Brake Bias

The built in bias control in the Corvette master cylinder, or most any semi modern master cylinder from a front engine car, keeps the pressure down on the back brakes and puts more pressure where the weight is. With GT40s we have to do the opposite, so we put the bias adjuster on the front brake line to hold it back and put more on the rears. This seems to me to be fighting the master cylinder's built in bias. This system seems to work on several cars: ERA, DRB, Sabre to name a few.

My cousin-in-law, Chuck, is a good ole country boy who lives on Lake Okechobee in Florida and is also the head mechanic for all of the county's equipment down there from police cars to bull dozers. I posed the issue to him. He suggested that I try just reversing the brake lines in the master cylinder to let its built in bias work for me rather than fighting against it with an auxiliary bias adjuster. It is possible that, alone, the built in bias might be too strong in the back this way, but it should be closer than the way it is done now or so it would seem. This might require dampening the rear brake lines a little.

Knowing that dampening the front brake line in standard position has worked for various cars, I haven't tried Chuck's suggestion, but I still think about it from time to time.

Any opinions on this. (Some of you who have been around a while might remember this; I think I posed this a long time ago. But then again, at my age, I am not sure. Anyway, I don't recall that there was a clear cut answer if I did

Lynn,

ERA's design uses standard Corvette calipers, but uses dual Girling master cylinders with a bias (balance) bar between them. 3/4" ID front and 5/8" ID rear.
http://www.erareplicas.com/gt/brakebox.htm

Dave

Lynn,
Have you considered the dual master cylinder route? It does not permit power assist, but has cable adjustable balance.

Gary

Lynn, my gut feeling tells me you would be creating more problems than you would solve, the rear of the master cylinder (front brakes) has a larger brake line and a larger passage from the reservoir to allow enough fluid through to fill the dual piston calipers. I suspect you could take it apart and drill the caliper to allow more fuid through but.... The other thing, and I dont know if this is true but I would think that the difference of swaping the lines would seriously under power the front and overpower the rear. On the other hand it wouldnt be too hard to try

I'm no expert on brake systems, but it would seem to me that a simple adjustable proportioning valve mounted so that you could tweak it as you drive would be the best and most effective way to deal with this problem. Isn't this what is fairly common current race car technology? The driver can adjust his bias when he takes on a load of fuel and ease it off as the fuel is consumed thereby keeping perfect balance, at least theorecically, throughout the race. Most race car technology usually ends up on the street, but this is a case where it is probably too high tech for production cars. Heck, can you seee granny adjusting her brake bias on her trip to the supermarket?
Regards, Blue

Blue, that is what I meant by dampening. I have a Wilwood proportioning valve on the front brake line so that I can reduce its pressure and put a bit more on the back where the weight is. What makes me wonder about this is the fact that the master cylinder has a proportioning valve build into it doing the opposite.

There is no problem with the lines. Both lines have a union just below the master. Well the front has the proportioning valve the rear has a union on to the brake line that was alread mounted through the tunnel. The flare is the same on both sides of the master, but, as you noted, the fittings are different sizes.

Lynn,
I have a strong feeling you are heading for disaster by putting the brake bias "where the weight is". A GT40 is not even close to a full reversal of a normal car, when it is slowing down, so fitting a reversed bias will not be the way to go....
The the moment you slow the car when travelling forwards,the weight shifts most definately to the front.
For example:
if we assume the wheelbase to be 95",
the center of gravity to be 14", and the static weight distribution to be 55% rear:
The weight transfer forward is 14/95 i.e. 14.7% per "G"
So, at only about one third of a "G", which is very close to 7.5 mph per second deceleration, the weight distribution is about 50/50.
So you could argue that on a system where the hydraulic ratio varies according to pressure, that 50/50 at the tires may be a good starting point.
And at 0.9 "G" ( getting near maximum with some tires), the weight works out to be 58% front, 42% rear.
At 1.4 "G" (a race tire, for example), the weight distribution would be about 65.5% front, 34.5 rear...almost 2:1

Given that you do NOT want the rear tires to lock first, and that there may also be a strong engine breaking component at the rear tires already, you can see that the brake force at the tires needs to be something like 60% front (or more) at hard braking on a dry road.
This brake bias is also directly effected by tire radius, disc swept area radius, and the friction characteristics of the pads themselves..
Feel free to give me a call should you need any details!

Rob

I use a twin master cylinder system on my car which has a fixed ratio between front and rear brakes. If a proportioning valve were fitted to one brake line to reduce its pressure, then I could not see how this would transmit any more pressure to the other brake line. Overall you have only reduced the total amount of pressure out there. Yes the ratio has changed but you have reduced your overall braking force.

To overcome this I would look at a balance bar system. This is fitted to the brake pedal and you have two separate master cylinders. By changing the leverage points of the balance bar in the pedal you will affect the ratio of balance front to rear but the overall amount of braking force remains constant.

I have all the parts to do this change but won't do it until I have a system of measuring the brake pressures so I know that I haven't got a daft setting and can compare one setting to another with facts to back it up. You can get pressure guauges and also telemetry sensors for this.

In the GTD club mag a few issues ago we published Tech Tip pages from the Rally Design catalogue (with their permission) and it covered the way to install a balance bar system.

Malcolm

Malcolm
As a member how would I go about getting some of the articles of back issues? Is there a fee? Would like to get some of those back articles.
Bill

The details of good braking and how to get it are beyond my expertise. However when I decided that my original GTD brakes needed to go I did the following: Had an experienced race driver take the car out and evaluate the stock system, result: way over braked in the front and under braked in general. Solution: Went from 10 inch unventilated rears to 12 inch ventilated Corvette discs redrilled to fit bolt pattern and replaced caliper with one of similar design but larger pad area (Mitsubishi Starion). Front: Fabricated new mounting brackets for Nissan 300ZX 4 pot calipers and 11 5/8" vented discs to replace the old 10 1/4" Granda discs. This system was all tested earlier by Ralph Todd (old Lola man) on his super modified "Scorch" (english Ford with twin turbo Nissan). Result: car stops without nose diving, the rear end doesn,t dance from unloading and I can run for 30 minutes of track time without over cooking the brakes or suffering fade. It is also easier to modulate the brakes to just short of lock up. You can also take the easier way and just buy a complete wilwood setup. My system is still using the original Master cylinder with a proportioning valve on the front brakes which I run slightly restricted.

Malcolm, Very good points. This is exactly what I was looking for. I agree that using pressure gauges will allow you to quantify the affect of changes you make to the system and get your initial settings in the ball park. But, I think more than setting numbers on the gauges, one will have to get on some tarmack somewhere and test the actual operation of the brakes to make certain that the balance is correct in real life use. Knowing where you came from and where you are going through the use of gauges will greatly reduce the guess work and time it takes to dial the system in though!

You are absolutely correct in you assesement of what the proportioning valve is doing when used on a double reservoir (isolated front back system). And this is the point I was missing in my thinking. But, I am not sure that it is necessarily as bad as it sounds. The system employed here was designed for a car that is nearly double the weight of my GT40. As long as the system has the clamping power to stop wheel rotation with a reasonable pedal pressure then you are ok, IF the system can also do this repeatedly, without fade. This latter requirement has more to do with the overall design of the system than it does with the amount of pressure in the brake lines. For instance, the ability of the lines to maintain a static dimension under the pressures exerted and with the heat built up from the braking. This has to do with mainenance of the pressure, but I suppose having a surplus wouldn't hurt . And, before anyone gets their panties in a wad, I know that the point of braking is NOT TO STOP wheel rotation, but the system must have the ability to take it to the verge of stoping.

Undoubtedly the balance bar method with two separate systems is the ideal to then fine tune the braking system to loads front to back. Unfortunately, not all cars were designed to employ this system and for normal road driving, which is where those of us in the states will spend the vast majority of our time, the expense and difficulty of converting may not be justified.

Lacking any impiracal data on the performance of the stock Corvette (85-87) vacuum assisted master cylinder combined with 85-87 Corvette front calipers/rotors and 88-92 Corvette rear calipers/rotors on a space frame GT40 with 17" tires, etc, etc, I will have to develop my own through testing once the car is done. But as a starting point, your observations, Malcolm, tell me I am probably starting with the proper configuration of my particular set up. If anyone has a braking set up similar to mine, I would love to hear about your experiences with it!!

Bill

We have a committee meeting this coming Sunday so will ask the question for you then but don't let me forget to come back to you!

Lynne

Definately the correct settings will be found from use of the car as opposed to static setup. However, when one day we were at Curborough Sprint circuit some years ago, Paul, Robin and myself found that we had Tom Hammond in his Pikes Pike Audi Quattro. This is the most awesome beast going with 720 bhp from 2.2 litres of engine. Paul was positively drooling over the spec. Anyway he told us the tale of when he did a run and spun off at the first corner. His grandson had sat in the car and twiddled the brake balance knob! Gave Grandpa too much rear brakes! Hence I want to be able to set the brakes up and then know what the static setting is from the rolling setup so checks can be made. Also different venues may require different setups?

If I remember correctly the car that GTD built for Jonathan Palmer (Ex F1 driver) had a proportioning valve. Car now in new ownership but still with proportioning valve as far as I know. He wanted this so that he could adjust his brakes in mid corner (!) to allow him the ability to give his passengers a good thrill by hanging the back end way out. He used the car as part of his Driving Experience package for a while before he changed to another type of car. He got through many tyres I believe. The valve was situated next to the gear stick. I think he had it in the rear brake line so that he could take away his rear brakes, brake the front wheels and gas the rears. Now children, don't try this at home, it seems suicidal to me!

I have waffled on about my thoughts on what a brake system should be able to do in much earlier postings (use search facility) so won't repeat here. But I would say again that the "old" standard GTD setup of Granada vented front discs and solid non vented rear discs was dreadful and anyone with this set up has a car with much more go than stopping power. You will also eat up brake pads if you get enthusistic enough.

Sorry but can't comment on Corvette bits as they are more rare than GTD's round my way! However with 17 inch rims you have no excuse to not being able to get a good system in that will more than satisfy your needs.

What Bud seems to have done and without having experienced it, is a good way to go or a style of system to copy. Although I would use a balance bar over a proportioning valve given a choice.

Malcolm

Everybody goes on about mounting the valve where you can reach it from inside the car. For track use, running the car at the edge, I can see this being very important. One would need to change the bias as fuel burns off. Which way you would change would obviously depend on the configuration of the car and where the gas tank(s)are. You can imagine how much change there is for the NASCAR boys with the weight stuck way out at each end of the car! Having a reachable valve would make tuning easier as well so you wouldn't have to stop between each run as you try to get the rears to lock an RCH ahead of the fronts (can I say that here?). I agree with your reaction to having no rear braking power (unless you are doing burn outs prior to a drag run

But, again for normal highway driving, I think one should probably set up for the "norm": probaby 3/4 tank of fuel and maybe a passenger and then possibly tweak as tires wear or maybe when the temperature makes large changes (summer vs winter)or when you get a chance to go to the track. (Boy, are we jealous of how much track time you blokes get!) I know myself and if it was within reach, I'd be fiddling with it all of the time. Sooner or later, I'd fiddle to much with it and end up spinning off a corner as you describe. My valve is just after the master, so once I get it tuned, I will lock it down and only make changes judiciously.

The beauty of the balance bar is its simple elegance. As an engineer, I have become a strong advocate of the KISS principle. Keeping a solution as near to fundamental physics with as little complexity as possible greatly increases the chances that it will work and continue to work and greatly decreases the chances that it will break or go wrong.

Lynn,
Did i read you right that you have your proportioning valve in the front line. Surely you would be better with it in the rear close to your reach for fine tuning while running.
I race hard over here in the UK in the Sports Racing & GT Series and was out testing at Brands Hatch yesterday where i did 60 laps of hard driving and braking with no problems whatsoever.
I run on 15inch rims so i do not have the much wider choice of caliper/disk setups that you have with 17 inch but my valve gives me perfect bias adjustment and it is fitted to the rear line.
If you intend to race your car i think your best bet is twin master cylinders/adjustable bias bar but at the end of the day the choice is up to you.Only you know the feel of your car and what you want to do with it.
Regards,
Graham Turner,
GTA.

Just a couple of comments.

The balance bar does allow the spliting of front to rear forces (for example 60% front, 40% rear) for all pedal forces. In other words gentle stops will still have the same split as hard stops. On the other hand the proportioning valve (if I correctly understand its operation) acts like sort of a releive valve and limits (or reduces) the pressure at the rear of the car. Thus up to the point of pressure limiting the rears could be doing more of the stopping (gentle stop). At the limit the rear valve would brake over and have a balanced system.

I have a program written in Matlab (free to whomever uses Matlab) that will study the braking balance of a car. It is set up for balance bar type systems, but could be used for single master cylinder systems also. It is fairly extensive in terms of what it is looking at and requires a lot of inputs like disk diameters, coef of friction for pads, stoping rate (say 1g), wheelbase, cg height, etc. What you get is the ability to play around with piston sizes for the calipers and master cylinders and get and idea of what the pedal force would be for the stop and what the balance bar setting should be (in the middle). I feel this will give you a good starting point then evaluate and adjust the balance bar at the track.

Graham, apparently you missed the part where I said I (we) are very jealous of you guys in England who get so much track time. We get very little and most of our driving is on the public highways. Unfortunate, but true.

Lynn,
I did see your comment but don,t get jealous about track time.It comes at some considerable cost, both financially and mechanically!!
For instance those 60 laps i did at Brands Hatch last week cost me well over £200 and finally finished off a set of expensive and scarce Pirelli Road/Race tyres.I am currently the only regular circuit racing 40 in the UKs Sports Racing & GT Series and each round rights off £200-£300 minimum with entrant fees, fuel, oil,etc,etc.After each race the car is gone over from top to bottom in the workshop to check for the rapid wear and tear that circuit racing brings -- far greater than normal road use i can assure you.So please if you do want to race or do occasional track work expect to pay for it one way or another.
Regards
GTA.

Actually the manufactures of separate master cylinder bias bar setups make a cable operated remote control for bias bars which allows bias bar adjustment while driving.
http://www.tiltonracing.com/pdf_files/pg6.pdf
http://www.tiltonracing.com/pdf_files/pg4.pdf
http://www.wilwood.com/products/pedal_assem/bias/bias.asp

the relatively new book "Brake Systems"
ISBN 1-55788-281-9

has up to date info for racing brakes and formulas and info for calculating master cylinder sizes, rotor sizes etc.

GTA,

You’re absolutely correct about the disadvantages. Now all we need to do is find a way to get rid of that person who is standing over us with the big whip, forcing us to go racing against our will. Then we can all get back to our true love of knitting and macramé (excuse the dry Australian humor).

But seriously folks, the buzz of being on that racetrack running shoulder-to-shoulder at the limits of the car’s and your abilities just can’t be beaten. For me, its worth all that money, even given that I’ve managed to roll a car once (and walk away). The only time when doubts form in my mind is (strangely) while I’m sitting on the starting grid, my pulse racing, waiting for the flag to drop, and thinking “what the heck am I doing here!!!” then we’re off and I remember exactly why I’m there.

Lynn,
I’m not sure I understand why you don’t have the opportunities to go racing in the land where the car is so appreciated. It sounds to me like there might be a market in the USA for a good race circuit... any entrepreneurs in the USA willing to make a buck?

I didn't say there were no opportunities, but they don't seem to be as plentiful as they are on your side of the pond.

In order to get on a track with a replica, one basically has to hook up with a car club and go with them. This might be a regional level BMW, Porsch, 'Vette, etc. organization who rents a track for a day and to help defray costs will allow others to join in.
The closest thing to your hill climbs are rallies in which a replica car is 1) somewhat frowned upon in general and 2) really doesn't fit well in the classes set up. There is a definitive bias against replicas in the US when it comes to organized competition. This is starting to change a little with events like the Run-N-Gun set up specifically for replicas.

I have never actually participated in these, nor have I researched this thouroughly yet (I am just trying to get the thing built at this point); so, if I am terribly misinformed, maybe someone from the US who has more knowledge on the subject of racing/track opportunities for replica cars would want to start a thread on the subject in the "All GT40" section. If this subject has already been ridden into the ground, I'll look it up when the car is done and I'm looking for the chance to test its limits.