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Descending technique (or lack thereof): please explain the physics

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Descending technique (or lack thereof): please explain the physics

Old 08-26-05, 09:12 PM
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'nother
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Descending technique (or lack thereof): please explain the physics

I am neither a physicist nor a good descender. But I am at least trying to get better at the latter. Maybe someone can help my curiosity on the former.

Today I was toying with a technique I've seen people do, just to see if it was something to tuck in my cap or not. I was turning my inside knee into the turn. I probably wasn't even doing it right, but maybe I was partway right, as I noticed right away that I felt more stable through the turn than I do if I just keep my knees forward as I have been doing. That got me wondering, why does that feel more stable (it's a little scary to do at first but after the first few turns it felt pretty natural and I think I was taking some turns a little faster than I normally would)? Also I noticed that I could "fine tune" my line with slight variations of the angle that my knee was extended. Pretty cool.

I should say here that I'm not really looking for a critique of my technique; still working on lots of things and I'm still pretty much a wuss at descents. I'm mainly just curious why this somewhat counter-intuitive measure results in at least the feeling of more stability. It certainly seems like something I'll keep.
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Old 08-26-05, 09:28 PM
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If you go here
roadbikerider.com
and sign up for the weekly email, you can download a free e-book that has lots of great tips in it, including Davis Phinney's counter steering method.

I found it quite helpful!
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Old 08-26-05, 09:49 PM
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Personally, I'm not a fan of Phinney's technique as it puts the bike at a greater tilt angle than necessary (rolls you off the edge of the tyre). I prefer the technique used by motorcycle riders that keeps the bike more upright and leans the rider deeper into the corner than the bike. Either way works just fine most of the time up to 95-98% of the theoretical cornering limit.

The sensation of stability has more to do with balance and fine motor-control of your muscles than anything else. By expanding your body to its limits, kinda like a flying-squirrel all spread-out, you have no more twitches and rocking motions. But it's a trade-off between stability and responsiveness. Like in swept-wing airplanes with canards, those things are far from stable, but they can make directional changes telepathically. I like as much twitchiness and responsiveness as possible, I even built my bike with 2" shorter wheelbase than most crit-bikes (36") to get as twitchy and fast handling as possible.

What really counts in cornering is the angle between the center-of-gravity between the bike+rider and the contact patches on the ground. If you're going around a corner at 1G, then the angle between the contact-patch and COG is 45-degrees. It doesn't matter if the bike is 40-degrees and the rider 47-degrees or the bike at 50-degrees and rider 43-degrees. It's the combined COG relative to the contact patch that matters. That's how velodrome riders are able to go around the corners without any leaning relative to the track-surface at all, because they've banked the track to keep it 90-degrees to the bike under cornering. Of course, now your head weights 2x as much and you gotta work out in the gym on your neck muscles to keep it from getting sore...
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Old 08-26-05, 10:56 PM
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^^ Im dizzy but I get the idea
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Old 08-26-05, 11:02 PM
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I too tilt my knee in to the corner. I find that it helps to indicate my intention of turn in with out lifting a hand off the bike. I also believes it helps to lower your center of gravity helping feel and control. but i really don't know
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Old 08-27-05, 12:46 AM
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Originally Posted by 'nother
I was turning my inside knee into the turn.
This always seemed counter to the intention to me. You stand on your outside pedal to shift weight to the outside of the turn to counterbalance forces. Hanging your inside leg further inside would seem to negate part of that effort.

Common racing practice for many - among newer riders mostly, because older ones seem to be tied to habit - is to tuck the inside knee into the top tube. In other words, moving that weight to the outside also. I started the "old" way, and changed to the "new" way some time ago, and I find it much more comfortable.

If you are uncomfortable with things like this kind of change, I wonder if you are just not comfortable with the cornering speeds you use at all. I think practice will take care of a lot of that.
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Old 08-27-05, 08:54 AM
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Originally Posted by gmason
This always seemed counter to the intention to me. You stand on your outside pedal to shift weight to the outside of the turn to counterbalance forces. Hanging your inside leg further inside would seem to negate part of that effort.

Common racing practice for many - among newer riders mostly, because older ones seem to be tied to habit - is to tuck the inside knee into the top tube. In other words, moving that weight to the outside also. I started the "old" way, and changed to the "new" way some time ago, and I find it much more comfortable.

If you are uncomfortable with things like this kind of change, I wonder if you are just not comfortable with the cornering speeds you use at all. I think practice will take care of a lot of that.
I believe what you are describing is the counter-steering method. I have tried that as well and did not get the hang of it but I think I was not doing it quite right, never got it to "click" as many people describe. NOTE: Please don't take this as an invitation to give lessons; see OP: I am just curious what's going on with the knee thing.

Indeed simply being comfortable with the speed is probably half of the "technique" regardless of what the legs are doing.
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Old 08-27-05, 10:32 AM
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Originally Posted by 'nother
I believe what you are describing is the counter-steering method.
While this may be a part of the countersteering technique, it is not for me. Simply what I think follows the physics. But then, I was never good at physics, so ...
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Old 08-27-05, 11:37 AM
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If you look at motorcyle races, they do an extreme version of this to get really low to the ground to get a tight turning radius at greater speeds.
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Old 08-27-05, 08:41 PM
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Originally Posted by fixedfiend
If you look at motorcyle races, they do an extreme version of this to get really low to the ground to get a tight turning radius at greater speeds.
actually motorcyclists are using their knee as a lean gauge and to a lesser extent to help pick the bike back up coming out of the turn. Their body position is what is used to complete the turn, not the dropping of the knee.
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Old 08-28-05, 12:12 AM
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Originally Posted by fixedfiend
If you look at motorcyle races, they do an extreme version of this to get really low to the ground to get a tight turning radius at greater speeds.
You can't really compare motorcycles to bicycles. A motorcycle rider is applying throttle and maybe feathering the brake to balance the centripital forces dynamically... a bicycle rider is just coasting and has to make do with whatever corner speed he arrives with.
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Old 08-28-05, 02:21 AM
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XYZ is exactly right above. Someone wanted to talk physics

In a corner you have to forces (or accelerations), straight and centripital. When you do not turn you have 100 percent straight forward motion. As you turn the centripital force (going in a circle) keeps you in a circle. Too much centripital acceration and you are going to turn too sharp, too little and you are off the road. When you have to much forward velocity you can not develop the centripital force required to make the turn. One of two things happen.

1. You go off the road to the outside of the turn
2. You try to use so much centripital acceration that you loose contact patch and your bike slides out from under you.

Now the counter steer method is tbe way the bike turns specifically. Bikes are unlike a four wheel vehicle that uses the tires for stability where you can use turning wheel motion to utilize centripital acceration. When the forward motion is greater than centripital the car slides to the outside of the turn. On a bike the other wheel does not hold the bike up, a car would lean if it only had two wheels, just look at contact patches in a tight curve. Therefore leaning the bike is the same exact motion that a car uses.

There is a limit to how much you can lean though, based on the coeficient of friction. With the center of mass in the proper place a cyclist can lean to a certain amount, say 60 degrees from vertical. Moving the handlebars more than 20 degrees or so will upset the balance and cause a wreck. The best way to turn then is to give a slight turn of the handlebars as you lean over (90 degree corner) or to just lean. The pushing down on the inside of the bars just facilitates the motion.

We can go lots farther but that is the basics.


As for crashes, the most common are...
1. Loss of surface friction at a given lean angle (relative to speed and corner)
2. Turning the handle bars too much with or without enough lean
3. In racing bumping wheels in a corner causing an additional force to the rider on the inside which may cause him/her to go down.
4. Braking inside a corner causing the wheel to skid.

Cornering is easy once you get the hang of it...


As for straight decending, simple

1. Better tuck - less air resistance, the main force opposing motion at speed - less frontal area
2. More weight - more potential energy, more speed in a direct tuck situation.
3. In the drops - lower center of gravity, more grounded steering is closer to the center of mass.

That is just the basics, you then have to deal with weight vs air resistance issues, pedaling (almost always up to speed faster due to increased forward propulsion, etc. BTW air resistance is very hard to calculate, requires numerical methods to solve... we will NOT go there!
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Old 08-28-05, 07:53 AM
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2. More weight - more potential energy, more speed in a direct tuck situation.

Nope, can't buy that one, Galileo proved that gravitational acceleration is the same regardless of mass.

In a vacuum a lead ball and a feather fall together. We don't ride in a vacuum? Then a more dense rider might acclerate faster due to decreased frontal area.

Any dense riders here to confirm?
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Old 08-28-05, 08:03 AM
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I guess I need to apologize for giving this thread a poor title.

While I do appreciate the physics lesson on descending in general, I'm specifically interested in the "point-the-inside-knee" technique. So far DannoXYZ is the only one that's addressed it and like ovoleg I'm a little dizzy but I get the idea Maybe I'm a dense rider.
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Old 08-28-05, 09:56 PM
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"actually motorcyclists are using their knee as a lean gauge and to a lesser extent to help pick the bike back up coming out of the turn. Their body position is what is used to complete the turn, not the dropping of the knee."

The motorcycle example is the opposite of Phinney's method although they both stick their knees out towards the ground. Look at the hips and the COG of the rider. The motorycle racer has all of this body on the inside and lower than the bike; thus keeping it more vertical and the tyre-patch stays flat on the ground. Phinney dumps his bike to the inside and lower than the body, making it lean more than the body's COG. I can tell you from experience that riding on the sidewalls of the tyres generates no cornering grip...

Another reason I don't like to stick out my knee on a bicycle is that it's a tremendous amount of wind-drag. The parachute effect probably has a lot to do with the feeling of stability as well. Personally, I try to stay as tucked as much as possible to preserve as much speed as possible. On the motorcycle, the power easily overcomes the drag and the emphasis is on cornering speed anyway. On my rice-rocket motorcycle, I use the knee to feel the exact angle of the bike's lean and if I'm already on the edge of the tyre and need a little more cornering speed, I'll slide down even further on the side of the bike. Randy Mamola was pretty insane with that one, the highest part of his body in the corners is his knee that's draped over the seat. The rest of his body's is about on the same level as the wheel-axles and his elbow's also grazing the ground...

"2. More weight - more potential energy, more speed in a direct tuck situation."

Well, almost. It's not density as all humans have roughly the same density. It's the weight to frontal surface-area that counts. The same rider of the same weight will downhill faster if he's in a tuck rather than sitting upright. A rider that's twice as big, will have roughly 2x the surface area facing the wind, but will weigh 8x as much. It's the much larger amount of mass pushing behind the slightly larger surface-area facing the wind that gives bigger/heavier guys an advantage on the downills.

But the tuck's still more important. Going head first, jumping out of a perfectly good airplane, I can get up to about 220mph vs. 125mph if I'm falling flat with my arms and legs spread out wide.

BTW, if any one needs visuals on the two riding positions, I can draw some pictures...
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Old 08-28-05, 10:13 PM
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Thanks again Danno for the insight. I agree with you on the wind drag thing, I can really feel it. Though I know for certain the added stability (or feeling of stability) has allowed me to take some corners somewhat faster than I normally would.

I've been watching and I see a considerably large number of people using this knee technique. Not that it's a popularity contest or anything but that's what got me thinking to give it a whirl, just to see what it's all about. I don't need to fine-tune the descending technique that much; I'm not a racer and don't intend to be one . . . again, mainly just curious.

But given the responses here, maybe I should revisit the counter-steering/knee-into-top-tube technique; I never really gave that a good college try and the logic does make a bit more sense.
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Old 08-28-05, 11:21 PM
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Well, I'll be the contrarian here.

I think trying to compare a motorcycle with a bicycle is just STUPID!

Motorcycle:300 lbs, rider 150 lbs. Motorcycle 185 HP, rider 0 HP. Motorcycle, specifically designed tires that "go around " corners 90% of the time; rider, tires designed to ride "straight up with minimum friction and corner 1% of the time without slipping out from under the bike!

There is NO comparison between a bike and a "bike"!!
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Old 08-28-05, 11:43 PM
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Both Phinney's technique and the body lean, bike up type of turns have their uses. The inside knee on the top tube?..never heard of it, never have seen it. Green eggs and ham... until I see really fast cornering people/descenders do it, I'll just have to say no thanks, Sam I am.
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Old 08-28-05, 11:54 PM
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"There is NO comparison between a bike and a "bike"!!"

You haven't ridden and race both have you? They're darn similar as the physics involved is identical. Just that the larger mass of the motorycle-rider makes everything happen slower, giving you time to adjust and learn the nuances of at-the-limit cornering. A bicycle's limits are so much higher than a motorycle and when you go over that limit, the crash happens in a blink of an eye. Racing a motorycle's really helped my ride my bike better as well, because I can practice all the same motions of counter-steering, leaning down into the corner, unwinding the counter-steering to go into steady-state cornering, balancing grip at the limit with slight steering corrections and body-weight shifting, etc. Things happen so fast on a bike at the limit that having time slowed by 1/2 on a motorcycle really helped me learn how to extract the last bits possible.

BTW, there are also "downhill" bike tires that are similar to motorcycle tires with a oval rather than round profile and a ledge of rubber on the sidewall. I've tried both and haven't gotten any significant differences in cornering-speeds or downhill times. That ledge of rubber's a little scary as well when you're riding on it, very squirrelly, but it scrubs off soon enough after a couple of drifts sliding into a corner..
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Old 08-29-05, 12:34 AM
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Originally Posted by DannoXYZ
"There is NO comparison between a bike and a "bike"!!"

You haven't ridden and race both have you? They're darn similar as the physics involved is identical. Just that the larger mass of the motorycle-rider makes everything happen slower, giving you time to adjust and learn the nuances of at-the-limit cornering. A bicycle's limits are so much higher than a motorycle and when you go over that limit, the crash happens in a blink of an eye. Racing a motorycle's really helped my ride my bike better as well, because I can practice all the same motions of counter-steering, leaning down into the corner, unwinding the counter-steering to go into steady-state cornering, balancing grip at the limit with slight steering corrections and body-weight shifting, etc. Things happen so fast on a bike at the limit that having time slowed by 1/2 on a motorcycle really helped me learn how to extract the last bits possible.

BTW, there are also "downhill" bike tires that are similar to motorcycle tires with a oval rather than round profile and a ledge of rubber on the sidewall. I've tried both and haven't gotten any significant differences in cornering-speeds or downhill times. That ledge of rubber's a little scary as well when you're riding on it, very squirrelly, but it scrubs off soon enough after a couple of drifts sliding into a corner..
Danno, if I thought you had a secret to cornering on a bike, I would take it seriously. The thing is, I see professional riders (bicycle) riding corners DIRECTLY over the frames of their bikes. I don't think they are stupid. If there was an advantage to "weight shifting" they would be using it. I see some extending a knee...maybe for aerodynamic steering, maybe for aerodynamic braking. A knee turned out to the inside of a turn will give a slight tendency to pull you into that turn. You can "feather" your brakes in a turn or you can use aerodynamics to do the same thing.

A bicycle turn is far closer to the physics involved in turning an airplane than a motorcycle. Bank angle and lean physics are almost the same. This crapolla about shifting the center of gravity on an 18 pound bicycle is not only absurd, it is beyond 99.9% of the riders ability on this forum....kind of like a carbon fibre seatpost making them any "faster"!
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Old 08-29-05, 12:39 AM
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Originally Posted by puddin' legs
The inside knee on the top tube?..never heard of it, never have seen it.
Just watch any pro race with a hill in it. The Vuelta is going on now.
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Old 08-29-05, 12:41 AM
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Originally Posted by RhumbRunner
A knee turned out to the inside of a turn will give a slight tendency to pull you into that turn. You can "feather" your brakes in a turn or you can use aerodynamics to do the same thing.:
Just watch them. Once the knee is in one of those positions, they never (not that I have ever noticed) change it during the turn.
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Old 08-29-05, 12:49 AM
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Originally Posted by 'nother
I guess I need to apologize for giving this thread a poor title.

While I do appreciate the physics lesson on descending in general, I'm specifically interested in the "point-the-inside-knee" technique. So far DannoXYZ is the only one that's addressed it and like ovoleg I'm a little dizzy but I get the idea Maybe I'm a dense rider.
Not accounting for bicycle riders who do the 'knee thing' as a conscious 'move', it is very much a natural result of cornering. The tighter the turns and the more linked that they are, the more it will prolly happen.
Proper cornering calls for 'looking' well thru the turn, as far thru the turn as possible. A well accepted and proper method is to not only turn the head but also rotate the shoulders and consequently the upper body towards the turn - regardless of where you center your weight (weight distribution being important, of course). This assures a solid, predictable platform for making good decisions. Without turning your head and shoulders, a rider then relies on the considerably poorer peripheral vision and makes it difficult to keep attention focused well forward rather than what the distraction of what is directly AND is already out of a rider's envelop of control.
Back to the knee. As one rotates the shoulders the natural motion/'reaction' is that the knee will 'flop' out as well. If you;re completely focused on coming turn and line, where the knee ends up is really inconsequential. In very tight radius turn trying to keep it tucked in diverts attention for some from the most imoprtant task at hand, completing the turn and determining the upcoming line.
Been accepted good technique for quite some decades. Its hard to find good pics of good descenders. Photogs like to take pics on the ascents, dramam of the climb and all that, plus its a lot easier to get the photo at 14 mph than at 50 mph. But nothing like watching a great descender - like watching Klammer win a gold metal. Lemond was dedicated technician and a hellacious descender, look for images of him. There are some decent photos of cornering and a very few of good descending. poke around - check footage of the tours. Review Jan Ullrich for how to NOT descend well.
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Old 08-29-05, 01:12 AM
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my tips is ... whatever you do .... don't press the damn break !
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Old 08-29-05, 01:33 AM
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Originally Posted by 'nother
I believe what you are describing is the counter-steering method. I have tried that as well and did not get the hang of it but I think I was not doing it quite right, never got it to "click" as many people describe. NOTE: Please don't take this as an invitation to give lessons; see OP: I am just curious what's going on with the knee thing.

Indeed simply being comfortable with the speed is probably half of the "technique" regardless of what the legs are doing.
He is not describing the countersteering technique. Countersteering is "counter" because you push forward on one side of the handlebar to initiate a turn in that direction. Normally, you would push the right side forward while going through a left turn, that is, turning the bars to the left. However, giving the right side a little forward push causes the bike (and rider) to slightly fall to the right, which immediately results in the handlebar, fork and front wheel to steer right, resulting in a right turn. Doing this on a motorcycle is more crucial because of the mass of the vehicle. It's easier to allow the physics involved (frame and fork design, NOT the gyroscopic effect!) to move the motorcycle's mass than for the rider to apply body weight to get it to lean. With bikes less so, but nonetheless effective for turning.
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