Well, in general, most of the braking is done by the front.
In this case, I would look at the calipers. I'd be willing to bey, with a smaller disk area that the fronts are dual piston calipers, while the rears are single piston.
The distribution of braking force from front to rear is more dependent on the weight distribution of the vehicle itself, suspension, vehicle speed, and design of the master cylinder than whether the rear brakes are disc or drum.
Brakes stop vehicles by changing kinetic energy to heat energy, and the faster the heat is dissipated, the more efficiently it can change more kinetic energy to heat energy. Since disc brakes are exposed to more air than drum brakes, they can get rid of the heat more efficiently.
Factors that affect brake rotor or drum diameter are wheel diameter, unsprung weight, vehicle weight, rotor and friction material composition, and standard designs from rotor manufacturers.
I suspect that the rotors on the Scion may appear smaller because the wheel has a larger diameter and so there is more space between the outside of the rotor and the inside of the wheel rim.
Oops, forgot to mention swept area as one of the biggest factors in rotor design. It is possible for a smaller diameter rotor to have a larger swept area than a larger diameter rotor.
Brake lining wear is more of a factor of lining material, brake rotor or drum hardness, weight, speed, etc. so it is not really possible to make a general statement that one type of brake will wear more quickly than another.
Designers face several challenges when designing brake systems, particuclarly with vehicles designed to carry heavier payloads. The problem with engaging rear brakes before front brakes is avoiding premature rear wheel lockup if the back end of the vehicle is lightly loaded.
My impression has always been that drum brakes in the rear
Rear brakes last longer than front brakes because the front brakes do most of the stopping. The heavier the vehicle, the more weight is shifted to the front of the vehicle when stopping, and the harder the stop, more weight is shifted to the front.
Braking force on any system is distributed about 60/40 front to rear. This explains why one goes through front brakes at a roughly 2:1 ratio over the back brakes.
Dang! What would be the size of the pads?!?!
If you picture a brake rotor as a top hat, with the brim being the swept area and the rotor's hub being the crown of the hat, a smaller hub would allow a larger brim, which would in turn allow larger pads.
If you want to see small rear brake pads, take a look at the rear pads on a Honda. The first time I did the rear brakes on my neighbor's Accord, I thought she bought Honda motorcycle pads when I opened the box.
Given that the pad area is the same, the bigger rotor has a leverage advantage over the smaller rotor. It's the same principle as having a longer handle on a wrench. The larger rotor also has the advantage of dissipating the heat over a longer area of the rotor. As you have mentioned, the more heat it dissipates, the more kinetic energy it can convert to heat.
It is more like 70-75%. This is true for most of the vehicles on the road. The only exceptions are rear and mid-engines. Since these vehicles are mostly high-performance cars (MR2, 911, NSX, 348's), they have stiff suspensions that reduce weight transfer to the front. These cars obviously use more of the rear brakes.
My 86 Toyota C&C work trucks normal weight with the parts, tools and equipment I need is close to 6,000 pounds. Plus I would not think twice about tossing 2 scoops of sand (a ton) anywhere in the bed. I doubt the sand even be noticeable when braking. That trucks rear axle does basically all the load and braking and the front none.
My point is braking force front/rear varies greatly.
My Supra's front pads, rear pads, front rotors, rear rotors all wear roughly 1:1 thank god and have no weak link.
The brakes also perform the same at any speed the car can do. But if you stomp the brake /lock up all 4 at 120 mph anyone not belted in is going to eat the dash hard
Dan
Funny you say that. Honda motorcycles use Nissin brand brakes. Their cars do too. Hmmm...
By the way, I would also like to add that the caliper is a big factor in breaking strength. The opposite force of the piston pushing against the pad is the force pushing the caliper away from the rotor. The less the caliper flexes, the more efficient the braking force.
I can't remember what car I was working on, it may have been the Supra, and I was STUNNED at how small the rear pads were!
IIRC, the Supra's rear pads are not that small. The Tercel's rear pads are pretty small but not as small as on a Honda.
Distributing 90+% of the load on the rear axle doesn't change things very much. The front brakes still do the work of STOPPING. There are hardware components that help to balance the braking duties from front to rear, but they are "stupid" devices that don't know how you have distributed the load.
Bottom line is, it's pretty safe to say that braking is distributed 60% to the front and 40% to the rear. There are notable exceptions -- the mid-engine cars and highly refined sports cars, that sort of thing -- but most passenger cars will be pretty close to the 60/40 distribution. The reason is the kenetic energy of the car itself. As you stop, the weight is thrown forward onto the front axle.
I do not recall that the OP listed his car among the notable exceptions.
equipment I need is close to 6,000 pounds. Plus I would not
doubt the sand even be noticeable when braking. That
back brakes.
1:1 thank god and have no weak link.stomp the brake /lock up all 4 at 120 mph
much. The front brakes still do the work of STOPPING.
front to rear, but they are "stupid" devices that don't
front and 40% to the rear. There are notable
thing -- but most passenger cars will be pretty
itself. As you stop, the weight is thrown forward
LOL I know we are off topic. anyway... I guess my Toyota C&C truck has huge brakes and 4 tires rear and wimpy front brakes for no reason then. 8)
I don't understand why the 'stiffness of the suspension' would have anything to do with 'braking distribution'...isn't that governed by the 'arm' of the lever effect? (how far above the axles the vehicle's sprung weight is)...seems to me that the
60/40 that I see here feels about right for any average sized automobile...no?.
On cars with 4-wheel discs, the front rotors are of larger diameter than the rear rotors for that reason (and are often vented as well).
Since the stopping capacity of the front brakes meet or exceed what's needed at the rear wheels, why not use the same rotors and calipers at all four wheels and make any necessary adjustments with a brake proportioning valve? Expenses could be lowered by having to procure, transport and stock fewer components.
Geoff
The reduced expenses by the factors you mentioned are probably not enough to offset the expense of having to install larger rotors, calipers, and pads. Also, Toyota uses a drum brake incorporated into the rear disc brake for a parking brake.
I don't understand why the 'stiffness of the suspension' would have anything to do with 'braking distribution'...isn't that governed by the 'arm' of the lever effect? (how far above the axles the vehicle's sprung weight is)...seems to me that the
60/40 that I see here feels about right for any average sized automobile...no?.
Under hard braking, inertia transfers weight to the front of the vehicle. When weight is transferred to the front of the vehicle, the springs at the front of the vehicle are compressed and the springs at the rear of the vehicle are stretched, compounding the effect of the weight transfer from the inertia. If the springs are stiffer, there is less compounding of weight transfer to the front of the vehicle.
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