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4 Channel Abs Anyone?


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#21 Ivor

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Posted 29 April 2022 - 06:19 AM

On the topic of removing the ABS;
I once had an Audi S3 (1st gen) that had a disturbingly soft brake pedal and after a long campaign of remove and replace/rebuild for literally every part of the braking system I was down to the ABS unit. Those ATE/Teves MK20 units essentially went bad after a few years and had fluid bypassing issues so your brake pedal pressure was getting taken up like a sponge before putting pressure to the calipers. A new unit was a £800 proposition and most were new old stock so I was not keen just to splurge without testing.

What I did was to get an aluminium block and drill passages into it.
I put in two 'master passages' and tapped them for the two feeds from the master cylinder. Into those passages I tapped two more holes (front left + right rear and front right + left rear, respectively) and that served as a split circuit.
That then allowed me to test the theory of the bad ABS unit by seeing what a 'direct' brake would feel like.
The downside was that the pressure to the rears was not limited and the rears locked really easily. This did not trouble me as it was just a test of my sanity vs. the ABS unit, but to make it usable I would have added pressure limiting valves to the rear brake lines - in my case there was probably a Audi/VW/Skoda valve available from a lower model (ABS was still an option in a lot of the cars from that period).

For your VX i'm not sure how you would do it for the rears - normally the movement of the trailing arm/5th link arrangement in a regular car and a little linkage actuate such a valve. There may be pressure limiting valves you can buy and then adjust, so you could go and do some testing on a private road and set it up to your tastes.
The only other option I could see would be a heavily imbalanced brake set up - 4 pot fronts on big discs on sticky pads and standard at the rear, or something like that.
I can't commend either, though.

After that it would be a dual master cylinder system and dial in your pedal bias, like a race car. I'm sure someone will have done that. If not on a VX then on Lotus.



I did get a secondhand low mileage TT 4wd unit for £65 and it was better but not great. I bought a few and chopped them up but never found a viable rebuild solution and I see the Audi/VW community still struggles with this issue.
They sold replacement valve blocks for the later MK70 Teves unit at a fair price, but I don't think anyone has put a MK70 in to any of those cars (delighted to be found wrong though).

Those of you who know that VAG cars generation, especially the 4wd models, know removing the ABS will light up the dash like a Texas bubblegum machine so no ABS was never a solution, plus the ESP and 4wd needed all sorts of feedback from the wider system so you'd lose valuable functionality.

Cliffe in his track car had a brake balance knob in the cockpit to adjust front rear brake balance, I'm guessing if you had such a valve you could find the sweet spot wrt F/R balance and fix it at that setting?

#22 Exmantaa

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Posted 29 April 2022 - 08:41 AM

I am just going to disconnect 3 of the sensors at the wheels after the MOT.  Speaking of which, 24 August is the earliest I can get as it has been sorn'd for a while - but I am on the cancellation list.  In NI we have to go to official MOT test centres and they are booked up for months after being closed for ages due to covid.  I still need to get new tyres and fix the bleed problem with the brakes as I can't get these working properly, oh and a geo set up.  But, at least it is on its wheels and running and moving now and with loads of new parts.  Just needs fettling ... 

 

Our rudimentary ABS also functions as a dynamic rear proportioning valve, so be careful when you also disconnect a rear sensor.  :dry:

I had loads of weird brake issues in my former track Speedster where I also disconnected a rear sensor to totally disable all ABS / proportioning functions, but in the end disconnecting only 1 front sensor worked best.

Also with a 4 channel ABS on track...  



#23 Ivor

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Posted 29 April 2022 - 10:25 AM


I am just going to disconnect 3 of the sensors at the wheels after the MOT. Speaking of which, 24 August is the earliest I can get as it has been sorn'd for a while - but I am on the cancellation list. In NI we have to go to official MOT test centres and they are booked up for months after being closed for ages due to covid. I still need to get new tyres and fix the bleed problem with the brakes as I can't get these working properly, oh and a geo set up. But, at least it is on its wheels and running and moving now and with loads of new parts. Just needs fettling ...


Our rudimentary ABS also functions as a dynamic rear proportioning valve, so be careful when you also disconnect a rear sensor. :dry:
I had loads of weird brake issues in my former track Speedster where I also disconnected a rear sensor to totally disable all ABS / proportioning functions, but in the end disconnecting only 1 front sensor worked best.
Also with a 4 channel ABS on track...
I was advised to disconnect one front wheel, worked ok for me, I've now cut the yellow wire to the abs unit to save messing with the Connectors at the wheel




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