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R31 vs 280zx rear brake calipers


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#21 MaygZ

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Posted 22 March 2012 - 04:08 AM

Pad area is the next consideration. When the pistons clamp the rotor, the same force is transmitted to the rotor regardless of pad area...it is just spread over a greater area (to change the force, you need to change the piston diameter, NOT the pad area). Consequently, provided the pad friction coefficient is the same, different size pads will produce similar braking effort for the same pedal force. What changes is pad heating and wear rate. Small pads concentrate heat more and hence get hotter, and will wear faster. The temperature change also affects their coefficient of friction (depending on the compound, it usually goes up as the pads heat up, and at some temperature they go off, lose their C of F until they overheat, disintegrate and fail.


Jamo, I need some help to understand this.  :o  My manbrain tells me that braking force will be determined by the amount of friction created at the disc surface. I would suggest - and I'm not an engineer or a physisist - that size of pad would be a significant contributor to the friction equation.  Obviously keeping pressure and pad material constant then pad area would make a massive difference.  I take your point that if you can increase pressure it will improve braking force, but I can't get my head around that different pad size will not affect braking force.

A little experiment that I did as I was trying to work this one out was:

I placed the pad of my middle finger lightly on my desk and dragged it towards me.  I then placed my whole hand on the desk, with as close to the same force (arm effort) and dragged that towards me.  Obviously this is a huge increase in 'pad' size, but the friction increase (resistance to drag) was substantial.

Please help me to understand.  I promise you that I am not taking the pi$$, I really do want to understand.

To the rest of you - stop dragging your hands across your desks and get on with your work!

#22 luvemfast

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Posted 22 March 2012 - 07:47 AM

To the rest of you - stop dragging your hands across you desks and get on with your work!

Guilty  :-[

#23 RLY240

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Posted 22 March 2012 - 08:10 AM

Jamo, I need some help to understand this.  :o  My manbrain tells me that braking force will be determined by the amount of friction created at the disc surface.
Please help me to understand.  I promise you that I am not taking the pi$$, I really do want to understand.


Total clamping force is a factor of pressure on a given surface area (say pounds per square inch) muliplied by the total surface area.  Keeping the piston pressure (force) constant and increasing the pad surface area certainly increases the fricton area but there is less pressure per square inch so the overall clamping force remains the same.

The only way to increase the braking force is to increase the pressure at the piston or to make the disc diamter larger and move the caliper outwards increasing the brake torque.

So what are the benifits of a larger pad?  Better temperature control as each square inch of pad is doing less work, better pad life (same reason) and less tapering or uneven wear over the pad area.
Roger


#24 d3c0y

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Posted 22 March 2012 - 07:12 PM

Something that might help explain it a bit more for you:
So in Roger's example the thing to take into account is that it is using a constant size piston(s).

So if you have a caliper with pad of size 'A' and piston size 'B' and you have another caliper with a pad of 2 x 'A' and a piston size of 'B' you get the case that Roger has given. This is not what is happening with your hand experiment, because if you pushed as hard as you could on one finger you would break it, unlike your palm. You can however feel the difference in heat dissapation over the larger area.

So tying it together:
If you have a caliper with both a pad 2 x 'A' AND piston size 2 x 'B' you can exert the same amount of pressure over the greater area - which is like your palm on the desk as you can push harder on your hand than you can on your finger.


Funnily enough at the end of all this R31 rear calipers do have a bigger piston than the 280ZX i think so there you go. I'll check later.



#25 jamo240

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Posted 24 March 2012 - 06:56 PM

Hey MaygZ...the other lads have done a pretty good job of explaining it, but I will add the formula....

The formula for braking force (F) is given by: coeficient of friction (u) x clamping force (N). F = uN. Braking force is the tangential force of retardation that the brakes create...this can then be translated into a torque figure for the purposes of determining whether the braking torque is sufficient to stop a tyre (lock up), and then a braking power figure (by introducing rpm) if you want to determine the power the brakes are dissipating at any given speed....we won't worry about all that for now.

u is the coefficient of friction, and is a function of the surface characteristics of the material. Brake pads are commonly in the 0.3-0.5 range. Note that u has no units...it is a constant, and therefore applies regardless of area. In the context of your finger vs hand comparison, we would assume that u for skin is the same whether we are talking about your finger or your whole hand!

N is the force in Newtons applied by the pads on the brake rotor surface. In the case of brake pads, the force is applied via the piston, so if the area of the pad is larger, the force being applied to it is identical via the piston, so the force it applies to the rotor is identical, just spread out over a larger area. It is the force component to the equation that takes into account the area of the pad (it's an inverse relationship...greater area = lower force).

So, using our formula, if we assume u = 0.4, and we applied say 800kgf to the pad via the piston, this would be the equivalent of 800 x 9.81N = 7,845N.

Now F = uN, so F = 0.4 x 7,845 = 3,138N, or 318kg force in a tangential direction. To convert this to a braking torque, we can take the radial distance the force is being applied from the centre of rotation (let's assume 150mm), then the torque in Nm is 3,138 x 0.15m = 470Nm.

While there are a number of factors that interact and effect real world braking performance, hopefully this helps you understand why the pad area does not relate to braking force. It relates to power dissipation, pad heating, wear rates, tapering and stuff like that.

Cheers

Jamo




#26 TBOWGN

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Posted 24 March 2012 - 08:50 PM

Good work lads! Wish they taught me this at school ;)

How about Front brakes? Who's using the standard 240/260 brakes, how are they, and who has upgraded to different ones??

As someone who has never driven a zed, but is building one from scratch as we speak, id love to know...

Lars

#27 jamo240

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Posted 24 March 2012 - 08:59 PM

Hey Lars...front brakes are ok for road driving. If you are going on the track though, you need vented and perhaps larger brakes. I have Porsche GT3 brakes on my car, and lordy it stops.

Jamo

#28 luvemfast

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Posted 24 March 2012 - 09:01 PM

Good work lads! Wish they taught me this at school ;)

How about Front brakes? Who's using the standard 240/260 brakes, how are they, and who has upgraded to different ones??

As someone who has never driven a zed, but is building one from scratch as we speak, id love to know...

Lars

Search is your friend.
Its been done many times.  ;)

(Surely you know who this dude is?  ???)

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#29 TBOWGN

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Posted 24 March 2012 - 09:10 PM

You're right, as soon as i posted it i found the sticky on the hilux/s13 conversion...  All good.

Is that Lars the drummer?? :)

#30 jamo240

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Posted 24 March 2012 - 09:20 PM

MaygZ...in case you still can't conceptualise it, think of it this way....

Imagine you had a pad that covered the entire area of the brake rotor (like aircraft brakes do), but was still only applied to the rotor with the equivalent force of a single piston...the force applied to the pad over this large area would be very low, and hence the brake would not provide any greater braking force than a conventional pad. But, the pad wear would be extremely low due to the low application force per unit area. Now, imagine you placed an array of pistons all the way around the rotor, so a great deal more clamping force was applied to the pad...this would result in a much increased braking force, and of course increased wear...you can see that the coefficient of friction does not change, but the clamping force does, and hence the brake capacity goes up.

Aircraft brakes are configured this way...they have an annular array of pistons around the brake rotors, and run a series of linings and rotors (like a multilplate clutch), and this is how they get such high braking efforts without over-heating...by effectively reducing the power density of the system.

Jamo

#31 d3c0y

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Posted 25 March 2012 - 11:04 AM

Search is your friend.
Its been done many times.  ;)

(Surely you know who this dude is?  ???)


Lol

I think we have scared off MaygZ...

#32 MaygZ

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Posted 08 April 2012 - 04:05 PM

Sorry guys, I've been away.

Thank you for all the answers to my question.  It all makes sense to me now.  So after the big 6 pot calipers, it's bigger disc time  ::)

This explains why the 11" discs on my old 800kg Alfa pulled it up like a dream - in fact too well at Calder when the discs stoped but the drive shafts kept turning (in-board discs attached to gearbox and drive shaft ran out to the wheels)

thanks again guys




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