I've seen bikes with front and rear disk brakes and noticed that how come the rear disk is reasonably small compared to its front disk brake. And some front disks are exltremely large almost same diameter as the front rim and the rear disk seems to be deprived of its size.

Below is an example. The Aprilia Nera

Thanks. :)

In most cases, except for heavy cruisers, 75% of your braking power comes from the front. Weight is transferred to the front when you put on your brakes and that's when your front brakes become more effective. The bigger, the better. Just as you observed, this is just the opposite for the rear. When weight is trasferred to the front, the back gets light and applying more brakes to the rear will make the rear tire skid, causing loss of control and a possible accident. Less brakes to the rear, is better.

IC, that clearly explains it. Great thanks. :)

guys how can you properly clean your disk breaks? Like what parts of it am i going to spray with for example with a wd40? oil? :?

I presume you mean to get rid of the brake dust? Don't use WD40, it won't evaporate and is designed to make things slip rather than grip. It will leave a greasy residue. Not desirable around brakes!. You can buy brake dust cleaner. Use that. If you can't find it I have also used carb cleaner effectively. Its CTC-based. It blows away the mess and leaves a shiny clean dry surface with no unwanted residue.

It may need a bit of help from an old toothbrush if the deposits have been there a while.

In most cases, except for heavy cruisers, 75% of your braking power comes from the front. Weight is transferred to the front when you put on your brakes and that's when your front brakes become more effective.

I have read in some forum that while driving on high speeds, most of the riders use only their front breaks. In doing this, the weight momentum will be transferred to the front. Won't this cause the bike to flip over? Just wondering!

Originally posted by siRei
I have read in some forum that while driving on high speeds, most of the riders use only their front breaks. In doing this, the weight momentum will be transferred to the front. Won't this cause the bike to flip over? Just wondering!

IMHO, it shouldn't flip the bike over. The worst that can happen is your fronts let go. The rear will only lift up and uver the front if

1. The front wheel is fully or nearly stopped.
2. The front wheel has full traction.
3. The weight transfer is forward and up of the front wheel.

This will explain why stoppies are just that stoppies. They happen at nearly full stop or at full stop.

Brakes work by using friction to convert forward motion to heat. I don't know of any braking system that can convert enough momentum into heat for a high speed stoppie.

To induce a 100 kph stoppie on Hiroki for example. Figuring a 2 second 100 to 0 decelleration rate is beyond my means of letting go the front brake lever.

Hiroki mass = approx. 174 kg. dry
My mass = 68 kg.
Combined Mass (CM) = 242 kg.

Start Speed (SS) = 100 kph = 27.8 m/sec
End Speed (ES) = 0 kph = 0 m/sec
Stopping Time (ST) = 2 secs
Deceleration (D) = 50 km./hr/sec. or 13.9 m/(sec^2)
Distance = (27.8 m/sec * 2 sec) + 1/2 (13.9 m/sec^2 * 2 sec^2) = 83.4 m

Thus the force (F) required to decelerate at this rate would be:
F = (242 kg x 13.9 m)/(sec^2)
= 3,363.8 kgm/sec2 or 3,363.8 Newtons

Work (W) therefore would be:
W = 3,363.8 Newtons x 83.4 m
= 280,540.92 Nm or Joules

It takes about 4.2 joules to heat 1 gram of water by 1 degree Celsius.

Basically, the brakes will be transforming the forward motion into enough heat to bring 1 gram of water to 66,976.46 degrees Celsius!

Imagine how the brake disc will look after this one.

Note: the calculation above were taken with a very loose level of precision and, mostly, calculated only to 2 decimal places.

I'm a nerd!

Whoa Boylit, awesome mathematical theory you have......you said it rigth bro. :D

Somebody correct me if i'm wrong ....... but.... ABS might help in avoiding flipping over when the front brake is accidentally pressed excesively. Because yes the bike might flip over if the front brake is suddenly pressed hard enough to stop the front wheel at high speeds. I don't know about cruisers but sport bikes might because of the rider's position unlike cruisers, the riders are leaned backwards.

I usually and almost always use both my brakes when slowing down or stopping, trying and sensing the brakes are applied evenly as possible.

Originally posted by Wolver888
Whoa Boylit, awesome mathematical theory you have......you said it rigth bro. :D

Somebody correct me if i'm wrong ....... but.... ABS might help in avoiding flipping over when the front brake is accidentally pressed excesively. Because yes the bike might flip over if the front brake is suddenly pressed hard enough to stop the front wheel at high speeds. I don't know about cruisers but sport bikes might because of the rider's position unlike cruisers, the riders are leaned backwards.

I usually and almost always use both my brakes when slowing down or stopping, trying and sensing the brakes are applied evenly as possible.

ABS is designed to prevent your tire from losing grip or breaking away in a panic stop. Basically, this means that when the ABS (electronic) detects that your wheel is decelerating at a higher rate than the preset threshold it will release the brake momentarily (the pumping action) to prevent a wheel lock.

Thing is, if your front wheel stops moving at lower than the preset limit and your forward momentum remains the same, you're still going to lift the rear end.

I was in a situation, just the other day, where I had to do a panic stop from 80 kph to a standstill with a backride who was also leaning forward. I was using both brakes. My fork took a dive. My backride slammed forward forcing me forward too. I heard the rear wheel skid (I figure from the sudden shift in weight to the front and the rear lifting up just a tad.) but only when I was near standstill already. I can safely tell you that there is definitely a reason why there are 2 large discs up front and a smaller one at the rear. All our weight was practically on that front wheel!

The funny thing was that during the loooooonnnnnngggggg seconds it took to stop (ever had that experience where seconds seem to take hours and everything seems to be in slow motion). My mind was going through every physics I took. Things like pumping the brakes, pushing back with my body when the rear started squealing and screaming "PUT*** IN* MO!" came so naturally.

I'm a nerd!

Boylit,
.... yeah... a sensible one.

Thanks for the theoretical input.
It makes sense why some riders have the guts to take on a full stoppie without being flipped over.

I have always feared applying too much pressure will flip me over.
Gives me more confidence to use more of my front brakes.
Thanks a lot.

Originally posted by siRei
Boylit,
.... yeah... a sensible one.

Thanks for the theoretical input.
It makes sense why some riders have the guts to take on a full stoppie without being flipped over.

I have always feared applying too much pressure will flip me over.
Gives me more confidence to use more of my front brakes.
Thanks a lot.

Just be wary that the front end can still break away. Losing grip in the rear during braking can be much easier to control than a fully loaded front end.

And i've seen some motorcycles too that has two disks and two braking mechanism on the front, both left and right ....... probably for the same reason as for the larger front disks.

Originally posted by boylit

Thing is, if your front wheel stops moving at lower than the preset limit and your forward momentum remains the same, you're still going to lift the rear end.

ABS equiped Motos like Beemers, Interceptor, FJR1300 have linked brakes. The hand brake lever actuates both Front and Rear to prevent the lift. The rear break pedals only actuates the rear.