Another Question about the Brake System

[SupraOfDoom]

They say your front brakes are 70% of your stopping power, I was wondering how is this determened? The front and rear brakes seem pretty identical? Is this more controled by the computer or what? Do the Calipers squeeze less in the rear? Is there less contact area for the pads in the rear?

Basicly why I ask, is because I was thinking about buying a system designed to fit the front, and put the same system thats going on the front, in the rear. Obviously I don't wan't 50/50 braking power, if the fronts were designed to have more stopping power. Any thoughts?

 

[cartel1_950]

don't know how much help this would be but on mx bikes they run a larger rotor up front cause it has more stoping power but i don't think there would be a problem mounting front brakes on the back as long as they mount up okay. i'm just stating what i know or have learned no idea how true any of it is. can anyone confirm this?

 

[jetjock]

Hello from JFK airport....

Look on the left fender under the master cylinder. See that little gadget the brake lines go into? It's called a proportioning valve.

 

[DaSuprawolf]

one reason is when you break, all the goes to the front and if your rear breaks were like the front or close, they'd just lock up, it'd be a fun for drifting but not really. the rears are useally preaty weak compared to the front, like you said 70/30. yeah, its really all they can do.

 

[IJ.]

Look at the piston area in the Callipers for the full story SoD

 

[SupraOfDoom]

Look at the piston area in the Callipers for the full story SoD

I'd imagine the pistons in the rear are much smaller? ( I will look when I have time ).

Hello from JFK airport....

Look on the left fender under the master cylinder. See that little gadget the brake lines go into? It's called a proportioning valve.

So, one question still remains if there is this Proportioning valve, will running the exact same brakes front and rear cause problems? If the piston in the rear is indeed different, will having the same calipers front and rear screw with the balance?

I will take my brake system apart and look at it more carefully soon, but I'd like to know anyway lol. Thanks

 

[IJ.]

Yes there's a non adjustable Prop valve under the booster on the firewall.

Running same size callipers f/r will cause issues with the stock valve in place you'd need to replace it with an adjustable one, I ran this setup on my Datsun with no issues.

When I specced my BBK I got AP Racing callipers that keep me within the stock valves operating range and it works perfectly.

 

[Finnon]

if the piston and calliper was lager then theres a chance they would lock up on the rears i should imagine, usually rear pads are maybe half the size of front pads?

 

[cjsupra90]

yes, typically (but not always the case) on a front engine/rear drive setup, the brake bias is set to around 70% front, 30% rear. The diameter of the rotors, size of the brake pads, total piston area of the calipers, master cylinder (on some cars) and proportioning valve all play a roll in braking bias.

On some newer cars with ABS and traction control, the brake bias in constantly changing to offer the best braking.

If you were to put the same system on the rear as the front, you would not end up with a 50/50 system because of the proportioning valve, but it still would be close and not a good Idea. This can be corrected for to some extent in a number of different ways, but one reason that makes it no practical is that you would loose the parking brake all together.

Another note about bias, is that in some cases more rear bias can be a very good thing, but take a lot of testing to get it correct. A number of factors can have a positive effect on braking performance by allowing more rear bias and more overall braking ability. Things like wider/ stickier rear tires offer more grip and can allow for a small increase in rear bias. An other thing for example is lowering the car, therefor lowering the center of gravity. This cause less dynamic weight transfer which therefor cause the rear wheel to stay vertically loaded and offer more traction which then offers another small increase in rear bias. An example of this is a factory stock Mazda RX-7, both the FC (2nd gen) which has a 65/45 bias and the FD (3rd gen) which has a 60/40 bias. This is because both of these cars have a very low CG (center of Gravity) and therefor very little dynamic weight transfer.

 

[cjsupra90]

being that you and Ian have posted a few times while my slow ass was typing the last post, The stock front pistons are 60mm and the stock rears are 38mm.

 

[Nick M]

The proportioning valve sends brake fluid to the rear first to prevent lockup. The metering valve controls how much goes to the front. Most cars today now have a combination valve, that does both. I would have to look in the repair manual to see what we have.

How much is in the front vs rear comes down to the type of suspension and drivetrain. Some say as much as 85% in the front on a front wheel drive with MacPherson struts.

When you brake, the weight shifts to the front. It is an unavoidable law of physics. Just like roll when cornering. So the brakes are set up to accomodate .

 

[SupraOfDoom]

Yes there's a non adjustable Prop valve under the booster on the firewall.

Running same size callipers f/r will cause issues with the stock valve in place you'd need to replace it with an adjustable one, I ran this setup on my Datsun with no issues.

So basicly, I could do this with an adjustable valve?

 

[cjsupra90]

a proportioning valve does not send fluid or pressure to the rear first on a 4wheel disc setup. it is only combination/metering valve used on front disc rear drum setups that do this and it is not to prevent lockup, it is to allow for a small amount of pressure to overcome the return spring tension in the rear drums. Also the proportioning ramp or curve is far different when used on drum brakes because of the servo action or self energizing effect of drum brakes.

 

[cjsupra90]

So basicly, I could do this with an adjustable valve?

Yes you can do it with an adjustable proportioning valve, but it may not be 100% effective. Also like I stated earlier, you are going to loose the use of the parking brake. Another thing that will help is choosing a lower coefficient of friction or CF # for the rear pads.

Just out of couriosity, do you have any specs for the kit that you are looking a getting or a link to a site that has info on the particular kit of interest?

 

[bluemax]

Brake bias is more than how big the brake cylinders are. Brake bias is determined by dynamics of the car then the brake system is proportioned to match to maximize brake performance. At least thats the way is should be done. When you brake the weight of the car shifts forward (assuming you are traveling forward). Available brakingl forces on the front tires are increased because the front tires now have more weight (thus more traction) on them. So the f/r brake bias can be increased to the front because the front tires can brake harder than the rear before the front and rear tires loose traction.

Lowering the car and the suspension setup will affect the bias.

 

[SupraOfDoom]

Yes you can do it with an adjustable proportioning valve, but it may not be 100% effective. Also like I stated earlier, you are going to loose the use of the parking brake. Another thing that will help is choosing a lower coefficient of friction or CF # for the rear pads.

Just out of couriosity, do you have any specs for the kit that you are looking a getting or a link to a site that has info on the particular kit of interest?

http://www.d2racingsports.com.au/gpage.html
They only make it for the front, and for the MKIV. But I'm willing to do some work, not sure if its a good idea to convert to the rear or not though.

 

[IJ.]

SoD: Not being a smartarse or anything but the system you're thinking of will be a disaster to tune and I don't think you'll save such a missmatch with an adjustable valve.

 

[bluemax]

Upgrading your brakes until you can lock them both up is the limit unless you use them constantly. Street cars, unless you live in the mountains or are Ricky Racer or are towing, don't have a heat problem because you are not constantly using them. Getting the coolest looking 8 piston caliper doesn't do anything for you but use your money (and I guess looks cool). The best bang for your buck would be to get performance pads all around and a set of super wide high performance tires. Upgrading the front drastically and not the rear is reducing braking performance because the front will be doing all the work and you're not getting anything from the rear brakes.

 

[SupraOfDoom]

SoD: Not being a smartarse or anything but the system you're thinking of will be a disaster to tune and I don't think you'll save such a missmatch with an adjustable valve.

Which is why I ask questions first .

Thanks all.

 

[mkiiSupraMan18]

I just read through this...

Not having looked into an upgrade brake kit, I always just figured from previous experiences that the front brakes did most of the stopping power because they were out front and actually stopping the forward movement rather than just being drug along like the rears. I learned the 70/30 rule the hard way back when I was learning to ride dirtbikes... a great lesson for all kids to learn, the rear brakes are OK to lock up, just don't do it in the front. :icon_bigg

Good info from all though.