Brake Bias Valve

[Guest carl leonard]

My disc and drum set up stops on a coin in a straight line so i hardly feel its dangerous unless someone is driving to close when i stop hard

 

 

I never fitted a bias valve and the brake test was perfect at the SVA (Watnall Road, Nottingham)

 

ONE MIAN POINT - Make sure you fit NEW pads, New discs etc and bed them in before the test.

 

I know of someone that build an excellent car and decided to fit the discs and pads from the donor. The car failed SVA, so a bias value was fitted from a Ford Fiesta!

 

This has been a gray area for so time but my opinion for what its worth would be not to fit it.

 

 

Your IVA, Your choice!

 

carl.

[Guest LukieZ]

Make sure you fit NEW pads, New discs etc and bed them in before the test.

 

I know of someone that build an excellent car and decided to fit the discs and pads from the donor. The car failed SVA,

 

well i did re-use the old disc and pads from the donor (the donor car stopped sharp and plenty of meat on the pads & good disks) and passed the IVA with no problems - however new pads are ideally need to be bedded in as could effect the bias of the braking effort

 

as regards the bias valve, probably don't need it on a standard setup (vented front disk, rear drums), the rears only ever locked up [briefly] once the first time i used the car (heavy too, thanks to a speeding van ) but worked fine ever since (sharp as a pin!) - i will add that i use the servo on my setup.

 

the only change as regards the IVA is (i think) is the use of a bias bar which must be permanentely locked off, i.e. welded so no further adjustment - which can cause problems as you don't know if the bias is right until you test it. this will be an issue for those that use a custom setup (4/6-pots etc, and individual master cylinders)

 

Luke

[ibrooks]

Everyone!

The weight distribution of the Sierra & Hood determine the braking forces needed for IVA/MOT front and rear. Hence the calculations. The system was designed for about 70:30 not 50:50 weight distribution. All Hoods are the same, in common with all other 7 type cars using standard Sierra brakes, drum or disc rears and transferred to a 7. The design says you are under braked at the rear! NOT ME, gravity.

 

Where the hell does this 50:50 figure come from? I could lift the rear end of my 2B in road trim by the spare wheel tube. Best of luck trying that at the front - a Pinto is HEAVY.

 

As far as all Hoods (and your implication all sevens) needing the same braking effort and distribution....

 

Some people use donor seats and some use composite seats. Some put the battery in the rear and some in the engine bay. People use different size fuel-tanks. All these things might only be a small individual amount of weight but considered as proportion of the total weight on a light car like a Hood they can each make a significant chunk. If you build a car with the heaviest of all the possibilities and all the non location critical components at the rear then you can get away with significantly more rear braking effort than a car built with the lightest of all these components and all the other stuff placed under the bonnet.

 

There is no simple answer to the original question.

 

Iain

[Guest mower man]

It,s logic Jim but not as we know it !! bye for now mower man

[Guest Alan_builder]

There is no simple answer to the original question.

 

Iain

Wrong (if this was the first time I explained this I would say "I think your WRONG")

 

You have missed the point about IVA, SVA and MOT tests. calculating breaking efficiency.

I do believe you have to declare the front and rear weight so the MOT braking can be calculated.

At SVA my car was weighed and the weights recorded for each end, with driver and full fuel (road going trim).

 

When you pass IVA or SVA the braking efficiencies are measured and calculated for the weight distribution and without the G valve (bias valve) working as the test is static on a rolling road.

 

As soon as you drive down the road the G valve becomes operational and reduces the rear braking force. This tends to make the car go into a front end slide as the tested figures are no longer in play, (your car is not road worthy!).

 

Ian, I don't want you to be at the wrong end of any litigation.

 

Don't fit the bias valve KEITH.

 

-----------------------------------

This is really off the question Topic, but the discussion is about the reason why the Bias valve should not be fitted.

 

I am content for a post listing the weights of our cars for front and rear axles, drive onto a public weigh bridge with only the front wheels and then only the rear wheels on the platform and take the 2 reading. (This is with half a tank of fuel and the driver).

 

Ian can you please post a picture of you holding up the rear end of a 7 type car with driver and half a tank of petrol.

 

Where the hell does this 50:50 figure come from? I could lift the rear end of my 2B in road trim by the spare wheel tube. Best of luck trying that at the front - a Pinto is HEAVY.

 

As far as all Hoods (and your implication all sevens) needing the same braking effort and distribution....

 

Some people use donor seats and some use composite seats. Some put the battery in the rear and some in the engine bay. People use different size fuel-tanks. All these things might only be a small individual amount of weight but considered as proportion of the total weight on a light car like a Hood they can each make a significant chunk. If you build a car with the heaviest of all the possibilities and all the non location critical components at the rear then you can get away with significantly more rear braking effort than a car built with the lightest of all these components and all the other stuff placed under the bonnet.

 

There have been posts in the past about building your car with a 50:50 weight distribution in mind for road holding. Building the rear end as light as possible is a silly thing to do as under acceleration the rear end will be too light and spin the rear wheels when setting off and give reduced road holding. I seem to recall this 50:50 was one of Trick Dickies selling points.

 

I don't want to argue with any one about the exact weight distribution of their car or the sierra. Just the general weight distribution and declared axle weights and how that effects the general requirement for a transplanted braking system as further evidence of why it is wrong to fit the brake bias valve.

 

My advice which you can ignore at your peril:-

1. To remove the standard Sierra brake bias valve if fitted.
   
2. When driving generally expect the front to lock up prior to the rear with standard transplant brake systems. (there will always be incidents otherwise due to outside influences).
   
3. Retract any advice to fit the sierra brake bias valve.
   

 

I again wish you all happy an incident free motoring. I am just thinking of your safety!

 

I my

[Guest mower man]

I am now confused Iget a slamming for saying I don't see the need for using a load sensing valve [ that is what the sierra valve is] then see the sme poster advise not fitting one .A bias valve is a different animal alltogether and MAY be even MANDANTORY in some circumstances but it is up to the induvidual to research this for his own situation and needs, my mono cars wieght distribution is 52%fr 48% rear weighed as dicussed earlier andmy set up works both to my own and 3 different MOT inspectors satisfaction bye for now mower man

[Longboarder]

When you pass IVA or SVA the braking efficiencies are measured and calculated for the weight distribution and without the G valve (bias valve) working as the test is static on a rolling road.

 

As soon as you drive down the road the G valve becomes operational and reduces the rear braking force. This tends to make the car go into a front end slide as the tested figures are no longer in play, (your car is not road worthy!).

I believe your bracketed conclusion is incorrect. IVA specifically requires the front wheels to lock before either of the rear wheels do.

 

The IVA allows for the fitment of an automatic hydraulic valve, sensitive to either load or deceleration. It does not allow manualy adjustable valves. It does allow a non adjustable bias bar. The sierra valve is by definition therefore permitted. If such a car were presented for testing then I would guess both roller brake test and deccelerometer testing would be required. Front to rear brake balance would be quite different static to dynamic but that is the intended design characteristic of such a valve. Apart from satisfying the individual braking requirements at each wheel and on each axle a decelerometer test would be required to activate the valve and prove that neither rear wheel locks before both the fronts have locked. This latter requirement must be satisfied or the car fails, regardless of the roller measured figures. Getting the correct angle on the valve body would have to be sorted prior to the test.

 

My personal experience and advice is still don't fit the sierra valve unless after IVA test your car has been shown to lock the rears before both the fronts. This is unlikely to happen but not impossible.

 

Nigel

[Guest Alan_builder]

I am now confused Iget a slamming for saying I don't see the need for using a load sensing valve [ that is what the sierra valve is] then see the sme poster advise not fitting one .A bias valve is a different animal alltogether and MAY be even MANDANTORY in some circumstances but it is up to the induvidual to research this for his own situation and needs, my mono cars wieght distribution is 52%fr 48% rear weighed as dicussed earlier andmy set up works both to my own and 3 different MOT inspectors satisfaction bye for now mower man

 

I think you miss read my original posting

[Guest Alan_builder]

I believe your bracketed conclusion is incorrect.

 

I am sorry I am unable to see any evidence to support your belief.

 

IVA specifically requires the front wheels to lock before either of the rear wheels do.

 

No comment I have not read the IVA manual so I am in position to comment on its wording.

Idealy of course all 4 lock about the same time with a bias towards the front locking first. ( my belief)

 

I believe your bracketed conclusion is incorrect. (your car is not road worthy!)

 

I don't think any of would set off with with our rear brakes working at 15% maximum, we would say the vehicle is not road worthy.

At 70 MPH you step on the brakes and the magic compensation valve reduces the rear braking to say 15%. It falls outside the IVA, SVA or MOT standard and would fail any of the tests. To me that is not road worthy! The car does not need to reduce its rear brake efficiency unless the backs are about to lock up and that has not been shown to be happening.

 

but that is the intended design characteristic of such a valve.

 

Yes but the brake compensator valve is intended for a Sierra as was the original braking system.

 

My personal experience and advice is still don't fit the sierra valve unless after IVA test your car has been shown to lock the rears before both the fronts. This is unlikely to happen but not impossible.

 

Nigel

 

I totally agree with this first part. The only way I can see the back locking first is if there is a fault with the front or rear brakes which should be sorted rather fit a compensator. I think this is also irrelevant as the poor front brakes would fail any way but as you say we never know.

 

I hope I have made a case for the "not road worthy issue", if not please give me a PM with tel Number so we can talk it through, obviously I can't see the point you are driving at. No hard feelings I just want the best for Keith and maybe we can all benefit from better understanding.

 

I my

[Guest mower man]

Just to add alittle more mischief my car has neither IVA orSVA certification so i suppose some of us would class it as illegal and unroadworthy? my answer to which is or would not be thought prudent on an open forum bye for now mower man

[Guest peter2b2002]

just a thought for my SVA I was told to put 600/400 for the axle weights , so how do you get the 50/50 weight distribution?

peter2b

[Guest Alan_builder]

Where the hell does this 50:50 figure come from?

Iain

 

Weight of my Robin Hood at SVA front 373 rear 369. I am sorry Ian that looks like 50:50 to me.

axel weights

 

 

Taken from Colin Ushers own site SVA test Note 1

Colin Ushers web

Q15 asks for the Design Weights, these are 600kgs Front Axle, 450kgs Rear Axle and Gross Weight of 1050kgs. (Official Robin Hood Sports Cars figures)

 

and that looks about 58:42

 

So I have some humble pie to consume saying the Sierra was 70:30, but hay just let me catch up with that guy. Just shows you need to check out all the information yourself.

 

PS

58:42 looks like you need to add in the region of 30% to the rear brakes to get to a point where the fronts just lock up prior to the rears (2% margin). So the last thing you need is the Sierra rear brake bias valve!

 

I my

[speedtripledan]

Wrong (if this was the first time I explained this I would say "I think your WRONG")

 

You have missed the point about IVA, SVA and MOT tests. calculating breaking efficiency.

I do believe you have to declare the front and rear weight so the MOT braking can be calculated.

 

 

-----------------------------------

My car has been MOT'd 3 times now and the guy does not use the rollers because there is no design axle weight in his dvla computer for my car (2B), he goes for a drive and tests with a meter that measures the deceleration. its also used on some 4 x 4,s for obvious reasons.

 

[Guest Alan_builder]

Wrong (if this was the first time I explained this I would say "I think your WRONG")

 

You have missed the point about IVA, SVA and MOT tests. calculating breaking efficiency.

I do believe you have to declare the front and rear weight so the MOT braking can be calculated.

 

 

-----------------------------------

My car has been MOT'd 3 times now and the guy does not use the rollers because there is no design axle weight in his dvla computer for my car (2B), he goes for a drive and tests with a meter that measures the deceleration. its also used on some 4 x 4,s for obvious reasons.

 

 

Dan, mine goes in the rollers.

But that has little to do thin Keith's question.

 

I my

[ibrooks]

I don't think any of would set off with with our rear brakes working at 15% maximum, we would say the vehicle is not road worthy.

At 70 MPH you step on the brakes and the magic compensation valve reduces the rear braking to say 15%. It falls outside the IVA, SVA or MOT standard and would fail any of the tests. To me that is not road worthy! The car does not need to reduce its rear brake efficiency unless the backs are about to lock up and that has not been shown to be happening.

 

How is this not roadworthy?. Step on the brakes at 70 and you MUST use a reduced rear braking effort to that which you could have used at 30. At those sorts of speeds under any sort of braking - especially heavy braking there is a huge weight transfer to the front wheels and almost any braking force applied to the rear wheels will cause them to lock. This is what an inertia type valve allows for.

 

At the SVA or IVA test they measure the braking force to ensure that the rear brakes are not putting out enough effort to lock the wheels in your above situation (the worst case scenario and therefore the minimal braking effort). BUT they measure it when the car actually has nearly it's theoretical MAXIMUM weight on the rear wheels and take no account for any dynamic situations although they do allow for valves that do. So for the test you can only produce the minimal braking effort on the rollers. With this in mind I would say no you shouldn't fit THE INERTIA VALVE for the test as it will only reduce your effort below the minimum that you are starting at.

 

Cars with different weight distribution need more or less rear braking effort and depending on the components used in your build and their placement the weight distribution of Keith's car will not be the same as anyone else's. With

different combinations of calipers, disc diameters, drum diameters and piston sizes you will get different braking efforts from the brakes themselves. Some of the Sierra brake setups can and do produce "too much" rear braking effort for the test and so you often do need to fit a pressure reducing valve of some sort.

 

As for Keith's original question of "do I need a bias valve?". We don't know - there are too many factors at work.

 

And I never said I could hold the rear of my 2B up - I could lift it, briefly I'll admit but I could get both rear wheels off the ground to shuffle it around the garage. No way could I do that with the front. No pictures or video of this as I sold it some time ago.

 

Iain