivalent incline at the other side.

Of course, an experienced rider must learn to control this--YOU keeping control over YOUR bike, no matter the terrain--is a key component of mastering riding (do I sound like a roadie yet??).

So... some of us here must have a physics background, or at least a lot of experience in downhill control.

Exactly when does this "oh ****" moment occur? How does a rider gain a feel for how to control the ride? And exactly what percentage of backpedalling to normal pedalling should one exerpt throughout the sections of a hill for maximum control?

We all know of shaloming (sp?) and other techniques to control speed, like skidding or skipping. Let's keep this scenario strictly to backpedalling to start.

Ideally, I think, one should strive to be able to come to a conplete stop without a handbrake within 20 feet of the end of a decent--or BY the end of the decent, if an intersection occurs at such a point.

So... have at it.

Within twenty feet of when stopping efforts commence?

I think this moment comes when you start to get bounced in the saddle, and especially when you try to coast. There's nothing too deep about it physics-wise imo. It just means you need to get better at spinning. You can try and stop without braking on a hill if you think it's fun, but i think it's too much work and too hard on my knees.

within twenty feet of the ground becoming fairly level -- as in, the downhill is over. I should have been more clear.

I'm just trying to think this through at a safety level... keeping a safe speed downhill and being able to stop after the hill is over.


I do see how it's more of a skill thing than anything... mainly getting the feel for exactly when to increase your backpedal ratio and muscle strength. There's obviously no way to do it without practicing on empty roads, and us who have been riding for at least some months are somewhat adept at it. However, I was riding downhill this evening and thought it would be interesting to see how others approach or handle a hill and what it looks like physics-wise.

twenty feet. hope you have a brake...a good one.
obviously, the grade of the downhill is going to have to be moderate enough so that you can still slow yourself by backpedaling to keep control, especially since you said no skidding/skipping. i usually just float the pedals down hills, or backpedal. i can easily spin out on flat ground, no need to try to spin out going downhill.

I'm not understanding why 20 feet is too small a mark if you're moderating your speed properly downhill anyway.

Someone from SF or something has gotta chime in. There are plenty of brakeless riders who need to stop for an intersection immediately following a downhill.

Certain authorities in the field of physics swear by shaloming.

http://www.neartexpress.com/art/imag...GS/PP30265.JPG

Since nobody has chimed in with some physics yet, I guess I'll take a crack. You use different muscles to apply backpressure during different parts of the stroke. Generally, the muscles you use to decelerate during one part of the stroke will be similar to/the same as the ones you use to _accelerate_ a half-stroke later. For example, (I believe) you use mainly your quadriceps to apply backpressure during the bottom of the stroke, and conversely, you use mainly your quadriceps to apply forward pressure at the top of the stroke.

Consequently, to slow down effectively with backpressure, you have to be able to activate large leg muscles/muscle groups and deactivate them before you reach the opposite side of the stroke (half-a-pedal-revolution later). The maximum rate at which you can activate and relax your muscles will undoubtedly vary with practice, muscle constitution, and other variables. When I pass this limit, I find that I can't synchronize my legs enough to actively slow down -- by the time I tell my quadriceps "Go!" at or slightly before the bottom of the stroke, I'm at the top of the stroke (or later), and I either accelerate or don't do anything useful at all.

I would guess that given how much forward walking practice most people have had, the maximum acceleration rate (RPMs) that a person can coordinate will be much higher than the maximum deceleration rate (again, RPMs); I've certainly found that this is the case for myself.

Furthermore, skipping, skidding, and other methods of slowing down at high RPMs are generally all methods of applying opposing force while having your legs rotate slowly enough to coordinate muscle activations.

i guess its pretty important to know bail techniques too. learning how to safely drop a bike is pretty crucial especially for hill bombing.

Oh, and about the 20 feet thing, that will vary a lot by person and with different equipment. The force of friction at the rear contact patch will vary with the contact force (which will itself vary with how much the rider weighs and how much of his/her weight is over the rear wheel) and the coefficient of friction (which will vary with the two contact materials, and _I believe_, some combination of contact patch area and unit contact force)

So, basically, what works for one rider may not work for another, and so on. Find what works for you.

Bottom of the hill is irrelevant if someone cuts across your path in a car. Use a brake.

I don't apply back pressure, I cease pushing down on the pedals and only use my legs to pull up, increasing the speed and causing me to pass all the coasting freewheelers... of course, if there's a light at the bottom I usually just do a series of skip stops to shave off speed, the skip stop mixed with slaloming is the most effective though. Applying back pressure down a hill is hell on knees, if you're that cautious about it just run a brake... or ted shred it :)

I generally start to apply back-pressure a block before intersections on the steep downhill I hit on my way home every night. There's a freeway on/off ramp that is right in the middle of the hill that has a flat spot before it and I come up to that one at a pretty good clip, but I've back-pedaled at that intersection so hard I've skidded without meaning to a couple times. There's another intersection that's at the bottom of a bridge that crosses the railroad tracks, but I've got the timing down on that one so that I just trackstand at the top of the bridge then bomb it downhill. If someone ever blows that light I'll probably be toast. The problem with that hill is the tire-grooves a the bottom from auto traffic. There are impressions about 4 inches deep that, while they haven't caused me to wipe out, have given me a really bad headache from jarring my brain so much. So I just cut across all the lanes to the other side of the road where it's smoother. The worst of the bumps is right in the center of the bike lane. So, not only do I have to time the light, but make sure I can cut over too. Fun!

Get a freewheel ya hippie.

30+ degree slope is 57.7% grade. The recognized steepest street is 35% grade. There is no way your friends bomb down 30 degree slope roads.

For me, the "oh ****" moment is when my cadence goes higher than what I would normally ride comfortably. I've recently switched to a lower gear (was riding 52-17, now riding 48-19), and I find that "oh ****" happens a lot sooner, because I don't have to be going as fast to have the pedals start pedaling me.

The rest of us were just letting that slide.

For me, going downhill means using my brakes. Tap, tap, tap, tap, Squeeze, squeeze, SQUEEZE!!! STOP! DANG IT! STOP NOW I SAY!! (I think I need better brakes)

I was estimating like hell. :p

I don't have the experience to really tell the exact slope of a hill (obviously), but it was intense.

Thanks for all the replies so far though, doods.

My oh sh*t moment comes when I'm riding down a hill and see bar lights to my left.

Shaloming... is this an anti-semitic spin on the word Slalom. Poor Einstein escaped the Nazis but couldn't escape BFSSFG

Shaloming? Anti-septic? What is going on here?

Another thing to realize is that when going downhill, most all of your weight is already shifted to the front of the bike, redering rear wheel braking all the more ineffective.

So we have:

Oh Sh** moment= (Grade of incline x effective gear inches / strength of rider's quads)^amount of cross traffic at next intersection

If you are running a front brake divide above calculation by a factor of 10.

Why can't I use a hand brake(s)?
I take it that you don't do any rides with very long descents on the fixie...... Short descents are tolerable up to a point...:)

ZIIIING

...you my friend are responsible for my new sig!