65 replies to this topic

[JG]

I'd say spill the beans Ray

[Tim_B]

What would you say if I told you it's none of the above?

I would be intrigued...

Fluid only just changed to ate super blue, doubt its that.
Will check vaccuum hose too, is this the one from servo to master cylinder?

[Adam_B]

I bet 12p and a cushion that unplugging the abs will sort it.

[Bargi]

I'd say spill the beans Ray

I could tell you, but you wont like the answer I give you

[dw1]

If the abs wasn't kicking in, then there is nO need to disconnect it

Slindborg and JG - you guys are a million times more mechanical and knowledgable in VX stuff than me, however I got a similar issue and no abs kicking in. From the moment I cut the abs it was cured so whilst you could well be right on the whole there may be exceptions here. So I say cut it and then see how you get on, you don't really lose anything by doing that and stand to gain.

My brakes are cracking (except when I had the CL flakey pads).

Edited by dw1, 19 June 2012 - 09:58 PM.

[Bargi]

I was going to post this up in a separate thread but this one is a good one as it's already covered the usual suspects.
Hard Pedal on track is what I'm talking about is not your regular "Ice Mode".

For the past year I've been getting the hard pedal on track under breaking. Most of the time following hard acceleration.
I found
- After long straights at high speed (maxing 4th/5th) even just lightly braking on he hard pedal the fronts would lock up
- After smaller straights (maxing 3rd) there would be reduced braking for a bit and then pedal would drop or needed to reapply (ie: within the braking limits of the tyres)
- It only happens going quickly from the accelerator to the brake
- It never happens on the road or motorway even doing big speeds
- It happens with or without ABS disabled.
- It can be eliminated completely by coming off the accelerator and coasting for 1.5-2seconds before

I fitted new pads, new discs, new fluid, new one way valves and no difference.
Had the system vacuum tested and no leaks and held normal vacuum.

As others mentioned it all pointed to loss of vacuum with lots of hard braking followed by lots of WOT and not allowing the booster to recover enough.
It seemed it couldn't be anything else, especially coming off the accelerator and coasting fixed.

Before going to Spa the other month I installed a Hela vacuum pump (OEM in Volvo, Audi) with a vacuum switch to kick in after one application of the brakes.
Made no difference whatsoever.
I then adjusted the vacuum higher and higher until the pump was running constantly... no difference

Not to be defeated I scoured the Net to see if our Lotus companions had come across this problem, turns out yes! (In fact turns out a certain German brands track oriented cars also suffer from it)

Turns out the issue is with the ABS unit, but not the ABS functionality.
The ABS unit has a second function as an electronic brake proportioning valve also called DRP (Dynamic Rear Proportioning).

Here's the official Lotus explanation of this braking issue:

"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."


Essentially the DRP thinks the rear wheels are going to lock up so locks out the rear brakes (hard pedal) and leaves the fronts with almost all the braking (reduced braking/locking up at high speeds)

So now I knew the cause I needed to find how to fix it, again after days of Googling and reading through masses of forum threads I came across what is the only post I could find that mentions how to turn DRP off
HERE
You just need to disconnect a front and a rear abs sensor…

ABS will disable with one sensor gone, but DRP can still function as long as there’s a front and rear sensor operating.

So Cadwell Park last Saturday, first session with a single ABS wheel sensor off and as expected hard pedal at the end of Park Straight and Mansfield.
Come in and remove an ABS sensor at both ends of the car, go back out on track and come barrelling into Park Straight … hallelujah!! no hard pedal! Completely gone!

For the nerds among us I think this is the patent describing how it works here

Final thoughts (first two common knowledge)
- It seems worse on the 2 channel ABS with 16/17 wheel combo. (possibly down to the wheel size difference)
- It seems it's not as bad with the 4 Channel (perhaps there’s some improvement to the DRP code as well as the ABS?)
- It only seems to pretty much only affect NA/SCs, I haven’t read/heard too many Tubby owners with the hard pedal problem once ABS/single sensor is disabled.
- Having said the above there a lots of people with NA/SCs that track their cars and don’t get the problem (perhaps this is down to driving style?)

A final, final thought as to why it happens to some and not others is the Rake of the car. I noticed at Spa I could barely get my low entry jack under DW1 and Wrightstuffs cars yet I have about4-5cm gap. Perhaps with a higher the tail and centre of gravity, under braking the rear goes much lighter which causes the DRP to kick in? (not being able to make heads or tails of the Patent details I’m not sure how it would even measure this happening)

Hopefully this helps other people who are experiencing the problem and please let me know.

Whatever the reason it's worse on mine, I'm just glad it's sorted an I can brake quickly without fear of going into the arse of the car in front!

Edited by Bargi, 19 June 2012 - 11:45 PM.

[Goosenka]

Fascinating stuff Ray. Well done for finding the details out

[dw1]

Ray, you legend! So glad you got it sorted Perhaps the weight of the turbo engine keeps that tiny bit more weigh distribution over the rear and reduces the chances of it triggering.

[siztenboots]

If you rapidly switch from 100% WOT , to brake pedal, there must still be a lot of torque in the gearbox from the engine / flywheel that the rear brake pads have to overcome. I think its a bit of driving style how you approach a corner and change down gears, I tend to change gear at the apex where I can use the weight shift to alter the cars balance.

[slindborg]

I think it could also be a software thing in that my very very very very early N/A didn't have any issues with hard pedal but others with later cars do etc... I believe there was a software update thing on the rework list for the ABS, perhaps the update caused this issue but solved others?

[MWS]

.

Edited by Matt_S86, 20 June 2012 - 08:20 AM.

[Bargi]

I think it could also be a software thing in that my very very very very early N/A didn't have any issues with hard pedal but others with later cars do etc...
I believe there was a software update thing on the rework list for the ABS, perhaps the update caused this issue but solved others?

X REG No. 60

[slindborg]

earlier than mine was then

[Craig S]

So everyone needs 2 ABS cut off switches

[Tim_B]

Excellent post, thanks!

[mbes2]

does pulling the sensor plugs from front & rear wheels also disables the ABS, or does that work fine still.? Also, does the ABS warning light come on ?

[MWS]

does pulling the sensor plugs from front & rear wheels also disables the ABS, or does that work fine still.?

Also, does the ABS warning light come on ?

Yes, when you pull the sensor plugs, the light comes on and it disables the ABS.

I have to have mine permanently unplugged because my control unit is fooked!

[mbes2]

or the real options are 1) press harder (while screaming) 2) fit ABS cut off switch 3) unplug sensors (same outcome as above) 4) upgrade to 4 channel ABS unit (which has newer code, which also has mixed reviews) 5) Brake earlier for bends 6) Buy a MX5

Edited by mbes2, 20 June 2012 - 09:00 AM.

[techieboy]

Remember, if you're pulling/interrupting a rear sensor, only do the drivers/offside sensor. Unless you want to lose the speedo and make the engine run roughly.

[siztenboots]

interesting read on the effect suspension has on brake distance, and how an active control antidive system performs http://scialert.net/...11.28.41&org=11