Dan515

This was me during my first Populaire a few months ago. Road rash healed up pretty nice now.

cyccommute 

You missed the other part of what I said. If you are simply stopping a wheel from rotating, that is not a skid. If you are stopping a wheel from rotating and converting from rolling friction to sliding friction, that is a skid. The contact patch has to be stationary and sliding to be a skid. Front wheels on single bikes in a straight line motion don't convert the rolling friction to sliding friction. Before that happens the center of mass of the system rotates around the handlebars and the rider is thrown over the bars.

The key is the center of mass. A bicycle rider in a "normal" position has a high center of gravity and they can't overcome the coefficient of friction between the tire and the road. Tandem, cars and motorcycles all have lower centers of gravity that are also further from the front wheels which keeps them from rotating around the front center of gravity. (Motorcycles can still be pushed into endos but it takes much more force and speed). Their front wheels can slide under braking but in the case of motorcycles and tandems, the riders will probably fall anyway since they no longer have the gyroscopic benefit of the wheels.

You can increase the amount of deceleration that you can get on a bicycle by a large amount (still not enough to keep you from going over the bars, however) by moving the center of gravity rearward and down when braking. I've used Wilson's calculation to see what the effect of moving 4" backward and 2" down on a bike is and the possible deceleration increases to around 0.9 g. Alternatively, if you move the CG forward, you can skid the rear wheel more easily and get into a nose wheelie more easily...if that is your goal.

Wilfred Laurier

The limit you are approaching is zero. The maximum deceleration when the wheel is six inches off the ground is less than the maximum deceleration when the wheel is two inches off the ground, and the maximum deceleration when the centre of mass is two inches behind the front axle is much less than when the wheel is six inches off the ground. And the maximum deceleration when the centre of mass is directly over the front wheel is zero.

Maximum deceleration occurs when the front brake is applied with such force that the rear wheel is unweighted but has not yet lifted off the ground. Nothing I have said contradicts this. Every millimeter that the rear wheel, and by extension the centre of mass, lifts off the ground, the lower the maximum braking.

cale

Great, we agree. When the wheel stops rotating and the bike and rider continue forward, the wheel starts to skid.

In so much that you're convinced of the soundness of this conclusion. I think you are failing to add the coefficient of friction to your mental equation for surely you'll agree that on a slick surface the front wheel can skid. I'm not sure at what level of friction a rider goes over the bars but a skidding front wheel is a distinct possibility for a great many riders.

cyccommute 

Try again. Why would the maximum deceleration be less when the wheel is 6" off the ground as opposed to 2" off the ground? There is no physical reason for this to occur. If you think it is, show some calculations to that effect.

The maximum deceleration would only be zero when the bike is directly over the contact patch if the bike has stopped. You don't necessarily have to stop when the bike is directly over the contact patch because you could continue forward of the contact patch if your momentum is greater than the maximum possible deceleration. If you are going too fast and try to stop too quickly, the rider is carried up and over the contact patch and falls to the ground.

That is what Wilson is saying. For a given weight and wheelbase used in his example, you can only develop about 0.56 g (5.5 m/s^2) of deceleration on a bicycle in a crouched position. If you need 0.57 g (5.6 m/s^2) of deceleration, you will continue past the contact patch and be thrown from the bicycle. The bicycle won't stop when the rider reachs the pitch over point.

Not according to Wilson nor many others. I used to think the same until it was demonstrated to me that maximum deceleration occurs when the rear wheel is completely unweighted.

As to your statement that "when the rear wheel is lifted, the rider has begun going over the handlebars", consider what happens if the rider uses only a rear brake. Weight shift occurs and the rear wheel is unweighted to the point where it may not have enough friction to keep turning...in other words, it skids. The ride is no where near "going over the handlebars" and, without something else stopping the rear wheel, can't go over the handlebars. This is exactly the same thing that is happening when the rear wheel just starts to lift while uses a front brake. The rider isn't at risk of going over the handlebars because the center of gravity isn't far enough forward yet.

cyccommute 

No, we don't agree. In a pitch over situation on a dry surface, the front wheel on a bicycle can continue to rotate. If you stop the wheel, the wheel doesn't slide but the rider and bike, i.e. the center of mass, continues forward, rotating around point 3 in the figure I posted. The hub allows the CG to rotate without the wheel converting from rolling friction to sliding friction. There is no "skid".

On a wet surface or even loose surface, the dynamics are going to be a bit different but not radically so. The front wheel may slide momentarily (we're talking fraction of a second) but the rider will quickly fall over because the front wheel has lost its gyroscopic ability to allow the rider to balance. The wheel will probably slip to the side because the rider is trying to compensate from the loss of the rotation in the front wheel and the bicycle and rider will crash rather than the wheel really "skid". You won't be sliding along on the bike with both wheels locked in the same way that you can slide a rear wheel.

cale

You're in a state of continuous change. You have not read what I've written and your responses are contradictory. I don't know how this works in person but it is a disaster online.

cyccommute 

I did read what you said and I see no contradictions. You can't skid a front tire on a bicycle on dry pavement. Period. The rider will be thrown to the ground before the front wheel slides. Even if you managed to stop the wheel but were in front of the contact patch, the tire won't slide. The bearings that we so carefully adjust on the wheels allow for free movement and the momentum of our mass will carry us over the bars before the tire slides. In fact, if you go over the bars, the front wheel is likely to come off the ground and rotate around the CG.

On a slick surface, the skid is of such short duration that it really can't be called a "skid".

Yellowbeard

Because the rider's center of gravity is higher and further forward. The force of braking acts at the front wheel contact patch, parallel to the ground, creating a moment around the center of gravity that tends to tip the bike forward. Under normal braking that moment is countered by the shifting of weight from the rear wheel to the front, which generates an opposing moment.

When the rear wheel lifts off the ground (assuming the rider stays in the same position relative to the bike) the moment from braking force gets stronger because the C of G is higher (as long as braking force stays the same) and the opposing moment gets weaker because the C of G is horizontally closer to the front wheel contact patch (assuming the rider doesn't spontaneously take on or jettison some form of ballast).

If braking force doesn't change, the only way a rider isn't going over the handlebars at this point is if they are going so slowly that they don't have enough kinetic energy to lift their center of gravity past the front wheel contact patch (or if the front wheel skids for some reason, but I'm not arguing about that).

You seem to be assuming that when someone brakes hard enough to lift their rear wheel they release the brake, and arguing against people who are assuming that braking remains constant throughout.

I can draw the force diagrams, but I don't have enough time before work. Maybe tonight.

cale

I don't disagree that bad things happen when the front wheel stops rotating. I also agree that lots of falls happen prior to this moment.

The idea of skidding, however unlikely, is a bit like your comment regarding the theoretical vs practical limits of maximum braking. It is beneficial to the discussion. That's all.

Yellowbeard

Yes it is. In it's simplest form (which is being argued here) it's a very straightforward equilibrium problem. Hell, it's not even three-dimensional.

This (bolded text) is untrue. Think about it, you've locked the wheel up with the brake so the bearings aren't turning.

Again, (bolded) is inaccurate, although only a tangential point. Locking up the rear wheel with the rear brake is NOT equivalent to unweighting the rear wheel through use of the front brake. The rear brake will lock up well before the rear wheel is carrying zero weight. When it does braking suffers because sliding friction is weaker than static friction but it'll still slow you down, which couldn't happen if there was no weight on it.

In fact it's an excellent demonstration of why the front brake is inherently more powerful.

YouthInAsia

I'm the dude who posted the original thread, just with a different screen name now. Over the past few weeks I've been using the front brake exclusively. I've been using it on my commutes to work (11.5 miles one way) and I've been using it on descents as well as inclines. It works beautifully.

When riding on loose ground, or while turning, or when the ground is wet, I prefer the rear brake. Tends to be a tad more stable in my opinion. Every other scenario I prefer the front brake. I'd say I use the front 90% of the time right now. It's dry here in California.

JonathanGennick 

As you can see, braking is well understood and all agree on the techniques. LOL!

I did pay closer attention on a couple of recent rides. I'm tending to - usually- squeeze both brakes, but I'm thinking with the front. My mental focus is on the front. Usually.

Leebo

This ^^^ is why I hated physics class. Im going to go pedal my mt bike, with disc brakes. And practice my nose wheelies on some downhill switchbacks. There is a thing of beauty in a properly executed nose wheelie within a tight downhill turn. Get the rear wheel up about 1-2', then perform a hip flick to what ever direction you wish to turn. Release the front brake in the new line your bike is now pointed. Way cool when performed correctly. Serious results may occur with poor execution. And gravity is a harsh mistress. Balance , concentration and focus needed, physics, not so much.

Rob_E

Oy.

I rely more on the front brake than the rear. I have one bike, which had a coaster brake, but now does not, and doesn't lend itself to any other kind of brake, that now has only front brakes. It actually stops very well. Although I still would like to get a rear brake on there that works.

I also tend to put the front brake controls on the right so that I can signal while using the front brake. But if I'm slowing fast, or slowing over a long distance, like a steep descent, I'll use both or switch between front and rear to keep the rims from warming too much. But mostly the front. So much so that last month my poor, front wheel had to be retired because the v-brakes had chewed through the rim. I should have been using the rear brake more because that wheel has been replaced multiple times for reasons unrelated to rim wear. But still I use the front.

Redhatter

I've never had the rear wheel lift when braking heavily on the front, even before I started loading the back up.

Today it'd be virtually impossible for me to lift the rear wheel as there's about 20kg sitting over the rear axle, dynamic friction on the front wheel would take over before that happened.

I find I use the front brake more (obvious because I wear out the brake pads faster there) but I do use both. The set up here is typical for bicycles in this country: left=rear, right=front. I have an indicator light system for signalling turns, and only use hand signals when that is out of action.

Going downhill and doing a right turn for me is a near impossibility whilst signalling with hand signals, since I need my right hand to control my descent, a sufficiently steep downhill grade will prevent the rear brake from being effective even with the extra load.

A couple of times I've been forced to ride places with only a rear brake, basically in the days before I upgraded my commuter bike to hydraulic brakes, there seemed to be a fault with the brake that would eventually wear out the brake cable and cause it to fray about the same point. A few times I've ridden to the bike shop with a brake cable on the verge of snapping: those runs I had to remind myself "Don't use the front brake!" and noticed just how poor the rear brake alone performs.

That said, I have seen bikes equipped with a rear brake (a disc, no less), and no front brake. I've often wondered how they handle.

cyccommute 

Yes, the rider center of gravity is shifted forward but that has no effect on the rate of deceleration. If the rider does nothing, the deceleration will remain the same regardless of the height of the rear wheel. Moving the CG forward increase the risk of going over the handlebars because it is easier to lift the rear wheel but it doesn't cause the deceleration to decrease.

Think about what happens when you start down a hill. Depending on how steep the hill is, a bicycle can easily have a vertical differential of 2" to 6". Does this mean that the bike's ability to slow down is less than on level ground? Of course not. If what Wilfred Laurier says is true, a bike headed down an incline would reach a point where it couldn't be stopped.

The CG of a bicycle rider is located around the mid-torso of the rider and moves forward but not up when the rear wheel lifts. But that still doesn't cause a decrease in deceleration. Moving the CG forward increases the normal force...i.e. gravity...on the contact patch. With the CG behind the contact patch, the rear wheel can still be pulled down to the ground if the brakes are released which means that there is still a part of the normal force pulling the rear wheel down. If the CG is directly over the contact patch, the normal force is at it greatest on the contact patch. That's why (some) cyclists can balance in a nose wheelie.

But none of this has any effect on the rate of deceleration.

You and Wilfred Laurier assume that the bicycle is going to naturally stop when it gets to the point of a nose wheelie. It won't. The rider has to feather the brakes and delicately adjust the CG to avoid falling forward. If the braking force doesn't change and if the rider is over the deceleration needed to put them over the handlebars, there is nothing that is going to stop them from falling.

No. You and Wilfred Laurier are assuming that someone who lifts their rear wheel is releasing their brakes as they approach the pitch over point. That's the only way you can balance in a nose wheelie. If you maintain constant braking and are over the maximum possible deceleration, the rider is going to pivot around the CG over the front wheel and do an endo.

To be clear, a reasonable rider will start to release the front brake when the rear wheel starts to skid. Part of the reason to use both brakes is that the rear wheel skidding is an indicator that you are headed towards going over the bars. Mountain bike riders know that when the rear wheel skids, you get off the front brake to stop the rear wheel lift. This is because they tend to ride on inclines where the CG is further forward and they have less deceleration to work with before they go over the bars.

Yellowbeard

It doesn't change the rate of deceleration, it changes the rate of deceleration at which the rear wheel stops rising. A cent


Firstly, there is a point where a bicycle can't be stopped going downhill. A hill steep enough that you're CG is over the front wheel. Secondly, those two situations are not equivalent. The force of friction between two surfaces is parallel to the contact between those surfaces. The normal force is called the normal force precisely because it is "normal" (i.e. perpendicular) to those surfaces. On flat ground, the force of friction acts horizontally. On a slope, it acts parallel to the slope (and the normal force is angled off-vertical). So the angle between the traction force and the force of gravity is different, which changes the moment forces.



Wut?

Yes, but if you're braking hard enough to lift the rear wheel (even if the wheel hasn't lifted yet) then the normal force at the front wheel is already at it's maximum because it's supporting 100% of your weight.

That's not what "normal force" means, or does. Gravity is pulling the rear wheel down, but there is no normal force on it when it isn't touching the ground.

Yes, but it doesn't reach it's maximum there, it reaches it's maximum as soon as the rear wheel is completely unweighted.

Not the actual rate of deceleration, that's determined by the limits of traction and the torque that the brake puts on the wheel. It does affect the maximum rate of deceleration possible without tipping further forward.

That's the exact opposite of what I'm assuming. It's also the exact opposite of what I said.

That's what I said.

Seriously, just let me put my dinner in the oven and find my tablet and I'll draw this out for you. It's clear that you have some misconceptions about the geometry of the problem, at least, and I don't think you have a grasp on the dynamics. We can calculate everything, at every point in time, easily.

*Edit: Bah. I suck at digital drawing. Lemme find some paper.

imi

Yes, would you guys please put some force diagrams up.
This is physics, not a peeing contest.

https://physics.wku.edu/phys201/Information/ProblemSolving/ForceDiagrams.html

Little Darwin

I see this has turned into a theoretical thread, but I will respond to the initial question.

Back in the 70's I rarely used the front brake at all. It was a feeling among us youngsters that it would too easily lead to going over the handlebar. In fact, at one point I had dual hand brakes hooked to the back wheel so I could stop the steel rim, even in in the rain.

Now, I tend to ride a lot of gravel and crushed stone, so I use both brakes... If stopping fast, I feel I have been successful at being near maximum braking when the rear wheel skids. I am not very coordinated, so I wouldn't be comfortable actually lifting the rear wheel, or skidding the front. I suspect I would skid the front wheel before lifting because of the surface, and the fact that I weigh a lot, and carry about 20 pounds of stuff between a seat pack and trunk bag.

I am impressed by riders who can lift the rear wheel of a bicycle or motorcycle in a controlled way... but, I am impressed by many things I will never try.

Little Darwin

As far as the physics, I have no training other than high school physics over 50 years ago..., but what little physics I think I understand would indicate that the lower the rear wheel is, the greater the braking force possible.

The reason is that the center of gravity is raised and moved forward based on any rotation caused by the wheel lifting while the direction of force (momentum) remains relatively constant. As with any other lever, the closer the center of mass is to the fulcrum (front axle) the less force is required to lift the mass. While technically the weight is always the same distance from the fulcrum in this case, it seems intuitive that it is effectively closer during braking if relating it to the direction of the force creating the rotation (momentum going forward).

I have not performed any experiments, or read the results of anyone else's... However, my hypothesis is that if you have the front wheel secured, and apply horizontal force directly forward (on the saddle), it would require significant force to lift the rear wheel... also, if you apply vertical force upward on the saddle, it should require less force to start rotation. Starting the bicycle at a 45 degree angle, I believe that the two forces would be closer to the same... So in short, the forces needed to rotate a bicycle using forward momentum should decrease as the distance of the rear wheel from the ground increases (until completely vertical). Since less momentum is required, I believe it makes sense that maximum braking possible is less.

What the impact of angle of force versus rotation is at various points in the rotation, I leave to real physicists.

I do also accept that changing the forward momentum to the upward rotation of lifting the rear wheel does take some energy, so this must be considered in the whole equation, and not just static forces as in my hypothesis... I just have no means to hypothesize the impact of that momentum change.

Thank you for taking the time to read my uninformed post.

Yellowbeard

Force diagrams coming soon.

imi

^^^ I'm holding my breath!

vatdim

I won't pretend I understand much of this physics debate. However, the way I see it - applying brakes on both wheels is the fool-proof method of actually coming to a stop without things becoming too exciting. Since I've had the "going over the handlebars" thing happen to me at a younger age, I now almost always press the rear brake first and a very short instant after that I apply the front brake quite hard. Personally, this makes me feel a lot safer and I've never even come close to flipping over again. I admit, there have been situations where I didn't manage to stop (right hook in right-hand traffic, anyone?), but in such circumstances one generally can't even begin pressing either brake lever before it's too late.

mozad655

I bought my bike 4 years ago. I have only used my front break. I have never used my rear break and for several years I took it off completely. I now ride with only a front break and I'm doing just fine. No problem. Would be a good idea though to reinstall the rear break just for emergency.