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Old 8th December 2008, 16:26   #1
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Default Tata safari rear brake release Valve(non ABS), disaster by design?

My safari was suffering from a unique problem,
Hard braking, even at 70kmph would mean the car fishtailing, and parking herself 90 degrees to the direction of travel.
With a high GC vehicle, this meant that if I ever braked at 100kmph, It would go sideways and then flip. Not a happy though.
Though the common consensus was that this will happen in any happy SUV, I was not convinced, and I demonstrated the skid to to TASC engineer, who was also very surprised.
According to him, slight fishtailing tendency is there, but with steering control it should not happen.
On inspection they found the culprit, pressure release valve.
For the rear brakes there is a pressure release valve, which releases pressure when it senses rear wheel lockup. Now this valve is not perfect, and works on vehicle movement detection, but it should do its job to a reasonable level.
In this case, the adjustment was totally off. Apparently PDI should have taken care of it, but they did not care.
After adjustment its much better, and at 70kmph atleast I am able to control the fishtailing, but still its a big concern, esp since I have not been able to understand whats the principal of operation of this pressure release valve.
In emergency braking conditions, which are not rare on highways, what should I do?
Should I slam the brakes so hard that front also locks up, and not just the rear, so that the skid is straight?
This means no more threshold braking like I do on indica(its become a reflex action now).

Other safari/Scorpio non ABS owners, have you tried hard braking at high speeds? If you have not, please try on empty roads at around 70-80kmph, and if you feel tail stepping out, release the brakes.


As of now, skids in the safari are straight line(70kmph), but I do not have the courage to try at higher speeds.
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Old 8th December 2008, 16:39   #2
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hey tsk,

i've not had such serious problems, even when braking(never slammed the braked just a bit of hard braking) at around 100kph or so... there is a bit of fishtailing , but manageable . Just tried to keep the vehicle in straight line thats all & it would come down with a bit of screeching when nearing halt!!
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Old 8th December 2008, 16:46   #3
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Front-rear Brake bias setting is also a reason for rear fishtailing at high speed braking.
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Old 8th December 2008, 17:42   #4
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I remember this happening to me in 2007 Nov when going to Goa from Hyd

I was at around 95-100kmph and had to break with full pressure (Auto came in wrong lane right in front) and my bull moved from back at appx. 45-60 degrees. But not sure if it ever happened again,

It has been very good so far and no fishtailing except that confidence levels at hard breaking are less, but so as with my friend's verna and another friend's Maruti Wagon R
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Old 8th December 2008, 18:28   #5
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I would say imagine a pressure release valve as a "Compromised rear wheel ABS".

Although I am not sure of the specifics used by TATA but lets assume a rear wheel is about to lock up due to extreme braking. The ECU module monitoring the lockup detects the onset of a skid through the sensor in this wheel and reacts by releasing the brake pressure slightly by rapidly opening a pressure release valve. If this corrects the locked wheel, the hydraulic motor will build up the pressure again to the optimum braking force.

Now going again purely by ABS architecture and assuming a partial application of these in Safari, Two types of system can be used, Three channel systems & Four channel systems. Three channel systems control the rear wheels together as a single unit. Four channel systems control the brake force on each wheel independently.

In three channel systems, although both rear wheels are monitored by sensors, if wheel lock is detected on a single wheel the hydraulic braking pressure is reduced equally on both wheels, therefore it would be safe to say that the three channel versions are a compromise and are usually fitted to save on the costs.

Let me re-iterate, I dont know whats being used in Safari, am just applying the ABS principle to the Safari PRV.

Last edited by dadu : 8th December 2008 at 18:30.
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Old 8th December 2008, 18:30   #6
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@tsk: pressure release valve??? this is an interesting thing, do share more details/pics, if you have. Brake bias as nitrous pointed i know off, and its preset and doesnt need/have an adjustment as far as i know in regular production cars.

Last edited by Jaggu : 8th December 2008 at 18:32.
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Old 8th December 2008, 19:01   #7
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Question Front wheel or rear wheel drive?

Most most stupid question to ask but now that i've on my mind i might as well blurt it out: Is safari 2 WD a front wheel or a rear wheel drive vehicle?
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Old 8th December 2008, 20:13   #8
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Hi tsk1979
I dad a very similar experience on my way back from Chandigarh.
Was driving at 110 and overtaking a truck, which suddenly decided to overtake the one ahed of it. This is very good but a little narrower than other 4 lane highways (Nasirabad- Chittorgadh). On applying brakes the back skidded towards the truck. Somehow the reflexes saved us and the trucker was as much surprised as I was. not a very pleasent situation to be in.
On reporting this to the TASS I was told that its probably the wheel alignment that needs to be checked. Will be going to the SC tomorrow to sought out a few niggles: AC fuse 25Amp blew off twice (have 4 in stock now), The driver side power window is too sluggish and often has to be pushed up the other hand, The katack sound on full turn. All these are the predictible issues as all of us know.
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Old 8th December 2008, 20:29   #9
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Quote:
Originally Posted by abk View Post
Most most stupid question to ask but now that i've on my mind i might as well blurt it out: Is safari 2 WD a front wheel or a rear wheel drive vehicle?
No question is stupid...its RWD.
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Old 8th December 2008, 22:21   #10
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Tsk1979,

Its mentioned in the mannual.

The valve is actually Load Sensing pressure valve. It regulates the pressure of brake fluid to the rear wheels as the load increases or decreases. There is no electronic thing associated with it. It has a spring kinda stuff attached to it (or something whatever it is i cant recollect rightnow) which according to the load regulates the brake fluid going to the rear wheels. In my yesteryear 3.0 dicor, though not fishtailing of voilent types, the rear wheels used to lock up on sudden brakes @50-60kmph when alone in it.

So in one of the services, they bled the rear lines to be sure for any kinda kachra stuff in the lines and adjusted the LSPV (load sensing pressure valve.)

My suspect is that u should also get ur rear brakes adjusted apart from LSPV setting.

Hope this information helps

Last edited by Rahulk76 : 8th December 2008 at 22:22.
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Old 8th December 2008, 23:04   #11
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@Dadu: Nothing as complicated and no sensor but a spring loaded valve on the rear brake circuit. It is a mechanical system that has been around since ages. I saw the working and settings even on dead rear axel of Bajaj tempo in 1980's. As you load more weight on the rear axle more force is allowed for transfer to rear brakes.

@tsk: As the TASS said proportioning was off and needed readjusting. Probable you need to reduce further the pressure in rear circuit. You can also (wallet permitting) look at slightly larger pattern tyres for the rear to increase surface area and avoid wheel lok ups.
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Old 9th December 2008, 01:07   #12
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If instead of threshold kind of braking, I start slamming pedal full force in emergency situations, that will help? I thought of this because it means front wheels will also lock up
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Old 9th December 2008, 07:38   #13
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Nein! That would make things worse. With wheels locked you are increasing the barking distance. Best is to get the proportioning valve readjusted to your normal load conditions.

Do a test with beast loaded with weight at the back. Does it make difference?

May be a google search on LSPV is good idea.
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Old 9th December 2008, 07:54   #14
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@tsk: As the TASS said proportioning was off and needed readjusting. Probable you need to reduce further the pressure in rear circuit. You can also (wallet permitting) look at slightly larger pattern tyres for the rear to increase surface area and avoid wheel lok ups.[/quote]

Hi sudev
Can one fit a larger pattern tyre at the rear to help traction? Is it a good idea to have front and rear tyres different. Have been wondering about this for a while and went thru a lot of threads but could not get help. In a rear wheel drive this would have a great sense!
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Old 9th December 2008, 08:03   #15
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Quote:
Originally Posted by nitrous View Post
Front-rear Brake bias setting is also a reason for rear fishtailing at high speed braking.
This is what I was thinking as well. Is it possible to adjust the bias so the front wheels get more of the braking force?
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