Brake line size?

[QSPres]

I'm almost at the point where I need to order brake lines for my car. 70' A-body.

Its got C6 Z06 front and rear brakes and will most likely have a hydroboost unit.

I was thinking of ordering a set of the stainless reproductions, but before I did, I wanted to make sure that the stock diameter lines would be sufficient for these new, modern brakes?

[Apogee]

Yes, OE size is fine. You could run metric tubing if you really wanted, but why? The car won't know the difference and neither will the driver ;)

I prefer the steel Bundy tubing over the stainless, but I don't live in the rust belt either. I've had issues with stainless fatiguing at the fittings if I need to remove and reinstall the hoses more than once.

[chicane67]

Actually... if you are going to order them and they are going to be for Z06 calipers... I would run 3/16" to both so that you maintain the stock PV ratio. It will make tuning the bias and master cylinder selection much easier.

[MonzaRacer]

AS for me I would run 3/16 stainless all the way around. If you run the 1/4in to the rear it can give you some issues with tuning.
Most companies can bend any diameter you want but as for metric the only metric part of brake lines are the fittings that screw in the tubing is still 3/16 or 1/4.
The only reason to use metric thread /bubble flair fittings is if you need them for what ever your installing.

[Apogee]

I'm curious, what are the tuning issues people are supposedly experiencing running 1/4" hardlines to the rear versus 3/16"? Educate me.

My understanding hydraulically is that there's virtually no difference due to tubing size except that larger diameter tubes expand slightly more...very slightly...under pressure and the fluid velocity difference associated with the different cross-sectional areas. So long as the fluid doesn't cavitate through the fittings, I would think that whould be a moot point as the volumetric flow rate isn't effected by diameter.

[6'9"Witha69]

Think of it as the effect acros the length of the tubing. With 3/16" tubing, 1/2" of pistn stroke is 1/2" of fluid movement into the atrium of the calipers (I am leaving the calipers themselves out of the equation as that is particular to the volume needed for any specific caliper). Now, use a 25% larger i.d. tubing from the dist block to the rear and going from 3/16 line out of the MC to the dist block (without doing exact math) 1/2" of piston stroke will be about 3/8" into the calipers. but the fronts will not experience this drop as they use the smaller i.d. lines.
This effectively increases the front bias already built in to the MC.

I hope this helps, my attempt at a Monday morning tech answer.

[speED]

Think of it as the effect acros the length of the tubing. With 3/16" tubing, 1/2" of pistn stroke is 1/2" of fluid movement into the atrium of the calipers (I am leaving the calipers themselves out of the equation as that is particular to the volume needed for any specific caliper). Now, use a 25% larger i.d. tubing from the dist block to the rear and going from 3/16 line out of the MC to the dist block (without doing exact math) 1/2" of piston stroke will be about 3/8" into the calipers. but the fronts will not experience this drop as they use the smaller i.d. lines.
This effectively increases the front bias already built in to the MC.

I hope this helps, my attempt at a Monday morning tech answer.

I have also been debating over brake line diameters and brake performance. Can you try to explain this one more time, I feel as though I'm not completely understanding your explanation.

[shep]

I think what he is saying is that using the larger line in the rear and the smaller up front the rear calipers will not move as far as the fronts as fast. Making the front calipers lock up before the rears. Increasing the front bias.

[chicane67]

Small diameter... builds pressure quicker, but moves less volume.

Larger diameter... builds pressure slower, but moves more volume.

Master cylinder, line size and caliper selection... if done correctly will take into account for 'pressure to volume ratios' to maintain a 'bias window' all while maintaining good pedal modulation and feel. Both, are defined by the pedal ratio and the effort required for correct operation.

Case in point, drum brakes. They will require more fluid volume to operate as they need to span a greater distance. Disc brakes only have to span a fraction of that and require more pressure due to the volume of the calipers themselves.

[Apogee]

The volume of fluid moved through a brake system is determined by the MC piston bore and the caliper piston bores. Assuming that the fluid is incompressible, then what goes in, must come out. If we ignore line size and push a volume of 10cc of brake fluid in one end of a brake line with the MC, 10cc will come out the other end, regardless of diameter, length, or pressure. The brake line could be .06" or 6" in diameter and that would still be true as long as we assume that the tubing doesn't expand.

I still don't believe or understand the assertion some are making that line size effects the "tunability" of a system, because in my experience and realm of knowledge, it does not.

[QSPres]

That seems to make the most sense to me. Once the calipers and lines are 100% full of fluid, it isn't going to take much (given everything else is sized correctly) to activate the brakes.


This is a street car anyway, so tunability won't be a concern.

[parsonsj]

Let's try this: when one presses the pedal and the master cylinder pushes the fluid down the tube to the rear brakes, it is exerting pressure all along the way. Not just at the caliper. So this pressure (representing work by the driver) is dispersed by all elements of the braking system. That's why SS brake lines make your pedal firmer than rubber lines. Bigger diameter brake line "drains" pressure away from the caliper because there is more surface area for the fluid to exert pressure.

So why not use 1/8" brake lines? 1/16" brake lines? If smaller is always better, then why not?

The answer is that volume is also important. Enough fluid has to move easily to allow the input work at the master to be translated to caliper.

So, since smaller is better, but we have to allow for volume, 3/16" brake lines have been proven effective. 1/4" is just giving energy away to the atmosphere that is better used on the caliper.

jp

[silver69camaro]

Splitting hairs if you ask me. I never include, or ever seen used, brake line diameters and/or volumes when doing a brake analysis.

But I do prefer 3/16" all the way around.

[QSPres]

Bigger diameter brake line "drains" pressure away from the caliper because there is more surface area for the fluid to exert pressure.

jp

The calipers are designed to work once a certain pressure is applied to them by fluid.. The volume is dictated by the cavity inside the caliper.

Larger lines would only seem to be required if the volume was inadequate with the smaller lines.

Bigger lines simply hold more fluid. If they're built correctly and the master cylinder is not applying an unsafe amount of pressure, the wall thickness of the larger lines should not allow any more flex than the smaller set.

The statement that the larger lines take more pressure off the caliper can't be correct, if they flex no more than the smaller lines, as the system is just as full with fluid as the system with the smaller lines. Therefore the fluid would have no where else to go once the caliper is full.

[6'9"Witha69]

Bigger lines simply hold more fluid. If they're built correctly and the master cylinder is not applying an unsafe amount of pressure, the wall thickness of the larger lines should not allow any more flex than the smaller set.

But as stated the larger diameter allows more surface area for the fluid to act against under pressure, thus allowing more expansion yielding less pressure.

[QSPres]

Assuming, once again, the master cylinder is capable.

But.....


It seems to me, it would be easier to get a MC to work correctly with one size of line than to intentionally weaken its "signal" by running two sizes of lines and call it "tuning"... Sounds to me like a buzz word that someone made up when the messed up a job. LOL

[parsonsj]

if they flex no more than the smaller lines, as the system is just as full with fluid as the system with the smaller lines.

That's an incorrect assumption. The wall thickness for standard tubing used in brake lines is .035". Both 3/16 and 1/4 tubing that I'm familiar with use that wall thickness. Given that, the bigger line will allow more flex, and reduce pressure at the caliper.

Now, as Matt opines, are we splitting hairs? Probably. If it were my car, and I had an existing 1/4" brake line in good condition already in place, I'd use it. On the other hand, if I was buying or making new brake lines, I'd use 3/16 material, if for no other reason than to minimize tool needs and fitting types.

jp

[parsonsj]

Let's try this calculation: we can assume a given area of metal has a material constant for net expansion (or flex). We also know that a given length of tube has a surface that expands according to the square of the radius (A = Length * pi r squared). That means that the expansion of a tube varies according to the square of the radius of the tube times its material constant.

There's just no getting around it: the bigger tube will expand more under pressure if we make the following assumptions: (which are true in this case)
1. the wall thickness is the same,
2. the material is the same.

All one can argue is the material's expansion constant is so small that the difference caused by 1/32" increase in radius is negligible. Which is probably true.

jp

[QSPres]

Unless, of course, you don't assume that the smaller diameter tubing is overly thick and the larger is thick enough to still not allow significant flexing. I doubt either line was designed much differently than anything else back then. See where it bursts and then double it...

I'm going with whatever kit is readily available for my frame...

[CarlC]

On mine......

1/4" stock steel rear brake line with SAE flare fittings. No rear brake lockup, but rear brakes effective.

3/16" stainless line with AN fittings. Rear brake lockup possible.

No other changes to the system.