Hi I'm a new member with a GTV-6. Currently having the rear brakes repaired and require a proportioning valve to suit 1985 GTV-6. I understand these are not rebuildable and have to be replaced. Can anyone tell me where I can buy one in Aust.
Cheers
Pasquale
Pasquale,
If you suspect that the valve is faulty and not fuctioning the alternative to replacing it is to simply do away with it and install a rear adjustable proprtioning valve substitute. This will give you a limited ability to adjust the rear brake bias based on the size and nature of your rear tyres, the type of barke pad material you are using and payload.
While you are at the workshops, make the enquiry about such a device and they may install one while the other work is being done. Best time to do it.
Thanks guys, apparently International Auto have them in the USA but they are currently out of stock untill 15 Oct, so I'll try Monza motors for 2nd hand option.
Pat
i have a secondhand 1 if u need
Pasquale,
Regrettably unless the valve is removed from a car that was running recently or the braking system has not been pulled apart, there is a very good chance that the valve will have internal corrosion from having fluid in it and absorbing moisture from the air once it was removed and put on the shelf.
My recommendation is either fit a new one original type, a new adjustable type or simply by-pass it altogether and leave it out.
My brakes on my GTV6 actually work better without it.
be careful with after market adjustable valves, some of them dont have a full range of adjustment, leaving you with not enough or too much bias to the rear..
Hi Martin, I'm about to upgrade my front brakes & will probably need an adjustable brake proportioning valve. Do have details of one that has adequate adjustment. Conversely which ones are no good.
Thanks
I think this topic is heading for a discussion.
My experience with the transaxle cars is that the difference in pad size (front > rear) alone provides sufficient front bias to prevent rear lock ups. Matter of fact, there is insufficient braking at the rear even if you completely remove the proportioning valve. This was not the case with 105's I have had in the past. I believe this is due to the weight differences at the rear between these cars. I have driven many thousands of kilometers in transaxles and I cannot recollect EVER having a rear wheel lock up with or without the valve in place.
I am talking about street cars only as racing applications vary too much to make a simple conclusion.
I would be interested in other peoples experiences in the same context.
yes, i agree with you md, and have had similar results, except for when i have used vastly varied pad compounds front to rear. ie a much better pad at the rear and a 10 dollar set of cheapies at the front, but even then, was only slight, and when driving spirited on private roads :)
another concern could be an insurance scenario, and if some strange reason the brake system had to be looked at, like if someone was killed or something in an accident.
Thanks guys, I've had similar advice from other sources. I am fitting V6 vented discs & Commodore calipers on the front & the car will only be used on the road. I was a bit nervous about removing the proportioning valve & locking the rears. If I'm going to slide off the road with the brakes locked up I'd like to do it frontwards ;)
Bob,
I am no authority in this stuff. It is the realm of hydraulic engineers to calculate the total fluid and pressure balances for modified brakes but just going by things that I have tried in the past, there may be master cylinder issues that evolve and just about certain that the rear brakes will be under performing compared to the fronts and you will have too much front braking and insufficient rear braking.
Best of luck with it anyhow..
Hello MD and others,
It seems that there are a host of options that could work its just a matter of deciding which option best suits the specific need. In my case, I've had the rear brake calipers fully reconditioned and new pads fitted. The proportioning valve in my GTV6 is still working but is just starting to show signs of some leakage, but not much. I want to maintain originalilty so will be fitting a new replacement as it becomes availble from International Auto in mid October or thereabouts.
Cheers
Pasquale
Highwood Alfa in the UK, I bought one for my 1983GTV6, the 85 model is a little more expensive but they have them and Chris is fantastic to deal with, the best i have ever bought from.
Thanks, I'll check them out.
Pasquale
My apologies for partially hijacking this thread, but does anyone know if fitting an aftermarket adjustable proportioning valve (eg Wilwood, Tilton, etc) is road legal in Victoria?
Why not put the question to the transport authority direct?
I have found Compbrake in the UK fantastic to deal with.
Check out their bias valve,
(http://www.compbrake.co.uk/images/BUTTONS/BRAKEACCESSORIES/BUTTON_BACCESS_BIASVALV_OVER.jpg)
http://www.compbrake.co.uk/BRAKE%20ACCESSORIES/BRAKEACCESS_BIASVALVE.html (http://www.compbrake.co.uk/BRAKE%20ACCESSORIES/BRAKEACCESS_BIASVALVE.html)
29 GBP = about $46 AU
QuoteMy experience with the transaxle cars is that the difference in pad size (front > rear) alone provides sufficient front bias to prevent rear lock ups.
This isn't exactly correct, pad size actually doesn't have any impact on the braking force generated by the braking system. Forces generated by friction are independent of surface area,(this does not stand true for tires as they do not work purely by friction).
The main factors that directly influence brake force are
Pedal ratio
Master cylinder diameter/Brake cylinder diameter
Effective disc radius
Wheel size
Friction material
Pad size will effect the heat capacity of the system, important consideration for fading
Just a quick point, I've had trouble with bias valves in the past as they can slow the flow of fluid back from the brakes causing pressure to remain after the brakes have been released. In my opinion a tandem master cylinder with bias bar is a much better option.
QuoteMy apologies for partially hijacking this thread, but does anyone know if fitting an aftermarket adjustable proportioning valve (eg Wilwood, Tilton, etc) is road legal in Victoria?
Generally any modification to a vehicles braking system will void its ADR compliance.
Quote from: Colin Byrne on July 27, 2010, 09:17:58 PM
QuoteMy experience with the transaxle cars is that the difference in pad size (front > rear) alone provides sufficient front bias to prevent rear lock ups.
This isn't exactly correct, pad size actually doesn't have any impact on the braking force generated by the braking system. Forces generated by friction are independent of surface area,(this does not stand true for tires as they do not work purely by friction).
I have heard this before and I can't say I believe it,personally I believe the swept area change must change the overall effectiveness of braking efficiency.If you are applying the same clamping force over a much larger area the amount of friction must increase.The valves we are discussing are Pressure control valves not brake bias valves.
Perhaps you could explain it to me,engineering theory doesn't always translate into practice.
I have been using pressure limiting valves on the rear brakes of my Alfetta GTV for years and find it vital for balancing up brake balance which varies due to track conditions especially wet to dry.
When I went to slotted vented rear discs and put front pads in the callipers I suddenly had to much braking on the rear with Volvo 4 spots on the front.Once I slotted the fronts and adjusted the pressure to the rear I got my balance back.Hawk hp plus pads front and rear.
Cheers
Andrew
QuoteIf you are applying the same clamping force over a much larger area the amount of friction must increase
Fair enough Andrew I know it can be hard to visualise this sometimes but this comment does go against the second law of friction
(from http://en.wikipedia.org/wiki/Friction)
2. The force of friction is independent of the apparent area of contact. (Amontons' 2nd Law) (Amontons' 2nd Law does not work for elastic, deformable materials. For example, wider tires on cars provide more traction than narrow tires for a given vehicle mass because of surface deformation of the tire)
I just checked out my copy of Millken/Millken Race Car Vehicle dynamics and I've attached the equation for longitudinal brake force vs Brake fluid pressure. As you can see the relationship between the two is independent of pad surface area.
Quoteengineering theory doesn't always translate into practice.
Been working out pretty well for me so far!
Quote
the valves we are discussing are Pressure control valves not brake bias valves.
If your controlling the pressure at one axle isn't that changing the brake bias?
Quotehave been using pressure limiting valves on the rear brakes of my Alfetta GTV for years and find it vital for balancing up brake balance which varies due to track conditions especially wet to dry.
Absolutely, getting correct brake balance is critical in decreasing stopping distance and, as you said, in different conditions such as wet vs dry, or anywhere else where weight transfer various due to different deceleration rates, then adjustment is required, I was just saying that I prefer a mechanical brake balance bar rather than a single pressure control valve, but like I said that's just my opinion.
Cheers
Hi Colin,
Thank you for the formula,it does indeed look like a very usefull tool to calculate the brake pressure required for a given set up but that then of course leads to whole lot more calculating of master cylinder diameter and the rest of the other factors you mentioned earlier to generate that pressure.
I am not an engineer but to ignore the pad size in braking efficiency is still not logical in my opinion as it still affects the overall work done by the brakes,which in turn affects the heat generation and disapation,contact pressure etc refered to before.
I found a little bit of information to support my thinking on the web from the US.It is called "brake pad prognosis system",which states"Brake work includes determining braking force as brake pressure times brake pad area times brake pad coefficient of friction.
The amount of work done affects the pad life and is used to predict it.
Thanks for the discussion even if I am "hijacking" the thread.
Cheers
Andrew
As I anticipated earlier this topic would generate discussion. Let's face it, it's inevitable.
Colin,
QuoteThis isn't exactly correct, pad size actually doesn't have any impact on the braking force generated by the braking system.
I gotta say I disagree with that. Let's look at this example. In a braking system without any other parts than the calipers, master cylinder and brake lines, the line pressure is the same at all points. I trust we are in agreement there. Now if the we apply the same pressure to all pads equally and the pad materials are all identical so the friction coefficients are the same, we would expect the same amount of braking forces at all wheels so why dont we have the same sized pads all round?
It is a common practice to reduce the pad area to rear brakes compared to the front brakes to achieve a better braking balance. The usual reason why this is so is because the rear of most cars are lighter than the fronts and the weight transfer to the front axles under braking also takes place making it even lighter. Applying the same line pressure to the same size pads would produce rear wheel lock up. Therefore designers install smaller calipers and pads to effectively try to reduce the effect of the equal line pressure and create less friction with the smaller pads at the rear.
My point was/is that the front brakes have a "natural" bias and negating this bias can be done in other ways than proprtioning valves. It can done by varying rear friction coeficients, rotor diameters.
Paul,
I can't see that gorilla behind the wheel in the 35 Gp getting past CAMS..although a good helmet... ;D
QuoteI gotta say I disagree with that. Let's look at this example. In a braking system without any other parts than the calipers, master cylinder and brake lines, the line pressure is the same at all points. I trust we are in agreement there. Now if the we apply the same pressure to all pads equally and the pad materials are all identical so the friction coefficients are the same, we would expect the same amount of braking forces at all wheels so why dont we have the same sized pads all round?
Because the work done, and therefore the heat generated, at the rear of the car, is much less than the front of the car. Less heat, means smaller pads will be able to stay within in their operating range, smaller pads, dics ect are cheaper, and more importantly in racing applications, are lighter.
MD in your example you didn't appear to take into account one very important factor, and that is the surface area of the brake calliper piston. I've attached an exert from a training manual we use here at work, hopefully this will help illustrate the previous equation I posted
If you still disagree I'm more than happy to keep discussing, but is there a specific part of the equation that you disagree with? That equation comes from one of the most respected vehicle dynamics text books, but that's not really important as it can be derived from first principles very easily, in fact the attached diagram almost does this
Let me know what you think
Cheers
PS nice gorilla
QuoteLet me know what you think
...er, I let you know after I consult the gorilla. ;D
Sorry mate, there's the number crunchers and the geasy knuckles brigade. I am in the latter group.
Hi Colin,
MD did say reduced pad and calliper size which to me implies the piston would be smaller-anyway,I would like to understand the diagram you posted last time,I don't recognise the symbols and the equation.
What do those equations tell you?.If you have the time to explain it I would appreciate it.
Cheers
Andrew
Happy to explain the diagram a little better
Starting at the brake fluid end, the master cylinder creates pressure (p), pressure is measured in force per unit area, so if the pressure is acting on a known area, in this case the brake piston Apiston we can calculate the resultant force, in this case the force of the piston (Fn). This gives us the first equation
Force of the piston = Brake Pressure x Area of the piston
Fn = p . Apiston
(the term "Normal force" just implies a force that is perpendicular or at 90deg. To a surface, in this case the brake disc)
Because the piston is directly acting on the pad and the pad to the disc we can say that the resulting force on the disc is the same as the piston (Fn),
Friction coefficient, mue, is unitless, its this way because it is a ratio, and it is a ratio of the resulting force due to friction in the direction of travel to a normal force. This gives us our second equation for brake force (Fr)
Brake Force due to friction = Friction coefficient of pad x Force of the Piston x 2
Fr = 2 . muepad . Fn
(the 2 is because it is a slideing calliper so the force acts on both sides, no different to if it just had another piston on the other side)
A torque is simply a force at a certain distance, now we have our brake force from our last equation, to get our braking torque (T) we simply multiply it by the effective radius of the disc (Ra)
Brake Torque = Effective radius x Brake Force
T = Fr . Ra
Following this to get the longitudinal force (Fl) at the tyre we simply divide this torque by the radius of the wheel Rw
Longitudinal force = Brake Torque / Radius of the wheel
Fl=T/Rw
So just for completeness I've re-arranged these equations and put them in the form of the first equation I posted (attached)
And just for the record, I'm not just a number cruncher!
Hi Colin,
Thank you,I don,t think for a minute that you are just a number cruncher.You have shown me the more scientific/technical side of things I practice and made me learn a few things,thank you.
One last question, what is Cp in the equation from the bosch book please?.
Cheers
Andrew
The Cp is just a compilation of all the physical properties of the braking system in one term, to give a very simple way of calculating resultant brake torque for a known pressure, this is important for the kind of work I do, but not so important for the explanation so I left it out. Drum brakes are sometimes specified with a Cstar value from manufactures, and this is the same thing