trap125

Where would you advise I set my brake bias? I've recently bought a 997.2 Cup (no ABS) and I'm just looking for a basic set up to get me going... Do I just set it to 0 for starters?

Also, should I also set the blades on the anti roll bars to Front 3 and rear 4 as per the technical manual or would you advise something different?

Any advise would be much appreciated. Many thanks.

coryf

Be careful with the **** setting on the display. If the bias cable was ever disconnected from the balance bar, it might not be a comparable number to another car. If it has front and rear pressure sensors, start out with the pressure about 5% more to the front. It's never a bad idea to check the balance bar and master cylinder rod adjustments to make sure they are set up correctly.

zracer196

Exactly what I am dealing with now...Have a 981 Clubsport with the GT3 brakes (option)...No display to check. Cable was disconnected and have no idea where it was set. Shut TC, etc. off and the car wanted to spin under braking...So there must be rear bias...But how much? I need to read the manual and see how to "zero it out", etc. I guess!

ProCoach

+1,000,000

Almost none of the older cars that are not looked after regularly by teams the quality of Autometrics, Goldcrest and a few others are adjusted correctly.

When I’m working with a client with a new (to them) car, that is the first thing I check now, as I am going to ask them to do some exercises that will stress and uncover any misadjustment.

Because of the rearward weight bias of the 911 platform, you can afford to dial in more front than a conventional front-engine car. Braking is one of the advantages a rear (and, to a lesser extent, mid) engine car has.

Too much rear brake can be diabolical and, in some cases, dangerous. Going into T1 downhill at the Glen, for instance.

Why would someone change it? Well, in the wet, you would want more rear to spread the load because you’re not using the pedal pressure you would in the dry.

RTM, for sure. The initial adjustment will get you in the neighborhood.

ProCoach

From Darrick Dong at PFC, for a 997.1 Cup:

Hi

Thanks for the update! Yes your Motec zero is off. In theory, a 997 in terms of PSI bias tends to be in the 48 to 50% front range but don't feel compiled to run that bias if you're comfortable now. Here's a few tricks.

1. From where the bias bar is, count how many turns it takes to get full front, then count how many turns it takes to go full rear.
2. If we assume 14 turns total, the 7 would be the 50/50% position or the neutral position. Note where the bal. bar position was relative to the 50/50% position. Zero out the Motec.
3. At the 50/50% placement, Then try this procedure:

Old School-checking dynamic brake timing
Place the car on jack stands, wheels are on, brakes are bleed. Balance bar is dead in middle of the pedal tube.

1 person is stepping on the brake pedal slowly and ever so lightly while 2 people are vigorously rocking the wheels to see which wheel bites 1st.

If the rears bite before the fronts, shorten the rear push-rod and lengthen the front push-rod on the master cylinders until both circuits bite at the same time, or vice-versa if the front’s bite before the rears. Once satisfied as this is repeatable, change the front person with the rear person and vice-versa to insure uniform dynamic timing of the brake apply is present.

Now step on the pedal hard and release slowly to insure both wheels release at the same time. This is more important than uniform engagement as on real fast tracks, you might prefer the fronts bite a tick early.

Once satisfied as this is repeatable then change the front person with the rear person to insure uniform dynamic timing of the brake engagement and release is present.

4. Once you’ve done this dynamic test at 50/50 on the B.bar, do a few laps at 80% to see how the balance is. Play with bias adjustment and you’ll note a click will be felt now.
5. When satisfied with the balance see how much difference if any from your previous settings.
6. Push or Pull rods lengths on the master cylinders determine timing
7. Bias adjustment determine PSI balance


Good Luck, have fun.


Darrick Dong
Director of Motorsports
ddong@pfcbrakes.com

ProCoach

Would certainly check to see that the bore size split front and rear is correct if the masters have been replaced, too.

I look at Wheelspeed data, plotting all four wheel speeds over each other, in ANY car that the balance is adjustable.

The goal is to, when braking hard in a straight line with good tires and everything warmed up, to see the front wheel speeds have some small, short jaggies downward, especially toward the end of the brake zone.

Too much front is ALWAYS preferable to even a hint of too much rear.

ProCoach

Lastly. pad compound can adversely effect the initial bite and make issues more complicated.

The braking system of the factory-built cars is a SYSTEM, one that requires coordination between all the parts and adjustment to function at a high consistent level.

I’ve see people struggle for years, only to find out the resolution was proper setup and components.

zracer196

Thanks for ALL of the tips above!!

ProCoach

The answer is NOT simple. Have to do your homework, but the answers are out there.

I will tell you that potentially winning advantages come from your confidence in the car's stability under braking and your ability to leverage maximum sustained deceleration in the shortest distance.

If everything is working right, it can make the difference between an "easy pass" and heartbreak....

RobT 394

If you have data like a Motec C125 highly recommend putting sensors on the car to look at the bias. Our car was way out a whack and while we thought we were close the sensors proved we had a lot more front bias to gain. With the ABS it is harder to feel than in the older Cups where you adjust until you get lockup and refine from there. After correcting the bias our braking improved dramatically! We are on street tires and difference in both feel and stopping distance was signficant.

Matt Romanowski

Bias is a critical adjustment if you have dual master cylinders. Tilton has a great article on it at https://tiltonracing.com/wp-content/...lance-Bars.pdf Carrol Smith has a big portion in Prepare to Win (I think it's this one) about setting bias before hitting the track.

I also can't recommend enough if you have dual masters, you need pressure sensors on both circuits. Then you can not only set the bias front to rear, but monitor it on track and see what bias is giving you the most negative long G and lowest lap times. I posted about this just this week on my Instagram https://www.instagram.com/trailbrakematt/ Once you have the tools to measure bias, you also need to check the migration.

When setting bias with pressures on the dash (or old school gauges), make sure to take the readings consistently in the same manner. This means that you should push on the pedal to get up to XXX psi in the front circuit and read the bias. Do not go to something over the front pressure and release to XXX pressure. Going over and release to the pressure will give you different bias readings.



Front versus rear pressure gives you bias migration and compliance in the system.

Bias changes as the pedal pressure changes. This is critical to monitor for consistent car behavior.

Checking compliance in the system is easily done with bias vs front brake pressure. You want the spread to be under about 5% percent. Poor bias control and compliance can lead to 12% plus!

trap125

Many thanks for all the great info.

Assuming that the brake bias system is functioning correctly where would you advise I set my brake bias? Do I just set it to 0 for starters? What percentage does 0 represent front vs rear? Please keep in mind that I'm a complete beginner!

Many thanks.

Matt Romanowski

If it's all unknown, I would start with the car in the air and have 2 helpers. You push on the brake pedal and the helpers each spin a front and rear wheel. Start applying brake pressure and see which end starts to lock first by just a little.. Make sure it's the front! Once you have done that, you're track tuning.

CRex

There is some good info here from experienced guys. But OP please do not take anyone's word as gospel. Optimal bias varies from track to track and tire to tire. It changes throughout the evolution of your brakes and the course of tire life.

It's all relative: in the course of a race you'd see more and more rear bias dialed in (our cars are front-axle limited and lose grip there first).

A couple key concepts: Bias is not an absolute number. It's a percentage: (front BP / total BP) * 100%.

That single-digit number on the 997 ADL2 really means nothing. It's only there to tell a driver how much he's turned the red ****. A "1.0" readout on one car will most likely mean a different % in another car showing the same "1.0". In the paddock we only compare bias in % terms.

How do you get to that %? You need to configure the ADL2 to show BP F and BP R (assuming you have the BP sensors installed). Hold the brake pedal so it reads a constant 40 or 50 bars on the rear axle. Take note of what the front axle reads at this point. F / (F+R) * 100% gets you that %.

I've generally found 43-45% workable in the 997.1 and 997.2 on Pirelli DH's and RST pads. You will most certainly have a different optimal % for your circumstances. The key is knowing what you're trying to achieve.

For me, I'm quick to feel (and hear) rear lock and have learned to bleed off BP before the rear steps out. I'd be happy to have my bias set so the rears are first to lock, but if I move bias to the front by just a notch or two (this is when the dash number gets mentioned) the fronts will become the first to lock.

Also consider: in most race situations we have a fragile front axle to protect. Compromise the front tire and our edge is good as gone. I do my utmost to not risk a flatspot on the front tires.

In the end we can only give you the concepts but you have to find out what works best FOR YOU through experience (i.e. killing tires). You need to drive the car at speed to find out--there's no other way to mimic the weight transfer and the dynamics affecting your contact patches.
Good news tho: once you get the concept and acquire the feel, you'll carry these skills throughout your career

trap125

Thank you all for your great replies. I now have a better understanding...