View Full Version : Questions about bikes and brakes
Okay, me and my friend ( who disagrees about EVERYTHING ) were talking about bikes and their brakes after my other friend fell off his and hurt himself badly.
He thinks that if you hit both breaks, you will immediatly flip over. He thinks, if you are going fast, down a hill or just flat and if you hit both breaks, front and back, you will flip over.
I disagree with this.
If you weigh 120-200, and are going down a hill fast, or just down a road and hit both breaks, would you flip over?
What are your opinions? or facts..
DieselDan
05-10-05, 07:41 PM
You won't endo if you keep your weight over the rear wheel and apply the rear just before the front.
http://sheldonbrown.com/brakturn.html
Read the above article. I learned a lot from this one.
ivan_yulaev
05-10-05, 08:47 PM
If you hit the brakes very hard, you will flip.
However, I dare him to try to go down that hill and make an emergency stop with just the rear brake. Better yet, an emergency stop going around a corner, with just the rear brake.
catatonic
05-10-05, 09:32 PM
Here we go, some facts about brakes in general...I'm talking about rim brakes, since those are the most common:
*the pad grips best at a certain temperature...this means the brake will GAIN stopping power shortly after applied, then LOSE power after it heats up to another certain point....this helps contribute to "endoing" when doing sudden stops. this can be alleiviated by "heating" up the pads by slowly adding force during a hard stop...if you do this right, you can have teh lever as far back as it can go on a downhill without weight shifitng and get tremendous stopping power. It's all about modulation
* you have to be familiar with how your bike reacts to brakes...rigid bikes react more elegantly than front suspension bikes, which tend to dip towards the road, making it EASIER to endo on overtly hard braking, especially on downhills.
* modulation: on long descents riding hte brakes will cause pad failure, which results in inability to slow down. There are tricks on how to do this and it varies by rider to rider...I liked fading my brakes from front to rear as I go down, but it still heats the pads up pretty bad on long descents, so keep that in mind. Modulation also means knowing what braking force is needed for what stop...sometimes it takes pulsing the brakes to do it. This is true especially in wet weather.
Basically you need: bike familiarity, knowing how to modulate your brakes, and controlling how bad your brake pads heat up. Once you got all these down, you can stop on a dime with little issues.
Just don't pull the brakes all the way back suddenly...that will cause a flip. Yo0u cannot flip on the rear brake, but you can lose control of your bike if you go too hard on the back brake around a turn or on a rough road/trail.
Dchiefransom
05-10-05, 09:33 PM
If you're going fast and you hit both brakes hard, you should break loose your traction on your wheels, and go into a skid.
DCCommuter
05-10-05, 10:09 PM
It pains me to disagree with the esteemed Sheldon Brown, but I believe that you should use only your front brake under most conditions. The rear brake should normally be used only as a back-up in case of mechanical failure of the front brake.
Applying either brake causes weight to shift from the back wheel to the front wheel. Except in low-traction conditions, or on a long-wheelbase bike like a tandem or some recumbents, it is not possible to skid the front tire -- the rider goes "over the bars" instead. The maximum stopping power comes when 100% of the weight is on the front wheel and the bike is on the verge of sending the rider over the bars. You can acheive this using only the front brake, and since the rear tire is carrying no weight, any use of the rear brake will cause the rear tire to skid. When the rear tire skids it no longer contributes any directional stability to the bike, and the bike has a nasty tendency to fold about the steering column and down you go. If you are using only the front brake, neither tire will skid, and you maintain control, you just have to worry about not flipping over -- but it is easier to modulate your braking with only one brake. If you use the rear brake it is possible to skid and lose control before the front brake is delivering maximum stopping power.
The exception to this rule is conditions where it is possible to skid the front tire -- either low traction such as ice, or a long-wheelbase bike. In that case you want to apply each brake until it is on the verge of skidding. Note that a long-wheelbase bike will have superior stopping power because it actually uses the full power of both tires.
Earlier this year I commuted for about four months with only a front brake and it really opened my eyes. I then sat down and calculated the physics, which confirmed my observations. I can post my calculations if anyone is interested.
I slow with my rear and stop with my front. That's how I've always done it and will continue to do so
On two wheeled vehicles, such as bikes and motorcycles, the front brakes provide about 70% of the stopping power... it is quite possible to flip a bike, but having never actually done that, I think it is a rare event. I have quite successfully locked up the front wheel and skidded on said wheel while not flipping a bike, nor shifiting my weight.
Riders should practice quick stops using both brakes and applying as much pressure as needed to quick stop.
I often use the front brake.. perhaps more than the rear brake, as the effect is so much greater. I tend to usually apply both brakes in most circumstances and just use the rear alone for slowing.
On long decents in dirt, I tend to just use the rear.
On any panic stop, I try to shift my weight to the rear.
lilHinault
05-11-05, 01:05 AM
Frankly unless you're going downhill on a mtb and hit a log, it's really hard to flip. I was told that old story too, heard it at Spanky's Bike Shop, when there with my dad, and wow what a load of hooey. And, 70% or so of your braking is in your front brake, that's just physics. You don't even need to move your weight way back off of the seat or anything, just brace yourself against the bars when you brake hard, which comes naturally.
If you suddenly apply the brakes hard enough to lock the front wheel, one of two things will happen - you will skid or you will have a tendency to go over the handlebar. Which one will happen depends on your speed, weight, weight distribution, the center of gravity of the bicycle and rider and the amount of friction between the wheels and the pavement.
I've skidded the front wheel numerous times. A few times I have had enough traction that my rear wheel lifted off the ground and I lost my balance and fell. I've come close enough to going completely over the handlebars that I'm convinced it's possible, at least on some bikes. On other bikes I suspect that "taking a header" would be virtually impossible unless you're trying to do it. (Actually I once did go completely over the handlebars - worst accident I ever had; I spent a few days in the hospital many, many years ago - but it wasn't due completely to braking; there were other factors involved, like an immovable object.)
Prevention is simple: don't lock up the wheels.
Practice, practice, practice. The quickest stop is achieved by modulating both front and rear brakes and slowing each wheel to the point that the tire just begins to skid but doesn't. Weight will transfer to the front so the rear wheel will have much less traction. However, as long as there is some weight on the rear wheel, the rear brake will provide some stopping power (perhaps not much in a quick stop where weight transfer is severe).
joejack951
05-11-05, 05:18 AM
Applying either brake causes weight to shift from the back wheel to the front wheel. Except in low-traction conditions, or on a long-wheelbase bike like a tandem or some recumbents, it is not possible to skid the front tire -- the rider goes "over the bars" instead. The maximum stopping power comes when 100% of the weight is on the front wheel and the bike is on the verge of sending the rider over the bars. You can acheive this using only the front brake, and since the rear tire is carrying no weight, any use of the rear brake will cause the rear tire to skid. When the rear tire skids it no longer contributes any directional stability to the bike, and the bike has a nasty tendency to fold about the steering column and down you go. If you are using only the front brake, neither tire will skid, and you maintain control, you just have to worry about not flipping over -- but it is easier to modulate your braking with only one brake. If you use the rear brake it is possible to skid and lose control before the front brake is delivering maximum stopping power.
The exception to this rule is conditions where it is possible to skid the front tire -- either low traction such as ice, or a long-wheelbase bike. In that case you want to apply each brake until it is on the verge of skidding. Note that a long-wheelbase bike will have superior stopping power because it actually uses the full power of both tires.
Earlier this year I commuted for about four months with only a front brake and it really opened my eyes. I then sat down and calculated the physics, which confirmed my observations. I can post my calculations if anyone is interested.
"Weight shift" is really a misnomer. Assuming the rider doesn't physically move himself forward or rearward on the bike, no weight is actually shifting. The high center of gravity of the bike/rider coupled with a force being applied at the contact patch of the tire creates a torque that tends to rotate the system about the front wheel. Same deal with a car although the CG does shift slightly thanks to the suspension compression.
As long as you have any weight on the rear tire (which any safe rider should) you wil get an added braking benefit by using the rear brake along with the front brake. Just like with cars, you need more front braking bias because of that torque created by your CG.
The same exact principles apply when riding a tandem, the only difference being you now have the weight of two people forcing the bike down instead of just one, plus because of the more rearward position of the rear rider, you create a much higher counter-torque (weight of rear rider X distance to front wheel).
It pains me to disagree with the esteemed Sheldon Brown, but I believe that you should use only your front brake under most conditions. The rear brake should normally be used only as a back-up in case of mechanical failure of the front brake.
You're not really disagreeing with him, as this is pretty much Sheldon's sentiment as well.
JohnBrooking
05-11-05, 08:55 AM
I don't mean to hijack the thread, but this seems the most natural time to ask this: Why is it that the front brake works so much better than the rear brake, anyway? Everyone always says so, and they're right, but I've never heard anyone explain why that is. Even Sheldon Brown's article (http://www.sheldonbrown.com/brakturn.html) on brakes doesn't explicitely explain why, although he hints it has something to do with traction.
So here's my guess: When you are slowing down, regardless of which brake you are using, most of the weight is shifting forward, even though you are sitting in the middle and maybe even sitting upright, depending on your bike. This causes the front wheel to have much more traction than the back. Hence Sheldon's advice on that page that "It is important to use your arms to brace yourself securely during hard braking, to prevent this. Indeed, good technique involves moving back on your saddle as far as you can comfortably go, to keep the center of gravity as far back as possible."
Does that sound right?
DCCommuter
05-11-05, 11:55 AM
OK, here's some of the physics involved. Imagine you are looking at a bike at rest, from the left hand side. The weight of the bike is divided between the front and rear wheels. The ground pushing up on the rear wheel tends to rotate the bike counter-clockwise, and the ground pushing on the front wheel tends to rotate the bike clockwise. Since the bike is at rest, these rotational forces are balanced, and the weight is distributed so that they cancel each other out.
Applying either brake exerts a force that pushes the bottom of the bike backwards, which rotates the bike counter-clockwise. In order for the bike to remain balanced, more weight has to move to the front wheel to provide a countering clockwise force. There are two possible limits to this behavior. In one case you reach the limit of the tires ability to provide stopping force, and the tires skid. In the other case, 100% of the weight gets transferred to the front wheel, and any additional braking creates an unbalanced rotational force that causes the bike to rotate and dump the rider.
Which one of these two will happen? It depends upon the geometry of the bike, and the friction between the tire and road. If you don't skid, the maximum breaking force is given by:
B=W* Cotan(A)
Where B is the braking force, W is the weight of the bicycle and rider, A is the angle between the ground and a line drawn between the contact patch of the front tire and the center of gravity of the bike and rider, and Cotan is the CoTangent function. At this point the torque from the breaking force is B*Sin(A), the torque from the weight acting on the wheel is W*Cos(A), and they are equal, which simplifies to this formula.
If the wheels skid, the maximum braking force is:
B=W*K
Where K is the coefficient of friction between the tire and the road.
The limiting case will be whichever formula is less. So if K < CoTan(A), the bike will skid, and if K > CoTan(A) the bike will flip before it skids.
Typically, on dry pavement the value of K will be around 1, which represents an angle A of 45 degrees. Most ordinary bicycles have an angle greater than that, and will flip before they skid on pavement.
If the bike has a tendency to skid, some weight is borne by each wheel, and you use both brakes to get maximum braking. If the bike has a tendency to flip, all of the weight ends up on the front wheel, and you should use only the front brake. Using the rear wheel in this case will cause it to skid very easily because it has no weight on it. On a two-wheel vehicle skidding tends to cause you to fall over because the wheels stop rotating and lose any contribution to directional stability.
Note that the values in these formulas are not constants. The coefficient of friction depends upon the tires and the surface. The same bike might flip on pavement and skid on dirt or ice. Also, you can change the angle A by moving your center of gravity backwards or lower.
I hope that wasn't too long-winded.
Nicodemus
05-11-05, 12:24 PM
CWK150: Sure it's possible, but difficult in most conditions. But it sure as damnation is possible. I had a right vicious bike-in-the-tree head over heels flight once. But then those were extreme conditions - 20mph down very steep hill with instant panic stop at the magical sight of a 5ft drop. And I weighed 240lbs.
DCCommuter: I agree, your explanation makes sense. However, the "reality" factor should be considered - that it may be safer to use both brakes because one cannot be such a master of their machine that they can accurately judge at which point the 100% maximum front stopping power will be reached, and that a balanced brake usage lowers the likelihood of flipping. You may have excellent balance and control of your bike, but many don't.
Re your second post - Aaah! Technical speak! University flashbacks! Aaaaaagh!
JohnBrooking: If you managed to follow DCCommuter's explanation, excellent. Otherwise, I see it as this:
The maximum stopping power of a front brake is larger than the maximum stopping power of the rear brake, simply because the front has the benefit of increased inertia from your weight as you increase braking power. The rear doesn't.
To expand on that, since the full range of braking power is greater on the front, it means you have more ability to accurately control the amount of power you use.
Hi,
I hate to disagree with Sheldon Brown but. I was on the Burlington Bike Path a month ago going down a slight hill when my riding partner hit her brakes and turned to talk to me. In a panic to avoid hitting her I slammed on my front brake. It’s a month later and my mouth still does not feel right. I am still healing from a large cut in my lower lip and I still have numbness there. I also ride a motorcycle and know from there that the front brake does most of the stopping but now when I bicycle I hit the back brake with steady pressure while hitting the front brake intermittently. I am flat out paranoid about flying over the handlebars again.
My friendly Dentist did replace a partial bridge made of porcelain with plastic. If it doesn’t hold it will be a two thousand-dollar bill to fix. That’s more than I paid for my bike – Ouch! I was ridding my Cannondale Hybrid which is a great bike but the write-ups on most Dale’s often mention a weakness in braking. If anyone can suggest an upgrade I would love to hear about it.
Thanks,
Ron
Sheldon Brown
05-11-05, 01:37 PM
Hi,
I hate to disagree with Sheldon Brown but. I was on the Burlington Bike Path a month ago going down a slight hill when my riding partner hit her brakes and turned to talk to me. In a panic to avoid hitting her I slammed on my front brake. It’s a month later and my mouth still does not feel right. I am still healing from a large cut in my lower lip and I still have numbness there. I also ride a motorcycle and know from there that the front brake does most of the stopping but now when I bicycle I hit the back brake with steady pressure while hitting the front brake intermittently. I am flat out paranoid about flying over the handlebars again.
Ron
You should never "slam on" the front brake. This is a classic example of what happens when a rider is used to relying on the rear brake, and hasn't developed a good feel for modulation of the front brake as a result.
If you had been in the habit of using the front brake, you wouldn't have "slammed" it. Then again, you might also have crashed into your careless riding partner.
People disgree with me about stuff all the time, and I make a general policy of avoiding getting into arguments about the fine points of bike mechanics and the like...except when there's a safety issue involved, because sometimes bad information or bad habits can cause accidents and injury, as may have been the case with Ron.
See: http://sheldonbrown.com/brakturn for my general article on this topic. Many readers have written to me reporting that the advice on that page has saved them from serious crashes.
Sheldon "Stop!" Brown
+--------------------------------------------------------+
| The ultimate measure of a man is not where he stands |
| in moments of comfort and convenience, but where |
| he stands at times of challenge and controversy. |
| --Martin Luther King, Jr. |
+--------------------------------------------------------+
I will never understand the physics, but I know from experience that you only need the front brake. I am ashamed to admit that my rear brake has been disconnected for over a week now, due to laziness on my part. I made several practice panic stops and one for real. They all worked fine.
Not that I would make a habit of this. (Well, I guess I have made a habit of it, but I shouldn't!) As others mentioned, the rear brake does provide additional stopping power (especially downhill?), it serves as a backup, and it probably decreases wear and tear on the front brake.
operator
05-11-05, 02:46 PM
Regardless, you'll want to have a rear brake, even if it's not doing most of the work. Or in case your front fails for whatever reason.
phidauex
05-11-05, 05:53 PM
Motorcyclists frequently perform 'stoppies', which is when you slow down so fast that your rear wheel lifts off the ground, and you roll along on only your front wheel. Lots of fun! At the moment that occurs, the rear brake is now worthless, because the rear wheel isn't on the ground. Of course, on a bicycle you rarely stoppie (though it is possible to do without endo'ing), but the concept is the same.
The possibility of a skid changes in proportion to the force on that wheel.
If you have two objects pressing against one another, they have a given coefficient of static friction (and a wheel is using static friction, because the contact patch is static, even though the bicycle is not). That coefficient depends partially on the force that holds the objects together. As you stop a bicycle, your downward force vector angles forward (this is what people mean by a 'weight' shift, as opposed to a 'mass' shift, which would indicate that the cyclist changed position). This movement of the force vector lowers the force connecting the rear wheel to the ground, and increases the force holding the front wheel to the ground. Hence, the front wheel is becoming less likely to skid, and the rear wheel is becoming more likely to skid.
Endo'ing is certainly possible, and I've done it myself (sorry to hear about those who've done it more recently), but it isn't because the front brake shouldn't be used on its own, its because you managed to stop the wheel before it skidded, locking it up and flipping yourself over. Better braking modulation and more practice will solve that problem.
In practice, I use both brakes, but rely on the front (same as on my motorcycles). For general slowing and stopping, both brakes contribute to your stopping, and you won't wear out rims and pads as fast. But if you were to test out how fast you could possibly stop your bike, IE, the minimum stopping distance for a given speed, you'd find it was the same distance whether you had a rear brake or not, since it can never contribute more stopping force than it takes to lift itself off the ground, and you can continue to bleed speed even while your rear wheel is off the ground with good modulation.
Confusing stuff... But it works.
peace,
sam
catatonic
05-11-05, 08:56 PM
all "endo"ing usually consits of is the rider moving forward at a faster rate than the bike. The other way it can happen is by too much braking power combined with tires grippy enough to bite hold of the pavement instadd of slidingout, which is pretty rare.
DCCommuter
05-11-05, 10:58 PM
I was ridding my Cannondale Hybrid which is a great bike but the write-ups on most Dale’s often mention a weakness in braking. If anyone can suggest an upgrade I would love to hear about it.
Disc brakes.
I went over the bars three times on my Cannondale before I upgraded to discs, and haven't since then. The V brakes were just too grabby. The Avid discs I have now have so much better modulation that I can stop much quicker and I've yet to feel even close to flipping.
You only really need to do the front, which means a new fork and a new front hub.
DCCommuter
05-11-05, 11:05 PM
It pains me to disagree with the esteemed Sheldon Brown
I just read the linked article, and it pains me even more to admit that I was actually agreeing the whole time!
Maximum braking= Front brake only.
Maximum control= Front brake only.
Rear brake= longer stopping distance and fishtailing.
It's counter-intuitive, but it's true.
ivan_yulaev
05-11-05, 11:42 PM
Usually, what happens when a person endos is NOT that he brakes hard. Rather, it is that he in unfarmiliar with using the front brake. So, when he is suddenly forced to use it, he fails to brace himself with his arms. Therefore, the rider slids forward, and goes "over the bars"
JonnyBoy
05-12-05, 08:51 AM
Yeah, DC Commuter and Phidauex are both right.
At the point of maximum braking a rider would have almost no weight on the rear wheel. When riding very steep down hills off-road, I am on the verge of an endo the entire descent. Also, when panic stopping in traffic, grabbing the rear brake always results in a rear wheel lock up due to all of my weight being shifted to the front wheel even though I stretch out and hang my butt behind the seat and over the rear tire. Using my rear brake in these situations doesn't do a lot of good in terms of stopping. I'm not saying that a rear brake is useless; but when trying to stop in a straight line, in the least amount of time, utilizing just the front brake is as effective as using both. These same forces apply on level road, and if you are traveling fast enough and brake hard enough, even uphill.
I try to practice stoppies or endo's when I can. It helps me get comfortable with brake modulation and weight transfer, which aids in performing successful panic stops without going over the bars.
Thanks for the advice. I know Sheldon Brown is correct on use of the front brake. I certainly should not have slammed it on. I also agree that my weight was to far forward and I did not use my arms properly. I also ride clipless (SPD) and my pedals did not release allowing me to use my legs properly. As is the case in most accidents things just happen so quick and unexpectedly and my reflexes just aren't what they use to be as a young man. Just yesterday I came around a curve in a road to see a car sitting stopped in the middle of the road while the driver talked to a pedestrian. Fortunately there was not a car coming from the other direction. This time I used my brakes properly and avoided another crash. After thousands of mile of accident free riding I can’t believe what is happening this spring. They say things come in threes so I am definitely going to be on my guard.
Ron
From Quotes by kids: "You should never pick on your sister when she has a baseball bat in her hands."
Joel, age 12
Thanks for all the explanations--especially sam. I finally got my rear brake working but I still don't use it much. Yesterday a kid on a BMX and I on my commuter almost collided at blind intersection. Neither of us was going real fast. We both had the same reaction. We locked our front wheels, turned sharply, planted one foot on the ground, and swung the rear end of the bike in the opposite direction. I think this clearly demonstrates that our rear wheels were off the ground. I could see that his was about 4 to 6 inches in the air as he swung his rear end around.
JohnBrooking
05-12-05, 02:23 PM
DCCommuter: Thank you for the detailed technical explanation. I think I sort of follow it, but I'm still not quite understanding how it answers my question.
First off, I've never heard the bit about the ground pushing the wheels tending to rotate the bike in opposite directions. I'll take your word for it, but it sounds really bizarre and I can't imagine what would cause that. Does it happen to cars, too, since they can also skid upon sudden braking?
Anyway, assuming it does, then you say the braking force pushing the bottom of the bike backwards means that in order to remain balanced, more weight has to move to the front wheel (because of the aforementioned rotating tendency). So by "has to", do you mean "does"? I can imagine weight shifting to the front due to inertia, but I have a harder visualizing it happening as you describe. Is what I am calling inertia really the same thing: The bottom of a slowing object wants to pull back, so the structural integrity of the object causes an equal forward movement to prevent the object pulling apart in the middle? On the other hand, that doesn't seem to have anything to do with rotating.
Is the bottom line, as Nicodemus says, that it is the inertia to the front tire that accounts for it?
Last night, I was riding with my two boys, ages 5 and 7, on the sidewalk. (Yes, I will teach them about VC when they are a bit older.) As an experiment, I tried applying just my rear brake, and it hardly worked. I probably wasn't going over 7 MPH when I started braking, and I didn't feel any forward intertia in my body at all. Yet the rear brake alone hardly worked. Would these forces still account for that, even at those low speeds? Or maybe I have a mechanical problem with my rear brake! :)
First off, I've never heard the bit about the ground pushing the wheels tending to rotate the bike in opposite directions. I'll take your word for it, but it sounds really bizarre and I can't imagine what would cause that. Does it happen to cars, too, since they can also skid upon sudden braking?OK, I'll bite - er, I mean I'll take a stab at trying to explain it.
It happens to cars, too.
It's simple physics. The greater the weight on a tire, the greater the stopping power it has. Because of weight transfer during braking, the front wheel will have more weight on it.
But, you ask, what the heck is weight transfer?
In order not to get too confusing, I'll use the same formula that DCCommuter used.
B = W * K
Where K is the coefficient of friction between the tire and the road.
B is the braking force. W is the weight or, more precisely, the force perpendicular to the ground where the tire meets the ground.
Force B is applied at the point where the tires meet the ground and resists the force due to momentum which acts through the center of mass of the bicycle/rider. Since the two forces are not in line, a torque is created which tends to rotate the bicycle (counter-clockwise to use DCCommuter's description). This is resisted the only place it can be resisted, by a force perpendicular to the ground at the front wheel.
So, back to the formula:
B = W * K
The torque created by the braking force (at either wheel) is resisted by an increase in the force, W, at the front wheel, combined with a decrease in W at the rear wheel.
In other words, braking causes W to increase at the front wheel. To keep things in balance, W must decrease at the rear wheel by the same amount (because the total weight is supported by the front and rear wheels and only the front and rear wheels).
To put it another way:
W (front) + W (rear) = W (total)
The increase in W at the front tire and the decrease at the rear tire is the phenomenon of weight transfer.
Now, to look at the braking force at each wheel individually:
B(front) = W (front) * k
B(rear) = W (rear) * k
Because of weight transfer, W (front) is greater than W (rear). B (front) is therefore greater than B (rear) by a factor that is a function of the weight transfer.
Maximum total braking force is:
B(total) = B(front) + B(rear)
As long as there is weight on the rear, B(rear) will not be zero, so maximum total braking force can only be achieved by using both brakes - at least in theory.
In practice, it's very difficult to apply the rear brake without locking it up so, in a quick stop, only the front brake, which is the one with most of the weight on it, is effective. The quicker the stop, the more this is true.
Try this:
Stand next to your bike and apply both brakes. Now push the handlebar forward. What happens?
The rear wheel comes off the ground. The entire weight of the bicycle is now on the front wheel - weight transfer.
Now apply just the front brake and push the handlebar forward. The same thing happens - the rear brake has no effect.
Now apply just the rear brake and push the handlebar forward. If you push hard enough, the rear wheel will slide along the ground. More proof of the ineffectiveness of the rear brake.
JohnBrooking
05-12-05, 09:12 PM
I understand about weight transfer, friction, traction, etc. What I don't understand is:
The ground pushing up on the rear wheel tends to rotate the bike counter-clockwise, and the ground pushing on the front wheel tends to rotate the bike clockwise. Since the bike is at rest, these rotational forces are balanced, and the weight is distributed so that they cancel each other out.
I understand forces going forward and backward, and I guess I could even understand if the back of the bike still wants to go faster than the front, it needs to wrap around the stem in some direction, but this explanation makes it sound like each tire always wants to push the bike in a certain lateral direction all the time, and one always does it one way, and the other always does it the other, and that's what I don't get. :(
DCCommuter
05-12-05, 10:06 PM
I understand forces going forward and backward, and I guess I could even understand if the back of the bike still wants to go faster than the front, it needs to wrap around the stem in some direction, but this explanation makes it sound like each tire always wants to push the bike in a certain lateral direction all the time, and one always does it one way, and the other always does it the other, and that's what I don't get. :(
When I say "clockwise" and "counterclockwise" I'm talking about the bike as viewed from the left. Imagine your bike is sitting on two sawhorses, one under each wheel, and you are standing to the left. You kick out the sawhorse under the front wheel. What happens? The front wheel falls down, because only the rear wheel is supported. Viewed from the left side, it rotates counterclockwise. Similarly, if you kick out the rear sawhorse the bike rotates clockwise, as viewed from the side.
john999
05-12-05, 10:18 PM
It's very easy to flip the bike over if V-brakes are set in the ^ configuration that cycling books recommend, and you are not used to it.
I flipped a bike over like that riding at walking speed. It was like someone grabbed the saddle and threw me upwards !
Foot brakes work more effectively on the rear because you shift most of your weight to the rear axle. (And in BMXs, the rider sits over the rear axle).
JohnBrooking
05-13-05, 06:28 AM
When I say "clockwise" and "counterclockwise" I'm talking about the bike as viewed from the left. Imagine your bike is sitting on two sawhorses, one under each wheel, and you are standing to the left. You kick out the sawhorse under the front wheel. What happens? The front wheel falls down, because only the rear wheel is supported. Viewed from the left side, it rotates counterclockwise. Similarly, if you kick out the rear sawhorse the bike rotates clockwise, as viewed from the side.
AAAAAAHHHHHHH!!! There's my confusion. I thought you meant from side to side, as viewed from the top. I was trying to picture the back wheel wanting to move the bike left (counterclockwise, viewed from top) and the front wheel wanting to move it right. That didn't make any sense at all. But you mean basically up and down from the front or back. I get it now.
So now I'll have to read the whole thread over again in this new light! :rolleyes:
Sorry again for the thread hijack. I hope others besides me got something out of it.
Stregone
05-13-05, 04:59 PM
Usually, what happens when a person endos is NOT that he brakes hard. Rather, it is that he in unfarmiliar with using the front brake. So, when he is suddenly forced to use it, he fails to brace himself with his arms. Therefore, the rider slids forward, and goes "over the bars"
This is really the prime factor in endos in my experience. Just the other day I was on a smooth section of trail, and was very casually pedaling along. I didn't notice a small log on the trail, and when the front wheel hit it the bike stopped and I kept going forward because I was in a very relaxed position. I went over the bars and my body pushing on the bars pulled the bike over on top of me. Since then I can just keep my arms braced and ride right over the log without any hopping, unweighting, or pulling up on the bars at all.
You can brake pretty dang hard if you you really brace yourself and lean back like you are going down a steep hill. If you are familiar with relativity you know that acceleration/deceleration is basically the same thing as gravitational attraction(indistiguishable in some situations), so it all makes since if you think of braking as going down a hill.
vBulletin® v3.7.3, Copyright ©2000-2008, Jelsoft Enterprises Ltd.