DCCommuter

You're missing my point. The 90% weight transfer is using the rear brake alone. Using both brakes at their full potential should result in 100% weight transfer to the front wheel. The only effect of using the rear brake is whether the rear wheel skids or rolls.

To recap:

Rear brake alone: 0.1g braking, rear wheel skidding
Front brake alone: 0.5g braking, no skidding
Both brakes together: 0.5g braking, rear wheel skidding

DCCommuter

Supcom is correct in his derivation, and what is important to note is that the position of the braking force is not a factor in calculating the rotation -- the same amount of braking on either wheel results in the same amount of weight transfer from back to front.

cyccommute 

Most everyone I know uses both front and back. It's not that difficult nor does it take special skill. Even my children used both. Mastering a skid is something any 10 year old boy is able to do. It's not that hard.

cyccommute 

I don't agree and feel you are missing my point as well. In the previous post you stated that the weight transfer using either brake is equal. But obviously it isn't since you can't endo using only the rear brake. The rear brake isn't just along for the ride.

I use both brakes. I always have and always will. Every time I apply the front brake, it doesn't result in a skidding rear tire. Even in high angle descents and high speed decelerations on both hard and soft surfaces, application of both brakes does not result in loss of rear wheel traction, at least not of done properly. In fact, if I don't want to skid the tire, I don't. There are other times...showing off...when I want to skid the tire. Slide forward towards the bars and lock the rear wheel and you can slide for a very long time. It's fun...it brings out the 10 year old in me This is accomplished by proper modulation of the brakes and proper body position. Proper braking is also accomplished by the same mechanism.

Application of the a rear brake alone doesn't always result in a skidding rear wheel, either. Bicycles with coaster brakes can be stopped without skidding the tire. As pointed out by others, the rear brake can be very useful in situations where application of the front brake would result in a front wheel skid...far worse than a rear wheel skid The use of brakes is far more subtle and complex than your recap.

One thing, which helps prove my point about using both brakes, is your deceleration numbers. 0.1g is small but if the overall deceleration is 0.5g, that's 20% of the overall deceleration available to the bicycle. It's not a good idea to throw away 20% of anything...especially deceleration!

cyccommute 

Thank you for the analysis, supcom. Very well done.

It clears up some points. One of the most important is the one I've highlighted above. Moving the center of gravity rearward and down, increases the rate of deceleration...exactly what any mountain biker knows by experience.

Additionally, moving the center of gravity, i.e. changing the H/V ratio, will also change the u term. If you want to skid a tire, more forward and up. If you want to use the friction of the tire for braking, move rearward and down.


By the way, I love your avatar. Too bad they changed the highway designation

graywolf

But the very point we are trying to make is that for maximum braking one moves the center of gravity back and down. Secondly there is no way to take a header with rear braking only because the friction patch is behind the center of gravity. So in the end your analysis, despite plugging numbers in to formulas you do not quite understand, is exactly the same as the guy who said Sheldon Brown is an expert on the subject therefore we who are saying something different are wrong. In other words, you believe something without understanding it. Which is not intended as any kind of put down, but to show you where you are going wrong with your thinking.

Furthermore, any simple modeling formula like you are using only approximates a complex dynamic situation like we are talking about very poorly. It works pretty well if all you are talking about is some dummy (real or proverbial) simply sitting rigidly on the saddle and squeezing the brake levers, but that is not what happens in real life situations.

Also the coefficient of friction varies with a bunch of things like the size and rubber compound of the tires, and the nature of the surface you are on from wet ice to dry concrete. In fact I have skidded both brakes on occasion at high speeds on dry asphalt pavement. At low speeds there is not enough energy involved to exceed the CoF which is technically why childrens bicycles are not usually fitted with a front brake for safety reasons. However, you hardly need a front brake if you never exceed 5 MPH. And this says that proper technique would be to ease off on the brakes as you approach a stop, being completely off the front brake the last few feet.

Or to put all that another way, your simple physics are too simple

DannoXYZ 

Problem is that braking is not a static equation. The COG will actually move as the rider leans back and down. Thus altering the weight-balance between the wheels. The wikipedia post makes too many assumptions that's simply not true. For example, the exact weight balance is not always 50/50 nor is the COG always located exactly equidistant between the contact-patches.

coldfeet

No, sorry, perhaps I was unclear, NO!
It is irrelevant that you are using both brakes, unless you slide the tire(s), ( note the plural ) the ultimate limitation on braking is flipping the bike. if you slide the rear tire it's doing 10%, of the potential maximum. If you then use the front brake to it' s maximum capacity, the rear brake becomes totally unweighted, and totally, ineffective,.Any use of either brake results in weight transfer. Refer to Supcom's post. A high enough braking force results in enough weight transfer to completely unweight the rear tire causing it to skid. It is very hard on a bicycle, on the road, to cause the front tire to skid before flipping. If you try to reserve some of the braking to the rear, you eliminate your ability to get the last bit of the braking from the front. The front has the potential to give you everything you gave up at the rear, and more.

I have stopped with the rear wheel OFF the ground, yes, it's mostly a stunt and not recommended, it IS possible.
If you accept that braking of either wheel causes weight transfer, ( Do you? If not why? ) Then how can you not accept that maximum braking can be done through the front tire?

I will grant you that most riders should split their braking between tires, because they are not good enough to get the most out of their front tire. I am not, unless thinking very hard in a prepared situation, ( or a fluke ) able to get the best result out of front tire only braking. I can do it, I have done it.

I have just re-read carefully your comment. It is possible that you are saying the same as I just said, that is, that people should not rely totally on the front because they are not good enough? I've just gone back an re-read some of you earlier posts, this does not seem to be the case. The only way I can see to convince you is to have someone of similar weight and build, on a similar bike, who is a better rider than you, have a brake off against you, with rear brake disabled.

If you want to take me up on that, you'll have to come North of the border, I refuse to cross it until you get a sane administration. ( If that's not an oxymoron )

cyccommute 

You won't get anywhere shouting. I do not agree that the ultimate limitation of braking is flipping the bike. That is past the point of ultimate limitation. Flipping the bike is a failure of the braking system. The ultimate limitation to any braking system is converting the kinetic energy to heat energy in the shortest possible time while still maintaining control of the vehicle. Any vehicle can be stopped instantaneously by putting a large enough object in front of it. That's an extreme example of ultimate braking but it is hardly the best method.

Using the front brake to the point where the rear wheel losses contact with the ground is not a best braking practice nor is it a measure of the ultimate braking capacity of the system. As has been pointed out before, a skidding wheel is an unstable wheel and adds nothing to control of the bike. I consider a skid to be a failure of the operator to understand the dynamics of braking...and poor technique.

As for weight shift during braking, a simple slide forward a very short distance on the bike is enough to unweight the rear wheel and cause a skid. Any one can do it. You can even do it with the rear brake alone. A slight movement rearward will also keep the rear wheel from skidding even if you are only using the rear brake.

Let's look at supcom's post

Quote:

Originally Posted by supcom

... the maximum deceleration is related only to the horizontal and vertical location of the center of gravity (CG) and is equal to g*H/V, where g is acceleration of gravity (9.81 m/s^2), H is the distance of the CG behind the front wheel contact patch, and H is the vertical distance above the ground of the CG. For a typical CG (according to wikipedia) of 60 cm back and 120 cm up, the maximum braking equates to 4.9 m/s^2, or 0.5g. Moving the CG back and/or down increases this.

Note that he states that moving the center of gravity back and/or down increases the deceleration. For this statement to hold, moving the CG back and down can only have an effect on the braking ability of the rear wheel. This means that weight is not transfered to the front wheel in as great an amount as when the CG is higher and further forward.

Quote:

Originally Posted by supcom

...the maximum deceleration is limited by the force required to overcome the static friction force of the tires against the ground. We'll assume that both tires are the same material, as is typical. The static friction force is equal to u*M*g, where u is the coefficient of friction of the rubber against the riding surface and M is the mass of the riser plus bike. In this case, the rider position does not affect the ability of the bike to skid assuming the rider is competent enough to use differential braking to keep one wheel from skidding before the other. The horizontal force exerted by the rider is equal to mass times acelleration, or Ma. Setting this equal to the friction force to find the point at which the bike skids, we get a=u*g. So, the maximum deceleration possible before a bike skids (assuming it doesn't flip) is limited only by the coefficient of friction. Internet source give a wide range for this, but generally state it as being somewhere between 0.5 and 1.0.

The important phrase in this scenario has been highlighted. The maximum deceleration is reached only if the the bike does not skid. Maximum deceleration is the point where friction between the bike and the ground is at its maximum, i.e. both wheels have to stay on the ground. Since the rear wheel provides 0.1g out of 0.5g of the deceleration (20%), it's best to use both brakes to maximize that deceleration. And, from the other quote, moving weight rearward increases that deceleration.


I never said stopping with the rear wheel off the ground is impossible. I've even listed the several names for it. I can stop with the wheel off the ground too (I'm not brave enough to do it with the wheel very high but I've broken enough bones and have enough scars, thank you very much). I can skid a bike for a few tens of yards too. It's not hard to do on just about any surface.

I also accept that braking causes weight transfer. I have never said differently. I disagree on the magnitude of that weight transfer and on mechanisms to mitigate it, however. Certainly using the rear brake alone does cause weight transfer but it isn't as great as using the front brake. If it were, you'd be able to endo a bike with the rear brake...clearly an impossibility.

I'll even accept that you can stop a bike using only the front brake...I'd be a fool to suggest otherwise. However, the discussion has become one where people are saying that you should use only the front brake and some have even suggested that using the rear brake is dangerous. Since most people use both without incident, including people using their brakes in less than ideal conditions (MTB being the most common example) , clearly using both is not dangerous and is actually advantageous. Add to this the fact that the rear brake provides 20% of the deceleration capacity of the bike. If you are not utilizing up to 20% of your braking capacity, you are not reaching full braking potential of your bike...just as you would if you were unable to modulate the brakes so as to keep the rear wheel rolling and not sliding.

I think most riders split their braking between tires because they understand on an empirical level that doing so gives them more benefits than not doing so. They understand that using both brakes stops them faster than using only one brake does. Mountain bikers understand this as much or more than do strictly road riders do because they deal with it on a nearly constant basis. Just about every mountain biker with a little experience knows that a rolling tire stops better and faster than a sliding one. Since a sliding front tire is a rarity, as well as a disaster, that usually means not skidding the rear tire.

What I am saying is that arguing that using the rear brake is dangerous or that it contributes nothing to slowing and stopping the bike is as silly as saying 'never use the front brake'. It makes no sense given what I know from many years of high speed, high angle bicycling on both road surfaces and mountain bike surfaces. Since this has started, I have talked with other cyclists as well as motorcyclists and all of them have looked at me like I had two heads when I asked if they used both brakes. The answer has been a unanimous 'Of course!'

As for testing, try it yourself. Stop with both brakes, stop with just the front one, stop with just the rear. Slide your CG forward of the pedals and rearwards of the pedals as well as centered over the pedals. Measure stopping distances and time. I'd be very careful on the front only and sliding the CG forward...that's gonna be the scary one and the one that'll hurt

coldfeet

Apologies for the shouting, it was the end of a long day and I may have had a little bit too much to drink.

I use both brakes as a matter of habit, sometimes I use the rear only as it is too risky to use the front. Those times are when grip levels are far too low to permit front braking because all the front's grip is going into steering. In such situations as a matter of practicality if not physics, the rear gives more than 20% of the potential maximum.

In straight line stopping, when there is enough grip, the front can generate all the *********** that is possible. The fact that the rear cannot lift itself is beside the point. It can generate enough weight transfer to unweight itself and slide. The front cannot unweight itself, it can transfer weight to itself, and in doing so provide greater grip. The idea of a stoppie seems to be providing a red herring here, I brought it up to try to demonstrate the fact that it is possible to stop at maximum possible rates of deceleration without the rear brake. It is my contention that the amount of weight transfered from the rear wheel is in direct relationship to the rate of ***********. Therefore, the fact that the rear wheel is off the ground demonstrates that it is not providing any *********** in maximum effort stops.

I have never suggested that anyone stop using the rear brake, i don't even say you should always use the front brake, my point has always been that Graywolf's original assertation that not using the rear brake is giving up a portion of your maximum possible braking, and your defense of it that is wrong. Using both brakes under most circumstances is the way to go, it does not mean that under good conditions that the last 20% can only be provided by the rear.

Visualize the force diagram described in Supcom's post. It describes the limitation as being when the force vector intersects the front contact patch.
"A. The force vector exerted by the center of gravity of the bike plus rider to intersect the contact patch of the front wheel."
A stoppie is basically keeping that force arrow pointed through that contact patch, keeping everything balanced at maximum ***********.

CdCf

This is how I visualise it, and I agree with supcom and others, that maximum braking occurs when the combined force vector of gravity and *********** goes through the contact patch of the front wheel, at which point no appreciable force should be on the rear wheel.

cyccommute 

I think we are in more agreement than you think. I seldom use one or the other brake. The issue is not that simple. I use both and I usually don't really think about how much force I put on either. I don't quantify, nor do I think anyone can reliably quantify, brake lever pressure. I have other things to think about while braking and deciding if I'm using more pressure on the front or rear doesn't enter into my thinking. As long as I'm not skidding or going over the bars and the bike is coming to a stop, I'm happy.

Here is where we differ. The moment the rear wheel loses contact with the ground, you gone beyond the maximum deceleration. Think of it this way: The entire bike system has 0.5g deceleration. The split is 80% front, 20% back. The friction between the contact patches on the tires is at its maximum when both are in contact with the ground. The moment the rear contact patch loses contact (the tire skids or lifts...it doesn't matter), you have now transferred all of the friction to the front contact patch and you are asking the front wheel to take that extra 20% deceleration duty. The brake will handle it but something has to give. You've halved your friction with the ground. The coefficient of friction on the front tire can't suddenly double, so the deceleration rate has to change. The only way for it to change is to increase the time of deceleration which means longer stopping distance. Maximum deceleration is way back when you still had both tires...and both sets of brakes...engaged.

It is not the last 20% but it is still 20% of the overall deceleration rate of the bike. The system works as a whole. Remove one piece and the conservation of momentum is going to raise it's ugly head and something will have to give. That something will be stopping distance.

But what you are missing is the force vector through the second contact patch. It is not zero in either of supcom's scenarios. The contact patch of the rear wheel isn't just along for the ride. It provides a significant enough contribution to the overall braking that it can't be neglected. If the rear wheel's 0.1g deceleration contribution were a part of a much larger deceleration rate, then I'd have no problem neglecting it. But it's way to large to just say ignore it.

plodderslusk

If this is a only a theoretical discussion of the physics of braking and has no bearing on riding your bike I apologize for butting in with some practical cycling . I suggest we put mister "one brake is all it takes " on top of Col de la Bonette together with one of the better pro tour decenders, like Thor Hushovd, and then we will see who is first into Barcelonette 1700 meters below. What might be correct for riding a straight line on tarmac is IMHO not correct for riding serpentines as fast as possible downhill.

Abneycat

I don't mean to interrupt the theoretical discussion, which is pretty neat really, but can I ask something?

When it comes down to it, you typically will either find yourself in a position to enjoy a decent stop, which can be done in any decent method there is, or you find yourself facing imminent peril.

In the latter situation, perhaps there should be the discussion of *simplicity* as well. I mean, you could argue for rolling the brakes, using the front at 99%, riding forward on the front with your rear in the air, whatever you like,

But I mean, does the average rider think about this with 2 seconds to stop? In a panic situation, the human brain reaches for the simplest solution that comes to mind. What brakings solution are: efficient, safe *and easy*, requiring no elegant balancing, tactical use of the levers, or thought?

coldfeet

Where are you getting that from?

DieselDan

SHUT UP! This isn't very hard. Just use the rear brake to slow down, and both to stop.

cyccommute 

Supcom's analysis give maximum deceleration at 0.5g.


DCommuter gave this quote which I have seen elsewhere, as well

Note, I'm not talking about weight distribution. I am talking about the deceleration of the bicycle and the contribution of each wheel to that deceleration. Transfer of weight to the front wheel is very high in almost all braking situations...including rear brake only. I don't agree that the weight transfer is equivalent in all situations...such as when using the rear brake only or when changing the center of gravity of the system...but the differences are quite small. The weight transfer is enough to skid the rear wheel with the rear brake which is lifting the rear but it's not enough to cause pitch-over as you'd get with front wheel braking only.

cyccommute 

I'd put the interval of reaction time 1 to 2 orders of magnitude smaller than 2 seconds. 2 seconds is enough time to think about getting chewed out by your boss, having sex, do the grocery list and plan your stopping strategy

But I agree that most people are going to take the easiest route and yank both brakes as hard as they can and hope for the best. Through many years of mountain biking, I instinctively thrust my body down and back and put my feet parallel to the ground. I basically crouch over the saddle. Even in nonpanic stops, I take that stance...I just find that my stopping distance is shortest that way.

cyccommute 

The last time I noticed, to read the posts in any given forum, you have to log on to the computer, find the website for the bikeforums, go to a specific forum, click on that forum and find the thread, then find out what is going on, click on reply, type some response and click on submit reply. That's a whole lot of effort to go through to get the the point where you are shouting at people to stop a rather pleasant, and stimulating, discussion about something that is far more complex than just 'us[ing]...the brake'.

While I may not agree with everything the other posters have posted, I respect their points of view and see validity in some of their arguments...they are completely wrong, of course...and enjoy doing this intellectual battle we are engaged in. I admire their intelligence and knowledge...but most of all, I admire their civility, which can be lacking in these forums.

However, sir, if you don't like the discussion then don't participate! You don't have to read it. You don't have to open the thread. You don't have to invest any amount of time here at all. Go ride your bike. Find the joy in the wind in your face, the wheels singing below you and the world slipping by on some pretty dappled lane. I assure you that I will be doing the same tomorrow but for now I'm enjoying the stimulating conversation...as I think are many others.

coldfeet

"(This number comes from the book "Bicycling Science" where it is shown that the rear brake of a upright bicycle can only produce about 0.1 g deceleration at best)."

That there is the key point, "at best." If you are using the rear brake only, you can generate about 20% of the potential maximum. If you start using the front at all, then you start taking that potential away from the rear. You can't maintain that 20% and use the front in any serious way.

You can get some braking from the rear if you are not going all out, and that is what I recommend, in most situations. I also recommend carefully practicing with the front brake, to get a better idea of how much force you can generate and get away with. Care is necessary in this because you can crash the bike easier with the front.
I also suggest they practice that technique you describe,
"Through many years of mountain biking, I instinctively thrust my body down and back and put my feet parallel to the ground. I basically crouch over the saddle."
I was suggested another technique, which is a more extreme form, putting your pelvis behind the saddle. This would have even more effect on the weight transfer, but I found very disconcerting. Thinking about it, I believe I use the technique you described without conscious thought. I wonder if I will lose that habit when I get my Longtail?

coldfeet

No he/she doesn't, but they should have some idea of what their brakes are capable of, and have some skill in using them. This is not an order, more of an observation of common sense. The more practice you have had with the brakes, the more likely you will remember in that fractions of a second, what to do. I remember a serious chain reaction crash that happened here some years ago, it happened on a stretch of road notorious for icing up, ( which should have given them a heads up anyway ) When they interviewed a number of survivors afterwards, they all said with very little variation, "I came round the curve and saw the crash, so I hit my brakes" None of them gave any indication of having tried to modulate the brakes or any attempt to steer instead. When driving, I am constantly amazed and disgusted at the level of oblivion that other drivers seem to have. You can tell just from watching that half of them have no clue what other vehicles are around or what they are doing. Having that same level of oblivion on a bicycle on traffic is just asking for it.

graywolf

You make a very good point.

What I think is that if you use proper braking technique all the time, that is what you will do in an emergency situation. Which in a way is one of the things that I and some of the other posters are trying to get across, for in all reality as you leisurely roll to a stop at a cross street it does not make any difference what technique you use.

However If you are coming down a mountain at 35-40 MPH, or more, and suddenly you see a patch of gravel on the pavement in the curve ahead of you, you either slow as fast as possible, or you get hurt. At that point I hope you have been practicing the most efficient way of braking.

In a way, I think we should leave stopping out of the equation, as that is only the last second or so of a potentially very interesting event that may have lasted a mind numbing couple of minutes. Sitting in front of your computer reading this thread and perhaps arguing about it is intellectually stimulating, but the actual event you may become involved in is physically and psychologically terrifying (I still have nightmare visions of the empty space on the outside of that curve mentioned in the above paragraph). It is best if you know what to do before it happens.

Maybe flatlanders do not need to worry about it too much, but if you are up in the mountains you use your rear brake a lot as a drag to control your speed. If you only had a front brake in such situations you could find that your brake is over heated and does not work as well as it should when you need it.

If you seldom ride faster than, say, 10 MPH then you can get along with just a rear brake as on most toy bicycles. If you usually ride faster that that you should have a front brake as well. The thing that bothers me, and the reason I started this thread, is that so many "experts" claim you only need a front brake. People can do what they want to do, I have no argument about that, but they should not put out false information as the "TRUTH".

coldfeet

Agreed, saying you should only ever use the front brake is stupid. I have never said that, and I don't think I inferred it. My disagreement with you and cyccommute is that you seem to be saying that there is a portion of braking force which can only be delivered by the rear, irrespective of what you are doing with the front. My contention, is that the front can make the rear irrelevant in a maximum effort stop, under good conditions. Discussing it is a mental exercise if you will, I just don't think you understand the physics.

1.It's a good idea to practice braking. carefully.
2.Most of the time you should use both brakes. Even alternating them on steep long downhills.
3.When conditions permit, The front brake gives you the greater part of the total braking effort. In extreme, flat, road, situations, the rear brake becomes irrelevant. It doesn't matter if you use it or not. If you can get the most out of the front, in such extreme situations, the rear brake is just along for the ride.

EDIT: Correction, added the word ever in the first paragraph

CdCf

The thing people are trying to explain to you is that if you have a working front brake, you don't need a rear brake to make you stop faster, because it won't be able to do that (Ye canna' change the laws o' physics!). The rear brake is still useful as an emergency backup, in case the front brake fails or fades, as well as a primary brake on "slippery" surfaces (ice patches, gravel). The rear is also useful as a "drag brake" during longer descents.

So, in my opinion, a bike is neither complete, nor safe unless it has a front brake and a rear brake (fixed gear doesn't count) independent of each other.

coldfeet

It's been mentioned before, but here is a direct link
https://www.sheldonbrown.com/brakturn.html
Mods? No problem with using that link is there?

Sheldon, as always, covers pretty much everything. You will note he does suggest using the rear sometimes.