De-activated ABS hazard

xr4racer · 09-10-2008, 08:42 PM

Just for info - the XR4 (or any ABS equipped Fiesta) does not have pressure limiters fitted in the rear brake lines . This is because brake bias (front to rear) is left to the ABS to sort out rather than having balanced basic brake design. If the ABS were to be de-activated for any reason (even a blown fuse) the brake system will have far too much rear bias. This causes rear wheel lockup and the car will spin if you apply the brakes hard.

Mike
www.ttcracing.com

Kariharnett · 10-10-2008, 08:56 AM

is that cos of that ebd?

fairmont1998 · 10-10-2008, 10:26 AM

If the rear bias is left to the ABS module to control wouldn't you feel it through the pedal all the time? I've done plenty of hard stops in mine and never feel the pedal pulsing to control the rear brakes.

xr4racer · 10-10-2008, 10:53 AM

no. front and rear brakes are controlled in exactly the same way and EBD is really just the PCM adding overrides so that the system can work side to side in circumstances like having 2 wheels in the dirt and 2 on bitumen.

The the brake circuits are connected diagonally front and rear. This is so that in the event of hydraulic failure at least 1 front wheel still has braking. The theory behind this is that FWD hatches have so much front weight bias that if only the rear brakes worked you would take forever to pull up.

The pulsing that ABS systems you to give has been eliminated in current, high quality systems by using faster PCMs (more computing power).

Mike
www.ttcracing.com

Piotr · 10-10-2008, 12:48 PM

If your ABS is fault your shouldn't be braking hard. Im face is your have any sort of warning you should be driving carefully

Adsy01 · 10-10-2008, 04:39 PM

Piotr, i think Mike is maybe telling us he learnt the hard way while on a race track?

While we’re on the subject, does anyone actually feel the ABS pulsing on their fez's?

I've had several emergency stops in mine and I can’t feel anything through the brake pedal... on the same note, I’m unable to lock the wheels, so it is working lol

Kariharnett · 10-10-2008, 05:21 PM

i can feel mine alot and it makes a terrible thumping noise and i think ive said it bbefore on here somewhere if your over 110km/h and you slam the brakes on the hazzards start flashing automatically. im still intrigued (small things amuse small minds)

fairmont1998 · 10-10-2008, 08:48 PM

You can over ride that hazzard feature in your settings menu. Its annoying.

xr4racer · 10-10-2008, 08:50 PM

Thats a good party trick !

xr4racer · 10-10-2008, 08:55 PM

Yes. Learnt on racetrack. Just giving everyone a warning that if the ABS warning light comes on, you should drive carefully until it is fixed because the standard brake bias is quite unbalanced on the Fiesta.

xurbanx · 10-10-2008, 09:49 PM

Thanks for the heads up; there was only one time where I felt the brake pedal shudder after braking hard from 80km (slamming the brakes). Would it have been that>?

xr4racer · 10-10-2008, 11:10 PM

The ABS unit has a small 'accumulator' built into it and when the computer senses a lockup a solenoid opens and bleeds off some fluid into it to reduce the hydraulic pressure that is locking the brake. If the wheel unlocks and you dont just keep pushing the pedal harder the fluid will be released from the accumulator back into the brake line without you feeling any shudder.

If you just keep pushing the pedal harder, the accumulator gets too full and the excess pressure must be fed back to the Master Cylinder (the bit that the pedal pushes). This is the shudder you feel in the pedal.

I am sorry, but if this is happening to you, then your brake application is very unskilled and you would probably have an accident or at least a very severe brake lockup in a non-ABS equipped vehicle. Practice your braking in a non-ABS vehicle to become a better driver. Wind down the drivers window and you can hear the tyres growling (not squealing) when you achieve maximum braking effort, a little bit more pressure will lock the wheels. Learn to do this well, then add ABS and you willbe very competent at stopping your car.

Piotr · 13-10-2008, 12:03 PM

That is incorrect. If you are making an emergency stop and you feel the brake pedal pulsing the worst thing you can do is reduce brake pressure. Keep the same force or apply more to the pedal. In an emergency let the ABS do its job because it will do it a million time better than anyone.

xr4racer · 13-10-2008, 09:09 PM

Piotr,

re-read the post! nowhere have i said that you should reduce pedal pressure if you feel it pulsing due to abs.

Also, a skilled driver can outbrake ABS in many, but not all, situations . It just takes practice.

A driver that gets used to just mashing an ABS pedal to the floor is unskilled and a liability in a non-abs vehicle.

Piotr · 14-10-2008, 01:32 PM

Quote:

Originally Posted by xr4racer

Piotr,

re-read the post! nowhere have i said that you should reduce pedal pressure if you feel it pulsing due to abs.

Also, a skilled driver can outbrake ABS in many, but not all, situations . It just takes practice.

A driver that gets used to just mashing an ABS pedal to the floor is unskilled and a liability in a non-abs vehicle.

ABS is not designed to reduce braking distance

Kariharnett · 14-10-2008, 02:46 PM

Does the ABS on the fiesta lx 5 door work on the back wheels?
or does it not cos of the drum brakes?

xr4racer · 14-10-2008, 10:56 PM

Yes it does.