Brake system rubber bits - replace?
07-02-2006, 05:52 AM
#1
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Brake system rubber bits - replace?
I'm going to be replacing my pads and rotors soon, so I'm thinking about what other bits of the brake system should be replaced proactively.
I don't believe the brake fluid has been replaced, certainly not recently, so that's a definite. Since I'm doing that, I figure I should do the rubber brake hoses as well.
Any opinions on whether I should rebuild the calipers? Was a time I would have said yes, but I've been reading things that say as long as the pistons aren't visibly scored or pitted, not to bother.
This is a 99 A8 with the single-piston calipers and 17" wheels. If I decide that I want to, is there any reason I can't buy the S8 calipers and rotors and drop them right in? As long as I did it on all four wheels, wouldn't the force proportions still be equivalent to the stock system -- so I wouldn't have to change a proportioning valve or anything? It's not like I need the extra heat-dissipation -- I don't track this car, and I don't even drive it all that hard -- but a few extra feet of stopping distance could make a big difference under an unfortunate circumstance.
I don't believe the brake fluid has been replaced, certainly not recently, so that's a definite. Since I'm doing that, I figure I should do the rubber brake hoses as well.
Any opinions on whether I should rebuild the calipers? Was a time I would have said yes, but I've been reading things that say as long as the pistons aren't visibly scored or pitted, not to bother.
This is a 99 A8 with the single-piston calipers and 17" wheels. If I decide that I want to, is there any reason I can't buy the S8 calipers and rotors and drop them right in? As long as I did it on all four wheels, wouldn't the force proportions still be equivalent to the stock system -- so I wouldn't have to change a proportioning valve or anything? It's not like I need the extra heat-dissipation -- I don't track this car, and I don't even drive it all that hard -- but a few extra feet of stopping distance could make a big difference under an unfortunate circumstance.
07-03-2006, 04:47 AM
#3
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I would change the pads, bleed the fluid, and leave the rest
If you try replacing the rest of the items, make sure you have a contingency plan in place, it might not be fun if something goes wrong.
The pieces should be just fine if you are not racing your car on the track every day.
The pieces should be just fine if you are not racing your car on the track every day.
07-03-2006, 05:50 AM
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Here's the problem trying to put S8 front calibers and rotors on an A8. ...
The S8's Brembo is a radial mount caliper, the A8 uses an axial mount caliper. See <a href="https://forums.audiworld.com/a8/msgs/88966.phtml">An Expensive Project</a>.
Also, if you can activate the ABS system with your current brakes, tire grip is the limiting factor, and you won't shorten stopping distances with an upgrade to S8 brakes. You will gain some additional resistance to fade from repeated stops (such as on track), but upgrading tires is the best thing you can do to shorten stopping distances, and the tires will have a huge effect.
Also, if you can activate the ABS system with your current brakes, tire grip is the limiting factor, and you won't shorten stopping distances with an upgrade to S8 brakes. You will gain some additional resistance to fade from repeated stops (such as on track), but upgrading tires is the best thing you can do to shorten stopping distances, and the tires will have a huge effect.
07-03-2006, 05:39 PM
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Custom rotors will also be needed.
Almost certainly, the centerline of the caliper mounted on an adapter will not match the centerline of the OE S8 rotor. As a result, a 2-piece rotor with a custom hat will be needed.
The adapter also determines the radial location of the rotor, which is extremely critical. In the worst case, the caliper can be destroyed when the rotor gets hot, expands, and grinds into the bridge of the caliper.<ul><li><a href="http://www.prosystembrakes.com/technical_info.htm">Brembo Tech Stuff</a></li></ul>
The adapter also determines the radial location of the rotor, which is extremely critical. In the worst case, the caliper can be destroyed when the rotor gets hot, expands, and grinds into the bridge of the caliper.<ul><li><a href="http://www.prosystembrakes.com/technical_info.htm">Brembo Tech Stuff</a></li></ul>
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07-03-2006, 09:52 PM
#8
"if you can activate the ABS ... tire grip is the limiting factor". But does EBD feel like ABS?
I agree in principle with your quote, and it is undeniably true in cars without electronic front-to-rear brakeforce distribtion (EBD).
But does EBD feel like ABS? Is EBD really nothing more than using ABS to rein in deliberately excessive rearward brake bias? The manual says that the rear wheels may lock prematurely if the electronics are off line.
Because if EBD feels like ABS, its activation might not mean the tires are the limiting factor. It could just mean that the EBD is compensating for the weight transfer of hard braking -- something that happens with the grippiest tires, and which a pressure limiter valve attached to the rear axle would have handled on an old Golf.
Tom
But does EBD feel like ABS? Is EBD really nothing more than using ABS to rein in deliberately excessive rearward brake bias? The manual says that the rear wheels may lock prematurely if the electronics are off line.
Because if EBD feels like ABS, its activation might not mean the tires are the limiting factor. It could just mean that the EBD is compensating for the weight transfer of hard braking -- something that happens with the grippiest tires, and which a pressure limiter valve attached to the rear axle would have handled on an old Golf.
Tom
07-04-2006, 07:20 AM
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I don't know what EBD is really supposed to do. ...
The model guide says our A8/S8's have <i>"electronic rear brake pressure proportioning"</i>, but that's no explanation. Without ABS, the maximum possible braking g's (the shortest stopping distances) depend on an optimum front-rear brake bias, which is a function of ...
1. Weight of the car
2. Static front-to-rear load distribution
3. Tire coefficient of friction (which varies with load)
4. Wheelbase
5. Height of the center of gravity
I calculated <a href="https://forums.audiworld.com/a8/msgs/36145.phtml">Here</a> that the optimum brake bias is 2.23 for an S8. When you add weight to our car, either something in the trunk or passengers, you change both the total weight and the F-R load distribution, which affects the optimum brake bias and max braking g's. [Later I compared the <a href="https://forums.audiworld.com/brakes/msgs/148.phtml">Braking g's</a> of various cars.]
So what could EBD do? ... Perhaps the electronics are clever enough to use the Xenon headlight load level sensors to adjust brake bias (in addition to adjusting the Xenon's aiming height). In principle, stopping distances could be marginally shorter with EBD, even with ABS. If the car had too much front brake bias, for example, in the very few msec's between the time that you stab the brake pedal and the front brakes go into ABS, the rear brakes could have been doing a bit more work. This certainly could account for a few feet of stopping distance from high speeds.
Note that this doesn't correct for the total weight of the car, and it certainly doesn't correct for grippier tires or reduced traction in rain. In the end, the car needs to rely primarily on ABS. I do fine tuning of brake bias by using different pad compounds in my front and rear brakes. Even on track, ABS is normally activated in the front before the rear. I've sometimes overheated just my rear tires, though, and with their reduced grip, ABS then activates in the rear first. It's surprisingly easy to feel the difference, the car gets slightly squirrelly when rear wheel ABS activates first.
1. Weight of the car
2. Static front-to-rear load distribution
3. Tire coefficient of friction (which varies with load)
4. Wheelbase
5. Height of the center of gravity
I calculated <a href="https://forums.audiworld.com/a8/msgs/36145.phtml">Here</a> that the optimum brake bias is 2.23 for an S8. When you add weight to our car, either something in the trunk or passengers, you change both the total weight and the F-R load distribution, which affects the optimum brake bias and max braking g's. [Later I compared the <a href="https://forums.audiworld.com/brakes/msgs/148.phtml">Braking g's</a> of various cars.]
So what could EBD do? ... Perhaps the electronics are clever enough to use the Xenon headlight load level sensors to adjust brake bias (in addition to adjusting the Xenon's aiming height). In principle, stopping distances could be marginally shorter with EBD, even with ABS. If the car had too much front brake bias, for example, in the very few msec's between the time that you stab the brake pedal and the front brakes go into ABS, the rear brakes could have been doing a bit more work. This certainly could account for a few feet of stopping distance from high speeds.
Note that this doesn't correct for the total weight of the car, and it certainly doesn't correct for grippier tires or reduced traction in rain. In the end, the car needs to rely primarily on ABS. I do fine tuning of brake bias by using different pad compounds in my front and rear brakes. Even on track, ABS is normally activated in the front before the rear. I've sometimes overheated just my rear tires, though, and with their reduced grip, ABS then activates in the rear first. It's surprisingly easy to feel the difference, the car gets slightly squirrelly when rear wheel ABS activates first.
07-04-2006, 10:50 PM
#10
If you can activate ABS in front first, then tires must be the weak link in that case
I wasn't sure whether what I felt when braking hard in a straight line was genuine ABS or just EBD, so I asked. With stock pads, it could still be EBD, right?
Still don't know whether EBD is anything more than ABS with a rearward brake bias. I don't think it uses the headlamp leveling sensors because some cars don't have them, and they'd be of little use on A8s with the self leveling rear suspension.
Speaking of self leveling and brake balance, the picture shows the DS/SM Citroen arrangement for balancing brakes using the hydraulic pressure in the suspension cylinders which hold up the car. (These cars have fully powered brakes. There is no master cylinder, but instead a pair of spool valves behind the mushroom shaped pedal which sends stored pressure to the brakes.) There are several clever bits: (1) weight on the rear wheels dynamically adjusts the fulcrum of the balance bar; (2) maximum pressure available for the rear wheels is that in the rear suspension cylinders -- directly proportional to the weight on the back wheels; (3) using the rear suspension as a pressure source for the rear brakes keeps them working a while if main pressure fails (a check valve and dedicated accumulator do the same for the front brakes); and (4) although the front and rear spool valves (and pedal) barely move, they are made force-linear by ducting pressure destined for the calipers to the backsides of the spool valves (the helical springs behind the spool valves are very weak).
The results are instantaneous (no vacu-mush), undemanding of needless movement, light and force-linear, like all car controls should be for those who drive with any finesse. I wish I had something so good now, but am not willing to give up eight airbags and the structure which makes them relevant to get it.
Tom
<img src="http://www.tuling.com/DSBrakeValve.gif">
Still don't know whether EBD is anything more than ABS with a rearward brake bias. I don't think it uses the headlamp leveling sensors because some cars don't have them, and they'd be of little use on A8s with the self leveling rear suspension.
Speaking of self leveling and brake balance, the picture shows the DS/SM Citroen arrangement for balancing brakes using the hydraulic pressure in the suspension cylinders which hold up the car. (These cars have fully powered brakes. There is no master cylinder, but instead a pair of spool valves behind the mushroom shaped pedal which sends stored pressure to the brakes.) There are several clever bits: (1) weight on the rear wheels dynamically adjusts the fulcrum of the balance bar; (2) maximum pressure available for the rear wheels is that in the rear suspension cylinders -- directly proportional to the weight on the back wheels; (3) using the rear suspension as a pressure source for the rear brakes keeps them working a while if main pressure fails (a check valve and dedicated accumulator do the same for the front brakes); and (4) although the front and rear spool valves (and pedal) barely move, they are made force-linear by ducting pressure destined for the calipers to the backsides of the spool valves (the helical springs behind the spool valves are very weak).
The results are instantaneous (no vacu-mush), undemanding of needless movement, light and force-linear, like all car controls should be for those who drive with any finesse. I wish I had something so good now, but am not willing to give up eight airbags and the structure which makes them relevant to get it.
Tom
<img src="http://www.tuling.com/DSBrakeValve.gif">