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Braking

Old 01-13-06, 10:23 PM
  #1  
sajikumar
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Braking

Hi

I like to know how much intensity (ratio) should we use while braking for front and rear brakes (like 40% front and 60% rear).Is there anything like optimum level of above ratio? And Is it safe to use front brake while biking downhill?.
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Old 01-14-06, 06:25 AM
  #2  
MichaelW
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First , make sure that your brakes are well adjusted and do not stick or catch on the rim.
I only use the rear brake for emergency stops or to reduce my speed on long descents (where I alternate front and rear).
You can brake with the front on downhills but you need to be very responsive. If you feel the wheel starting to lose traction you should release some of the braking pressure.
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Old 01-14-06, 06:55 AM
  #3  
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This article explains things much better than I ever could.
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Old 01-14-06, 07:53 AM
  #4  
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Good braking technique comes with experience. I favor the front brake most of the time unless I have questions about the traction. For instance on wet paved downhills, I might favor the rear more than usual. Lose that front end in a skid and you can go down in a heart beat. Skidding in the rear can be be compensated for with body shifting. I very rarely use both brakes at the same time. The one thing I try to do is not use my brakes. They are the last resort. Nobody taught me to do this, I just sort of figured it out over the years.

I do not pretend to be an expert. I will still occaisionally get myself into trouble with poor brake usage. Usually on some trail where I miscalculate the trail conditions. Hence the no brake policy whenever possible. A good way to learn braking is to ride on ice.
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Old 01-14-06, 01:15 PM
  #5  
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just dont hit the front brakes too quickly. Ive never really heard of any particular techniques, i just do what works for me. in this case, it is pretty much on par with whats been said here. Except on my Giant, becuase it only has front brakes...and they dont work anymore...still waiting on new brake pads...

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Old 01-14-06, 01:23 PM
  #6  
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Originally Posted by SteveFox
just dont hit the front brakes too quickly.
Yeah, that will build braking forces too fast for you to adjust. I think the OP wanted a simple answer, so here it is:

F/R balance__braking-force
50/50 - initial braking, under 0.25g deceleration
60/40 - 0.4-0.6g
70/30 - 0.6-0.9g
80/20 - 0.9-1.3g
90/10 - 1.3-1.5g
100/0 - 1.5g+
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Old 01-14-06, 02:35 PM
  #7  
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Here's what Sheldon Brown has to say about braking:
http://sheldonbrown.com/brakturn.html

A very good article.
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Old 01-14-06, 03:37 PM
  #8  
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Originally Posted by DannoXYZ
Yeah, that will build braking forces too fast for you to adjust. I think the OP wanted a simple answer, so here it is:

F/R balance__braking-force
50/50 - initial braking, under 0.25g deceleration
60/40 - 0.4-0.6g
70/30 - 0.6-0.9g
80/20 - 0.9-1.3g
90/10 - 1.3-1.5g
100/0 - 1.5g+
Now apply those figures (calculating in your head) while braking.

Only one answer - practice in different conditions in a number of places.
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Old 01-14-06, 04:40 PM
  #9  
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Originally Posted by DannoXYZ
Yeah, that will build braking forces too fast for you to adjust. I think the OP wanted a simple answer, so here it is:

F/R balance__braking-force
50/50 - initial braking, under 0.25g deceleration
60/40 - 0.4-0.6g
70/30 - 0.6-0.9g
80/20 - 0.9-1.3g
90/10 - 1.3-1.5g
100/0 - 1.5g+
Where are you getting those numbers? Forester, who I'll believe on this issue because he's an engineer, says you can do about 0.3g max using the rear brake, and about 0.6g max with the front brake before you go over the handlebars. Using either brake causes weight to shift from the rear wheel to the front; at the limit of braking all of the weight is on the front wheel and the rear wheel contributes nothing.

I normally only use the front brake. The only case where you can get more braking power using both brakes than just the front is when conditions are so slippery (or your front brake is in such bad repair) that you can't generate enough braking with the front brake to lift the rear wheel.

I'd be surprised to see any vehicle with rubber tires pulling 1.5 g.
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Old 01-14-06, 04:55 PM
  #10  
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Originally Posted by DannoXYZ

Taking this graph beyond auto applications since the bike has much better brakes, you'll actually see the graph head back down again. More front-brake is added while the rear-brake is actually released. This is in 1.5g braking where the rear tyre starts to slide, you gently lighten up on the rear brake while increasing the front brake.

At some point around 2.0g of braking, the rear tyre is completely off the ground and ALL braking is done with the front. In this case, in addition to having released ALL of the rear brake, you'll also want to have your butt scooted off the back with your belly resting on the saddle to keep the rear-tyre down.
Only two nits to pick there; the car has better brakes, the bike just has much less mass.

I really don't think you're going to get much over 1 G on any normal two wheeled vehicle, and that would take conscious technique to keep more weight over the back. Probably less on a wedgie bike than on a motorcycle because the CoG is so much higher (although the pivot point (front axel is also higher...). Cars can go way over 1G streat cars rarely do, but purpose built race cars go way over.
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Old 01-14-06, 04:57 PM
  #11  
danch
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Originally Posted by DCCommuter
I'd be surprised to see any vehicle with rubber tires pulling 1.5 g.
4 wheels and a low center of gravity will do it. Way over 1.5 g, for F1 cars (whose tires are as 'rubber' as anything else - i.e not much)
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Old 01-14-06, 05:55 PM
  #12  
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Originally Posted by DCCommuter
I'd be surprised to see any vehicle with rubber tires pulling 1.5 g.
I attended a Skip Barber racing school course in Lime Rock Park (CT) in 2005 and let me assure you that those little open wheel carts that we drove were more than capable of pulling over 1.5g (ESPECIALLY when braking).
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Old 01-14-06, 08:58 PM
  #13  
danch
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Originally Posted by DannoXYZ
I make that assessment based upon stopping distances; there are zero cars on the market that can stop as fast as a bike, period.
Demonstrate please. I think that you're assuming that bikes can decellerate over 1G and/or that Cars 'on the market' cannot.

I look at the RotorDiameter-to-mass ratio. Bikes with their big rims require minimal amounts of clamping force to generate resistive torque on the wheels while the car's smaller discs require much more torque.
Sure, but the car can clamp harder on its disks than the bike can, _because_ of the greater mass (greater inertia, better traction mean that the wheel will continue to turn with higher clamping forces. Also, the car most likely has much better friction compounds in its brake pads.

I also look at the mass-to-BrakeSwept area. This determines how much heat a braking system can shed for its total braking-force requirements. The total surface-area of the braking surface on a bike is much larger relative to the mass when compared to a car, thus a bike's brakes will not heat up as much for a similar deceleration rate as a car; it can shed more heat/second/lb than a car.
Isn't that "... a similar deceleration energy..."? The bike starts with much lower inertial energy in the first place.

<snipped the discussion of traction of a rubber tire fs. friction - I was going to point that out, but you beat me to it!>
Back to bike tyres, yes, due to the lower mass of the bike+rider combination, along with the high-pressure tyres that really bite into the road, a bike can ALWAYS stop much, much faster than a car.
Lower pressure tires grip better by heating up better and by deforming to fit the surface of the road better. They also have much more surface area, which as you point out in the bit I snipped above is an advantage.

Going from 40mph on a bike down to zero requires only about 40-50ft. Calculate deceleration-G from that. Try that in a car... heh, heh... Well, a $450K Porsche CarreraGT with 60-0mph braking distance of 101ft actually comes close, but not any off-the-showroom floor you're likely to see out there...
I want to see you accelerate at over 1 G for any appreciable length of time. I need evidence, here.

Due to the change in weight-shift to the front-tyre, optimum F/R brake-balance MUST always change with deceleration rate. There's no way anyone can claim that a 60/40 split will work all the time, or a 70/30 split. It HAS to be customized to the particular deceleration rate.. And there's such a thing known as training. We spend years and years training firefighters, doctors, underwater welders, F1-drivers, fighter-pilots for a reason. There IS a way to get maximum braking-force out of a bike-tyre so that you're sliding the front-tyre with the rear-tyre off the ground without going over the bars... Until you've slid the front-tyre under maximum-braking, you really have no idea how powerful the brakes are on a bike (refer to disc-brake thread somewhere).
Absolutely, my dispute is that the limiting factor (on a good, dry surface) is going to be the simple of the leverage of the mass and the force the caliper applies to the fork. I may be mis-generalizing from my experience on motorcycles, which tells me that this is absolutely true. My back wheel comes off the ground before I get to 1G of decelleration. I may be wrong, prove it with data or a vector diagram! I know you're up to it
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