Brake bias
05-01-2018, 11:22 AM
#18
RL Community Team
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Half the time I wonder what's going on in Stuttgart. They are all over the map with their brakes systems over the last 15 years on a car design that hasn't changed that much. The only constant has been the fact that their brake systems have continuously gotten larger in diameter, especially in the rears. Which proves 2 things to me. 997's are under braked for their performance level (350/350mm should be on all base 911's, not 330/330, IMO). Rear brakes do a lot more work in a 911 than any other car design due to all that weight in front of the rear axle, and should be doing more work, especially because their is not a lot of weight on the front axle of a 911.
Their Motorsports division got it right. 380/380mm iron brakes that fit under 18" wheels.
Their Motorsports division got it right. 380/380mm iron brakes that fit under 18" wheels.
Peace
Bruce in Philly
05-01-2018, 12:09 PM
#19
Three Wheelin'
Thread Starter
Half the time I wonder what's going on in Stuttgart. They are all over the map with their brakes systems over the last 15 years on a car design that hasn't changed that much. The only constant has been the fact that their brake systems have continuously gotten larger in diameter, especially in the rears. Which proves 2 things to me. 997's are under braked for their performance level (350/350mm should be on all base 911's, not 330/330, IMO). Rear brakes do a lot more work in a 911 than any other car design due to all that weight in front of the rear axle, and should be doing more work, especially because their is not a lot of weight on the front axle of a 911.
Their Motorsports division got it right. 380/380mm iron brakes that fit under 18" wheels.
Their Motorsports division got it right. 380/380mm iron brakes that fit under 18" wheels.
My observation is that Porsche increases their rear rotor size throughout the generations to tame the rear heavy design. The front brakes having a tendency to lockup is very obvious due to the lighter front end. I've confirmed this in a followup parking lot test and was very aware that the ABS pumping activation on the front only. Perhaps with the capability of the ABS, that will take care of any lockups, brake bias is not a concern as in the old days. I brought this subject up not as any concern of Porsches' brake design, or doubtful of their engineering aptness, but as an observation of its brake bias. Surely more than adequate for normal street usage, but it is evident when it comes to track/racing, which does not apply to me, that the brakes bias are very different from our stock cars.
05-01-2018, 12:41 PM
#20
Half the time I wonder what's going on in Stuttgart. They are all over the map with their brakes systems over the last 15 years on a car design that hasn't changed that much. The only constant has been the fact that their brake systems have continuously gotten larger in diameter, especially in the rears. Which proves 2 things to me. 997's are under braked for their performance level (350/350mm should be on all base 911's, not 330/330, IMO). Rear brakes do a lot more work in a 911 than any other car design due to all that weight in front of the rear axle, and should be doing more work, especially because their is not a lot of weight on the front axle of a 911.
Their Motorsports division got it right. 380/380mm iron brakes that fit under 18" wheels.
Their Motorsports division got it right. 380/380mm iron brakes that fit under 18" wheels.
Dunno. Pretty sure though that when the car is moving fast and you brake hard then a lot of that weight, regardless of where it was to begin with, shifts forward so that the front tires now have huge weight pressing down on the road enabling them to brake harder. Which only works if the brakes at that end are powerful enough to exploit all that pressure. So they make them bigger. And as tires and suspension continue to improve, guess what? Bigger brakes. Pretty hard to see how they have been "all over the map." What I see is 60 years of continual gradual improvement.
Then there's another old-school thing called a traction circle. Friction circle. Or maybe that's changed too. But back when rear meant behind the friction circle was a way of visualizing that tires only have so much traction to go around. Get it? Around! Basically, you can brake at 1g, or corner at 1g, but not brake and corner at 1g at the same time.
Get it?
So they have to balance all these things. Sure they could add more rear brake. Did you not read my post above? No brake changes necessary! Simply swap out a valve. You can dial in as much rear brake as you want. People do it all the time. It will NOT overtax the rears because you never can use much rear brake because.... but I just explained all that.
Please try and understand, because until you do you will continue to draw all the wrong conclusions.
For example, about the 997. A lot more changed between the 997 and the 991 than brakes. Whole new suspension geometry. Increased wheelbase. Larger diameter tires. Every single one of these things improved the ability of the rear tires to handle both braking and cornering simultaneously. Made the friction circle bigger, so to speak. So of course they could then use more brake. Because the car was better able to use it. Not because they just decided it would be fun to put puny brakes on the 997.
So now you no longer have to wonder what's going on at Stuttgart. Its called engineering.
05-01-2018, 01:22 PM
#21
Which car are you talking about? Because "all that weight" on a 911 is behind the rear axle. Even seem to recall something about the 911 being rear engine. But so much social upheaval these days, 57 different genders, maybe rear is the new front?
Dunno. Pretty sure though that when the car is moving fast and you brake hard then a lot of that weight, regardless of where it was to begin with, shifts forward so that the front tires now have huge weight pressing down on the road enabling them to brake harder. Which only works if the brakes at that end are powerful enough to exploit all that pressure. So they make them bigger. And as tires and suspension continue to improve, guess what? Bigger brakes. Pretty hard to see how they have been "all over the map." What I see is 60 years of continual gradual improvement.
Then there's another old-school thing called a traction circle. Friction circle. Or maybe that's changed too. But back when rear meant behind the friction circle was a way of visualizing that tires only have so much traction to go around. Get it? Around! Basically, you can brake at 1g, or corner at 1g, but not brake and corner at 1g at the same time.
Get it?
So they have to balance all these things. Sure they could add more rear brake. Did you not read my post above? No brake changes necessary! Simply swap out a valve. You can dial in as much rear brake as you want. People do it all the time. It will NOT overtax the rears because you never can use much rear brake because.... but I just explained all that.
Please try and understand, because until you do you will continue to draw all the wrong conclusions.
For example, about the 997. A lot more changed between the 997 and the 991 than brakes. Whole new suspension geometry. Increased wheelbase. Larger diameter tires. Every single one of these things improved the ability of the rear tires to handle both braking and cornering simultaneously. Made the friction circle bigger, so to speak. So of course they could then use more brake. Because the car was better able to use it. Not because they just decided it would be fun to put puny brakes on the 997.
So now you no longer have to wonder what's going on at Stuttgart. Its called engineering.
Dunno. Pretty sure though that when the car is moving fast and you brake hard then a lot of that weight, regardless of where it was to begin with, shifts forward so that the front tires now have huge weight pressing down on the road enabling them to brake harder. Which only works if the brakes at that end are powerful enough to exploit all that pressure. So they make them bigger. And as tires and suspension continue to improve, guess what? Bigger brakes. Pretty hard to see how they have been "all over the map." What I see is 60 years of continual gradual improvement.
Then there's another old-school thing called a traction circle. Friction circle. Or maybe that's changed too. But back when rear meant behind the friction circle was a way of visualizing that tires only have so much traction to go around. Get it? Around! Basically, you can brake at 1g, or corner at 1g, but not brake and corner at 1g at the same time.
Get it?
So they have to balance all these things. Sure they could add more rear brake. Did you not read my post above? No brake changes necessary! Simply swap out a valve. You can dial in as much rear brake as you want. People do it all the time. It will NOT overtax the rears because you never can use much rear brake because.... but I just explained all that.
Please try and understand, because until you do you will continue to draw all the wrong conclusions.
For example, about the 997. A lot more changed between the 997 and the 991 than brakes. Whole new suspension geometry. Increased wheelbase. Larger diameter tires. Every single one of these things improved the ability of the rear tires to handle both braking and cornering simultaneously. Made the friction circle bigger, so to speak. So of course they could then use more brake. Because the car was better able to use it. Not because they just decided it would be fun to put puny brakes on the 997.
So now you no longer have to wonder what's going on at Stuttgart. Its called engineering.
Very good post.
Some of these posts boggle my mind. Like I said this thread was done with its value at post #4.
05-01-2018, 04:27 PM
#22
Rennlist Member
997.1 GT3 cars were notorious for burning up rear brakes. Sometimes twice as fast as the fronts. So do I feel that RWD 997's are under braked, especially in the rear on the track? Yes that's my opinion, which I've concluded based on how my car brakes, my own personal research, and my conclusions from what Stuttgart has been doing over the years. IMO, the 991 isn't drastically different than the 997, yet the 991 brakes keep getting bigger, and it's not marketing. Stopping distance tests don't lie.
Unlike most in this thread, I will be doing something about my brakes in the near future to remedy them, as they are not up to my standards. RB's 394/390mm 6 piston/4 piston CCM kit will most likely be going on the car, funds providing. That will really fix the bias issue.
Could I mix pad compounds to change brake bias? Sure,I could run a more aggressive rear pad. But that would introduce even more heat into an already thermally inadequate System.
Unlike most in this thread, I will be doing something about my brakes in the near future to remedy them, as they are not up to my standards. RB's 394/390mm 6 piston/4 piston CCM kit will most likely be going on the car, funds providing. That will really fix the bias issue.
Could I mix pad compounds to change brake bias? Sure,I could run a more aggressive rear pad. But that would introduce even more heat into an already thermally inadequate System.
05-01-2018, 04:48 PM
#23
997.1 GT3 cars were notorious for burning up rear brakes. Sometimes twice as fast as the fronts. So do I feel that RWD 997's are under braked, especially in the rear on the track? Yes that's my opinion, which I've concluded based on how my car brakes, my own personal research, and my conclusions from what Stuttgart has been doing over the years. IMO, the 991 isn't drastically different than the 997, yet the 991 brakes keep getting bigger, and it's not marketing. Stopping distance tests don't lie.
Unlike most in this thread, I will be doing something about my brakes in the near future to remedy them, as they are not up to my standards. RB's 394/390mm 6 piston/4 piston CCM kit will most likely be going on the car, funds providing. That will really fix the bias issue.
Unlike most in this thread, I will be doing something about my brakes in the near future to remedy them, as they are not up to my standards. RB's 394/390mm 6 piston/4 piston CCM kit will most likely be going on the car, funds providing. That will really fix the bias issue.
What you are saying is not supported at all by your argument though. The porsche 991 uses 340mm/330mm discs. The 991S uses 350mm/330mm. The 997 uses 318mm/299mm. The 997S uses 330mm/330mm.
In other words, there is no drastic change to increasing rear brake size on the 991 as compared to the 997 like you say. Like Chuck said, they are increasing the brake performance because the cars are getting both faster and having more grip. These are the reasons you need to increase brake performance. If you're brakes are not working for you because you are tracking than its likely because you have also increased your speed compared to the average driver and increased your grip. So you need better brake capacity (whether it be through pad, fluid, or rotor size upgrades).
If you want to increase your rear bias, just change your pad compounds. Easy peasy.
I would NOT recommend the OP go trying to change his brake bias. I mean no disrespect at all but if he doesnt know if the ABS is working from one run to the other than you DO NOT want to go messing with brake bias. Thats not the level of experience a person should have who should be messing with brake bias. You're going to shoot your eye out.
Mike
05-01-2018, 04:55 PM
#24
RL Community Team
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05-01-2018, 05:22 PM
#25
Rennlist Member
For what it's worth the 997 i don't think is underBRAKED but I definitely think it is underCOOLED at the brakes. Maybe the base carrera is fine but the GTS has the same brakes as the 997.1 S with an extra 50+ hp.
05-01-2018, 07:40 PM
#26
Nordschleife Master
05-01-2018, 10:45 PM
#27
997.1 GT3 cars were notorious for burning up rear brakes. Sometimes twice as fast as the fronts. So do I feel that RWD 997's are under braked, especially in the rear on the track? Yes that's my opinion, which I've concluded based on how my car brakes, my own personal research, and my conclusions from what Stuttgart has been doing over the years. IMO, the 991 isn't drastically different than the 997, yet the 991 brakes keep getting bigger, and it's not marketing. Stopping distance tests don't lie.
Unlike most in this thread, I will be doing something about my brakes in the near future to remedy them, as they are not up to my standards. RB's 394/390mm 6 piston/4 piston CCM kit will most likely be going on the car, funds providing. That will really fix the bias issue.
Could I mix pad compounds to change brake bias? Sure,I could run a more aggressive rear pad. But that would introduce even more heat into an already thermally inadequate System.
Unlike most in this thread, I will be doing something about my brakes in the near future to remedy them, as they are not up to my standards. RB's 394/390mm 6 piston/4 piston CCM kit will most likely be going on the car, funds providing. That will really fix the bias issue.
Could I mix pad compounds to change brake bias? Sure,I could run a more aggressive rear pad. But that would introduce even more heat into an already thermally inadequate System.
That must be it! Like you said, they are not up to your standards. Have you thought of writing Porsche about a position?
One bit of advice first though. Changing brakes will not change the front rear brake bias. Third time saying this, don't strike out: brake bias is controlled by a proportioning valve. You want to change the bias, you change the valve. Even if you don't believe this and are determined to install bigger rear brakes, you will still need the proportioning valve. Need it even more in fact. Because you will now have rear brakes out of balance with the fronts. You will need some way of tuning that balance. That way is called a proportioning valve.
Plus you can include the experience in your cover letter along with your resume to Porsche.
05-02-2018, 01:49 AM
#28
Rennlist Member
By golly, I think you got it! Porsche really did put inexcusably puny brakes on the worlds best track car. A car with some of the most highly regarded braking in the world. Which hypnotizes even owners into not noticing. So that only you among all the worlds reviewers, drivers and owners has been able to figure this out.
That must be it! Like you said, they are not up to your standards. Have you thought of writing Porsche about a position?
One bit of advice first though. Changing brakes will not change the front rear brake bias. Third time saying this, don't strike out: brake bias is controlled by a proportioning valve. You want to change the bias, you change the valve. Even if you don't believe this and are determined to install bigger rear brakes, you will still need the proportioning valve. Need it even more in fact. Because you will now have rear brakes out of balance with the fronts. You will need some way of tuning that balance. That way is called a proportioning valve.
Plus you can include the experience in your cover letter along with your resume to Porsche.
That must be it! Like you said, they are not up to your standards. Have you thought of writing Porsche about a position?
One bit of advice first though. Changing brakes will not change the front rear brake bias. Third time saying this, don't strike out: brake bias is controlled by a proportioning valve. You want to change the bias, you change the valve. Even if you don't believe this and are determined to install bigger rear brakes, you will still need the proportioning valve. Need it even more in fact. Because you will now have rear brakes out of balance with the fronts. You will need some way of tuning that balance. That way is called a proportioning valve.
Plus you can include the experience in your cover letter along with your resume to Porsche.
Secondly, Porsche themselves admitted the rear brakes on the 997.1 were not optimized correctly. The 997.2 GT3 has rear cooling ducts added where the 997.1 did not and the 997.2 PSM was revised to correct for this.
I'm not sure you know what you're talking about.
Last edited by nwGTS; 05-02-2018 at 02:32 AM.
05-02-2018, 06:55 AM
#29
A couple things. You contradict yourself here.
Secondly, Porsche themselves admitted the rear brakes on the 997.1 were not optimized correctly. The 997.2 GT3 has rear cooling ducts added where the 997.1 did not and the 997.2 PSM was revised to correct for this.
I'm not sure you know what you're talking about.
Secondly, Porsche themselves admitted the rear brakes on the 997.1 were not optimized correctly. The 997.2 GT3 has rear cooling ducts added where the 997.1 did not and the 997.2 PSM was revised to correct for this.
I'm not sure you know what you're talking about.
mike
05-02-2018, 01:12 PM
#30
Rennlist Member
By golly, I think you got it! Porsche really did put inexcusably puny brakes on the worlds best track car. A car with some of the most highly regarded braking in the world. Which hypnotizes even owners into not noticing. So that only you among all the worlds reviewers, drivers and owners has been able to figure this out.
One bit of advice first though. Changing brakes will not change the front rear brake bias. Third time saying this, don't strike out: brake bias is controlled by a proportioning valve. You want to change the bias, you change the valve. Even if you don't believe this and are determined to install bigger rear brakes, you will still need the proportioning valve. Need it even more in fact. Because you will now have rear brakes out of balance with the fronts. You will need some way of tuning that balance. That way is called a proportioning valve.
One bit of advice first though. Changing brakes will not change the front rear brake bias. Third time saying this, don't strike out: brake bias is controlled by a proportioning valve. You want to change the bias, you change the valve. Even if you don't believe this and are determined to install bigger rear brakes, you will still need the proportioning valve. Need it even more in fact. Because you will now have rear brakes out of balance with the fronts. You will need some way of tuning that balance. That way is called a proportioning valve.
Try doing some research before you say erroneous bull**** ( nonetheless 3 times). http://www.stoptech.com/technical-su...alance-matters
Look at the first thing on the chart below that dictates a change in bias. If you do more research, most, if not all brake companies DO NOT recommend changing the proportioning valve on a car that sees any streets. Manual proportioning valves, dual master cylinders, etc are for race cars.
So what influences brake bias?
If we look at the equations we have developed, we see that all of the following factors will affect the weight on an axle for any given moment in time:
· Weight distribution of the vehicle at rest
· CG height – the higher it is, the more weight gets transferred during a stop
· Wheelbase – the shorter it is, the more weight gets transferred during a stop
We also know from fundamental brake design that the following factors will affect how much brake torque is developed at each corner of the vehicle, and how much of that torque is transferred to the tire contact patch and reacted against the ground:
· Rotor effective diameter
· Caliper piston diameter
· Lining friction coefficients
· Tire traction coefficient properties
It is the combination of these two functions – braking force at the tire versus weight on that tire – that determine our braking bias. Changing the CG height, wheelbase, or deceleration level will dictate a different force distribution, or bias, requirement for our brake system. Conversely, changing the effectiveness of the front brake components without changing the rear brake effectiveness can also cause our brake bias to change. The following table summarizes how common modifications will swing bias all over the map.
