61 replies to this topic

[JohnTurbo]

Road brakes are always biased to the front as locking rears is more dangerous. Ask SteveA. Im of the opinion that that the abs unit does very little when inactive. Im also unconvinced about this rear bias thing in an emergency. Sounds bloody stupid and ive seen no documents supporting its existence. Anyone? Id keep the servo if it was me, but thats just a personal choice as ivenhad cars with no beake servo ans wasnt a fan.

[siztenboots]

I recall my friends Fords had a compensator valve in the rear lines, connected to trailing arm

[LY_Scott]

I recall my friends Fords had a compensator valve in the rear lines, connected to trailing arm

GM did this too. put more bias to the rear if there was weight there (passengers). My old corsa had it and it would stick as they all had a reputation for. Not fun .

[Firthy]

Road brakes are always biased to the front as locking rears is more dangerous. Ask SteveA.

Im of the opinion that that the abs unit does very little when inactive. Im also unconvinced about this rear bias thing in an emergency. Sounds bloody stupid and ive seen no documents supporting its existence. Anyone?

Id keep the servo if it was me, but thats just a personal choice as ivenhad cars with no beake servo ans wasnt a fan.

Have you driven a car with the 4 channel setup? The initial bite (for me) is on another level entirely.
Now either I need my calipers refurbed or there's something going on with that ABS unit. Me and my friend have the same pads RS14's both new... the way his car stops compared to mine is on a different level.

I'm of the opinion that that the abs unit does very little when inactive - Bargi's link is around here somewhere but when you read it makes a lot of sense...

Edited by Firthy, 25 April 2013 - 10:47 AM.

[Mangham54]

I recall my friends Fords had a compensator valve in the rear lines, connected to trailing arm

GM did this too. put more bias to the rear if there was weight there (passengers). My old corsa had it and it would stick as they all had a reputation for. Not fun .

The 2008 Focus I have goes from decent, to unbelievable by having some weight on the rear axle. The difference cannot be down to grip, as I never lock up the brakes and the handbrake is pretty much non-existent.

[JohnTurbo]

Ive messed with brakes a lot. I did have a four channel unit...it was crap compared to the 2010 and 2008 cars ive owned but it stopped brakes locking so was good on the road. Im nearly certain my old ds3 would outbrake a stock vx with 4 channel abs. Im upgrading brakes even further soon.

[Firthy]

Ive messed with brakes a lot. I did have a four channel unit...it was crap compared to the 2010 and 2008 cars ive owned but it stopped brakes locking so was good on the road.

Im nearly certain my old ds3 would outbrake a stock vx with 4 channel abs.

Im upgrading brakes even further soon.

Ive messed with brakes a lot. I did have a four channel unit...it was crap compared to the 2010 and 2008 cars ive owned but it stopped brakes locking so was good on the road.

Very Interesting John..... correct me if I am wrong but my understanding is the only difference between the VX and the 2010 and 2008 cars is the ABS brain.....

Bargi has done some experiments where he has taken both hub sensors front and rear off the car which completely stops the ABS interfering with the rear bias and gives better feel. I have not tried this yet but no doubt will soon!

Do you have any ideas why our brakes seem to be worse than the other cars you have driven.... for me I keep coming back to that ABS module... given the rest of the hardware is the same.

Firthy

[Bargi]

It's real John, but it's random as to which cars it'll affect but also down to the driver and how quick to go from accelerator to brake. I had a theory it was related even to the high rake I had that was triggering it more. But I've since had new dampers and ride height adjusted and no different. First lap out at Goodwood it kicked in straight away so took it in and unplugged F/L and R/R ABS sensors and it was fine. "The symptoms being described are a result of the Electronic Brake Distribution (EBD) system operating. This system is also referred to as Dynamic Rear Proportioning (DRP) and is, as the name implies an electronic system which, through the ABS control valve block restricts the line pressure to the rear brakes automatically to a pre-programmed algorithm. You can consider it as an electronically controlled proportioning valve which measures parameters like the rate of deceleration and rate of pedal application and uses this data to anticipate a rear wheel lock-up and then reduces the braking effort at the rear wheels as necessary. If the ABS system is left to do this, it can only react to a wheel as it starts to lock and therefore the car can already start to spin before the ABS can start to work. In extreme circumstances, if the driver brakes very suddenly the EBD system can lock off the pressure to the rear wheels completely; what pressure was at the rear brakes as the EBD system engaged remains there and the rear brakes are still working as a result, but further increases in pedal effort will not increase the braking at the rear of the car because the pressure to the rear brakes cannot increase. When this happens the brake pedal goes hard, as it is now pushing against the front callipers and a closed valve only, instead of against the front and rear callipers. The rear callipers are single piston and therefore quite flexible, so they are a major factor in making the brake pedal feel 'soft'. When the valve closes, the brake pedal pressure no longer flexes the rear callipers, hence the increase in pedal hardness. The front brakes are still working just as well as before the valve closed and will give more braking if the pedal effort is increased, while with the rear brakes working as hard as they can the braking is NOT affected. The problem is the driver feels like braking is reduced (even though it is not) because of the change in pedal feel. If the driver continues to push hard on the pedal, the car will continue to slow as fast as it possibly can in the circumstances. If he increases the pedal effort the front braking effort will increase and the rear effort will remain where it was. If he was to back off the pedal for a fraction of a second, the valve will reopen and the rear brakes will operate as normal again, with the pedal feel going back to normal. In the case of releasing and re-engaging the pedal the car should not be able to slow any faster than it was with the system engaged unless either 1: the driver triggered the system in the first place by stamping on the pedal too fast or 2: the system triggered because a rear wheel was unloaded when the brakes were applied and would have locked up but is now fully loaded once again and able to sustain a greater braking torque. If the rate of deceleration does improve when the pedal is reapplied then it is telling the driver that he is over braking either in terms of the ultimate ability of the brakes (cause 1 above) or the track condition (cause 2 above) and needs to adjust his driving style to suit. If the system were not fitted or disabled and he continued to drive that way he would be in danger of spinning when applying the brakes. The suggestion that the system is running out of vacuum is just plain wrong. The system carries an internal reservoir of vacuum sufficient for three full brake applications. As with every servo system ever fitted to a car there is a one way valve which prevents the vacuum being lost when the car is on boost. The only way this reserve can be depleted is if the driver is maintaining boost while applying full brakes, i.e.: left foot braking very badly. In this instance I would argue that depleting the vacuum is probably a good thing as it should provide him with a warning that he is doing something awful to the car and it may reduce the speed of impact when he finally hits something as the brakes fade to nothing!! In normal use the throttle is closed when the brakes are applied, there is therefore no boost and the vacuum is automatically replenished as it is used."

[JimmyJamJerusalem]

Great post raymondo!! Cadence braking FTW then lol. I wonder then without the abs unit connected (ie no DRP), 4 pots up front and standard rears I wonder if they'll work we'll...? Might even be able to remove the servo completely then!!

[Mike (Cliffie)]

I have an adjustable bias valve so I can limit effect from the rear. Having 16 pistons instead of the standard 6 makes for longer pedal travel but when they kick in boy oh boy are they good. Had no feel at all withot the servo though. Should getter a bigger master cylinder I guess but it looks like too much work TBH.

[JohnTurbo]

Not a massive job! Mostly its just turning up a plunger adaptor. I suspect the processors in modern abs units are an order of magnitude higher than that aged lump in the VX. The sensors will be sampled much much faster. They also have stability control as standard now. Hmm. Sounds like bollocks even if it is from Lotus. I'm not even convinced the system knows what a rear wheel is in terms of sensing - consequently it would be a guess at best when this value would be actuated. The idea is also pretty ludicrous - that a system designed to stop wheels locking would reduce braking to the rear to stop it locking?! - Hardly fullfilling its primary purpose is it? I suppose the idea is that under heavy braking you unload the rear axle and perhaps cant exert as much braking effort - but the VX is hardly a soft barge is it? In any case its been years since my ABS system has been both 2 channel and active so I don't really have any experience of the hard pedal.

[JG]

Great post Ray (even better than the first time you posted it ) There is one problem though. I suffered terrible hard pedal syndrome. I changed quite a lot so i'll never know what cured it, but i've not been able to replicate it again with: -4 pots -2012 Exige abs unit -new discs -new pads -new servo hose.

[CHILL Gone DUTCH]

Great post Ray (even better than the first time you posted it :P/>)

There is one problem though. I suffered terrible hard pedal syndrome. I changed quite a lot so i'll never know what cured it, but i've not been able to replicate it again with:
-4 pots
-2012 Exige abs unit
-new discs
-new pads
-new servo hose.

I solved my hard brake syndrome buy turning ABS off

[JohnTurbo]

The stock ABS being shite is not something many would argue with anyway!

[Bargi]

Great post Ray (even better than the first time you posted it :P/>)

There is one problem though. I suffered terrible hard pedal syndrome. I changed quite a lot so i'll never know what cured it, but i've not been able to replicate it again with:
-4 pots
-2012 Exige abs unit
-new discs
-new pads
-new servo hose.

Not a problem at all, I've done all bar Exige abs and 4 Pots. I'm trying to avoid 4 pots like the plague so tell you what, come to Abingdon and let me try out the ABS to confirm.

[JG]

I avoid MSE like the plague so won't go within 10miles of them. Any other TD though

[Bargi]

That's fine, I didn't think you would, I just want to try out the ABS unit Snett 5th?

[slindborg]

I have an adjustable bias valve so I can limit effect from the rear.

Having 16 pistons instead of the standard 6 makes for longer pedal travel but when they kick in boy oh boy are they good.

Had no feel at all withot the servo though.

Should getter a bigger master cylinder I guess but it looks like too much work TBH.

Technically, smaller master cylinder but needs longer travel.
Bigger will move fluid but be at a lower pressure for a given peal position, so in a similar boat to now.

[Gedi]

Not a problem at all, I've done all bar Exige abs and 4 Pots. I'm trying to avoid 4 pots like the plague so tell you what, come to Abingdon and let me try out the ABS to confirm.

IMO, the 4 channel ABS is probably the best mod you can do on a VX

[Bargi]

Actually I'd like to see what the CF setup is like and how the rear ARB creates a rip in the time continuum as well as the pump access hole...