How Brake System Plumbing Affects Brake "Feel"

[forwardgear]

Hi all,

First post. My question is regarding an attempt to discover why changes to brake system plumbing change the way the brakes "feel" to the vehicle operator.

More specifically, I'm concerned only about changes to the flexible hoses in terms of material and/or internal diameter. An example could be replacing rubber brake hoses with hoses made from stainless steel braided PTFE, but I would also like to explore replacing larger diameter braided stainless hoses with smaller inside diameter hoses.

My core question is, "Why exactly do these changes change the feel of a brake system?"

Would you agree that the change from rubber to braided stainless changes system feel because of the reduction in hose expansion? That alone?

Then regarding the change from larger ID braided stainless hoses to smaller, is the change in system feel in that case also due to a reduction in expansion? Some other aspect, such as the reduction in fluid volume, or the smaller fluid pathway?

If all of these effects have an impact on system feel, is there a rank order to them in terms of their impact?

I'd further like to discuss whether, given a fixed plumbing spec, can feel be changed via restriction of fluid flow through addition of an orifice of some kind.

Thanks!

Chris

 

[patprimmer]

A real test of feel is to see if someone unaware which way the changes were made consistently notices a change in feel that matches changes vs no change.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[SomptingGuy]

I suspect (with no physical evidence) that braided hoses are expected to work better and so are fitted to all braking systems that are already better. I would be suprised to see an expensive 6-pot calliper with anything else attached to it. Likewise, my old GPz and its twin-pots only had good old rubber.

I wonder if metallic blue banjo fittings give a better feel too? Or (on cars) callipers painted red?


- Steve

 

[GregLocock]

I was under the impression that the braid is a fairly loose sleeve over the rubber. A such it seems unlikely that braid is the difference between a radially stiff hose and a compliant one.



Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[MacGyverS2000]

SG,

Hey now! My calipers are painted red... of course, I think that was my attempt to hide the rust, but that's another story!

Dan - Owner

http://www.Hi-TecDesigns.com

 

[ivymike]

I have a coworker who spent an entire week of vacation carefully painting the calipers on his BMW. It was made more difficult, I imagine, by his unwillingness to unbolt anything other than the wheels (for fear what might happen). I'll have to ask him whether the feel is improved. He certainly wasn't impressed when my old floor jack left rust in the trunk (I guess that's the trouble with borrowed tools, eh?)

 

[thruthefence]

The teflon lined, stainless braided hoses I've seen are pretty stiff. Maybe this leads to the perception (or fact) that they have less "give" to them under pressure, thus leading to the better "feel". And as has been mentioned, they cost twice as much, so they "must" be better.
I would think careful sizing of master cylinder vs wheel cylinder, and pads specific to the application, (street/track?) As a previous poster noted (among other things)would have the greatest effect.

 

[swall]

The subject of braided ss brake hose comes up all the time on the motorcycle discussion boards. 99% of the time, the "improved braking feel" mantra is cited. And they have a racing image, which appeals to the "cool crowd". But people are woefully ignorant of the disadvantages of this hose. They are easy to kink if the installation is sloppy and/or the hose is not properly supported. The wire strands are also vunerable to rock damage and other sources of nicks, which can lead to fatigue of the strands. The fabrication process for this type of hose is also critical--it is easy to get some frayed strands that won't seat in the ferule all the way or get some that double over and you won't see this. Leads to either a fatigue failure or a creep type of blow-off somewhere down the road. Finally, the ss braided brake hose have a problem passing the whip test which is part of the SAE J1401 brake hose spec, which is why the OEM's tend to stick with rubber hose.

 

[evelrod]

A bit more---

Greg, the vintage looking hoses I put on the front of my 23 street roadster are a glass weave with a fiber "braid" covered in a heavy outer "rubber" coating with swedged brass banjo fittings. Pretty high dollar, but "necessary" for the proper look of a 50's hot rod.

Yes, the SS braided PTFE AN -03 is stiff, but, in the right hands is very easy to work with and virtually problem free...IN THE RIGHT HANDS! They require the proper equipment and training to assemble. Not difficult when you have done a few.

Lastly, several years ago I updated my inventory as I normally keep a small stock of -03, -06, -08 and, -10 plus a few fittings on the shelf. The hoses were counterfeit AN hardware from "China"! (If it can happen to Boeing, it can happen to you!)
I've run into counterfeit bolts before, but not hose. Fortunately it was SOOOO bad we caught it in the shop.

Rod

 

[NickJ67]

My observations as noted on a 60's car with no servo is that the SS externall braided hoses firmed up the pedal very noticeably even though the OEM hoses they replaced were quite new. OEM hoses that I have cut in the past have been internally glass or nylon braided rather than anything metallic. Motor cycle ones, especially long ones, may be different though.

To Pats comprehensive list I add the compliance of the pad itself. Having experimented with 5 different pad materials in the last 2 years on this particular car (having finally run out of the rather excellent but asbestos based OEM pads) I was really surprised at how much difference there was between them.

Of course it becomes more subjective to assess at this point as you have also the frictional characteristics of the pad coming into to play. While this doesn't affect the actual pedal travel it will have a strong subjective effect as the brain correlates braking g-force the distance your foot moves and pedal pressure. My personal favourites are actually quite compliant and have excellent bite giving the nicest braking feel and greatest controllability.

I have have to say that the performance and feel difference between the pads ranging from the cheap motor factors "compressed cardbord/lard composite" efforts to the £100 performance pads I'm now using was nothing short of astounding. The sad thing is that the expensive ones are little improvement on the original asbestos ones. The cheap motor factor pads were downright dangerous giving a rock hard pedal and so little bite it was like the onset of pad-fade the whole time.

My point? I think you will find more difference in braking feel by changing pad compunds than you will by changing hoses.

The compressibilty of the fluid can have a surprisinig effect too. The most notable variation is with the DoT 5 silicone fluid which is very noticeably "squashy". I hated it and went back to the real stuff after a week.

Nick

 

[evelrod]

A major problem with the silicone based brake fluid is the "gassing" of the fluid when things get really hot as in racing or even a long run down the mountain on a curvy road.
Yes, conventional fluid will boil and cause spongy pedal, but at least you have a warning...with silicon fluid you get no such advanced warning...one push you have brakes the next you have nothing!

It did not happen to me, but back in the "good ole days", one of the more notable drivers/writers, Sam Posey, had the problem arise while testing a car at RIR (for a magazine I think). Going up the hill from T-5 to T-6 the pedal went to the floor and Sam instinctively spun the car, a Ferrari and saved it. I was standing on the inside of the turn at the time and jumped about ten feet back. Later in the pits his crew determined it was the brake fluid had overheated and caused the temporary failure. Some time later SP wrote about it in a tech article that I saved to remind me NEVER to use silicone fluid.

Rod

 

[swall]

Rod--is it possible that water contamination could have been the culprit?

 

[evelrod]

That happened many years ago. Subsequent tests have shown it is just inherent with the type of fluid. I've even heard of it happening on the street...But...you know my old tag line..."believe only half of what you see and none of what you hear".

I'm pretty sure, however, that the car in question was from a notable, pro team and if they were as diligent about the brakes as I am, the fluid would have been new and uncontaminated. Just a guess.

Rod

 

[NormPeterson]

I thought the issues with DOT5 fluid were more about entrained air and cavitation rather than boiling. Same end result - gas where you need liquid.

Though simply replacing DOT3 with DOT5 may not have evacuated all of the moisture, which would of course boil if you got things hot enough.


Norm

 

[evelrod]

You have me there, Norm. I'm not a chemist...I am a racer and in that capacity I would not choose the DOT5 silicone based brake fluid for anything I own. I have used several brands, Castrol, Motul and such that have boiling temp up near 500f, sufficient for any race car I have ever driven.

http://www.pegasusautoracing.com/ProductSelection.asp?Product=MT600&iorb=4764

Rod

 

[NormPeterson]

I wouldn't use it either, preferring also to stick with what I know will work and what will give the "feel" that I am expecting. For my newer cars, I'd add the reason that DOT5 apparently doesn't play nice with the ABS bits.


Norm

 

[NickJ67]

Something I read about the silicone stuff was that whereas conventional brake fluid is hygroscopic and readily absorbs water, the silcone does not absorb water. Indeed this is touted as one of it's benefits.

Problem is though, the silicone being less dense than water, any water in the system will sink to the lowest point which could well be the calipers - right where you don't want it, where things get hot.

Conventional fluid will hold the water more uniformly thus needs a larger amount of water to make things dangerous. I also remember reading (on a brake rebuild specialists website I think) that this free water creates a geater corrosion hazard and is the reason that they will not honour warranties where silicone is used used.

Think I'll stick to the conventional stuff.

Nick