Descending - Cornering - the geometry?

[Boeing]

A higher centre of gravity can increase grip in certain conditions

A higher centre of gravity increases front wheel traction under heavy braking. When climbing a steep incline on dirt, a higher centre of gravity reduces rear wheel spin by increasing rear wheel traction. At higher degrees of lean, a rider whose mass is more inside the turn than the rider who leans the bike as a unit, will have more side grip. The lower his centre of gravity, generally the less grip his tyres will have.

I reckon weighting the outside pedal and allowing the bike to pivot underneath you as a mid corner steering technique during mid to low speed / high lean angle downhill cornering, whilst trying not to get too low on the bike will give you the most control and highest corner speed.

well said.

This can evolve into a big physics discussion of which I am not qualified.

As some have mentioned The "knee" question is something I have always debated with friends.

As you suggest, Speed is the key to much of technique regardless of terrain

I agree with your high speed, high center, high lean....at speed. What most riders fail to recognize is the timing of what I call the "cross over" of ones high center of mass turn to turn. It takes longer to move the high mass over center into the next turn. (as opposed to maneuvering the bike beneath you). A low center of mass is easier etc. So at speed one reason many descenders move their inside knee into the turn is to quicken the mass transfer and quicken the cross over with out lowering center.

I am no expert. and am really only speaking from my own trial and error.

Interesting too is this is a topic more and more in the mountain bike realm as it pertains to 29er vs 26er. I am a tall rider who prefers a 29er now at every level except high speed technical cornering....another discussion I guess

Thoughts?

 

[Hangdog98]

Even though the bike turns the corner because of the tyres, the forces acting upon it are like those of the ball on the string. The bike draws an arc, that arc has a radius being the centre-point of an imaginary circle, that radius is the piece of string. The bike is the ball.

Bike #1 carries an amount of mass in a high position with the rider in a more upright position. When it is leaned over 45 deg for the turn, that mass moves one hundred centimetres* towards the centre-point of the imaginary circle. The centre of mass of bike #1 is 'A'.

Bike #2 carries the same amount of mass but with the rider in a low position. When it is leaned over 45 deg for the turn, that mass moves only ten centimetres* toward the centrepoint of the imaginary circle because the rider is lower on the bike. The centre of mass of bike #2 is 'B'.

A is closer to the centrepoint of the imaginary circle than B is.
A's velocity is therefore less than B's.
The centripetal force acting on bike #1 is less than that acting on bike #2 because A is slower than B.
The demand on tyre grip and therefore lean angle is less on bike #1 than it is on bike #2

Bike #1 can now go faster than bike #2 and bring the velocity of A to be same as B. Even though bike #1's speed is now higher than bike #2, the centripetal forces are the same.

Normally people assume that vertical mass distribution doesn't affect the lean angle in a corner. It does affect the effective radius of turn and therefore the centripetal force {mw^2r} and therefore the lean angle {tan-1 (mw^2r/mg)}. But suppose we can move the CG up and down through 300 mm; in a 45 degree lean that is 200 mm difference in r, compared with a starting value of maybe 10 cm. It only makes a 2% difference in the tyre grip needed. And yes, the effect is larger in tight, high g turns.

The fun comes when we get beyond steady state, first we can look at how the rider balances the bike, which is tiny movements about the steady state point, then we can look at taking the bike in and out of turns, which are large movements. Inertia and mass distribution are important for both. When you take the bike in and out of turns, the forces to roll and yaw the bike have to come from tyres, and I think you'll find in those instances the mass distribution makes a difference.

The good news is that the best cornering technique varies between riders but it mostly depends on how you initiate the turn and what you're doing with brakes and body position when you make that direction change. These discussions can often become math debates but the dynamics of the action is so varied and personal that you need to simply try stuff that the fast guys are doing to see what fits your style.

 

[hiero2]

hangdog, what do u do for a living, if i might ask? You seem to feel at home with these physics equations?

Tony Foale's book about motorcycle chassis and design was mentioned earlier. It is available on Google books in preview form (small bits missing). I've had a chance to review some of it - and it has parts that are quite pertinent. I'll have to spend time on it tho - it is not simple.

CozyB's interview w/ Jobst had me looking at his website - and he also interviewed the author of Bicycle Science - which I haven't seen in 20 years or more. I'll have to revisit that one, as well. It might have something pertinent.

 

[hiero2]

Even though the bike turns the corner because of the tyres, the forces acting upon it are like those of the ball on the string. The bike draws an arc, that arc has a radius being the centre-point of an imaginary circle, that radius is the piece of string. The bike is the ball.

Bike #1 carries an amount of mass in a high position with the rider in a more upright position. When it is leaned over 45 deg for the turn, that mass moves one hundred centimetres* towards the centre-point of the imaginary circle. The centre of mass of bike #1 is 'A'.

Bike #2 carries the same amount of mass but with the rider in a low position. When it is leaned over 45 deg for the turn, that mass moves only ten centimetres* toward the centrepoint of the imaginary circle because the rider is lower on the bike. The centre of mass of bike #2 is 'B'.

A is closer to the centrepoint of the imaginary circle than B is.
A's velocity is therefore less than B's.
The centripetal force acting on bike #1 is less than that acting on bike #2 because A is slower than B.
The demand on tyre grip and therefore lean angle is less on bike #1 than it is on bike #2

Bike #1 can now go faster than bike #2 and bring the velocity of A to be same as B. Even though bike #1's speed is now higher than bike #2, the centripetal forces are the same.

Normally people assume that vertical mass distribution doesn't affect the lean angle in a corner. It does affect the effective radius of turn and therefore the centripetal force {mw^2r} and therefore the lean angle {tan-1 (mw^2r/mg)}. But suppose we can move the CG up and down through 300 mm; in a 45 degree lean that is 200 mm difference in r, compared with a starting value of maybe 10 cm. It only makes a 2% difference in the tyre grip needed. And yes, the effect is larger in tight, high g turns.

The fun comes when we get beyond steady state, first we can look at how the rider balances the bike, which is tiny movements about the steady state point, then we can look at taking the bike in and out of turns, which are large movements. Inertia and mass distribution are important for both. When you take the bike in and out of turns, the forces to roll and yaw the bike have to come from tyres, and I think you'll find in those instances the mass distribution makes a difference.

The good news is that the best cornering technique varies between riders but it mostly depends on how you initiate the turn and what you're doing with brakes and body position when you make that direction change. These discussions can often become math debates but the dynamics of the action is so varied and personal that you need to simply try stuff that the fast guys are doing to see what fits your style.

O - and btw, if I understand one of Foale's treatments correctly, the lateral force exerted in the case of B will be less than that of A. He also demonstrates that the contact patch will remain in the same shape, but will increase in size slightly when leaned over, probably due to the additional force exerted upon the tire (but I haven't completed reading that much). I would think it would follow that the lower CoG B could then lean over more than the 45 degree A, until B's lateral force = that of A.

 

[Black-Balled]

I want to add this to the thread, since this vid clearly shows Taylor Phinney dropping his knee.
...If Taylor isn't getting the best advice and techniques this planet has to offer, then who would? We should be able to better answer this question by now.

I think your logic is not correct. Taylor has better than advice: he has Carpenter/ Phinney genes. That totally trumps technique, trust that.

Haiku...alliteration. This place rocks!

 

[hiero2]

. . . centripetal force {mw^2r} and therefore the lean angle {tan-1 (mw^2r/mg)}. . . .

Let me make sure I understand the formulae, given the limits of the medium.

centripetal force = mass time weight (squared) times radius = "cf"

lean angle = (tangent minus 1) times cf divide by "mg")

What is mg? And I'm sure I've got the 2nd formula wrong.

 

[Black-Balled]

I think your logic is not correct. Taylor has better than advice: he has Carpenter/ Phinney genes. That totally trumps technique, trust that.

Haiku...alliteration. This place rocks!

Therefore the theory that Taylor takes tips...

 

[Hangdog98]

O - and btw, if I understand one of Foale's treatments correctly, the lateral force exerted in the case of B will be less than that of A. He also demonstrates that the contact patch will remain in the same shape, but will increase in size slightly when leaned over, probably due to the additional force exerted upon the tire (but I haven't completed reading that much). I would think it would follow that the lower CoG B could then lean over more than the 45 degree A, until B's lateral force = that of A.

Yes, Tony Foale's work is highly regarded. The difference in corner speed seems more dependent on the mental connection between tire and rider. Large changes in CoG only make small changes to actual grip (by my calculations in the range of 2%). Remember also that a larger contact patch does not automatically equate to more grip. Higher grip (and faster wear) comes from softer rubber. Larger contact patches reduce wear and allow softer compounds, not normally an option, or necessity, for road racing bicycle tires. It may create more turning friction depending on the shape of the patch, but not more actual grip.

If what we are seeking is higher corner speed when descending on a bicycle then I think we are looking for faster and more confident corner entry speed which is dictated by the action of releasing and trailing brake technique and lean initiation by the rider. This is the point in the corner where the rider will either feel confident in his front tire... or not. The forces acting on that front tire, controlled by the rider, affect both braking and side grip and are not necessarily relative to bike speed. Two riders on the same bike, same tires, same corner etc, with different technique will yield different results. One may lose the front and crash at a lower speed than the other guy who sails around the corner like he's on rails. Body position and human input. Movements around the steady state point.

 

[Hangdog98]

Let me make sure I understand the formulae, given the limits of the medium.

centripetal force = mass time weight (squared) times radius = "cf"

lean angle = (tangent minus 1) times cf divide by "mg")

What is mg? And I'm sure I've got the 2nd formula wrong.

Vertical forces.

 

[TexPat]

Hmmm, as I recall, Cycle mag's article didn't come to the same conclusion - but it was a long time ago. It would seem to me, though, that the wheelbase, at a minimum, would change. And, part of cornering technique, as I recall, was to control (in rally driving, using braking, and on motorcycles also using body language) HOW the vehicle "sat down" on the suspension. In rally driving, I'm sure that kind of braking is passe, since the technology of 4-wheel suspension has changed so much. On motorcycles, it might not have.

Very interesting observation about the Hailwood technique. I'll have to check on that and (from Elagabalus).

Motorcycle tires also have a different inflated shape than a bicycle tire. I don't know how they build tires to do that, but they do! I've heard said that this impacts steering characteristics and practice.

As for the MTB descending - wonderful little anecdote! Could this be due to tire size (larger contact patch, greater feeling of security, etc)? You said you thought it was how you took the hairpins - but I don't quite get the picture of what you are doing in the hairpin bend - how do you take the same corner on your road bike?

And Cozy - I'm eagerly waiting to see if we get responses from

Many thanks again.

I've also reached greater speed descending on an mtb through chicanes and hairpins. An astonished German driver caught up with me at the bottom of a descent near Heidelberg Germany to congratulate me on passing him in his BMW sedan through a network of steep hairpins at 80kmh.
Though straight line speed seems more stable on a road bike (up to 60mph as my pb), I'd still rather descend turns on an mtb.
Regarding bb height and CoG: I have two cx bikes. One is old school with a high bb, the other new with a bb drop close to road bikes. The oldie handles far better in all situations!

 

[Apolitical]

The fun comes when we get beyond steady state, first we can look at how the rider balances the bike, which is tiny movements about the steady state point..

The good news is that the best cornering technique varies between riders but it mostly depends on how you initiate the turn and what you're doing with brakes and body position when you make that direction change. These discussions can often become math debates but the dynamics of the action is so varied and personal that you need to simply try stuff that the fast guys are doing to see what fits your style.

Reading about cornering, whether cars, motorcycles, or bicycles, gives me a headache. There are so many factors. Nobody has yet addressed the best arc through the turn. Where is the apex? Is the fastest turn done by winding (leaning the bike in) into the turn and then, at the apex, unwinding (letting the bike come upright) out of the turn? Thus, making it a 2 step process?

It does seems from a simple pov, e.g. the rider is a point mass, speeds are constant through the turn, the arc is constant, that it's not that (or as) difficult. It's the human element that changes everything. A lot of what is done is to put the rider into a position where he can make adjustments to compensate for external conditions. For instance keeping your body more upright in the turn allows for easier adjustment to the external conditions than when we're leaning into it. Do we really need to counter steer to initiate the turn instead of just leaning? Or is this just a mechanism that puts our body into a good position?

BTW, I always thought I was faster decending on the mtb because of the longer wheel base. I recall having a lemond/hinault geometry bike in the 80's (slightly relaxed angles with a fraction longer wheel base) and being able to decend on it faster than my future rigs. Granted, it felt slower in crits. However, it was less twitchy which made for nice carving through the turns.

 

[hiero2]

Top-posting - apologies if they are considered due. I've got some work ahead of me - and I'm taking a cert test next tue, so it'll be a little b4 i get back to this - unless i get my study work done early.

First, though, it seems we need to define the question. We'll have to control some variables for now. So here is what I think I'm asking:

Given - identical bicycles and rider weights, identical speed, no brakes being applied at the initiation of the turn.

Question (s) - which rider gets through the corner fastest, and what does he do to get there?

This is going to break down to body position, and how it will impact the turn. Corollary question is why do most of the racing peloton drop their knees in a hard turn?

Definition of an answer: how does the body position described in the booklet early in thread, and in the mtb downhill anecdote, impact cornering forces, ditto dropping the knee.

After I feel like I've got an answer to that, then I'll branch out into other variables - maybe.

See below for more comments.

Yes, Tony Foale's work is highly regarded. The difference in corner speed seems more dependent on the mental connection between tire and rider. Large changes in CoG only make small changes to actual grip (by my calculations in the range of 2%). Remember also that a larger contact patch does not automatically equate to more grip. Higher grip (and faster wear) comes from softer rubber. Larger contact patches reduce wear and allow softer compounds, not normally an option, or necessity, for road racing bicycle tires. It may create more turning friction depending on the shape of the patch, but not more actual grip.

If what we are seeking is higher corner speed when descending on a bicycle then I think we are looking for faster and more confident corner entry speed which is dictated by the action of releasing and trailing brake technique and lean initiation by the rider. This is the point in the corner where the rider will either feel confident in his front tire... or not. The forces acting on that front tire, controlled by the rider, affect both braking and side grip and are not necessarily relative to bike speed. Two riders on the same bike, same tires, same corner etc, with different technique will yield different results. One may lose the front and crash at a lower speed than the other guy who sails around the corner like he's on rails. Body position and human input. Movements around the steady state point.

We need to define grip. Also, if what is shaping up is what I'm suspecting, then the lower CoG can utilize the SAME grip to corner faster - ALL OTHER FACTORS BEING EQUAL (as proposed in the question definition above). From what I've read in Foale so far, for the same cornering attitude (degree of lean), a lower CoG will have less "F" and more "mg" than a higher CoG (I'm sure the total forces should probably be equal).
lower = subL
higher = subH

mgsubL + FsubL = mgsubH + FsubH
mgsubL > mgsubH
and
FsubL < FsubH

Reading about cornering, whether cars, motorcycles, or bicycles, gives me a headache. There are so many factors. Nobody has yet addressed the best arc through the turn. Where is the apex? Is the fastest turn done by winding (leaning the bike in) into the turn and then, at the apex, unwinding (letting the bike come upright) out of the turn? Thus, making it a 2 step process?

It does seems from a simple pov, e.g. the rider is a point mass, speeds are constant through the turn, the arc is constant, that it's not that (or as) difficult. It's the human element that changes everything. A lot of what is done is to put the rider into a position where he can make adjustments to compensate for external conditions. For instance keeping your body more upright in the turn allows for easier adjustment to the external conditions than when we're leaning into it. Do we really need to counter steer to initiate the turn instead of just leaning? Or is this just a mechanism that puts our body into a good position?

BTW, I always thought I was faster decending on the mtb because of the longer wheel base. I recall having a lemond/hinault geometry bike in the 80's (slightly relaxed angles with a fraction longer wheel base) and being able to decend on it faster than my future rigs. Granted, it felt slower in crits. However, it was less twitchy which made for nice carving through the turns.

Ya, it gives me a headache, too! I'm saving the arc for later - it'll be one of the variables we keep constant or ignore for now. I'm going to deal with lean first. I will say, tho, and I think this was from Jobst - but somebody was saying that the arc should be different than a powered vehicle, since there is no power applied to exit the turn. If I recall - he posited exiting the arc much later to avoid scrubbing speed on entry.

As for counter-steering, I think, that the answer to your question is no, but a very small counter-steer will make the bike lean faster. A big counter steer will have you head over teakettle! I have wondered, tho, if when you lean, the bike doesn't enact a very tiny countersteer that is smaller than what one is normally aware of. And, I'm thinking that a lot of the differences we are discussing are very small actions, and small results, but it is the cumulative impact that nets a one or 2 kph difference.

 

[Apolitical]

Ya, it gives me a headache, too! I'm saving the arc for later - it'll be one of the variables we keep constant or ignore for now. I'm going to deal with lean first. I will say, tho, and I think this was from Jobst - but somebody was saying that the arc should be different than a powered vehicle, since there is no power applied to exit the turn. If I recall - he posited exiting the arc much later to avoid scrubbing speed on entry.

As for counter-steering, I think, that the answer to your question is no, but a very small counter-steer will make the bike lean faster. A big counter steer will have you head over teakettle! I have wondered, tho, if when you lean, the bike doesn't enact a very tiny counter steer that is smaller than what one is normally aware of. And, I'm thinking that a lot of the differences we are discussing are very small actions, and small results, but it is the cumulative impact that nets a one or 2 kph difference.

It seems to me from experience the 2 most important part of cornering are the braking and transitioning into the turn (especially on really steep roads) and picking the best line through the turn. Mess up either of these you end up needing to make big corrections which usually means braking and slowing down.

I don't believe lean makes that big a difference. Especially on smooth even gradient roads. In most real life situations however, if you're leaning with the bike it's harder to make to make corrections than if you're upright. In ice skating we'll say "leaning out of the turn" - though not really - for the same reason. If you're leaning in and make an error it's hard to compensate. Most likely because you have to move your entire body weight as opposed to making a small correction.

 

[hiero2]

I'm pretty sure I've found the answer to one question that we posed here - what is the fastest line through a curve, and where is the apex of that line.

Assuming that THERE ARE NO MITIGATING FACTORS - like other cyclists, taking the wrong line coming in, coming in too hot, etc - the fastest line will have its apex at the center of your turn. This should be true regardless of your speed. This does assume that you choose the correct entry line for your turn radius and speed. Realizing that this was true was simpler than I thought. When taken at maximum speed for the given roadspace, the apex of your turn will also be the centerpoint of the turn in the road.

This may change when you continue braking after entering the turn. This may change when descending - since when descending you might have a greater acceleration capacity than when on the flats.

If you want to posit that you will slow down somewhat because the turn is unpowered (coasting), then, add in to the equation the fact that for a given lean angle, a faster speed requires a larger turn radius. Ok, then if I'm slowing through the turn, the lean angle I require to finish the turn reduces -or the same lean angle produces a sharper turn. Accounting for this, your turn would no longer be a symmetrical shape - but would become a lopsided curve. The optimum apex would be, then, slightly after the midpoint of the curve. However, I would think that the difference in speed - the negative acceleration - resulting from coasting, would be so small as to make this negligible.

Something else I've learned - that someone more au courant with physics would have known - is that centripetal forces are the opposite of centrifugal forces. To me, realizing this brings the whole "centripetal force" issue back to plain English. It's like muscles - for a puller you must have a pusher. "Equal and opposite reaction", all that. The centripetal forces in a turn are those that are opposed to the centrifugal forces. Simple. Part of why we lean in to a corner on a bicycle is NOT to turn, but to counter the centrifugal forces, which would have us *** over teakettle otherwise.


I'm still studying.

 

[Apolitical]

.... Simple. Part of why we lean in to a corner on a bicycle is NOT to turn, but to counter the centrifugal forces, which would have us *** over teakettle otherwise.

I'm still studying.

I think you need to start at the basics. Why does the bike stay upright? Look at a single wheel, if you stand it up it falls over. But if you roll it it doesn't fall. And, the faster it goes the harder it is to make it fall. This is because of conservation of angular momentum (Iw). If a wheel is rolling and tries to fall over a torque will straighten it up again. Remember the experiment where you roll a wheel and then turn it on a stool that can rotate? This is why we can ride a bike and not fall over.

Thus, the bike wants to go straight. To counteract these wheel forces and turn, you must lean. The faster you go the harder it is to turn and the more you must lean.

At slow speeds you can steer (like a 2kph hairpin turn on an mtb), as opposed to corner, because there isn't much torque generated by the wheels trying to keep the bike straight and upright.

Just more to think about.... it still hurts my head.

 

[hiero2]

Now, why would you have me repeat a class? Are you flunking me here, dude? Anyway, I've been through all that - long time ago - working on stuff a couple of steps down the road now.

 

[Apolitical]

Now, why would you have me repeat a class? Are you flunking me here, dude? Anyway, I've been through all that - long time ago - working on stuff a couple of steps down the road now.

Dude! First, no one brought up what I mentioned in my last post about balance and why a bike doesn't fall over once in motion. Second, reread the third post where the centrifugal force was brought up, though not spelled out.

Cheers

 

[Hangdog98]

Dudes (assuming that is the new vernacular for inquisitive cyclists , there is no good line down a mountain pass. There's way too many variables, including road surface, camber etc etc. You descend faster when you feel confident, mostly in the way your bike makes the change of direction at the 'Turn-in'. Some days your technique is good and your confidence is high and you fly. Other days you ride like a shop mannequin and you suck.

Centrifugal force contributes very little to bike control. The bike stays upright because of steering micro adjustments which cause the portion of the bike behind the head tube to want to pass the head tube causing the bike to roll in the direction opposite* to the steering input. (*turn left, leans right). The rotating wheels and subsequent conservation of angular momentum slow the roll process down but don't keep the bike from falling. That's our job as riders.

Now, the geometry of the bike plays a major role in creating that cornering confidence. Some bikes I've owned descend better than others. I find that longer chainstays work better for me. I also find a higher bottom bracket and flat bars contribute to better feeling and higher corner speeds. I remember a prototype cyclocross bike I made with disc brakes that was awesome on a favorite downhill run. It had drop bars but I rode it with my hands on the hoods because the brakes needed only a gentle nudge with a single finger to get all the braking I needed. Downhill, I flew past everybody on that thing, even in the wet it was brilliant. There are too many variables to pin down what made it fast for me but there are a few design features that crop up in all my good descending experiences.

Dudes, don't get caught up in the math, it's all about confidence and finding a bike and a technique that gives it.

 

[Boeing]

Vertical forces.

that dude is on a mountain bike and his pedals are level

 

[Boeing]

i think what Dude is saying to Bro is: "it depends"

apart from that...Clearly this is a Roadie discussion

 

[DirtyWorks]

Hips

Some of you grounded in physics might turn the following observations into more precise language, but I think this might help the discussion.

I teach kids that the key to turning is in the hips. They learn to treat their hips as the turning point with their eyes on the horizon as the guide. It's possible to turn at the shoulders without using your hips, but scary for kids. That's why talking about steering at the handlebars just doesn't work well. Once the idea that where they look is exactly where they will go and their hips take them there, cycling is more fun. It takes a while for the concept to stick, but that's being a kid...

I would argue the Davis Phinney description is manipulating the body's position at the hips. The outer leg being straight affects the body's center of gravity. (the hips) The lost part of the Phinney description is timing the shifts he is describing. Also not described is the position of the head. Eyes on the horizon! (roughly down the road)

When I need to brake into a corner, I picture making turns into elbows. The "elbow" is the time in space where I'm changing direction. Timing the shift of the hips Phinney describes coincides with when the head turns towards the exit of the elbow. Turns I don't have to brake into, I can reasonably make into an arc using counter-steering.

Once you fully comprehend the Phinney's/my method, the speed in descending is entirely mental.

Lastly, if you want some rich information on steering talk to tandem captains whose new partners are blind. If the captain does not share corner information, cornering is a mess.

 

[DirtyWorks]

Hips

Some of you grounded in physics might turn the following observations into more precise language, but I think this might help the discussion.

I teach kids that the key to turning is in the hips. They learn to treat their hips as the turning point, not their shoulders/arms/handlebars. It's possible to turn at the shoulders without using your hips, but scary for kids.

I would argue the Davis Phinney description is manipulating the body's position at the hips. The lost part of the Phinney description is timing the shifts he is describing.

When I need to brake into a corner, I picture making turns into elbows. The "elbow" is the time in space where I'm changing direction. Timing the shift of the hips Phinney describes coincides with just after entering the "elbow." Turns I don't have to brake into, I can reasonably make into an arc using counter-steering.

The speed in descending is mostly mental.

Lastly, if you want some rich information on steering talk to tandem captains whose new partners are blind. If the captain does not share corner information, cornering is a mess.

 

[hiero2]

DUDES!, and DUDETTES! (altho we have yet to see a dudette posting in this thread), what a fine conversation!
. . .
[cue background conversation, like stereo or radio] "Dave's not here, man!"
. . .
Ya, I get caught up in phrasing from long ago and far away sometimes.

Dude! First, no one brought up what I mentioned in my last post about balance and why a bike doesn't fall over once in motion. Second, reread the third post where the centrifugal force was brought up, though not spelled out.

Cheers

Aha - ok, then, I think I understand where you're coming from. But, what I want to know has, I think, less to do with the gyroscopic forces and how trail geometry affects the bicycle, and more to do with how does the rider maximize these forces in a cornering situation. So far my readings and study have supported that what I am looking for is not centered in the gyro forces and geometry. One thing upcoming (from me) that you might be interested in. While reading Foale's book, it seems that Foale is saying that you CAN'T have a turn without an initial countersteer. I'm thinking of locking a bicycle headset so that there is NO steering available, and testing this. If I understand Foale correctly, this means that the bike will have to become unstable before it can turn, and I will likely do a faceplant, so say a prayer for my old bones - and that I don't break any.

Dudes (assuming that is the new vernacular for inquisitive cyclists , there is no good line down a mountain pass. There's way too many variables, including road surface, camber etc etc. You descend faster when you feel confident, mostly in the way your bike makes the change of direction at the 'Turn-in'. Some days your technique is good and your confidence is high and you fly. Other days you ride like a shop mannequin and you suck.

Centrifugal force contributes very little to bike control. The bike stays upright because of steering micro adjustments which cause the portion of the bike behind the head tube to want to pass the head tube causing the bike to roll in the direction opposite* to the steering input. (*turn left, leans right). The rotating wheels and subsequent conservation of angular momentum slow the roll process down but don't keep the bike from falling. That's our job as riders.

Now, the geometry of the bike plays a major role in creating that cornering confidence. Some bikes I've owned descend better than others. I find that longer chainstays work better for me. I also find a higher bottom bracket and flat bars contribute to better feeling and higher corner speeds. I remember a prototype cyclocross bike I made with disc brakes that was awesome on a favorite downhill run. It had drop bars but I rode it with my hands on the hoods because the brakes needed only a gentle nudge with a single finger to get all the braking I needed. Downhill, I flew past everybody on that thing, even in the wet it was brilliant. There are too many variables to pin down what made it fast for me but there are a few design features that crop up in all my good descending experiences.

Dudes, don't get caught up in the math, it's all about confidence and finding a bike and a technique that gives it.

You are absolutely correct, and yet not entirely correct. The variables are EXTREMELY numerous, and I am certain that what you have said is a primary reason why this has not been looked at in more detail in the past. Cornering technique does not offer the pay-off in bicycling that it does in motor sports. In fact, cornering technique is a minor component of getting from point A to point B, and is rarely decisive, and thus rarely practiced.

However, I disagree that it is not possible to determine a "best" line. The reason I started this was because I have observed a difference in body language application through the turn in theory and in practice. Since I respect both the theorists and the practitioners, it occurred to me to wonder why or how both could be right - since they both obviously believe themselves correct, and have, at least, anecdotal experience to lend substance to their position.

I'd like to touch on two examples of possible variables, both of which have come up in this thread. A couple of people have mentioned quick descending on mountain bikes. I might guess those bikes had fatter tires than their road bikes. Fatter tire = bigger contact patch = HUGE difference in ability to hold the road (technically coefficient of friction). You just mentioned a mountain bike with disc brakes. Since, in descending, braking plays a HUGE role in speed control, if you had confidence in your braking system, and it was more responsive than a traditional rim brake, one could quickly reason that this could make more difference in your "time to the bottom" than your cornering technique. This validates your "too many variables" position, but . . .

But, all things equal, given similar hardware, etc. why do competitive cyclists use the knee drop when serious objections have been raised as to the validity of this technique?

Some of you grounded in physics might turn the following observations into more precise language, but I think this might help the discussion.

I teach kids that the key to turning is in the hips. They learn to treat their hips as the turning point with their eyes on the horizon as the guide. It's possible to turn at the shoulders without using your hips, but scary for kids. That's why talking about steering at the handlebars just doesn't work well. Once the idea that where they look is exactly where they will go and their hips take them there, cycling is more fun. It takes a while for the concept to stick, but that's being a kid...

I would argue the Davis Phinney description is manipulating the body's position at the hips. The outer leg being straight affects the body's center of gravity. (the hips) The lost part of the Phinney description is timing the shifts he is describing. Also not described is the position of the head. Eyes on the horizon! (roughly down the road)

When I need to brake into a corner, I picture making turns into elbows. The "elbow" is the time in space where I'm changing direction. Timing the shift of the hips Phinney describes coincides with when the head turns towards the exit of the elbow. Turns I don't have to brake into, I can reasonably make into an arc using counter-steering.

Once you fully comprehend the Phinney's/my method, the speed in descending is entirely mental.

Lastly, if you want some rich information on steering talk to tandem captains whose new partners are blind. If the captain does not share corner information, cornering is a mess.

Exactly. Particularly

I would argue the Davis Phinney description is manipulating the body's position at the hips

. This is exactly what I am trying to find some more serious validation for. Phinney's description of cornering would seem to be at odds with the knee-drop technique, and is certainly at odds with the "body language doesn't matter because it's all in the coefficient of friction" idea. I'm assuming that Phinney, and Spartacus (check YouTube for Cancellara descending for some good vids of his descending technique), are on to something when they don't stick with a straight body in the corner. Once you've made that step, one has to ask "why" or "how" they can be right, when it is also true that the rules of physics apply, and the coefficient of friction is the ultimate truth.

Right now I'm trying to work out, in my head, why motorcyclists hang off. I think understanding this has a key to the various body language cornering treatments we see in cycling. In Foales book, he points out that the hanging-off reduces the gravitational component of force when leaning, and increases the centrifugal component. I'm currently trying to figure out how it WORKS, then, as this is opposite to what I would have informally conjectured. And, leaves me doubting the validity of the usefulness of hanging off - which, however, is now thought to be a universally race-proven technique on getting thru a corner faster.

 

[Hangdog98]

OK, just two things homie,

1). A bigger contact patch offers less grip with the same rubber compound and;
2). Racing motorcyclists hang off towards the inside of the turn to get their knee on the ground which turns the bike into a trike and allows the tires to slide all over the place without falling down. The practical side of hanging off is that the further you do it, the less lean angle you require for a given turn at a given speed and thus you can carry more speed for the same lean angle.

Racing motorcycles run out of ground clearance before they run out of side grip and so lean angle is a precious commodity. Bicycles on the other hand run out of side grip before they run out of lean angle so hanging off is not for the same purpose. I put my knee out on the bike and it helps to rotate the pelvis towards the turn which in turn makes it more natural to turn the head (keeping the eyes level) and to look through the turn.

and thirdly, the best line for a corner depends on;
a. radius (constant or variable).
b. Flat, uphill or downhill or a combination.
c. approach speed.
d. traction (is there gravel or moss on the roadway).
e. knowledge of the corner (have you been here before and seen how it tightens up and goes off camber causing you to run wide towards to the 1000 foot drop off?).
f. are you alone or in a bunch (gruppetto of dudettos).
g. Braking or rolling or pedaling or a combination.
h. is it the last corner before the bunch sprint to the line.
i. is it damp and do the white lines (road paint) offer any grip.

It doesn't take your style of riding into account nor your state of tiredness or even rider weight which plays a huge role in braking distance and traction.

This is just some considerations that we make with our massive cycling brains when approaching a corner before deciding what line we'll take.

 

[DirtyWorks]

motorcycles != bicycles

Right now I'm trying to work out, in my head, why motorcyclists hang off.

Bicycles are not motorcycles. Two factors *vital* to keeping the distinction are:

1. Motorcycles have insane amounts of power available for manipulating the vehicle. Bicycles do not.
2. Motorcycles sans rider are **much** heavier. That along with #1 invalidate comparisons between bikes and motor bikes.

Lastly, if in fact one counter steers on a bicycle, the rear wheel can slip something vaguely like a motor bike and you'll live to tell about it. It alters your line slightly, but that's about it. 'cross riders who corner on the rivet can back me up on this. I've done it on the road too. Dropped knees not required.