soft brake pedal with Big Reds
02-17-2009, 08:46 PM
#46
Rennlist Junkie Forever
You know you could simply change your pedal ratio by drilling another hole in the pedal about 1/8" farther from the pivot point. This will affect the hydraulics exactly like going to a bigger master cylinder, and will firm up, and quicken the brake response.
I've got several holes in my brake pedal, deciding which one did the job.
It's easy and free.
TonyG
I've got several holes in my brake pedal, deciding which one did the job.
It's easy and free.
TonyG
02-17-2009, 09:05 PM
#47
Rennlist Member
Yes I've been down this path before and I don't think it's mutually exclusive to BReds either. I tried to get Sam Lin to expand on his post but he's not very forthcoming. I pursued the larger m/c with a number of sources. I eventually got a model number for it and bought one from Ebay. It's now in Evil's garage and hasn't been installed into anything. Quite likely it's the wrong model or size from what I can gather.
What brake rotors do you have Eclou? I have just changed over to the 965 drilled disks and hope to eliminate what we believe could be the cause for the soft pedal. There can be a build up of gases between pad and rotor which can affect the feel/travel of the pedal. I am due to do a track day tomorrow with the latest setup if it's not raining.
My setup is BR fronts-S4 rears, s/s lines, SRF fluid, 5/33, tube air ducting, either Pagid Blacks or PFC pads and a new OEM m/c.
What brake rotors do you have Eclou? I have just changed over to the 965 drilled disks and hope to eliminate what we believe could be the cause for the soft pedal. There can be a build up of gases between pad and rotor which can affect the feel/travel of the pedal. I am due to do a track day tomorrow with the latest setup if it's not raining.
My setup is BR fronts-S4 rears, s/s lines, SRF fluid, 5/33, tube air ducting, either Pagid Blacks or PFC pads and a new OEM m/c.
It's not the gas, I've driven it and it's soft from the first application.
02-17-2009, 11:06 PM
#48
Rennlist Member
You know you could simply change your pedal ratio by drilling another hole in the pedal about 1/8" farther from the pivot point. This will affect the hydraulics exactly like going to a bigger master cylinder, and will firm up, and quicken the brake response.
I've got several holes in my brake pedal, deciding which one did the job.
It's easy and free.
TonyG
I've got several holes in my brake pedal, deciding which one did the job.
It's easy and free.
TonyG
02-17-2009, 11:49 PM
#49
I think it's an altogether bad idea. Here's why: ideally your pedal travel should equal (nearly) the piston travel of the master cylinder. If you alter the pedal ratio and cause increased piston travel, you can bottom out the piston in the master cylinder bore, you can rupture seals this way and end up with no brakes. You could have the piston bottomed out when the brake pedal is half way to the floor.
Choose this approach at your own risk.
Choose this approach at your own risk.
02-18-2009, 12:00 AM
#50
You know you could simply change your pedal ratio by drilling another hole in the pedal about 1/8" farther from the pivot point. This will affect the hydraulics exactly like going to a bigger master cylinder, and will firm up, and quicken the brake response.
I've got several holes in my brake pedal, deciding which one did the job.
It's easy and free.
TonyG
I've got several holes in my brake pedal, deciding which one did the job.
It's easy and free.
TonyG
Not true at all. This approach does not change the hydraulics one iota. It makes the master cylinder piston move farther and faster with the depression of the pedal. The hydraulics moves a specific volume of brake fluid based on the stroke and the bore of the cylinder - and this hasn't changed. When you run out of piston stroke, you're done - no more braking authority available.
This is why you need to consider a larger master cylinder when you install larger calipers with more or larger pistons. The fact that you have altered you pedal ratio without a failure does not guarantee that someone else experimenting with pedal ratios will not have a catastrophic failure due to running out of stroke or seal failure.
02-18-2009, 01:41 AM
#51
Rennlist Junkie Forever
I think it's an altogether bad idea. Here's why: ideally your pedal travel should equal (nearly) the piston travel of the master cylinder. If you alter the pedal ratio and cause increased piston travel, you can bottom out the piston in the master cylinder bore, you can rupture seals this way and end up with no brakes. You could have the piston bottomed out when the brake pedal is half way to the floor.
Choose this approach at your own risk.
Choose this approach at your own risk.
Huh?
The master cylinder will always lock the brakes at the same relative position. The pedal position is irrelevant. The pedal could be 1" higher or 1" lower, but the master cylinder is in the same spot when you're at maximum hydraulic pressure. The only difference is the mechanical leverage, and the pedal ratio, acting on the master cylinder. But in the end.... locking the brakes will occur at a given master cylinder position irrespective of where the pedal is, no matter the pedal ratio.
Increasing the pedal ratio moves the master cylinder at a quicker ratio. It does not move the master cylinder farther in its stroke. Thus the notion the that you can tweak the seals is makes no sense.
Increasing the leverage of the pedal means that you would activate the brakes in a "slower" ratio. While this is not what the poster of the thread is looking to do, you might be able to slightly push the master cylinder farther in it's bore due to your increased mechanical leverage. But this is the opposite of what the poster is looking for.
To fix the problem that we're talking about here... you would need increase the ratio, decreasing the mechanical leverage, which would activate the brakes faster with a proportionally firmer pedal feel. And, the ability to push the master cylinder past its normal operating stroke range would be "decreased" as a result.
That said, adjusting pedal ratios is a normal brake tuning tool/parameter of setting up the brakes.
Case in point my brakes..... 993TT brakes. I adjusted the pedal ratio. The brakes are perfect. Two track seasons and zero problems.
TonyG
02-18-2009, 01:48 AM
#52
Rennlist Junkie Forever
Not true at all. This approach does not change the hydraulics one iota. It makes the master cylinder piston move farther and faster with the depression of the pedal. The hydraulics moves a specific volume of brake fluid based on the stroke and the bore of the cylinder - and this hasn't changed.
Pedal ratio is part of the hydraulics.
In fact, pedal ratio adjustment is discussed in several "how to make your car handle" publications, such as the one from Fred Puhn.
TonyG
02-18-2009, 02:35 AM
#53
Rennlist Member
Just an update. I went to the track today. Once the rain went and the track dried I got a chance to test the drilled rotors. Well it was perhaps ultimately inconclusive. The brakes were good in their retardation but they did get a little bit mushy a couple of times through the day. I didn't do a lot of laps as the car just chews up the E85. Probably 25 laps via 3 short sessions. So the brakes did get hot enough to start to get softer. You just push a bit harder and they still work, but it's not absolutely ideal. Especially with 200 more hp heading my way pretty soon! To be honest I'm concerned that the only way I can keep up with the incoming power hike is to increase my braking yet again? Until I can put wider rubber on the car which requires some body mods, I probably won't be able to go much faster than I already am...without running out of corner due to not being able to brake like those damn GT3s. I know their ABS is way superior to ours but I don't really notice mine that much and I know I'm going in deep into the corners.
Also to answer slivel's assertion that you must increase m/c if you go up in piston size/amount. Well this is true enough, but the BRs and the S4 calipers have the same size and amount of pistons.
http://www.pelicanparts.com/MotorCity/wmv/brakes.htm
btw, where is the best place to get brake duct tube? Mine's a bit tatty and I need some more pronto.
Also to answer slivel's assertion that you must increase m/c if you go up in piston size/amount. Well this is true enough, but the BRs and the S4 calipers have the same size and amount of pistons.
http://www.pelicanparts.com/MotorCity/wmv/brakes.htm
btw, where is the best place to get brake duct tube? Mine's a bit tatty and I need some more pronto.
02-18-2009, 10:09 AM
#54
Basic Sponsor
Rennlist
Site Sponsor
Rennlist
Site Sponsor
Our track car is a standard 88 Turbo with Big Reds, stock rear calipers and stock (still original) master cylinder. Pedal is rock hard. We do have a adjustable bias valve that must be opened up all the way before bleeding then adjusted back. If you don't, you cannot properly bleed and can try all day long. Perhaps this is more a bias valve issue for some.
Not to discount anybodies mechanical skills, but are you bleeding the brakes correctly? Assuming you have you calipers mounted correctly with bleeders up and pistons in correct orientation... First to know is you start with the furthest caliper from the master cylinder (RR, LR, RF the LF on a LHD Car) and the outside bleeder before the inner. It makes a difference when it comes to getting all the air out. You should pump the pedal several times until it gets as firm as it's going to get. Then crack the bleeder while still depressing the pedal. The pedal will sink to the floor but "Don't" lift or quit depressing the pedal until the bleeder is closed first or you suck air back in the bleeder. A small clear hose on the bleed submerged into a small glass container of brake fluid helps here. Keep working that bleeder until you no longer see any air or no further improvement in pedal feel then move to the next one. Once you work all 8 bleeders, and if the pedal doesn't feel firm as hell, start over since you will likely find a little more air.
Keep an eye on the fluid level in the reservoir because if you go low and suck air, you start all over.
Not to discount anybodies mechanical skills, but are you bleeding the brakes correctly? Assuming you have you calipers mounted correctly with bleeders up and pistons in correct orientation... First to know is you start with the furthest caliper from the master cylinder (RR, LR, RF the LF on a LHD Car) and the outside bleeder before the inner. It makes a difference when it comes to getting all the air out. You should pump the pedal several times until it gets as firm as it's going to get. Then crack the bleeder while still depressing the pedal. The pedal will sink to the floor but "Don't" lift or quit depressing the pedal until the bleeder is closed first or you suck air back in the bleeder. A small clear hose on the bleed submerged into a small glass container of brake fluid helps here. Keep working that bleeder until you no longer see any air or no further improvement in pedal feel then move to the next one. Once you work all 8 bleeders, and if the pedal doesn't feel firm as hell, start over since you will likely find a little more air.
Keep an eye on the fluid level in the reservoir because if you go low and suck air, you start all over.
__________________
Mike or Dave Lindsey
www.lindseyracing.com
U.S. 1-877-943-3565
Other 1-405-947-0137
Mike or Dave Lindsey
www.lindseyracing.com
U.S. 1-877-943-3565
Other 1-405-947-0137
Last edited by Mike Lindsey; 02-18-2009 at 02:53 PM.
02-18-2009, 10:42 AM
#55
Addict
Rennlist Member
Rennlist Member
I have an '87 non-abs turbo with new Big Blacks, stock rears, and the 5/33 valve. I used the Motive bleeder and have a rock-hard pedal. I had to replace clutch and slave cylinders last fall so I did the brake master as well. Bled everything with the Motive and still have good pedal feel on both left pedals. I have also replaced the rubber lines with SS when I had stock brakes and that seemed to help stiffen the pedal.
Bigger pistons should require more fluid to move them the same distance but if everything in the system is incompressible and the pads are touching the rotor, the piston should not need to move very far. Piston seals, hoses, and brake pads all have some compliance but air in the lines will have a much greater affect on pedal feel. When bleeding, I follow up the initial bleed after I have driven on a pot-holed road to shake up the fluid and get the bubbles to combine. I also tap the calipers with a soft mallet while bleeding to try to loosen up the bubbles as well.
Bigger pistons should require more fluid to move them the same distance but if everything in the system is incompressible and the pads are touching the rotor, the piston should not need to move very far. Piston seals, hoses, and brake pads all have some compliance but air in the lines will have a much greater affect on pedal feel. When bleeding, I follow up the initial bleed after I have driven on a pot-holed road to shake up the fluid and get the bubbles to combine. I also tap the calipers with a soft mallet while bleeding to try to loosen up the bubbles as well.
Last edited by tjbreen; 02-18-2009 at 11:12 PM.
02-18-2009, 11:28 AM
#56
Rennlist Junkie Forever
The other thing people with caliper adapters should check is the squareness of the caliper adapter and its mounting holes.
If the adapter is off even a tiny tiny amount, the caliper won't be sitting squarely on the rotor and will for sure have mushy brake pedal feel that will never go away. This is because the pads won't be pushed squarely against the rotor.
The degree to which it's mushy depends on how far off the bolt holes are.
There is no room for error here. They have to be perfect.
TonyG
If the adapter is off even a tiny tiny amount, the caliper won't be sitting squarely on the rotor and will for sure have mushy brake pedal feel that will never go away. This is because the pads won't be pushed squarely against the rotor.
The degree to which it's mushy depends on how far off the bolt holes are.
There is no room for error here. They have to be perfect.
TonyG
02-18-2009, 11:33 AM
#57
Huh?
The master cylinder will always lock the brakes at the same relative position. The pedal position is irrelevant. The pedal could be 1" higher or 1" lower, but the master cylinder is in the same spot when you're at maximum hydraulic pressure. The only difference is the mechanical leverage, and the pedal ratio, acting on the master cylinder. But in the end.... locking the brakes will occur at a given master cylinder position irrespective of where the pedal is, no matter the pedal ratio.
Not necessarily true. Slight warpage in the rotors will kick the caliper pistons farther back in their bores and sometimes sand or other debris can collect on the rotor causing an initial "grab" but then the pedal sinks farther.
Increasing the pedal ratio moves the master cylinder at a quicker ratio. It does not move the master cylinder farther in its stroke. Thus the notion the that you can tweak the seals is makes no sense.
The effect of changing the pedal ratio can be either you move the piston farther in its bore OR you move it less than before but apply a higher force on it. With a longer moment arm, you move a greater distance on one side of the pivot, but travel will decrease at the object end on the other side of the pivot or fulcrum - or the reverse if you shorten the moment arm where you apply force, on the other side of the fulcrum you get a longer travel. Depending upon which way you went on your modification you either had longer travel of the pedal and higher mechanical advantage (force) on the master cylinder piston, or you had shorter pedal travel and lower force on the M/C piston.
Increasing the leverage of the pedal means that you would activate the brakes in a "slower" ratio. While this is not what the poster of the thread is looking to do, you might be able to slightly push the master cylinder farther in it's bore due to your increased mechanical leverage. But this is the opposite of what the poster is looking for.
To fix the problem that we're talking about here... you would need increase the ratio, decreasing the mechanical leverage, which would activate the brakes faster with a proportionally firmer pedal feel. And, the ability to push the master cylinder past its normal operating stroke range would be "decreased" as a result.
I think that with power assisted brakes, the ability to get the piston travel farther than intended would be relatively easy. There was a well regarded poster on here some time ago ( I think it was Markus) who said that when bleeding brakes and using the pump the pedal method that you sometimes end up with the seal failing. This happens because over time sludge, rust , and other debris collect in the bore and when you push the piston outside of its normal range of travel, the seal rides over this stuff and tears. So unless you are replacing the M/C, you run the risk of running the piston outside of the normal range of travel by changing pedal ratio.
That said, adjusting pedal ratios is a normal brake tuning tool/parameter of setting up the brakes.
Yes, it is a tuning technique, but only if you fully consider what you are doing. Blindly drilling holes in your pedal to change ratios until you get what feels like an improvement is not a good idea. You must take into account the design parameters of the master cylinder. What is the operating range of travel for the piston? If you exceed this by changing the pedal ratio, you are risking brake failure.
Case in point my brakes..... 993TT brakes. I adjusted the pedal ratio. The brakes are perfect. Two track seasons and zero problems.
I'm happy for you. I'll give you extra wide berth if I see you on the track.
TonyG
The master cylinder will always lock the brakes at the same relative position. The pedal position is irrelevant. The pedal could be 1" higher or 1" lower, but the master cylinder is in the same spot when you're at maximum hydraulic pressure. The only difference is the mechanical leverage, and the pedal ratio, acting on the master cylinder. But in the end.... locking the brakes will occur at a given master cylinder position irrespective of where the pedal is, no matter the pedal ratio.
Not necessarily true. Slight warpage in the rotors will kick the caliper pistons farther back in their bores and sometimes sand or other debris can collect on the rotor causing an initial "grab" but then the pedal sinks farther.
Increasing the pedal ratio moves the master cylinder at a quicker ratio. It does not move the master cylinder farther in its stroke. Thus the notion the that you can tweak the seals is makes no sense.
The effect of changing the pedal ratio can be either you move the piston farther in its bore OR you move it less than before but apply a higher force on it. With a longer moment arm, you move a greater distance on one side of the pivot, but travel will decrease at the object end on the other side of the pivot or fulcrum - or the reverse if you shorten the moment arm where you apply force, on the other side of the fulcrum you get a longer travel. Depending upon which way you went on your modification you either had longer travel of the pedal and higher mechanical advantage (force) on the master cylinder piston, or you had shorter pedal travel and lower force on the M/C piston.
Increasing the leverage of the pedal means that you would activate the brakes in a "slower" ratio. While this is not what the poster of the thread is looking to do, you might be able to slightly push the master cylinder farther in it's bore due to your increased mechanical leverage. But this is the opposite of what the poster is looking for.
To fix the problem that we're talking about here... you would need increase the ratio, decreasing the mechanical leverage, which would activate the brakes faster with a proportionally firmer pedal feel. And, the ability to push the master cylinder past its normal operating stroke range would be "decreased" as a result.
I think that with power assisted brakes, the ability to get the piston travel farther than intended would be relatively easy. There was a well regarded poster on here some time ago ( I think it was Markus) who said that when bleeding brakes and using the pump the pedal method that you sometimes end up with the seal failing. This happens because over time sludge, rust , and other debris collect in the bore and when you push the piston outside of its normal range of travel, the seal rides over this stuff and tears. So unless you are replacing the M/C, you run the risk of running the piston outside of the normal range of travel by changing pedal ratio.
That said, adjusting pedal ratios is a normal brake tuning tool/parameter of setting up the brakes.
Yes, it is a tuning technique, but only if you fully consider what you are doing. Blindly drilling holes in your pedal to change ratios until you get what feels like an improvement is not a good idea. You must take into account the design parameters of the master cylinder. What is the operating range of travel for the piston? If you exceed this by changing the pedal ratio, you are risking brake failure.
Case in point my brakes..... 993TT brakes. I adjusted the pedal ratio. The brakes are perfect. Two track seasons and zero problems.
I'm happy for you. I'll give you extra wide berth if I see you on the track.
TonyG
02-18-2009, 12:00 PM
#58
Addict
Rennlist Member
Rennlist Member
Thread Starter
Slivel I don't think changing the mechanical advantage of the brake pedal can cause the move the piston any further into the bore than what is already being achieved - i.e. if the change to larger front brake pistons allows further travel of the MC piston then the problem of overtravel will be there regardless of the brake pedal pivot point
02-18-2009, 01:21 PM
#59
Rennlist Junkie Forever
Slight warpage in the rotors will kick the caliper pistons farther back in their bores and sometimes sand or other debris can collect on the rotor causing an initial "grab" but then the pedal sinks farther.
Yeah so...? This isn't relative to the effect of altering the pedal ratio.
The effect of changing the pedal ratio can be either you move the piston farther in its bore OR you move it less than before but apply a higher force on it. With a longer moment arm, you move a greater distance on one side of the pivot, but travel will decrease at the object end on the other side of the pivot or fulcrum - or the reverse if you shorten the moment arm where you apply force, on the other side of the fulcrum you get a longer travel. Depending upon which way you went on your modification you either had longer travel of the pedal and higher mechanical advantage (force) on the master cylinder piston, or you had shorter pedal travel and lower force on the M/C piston.
We understand mechanical leverage and ratios.
I think that with power assisted brakes, the ability to get the piston travel farther than intended would be relatively easy.
There is plenty of "headroom" in the m/c addembly to push the pedal well past what is required to lock up the brakes.
There was a well regarded poster on here some time ago ( I think it was Markus) who said that when bleeding brakes and using the pump the pedal method that you sometimes end up with the seal failing. This happens because over time sludge, rust , and other debris collect in the bore and when you push the piston outside of its normal range of travel, the seal rides over this stuff and tears. So unless you are replacing the M/C, you run the risk of running the piston outside of the normal range of travel by changing pedal ratio.
Sure. The same thing could happen if you push the pedal down farther than it was pushed in the last 3 years.
It's silly. If you have that problem, then you needed a new m/c to begin with and it was a failure waiting to happen.
Yes, it is a tuning technique, but only if you fully consider what you are doing.
It's a tuning technique, which is why I brought it up in the first place.
Blindly drilling holes in your pedal to change ratios until you get what feels like an improvement is not a good idea.
There's nothing blind about it. Different strokes for different folks. Differerent people like different rates of engagement and different brake "feel"... which is why we run different brake pads for example.
2/16" to 3/16" That should do the trick.
You must take into account the design parameters of the master cylinder. What is the operating range of travel for the piston? If you exceed this by changing the pedal ratio, you are risking brake failure.
And changing the pedal ratio will affect m/c piston travel far less (if at all) than going to a bigger set of brakes with greater caliper piston area (as tons of people have done).
Consider what happens when you go from regular 951 brake calipers to Big Red bake calipers, which have larger pistons... What then?
I'll give you extra wide berth if I see you on the track.
TonyG
02-18-2009, 01:48 PM
#60
Nordschleife Master
I think Sivel is missing the fact that when you change the pivot point on the brake pedal you also adjust the pedal STOPPER to make sure that the master cylinder doesn't travel farther than it did before. Otherwise he would be right.
I'm pretty sure that when you start doing things like changing rotors and calipers, you have to adjust everything else to make it perfect, bias, master cyldiners, pedal piviot, etc etc. You can't just put new rotors and calipers and expect it to work perfectly.
This thread is awesome. Lots of good info for me right at the point when I'm about to mount my big reds!!
PS is anyone running a custom Tilton setup? Or has anyone re-inforced the firewall to prevent flex? I think this is also a serious issue that needs looking into.
I'm pretty sure that when you start doing things like changing rotors and calipers, you have to adjust everything else to make it perfect, bias, master cyldiners, pedal piviot, etc etc. You can't just put new rotors and calipers and expect it to work perfectly.
This thread is awesome. Lots of good info for me right at the point when I'm about to mount my big reds!!
PS is anyone running a custom Tilton setup? Or has anyone re-inforced the firewall to prevent flex? I think this is also a serious issue that needs looking into.