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

 

[evelrod]

The "rule of thumb" in racing has been to use the smaller dia braided SS AN/PTFE hoses for flexible connections. It is so ingrained in our modern race culture as to be thought of as a "given"!
Now, since I have spent almost all of my adult life building and racing various cars and bikes...I will disagree with most of what is posted on the race car sites. What brake hoses/pipes are best for any single application, vis a vis "feel" will vary ... A LOT.
For some vintage iron, the OEM type hoses often suffice and no advantage can be felt with AN hardware...For modern cars, the "feel" with AN hoses over OEM is pretty subjective. Not much change from my point of view.

The real reason I choose braided hose over OEM is more availability/cost and, I have the ability to fab any application as needed---No parts chasing. Maintenance and longevity are much the same, but cost of custom "rubber" piping can be enormous. I only choose to use it where authenticity is paramount, such as my 23 T tub 50's era track roadster (OEM style front hoses ~$79).

Bottom line...Purely racing applications...Braided SS
High performance street...Either braided SS or performance grade OEM pieces. Any "change" in feel is usually attributable to replacing "old" parts with "new" and not from "rubber" to "braided".

Feel is subjective. If that's your primary concern you may or may not see a change. If you want it to "look" racery, go for the shiny stuff.

Rod

 

[forwardgear]

Thanks Rod,

Would you agree that the ideal design for brake plumbing would be 100% solid tubing, but since we need to accommodate suspension travel and steering movement, the next best thing is a hose with as little expansion as possible?

Regarding old to new vs rubber to braided, what if both were new (rubber and braided)? If there then was a change in feel, would it be attributed to the change in expansion spec or something else? Then what if you swapped the still new stainless hoses out with stainless of a smaller ID and had a change in feel again? Would it be attributed to the reduction in expansion? The smaller ID allowing less volume of fluid in the system? The smaller channel for the fluid to move through?

And finally, all other things remaining equal, would there be any merit to having the ability to insert and change the size of a fluid "restrictor" inline in the brake system, in terms of adapting feel to preference?

Thanks,

Chris

 

[ivymike]

what would be the goal of the restrictor? to make it hard to get the pedal down quickly, but give the same force once the pedal is down?

 

[forwardgear]

The restrictor would be used to modify the feel of the pedal. I wouldn't want to say make it hard to move the pedal, but rather to change the way the pedal feels via changing the size of the restriction.

Chris

 

[MikeHalloran]

A restrictor small enough to be effective would be dangerous in a brake system.

My Camaro's hydraulic clutch has a restrictor to limit the drivetrain shock damage you can cause by sidestepping the pedal, but that's a different application.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

You can build a very simple model of the hydraulic circuit, and you need to know the stiffness of your calliper/pad setup and then you can use it to create a budget of compliance in the system.


Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[forwardgear]

So, to parse out a portion of one of my above posts:

Is the ideal design for brake plumbing 100% solid tubing?

Since we need to accommodate suspension travel and steering movement, the next best thing is using hose with as little expansion as possible?

Once we have that in place, have we done all we can do as far as optimizing the plumbing or is adding a restrictor to the mix a value-add prospect?

Chris

 

[GregLocock]

Do not add a restrictor.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[forwardgear]

Fair enough.

Do you think, then, that any change in the feel of a brake system caused by a change to the flexible hoses is specifically and only because of differences in the expansion characteristics of those hoses?

Chris

 

[forwardgear]

What if it's just a fractional restrictor? Say the lines have an ID of 3.2mm and the restrictor has an ID of 3.0mm?

Chris

 

[forwardgear]

Sorry to keep bringing the restrictor up. Just exploring an idea...

Chris

 

[evelrod]

Chris, forget the restrictor idea, period!

All steel is probably the best as far as line expansion goes. I doubt that such a system could be made viable. Even if it could, I seriously doubt you could tell the difference in everyday use.

Let's get another idea out of the way while we are at it. The modern OEM brake hoses and piping do not "expand" any more or less than the AN -03 PTFE stuff that I use...At least to any degree that you can feel under you foot. OEM lines are still "braided" material simply coated with a "rubber" outer for resistance to weather---Something that AN braided SS is not ANY better at than OEM. In racing, the brakes are attended too after each race. Treat OEM stuff the same way and they will be just as effective as AN hardware. Like I said, safety, expansion, wear resistance/longevity are NOT why I use AN hardware---Cost, availability and, custom applications are!

No OEM puts substandard braking systems on new cars, at least none that I know of. It would be suicide.

Rod

 

[forwardgear]

The application I'm focused on is motorcycles. My gut guess would be that the type of hose makes more difference on a motorcycle because it composes the bulk of if not 100% of the plumbing. Contrast that to a car, where the flexible portion of plumbing is a small percentage.

That said, you can put a bike with OE rubber lines next to a bike with SS lines and feel a distinct difference at the lever between the two.

My effort here was to try to get to the bottom of what about the lines creates that difference in feel. If not expansion, what?

The specs I have for SS hose shows an "elongation" allowance of +2%. I'm going to quasi-equate that to expansion specs.

Does anyone have comparable specs for OE rubber lines, or comparison specs? Maybe I should visit the Eaton website...

Chris

 

[thruthefence]

Isn't "feel" a kinda subjective criteria?

 

[ivymike]

well, it'll sure change the "feel" if you stop the fluid getting to the calipers and/or back out... they'll start to feel less like functional brakes.

 

[patprimmer]

It is really VERY straightforward, to the point that a detailed explanation is really quite insulting to a real engineer.

The restriction only limits fluid flow rate vs pedal force. Restricting flow makes the pedal feel heavy or slow to apply the brakes.

Once the pad hits the disc and starts to clamp the restriction will have no influence as the pressure equalises in the system.

The clamping process after contact has much less travel than take up of clearance, so the restriction has little influence as it only impacts on flow rate, not pressure at equilibrium.

Off the top of my head, the things that effect feel once a pad is being clamped are:-
Compliance in the lines.
Compliance in the callipers.
Compressibility of the fluid.
Co-efficient of friction between the pad and the rotor.
Compliance of the rotor surface.
Projected area of the master cylinders piston.
Projected area of the calliper piston or pistons.
Compliance in the pedal assembly.
Effects of changes in temperature of various components.

Note

As far as I know, rubber brake lines are steel braided. It is just that the braid is not stainless and it is covered by rubber both inside and out.

Solid steel lines are much more rigid than flexible lines.






Regards
Pat
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[forwardgear]

Thank you.

 

[BrianPetersen]

On most newer motorcycles, replacing the brake lines with braided steel makes scant if any difference to brake lever feel on its own. BUT.

If you want to change the location of the handlebars for roadracing or whatever, or change the master cylinder or calipers to ones that have different orientation of the end connections, often you can't use the original-equipment lines, and as others noted, it's a whole lot easier (and generally less expensive) to make something up with braided steel to suit than it is to chase around looking for some other OEM brake line that will coincidentally fit what you have done.

The other thing is that quite a number of late model Suzuki and Kawasaki motorcycles have a brake line routing in which a single brake line goes down to one caliper, then there is a double banjo fitting at that caliper and a cross-over hose to the other side. This causes a situation where the cross-over hose has a high spot, and if any air gets into the brake system (or if you have to service the brakes during a race weekend) it's really hard to bleed that air bubble out quickly using the equipment available at the racetrack. If you change it to the normal arrangement (two separate hoses to each side from the master cylinder) then bleeding the brakes in the field is a lot easier - but can't do that with the original-equipment hoses. The OEM doesn't care about this, because they can just pull a complete vacuum on the system (not just "suction" like a brake bleeding kit uses - I mean a complete vacuum so that there is no air left), but you can't do that with what's available in hot pit lane ...

 

[evelrod]

Okay, I'll still say that feel is subjective. I owned a 1977 KZ 650 Custom. Rode it to Mexico City and back. It had dual front disc brakes with two "rubber" hoses that ran from a "t" near the triple tree. Quite long, considering. At around 30,000 miles I rebuilt the brakes and changed the hoses to AN -03 braided SS for many if not most of the reasons you seem bent on accomplishing. Again, in retrospect, I don't recall any great changes in "feel", aside the obvious that I WANTED AND EXPECTED to "feel" BETTER !!!
Looking back, that little bike always had superior brakes, before AND after I fooled around with them. It was the first bike I owned with fully hydraulic disk brakes. I finally sold it at 39,000 still with the ORIGINAL 'O' ring chain !!!(Gettin' older, traded for an '80 Goldwing)

Hard to grasp "feel" if you are too closely involved in the changes.

Rod