Flexible brake lines, braided or rubber, which is FASTER?

A conversation recently got me thinking about this. Generally, stainless steel braided brake lines are recommended 'because race car', and I get it, theoretically giving the driver more direct control of the brake pads' pressure on the disk seems like a potential performance bonus. 

But what is the actual performance delta (in the metric that matters, the time saved to get around the track)? It doesn't seem like the advantage would be available to all drivers either, so what level of driver do you have to be to benefit? I also admit that there may be different advantages to braided lines that aren't based on lap time. Maybe they do better when everything gets smoking hot, for example. Or maybe they are easier to visually inspect for damage between races? But in this thread I'm focused on lap times. 

Has anyone actually tested braided vs rubber for time?

For me it's purely modulation; that extra bit of feedback makes it easier to be consistently aggressive with the brakes. I may have some bias as on motorcycles there was a night and day difference. On my Datsun the pedal feel also improved a lot.

I also think there is a secondary consideration on race cars as the braided lines are more resistant to damage

With modern ABS braking systems the advantage is probably less than I has a driver imagines it is.

This isn't the kind of thing that's going to improve the theoretical best lap time.  What it's going to do is make the car easier to drive and perhaps drop the average lap time with an average driver by a little bit.  No idea how much, it's going to depend on a lot of other things.

Yeah any difference in lap time is probably too small to measure, it just makes a slight difference in pedal feel, not worth the packaging issues that come with these lines IMO. I've done them once and I think I'll stick to rubber in the future.

They can actually be harder to inspect for damage since they tend to fail from the inside-out, while rubber lines tend to fail from the outside-in.

In reply to Robbie (Forum Supporter) :

I switched from Rubber to braided steel lines in about 1976 on the Black Jack.  If you go to the Packard Museum you'll see the same  set of brake lines. So durability should be proven.  
 Regarding transmission of brake pressure. Rubber vs braided steel has been proven to my satisfaction but I suppose  there might be a rubber line that nears the transmission of braided stainless steel. 

I don't believe there is any performance advantage.  "Feel" improvements are just that "feel".

The reason I say this?  Modern performance ABS systems provide perfect multi channel threshold braking using RUBBER lines.  If there was any performance loss in the ability to detect the bite point and provide the optimum tire slip angle due to "squishy" fluctuations in line pressure caused by rubber lines they would run different kinds of brake lines.   I assume race cars run braided lines mostly for durability reasons vs pure performance.  

It would be an interesting test though to take something with really good Stability control and braking like a 911 or C8 Vette and then swap ONLY braided lines and see if there really is any improvement.  Do it both with all driver aids on and off.  

The main advantage to braided lines is you can do your brake system in -3 AN and get rid of all of the weird banjos and double flares and bubble flares, so you need to carry fewer spares and can even make your own lines at the track.

The main downside is that braided lines seem to be a lot more fragile.  When pinched or hit the braid gets damaged and the line blows out.  So you get to gain more experience making your own lines at the track.

Direct fit braided lines seem like the worst of both worlds to me.

nocones said:

The reason I say this?  Modern performance ABS systems provide perfect multi channel threshold braking using RUBBER lines.  If there was any performance loss in the ability to detect the bite point and provide the optimum tire slip angle due to "squishy" fluctuations in line pressure caused by rubber lines they would run different kinds of brake lines.   I assume race cars run braided lines mostly for durability reasons vs pure performance.  

I do wonder about the validity of comparing what a computer controlled system can achieve vs what a human can do.  Everything about the  ABS system is optimized around the rubber lines so things like the volume and frequency of each pulse will take the system compliance into account.  The human on the other hand (or foot in this case) can't move at the same speed or with the same ability to go directly to a pressure...I'm pretty sure switching to braided lines on an ABS car does nothing helpful, but I'm also pretty sure human controlled braking will be better with braided lines.

In reply to mke :

I'm not sure.  Compliance in the brake hose should be fairly linear based on the spring constant of the rubber material.   If the primary feedback for threshold braking is brake pad forces on the caliper piston then I could see an impact on human controlled braking as there would be a very slight dampening of this pressure feedback.  But I don't believe that's the case.  I believe you are generally trying to feel tire forces through seat of pants and some steering weighting and that is the feedback loop that makes you apply pressure through your foot.  I think in this feedback loop the rubber hoses are a relatively constant in the system similar to caliper flex, and general flex of the brake components.  

But it is absolutely worth a test to see.  And the comparison of computer control to human would be really interesting.

Well, 2 things strike me. 

1. It should be easy to measure the "compliance" of a rubber line per foot or line diameter or whatever. How much more compliant is a brake line than a stainless line (probably not all braided lines are created equal either)? How much more volume goes into a rubber line at a normal threshold pressure? If the real difference in compliance is small enough, there's our answer.

2. Theoretically I could see that a little compliance is actually a good thing. The ground is never flat, so the forces going into the tire are never constant, so the forces going into the wheel are never constant and so forth. That means that if you graphed the maximum threshold pressure in the caliper over time you would not see a flat line. If the rubber line has a bit of compliance in it, then you would better "average" those peaks and valleys than a line with no compliance. 

But I also know that theory is not always a good predictor of practice. Which is why I'm curious if anyone has ever tested the lines back to back for time advantages.

I think whether or not the car has ABS is an important question.  With ABS you pretty much just mash the pedal to the floor until it starts pulsing then back off a hair.  Without ABS it requires a lot more feel to brake close to the threshold without locking up.  Either way, a good driver will adapt as long as the car's behavior is consistent.  Rubber lines may feel mushier, but as long as they're consistently mushy then I don't think they're any slower than braided lines.

All of my real track experience was on motorcycles in the 90s so everything could be different now.  At the time though there was almost no such thing as a bike without braided lines...even in the production classes that was 1 of the only mods allowed because they considered the mushiness of rubber lines a safety issue...the difference in brake feel and response was just night and day. But that was then, I really don't know what is done today...which is sad 

In reply to mke :

That's really interesting!

High end bikes are basically the closest thing you can buy to "full race car" for the street so I'm doubly surprised that they came from the manufacturer with parts that were so clearly inferior for track work. I wonder why...

In reply to Robbie (Forum Supporter) :

I have a similar experience to mke (late 80s motorcycle road racer); bikes in the 90s are not as high end as you think. 

I'll add that, at least at the time, braided lines were not DOT approved so were technically street legal.  I read somewhere the issue was if it gets crushed, the braid can puncher the hose so rubber hoses were the factory option.  Again, I have not followed it and don't know the current state.

Self made braided hoses are not legal because of the compression fitting. A pre made braided hose with crimp fittings can be DOT legal. I have such items on both my street cars. That said I have never seen a compression fitting type brake hose fail without a major collision. I have seen quite a few complete suspension corners hanging from the hose though.

I think a lot of people attribute the improved feel of their braided lines to the fact that they are braided rather than the fact they are taking off years-old rubber ones.

GCrites80s said:

I think a lot of people attribute the improved feel of their braided lines to the fact that they are braided rather than the fact they are taking off years-old rubber ones.

Or that they had to bleed their old fluid out in the process of installation.

I took a 1990 bike to the track in 1991, I promise the line change was a night/day upgrade and the "old" lines and fluid just to OEM new specs

Yes on bikes it indeed makes a difference. Stock first-gen KLR650 brakes are pretty bad but less bad with the stainless lines I put on. Of course motocross bikes have almost all come with stainless lines since their switch to discs in the mid '80s, but the seldom-updated old-style dual sport bikes like KLRs, DRs and XTs were a lot less likely to come with them.

I admit to not being fully aware of the hydraulic ratios involved in a bike but I can imagine they are huge compared to automotive, given that fingers are weaker than legs and brake levers have a lot less travel than a brake pedal.

Also a lot of bikes have zero hardline in the system.

I will second what mke has been saying; we took brand new lines of the bikes and replaced them with braided lines as there is a huge difference. 

My Beta 520RS came with them; if there wasn't a worthwhile difference Beta wouldn't have bothered.

Aeroquip style metal braided lines with PTFE inners are certainly good at improving feel over even a new regular rubber line. I am not so sure whether the reduce slowing times on the track. 

I do know that if a single one of the steel wires that make up the outer sheath breaks it can cause failure and I had one instance when there were no broken wire bits yet the PTFE liner had somehow rubbed on the inside of the braiding on a line on one of my race cars, which resulted in a pinhole leak that fortunately manifested itself when I was under the car bleeding the brakes - all of a sudden a really fine stream of fluid shot out!

I know that I prefer the feel of braided steel lines over rubber enough that I'll run them if the rules and budget allow but are they actually faster?  I don't know.  I suspect the answer is, it depends. 

Rubber lines not only increase pedal travel they also make the pad clamping pressure to pedal displacement non-linear.  On the initial pedal movement the pads move out till the contact the rotors.  Once they're in contact the pressure increases in the system linearly as the pedal if moved further until it reaches a point where the lines start to swell.  At that point continuing to move the pedal increases system pressure at a lower rate until the lines stop expanding (or the rate of expansion slows) at which point the original pedal movement\pressure increase ratio is resumed. 

It's that non-linearity that's perceived as lack of feel.  If threshold breaking requires pressure modulation in and around the range where the change in the pedal movement\pressure ratio occurs then the impact on lap times could be significant.  Otherwise it's probably just a driver comfort/confidence thing and that's harder to quantify.