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Old 02-19-2016, 01:45 PM   #1
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brake bias

hi. got a question about brake bias. i'm thinking the boxster comes w excessive front bias out of the box (just as it comes with built-in understeer - designed for street driving). I run on a (relatively) technical track and think that more rear bias would keep my front tires from overheating, and help me rotate through corners better. I've read of folks running pagid yellows on the front and blacks on the back to get more rear braking. I've also read of folks that, on an 's' car, put base front calipers on the rear (direct bolt on, same diameter rotor, but the pistons are 40% larger).

anyone with experience playing with brake bias on these cars?

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Old 02-19-2016, 03:10 PM   #2
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If your front tires are overheating, it's probably because your car is understeering. Too much front brake bias shows up as the fronts locking up before the rears, not overheated front tires.

Trying more rear bias to get the car to rotate will likely introduce all sorts of unexpected consequences. If you want the rear to have less grip to "rotate" the car, there are a lot of better ways to address that than messing with the brake bias. Try a stiffer roll bar, or stiffer springs. Better yet, increase front grip with a softer front bar or springs. You can also try different alignment settings.

In my experience, screwing with something that isn't causing problems (like brake bias) to try and change other behaviors in a car breaks some of the fundamental rules of proper car preparation. If your brakes aren't giving you problems, then don't mess with 'em! Your brakes are the single most important system on your car. Believe me, dealing with handling issues is easier than dealing with braking issues!

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Old 02-19-2016, 03:16 PM   #3
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The first question is: What tires are you running F/R? Tire size affects brake bias as much as anything. Drivers running square figure out quickly that they can put their car on it's nose without too much effort. Once you have determined your tire size and compound, then choose brake compounds that make the most of your contact patch in braking zones. Running yellow (F) and Black (R) is one good way to move bias aft on a square setup. Another is to run rear pads on the front to even things out some. Always answer the tire question first.

Changing rear brake calipers and rotors or changing brake bias in the hydraulics is probably a last resort. I have not yet seen a 986 Boxster where this was necessary.
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Old 02-20-2016, 08:04 AM   #4
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thanks folks. i'm 235 front 265 rear but moving to a 245/275 setup this spring. i suppose increasing the front tire relative to the rear would have moved bias forward.

i feel the abs coming on in the front before the rear. this was happening before the tire change and think this is how it comes from the factory (similar to the inherent understeer that the car comes with in the name of street safety for the lowest common denominator driver - and something we actively tune out of the car for track use). if i am locking the fronts before the rears then on a technical track where i am braking hard and frequently then i would assume i am getting more heat into the fronts. so, i was wondering if any of you folks who see more boxster racing than i do up here are aware of folks playing w bias, either through different pads, different rotors/calipers, bias valves, etc.

what i am hearing is that it's really only done when needed to address the additional front bias caused by bigger front tires.
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Old 02-20-2016, 11:39 AM   #5
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TRK....any difference side to side? My front right would want to lock up ....likely due driver weight. Is there a safe side to side differential in pad compound that one can do?

I have completely melted front street tires at Shannoville (if you know it) presumably at a beginners pace; a very technical track. It seems be a common thing with the base set up of these cars. I did have a spaced rear track though. Tire pressures could have been high but wear indicators were showing good scrub
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Old 02-20-2016, 12:00 PM   #6
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Quote:
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TRK....any difference side to side? My front right would want to lock up ....likely due driver weight. Is there a safe side to side differential in pad compound that one can do?

I have completely melted front street tires at Shannoville (if you know it) presumably at a beginners pace; a very technical track. It seems be a common thing with the base set up of these cars. I did have a spaced rear track though. Tire pressures could have been high but wear indicators were showing good scrub
occasionally, but that i attribute to varying road surface. in your case i would consider a corner balance if possible?
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Old 02-20-2016, 02:21 PM   #7
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As Topless said, start with tires first.

Based on my experience, I think that you still have too much stagger between front and rear tire width. The stagger creates an understeer bias to a large extent. And even though you have went wider on the front for more grip, you are also increasing rear width/grip. The net effect is a only slight reduction in overall understeer.

And as RacerBoy said, understeer is probably what is over-heating the front tires. Overheated tires don't grip as well and then the ABS comes on earlier.

I would suggest increasing the front width to 245 as you have planned but keep the rears at 265. The reduction in stagger to 20mm might be all you need to keep the front end planted while loosening up the rear to help the car to rotate easier.

Another suggestion is to use driving techniques specific to helping the car to rotate. At some tracks, I will abruptly come off of the throttle and turn in sharply at the same time; this unloads the rear end and starts it sliding. Then I immediately get back onto the throttle once the car has started to rotate to stop the slide and continue through the turn. It takes some practice but it works really well.
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Old 02-20-2016, 03:22 PM   #8
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thanks for the tips. i guess the intent of this thread was to initiate discussion about brake bias as it applies to the boxster, as i've not seen much dialog on this topic here. i mentioned tire heating and rotation purely as discussion points as they might relate to brake bias.
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Old 02-20-2016, 04:10 PM   #9
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Seek out Bill Verburg on Pelican He would be THE Porsche brake guy in my opinion. His knowledge about brakes is nothing short of astounding.
My thoughts are if your fronts are triggering the ABS before the rears its a sign the front tires have less grip than they need for the applied brake force. and a larger front tire /less stagger is indicated as others have said above.
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Old 02-20-2016, 06:08 PM   #10
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base setup = front wheels loose traction first = abs kicks in on the fronts first.

how to make abs activate front and rear at the same time?

- increase front traction = wider tires = more grip = more stop before abs kicks in.
- increase rear clamping force = grabbier pads/taller rotors/bigger calipers/change the rear porportioning valve.

i think that things that detract from the performance of the car should not be considered: less front clamping force (ie, smaller front pads) less rear traction (narrower rear tires). further, folks that say "don't mess with brake bias" don't realise they are already doing it by changing tire width.

i guess the question that i'm asking is: on the boxster, what would be the effect of increasing rear braking? i would *think* that:

- less heat into the front tires because they are not working as hard to stop the car
- less dive on braking so flatter corner entry
- more pronounced rotation when trail braking

i am not asking to address any particular issue i am seeing at the track, i'm just curious.
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Old 02-21-2016, 02:55 AM   #11
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If you feel you have premature front lockup (ABS trigger point) Have you checked corner balance ? The 911,s pre ABS were front biased and a sure sign of a corner balance issue was premature front lockup one side first. Today an ABS equipped car would disquise the problem by triggering the ABS early because one front wheel was light
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Old 02-21-2016, 07:23 PM   #12
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Remember that due to driving dynamics, at least 70% of your braking power is always on the front tires because that is where weight transfer moves under braking. The last thing you want is to have the rear wheels go into ABS before your front tires have reached max braking power. That would greatly lengthen stopping distances. We want the front to go into ABS first for maximum braking but getting the rear closely balanced is a valid goal.

Try Pagid Yellow F/ Black R to create more clamping force on the rear as many Spec cars do. Reducing stagger by running 245/265 or 255/275 will add grip forward and also move braking bias forward. More grip up front also means more heat in the front brakes. No problem if you run the right pads and fluid.
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Old 02-22-2016, 05:49 AM   #13
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I remember reading this story a while back, about moving the front brakes to the rear and installing S brakes on the front in a base Boxster - in this instance it caused the rears to lock up a fraction of a second and activate the ABS before the fronts and it resulted in a spin:

Chronicles of Sebring, part 1 Technical Article by Pedro
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Old 02-22-2016, 06:40 AM   #14
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thanks folks. what I was looking for. I wonder if anyone has feedback on how the change to blacks on the back affected dynamics. I wonder if pedro had put wider tires on the back he might have avoided the rear lockup issue. I guess the one take home is that the margins are narrow when it comes to futzing w brakes.

I guess I have a decision to make on my rears when I put the new tires on this spring. my concern w a 245/265 ratio is that I don't want to loosen up the rear too much. regardless of what I chose, my next pads will be yellow front/black back and i'll report back on any changes I experience.
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Old 02-28-2016, 07:07 AM   #15
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Quote:
Originally Posted by The Radium King View Post

I guess I have a decision to make on my rears when I put the new tires on this spring. my concern w a 245/265 ratio is that I don't want to loosen up the rear too much. regardless of what I chose, my next pads will be yellow front/black back and i'll report back on any changes I experience.
A follow-up question for this thread: Has anyone tried running Pagid Yellows front and black rears with the ABS disconnected?

I would like to find a way to run with ABS disconnected at times, but there is way too much inherent front brake bias to make this work. I have tried disconnecting ABS with the same pad compound front and rear, which resulted in immediate front brake locking. I thought about trying to rig up a Tilton Prop valve, but have not tried that yet. Wondering if Yellow front / Black rear would do the trick.

Thanks for any info.

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