Caravelle 1965

[4cvgordini]

Well, I have ignored fashion & "group think" in my academic life to the point of being almost officially declared feral. But: unfashionableness does not entail truth & fact citation should be relevant to the task at hand. Nothing in Harrisonati's document contra-indicates front brake lock-up in extremis.

My appraisal of Renault's design of brake-pressure management is that it was dangerously incompetent.

 

[JohnW]

Well, I have ignored fashion & "group think" in my academic life to the point of being almost officially declared feral. But: unfashionableness does not entail truth & fact citation should be relevant to the task at hand. Nothing in Harrisonati's document contra-indicates front brake lock-up in extremis.

My appraisal of Renault's design of brake-pressure management is that it was dangerously incompetent.

All of which is exactly the benefit of our open forum I'd say.

 

[harrisonati]

Well, I have ignored fashion & "group think" in my academic life to the point of being almost officially declared feral. But: unfashionableness does not entail truth & fact citation should be relevant to the task at hand. Nothing in Harrisonati's document contra-indicates front brake lock-up in extremis.

My appraisal of Renault's design of brake-pressure management is that it was dangerously incompetent.

I do realise that my views are unfashionable and I really don't want to start WW3. However .....
Consider the case of a RER with a PL valve which had failed in the closed position. Braking would improve with the PL valve removed, because the failed PL valve wasn't allowing the rear brakes to do any work. I believe that is the situation when most people replace their PL valve with a T piece.
Consider the case of a RER with a properly operating PL valve. It would limit the pressure to the rear brakes only at high braking pressures. Those high braking pressures would normally be generated only in the dry and not in the wet when the driver was being careful! If the driver did lock the front wheels in the wet, it would likely be at pressures not high enough to not cause the PL valve to limit the pressure to the rear brakes.
Consider the issue of locking front wheels in the era before anti-lock brakes (with the probable exception of hydraulically suspended Citroens). You could easily lock the front wheels of most cars by jamming the brakes hard on before weight transfer had occurred. It was even easier to do that in the wet. The ability to lock the front wheels with or without the PL valve proves little.
As Greenpeace observed earlier, the best illustration would be a 'back to back' test - same car, but with one test being with a properly operating PL valve in place and the other with the PL valve replaced by a T piece.
Finally, I think that there is a case for some small limitation of stopping power in the interests of stability. I was never as happy with the extreme braking performance of my R8 as I was with my R12. The R8 was probably stopping faster but sometimes felt decidedly 'iffy'. The R12 was always the model of stability! With the R12, you could use 100% of it's performance (not just braking) just about everywhere - something that you couldn't say about an R8.

 

[56 Fregate]

.

 

[Greenpeace]

Going by what I have read on other forums the biggest problem these valves have is "gunking up". I would say this is due to the probability that many drivers would simply never have braked hard enough to activate the valve in normal driving conditions anyway.
The lighter of the two (known) early Renault valves has a close to 800 psi trip point, that is getting into near panic stop pressures. Given the widespread availability of replacement valves, one would assume the failure rate, and therefore replacement rate, is high?
Anyhoo, I find it odd that on other AF threads, empirical data is apparently the only acceptable evidence in regards to the vehicle's (and therefore the components being tested) performance, and that anecdotal evidence is non-admissible.
Yet with brakes a conclusion can be drawn, based only on anecdotal evidence, and with zero supportive empirical data.

 

[JohnW]

I do realise that my views are unfashionable and I really don't want to start WW3. However .....
Consider the case of a RER with a PL valve which had failed in the closed position. Braking would improve with the PL valve removed, because the failed PL valve wasn't allowing the rear brakes to do any work. I believe that is the situation when most people replace their PL valve with a T piece.
Consider the case of a RER with a properly operating PL valve. It would limit the pressure to the rear brakes only at high braking pressures. Those high braking pressures would normally be generated only in the dry and not in the wet when the driver was being careful! If the driver did lock the front wheels in the wet, it would likely be at pressures not high enough to not cause the PL valve to limit the pressure to the rear brakes.
Consider the issue of locking front wheels in the era before anti-lock brakes (with the probable exception of hydraulically suspended Citroens). You could easily lock the front wheels of most cars by jamming the brakes hard on before weight transfer had occurred. It was even easier to do that in the wet. The ability to lock the front wheels with or without the PL valve proves little.
As Greenpeace observed earlier, the best illustration would be a 'back to back' test - same car, but with one test being with a properly operating PL valve in place and the other with the PL valve replaced by a T piece.
Finally, I think that there is a case for some small limitation of stopping power in the interests of stability. I was never as happy with the extreme braking performance of my R8 as I was with my R12. The R8 was probably stopping faster but sometimes felt decidedly 'iffy'. The R12 was always the model of stability! With the R12, you could use 100% of it's performance (not just braking) just about everywhere - something that you couldn't say about an R8.

Blow fashion. It is an interesting discussion, for which thanks.

I used to have the opposite problem wth my PL valve. It would click in loudly with low-ish pressures. Just bleeding the brakes was difficult and the rear pads wore very little. My diagnosis was the opposite of gunked up, rather it activated at much too low a pressure. Our car was known to have had regular fluid changes, unlike most I agree. Back then I knew little about these things and the local dealer seemed to know less.

 

[4cvgordini]

.

I disagree with every point you make here.

 

[harrisonati]

Going by what I have read on other forums the biggest problem these valves have is "gunking up". I would say this is due to the probability that many drivers would simply never have braked hard enough to activate the valve in normal driving conditions anyway.
The lighter of the two (known) early Renault valves has a close to 800 psi trip point, that is getting into near panic stop pressures. Given the widespread availability of replacement valves, one would assume the failure rate, and therefore replacement rate, is high?
Anyhoo, I find it odd that on other AF threads, empirical data is apparently the only acceptable evidence in regards to the vehicle's (and therefore the components being tested) performance, and that anecdotal evidence is non-admissible.
Yet with brakes a conclusion can be drawn, based only on anecdotal evidence, and with zero supportive empirical data.

.

I disagree with every point you make here.

I just thought that I'd left 56 Fregate speechless!

 

[56 Fregate]

...I was just pointing out the bleedin' obvious.

 

[Fireblade]

Here is a good explanation of pressure limiting valves work and their rationale

How Does a Proportioning Valve Work? And How Do You Adjust It?

When setting up a street car for performance driving or converting a classic to disc brakes, it is essential to include an adjustable proportioning valve in your brake system. This valve is typically plumbed into the rear brake line or built into the combination valve below the master cylinder...

shop.wilwood.com

And a graph from said article.

The Scientific American workshop manual for the Renault 10 suggest the kick point is 415 psi with a 50% reduction in pressure. I think these graphs are for 57% reduction in pressure. Interpolating from the graph - 800 psi at the master cylinder - rears would see 600 psi - a fairly modest reduction. At 1200 psi (which is probably about where the pedal hits the floor) rears see around 800 psi. Under most driving conditions, the effect is quite subtle - and doubt any of us would really pick whether the limiter was there or not.

 

[JohnW]

Here is a good explanation of pressure limiting valves work and their rationale

How Does a Proportioning Valve Work? And How Do You Adjust It?

When setting up a street car for performance driving or converting a classic to disc brakes, it is essential to include an adjustable proportioning valve in your brake system. This valve is typically plumbed into the rear brake line or built into the combination valve below the master cylinder...

shop.wilwood.com

And a graph from said article.

The Scientific American workshop manual for the Renault 10 suggest the kick point is 415 psi with a 50% reduction in pressure. I think these graphs are for 57% reduction in pressure. Interpolating from the graph - 800 psi at the master cylinder - rears would see 600 psi - a fairly modest reduction. At 1200 psi (which is probably about where the pedal hits the floor) rears see around 800 psi. Under most driving conditions, the effect is quite subtle - and doubt any of us would really pick whether the limiter was there or not.
View attachment 229692

Thanks for those curves!

 

[4cvgordini]

I do realise that my views are unfashionable and I really don't want to start WW3. However .....
Consider the case of a RER with a PL valve which had failed in the closed position. Braking would improve with the PL valve removed, because the failed PL valve wasn't allowing the rear brakes to do any work. I believe that is the situation when most people replace their PL valve with a T piece.
Consider the case of a RER with a properly operating PL valve. It would limit the pressure to the rear brakes only at high braking pressures. Those high braking pressures would normally be generated only in the dry and not in the wet when the driver was being careful! If the driver did lock the front wheels in the wet, it would likely be at pressures not high enough to not cause the PL valve to limit the pressure to the rear brakes.
Consider the issue of locking front wheels in the era before anti-lock brakes (with the probable exception of hydraulically suspended Citroens). You could easily lock the front wheels of most cars by jamming the brakes hard on before weight transfer had occurred. It was even easier to do that in the wet. The ability to lock the front wheels with or without the PL valve proves little.
As Greenpeace observed earlier, the best illustration would be a 'back to back' test - same car, but with one test being with a properly operating PL valve in place and the other with the PL valve replaced by a T piece.
Finally, I think that there is a case for some small limitation of stopping power in the interests of stability. I was never as happy with the extreme braking performance of my R8 as I was with my R12. The R8 was probably stopping faster but sometimes felt decidedly 'iffy'. The R12 was always the model of stability! With the R12, you could use 100% of it's performance (not just braking) just about everywhere - something that you couldn't say about an R8.

First, allow me to express my appreciation for the civility & thoughtfulness of your remarks. However, you're wrong. Why?

I am in the unusual position of fanging the same vehicle from when it was new (1966) until now. Some empirical data (yes, anecdotal but . . .):

When new (thus when valve-new) it locked fronts wet or dry unless I deliberately cadence-braked (didn't know about threshold-braking at the time). This was exacerbated by the atrocious Michelins with which it was sold. Fine, but in an emergency I am not good enough to be cadence or threshold-braking; I just hammer the brakes. And, for anyone, an emergency braking episode can be, & is most dangerously, in the wet.

This front-locking trait continued even when better tyres went on the front. And the valve being deleted occurred with the same tyres as just before it was deleted. The fronts locked early before & almost didn't after.

I am tyre-interested & always play when tyres change & certainly played when the valve was deleted. It made a difference & the (old valve) performance was not noticeably different to the (new valve) performance in the sixties when the car was new.

I submit that the valve is not malfunctioning owing to age, poor maintainance or whatever but functioning dangerously owing to ill-conceived-of design.