i dont think the brake fluid exposed to air goes bad that quickly.. be interesting to see how long its been measured to take to become saturated with 3.7% water in a high humidity environment.

i also do what you do now too. suck out the old stuff, usually with a siphon and then re-fill and bleed. probably a good idea than pumping all the bad stuff around the seals

 

Mark, you don't believe that people can feel compressibility. We got it. No need to repeat. The number of times you repeat it does not increase the chances someone will suddenly change their mind and side with you.

And questioning Tim's integrity because he has a different opinion than your does nothing for your credibility. It was uncool and uncalled for.

 

Mark,

You can go on and on all you want about it not being possible to feel the difference between various brake fluids when fresh. It would seem to be a logical position to take considering the compressibility data of the various high quality fluids.

However, in the real world, conclusions based on "logic" do not always prove to be correct. Heck, I used to discount the idea as well. But, when I raced bikes, I heard statements from racers with excellent feel stating that liked the feel of some brake fluids over others. Early in my motorcycle racing career, I used Ate Super Blue and it worked just fine except that I had to flush it after every race weekend. A friend suggested I try Castrol SRF as it lasted almost all season for him. So I did. The first thing I noticed was the improved lever feel. This was comparing fresh Ate Super Blue to Fresh Castrol SRF. If you looked at the compressibility numbers, you would think there is no way I would feel a difference. Yet I did. And it wasn't a placebo effect.

Is something else going on here to change the feel other than compressibility? Heck if I know. All I know is that too many skilled people have said that they can feel the difference from one fluid to another for me to discount it out of hand.

The way you treated Olsen was deplorable.

 

Everyone here needs to chill out. It's absurd to be carrying on like this about brake fluid. We have differences of opinion. So what else is new?

There are a number of good choices in fluids, with reasons in everyone's mind about which is the best.

Why don't we discuss oil now? :-)

 

Something that might be going on to affect feel beyond the underlying compressibility is any difference in aeration propensity between fluids. Makes almost no difference at the 5000 PSI test pressures in the data Mark posted, but a lot more at typical brake system maximums like 1500-2000 PSI (Bulk Modulus is resistance to compression):



But I don't actually think it is necessary to reach that far to account for the perceived difference in the feel between fresh SRF and most other super Dot 4s.

If people can feel the difference when swapping brake hoses or pads, then it's not so strange to feel a difference between fluids, even if there is only a 25% difference in compressibility between them. I'm fairly inclined to believe those who have published specialist books on automotive braking (Braking of Road Vehicles, Andrew J. Day - and obviously we're the MC + 4 Caliper scenario), and they credit fluid compressibility with twice the contribution to pedal softness as the brake hoses, and more than 3 times that of the pad compressibility:



Once the initial slack from the pad-disc offset is taken up, fluid compressibility then accounts for about a third of the compressibility under normal use (moderate braking at 725PSI in the table above) in a road car. If we believe we are running stronger hoses and pads than the average road car, and we run higher volumes of brake fluid in our larger calipers, it's fair to say we'd be pretty safe in considering the compressibility of brake fluid to make up at least half of the initial pedal softness in our cars (before heating and boiling effects take over). The table looked at a fluid with a modulus of 0.007 though, versus our racing fluids at 0.0004 or 0.0005, so lets be conservative and go back to our fluids only contributing a third of the system's sponginess in moderate braking (eg when trailing into a corner). Going from a compressibilty of 0.0004 to 0.0005 is a 25% increase in the sponginess of a factor making up a third of the sponginess total. In other words, that means that the system as a whole is expected to become 8% more spongy in normal use due to changing between those fluids.

Could racing drivers who are trailbraking on the limit into corners feel an 8 percent increase in sponginess/hysteresis ie 8% less direct controllability? I guess that's like asking if a golfer can feel the difference with a club that's 8% stiffer. I reckon that probably many of the better ones would.

And for the record, per previous posts, I'm happily running SRF for years without bleeding so not in a hurry to swap. And I'll happily continue to trade 8-10 percent more sponginess on the first few laps to save massively more sponginess on the last laps when stuff like fresh RBF600 was starting to boil in my relatively underbraked car (or did actually boil - with streams of air bubbles at bleeding). And yes, I'm with you Winders in deploring when some imply/tell contributors that they are stupid for kindly sharing their direct experiences.

 

I run Endless RF650...pedal feel is terrific and is noticeably different to me when compared to the Motul 600 that i had used in the past. Never measured compressibility, have no idea if it is a "thing" however the pedal feel is different in the race cars. I flush it once per season.

 

Your right... and my intent was not to question his integriy, because i mentioned folks trust his experience and data, just that i thought i could help with bringing up the facts of the scale of the levels of compressibility for fluid comparison. In racing and many other sports , there is a belief system because, there is no doubt , many times , in what folks feel, but the causes might be other things than what they are thought to be. sometimes with the facts, this becomes clear and we can look at other causes.

Again, I totally agree. but your point that looking at something else is all i was trying to spark here, based on the numbers. the fact that you "felt" something differnece is the basis of the discussion. I know you understand the compresiibility numbers, but maybe not even to the extent that you think (heck, i didnt at first) we can actually tell you how much a fluid would move the pedal from one compressibilty valued fluid vs another (based on volume and size of the lines in the system) its going to be fractions of a mm. but the point here is what are you feeling? you change the fluid, and maybe with ATE, you dont get all the air out, maybe it seals differently with the master cylinder due to the viscosity. maybe the viscosity itself is the main factor . think about it, the viscocity numers of the brembo were not even to SAE standards, well thicker than recommended.... that could be the smoking gun. I dont know.
but, again coorelation doesnt mean causation.

as far as Olsen.... very sorry about what i said being taken as disrespectful. i was too focused on the discussion of the facts and ddint mean to discredit him, only to show indicators to spark thoughts in finding better reasons of why there are different "feels" with the different fluids.
OLSEN, if you are reading, sorry for my post if it was taken in any other way.

EDIT: i just read my post again to Tim Olsen, and i didnt see my post as being insulting. I acknoledged his valid expereinced and offered some thought to recommendations based on what we are talking about here. i asked the question many times of "is it possible to detect these types of fluid differences".
However, again, Ill try to take a more personable approach in the discussion and less of a perceived attacking one.

Great points. again, i dont discount anyone's observations here. certainly there could be something else going on as you allude to. however, you compare the pro golfers sensitivity of a 8% stiffer club, to a .00005% higher compressibile fluid. (we might be talkng mm's with the club difference that could be felt, while with the fluid, it might be fractions of a mm) i personally dont think that would logically be a valid comparison or analogy, because of the scale . when you and the books compare compressibility contribution of 3x the hose or 2x the pads, what are those values to begin with . we know there is compressiblity in the system, but what really is the source? saying there is 25% difference in compressibility from one fluid to another is a very misleading data point, correct? this is because 25% of .00005 (plus or miinus) is a very very small number. a fluid is generally in compressible for most all applications. HOWEVER, what we might be feeling is the other things that you have mentioned. Aeration?? thats a GREAT possibility, right? fluid viscosity?? and then of course , boiling of the water in the fluid when driving , and or boiling of the fluid itself in very hot caliper conditions. this might be the most important value.

Back to your values, you mention that once the fluid has pushed the pads to the rotors, the fluid is responsible for the "sponginess". however, i would again add, that you are leaving out HUGE factors that could be responsible for this. besides the line expansion, there is flex in the pedal. there is flex in the master cylynnder mounting plate and firewalll, and mostly there is dynamic flex in the seals, mostly on the mastercylinder and in the calipers themselves.

I like your accounting method though. 25 % of the difference in compressibility, and then it only being a 3rd of the total, so there could be an 8% difference, but i think that is flawed knowing that the 25% difference and effect is a number so small that its almost beyond normal imagination. remember also that under less than the 6000psi test, at 2000 psi for example and down at the 750psi that you mention the compresibility value will be a LOT less.
Im not saying im absolutely right here. but it seems logical to me. SO, with so many other factors, i think its fair to say we need to look for the smoking gun still.

 

Tim, since it seems that Torque has 30% less compressibility than SRF or Endless, and comparing the later two has only a 5% difference between them, have you heard from clients that torque has the better feel? since there is SUCH a great difference in the compressibility number you would think that torque would stand head and shoulders above Endless and SRF. And, for what i feel is most important, the wet boiling point is 50C higher with SRF. so, im wondering if endless has provided such a great result in reality, vs SRF, what it is due to and why Torque wouldnt provide the best pedal feel and a higher boiling point than Endless.

 

Mark have you noticed that every time you post in a thread it becomes controversial? Your post to Olsen was insulting since you implied that he was a vendor shilling for a particular brand. He is not a vendor - but you jumped to that conclusion. You are skating on thin ice Mark. Contribute - sure but bombastic statements and rude/argumentative posts will not be tolerated.

By the way, stop reporting posts you consider insulting while the rest of us applaud their exactness.

On Rennlist it takes approval of 3 mods to ban someone. Two have voted yes and I am the holdout - so far!

 

This has more to do with location. Out in Kali - probably minimal. In Florida, in the summertime, when it's 90% humidity and it rains everyday - I shorten the time period between bleeding and driving on the track. I have been told that in a vented system that brake fluid does in fact draw in moisture.

BTW - my last car was a 1974 911- non boosted brakes - and I could tell a difference between the compression rates of different fluids - less so on a boosted car

 

I know Tim Olsen. His integrity and knowledge is beyond reproach. And I would wager that he knows more about pretty much anything we ever discuss here than pretty much anybody else here.

 

again, i didnt mean it to be insulting and sorry it came across that way.

If i did, consider this my apology.

I do like the discussion and many have PM'ed or email that they do as well, to me its a great one to discuss because there is a science vs experience factor . what those factors are , is all im trying to understand if there is more to the story to the compresibility values. I think the controversy always start when things go against the belief or experiences of folks on the list. there are a lot of folks that have strong beliefs, and that's very characteristic of racers in general ive found. That's what makes the discussion so valuable as we all voice our opinions, data and experiences and we might all learn from the interaction in the end.

Thanks for allowing my participation here.

 

Tim - have you tried the Torque RT700 fluid? 685F dry boiling point and very consistent braking in my SPB, no fade or pedal drop at all...

 

Tim Olsen absolutely knows what he is talking about just as VR said. Ask around. Beyond reproach.

 

Here are a couple of charts i found on the moisture absorbtion rate and the Brembo racing fluid pedal pressure vs pedal movement graph (marketing from brembo)
looks like fluids take on moisture over a few months based on the graph.
also looks like Brembo racing fluid saves a few mm of pedal travel at near 1200psi. I would love to see the test results and methods for this experiential and who the competitor was. If true, that is a value that some would say is perceptible.... small, but certainly perceptible.
interestingly, if the typical competitor was say, Castrol SRF, it has a 10% reduction in compressibility, but would lose to Torque700 as its compressibility numbers seem to be near 25% better than SRF