Caravelle 1965

[Bustamif]

Right as usual Frans, good basic engineering will usually answer the question. Brake temps will confirm why the rear pads on a standard R8/R10/Caravelle setup don't wear. The answer is they are not doing any braking.

Standard rear engine Porsches of the late 60's did not use a pressure limiter on the rear, the balance was achieved by correct piston size front and rear. Throw the rear limiter away on an R8/R10/Caravelle and it will stop like a Porsche.

No need for a brake booster in answer to the original question

 

[Fireblade]

Have you considered that the main purpose of the pressure limit valve is not "to prevent rear wheel lock up"? I think the valve is masterpiece of engineering and must be taken in the context of the whole design of the brake system. I propose it has three main functions:

1. Improve pedal height
2. Improve the progressive feel
3. Provide dynamic front-rear brake balancing - rear brake bias under light braking and forward bias under heavy braking

1. Improve pedal height:

First of all, I have pulled apart a limiter on the Caravelle, and there is a pressure return valve in there similar to the one in the master cylinder - to keep the pistons from pulling back into the callipers. Less distance for the piston to travel on initial application = better pedal height. If you are going to delete the limiter, you should at least consider a low pressure valve suitable for discs.

2. Progressive feel

My partner's VW has very touchy heavily boosted brakes. My Laguna had seemingly soft brakes that progressively got harder the more you push on the pedal. Plenty of feel and travel for mild to moderate braking - solid brakes under pressure. I think this approach has its roots in Renault's design approach. The effect of the limit valve coming into operation only above a set pressure is to stiffen up the brakes under heavier application.

3. Dynamic front-rear braking balance

Why did Renault specify smaller pistons for the rears when the car has the same disc size/swept area front and rear? Without any pressure limiting, the difference in piston size means the brakes are biased TO THE REAR. Yes, I know a lot of you will argue it works the other way round, but look it up, smaller pistons = more force on the pads for a given pressure and swept area.

So the braking engineers have designed the set up so that for mild braking, the balance is weighted to the rears, under moderate braking (when the pressure limiter starts to kick in) it will move through neutral set up, and under heavy braking it will weight towards the front.

So in summary, if you want to maintain good pedal height, a progressive feel and well mannered, balanced braking - keep your limit valve working in good condition. If you want to do screaming drifts on a dirt track, by all means, delete the valve.

 

[Frans]

Why did Renault specify smaller pistons for the rears when the car has the same disc size/swept area front and rear? Without any pressure limiting, the difference in piston size means the brakes are biased TO THE REAR. Yes, I know a lot of you will argue it works the other way round, but look it up, smaller pistons = more force on the pads for a given pressure and swept area.
e.

I am sure you have it wrong. The smaller the master, the better the clamping of the discs but then it requires more travel. Remember that the hydraulic jack has a small piston on the lever (foot) and a big piston in the cylinder. (caliper).

Regards Frans.

 

[Fireblade]

With respect Frans, your example only proves my point. The bigger the piston on the jack - the less movement there will be - and similarly, the less "squeezing" of the brake disc. Don't confound the "ratchet" effect of a hydraulic jack with pedal effort.

If what you say was true, then the brake engineers would - in effect - have designed the system to weight the fronts on mild braking, neutral on moderate braking and the rears on heavy braking - what would be the sense in that?

 

[Greenpeace]

I found a period Australian article (1965) on the R8 Gordini. It said earlier on in the piece that the brakes were pretty much the same as the regular R8 other than harder pads and the booster.
It has alrèady been mentioned that modern pad material can be harder than what was used "back in the day". Sans the availability of "soft" pads, wouldn't a booster be a logical upgrade?
The OP hasn't been back since he started the thread, but one would assume he's just after his brakes feeling a bit more like a modern car, rather than that he's setting it up for track days?
Anyhoo the tester in the Australian Motor Sports magazine article speaks quite highly of the boosted Gordini brakes.

 

[JohnW]

Well, Mr Fireblade, ever so politely I disagree, not least on the basis of personal experience. The valving cannot maintain back pressure in these systems or the pads won't retract from the disc, because the 'o' ring torsion does that. I finally threw away my R8 non-return valve after 30 years of ownership and the braking improved significantly in progressiveness and the rears actually were working much of the time. Nice and light they are. I have fitted the largest master cylinder diameter, which minimised free movement although it makes pedal pressures a bit higher.

They are subject to a wee bit of knock-off but there is no doubt that the braking is improved by throwing away that valve. I also agree with Frans. It is the ratio of the master cylinder diameter to the calliper piston diameter that controls the force applied and it must be a greater force at the front with the larger diameter pistons.

 

[Greenpeace]

Different application, but same principles as being discussed.
I fitted a complete drum brake Borg Warner diff out of an XA Falcon into a HK Holden. First drive under mild braking and the back wheels would lock up without warning every time. Went home and pulled the drums and wheel cylinders off both diffs. The Holden wheel cylinders had 9/16" pistons and the Falcon wheel cylinders had 13/16" pistons. After a fair bit of fiddling I fitted the smaller diameter Holden cylinders onto the Falcon backing plates, and problem solved.
So yes, with all other things being equal, larger diameter piston/s at the wheel/s will provide more braking force to those wheels.

 

[4cvgordini]

To Fireblade: re your last point (dynamic . . . .), what I don't want with a light-front car in a crash stop in the wet is more effort directed to the front.

 

[Greenpeace]

The bigger the piston on the jack - the less movement there will be - and similarly, the less "squeezing" of the brake disc. Don't confound the "ratchet" effect of a hydraulic jack with pedal effort.

Remember the pads are already skimming the discs, so there is negligible movement. Depressing the brake pedal is predominantly applying pressure rather than generating movement. With its greater surface area a larger piston will apply more pressure than a smaller one will.

 

[Frans]

So the braking engineers have designed the set up so that for mild braking, the balance is weighted to the rears, under moderate braking (when the pressure limiter starts to kick in) it will move through neutral set up, and under heavy braking it will weight towards the front.

The balance cannot be weighted to the rears because the lines from the master to the front calipers are direct and without restriction. The line to the back has the limiter in it, mechanically it cannot boost the pressure to the rears, it can only restrict it.

Pressure applied to any system, cooling, tyres, brakes, etc. will always be the same in all directions. That pressure will be the same in the braking system up to the limiter, there after it will be limited and can not be boosted to get more braking on the rears. It can only be boosted with some external devices.

The pad pressure is released/retracted by the "roll" of the rubber seal between the caliper and piston which is minute. That is why the pads will always be skimming the disc with only an air gap between them.

Regards, Frans.

 

[Greenpeace]

What the limiting valve does really depends on it's initial design parameters.
Given that the car in question has equal diameter rotors with 38mm front caliper pistons and 32mm in the rear, even without the limiting valve in place, the rears can only ever apply 70% of the force that the front ones do in a single circuit system.
I don't know the specs or indeed the exact design of the Renault valve, but I assume it is a regular 2 piston proportioning valve?
For discussion's sake, say it has a 400 psi line pressure trip point, up to that point the rear brakes will be working at their maximum potential relative to the front brakes (70%).
The diameter of the valve's pistons are unknown (in this thread anyway) but let's say there is a 50% difference in surface area. So the valve trips at 400 psi, the brakes get applied harder and the main line pressure rises to 800 psi so the valve will now transfer 400 psi plus 50% of the extra main line pressure to the rear, so at 800psi of main line pressure, the rears would be receiving 600 psi of pressure via the valve. At 1200 psi of main line pressure, the rears would receive 800 psi of pressure.
So you can see there is a curve where the the rear line pressure (relative to the front) falls away (as percentage of the front) as the main line pressure rises, which makes sense, as the higher the main line pressure, obviously the more the weight of the car is transferring forward under brakes.
Again I'm assuming the Renault has a two piston proportioning valve, as fitting anything else really wouldn't make sense?
So Fireblade is sort of correct in that the rear brakes should be working at their maximum potential (70%) up to the point where the limiting valve begins to take effect, and at that point the overall braking effort is gradually transferred, (at a predetermined rate by the limiting valve) to the front brakes.
One would have to think if rear pads are not wearing at all (with the limiting valve in place) that the valve itself is faulty?
Of course the Renault's braking was set up to work at its best ex factory, so once one relative element changes the whole thing is mucked up.
Even using different pad material front to rear will upset the effects of that predetermined brake balance.

 

[jaahn]

Different application, but same principles as being discussed.
I fitted a complete drum brake Borg Warner diff out of an XA Falcon into a HK Holden. First drive under mild braking and the back wheels would lock up without warning every time. Went home and pulled the drums and wheel cylinders off both diffs. The Holden wheel cylinders had 9/16" pistons and the Falcon wheel cylinders had 13/16" pistons. After a fair bit of fiddling I fitted the smaller diameter Holden cylinders onto the Falcon backing plates, and problem solved.
So yes, with all other things being equal, larger diameter piston/s at the wheel/s will provide more braking force to those wheels.

Yes the wheel cylinder bore size was used to match the braking force required for different models and weights. Years ago i built a buggy with a holden rear end and drums and fitted the lowest diff ratio. The front brakes were too powerful compared to the back so i fitted bigger rear wheel cyls and it was OK. Then after a while i moved more weight to the rear, engine, battery repositioned etc for better traction and needed more rear braking. So i fitted the biggest size cyls available and it was then perfect balance for dirt braking. The front discs stayed the same.
Jaahn

 

[JohnW]

"So Fireblade is sort of correct in that the rear brakes should be working at their maximum potential (70%) up to the point where the limiting valve begins to take effect, and at that point the overall braking effort is gradually transferred, (at a predetermined rate by the limiting valve) to the front brakes."

Yes, I wouldn't argue with that point. There were at least two types of limiting valve used over time, and I don't know of any that were actually tested for effectiveness when a few (or many) years old. The one I threw away certainly came in with a loud click if I braked hard (and made conventional bleeding difficult). Not a progressive device at all. In practice, the brakes improved without it. I think all it did was cut off the rear brakes if you braked hard enough. That's a 1964 device. the later ones look quite different and might even have been sophisticated in the way you suggest. The later FWD Renaults had a progressive pressure regulator controlled by a lever to rear suspension position. So full braking if you had a good load in the back, for example.

 

[Greenpeace]

"So Fireblade is sort of correct in that the rear brakes should be working at their maximum potential (70%) up to the point where the limiting valve begins to take effect, and at that point the overall braking effort is gradually transferred, (at a predetermined rate by the limiting valve) to the front brakes."

Yes, I wouldn't argue with that point. There were at least two types of limiting valve used over time, and I don't know of any that were actually tested for effectiveness when a few (or many) years old. The one I threw away certainly came in with a loud click if I braked hard (and made conventional bleeding difficult). Not a progressive device at all. In practice, the brakes improved without it. I think all it did was cut off the rear brakes if you braked hard enough. That's a 1964 device. the later ones look quite different and might even have been sophisticated in the way you suggest. The later FWD Renaults had a progressive pressure regulator controlled by a lever to rear suspension position. So full braking if you had a good load in the back, for example.

I found some info on the brake limiter valve.
So at a predetermined pressure the rear braking pressure flatlines, but doesn't reduce.
On the 1130 chassis this is at 50 bar and on the R1131 chassis this is at 75 bar.
75 bar is over 1000 psi of line pressure, so this valve on the Caravelle shouldn't be doing anything until some pretty serious braking is being undertaken.
The attached graph is for the 50 bar valve, as you can see at the trip point the main circuit pressure continues to rise, and the rear brakes maintain 50 bar of pressure.

 

[JohnW]

I found some info on the brake limiter valve.
So at a predetermined pressure the rear braking pressure flatlines, but doesn't reduce.
On the 1130 chassis this is at 50 bar and on the R1131 chassis this is at 75 bar.
75 bar is over 1000 psi of line pressure, so this valve on the Caravelle shouldn't be doing anything until some pretty serious braking is being undertaken.
The attached graph is for the 50 bar valve, as you can see at the trip point the main circuit pressure continues to rise, and the rear brakes maintain 50 bar of pressure.

View attachment 229224View attachment 229225

Thanks very much for digging that out. Interesting. I can note that I never had brake dust on the back wheels before I threw the thing away although it did click at me.

It obviously just shuts off the back line at 50 bars pressure, doesn't it. That's why hard braking on these cars used to lock up the fronts!

 

[Greenpeace]

Thanks very much for digging that out. Interesting. I can note that I never had brake dust on the back wheels before I threw the thing away although it did click at me.

It obviously just shuts off the back line at 50 bars pressure, doesn't it. That's why hard braking on these cars used to lock up the fronts!

The 50 bar is the R8 valve. The Caravelle uses a 75 bar valve. I'm not sure which the R10 has?
Even 50 bar (725psi) is quite a bit of line pressure for an unboosted system. I would have thought unless almost panic stopping, that a correctly functioning valve would remain fully open?
Most systems max out in the 1000 to 1200 psi range. Most brake testing gauges only read to 1500 psi.

 

[JohnW]

The 50 bar is the R8 valve. The Caravelle uses a 75 bar valve. I'm not sure which the R10 has?
Even 50 bar (725psi) is quite a bit of line pressure for an unboosted system. I would have thought unless almost panic stopping, that a correctly functioning valve would remain fully open?
Most systems max out in the 1000 to 1200 psi range. Most brake testing gauges only read to 1500 psi.

Curious isn't it. They are all essentially the same vehicle with the same braking system albeit with sight weight differences. The Caravelle overlapped with the R8 (and was first sold as the Floride S, with early R8 mechanicals).

I would have thought the same as you, that you'd need a near-panic stop to actuate the valve. And if avoiding rear lockup was the plan, that would only apply for crash stops.

The R8 was made over many years in several versions and various aspects changed over time, mine being a reasonably early one. Who knows. Florides were called Caravelles in USA to add to the confusion. Some Dauphines had the limiter too, also overlapping with early R8s.

Regardless of specifications and logic, I'm not the only one to report nicer, more progressive braking with the limiter removed. I can't see why, unless my limiter was blocked up or the valve spring had weakened (or it was the wrong limiter from the start?) but there you are...

Thanks again for the input.

 

[Greenpeace]

Curious isn't it. They are all essentially the same vehicle with the same braking system albeit with sight weight differences. The Caravelle overlapped with the R8 (and was first sold as the Floride S, with early R8 mechanicals).

I would have thought the same as you, that you'd need a near-panic stop to actuate the valve. And if avoiding rear lockup was the plan, that would only apply for crash stops.

The R8 was made over many years in several versions and various aspects changed over time, mine being a reasonably early one. Who knows. Florides were called Caravelles in USA to add to the confusion. Some Dauphines had the limiter too, also overlapping with early R8s.

Regardless of specifications and logic, I'm not the only one to report nicer, more progressive braking with the limiter removed. I can't see why, unless my limiter was blocked up or the valve spring had weakened (or it was the wrong limiter from the start?) but there you are...

Thanks again for the input.

Yes, you have to wonder if age or contamination is a factor. I found this period R8 road test, and it sounds like a new R8 braked very well with the valve in situ.

 

[rubyalpine]

Many theories and much information has been submitted on this subject,,,but "the proof is in the pudding".

It seems that all those who have removed the valve, have been satisfied with their decision.

Henry

 

[4cvgordini]

Yes, you have to wonder if age or contamination is a factor. I found this period R8 road test, and it sounds like a new R8 braked very well with the valve in situ.

View attachment 229227View attachment 229228

Ta; that is, I assume, the test I mentioned in post #6 (so: R8, not R10 -memory!) - he wouldn't have his hands out of the window if the rears locked! (Not that they would, even with limiter valve removed.)