RoadBikeReview.com's Forum Archives - Fixed Gear
Sorry to rehash, but front vs. rear braking...(14 posts)
|Sorry to rehash, but front vs. rear braking...||timfire|
Jun 9, 2003 9:57 AM
|Sorry to rehash this, but I haven't visited in a while and the other thread on brakes has been pushed down the page.
Anyway, on the issue of front vs. rear braking, the guru himself, Sheldon Brown, has this to say:
"The fastest that you can stop any bike of normal wheelbase is to apply the front brake so hard that the rear wheel is just about to lift off the ground. In this situation, the rear brake cannot contribute to stopping power, since it has no traction...
The rear brake is O.K. for situations where traction is poor, or for when your front tire blows, but for stopping on dry pavement, the front brake all by itself provides the maximum stopping power, both in theory and in practice...
Many cyclists shy away from using the front brake, due to fear of flying over the handlebars. This does happen, but mainly to people who have not learned to modulate the front brake...
Jobst Brandt has a quite plausible theory that the typical "over-the-bars" crash is caused, not so much by braking too hard, but by braking hard without using the rider's arms to brace against the deceleration: The bike stops, the rider keeps going until the rider's thighs bump into the handlebars, and the bike, which is no longer supporting the weight of the rider, flips."
If you want to read more go over his page and look around.
|That question pertained to going from fixed to SS ...||Humma Hah|
Jun 9, 2003 10:53 AM
|... which means the rear wheel has no braking ability on its own, unless brakes are added. This brings up a big difference between SS and fixed.
Fixed gear bikes generally can't corner aggressively because their leaning is limited by pedal clearance. That being the case, you slow down well before a turn, or you're doomed anyway.
Freewheeling bikes can be leaned over and turned aggressively by coasting thru the corner with the inside pedal up. For a maximum-performance turn, you brake into the first third of the turn, coast around the apex, and accelerate out. Once leaned over in the turn, you would be VERY unwise to do all your braking on the front wheel ... you want roughly equal braking front and rear.
For general braking in a straight line, on average you'll probably get about 80% of your braking on the front wheel, as most of us don't QUITE lift the rear wheel while braking. So that leaves 20%. That's a nice little margin when trying to avoid hitting the door that just swung open in front of you in traffic.
|with all due respect||Steve_0|
Jun 9, 2003 11:58 AM
|to Sheldon; I disagree. More friction=more stopping power.
...even if it WERE true, it would only be so on clean/dry roads. Front-wheel skids are certainly not the fastest way to stop (nor the most painless).
Jun 9, 2003 12:22 PM
|I tried. After reading Sheldon's thesis a couple of years ago, I occasionally have tried it. Grab a handful of front brake and squeeze as hard as possible. I never could lock the front on clean, dry pavement. The back tire darn near comes off the ground, and at least unweights enough that braking is nearly useless there.
I've ridden fixed gear with no rear brake, and climbing bikes with no rear. You really can brake sufficiently with just a front.
However, as Sheldon disclaims, sometimes you really do need a rear, as when turning, front blowout, or when descending mountains and not wanting to overheat the rim/brakes (divide the heat).
So, the need for a rear depends on the circumstances. For braking in a straight line on clean and dry roads, though, it's almost useless.
|speaking of trying...||Steve_0|
Jun 10, 2003 3:52 AM
|have you tried stopping faster with two brakes over one? I have.
Sheldon's statement holds water if the point of 'almost lifting the rear' is immediate (i.e., stopping on a dime). It's not. Using the rear brake for the duration of decellaration certainly adds friction and decreases stopping distance.
I agree that you can brake 'sufficient'ly in MANY circumstances. All it takes is one time, though.
Suprising, all the people who are so adamant about wearing helmets and gloves 'in case of an accident' dont take every precaution to avoid that accident. We're talking negligible weight here.
Jun 10, 2003 8:39 AM
|I think what's missing from the analysis is whether people really do use the front brake hard enough in emergency situations. The front and rear lockup threshold is drastically different, and people may be using too little front and too much rear.
Sure, I'd like to know the difference in stopping distance, if any, using both methods. Problem is doing a controlled test from a meaningful speed, not just 10 mph. Not sure I want to test this at 30 mph or faster, because to get a true test, you have to be just short of locking up, or at least trying to on the front. Could be scary.
If the difference is something like 1 foot, then I doubt it makes a significant difference in reality. In that last foot your speed would be pretty low, anyway; might be significant if a car were crossing your path at 90 degrees and you either get hit or not, I suppose.
I'll concede there are many situations a rear brake could make a difference. I have encountered them, once blowing a front tire on Highway 1 in California at 45 mph. Scary. I don't go quite that fast on a fixed gear, though.
|Another meaningful measurement is mph at impact.||dzrider|
Jun 10, 2003 11:44 AM
|You're analysis of the last foot seems correct to me, but if you hit something while slowing down, the more speed you've lost the better off you are.|
|I can offer no science to this discussion.||dzrider|
Jun 10, 2003 4:48 AM
|I do know that in a panic, I grab both brakes and squeeze as hard as I can, even when the left hand is just a hood with no lever. It also makes intuitive sense to me that until the front wheel locks and the back starts to lift, stopping the back wheel must be doing some good. I don't use my rear brake much, even with gears, but I feel a real need for some way to slow the rear wheel.|
|Pretty easy to test yourself||LC|
Jun 10, 2003 9:57 AM
|I am sure you have a geared bikes somewhere with 2 brakes, or go test drive one.
Find a good sized hill to come down and pick a marker about 2/3 of the way down the hill. Apply your brakes as you pass the marker, and see how far you get before you stop and note where that is. Now try it again with only the front brake.
I bet you anything that you stop quicker with two brakes! If you really want to see some major difference, try it in the rain. Sheldon Brown has some good info, but I think he was smoking something when he wrote that article ;)
Jun 10, 2003 10:24 AM
|Sheldon has created a blind following. I'm sure the brake-thing is one of VERY few errors on his part, but people sure latched onto it as gospel!
Of course, I'm sure a large part of the following is people who simply WANT it to be true; I wonder how many have removed rear brakes from their motorcycles and cars?
|Well, I won't rehash a second time...||timfire|
Jun 16, 2003 10:54 AM
|I must apologise, I never meant to imply that I was against rear brakes, or that rear brakes weren't helpful. I only said (in the original thread) that theoretically a front was all that was needed. I do believe (as Sheldon also says on his site) that there are times when a rear is helpful. But I do stand by my original statement that a rear is theorectical unneeded.
I believe to understand Sheldon's thesis you have to use the front brake to the point were the rear wheel starts to lift and lose traction. At this point the rear wheel CANNOT contribute to braking, since it will no longer be touching the ground (theorectically).
Now, you can argue that the average person doesnot have the skill to take the front brake to this point, but that does not diminish the idea.
As far as the car/motorcycle statement goes, though Sheldon never adreeses them specifically, he does talk about tandems. He says that because of a tandems longer wheelbase and weight distribution, a tandem's rear wheel won't lift off the ground like on a solo bike. Thus both brakes are needed for the most efficient braking (thought the front is still the primary). I assume that it's a simliar deal with cars and motocycles.
|Cars and motorcycles ...||Humma Hah|
Jun 16, 2003 3:32 PM
|Cars frequently have front disc brakes, rear drums. The fronts to the lion's share of the stopping in hard, straight-line stops.
Motorcycles: my old streetbike had front disc, rear drums. Most high-performance bikes these days seem to be dual front disc, single rear disc. My estimate of 80% of braking on the front wheel is actually a value from motorcycles. Motorcycles CAN be braked hard enough to lift the rear wheel in certain situations (dry pavement, warm tires, and relatively low speeds, typically).
Both types of vehicle, in racing situations where aggressive cornering is required, brake into the first third of the turn, generally quite hard. If that braking were all on the front wheel, the front wheel would be the most likely to slide, and that's a BAD thing. Distributing part of the braking to the rear wheel helps keep both of them glued to the pavement. This is a singlespeed concern only as fixies can't be cornered that hard.
|the ubiquitious squids on todays ZZ's and RX's prove that. nm||Steve_0|
Jun 17, 2003 4:48 AM
|center of gravity||DougSloan|
Jun 17, 2003 2:32 PM
|The center of gravity on a road bike is far higher than a car or motorcycle in relation to wheel base; thus, far more weight transfer and little traction at the rear tire contact patch when braking hard.