981 Brake Feel

[ralphmusic]

This is a question regarding 981, rather than 987 where it has been discussed at length.

The brake pedal gas always seemed mushy on first press but once through this it is easy to modulate the pressure.

I have recently upgraded my fronts to 991S 6 piston to have brakes that cope better with on track use. Unfortunately the effect of the mush and extra clamping force made for interesting brake locking in the wet at Goodwood today. OK so I quickly learnt to avoid locking but the mush remains and I wonder if anyone has actual 981 experience of improving/eliminating this mushy feel.

 

[Motorhead]

Ralph, I presume that you've been through the brake fluid change and full brake bleed routine to try to eliminate the problem, without success. Maybe there's been a fault with the master cylinder or the ABS module from new. You may be able to get your PC to swop out the master cylinder under warranty but having them replace the ABS module FOC might prove more challenging. Jeff

 

[ralphmusic]

Jeff, I have been through the pedal feel issue with the standard disks and I think it is one of the "there're all like that Sir" and actually is - others I've driven seem the same - which has been further challenged by the stronger callipers. I have reached out to some guys in the USA and Germany who race/track this combination to see if they have seen the same behaviour and whether there are are any system elements that they have improved upon before starting from scratch with the basics. I also have to consider the contribution of "wet and Pirelli" (5mm tread depth) to the problem as rear end grip was not particularly strong either. Mind you (it was a Lotus on Track day) the Lotus cars were really slow at the back of the circuit as they had less rain suitable tyres. The one exception was a BMW 3 Series track car running used Clio Cup rain tyres and he went around as though it was dry. Ralph

 

[Motorhead]

Yes Ralph, if it's an endemic problem with the series then it sounds as though you could be in for a long haul swopping out individual components. I presume that the 991 suffers from the same characteristic. It'll be interesting to see how the GT4 behaves in this respect with the GT3 brake set-up. Jeff

 

[tscaptain]

I presume by "brake locking" you mean the ABS kicking in, Ralph? Funny thing but when I drove a 981 I thought the pedal felt firm compared to mine and even had the PC check my brakes to make sure my pedal wasn't "mushy"!

 

[ralphmusic]

ORIGINAL: tscaptain I presume by "brake locking" you mean the ABS kicking in, Ralph?

Alan, I have been taught to build brake pressure rapidly aiming to get to the ABS threshold without actually tripping ABS into action and ease off into corner. It is quite difficult to find this through the mush. Moreover, the locked time (a second?) was/seemed more than I would expect from ABS. So is it ABS? could be but I doubt it. I was braking from around 115/120 before Woodcote which has claimed some late brakers in the wet, notably a lady in a Caterham at a Spring sprint a couple of years ago. The more I consider it, I am drawn to wet, Pirelli tyres, mush and me being too rapid on the pedal for all the elements on the day. So at present I am aiming first to see what those with track experience with larger brakes have done (anti mush), then dump the Pirellis, check fluid bleeding and other basics including having the ABS module tested before I use the car next month.

 

[Mark_Bennett]

Is this with PSM on? PSM uses the brakes to help "stabilise" the car under dynamic driving forces. Maybe it's interfering (especially on a wet track) and giving you the strange feedback you are experiencing?

 

[ralphmusic]

I am not brave enough to turn off PSM even in the dry... this was all in a straight line and the car was (naturally) stable throughout so good thought but unlikely.

 

[fbr]

Ralph Generally I have no problems with the 981 braking systems when using it all year round in the frozen North. I think what you are describing is the behaviour of a modern ABS/PSM system designed for road use. Look at this [link=https://vimeo.com/88706930]video link to one of two ABS seminars[/link] that describes the limitations of road going ABS systems used under track conditions, with some hints and tips on how to avoid the inevitable problems of tracking a modern road going sports car. I think I gave you this link in a previous answer to one of your brake related questions. In my 2yrs experience, albeit not track related, avoid instantaneously offloading the accelerator combined with a "gentle" activate the brake pedal, is the best way to get them to consistently stop/slow down. What you are aiming to do is avoid initiating the enhanced readiness of the braking system that pre-fills the system, which increases the pressure and gives the standard "rapid accelerator off/full brake on" leading to the standard 1g road-going emergency stop that the system is designed to deliver. You could of course go the full race designed ABS system - but then you would no longer have a road going car. I'm always a little surprised that folks don't seem to appreciate that they've bought a mass produced, full on road going car that you can occasionally race round an aerodrome . Frank PS whenever I get near a track I'm usually going slowly enough to hop out and take a photo whilst overtaking the opposition! You must have some pictures of you heroic on-track action.

 

[tscaptain]

With ABS I wouldn't expect the brakes to lock at all - the clue is in the name! I have tried a full emergency stop, on a wet road, with no locking, just a lot of juddering under my foot! A second of wheel(s) not rotating, which is what you suggest, seems either unlikely or there is a fault?

 

[fbr]

ORIGINAL: tscaptain With ABS I wouldn't expect the brakes to lock at all - the clue is in the name! I have tried a full emergency stop, on a wet road, with no locking, just a lot of juddering under my foot! A second of wheel(s) not rotating, which is what you suggest, seems either unlikely or there is a fault?

Under certain road conditions it's quite easy to lock up the fronts. Try any loose surface and they are quite common round here at this time of the year.

 

[Motorhead]

Ralph, Just to quote Porsche on PSM: PSM remains set to intervene if the vehicle is braked and ABS assistance is required. ABS and ABD remain switched on at all times. Also included with PSM are engine drag torque control (EDC), precharging of the brake system and brake assist. If you suddenly release the accelerator pedal, PSM automatically prepares for your next action: the braking system is precharged so that the brake pads are already in light contact with the brake discs. Maximum braking power is therefore achieved much sooner. Brake assist detects a panic braking situation and generates the brake pressure required for maximum deceleration. So with PSM switched on you're most likely to be in panic brake mode when on track, which means that a lot's going on between your foot and the brake caliper itself. It might be worthwhile experimenting turning PSM off on a clear straight in the dry [and with no-one behind you.!] when you're next on track to see if the problem persists. Jeff

 

[MrDemon]

fit RS 29 pads and pilot super sports/cup2's I only drive PSM off and it's never off on a normal non GT car ! I see the GT4 has 3 modes on PSM on, on with TC off and off. or somting like that. I'll see what the oem pads are like and fit RS29's if no bite. GT cars normally have a non branded pagid yellow pad though as standard.

 

[ralphmusic]

PSM remains set to intervene if the vehicle is braked and ABS assistance is required. ABS and ABD remain switched on at all times. Also included with PSM are engine drag torque control (EDC), precharging of the brake system and brake assist. If you suddenly release the accelerator pedal, PSM automatically prepares for your next action: the braking system is precharged so that the brake pads are already in light contact with the brake discs. Maximum braking power is therefore achieved much sooner. Brake assist detects a panic braking situation and generates the brake pressure required for maximum deceleration. OMG! I shall start agreeing with Mr Demon soon about computer assistance....

 

[tscaptain]

ORIGINAL: fbr

ORIGINAL: tscaptain With ABS I wouldn't expect the brakes to lock at all - the clue is in the name! I have tried a full emergency stop, on a wet road, with no locking, just a lot of juddering under my foot! A second of wheel(s) not rotating, which is what you suggest, seems either unlikely or there is a fault?

Under certain road conditions it's quite easy to lock up the fronts. Try any loose surface and they are quite common round here at this time of the year.

Agreed, but I didn't think we were talking about gravel, sand or deep snow here - unless the circuit at Goodwood has gone significantly downhill since I was last there![] Oddly, for Wiki whatsit, this piece seems quite reasonable on the subject http://en.wikipedia.org/wiki/Anti-lock_braking_system

 

[fbr]

ORIGINAL: tscaptain Agreed, but I didn't think we were talking about gravel, sand or deep snow here - unless the circuit at Goodwood has gone significantly downhill since I was last there![] Oddly, for Wiki whatsit, this piece seems quite reasonable on the subject http://en.wikipedia.org/wiki/Anti-lock_braking_system

But when it rains you can get loose material brought up or onto the road/track, and doesn't take much of this type of material to induce wheel locking. Quite small amounts of fine loose material will produce the desired effect. Thanks for the Wiki link, surprising what's available on the Interweb to enhance one's education . Frank