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?.

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.

This article explains things much better than I ever could.

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.

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...

steve

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+

Here's what Sheldon Brown has to say about braking:
https://sheldonbrown.com/brakturn.html

A very good article.

Now apply those figures (calculating in your head) while braking.

Only one answer - practice in different conditions in a number of places.

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.

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.

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)

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).

Demonstrate please. I think that you're assuming that bikes can decellerate over 1G and/or that Cars 'on the market' cannot.

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.

Isn't that "... a similar deceleration energy..."? The bike starts with much lower inertial energy in the first place.

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.

I want to see you accelerate at over 1 G for any appreciable length of time. I need evidence, here.

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