Strange brake fade on track - 86 951
06-20-2012, 12:52 PM
#17
Rennlist Member
Larry, that blue check valve is for the evap system, I believe. But I'm pretty sure the brake booster has a larger one crimped in-line on the big hose.
06-20-2012, 06:11 PM
#18
Rennlist Member
Can't imagine that it's the pads or at least the brand and model of pads anyway. Perhaps glazing but I've run the 01's front and 97s rear and never had this particular issue. Did find that moving to a slotted rotor helped too.
07-16-2012, 06:25 PM
#19
Pro
Thread Starter
As a follow up to this thread, the brakes seem to be fixed. All pads were replaced with PF 97s and it was bled with Motul RBF 600. But I don't think that's what solved the problem. Though we can't be completely sure, the pads that were removed seem likely to have been PF 97s, and they didn't appear to have been overheated. The new pads worked fine too. So changing the pads probably didn't change anything.
So I believe that the real problem, which I sort of dismissed at first, was that there was insufficient vacuum in the power booster during hot lapping. The check valve that prevents loss of accumulated vacuum (and infiltration of manifold pressure) into the brake booster was replaced. It's a $15 item. Afterward, during a recent track day, the hard pedal/inadequate deceleration problem was gone. Though the old check valve wasn't formally tested to see if it was leaking (and mouth pressure didn't show an obvious problem), it seems likely that it was the culprit.
As part of the diagnosis, I clamped shut the vacuum feed to the brake booster to see how the car felt with no booster. I was amazed at how much pedal force it took to stop the car, even from 25 mph. And for some odd reason, it seemed that no matter how hard I pushed, the deceleration did not improve. This is not what I expected. I've never driven another car where the absence of brake boost rendered the car nearly impossible to stop.
It's interesting that this problem acted just like hot, faded brakes. It only seemed to happen after several laps, when the car and driver were up to speed., and the brakes were hot. My theory is that when the driver was up to speed, the car was pushed down the straight at full throttle and the right foot went very quickly from the accelerator to the brake pedal at corner entry. The long straights at full boost allowed the leaking check valve at the booster maximum opportunity to dump whatever pressure was in the booster accumulator, with no chance of replenishment. Hence, it felt like brake fade. I found during testing that if I would pause for a second or two before applying the brakes after a long period of on-boost, full-throttle acceleration, the brakes would feel fine, but if I jumped from accelerator to brakes, I'd have diminished braking. This was the case even when the brakes were relatively cool. Hence, a couple seconds at closed throttle would build enough vacuum to give sufficient boost to the brakes.
So I think there may have been a small leak at the check valve that bled down under just these precise circumstances. A new check valve seems to have fixed the problem.
Rob
So I believe that the real problem, which I sort of dismissed at first, was that there was insufficient vacuum in the power booster during hot lapping. The check valve that prevents loss of accumulated vacuum (and infiltration of manifold pressure) into the brake booster was replaced. It's a $15 item. Afterward, during a recent track day, the hard pedal/inadequate deceleration problem was gone. Though the old check valve wasn't formally tested to see if it was leaking (and mouth pressure didn't show an obvious problem), it seems likely that it was the culprit.
As part of the diagnosis, I clamped shut the vacuum feed to the brake booster to see how the car felt with no booster. I was amazed at how much pedal force it took to stop the car, even from 25 mph. And for some odd reason, it seemed that no matter how hard I pushed, the deceleration did not improve. This is not what I expected. I've never driven another car where the absence of brake boost rendered the car nearly impossible to stop.
It's interesting that this problem acted just like hot, faded brakes. It only seemed to happen after several laps, when the car and driver were up to speed., and the brakes were hot. My theory is that when the driver was up to speed, the car was pushed down the straight at full throttle and the right foot went very quickly from the accelerator to the brake pedal at corner entry. The long straights at full boost allowed the leaking check valve at the booster maximum opportunity to dump whatever pressure was in the booster accumulator, with no chance of replenishment. Hence, it felt like brake fade. I found during testing that if I would pause for a second or two before applying the brakes after a long period of on-boost, full-throttle acceleration, the brakes would feel fine, but if I jumped from accelerator to brakes, I'd have diminished braking. This was the case even when the brakes were relatively cool. Hence, a couple seconds at closed throttle would build enough vacuum to give sufficient boost to the brakes.
So I think there may have been a small leak at the check valve that bled down under just these precise circumstances. A new check valve seems to have fixed the problem.
Rob
07-16-2012, 11:43 PM
#20
Rennlist Member
Thanks for the follow-up, Rob.
For those that are interested, I put a brake pressure sensor on my car, on the rear brake circuit. I believe the bore of that portion of the master cylinder is 19mm.
Max braking pressure I see on the track is about 800 psi. (turn 5 at Mosport). My calculations tell me that the area of the 19mm plunger, in square inches, is 0.439 in^2. That means, to build up 800 psi, it requires 1,822 lbs of force.
According to this: http://944hybrids.pbworks.com/f/1248...DL%20RATIO.jpg the brake pedal ratio is 4.25:1.
That means, without the booster, one needs to apply 429 lbs of force on the pedal to achieve maximum braking.
Because I'm a bit of a nerd... I just got out the bathroom scale and put it against the wall. If I sit on the floor (carpet) and push on the scale with my right foot, it reads 50 lbs before my butt starts sliding. As an educated guess, I'd say in the race car I'm applying 50-75 lbs of pressure on the pedal - the brake booster is doing the rest.
To all the guys out there with LS1 swaps and no power brakes: you guys are nuts and flirting with danger.
For those that are interested, I put a brake pressure sensor on my car, on the rear brake circuit. I believe the bore of that portion of the master cylinder is 19mm.
Max braking pressure I see on the track is about 800 psi. (turn 5 at Mosport). My calculations tell me that the area of the 19mm plunger, in square inches, is 0.439 in^2. That means, to build up 800 psi, it requires 1,822 lbs of force.
According to this: http://944hybrids.pbworks.com/f/1248...DL%20RATIO.jpg the brake pedal ratio is 4.25:1.
That means, without the booster, one needs to apply 429 lbs of force on the pedal to achieve maximum braking.
Because I'm a bit of a nerd... I just got out the bathroom scale and put it against the wall. If I sit on the floor (carpet) and push on the scale with my right foot, it reads 50 lbs before my butt starts sliding. As an educated guess, I'd say in the race car I'm applying 50-75 lbs of pressure on the pedal - the brake booster is doing the rest.
To all the guys out there with LS1 swaps and no power brakes: you guys are nuts and flirting with danger.
07-20-2012, 01:38 AM
#21
Three Wheelin'
Thanks for the follow-up, Rob.
For those that are interested, I put a brake pressure sensor on my car, on the rear brake circuit. I believe the bore of that portion of the master cylinder is 19mm.
Max braking pressure I see on the track is about 800 psi. (turn 5 at Mosport). My calculations tell me that the area of the 19mm plunger, in square inches, is 0.439 in^2. That means, to build up 800 psi, it requires 1,822 lbs of force.
According to this: http://944hybrids.pbworks.com/f/1248...DL%20RATIO.jpg the brake pedal ratio is 4.25:1.
That means, without the booster, one needs to apply 429 lbs of force on the pedal to achieve maximum braking.
Because I'm a bit of a nerd... I just got out the bathroom scale and put it against the wall. If I sit on the floor (carpet) and push on the scale with my right foot, it reads 50 lbs before my butt starts sliding. As an educated guess, I'd say in the race car I'm applying 50-75 lbs of pressure on the pedal - the brake booster is doing the rest.
To all the guys out there with LS1 swaps and no power brakes: you guys are nuts and flirting with danger.
For those that are interested, I put a brake pressure sensor on my car, on the rear brake circuit. I believe the bore of that portion of the master cylinder is 19mm.
Max braking pressure I see on the track is about 800 psi. (turn 5 at Mosport). My calculations tell me that the area of the 19mm plunger, in square inches, is 0.439 in^2. That means, to build up 800 psi, it requires 1,822 lbs of force.
According to this: http://944hybrids.pbworks.com/f/1248...DL%20RATIO.jpg the brake pedal ratio is 4.25:1.
That means, without the booster, one needs to apply 429 lbs of force on the pedal to achieve maximum braking.
Because I'm a bit of a nerd... I just got out the bathroom scale and put it against the wall. If I sit on the floor (carpet) and push on the scale with my right foot, it reads 50 lbs before my butt starts sliding. As an educated guess, I'd say in the race car I'm applying 50-75 lbs of pressure on the pedal - the brake booster is doing the rest.
To all the guys out there with LS1 swaps and no power brakes: you guys are nuts and flirting with danger.
Cheers,
Mike
07-20-2012, 09:29 AM
#22
Rennlist Member
Hi Mike, are you sure? If the area was 1in^2, then the force would be 800. If the area was 2in^2, wouldn't 400 psi of force be multiplied to 800 psi? And, thus, if the area was 1/2in^2, it would take twice as much force, or 1,600 lbs, to reach 800 psi?
07-21-2012, 12:33 AM
#23
Three Wheelin'
Pressure is not multiplied.
Force = pressure x area. (or, pounds per square inch x square inches = pounds).
To explain it another way: say you measure a pressure of 800 pounds per square inch. If you only have half a square inch, then that generates 400 lb of force.
If you have another half a square inch, then that also generates 400lb of force. If you add them together, you have 1 square inch and 800lbs force, which is what you expect.
Cheers,
Mike
07-22-2012, 07:02 PM
#24
Rennlist Member
Hi Van, yes I'm sure.
Pressure is not multiplied.
Force = pressure x area. (or, pounds per square inch x square inches = pounds).
To explain it another way: say you measure a pressure of 800 pounds per square inch. If you only have half a square inch, then that generates 400 lb of force.
If you have another half a square inch, then that also generates 400lb of force. If you add them together, you have 1 square inch and 800lbs force, which is what you expect.
Cheers,
Mike
Pressure is not multiplied.
Force = pressure x area. (or, pounds per square inch x square inches = pounds).
To explain it another way: say you measure a pressure of 800 pounds per square inch. If you only have half a square inch, then that generates 400 lb of force.
If you have another half a square inch, then that also generates 400lb of force. If you add them together, you have 1 square inch and 800lbs force, which is what you expect.
Cheers,
Mike
