Vt brake conversion Bias.

[VK SL 3800]

Hi all,

After doing some searching through the threads i couldnt find out what happens to the brake bias when you do a VT twin spot upgrade on the front. Im will be running Vp rears and im sure that they would be marginally smaller then a VT so how much does it affect bias? Is the rear piston just a poofteenth smaller from a Vt and should i just not worry about it or should i try to fit VT rear Calipers and rotors as well. (aware of handbrake issues in doing so).

Any help appreciated

[ari666]

i dont know the answer, but am bored and this is the most interesting question posted this week so wanted to join in

wouldnt you just need to get a new splitter to go on top off the diff? its my understanding that the splitter (proportioning valve) controls how much pressure goes to which wheel.

and when you say "marginally smaller" do you mean rotor width, or radius? cos radius shouldnt make bugger all difference. actually width wont really make a difference either. plus you always want the front to work better than the rear. (IMO)

[VK SL 3800]

Ive been doing brakes at tafe recently and you have to know all the theorems and all the technical stuff, im talking piston size in the caliper, larger the piston, the more force is applied. So creating more braking force to the rear with the larger piston giving more bias to the rear. With rotors it another ball game again as even though if you have good brake bias the pads and rotors may grip more possibly creating a stronger rear brake even though the hydraulic force applied is still the same but im not interested in that part atm that can wait as it doesn't alter alter braking performance to one particular wheel as much as piston size. MY VK is currently drum rear and i have a VP ute which im going to use for the rear brakes, AFAIK the proportioning valve in the vk is in the MC but i may be wrong? somebody with experience will have to let us know . So trying to get to the point, im wondering whether using VT twin spots on the front will drastically reduce rear brake performance or not, as even though fronts do most of the work you need the rears to be good to stabilize everything and also get good performance when you are towing something. Either way because the VT brakes are so much larger on the front you will lose some rear brake bias as the front will now be pushing harder even though the rears still are applied with the same force. Gets very technical when you look into it........... Also i might add, because the MC has dual circuits for front and rear does that mean there is no effect anyways? pretty sure it doesn't change things and you still get same bias issue.

[VK SL 3800]

Also will add, i am using VT master cylinder.

[ari666]

Ive been doing brakes at tafe recently and you have to know all the theorems and all the technical stuff, im talking piston size in the caliper, larger the piston, the more force is applied. So creating more braking force to the rear with the larger piston giving more bias to the rear. With rotors it another ball game again as even though if you have good brake bias the pads and rotors may grip more possibly creating a stronger rear brake even though the hydraulic force applied is still the same but im not interested in that part atm that can wait as it doesn't alter alter braking performance to one particular wheel as much as piston size. MY VK is currently drum rear and i have a VP ute which im going to use for the rear brakes, AFAIK the proportioning valve in the vk is in the MC but i may be wrong? somebody with experience will have to let us know . So trying to get to the point, im wondering whether using VT twin spots on the front will drastically reduce rear brake performance or not, as even though fronts do most of the work you need the rears to be good to stabilize everything and also get good performance when you are towing something. Either way because the VT brakes are so much larger on the front you will lose some rear brake bias as the front will now be pushing harder even though the rears still are applied with the same force. Gets very technical when you look into it........... Also i might add, because the MC has dual circuits for front and rear does that mean there is no effect anyways? pretty sure it doesn't change things and you still get same bias issue.

lol, i cant help but ask... was this a setup? knowing all that stuff but still asking the magic JC ball

[VK SL 3800]

I have been having trouble trying to put my thoughts into writing here, i just thought of the best way to explain it. Essentially the upgraded braking system will be VT spec, so imagine if using all VT parts at all 4 corners it will be the same bias as a VT, which may be to much rear bias for the VK as it is a much lighter car in the ass end, so could POSSIBLY have slight over kill on the rear brakes, but not enough to cause lock ups or any noticeable issues. But then could also be about right. either way. SO if i put VP brakes on the rear, i am essentially reducing rear brake power ( what you do if your rear end locks up commonly like XU1 Toranas). And maybe the VP sized piston is that little bit smaller to get better bias for the VK? Or it could be to small and not enough is pressure is applied. Any thoughts...........

[VK SL 3800]

lol, i cant help but ask... was this a setup? knowing all that stuff but still asking the magic JC ball

LOL, no im thinking to hard on this one i thinks, might have to end up going on a trail and error style approach.

[ari666]

I have been having trouble trying to put my thoughts into writing here, i just thought of the best way to explain it. Essentially the upgraded braking system will be VT spec, so imagine if using all VT parts at all 4 corners it will be the same bias as a VT, which may be to much rear bias for the VK as it is a much lighter car in the ass end, so could POSSIBLY have slight over kill on the rear brakes, but not enough to cause lock ups or any noticeable issues. But then could also be about right. either way. SO if i put VP brakes on the rear, i am essentially reducing rear brake power ( what you do if your rear end locks up commonly like XU1 Toranas). And maybe the VP sized piston is that little bit smaller to get better bias for the VK? Or it could be to small and not enough is pressure is applied. Any thoughts...........

i find hitting the enter key more often makes it a lot easier for people to read and more oftne they will answer.


also just wait... people who have a clue will log on tomorrow AM (no one is on tonight) and will have a much better idea of what you are on about.

[vkberlina]

I have always looked at the brakes like a hydrolic system, they will balence out based on the set bias, so if oyu dont change the bias even with the bigger calipers on the front they _should_ still balence as long at the MC can apply the appropriate pressure.

Having said that on my VK I am running VT MC & booster, VZ monaro twinspots and rotors on the front and VZ monaro ventilated rear discs and calipers on the rear I have done some very heavy braking and have not experienced any bias issues infact the car is more stable under brakes since doing the brakes.

cheers
Scott

[VK SL 3800]

What do you mean by the set bias?

[ari666]

yeah i was thinking that: if you apply (random made up figures) 100nm to the front, and the back gets 75 nm, then surely the pressue would travel to the back to equalise? so eventually they would both become 100nm unless you had a leak somewhere. for example. when you have a seized pot, all three other pots will actually work better cos' all the pressure is then applied to them.

[VK SL 3800]

Its really weird the way it all works, its a physics thing, it will not even out. Very hard to explain....................
It all to do with the cm3 sizes of your pistons, for example: Imagine master cylinder with single piston is 10cm3, front brakes are 40cm3, and rears are also only 20cm3, if you apply say 100nm of force to the master cylinder force that is produced onto the front brake pads is 400nm, but the rears are only 200nm. Even though you are only pushing your pedal at 100nm you get to different levels of force at the brakes due to size difference. Its like hydraulic advantage.
kinda like your bottle jack, if the piston you pump was huge it would take heaps more force for you to push it down, but with a small one you are able to create just as much lifting power and it is easier for you to pump but it takes more pumps to get there, but with the same size pump and a smaller lift piston it would get harder to pump. Same thing with brakes because rear brakes are smaller you get less advantage because you are trying to squeeze closer to the same amount of fluid into the rear piston as what came out of the master then the front, creating less pressure.

[Cheap6]

The relative piston areas front to rear is only part of the story. Brake torque also depends on the distance from the axis of rotation of the disc that the force is applied at. eg. Keep the disc dia. constant but change from a single piston to two (or more) pistons of the same area. Locate the pitch circle dia. of the pistons as far outboard as they will go in each case and the brake torque will be increased (for a given line pressure) with the multiple pistons.

To illustrate the effect of changing just the front brakes to VT from the heavy duty VL, VP S2-VS type and if I may borrow from my own post in another thread:

Some maths:

VN-VS calipers have a single piston of 54mm in diameter and, for the heavy duty brakes, a disc diameter of 289 or 290mm. The effective pad width, = radial dimension of the swept area, is ~45mm.

VT-VZ have two pistons of 42mm in diameter and a disc diameter of 296mm. The pad width is also 45mm.

Brake torque is the force on the pad x the distance from the centre of the pad (~; actually the distance to the centroid of the swept area defined by the pad but that => calculus) to the axle centre x the coefficent of friction between the pad and the disc x 2 disc/pad surfaces.

The force on the pad is proportional to the area(s) of the piston(s) => proportional to the piston diameter(s) squared.

Ignoring the factors that cancel out, the ratio with VT-VZ front brake components to VN-VS front brake components is:

[{(296/2)-(45/2)} x 2(42^2)] / [{(289/2)-(45/2)} x (54^2)] = 1.24

i.e. + 24%

Only looking at the piston areas also assumes that the line pressure is constant between the front and rear wheel cylinders.

Because the desirable brake torque distribution between front and rear changes with the rate of deceleration, a proportioning valve is usually included to reduce the line pressure acting on the rear wheel cylinders, so reducing the brake torque acting on the rear wheels.

Ideally the reduction in rear wheel line pressure would match the rate of deceleration. If you were to plot ideal rear line pressure vs front line pressure - assuming that the static brake torque matched the static weight distribution - you would get a curve that looks like a hockey stick with the toe starting at the origin.

A two stage proportioning valve approximates that curve with two straight lines. Initially one to one, fr:rr pressure, then after the crack point for the prop. valve, a line with reduced slope. The slope of the second part is determined by the design of the valve.

Incidentally, electronic brake force distribution allows the curve to be matched perfectly.

From my tech. stuff, the VT M/C prop. valve cracks at 3.5MPa vs the VP valve at 2.4MPa. This is in the right direction but the slope of the pressure relationship post crack point remains unknown. Without testing (which would be pretty easy to do), specs. or measurement you can't be sure that the characteristics of the valves will mean that the brake distribution is anywhere near correct.

For reference:

VN, VP, VR:

M/C: 23.835mm dia or 25.4mm dia.(HD)

Fr. disc: 271mm or 289mm (290mm VR)

Fr. caliper: 54mm piston dia.

Rr disc: 279mm

Rr caliper: 38mm piston dia.

The front pad on VN-VS, also VB-VL I think is 45mm radially.

VT:

M/C dia.: 25.4mm

Fr caliper: 42mm X two

Fr disc: 296mm

Rr caliper: 40.5mm

Rr disc: 286mm

The front pad is also 45mm in the radial direction.

You will have to measure the rear pad dimensions yourself.

[Philthy]

You are making this more difficult than it has to be. Put the brakes on and see how they feel. Don't try to setup a car by numbers, set it up by what feels good

[VK SL 3800]

Thanks for the tech info mate, very clear and easy to understand, will be using all to make my decision on what to do

And Philthy, yeh your probably right but when you start playing with really important stuff like the brakes i want to make sure i do it right, manufacturers put a lot of time and effort into these sort of things so an upgrade can quite easily end up a down grade.

[Cheap6]

You are making this more difficult than it has to be. Put the brakes on and see how they feel. Don't try to setup a car by numbers, set it up by what feels good

I disagree with both premises. It's not difficult and you can't set the brakes up solely by feel.

Given the piston, pad and disc dimensions it was a trivial effort to make the comparative analysis between old and new static brake torque distributions - less than a minute with a pen, paper and calculator. Zero cost.

That highlights that using the VT- front calipers will change the brake distribution from what has been established by the Holden engineers as satisfactory and that something else will have to be changed to compensate.

With the slopes for the proportioning valves, it will only be 5 minutes or so with a spread sheet to plot the dynamic brake torque distributions and compare them.

The info. on the proportioning valves is probably only an e-mail to PBR away. Less than 15 minutes total time. Again zero cost and probably less time than has already been collectively invested in this thread.

Alternatively, measure the primary and secondary piston diameters (=> areas) in the proportioning valve.

Even bench testing the M/C's to determine the prop. valve characteristics would only cost a couple of hundred dollars and an hour at the most.

Contrast that with buying all that is required for the conversion; M/C, M/C/booster adaptor, booster, discs/hubs, calipers and pads, hoses and add the time taken to fit them.

Having done all that and outlayed the full cost of the conversion, without the "numbers", you still don't know if it works properly. Certainly not by "feel".

To illustrate, assume that the brake distribution is absolutely wrong and biased 100% to the rear. You can actually implement this scenario by using the handbrake.

Make a gentle stop on a high friction surface, like a dry road. Right up to the point where the rear wheels lock, a stop will "feel" great; no lock up, stable, all "good".

Now try to stop at the same rate where the grip between the tyre and road is lower, like a wet road. Now it's not so good.

Try stopping at a faster rate. Also not so good.

If it's biased 100% to the front instead, braking at the point where the front wheels lock, while stable, will always be less than is possible with all four wheels making a contribution but still "feel" OK.

You can indeed reduce the braking capability of the car by "improving" one end of the car at the expense of the other.

Not adjusting the brake distribution properly can even reduce the braking capability of both ends of the car, within the same braking system, depending on the rate of deceleration/tyre grip available.

Without relying on the testing already carried out by the Holden engineers and a comparative analysis, carrying out similar testing or a full analysis from basic principles, there is no way to tell if a particular combination of deceleration rate and road grip will result in less than optimum - less than that possible with standard brakes - or dangerously unstable braking.

This may be of interest:

http://autospeed.com/cms/title_Brake...1/article.html

Even "professionals" can get it wrong sometimes.

[vkberlina]

What do you mean by the set bias?

By "Set Bias" I mean the factory bias setting in the MC

Stuffsnipped

That highlights that using the VT- front calipers will change the brake distribution from what has been established by the Holden engineers as satisfactory and that something else will have to be changed to compensate.

If it's biased 100% to the front instead, braking at the point where the front wheels lock, while stable, will always be less than is possible with all four wheels making a contribution but still "feel" OK.

You can indeed reduce the braking capability of the car by "improving" one end of the car at the expense of the other.

Not adjusting the brake distribution properly can even reduce the braking capability of both ends of the car, within the same braking system, depending on the rate of deceleration/tyre grip available.

Without relying on the testing already carried out by the Holden engineers and a comparative analysis, carrying out similar testing or a full analysis from basic principles, there is no way to tell if a particular combination of deceleration rate and road grip will result in less than optimum - less than that possible with standard brakes - or dangerously unstable braking.

If your taking a complete setup from a complete car say VT MC, Calipers etc keeping same size rotors the bias _should_ be correct. As the valves etc are set for the caliper pistons etc. Taking parts from different makes models & brands could lead you into trouble and to what your describing.


Nice topic though
cheers
Scott

[RED1M8]

I have always looked at the brakes like a hydrolic system, they will balence out based on the set bias, so if oyu dont change the bias even with the bigger calipers on the front they _should_ still balence as long at the MC can apply the appropriate pressure.

Having said that on my VK I am running VT MC & booster, VZ monaro twinspots and rotors on the front and VZ monaro ventilated rear discs and calipers on the rear I have done some very heavy braking and have not experienced any bias issues infact the car is more stable under brakes since doing the brakes.


cheers
Scott

What was involved in fitting the VZ rear discs and callipers to your VK ?

[VK SL 3800]

By "Set Bias" I mean the factory bias setting in the MC

If your taking a complete setup from a complete car say VT MC, Calipers etc keeping same size rotors the bias _should_ be correct. As the valves etc are set for the caliper pistons etc. Taking parts from different makes models & brands could lead you into trouble and to what your describing.


Nice topic though
cheers
Scott

Not only is the system balanced together, but remember also for the cars weight and other properties, just ANOTHER thing you have to take into consideration. Imagine a VT commodore set up in a Laser, the rears would lock for sure.........

Cheap 6, you really know your s**t!

[vkberlina]

actually wouldnt it be more weight distribution affecting bias rather than overall car weight? Of course the setup in a laser would cause problem different weight distribution, thats why we run a bias valve in race cars so as a fuel load lightens weadjust the brake bias.

scott

[dirt drifter]

to the OP. if it was me i would do the brake upgrade, and if it locks the rears first then throw in an adjustable proportioning valve in the rear brake line. shouldn't cost no more than $250 for the valve and install. then find a quiet and wide bit of road and do some testing.

yeah its not the most scientific or methodical way to go about it, but u will learn how a car with less than perfect brake balance behaves under heavy braking, and it will sharpen ya reflexes, and give ya some good practice at doing emergency stops in a non-abs car

[Cheap6]

to the OP. if it was me i would do the brake upgrade, and if it locks the rears first then throw in an adjustable proportioning valve in the rear brake line. shouldn't cost no more than $250 for the valve and install. then find a quiet and wide bit of road and do some testing.

One problem with doing it that way is that it will only be testing one particular braking condition. Even if you could do sufficient testing with sufficient skill to sort the balance out under those conditions, change the grip level between the road and tyre and the balance is not necessarily going to remain correct. The ideal F:R split varies dynamically with the rate of deceleration. At the limit of grip that means it changes with the road surface eg. wet vs dry, dirt vs asphalt. The calculations based on the testing Holden has already done are easier, cheaper and more reliable.

Also, if it doesn't lock the rears first, you have no way of knowing if you have just reduced the effectiveness of the rear brakes, and therefore the braking capability of the car, at the point where the front brakes lock. If that is the case, the car will stop slower. See above when I discussed 100% front bias, the most extreme case. Because the VT front brakes are more effective than the OEM VB-VK, even VL-VS HD, this will be the case unless the different proportioning valve in the VT M/C compensates. Without changing the proportioning valve i.e. M/C that is certainly the case.

One advantage of using an additional proportioning valve, outside the M/C, is that it would allow three stages of rear line pressure response (as distinct from two) and a closer approximation to the ideal curve, assuming you know what that curve is before you start and will be with the new valve(s).

VK SL 202 is correct in post #19; It is the static weight distribution and load transfer characteristics - which depend on CoG height and wheelbase length - that determine the required brake torque distribution. Unless these match - unlikely - just transferring a complete system from one car to another won't work very well.

As an aside, in discussing this it doesn't help when the Australian magazine devoted to modified Commodores provides misleading and innaccurate advice in its technical letters page. Not for the first time either.

[vkberlina]

What was involved in fitting the VZ rear discs and callipers to your VK ?

Mounting the calipers and calipers were easy, custom made brake lines, weld up the original bolt holes and drill new ones and fit it all up. Getting the hand brake to work was heaps of work basically had to remake the rear discs, I tried making sleeves out of cast iron with key ways cooling the inser and heating the disc and pressing them in but somehow they kept coming loose went through about 4 revisions of designs until we come up with a solution that works and doesnt weigh as much as a wheel.
They probably dont help that much over the original rears but I wanted to have a go since I had them sitting there. Really wank factor more than anything plus probably the only early commodore around with ventilated rears again wank factor :P
Perfomance wise it stops on a 10c piece very balenced inall conditions wet, dry, gravel, different tarmac surfaces you name it only thing it wont get tested on it dirt.

cheers
Scott

[dirt drifter]

for ventilated rear discs on an early live axle setup, how about the rear brakes off a vn group a? i bought my vs with a vt front- vn grp a rear setup already on it. as far as im aware its a fairly easy bolt on process. dba make the discs and calipers to suit shouldn't be too hard to find

[SLE-883]

I'm doing the same conversion on my VH.

I asked Race Brakes Sydney about what upgrade should happen with the rear brakes with my VT twin piston and VT booster/Master cylinder upgrade. Their response was "There is no decent rear upgrade to match the VT fronts so just use the std rear disc brake set up but fit better pads and slotted rotors".

So I guess i'll be using the standard VH rear brakes with slotted discs and so good pads.