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7 Series - E38 (1995 - 2001)

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  #1  
Old 08-30-2013, 04:02 PM
Showpony01 Showpony01 is offline
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Mein Auto: BMW 740il E38 1995
BMW E38 740il BREMBO BRAKES

Hi all I have aN E38 740il

When I purchased this car it had BREMBO 4 Piston calipers with slotted rotors all round although compared to my wife's E38 they have never really been that much better, so I had a look and the rear pads are worn, will this give me a spongy feel when applying??? if I hit them again quickly they feel better.

As I need to change the pads can any one tell me do I need to by special pads to fit these calipers or do the normal pads for normal calipers fit????

I have seen some pads on the net but they are so expensive.

Thanks
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  #2  
Old 08-30-2013, 05:09 PM
will williams will williams is offline
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Dont know the answer to your question because it will depend on which model is on your car. But if you want to sell them let me know.
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  #3  
Old 08-31-2013, 03:20 AM
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BMWFatherFigure BMWFatherFigure is offline
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Worn pads do not = spongy pedal. Air in the system or wrong pad material will.
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  #4  
Old 09-01-2013, 12:50 PM
7 series guy 7 series guy is offline
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When some people upgrade to a larger capacity brake system (more pistons at the caliper) companies fail to take the Master Cylinder in to account to sometimes and may also want to upgrade to braided stainless brake lines.
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  #5  
Old 09-01-2013, 12:59 PM
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ebida3 ebida3 is online now
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Mein Auto: 2001 740iL
Bleed the brakes.
Also apply slight pressure on the pedal while idling and see if the pedal slowly sinks. If it does then the master cylinder is bad. If it holds it's fine.
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  #6  
Old 09-01-2013, 04:43 PM
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supertech777 supertech777 is offline
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Re: BMW E38 740il BREMBO BRAKES

Quote:
Originally Posted by BMWFatherFigure View Post
Worn pads do not = spongy pedal. Air in the system or wrong pad material will.
+1

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  #7  
Old 09-01-2013, 10:15 PM
Showpony01 Showpony01 is offline
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Thanks Guys it seems I have 4 piston brembos on the front with standard calipers with brembo stickers on the rear gave them a bleed today will take for a drive to work tomorrow to see but very tiny amount of air came out we will see????
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  #8  
Old 09-02-2013, 04:58 AM
chrisn7 chrisn7 is offline
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Quote:
Originally Posted by 7 series guy View Post
When some people upgrade to a larger capacity brake system (more pistons at the caliper) companies fail to take the Master Cylinder in to account to sometimes and may also want to upgrade to braided stainless brake lines.
Yeah, I was wondering about this too. If more pistons at the caliper results in a given brake pedal force being applied over a larger area, wouldn't this result in a lower force per unit area being applied to the disc and hence reduced braking performance and therefore higher pedal pressure than before to stop the car?
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  #9  
Old 09-05-2013, 01:12 AM
Podmore Podmore is offline
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Quote:
Originally Posted by chrisn7 View Post
Yeah, I was wondering about this too. If more pistons at the caliper results in a given brake pedal force being applied over a larger area, wouldn't this result in a lower force per unit area being applied to the disc and hence reduced braking performance and therefore higher pedal pressure than before to stop the car?
Nope, you have it bass ackwards. The master cylinder will exert a given brake fluid pressure through the system; let's call it 100 psi. This means if you have a brake caliper piston with a surface area of 10 in.sq the brake fluid will apply a force of 1000 lb to the brake pad. So if you have a four-piston caliper with each piston having a surface area of 10 sq.in. the total braking force applied will be 4000lb. The larger the caliper piston, the lower the force which needs to be applied at the pedal for a given master cylinder diameter.

Problems may occur when front calipers are upgraded from OEM but rears are not; the brake force may not be properly apportioned between front and rear.

Spongy feel is usually caused by air in the lines; if a quick pump of the pedal resolves the squishy feel (and the pedal does not drop away under the foot when pressure is maintained) then it is almost certainly air in the lines. If the brake fluid has not been replaced for some years, it can absorb enough moisture that under hard and sustained braking, the water can boil off, leaving air in the system.

Suggest you do a complete brake fluid replacement - if the fluid appears cloudy or discoloured, it should definitely be replaced as it is contaminated either by rust coming back up from the calipers, or by moisture (or both).

Last edited by Podmore; 09-05-2013 at 01:13 AM.
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  #10  
Old 09-05-2013, 06:17 AM
will williams will williams is offline
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Well said Podmore.
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  #11  
Old 09-05-2013, 04:08 PM
chrisn7 chrisn7 is offline
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Your thinkng is flawed. If you have 100 psi from the master cylinder applied over 1 square inch, then that's clearly 100 lbf per sq inch. Raise the area to 4 square inches then that becomes 25 ibf per square inch. Think of it like this: ask your wife to stand on your foot with her stilettos on - it hurts. Now get her to do it with flat heels on - hurts a lot less. Why, because she still weighs the same but is applying that force over a larger area.

In your example you would have to raise the pedal pressure from 100 psi to 400 psi to achieve the same pressure over a piston area 4 times larger.

More contact surface area may help, but I guess you would have to change the master cylinder and /or servo as well to get that higher pressure
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  #12  
Old 09-05-2013, 07:23 PM
Podmore Podmore is offline
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Quote:
Originally Posted by chrisn7 View Post
Your thinkng is flawed. If you have 100 psi from the master cylinder applied over 1 square inch, then that's clearly 100 lbf per sq inch. Raise the area to 4 square inches then that becomes 25 ibf per square inch. Think of it like this: ask your wife to stand on your foot with her stilettos on - it hurts. Now get her to do it with flat heels on - hurts a lot less. Why, because she still weighs the same but is applying that force over a larger area.

In your example you would have to raise the pedal pressure from 100 psi to 400 psi to achieve the same pressure over a piston area 4 times larger.

More contact surface area may help, but I guess you would have to change the master cylinder and /or servo as well to get that higher pressure
OK, I was not sufficently clear (or accurate in terminology) in my original explanation. You appear to be confusing pressure with weight, and maybe don't understand the concept of 'work'. 'Work' is force times the distance moved (heard of 'newtons' as a measure of force? You can look that up in Wikipedia).

If you apply 400 lb of WEIGHT over 4 square inches, that will deliver 100 lb per square inch of FORCE. Deliver 400 lb per square inch FORCE over 2 square inches, will still give you 400 lb per square inch of FORCE but it is multiplied by the area over which it is applied. Deliver 400 lb per square inch over 4 square inches will give you 1600 lb of 'work potential' - 'work' being force x distance. So a smaller master cylinder diameter, for a given force applied to the brake pedal, will give you higher pressure in the brake system. The challenge is that the master cylinder piston also needs to be able to move a certain volume of fluid through the lines to make the caliper pistons move over a sufficient distance to make the pads contact the brake disc, so it can't afford to be too small, or the distance it has to travel will be too large. The principle of mechanical leverage as a force multiplier is already at work between the brake pedal and the master cylinder, and I won't go into that further here.

So your wife weighs 100 lb. She is applying that weight over the surface area of her stilletto, say 1/10 sq.in. So the FORCE she is applying to your foot is 1000 lb/sq.in. That's the pressure in the master cylinder and brake lines. If your foot does not move, her weight is doing no work (apart from hurting like hell). If her stilletto digs into your foot a distance of 1/2", she is 'doing work' on your foot equivalent to 500 in.lb. If she applies her weight with her flatties on, she won't sink in nearly as far - she is doing less 'work' because her weight, which is still 100lb, is applied over the larger surface area of her flatties.

If brake force is applied over a 4 sq.in piston, there is no work done until the piston moves. Say the the piston moves 1/4", then the WORK done by that 4 sq.in. piston, exposed to 1000 lb per square inch of force, is 4 X 1000 X 1/4" = 1000 in.lb of work; in this case, that is the braking effort applied or the 'work done' by the braking system.

The whole purpose of a hydraulic braking system is to enable force multiplication through the differential between master and caliper piston sizes. You've seen the size of a caliper piston, I assume; if things worked the way you think, the master cylinder piston would need to be several times larger than the combined surface area of all the pistons in the brake calipers to be able to achieve the mechanical advantage you would need to ensure you didn't have to grow a thigh like Arnie Schwarzenegger to be able to stop your car....

Hope that helps.

Last edited by Podmore; 09-05-2013 at 10:16 PM.
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