Deep Braking and Suspension Travel

[Jaybird180]

If it were possible to enter the same heavy braking area at the same speed and same braking pressure but only using different stability technique, which produces more fork travel and why:

1. The rider locks his arms and uses his hands to stay on the bike
2. The rider locks his knees on the tank and doesn't lock his arms but the arms are fully extended 

[Cobie Fair]

If the arms are locked, does that throw more weight on the front?  If they are locked, does it allow the front to correct for the imperfections?

[Jaybird180]

2 hours ago, Cobie Fair said:

If the arms are locked, does that throw more weight on the front?  If they are locked, does it allow the front to correct for the imperfections?

Has the rider’s mass been altered? Please, no suppositions about the squid who misses the braking point and involuntarily alters his suit’s mass. 😂

[Hotfoot]

On 10/22/2020 at 6:31 PM, Jaybird180 said:

Has the rider’s mass been altered? Please, no suppositions about the squid who misses the braking point and involuntarily alters his suit’s mass. 😂

No the rider's mass clearly doesn't change. However, we know the braking is causing a force on the rider. Where is the rider attaching to the bike in each scenario? Where does the force get transmitted to the bike, if the rider locks his arms arms and uses his hands to stay on the bike? What about if he uses his knees instead? How does the change the effect on the bike, on the forks?

Hint: consider the location of the bike's center of mass, and how close or far away the rider's attachment point is from that, in each scenario.   

[Jaybird180]

Let's talk about why I asked the question.

Re-reading some threads where we discuss attachment points, I saw consistently that it was a subject of contention. Opinions are all over the place in various internet venues, but they tend to be reigned-in here, which sometimes has the unfortunate side-effect of not allowing incorrect observations to be voiced.

Cobie began to address my question but did so by mixing two separate issues that I didn't do enough of a job to confine. Cobie explored: 1- the geometry change from deceleration and 2- the rider's relative level of control with each technique. Separate things, but important to note.

I read one place where (I believe KC) said, consistent cornering come from a consistent position on the bike. It sounds like something Mr. Code would say, and whoever said it, I agree with it and it makes sense to me (two separate things).

I've been led to believe that locking the arms during braking could cause the steering to sharpen too much as a result of deeper fork compression. After thinking about it the night before my OP, I beg to differ. I think, controllability and consistency concerns aside the braking forces and consequently the geometry changes would be the same in either scenario. The simple reason is that if the deceleration is the same, the amount of force on the front tire producing the deceleration would also be equivalent.

I asked the question the way I did, while not a pure troll post, I was hoping to get 2 camps on either side of the discussion so I could test my above idea with no risk of being wrong.

Now that I've outted myself, I'm fully prepared to be shown wrong.

 

[Hotfoot]

Oh, now I understand your question and see why you are asking. Yes the front tire has to stop the mass of the bike and rider. The total mass (and thus the braking force required to stop it) doesn't change. However, the location of the rider's center of mass CAN change, and if the riders C.O.M. is significantly higher than the bike's center of mass, that introduces a new element, which is the lever action of the rider's upper body. That introduces a "moment", which is a rotational force, which acts (if the rider is bracing on the bars and using the bars to hold up the upper body during braking) directly on the handlebars and thus the forks.

We can talk about the theory of it but really It is very easy for you to go test this - put a zip tie on the front fork of your bike, tight enough to stay in place but not so tight that it cannot be moved at all. Slide it up to the top. Set or choose braking markers so you can be consistent in braking distance, and go out and brake hard in a straight line, riding with your knees tightly locked to the tank, your chest low on bike, and keep your arms loose.

Measure or photograph how far the zip tie got pushed down. 

Then go back out and do the same braking run with the same approach speed*, but this time sit up tall and lock your arms, don't lock your legs or support your body with your core muscles, let the handlebars do all the work of keeping your upper body from tipping forward. See if the fork travel is different. *NOTE - wear all your gear and be careful doing this, it IS possible to get into a "stoppie" or create a front tire slide, the weight on the bars and the restriction on the bars can really change the handling under braking. You may want to do a few initial runs at a lower braking intensity to get a sense of what will happen. 

If the front end is not super stiff (spring weight too high for the rider, or compression damping way too high, or forks bound) you should see a measurable difference in fork travel between one riding position and the other. 

Or, even better you can come to a school and ride our Braking Bike, which has outriggers to help stabilize the bike in case of a front end tuck or slide, and try both ways and see how it impacts the bike behavior and your ability to retain control under heavy braking.

 

[Jaybird180]

Firstly let me say that I recognize that you’re helping me work this out. On my own, I haven’t been able to crucible this to ground truth. So your contributions are invaluable.

In my experience, whenever I’ve locked on using the tank my braking distance has been improved, controllability and everything I’d want is in-fact better.

However, I don’t think I, nor 99.9% of the riding population for that matter have the ability or the tools to get repeatable performance or accurate enough measurements to prove either premise.

I also don’t have the ability to program computer simulations to see how changing rider structural point makes a difference, similar to pressing weight on a foot peg to change a line; makes no real effect despite repeated claims to the contrary.

According to my Physics lessons, Newton did not observe center of mass to be mathematically relevant to acceleration (F= MA). As a believer in Newtonian physics, that’s all I have as a stable datum to rely upon.

Eliminating the above, the remaining choices leads me to believe that most likely we’re dealing with observational error.

I don’t know why lock on is better, but if I had to guess, I’d have to say that it’s due to body mechanics, that and having the right amount of input to begin with to allow the bike to self-correct (rake/trail geometry).

If I’m right on this then it means previously I’ve been investing attention unwisely when instead it should have been spent on getting the bike to the right speed (remember: “lately” I’d been riding a bike that doesn’t have tank shrouds wide enough to lock in, and it had been a source of concern) and getting setup for the corner.

[Hotfoot]

If the position of center of mass is not relevant, what do believe causes a bike to do a stoppie?

[Hotfoot]

I didn’t follow your reasoning on why you “or 99.9% of the riding population”  couldn’t try the experiment with the zip tie on the fork? We do very similar braking drills with students all the time, at schools.

[faffi]

If you lower the CoG, less weight will be shifted forward and you can stop harder before you do a stoppie. Which is also why a long and low bike can stop (and accelerate) harder than a tall and short one. However, I am not certain that the fork will compress more doing a stoppie while sitting upright with stiff arms, than doing a stoppie laying low with the resulting lower CoG. Since you can now slow down harder, would not even more force be fed into the suspension? But for the same rate of retardation, you probably will end up with slightly less compression of the fork laying down.

However, will - if the rider position is the same - the point of ancorage matter? If I imagine in my head a 200 lb steel pole being welded to the footpegs, and another identical pole being welded to the handlebars with imaginary weightless arms between the pole and said bars, my flawed logic would indicate that the pole welded to the pegs would carry a greater rotating force forward during braking than the pole supported at the handlebars.

Put differently; if the riding position is the same, would supporting the weight on the arms make the fork compress any differently to supporting it fully with the knees, since bars and tank sit at similar levels on a sports bike?

[Jaybird180]

9 hours ago, Hotfoot said:

If the position of center of mass is not relevant, what do believe causes a bike to do a stoppie?

Shameless Confession: I've never done a stoppie and only small wheelies or big ones unintentionally so this answer is based on observation sans competent experience.

In a wheelie, there's timing associated with the rider shifting rearward (and in some cases to cause the front suspension to rebound) and excess thrust brings the wheel up. A stoppie is excess braking and a rider timed movement to spring the rear shock. I've observed both being executed at various speeds.

I should have said that vertical position of CoM wouldn't be the differentiating factor for fork travel under braking. I think it matters for a stoppie and maybe (???) for a wheelie.

9 hours ago, Hotfoot said:

I didn’t follow your reasoning on why you “or 99.9% of the riding population”  couldn’t try the experiment with the zip tie on the fork? We do very similar braking drills with students all the time, at schools.

I've ridden the brake bike at the school. The intention of that drill is different. The goal of the brake bike drill is to practice getting the braking to the edge of a skid or over the edge, experience it and get comfortable being there and practice the recovery in a safe manner. It skids when the Coefficient of Friction between the pavement and tire is exceeded. I don't think this would be useful for this purpose.

In order to use your practical exercise to find the answer to the question, we have to ensure that all parameters are same and only change rider brace point. I already have a fork travel indicator installed on my forks; it's a little ring but not the same type of material as a zip-tie but with the same purpose and it works. I could then setup cones in a flat section of pavement for start and stop points and try my best to make speed consistent across several runs and then take the measurement. I could take all of the preload and compression damping off the forks to exacerbate the fork dive and increase the rebound damping to aid consistency; might be sketchy from a safety standpoint. I'd then dismount the machine, measure and record fork travel, reset and repeat. This only requires slightly less discipline than finding how many licks it takes to get to the center of a Tootsie Roll Tootsie Pop. It's possible, but I doubt its practicality. But being game, I reached out to a friend and asked if he'd help run a motorcycle experiment (no details given) and he said "Maybe?", which is exceedingly funny to me, knowing this friend.

...And after giving him the details, he agreed to help. It's too bad that we have to wait for weather to cooperate. It's cold and wet here.

4 hours ago, faffi said:

(Premise)If you lower the CoG, less weight will be shifted forward and you can stop harder before you do a stoppie. (Rationale offered as "proof" for premise) Which is also why a long and low bike can stop (and accelerate) harder than a tall and short one. However, I am not certain that (conclusion) the fork will compress more doing a stoppie while sitting upright with stiff arms, than doing a stoppie laying low with the resulting lower CoG. (Conclusion based on Rationale above)Since you can now slow down harder, would not even more force be fed into the suspension? But for the same rate of retardation, you probably will end up with slightly less compression of the fork laying down.

In logic, the conclusion flows directly from premises of a statement. The truth of the premise is not considered in evaluating the structure of statements. According to logic, it is presumed to result in a correct conclusion which is a flaw in classical logic.

In the red and blue statements above I've added color, italics and bold to indicate where I see premise, conclusion and rationale. The conclusion could actually be true, but we must stipulate the premise as true. This thread is about evaluating this very premise considering we have each in our own ways observed this apparent behavior. I think we are guilty of observational error.

4 hours ago, faffi said:

However, will - if the rider position is the same - the point of ancorage matter? If I imagine in my head a 200 lb steel pole being welded to the footpegs, and another identical pole being welded to the handlebars with imaginary weightless arms between the pole and said bars, my flawed logic would indicate that the pole welded to the pegs would carry a greater rotating force forward during braking than the pole supported at the handlebars.

Put differently; if the riding position is the same, would supporting the weight on the arms make the fork compress any differently to supporting it fully with the knees, since bars and tank sit at similar levels on a sports bike?

 I agree that you correctly transliterated my OP.

[Hotfoot]

Do we agree that if a rider sits up high, that the rider's COM is higher than that of the bike, producing a rotational "moment" that wants to make the bike rotate around the front tire's contact patch and create a "stoppie"?

If we agree on this point, then the remaining questions is: does that situation cause more fork compression? My thinking is that yes, it would, because the "moment" created is a new force, in addition to the normal braking/deceleration force from mass of bike & rider. (The farther apart vertically the COM are, the bigger that "moment" becomes.) That rotational force, which centers around the front tire contact patch, can be resolved into two components, where the rider connects to the handlebars: a forward force and a downward force. It is that additional downward force that puts a new vertical load on the bars and THAT is what I think causes more fork compression.

If the rider sits very low on the bike, and braces forward (instead of down) with his/her arms, I do not know if there would be any additional fork compression. My guess is that there would not be.  However handling would still be negatively affected simply because the pressure on the bars restricts handlebar movement and if there are any imperfections in the road, the bars cannot move as they need to, to compensate. And obviously if the rider is entering a corner it is much more difficult to make a precise steering input when one's weight is on the bars.

The reality on an S1000rr is that they have unbelievable stopping power, and at maximum braking it is quite difficult to hold on well enough with your lower body to have zero pressure on the bars, especially on a hard braking track over the course of a long race. That said, when would it be MOST important for rider to be light on the bars? During straight line braking or when entering a corner? When is the best time to get the hardest braking done, straight up or leaned over? If a rider does have some pressure on the bars during hardest braking, is it better to be sitting straight upright, or to be down low? Is it better for any bar pressure to be forward on the bars, or downwards? 

[faffi]

Well, I am probably guilty of everything you pointed out, Jaybird 😄 However, when it comes to lowering and/or lengthening a vehicle for better stopping or acceleration, it is a result of the vehicle's ability to resist flopping over forwards or backwards. Although only if you already slowed or accelerated hard enough for that to be an issue, of course. This partly explains it https://www.physio-pedia.com/Centre_of_Gravity#:~:text=Stability and the Centre of Gravity,-The direction of&text=When the line of gravity,is said to be stable. 

 

Sorry if I just dug myself a bigger grave here 🤪

[Jaybird180]

51 minutes ago, Hotfoot said:

Do we agree that if a rider sits up high, that the rider's COM is higher than that of the bike, producing a rotational "moment" that wants to make the bike rotate around the front tire's contact patch and create a "stoppie"?

I'll agree to the stipulation if we agree that a stoppie is by definition a condition where the rear tire is not supporting any weight.

 

51 minutes ago, Hotfoot said:

If we agree on this point, then the remaining questions is: does that situation cause more fork compression? My thinking is that yes, it would, because the "moment" created is a new force, in addition to the normal braking/deceleration force from mass of bike & rider. (The farther apart vertically the COM are, the bigger that "moment" becomes.) That rotational force, which centers around the front tire contact patch, can be resolved into two components, where the rider connects to the handlebars: a forward force and a downward force. It is that additional downward force that puts a new vertical load on the bars and THAT is what I think causes more fork compression.

I think you're saying that the rider has the ability to change the fulcrum of the point of rotation and that this action of having more mass further from the fulcrum creates more inertia for fork compression. As kinematic theory I believe it would produce the desired conclusion.

I believe that it fails in practice because (taking the extreme example) your rider can't alter her mass to be centered in her head anymore than she can focus it at her feet. It goes back to the discussion where a student asked if the rider weight should be on the pegs or seat during a turn and Dylan in summation said to put the weight in the seat for the simple reason of physical exertion, but to the bike it didn't matter.

51 minutes ago, Hotfoot said:

If the rider sits very low on the bike, and braces forward (instead of down) with his/her arms, I do not know if there would be any additional fork compression. My guess is that there would not be.  However handling would still be negatively affected simply because the pressure on the bars restricts handlebar movement and if there are any imperfections in the road, the bars cannot move as they need to, to compensate. And obviously if the rider is entering a corner it is much more difficult to make a precise steering input when one's weight is on the bars.

The reality on an S1000rr is that they have unbelievable stopping power, and at maximum braking it is quite difficult to hold on well enough with your lower body to have zero pressure on the bars, especially on a hard braking track over the course of a long race. That said, when would it be MOST important for rider to be light on the bars? During straight line braking or when entering a corner? When is the best time to get the hardest braking done, straight up or leaned over? If a rider does have some pressure on the bars during hardest braking, is it better to be sitting straight upright, or to be down low? Is it better for any bar pressure to be forward on the bars, or downwards? 

I'd imagine that if we did see a net gain, riders would no longer sit upright for straight line braking to get the aero braking effect.

For the purposes of this discussion, can we leave handling issues aside? I think it's universally agreed (???) that relaxed arms are in better control of steering and decoupling road induced oscillations that could happen if the rider were stiff and the bike needed to self-correct.

If you could slow from 200mph to 100mph, assuming same brake on/off points on Brand X but can't do it on a BMW using the tank only, then we're getting into ergonomic and possibly aerodynamic engineering. May I suggest this is also outside the scope of the discussion at this point and may distract from getting an answer.

50 minutes ago, faffi said:

Well, I am probably guilty of everything you pointed out, Jaybird 😄 However, when it comes to lowering and/or lengthening a vehicle for better stopping or acceleration, it is a result of the vehicle's ability to resist flopping over forwards or backwards. Although only if you already slowed or accelerated hard enough for that to be an issue, of course. This partly explains it https://www.physio-pedia.com/Centre_of_Gravity#:~:text=Stability and the Centre of Gravity,-The direction of&text=When the line of gravity,is said to be stable. 

 

Sorry if I just dug myself a bigger grave here 🤪

Yes, CoG may lie outside the body when the body is in certain positions or planes of motion with limbs away from the upright, resting CoM.

[Hotfoot]

14 minutes ago, Jaybird180 said:

 

For the purposes of this discussion, can we leave handling issues aside? 

What IS the purpose of the discussion, if you are not interested in handling issues?

If this is a purely theoretical discussion and not directly related to real world riding, this is probably not the right forum for that sort of discussion.

[Hotfoot]

8 minutes ago, Jaybird180 said:

I think you're saying that the rider has the ability to change the fulcrum of the point of rotation and that this action of having more mass further from the fulcrum creates more inertia for fork compression. As kinematic theory I believe it would produce the desired conclusion.

I believe that it fails in practice because (taking the extreme example) your rider can't alter her mass to be centered in her head anymore than she can focus it at her feet.

 

A rider cannot change the location of the COM within their body, no. However they CAN change the position of their body. Do we agree that when a rider hangs off the inside of the bike, their COM is moved farther to the inside of the corner?

[Jaybird180]

9 minutes ago, Hotfoot said:

What IS the purpose of the discussion, if you are not interested in handling issues?

The net gain in resolving this would be saved attention. If it doesn't matter where the rider locks on during braking then it's one less thing to "correct". If it were practical, the rider could stiffen during braking then relax when making the steering input. There might be some lost "cool points" for style, but who cares if it gets the rider progressively closer to the podium, right? This rider would also need to understand that some ability for the bike to correct for surface imperfections in the braking zone will be sacrificed if using this technique.

I also think it explains the Leg Dangle.

 

1 minute ago, Hotfoot said:

A rider cannot change the location of the COM within their body, no. However they CAN change the position of their body. Do we agree that when a rider hangs off the inside of the bike, their COM is moved farther to the inside of the corner?

I agree that the rider CoM is moved to the inside of the corner.

[Hotfoot]

14 minutes ago, Jaybird180 said:

If it were practical, the rider could stiffen during braking then relax when making the steering input.

Ah, this makes more sense, now I'm with ya.

Yes, that is EXACTLY what a rider would have to do, if they were not strong enough to get a good lock on with the lower body, and not strong enough to support their upper body with their core muscles, they would be forced to use their arms to support themselves under straight line braking, then do their best to get off the bars when it is time to steer. What else could one do, other than trying to add tank grip or other anchoring devices to make it easier to lock on to the bike? 

[Jaybird180]

18 minutes ago, Hotfoot said:

Ah, this makes more sense, now I'm with ya.

Yes, that is EXACTLY what a rider would have to do, if they were not strong enough to get a good lock on with the lower body, and not strong enough to support their upper body with their core muscles, they would be forced to use their arms to support themselves under straight line braking, then do their best to get off the bars when it is time to steer. What else could one do, other than trying to add tank grip or other anchoring devices to make it easier to lock on to the bike? 

This is my only option on my minimoto. I think Supermoto riders have the same issue.