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Descending - Cornering - the geometry?

Jul 10, 2010
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Ok - asking a favor - do you know of any good work regarding bicycle cornering characteristics? I found some old stuff from rec.bicycles.misc (or .tech, I forget) - but it was not definitive (relying on expert opinion), and I found contrary postings from sources just as well respected.

If I can't find something already there, I might do some work on this on my own.
Anybody know of any publications that applied physics, geometry, scientific method? Looking for stuff that relates to cornering and descending technique (essentially the same thing, since descending is just fast cornering with the added risk of going over the edge - aieeeeeeeeeeeeeeeee!).
 
Can you be a bit more specific? Cornering technique has been covered by many. I suggest you read Jobst Brandt's advice.

The physics of cornering a bike is pretty simple. The maximum lean angle at any given speed is limited by the co-efficient of friction at the tire-road interface and any good University level physics textbook should contain a mathematical treatment of it.

A lower center of gravity will also aid in the bike being more nimble. All things kept the same, a bike with a longer wheelbase will have a higher turning radius. Take two bikes, one short, one long. Turn the handlebars left slight. Mentally draw a straight line along the axis of the bar towards the extension of the rear axle axis. The point at which they meet, and the angle which those two lines form, is the turning radius. You're better off with a light and nimble bike at the cyclocross races.
 
Jul 15, 2010
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Steering dynamics

As you've discovered, you have asked a question about something that appears very simple but is in fact an enormously complex interaction between machine and rider. The dynamics of steering a bicycle descending at speed is at the more complex end of the process. It is quite different to steer a bicycle when rolling downhill or racing downhill and braking into a hairpin bend. You will also find a tremendous amount of conflicting information about bicycle steering dynamics as it's an area of cycling dynamics that is poorly understood as the micro shifts in moments of inertia play a huge part. Many in the motorcycle racing community call it a 'Black Art'.

I don't know what point you're at with your research so I'll cover off some basics for the benefit of everyone.

Simply, the bike turns a corner when the tyres are leaned over and the difference between the circumference of the tyre on the contact patch on the part of the tyre inside the turn is lesser than the circumference of the tyre on the contact patch on the part of the tyre outside the turn. Think of a disposable coffee cup rolling on it's side making an arc. Of course, that's only a small part of the process of riding quickly down a mountain road where the difficult part is initiating the turn and transition from upright (braking traction) to side grip (turning traction).

The practical aspect of initiating a turn involves a process called counter-steering. Newton's laws tell us that an object will travel in a straight line unless force is applied to divert its path. In the case of a bicycle, the force comes from the unequal friction applied to the tyre's contact patch mentioned above. To initiate a turn we need to lean the bike over and this can be achieved by shifting body weight to the inside of the turn. This however is really only part of the way we initiate a turn. The real work is done through small amounts of counter-steering.

Counter-steering can be better understood if you think of it as the jack-knife effect. Riding along a level road at 20 kph, moving the handlebars to the left will initiate a right turn. The reason for this is that the bike will try and drive past the steering head (head-tube/fork steerer) as the front tyre tries to go left. The bike will then fall to the right putting the tyres on the side which will then start turning right. To arrest the fall to the right, the rider will steer a little to the right and balance the lean process with micro adjustments to steering that cause the bike to exert force either side of the head-tube. Gyroscopic forces are commonly credited with this process but in reality they play a secondary role.

Riding down hills we find that the bike will be more difficult to initiate into a turn. There are many reasons for this including (but not limited to) the application of brakes. The jack-knife effect relies on the part of the bike behind the steerer trying to overtake the front part. As you approach a hairpin turn on a steep mountain road at 70kph you must first reduce your speed to get around the corner. To do this you apply both front and rear brakes to wash of a lot of speed. If you are still hard on both brakes when you try an initiate the turn, the bike will respond by not wanting to lean over as easily and by pushing the front tyre as you overwhelm the available traction. The feeling on the bike is that the bike wants to run straight ahead. This is caused by reaching the limit of traction, flattening out the tyre and by the practical effect of steepening the steering head angle which in turn reduces the amount of contact patch lateral shift under counter-steering. The usual response to this is to brake harder which exacerbates the problem and results in a slow corner or by making a bicycle shaped hole in the scenery:eek:. Letting off the rear brake forces even more weight onto the front tyre and makes it worse. Letting off the front brake has the opposite effect and makes for a more manageable turn-in, provided of course you've washed off enough speed beforehand, but it's not the fastest way around and if you stay on the rear brake and lose rear grip you'll fall.

This may seem to be at odds with the jack-knife effect which relies on the rear part of the bike trying to overtake the front but the friction and subsequent (and relative) slowing of the front tyre as it veers off in another direction is sufficient to initiate the turn-in.

Generally speaking, a gradual easing of the front brake before the point where you need to make your turn-in followed by a trailing rear brake and the practice of looking through the corner (to force your weight shift) will result in a faster mid corner speed. Why, by allowing you to manage the available traction for both braking and turning using brakes, body-weight shifts, counter-steering and talent. This part is a whole book by itself.

Different geometries move the contact patch to greater or lesser degrees for the same steering input. Longer wheelbase (as correctly mentioned) affect the lean angle required to make the turn. The center of gravity of a bicycle is less important than the centralization of mass and the moments of inertia created by the dynamic between rider and machine.

This is why some guys are faster than others on the descent. It's not about the bike, unless you're riding something outside the narrow range of accepted geometries, or racing on a unicycle.
 
Jul 10, 2010
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RE: steering dynamics and Jobst

Actually, Jobst and Sheldon are two of the expert opinions I was referring too, and attempting to do so without naming them. I have the highest regard for Jobst, but he is a curmudgeon. He is also an intelligent, educated and accomplished curmudgeon. The result is that when he is wrong, you will never get him to admit it. And, when he doesn't know the ultimate truth, he will also not admit that.

Since there are other expert opinions that disagree with Jobst's assessment of cornering geometry and physics, especially when it comes to center of gravity and body shifting - I want to find a more thorough treatment than just expert opinion.

Hangdog's reply is a good one - as is his explanation of counter steering. I wasn't clear, but that is more basic than what I'm seeking. Here is the nut of where I'm looking: Hangdog98 sed:
You will also find a tremendous amount of conflicting information about bicycle steering dynamics as it's an area of cycling dynamics that is poorly understood as the micro shifts in moments of inertia play a huge part. Many in the motorcycle racing community call it a 'Black Art'.

Yep. Somewhere in the 80's, or maybe even in the late 70's, Cycle magazine did an excellent, and thorough, article on motorcycle geometries and body language in cornering. The thing for cyclists, tho, is it by and large didn't apply - because a bicycle's geometry doesn't change, but a motorcycle's does (suspension, and I'm ignoring suspended mtb for the moment).

So to be specific, I'm looking to find validation, or not, for Jobst's theorizing on cornering geometry and c.o.g.
 
Jul 15, 2010
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body shift

Here's one example of body position affecting steering dynamics that may add to your research.

OK, so Jobst penned that putting your foot out into a turn has no value on a bicycle. When one is observing the actions of motion it sometimes helps to examine actions at the extreme end of the spectrum to see what happens. Motorcycles respond similarly to bicycles in corners. Their suspension adds a few variables but there is a lot of common ground. Recently, MotoGP riders began putting out the inside leg on the approach to hard braking corners. Valentino Rossi said he doesn't know why it works, but it does. What is mainly happening is a shift in the pivot point of the body on the bike. The rider can counter-steer and change the lean angle of the bike with him seated or standing. With both feet on the pedals (pegs) the change in lean angle of the bike includes a substantial amount of the riders body mass. This takes more input and causes different stresses on the side grip of the tyre. When Valentino takes his foot off the pegs and makes a counter-steer induced lean angle change to the bike, the bike pivots under his bottom and moves about 30kg less mass to effect the same change in contact patch position. This allows more finite micro-adjustments, enables more rider feel and results in better braking and steering control at the limit. This is why dirt bike riders do it. There are secondary forces at work here too and they relate to the proportion of mass inside the turn relative to the moment of the turn radius.

Jobst is probably right that un-clicking and sticking your leg out is not the best way around a turn on a road racing bike but, in my view, he has completely ignored the effect of changing lean angle with or without the mass of the rider.

A bicycle rider who lifts his backside off the seat a little and allows the bicycle to rock side to side between his legs is moving a 7kg mass. The rider who puts all his weight on this saddle needs to move a 70kg mass when making a lean angle change. This allows more finite adjustments and greater feel. Indeed, this method is employed by the fastest riders particularly when they reach extreme lean angles in the centre of a turn. Good riders employ both methods in each and every turn to varying degrees. At the end of the day it is the contact patch and its differential friction that makes the thing go around a corner. Using your body and your bicycle controls to change the forces acting on that patch is what makes it go around a corner efficiently.
 
Jul 10, 2010
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More cornering technique debate material

While I'm on this little run - and I've got the material in front of me, let's add the other primary expert opinions. Note that Jobst (Brandt) and Roger (Marquis) originally posted these in the rec.bicycles.misc usenet FAQs.

Sheldon Brown (posted on Sheldon's site) agrees with Jobst:
http://www.sheldonbrown.com/brakturn.html
Many people will tell you that it is dangerous to use your front brake in a turn; I would respond that this is so if your turning/banking technique is incorrect. The center of gravity of a bicycle/rider must lean into a turn; this is required by the laws of physics. There are three ways you can do this. One way is to keep the bike more-or-less upright, but to lean your upper body into the turn. Another is to keep your body more-or-less upright, and lean the bicycle under you. The third, and usually correct technique is to keep your body in line with the bicycle frame, lean the bicycle and rider together as a unit.

Leaning the bicycle and rider differently messes up the handling of the bike, by moving the center of gravity sideways from the plane of the wheels. If you apply the front brake while doing this, the braking force exerts a steering force through the now off-center headset. Jobst Brandt has an excellent way of proving this to yourself: try riding down a straight, but bumpy stretch of road while leaning the bike to one side and your body to the other. If you are brave, try applying the front brake very gently.

But, all that Jobst's test does is demonstrate that changing the body position creates steering forces. This is not the same thing as how it impacts steering geometry or dynamics.

Roger Marquis posted the original "Descending" FAQ, to which Jobst responded with his Descending FAQ post, if I'm not mistaken. They don't agree. Roger's FAQ can be found here: http://www.faqs.org/faqs/bicycles-faq/part5/section-3.html

One of the problems I have with Jobst's assessment of body language applied to cornering, is that we see that many expert descenders in the peloton continue to do things like dropping the knee. This is a bit like all the pediatricians telling us we should never spank children, against all the mothers who say, "that's fine to say, but it doesn't work in real life!" You have the real life enacter-type expertise against the theoretical (albeit intelligent and rational) expertise. Neither one is guaranteed to be right, and both have historically been proven wrong in various scenarios.

As hangdog98 mentioned, there are many questions left.
For instance:
* We all agree that shifting body position changes the center of gravity. How does this affect the bicycle in a turn, and can it be used to increase the speed with which a turn is taken?
* Jobst points out that a bicycle will not take a symmetric line through a curve due to the inability to accelerate at the same rate as deceleration into the curve. So, then, what is the fastest line, all other things held equal? What is its shape? Where does the apex occur? [Holding everything else the same, and determining the fastest line, since we can adapt that line as circumstances dictate, without wasting current time in sidetrack discussions of those circumstantial exceptions.]

I believe, that somewhere, somewhen, I saw someone's theoretical analysis (complete with diagrams) of shifting body weight and its impact on bicycle handling dynamics. Anybody else out there ever see this online?
 
Jul 10, 2010
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Hangdog98 said:
Here's one example of body position affecting steering dynamics that may add to your research.

OK, so Jobst penned that putting your foot out into a turn has no value on a bicycle. <snip>. . .

A bicycle rider who lifts his backside off the seat a little and allows the bicycle to rock side to side between his legs is moving a 7kg mass. The rider who puts all his weight on this saddle needs to move a 70kg mass when making a lean angle change. This allows more finite adjustments and greater feel. Indeed, this method is employed by the fastest riders particularly when they reach extreme lean angles in the centre of a turn. Good riders employ both methods in each and every turn to varying degrees. At the end of the day it is the contact patch and its differential friction that makes the thing go around a corner. Using your body and your bicycle controls to change the forces acting on that patch is what makes it go around a corner efficiently.

Exactly what I've observed. While I had looked at the motorcycle as a possible parallel, I found it not particularly useful, for three reasons: suspension, tire shape, and power application (acceleration). Braking, and body english, can be used to significantly (I believe) alter steering geometry, because they impact how the suspension responds to the turn. In turn, this change in geometry is significant for the motorcycle turn. I would imagine that the example you mention is an extension of this. (Interesting example, btw).

Motorcycle tires usually have a lot different shape than bicycle tires. This is apparently also significant. And, acceleration: on a bicycle we ain't got it. I think that Jobst's point about the symmetry of the line being impacted on this count is probably right. Pedaling through a corner requires a much more upright bicycle, and to hammer the pedals and get acceleration during a turning lean would, I think, be unlikely if not impossible.

Hangdog98 - great replies!
 
hiero2 said:
So to be specific, I'm looking to find validation, or not, for Jobst's theorizing on cornering geometry and c.o.g.

No problem. Let me ask Jim Papadopoulos, Arend Schwab and Andy Ruina. They may have pondered over this topic more than most people. I'll get back to you.
 
Jul 10, 2010
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Engineering analysis

--- never mind - you beat me to it! THANKS! D

Say, cozy beehive, I just went to your site - and you look like just the person to take on a little more sophisticated analysis of this whole topic! Uh, I could use a better index to your site material tho, it's hard to winnow through it.

But, I saw the bit about the endo dynamics. You analyzed fore-and-aft c.o.g. positioning and its impact there.


Thank you, and regards.
 
May 21, 2010
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If you can find a copy I would definitely check out Tony Foale's book "Motorcycle Chassis Design". I think Cozy mentions it on his site and I think you can still order it from Tony's website in a pdf. Even though it's about motorcycle chassis design he does cover a lot of the topics that you mention. The effects of head tube angle on turning etc. IIRC ( I lent someone the book and never got it back, Grrrrrrr) his theoretical starting point is a frame without suspension and goes from there so there is some information that would apply to bicycle frames as well.

He also discusses steering concepts. Basically there's a difference between turning and leaning which both together contribute to what is called steering. If you take your front wheel, spin it and then hold it out in front of you (both arms out stretched) when you "turn" the wheel to the left
(essentially keeping the axle horizontal) the gyroscopic forces in the spinning wheel will push the wheel to the right. However, if you lean the wheel to the left the gyroscopic forces will push the wheel to the left. So steering a bicycle around a curve is a combination of the two.

You also may want to check out some of Keith Code's books about Motorcycle racing techniques.

Spot on observation about Jobst Brandt!! btw
 
hiero2 said:
--- never mind - you beat me to it! THANKS! D

Say, cozy beehive, I just went to your site - and you look like just the person to take on a little more sophisticated analysis of this whole topic! Uh, I could use a better index to your site material tho, it's hard to winnow through it.

But, I saw the bit about the endo dynamics. You analyzed fore-and-aft c.o.g. positioning and its impact there.


Thank you, and regards.

Sorry about your inconvenience navigating to the post you're looking for. Thanks to Blogger.

I tell you, use that search bar on the left (you'll find it under the Vodpod roll). Its labelled "Beehive Search". Its usually hones in on the stuff you're looking for.

I will keep an index in mind for the future. That would be great.
 
Jul 15, 2010
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motorcycle suspension

Remember that when a motorcycle is leaned over in a turn, if properly set up, the front and rear suspension compress which keeps the steering geometry as it was when static. Indeed motorcycles compress the forks and change the steering head angle under brakes which is compensated for in the static settings ie, the steering head is a bit slack and the bike steepens up to be more like a road bicycle. The difference as you correctly noted hiero2 is the ability to apply power and drive the rear portion of the motorbike past the steerer. Having said that, motorcycles are also more difficult to ride fast around corners downhill as they too want to run wide for the same reasons.

Regarding tyre widths on motorcycles, the reason they're wider is for greater longevity on the heavy and fast machine. The turning friction principles are the same. Skinny tyres turn better but wear very quickly under high traction loads.

It was interesting reading Jobst assertions "The third, and usually correct technique is to keep your body in line with the bicycle frame, lean the bicycle and rider together as a unit." was something touted for motorcycle racing in the early 70's, known as the Hailwood technique. Of course that, along with motorcycle design, all changed. Having said that I don't think road bike design has any major changes in the near future.

On a personal note, I can negotiate downhill hairpin bends far more quickly on my MTB with slicks than I can on my roadie. The MTB has a longer wheelbase, higher BB and wide riser bars so one might assume the CoG is higher too. I believe that the thing that makes me able to fly around the hairpins is that I can lean into the turn under brakes then dip the bike down into the apex to make that tighter part of the turn without shifting my body position. I had a flat bar roadbike in Italy a few years back and rode down the Passa Pordoi using that technique with great success. I'd love to be able to descend like that on my roadie.:p
 
Jul 10, 2010
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Motorcycles, and mtbs

Remember that when a motorcycle is leaned over in a turn, if properly set up, the front and rear suspension compress which keeps the steering geometry as it was when static.
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
Tony Foale's book "Motorcycle Chassis Design".
(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
Jim Papadopoulos, Arend Schwab and Andy Ruina

Many thanks again.
 
Jul 29, 2009
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I have read all of your posts with interest.

Unfortunately I am now so confused trying to understand it I keep falling off every time I try to turn.
 
Jun 15, 2010
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sir les

know your lines entrance thru exit with surface knowledge
trust and know your tires
scrub before you enter
breathe

are you willing to pin it with the understanding of the possibility of crashing?
anyone with above avg handling skills expects to crash to progress. fact.
 
Jul 15, 2010
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motorcycle suspension comparison etc etc

Yeah, good discussion, and one that I always enjoy. You will find a tremendous amount of conflicting info on this topic. My knowledge comes specifically from motorcycle road racing, though as you probably know, most motorcycle road racers are very keen cyclists and this topic is discussed endlessly as motorcycle road racers earn their living from applying suspension theory to make bikes go faster and improve feel.

The wheelbase may or may not change on a motorcycle cornering with suspension compressed. This depends on the position of the swingarm pivot. If the swingarm pivot is above the rear axle then the rear axle will travel rearwards under compression. The front axle will also travel rearwards under compression. This can be adjusted to make the geometry the same as static under squat. This is adjustable on a race bike and is changed to affect the swingarm's effect under power or off-throttle. (rise and fall).

The tyres on a motorcycle, bicycle, MTB or townie all work the same way with respect to the way they steer. Motorcycles with wider rear tyres than fronts have different cross sectional shapes to compensate for the width. The rear needs to be bigger to last as long as the front. Yes, different tyres work and steer differently, as do different types of bicycle tyre or the same tyre on a wider rim.

What I meant by the cornering technique, with specific reference to the "dip" was that I leaned the body and bike as a "unit" al la Jobst and then as I neared the apex I unweight the saddle and pivot the bike at the pedals whilst pushing the bike over into the turn with the handlebars, thus causing the bike to turn tighter at the apex. I can't do it as well on the roadie because (I think) my hands are in a very different position (further forward) with more weight on them. I didn't invent it of course, I copied it from following a particularly talented WSBK rider and it made me go faster on the motorcycle too.

Finally, at the risk of sounding pedantic, wider tyres don't have more grip because they're wider. Grip is the the result of the rubber compound. Wider tyres can have a softer compound than skinny ones and offer more grip for the same wear. A wider tyre in the same compound as a narrower tyre will have less grip. Bicycle road tyres of varying widths almost always have the same compound.:eek:
 
May 11, 2009
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hiero2 said:
Ok - asking a favor - do you know of any good work regarding bicycle cornering characteristics? I found some old stuff from rec.bicycles.misc (or .tech, I forget) - but it was not definitive (relying on expert opinion), and I found contrary postings from sources just as well respected.

If I can't find something already there, I might do some work on this on my own.
Anybody know of any publications that applied physics, geometry, scientific method? Looking for stuff that relates to cornering and descending technique (essentially the same thing, since descending is just fast cornering with the added risk of going over the edge - aieeeeeeeeeeeeeeeee!).


I'm not sure if you are looking for theory or practical advice.

If the latter then you might want to read pages 14-16 of this booklet
http://www.north-stars.org/tips/ProSecrets.pdf (This same description is included in a training book but can't recall which one).
It describes what Davis Phinney did/does; I tried this technique and it works for me.
 
Jul 10, 2010
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avanti said:
I'm not sure if you are looking for theory or practical advice.

If the latter then you might want to read pages 14-16 of this booklet
http://www.north-stars.org/tips/ProSecrets.pdf (This same description is included in a training book but can't recall which one).
It describes what Davis Phinney did/does; I tried this technique and it works for me.

Theory or practical advice? A bit of both, actually. I'm looking for practical application of theory. To my knowledge on starting this, Jobst et al (previously mentioned) were the only publications I knew of that directly covered cornering on bicycles, and were referenced or available on the internet. Given the great growth in the application of science to bicycles in the past 40 years, I felt that someone has probably looked at this in greater detail. As I mentioned, and as noted by several responders, this is a controversial topic! Jobst's advice, for example, seems directly opposed to peloton practice. I, for one, have a certain respect for the wisdom of the general masses. I also respect Jobst - so which to believe?

The answer lies in science - proving the answer, either through repeated and repeatable practice, or thru repeatable scientific experiment. So I wanted to know if anyone had done this.

The booklet you link (prosecrets.pdf) is a great reference, btw! I hadn't seen it before, and when I did a google on cornering, I didn't find it. A lot of other good stuff in there. It's copyright RBR pubs, tho, or I'd include some quotes.

But I'm going to summarize a bit of what's been said, and how it fits together for me. I haven't had the opportunity to read the books we mentioned earlier yet, btw.

The RBR booklet quotes Davis Phinney in its cornering methodology. That description sounds, to me, quite similar to the technique earlier described by Hangdog98. Both are describing a technique where some of the body weight is not leaned with the bicycle, but kept more closely to the upright.

Physics wasn't my strength, but I think this is also what Hangdog98 was pointing out when he talked about CoG opposed to centralization of mass, as center of gravity is probably a measurement made in the plane of the bicycle, and centralization of mass is probably measuring the combined, 3d action of the bike and rider as a total.

Jobst based his thinking on the coefficient of friction for the contact patch. He felt that this doesn't change. His thinking was that 'you're gonna get x degrees of lean, period. Not x plus 1, or x minus 1.' Changing the relationship of the body to the plane of the bicycle would not change this lean angle.

The countersteering method covered by the RBR booklet, I think, does three things, on top of the basic lean angle. Not in order of importance, but first, the countersteering: altho this is done on practically a microscopic level, it still effectively shortens the wheelbase. Secondly, the body is positioned to put the body weight as low and evenly distributed on the bicycle as possible, and also to increase the body's contact with (and control of) the bicycle (by using the thighs and knees to hold the frame). This will present a lower CoG, and should increase the stability of the body and the bicycle as a single unit. Thirdly, the body positioning is used to (I think) move the body weight downward at the same time the plane of the bicycle lean is being increased. This last bit is the most complex part of the interaction, I think. We can safely surmise that each of these has a real impact, because they experientially have done, and this is repeatable.

The last bit - shifting the body weight - is interesting. The RBR booklet has you putting the inside knee against the top tube, and it mentions that this will rotate your hips away from the turn. This, I think, effectively should move your weight down and to the inside of the corner. This seems to be countered then, by locking your outside thigh (now on top) against the saddle and pushing down, which I think would move your upper body slightly back up towards the vertical plane, and could counter the hip shift. Taken a little farther, we have precisely what Hangdog98 described - "dipping" his bike in the apex of the turn while not changing the body position.

But the way that Davis (RBR) describe the turn, it reminds me of Jobst's description, since the body language being applied would be hard to measure, if one could stop the cornering bicyclist in mid-action, and begin to actually apply physical measurement. I think the Davis/RBR description applies force on a very micro-level.

All in my humble opinion. Forward and onward to greater knowledge!
 
Jul 17, 2009
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Cozy Beehive said:
Can you be a bit more specific? Cornering technique has been covered by many. I suggest you read Jobst Brandt's advice.

The physics of cornering a bike is pretty simple. The maximum lean angle at any given speed is limited by the co-efficient of friction at the tire-road interface and any good University level physics textbook should contain a mathematical treatment of it.

A lower center of gravity will also aid in the bike being more nimble. All things kept the same, a bike with a longer wheelbase will have a higher turning radius. Take two bikes, one short, one long. Turn the handlebars left slight. Mentally draw a straight line along the axis of the bar towards the extension of the rear axle axis. The point at which they meet, and the angle which those two lines form, is the turning radius. You're better off with a light and nimble bike at the cyclocross races.


good link. I like the 'drifting' info he writes

reading through it I thought I'd share for what it is worth. with a BMX, mountain and DH type dirt jump slope style downhill background my brother and I tried it in past years with painful results on a road bike. pavement has to be consistently smooth and not slick or grippy. I have never seen anyone do it in a crit or local group ever

however recently around these parts the fixie kids (not the college hipsters but the jr high kids riding like bmx) have developed a "Drifting" type racing. They poach the multi story business parking lots late night and sprint to the bottom. The slick concrete allows them to drift through the corners and lock up the cranks and pedal out. they hit some crazy speed.

Like Tokyo Drift on fixies.

you might find some youtube but ours took theirs down because it is frowned upon my authorities now and they dont want to be id'd or their bikes id'd
 
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CoG

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.
 
Jul 10, 2010
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Hangdog98 said:
. . . 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.

I don't quite follow that first part about "more inside the turn". It sounds like you are describing a body position lower than the bike - in extremis exemplified by motorcycle riders hanging off to the inside on a turn. But, after carefully reading what you wrote (at least 5 times), I think you are actually describing the body position I described earlier - and must credit to the RBR booklet. That position would be higher than the bike, even if on a micro scale. "Inside the turn" isn't clear in this usage. But let's say that your following paragraph describes the cornering position. Then "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." Why? If I were to guess the force application, I would guess that moving your body upwards, and closer to the vertical axis would create more force on the vertical axis. If this is true, then perhaps conversely, moving the body down, and lower in space than the plane of the bicycle would create more force on the horizontal plane - and thus would account for your "generally the less grip his tyres will have".

An interesting thing about this is that "lowering the CoG" is not clear here, and the off-road examples are a little confusing to the issue. I think "lowering the CoG" would technically mean moving it closer to the horizontal axis, regardless of the plane of the bicycle. Whereas, what I'm attempting to describe would be "lowering the CoG" in the plane of the bicycle. The RBR technique description would do just that - move the CoG closer to the junction of the tire and the road. But, unlike hanging off, it also moves the force vector closer to the vertical plane.

All of this is contraindicative to "hanging your knee" - but it is too widely used to be so easily dismissed, and it must somehow be at least somewhat positively functional.

At this point, with Jobst's technical background, his input might be valuable, if he didn't just completely blow me off for calling him a curmudgeon! Of course, I don't know if you guys ever see him on these forums.

Cozy Beehive - did your guys have anything to say?

RSVP.
 

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