Hardlines vs Flexlines.

[RollingCamel]

Last year we used Fedhill's copper-nickel alloy brake hardlines and then speedflex lines to the calipers. However, in the competition I saw many flexlines all the way and I can't rational which to go.

Using flex lines all the way will remove the connector between the lines, so something less could go wrong.

However, I don't know the weight/length for each whether there is a considerable difference or not. The other thing is that Fedhill lines cost/foot ranges from $2.2-$3 and speedflex $3.62-$4.16, though we got the lines and fittings for free from Fedhill.

I'm not aware of other differences.

[exFSAE]

Flexible line is used where things need to.. well.. flex.

This should be straight forward.

[RollingCamel]

Then why I saw many ppl using it all the way?

[exFSAE]

Because people in FSAE do a lot of dumb crap.

[Mehul Botadra]

The advantage of hard lines is that you dont have a pressure gradient since the surface is solid which is not the case in flexible lines. However this gradient is negligible and you still get a good amount of braking pressure. Flexible lines will only eliminate the connector required. Flexible lines also have a problem proper routing since they need to be away from the differential chain which is totally not required for solid lines. I think you can use a smaller diameter solid lines, reduces packaging issues to an extent. This is the case only when you have everything running with your electronics wires.

[ibanezplayer]

Originally posted by exFSAE:
Because people in FSAE do a lot of dumb crap.

^This

[RollingCamel]

Originally posted by Mehul Botadra:
The advantage of hard lines is that you dont have a pressure gradient since the surface is solid which is not the case in flexible lines. However this gradient is negligible and you still get a good amount of braking pressure. Flexible lines will only eliminate the connector required. Flexible lines also have a problem proper routing since they need to be away from the differential chain which is totally not required for solid lines. I think you can use a smaller diameter solid lines, reduces packaging issues to an extent. This is the case only when you have everything running with your electronics wires.

Thanks.

@ exFSAE
If ppl do a lot of dumb crap, explaining why it is dumb crap could be helpful.

[exFSAE]

I think you explained it well yourself. Hardlines are smaller, lighter, and less expensive.

[Ockham]

It's worth noting that for the typical -3AN lines used for FSAE brakes, the burst pressure of a hard line is about 10x the burst pressure of a soft line. It's a pain to route hard line, but a higher FS with lower weight is pretty much a race engineer's Holy Grail.

[Crispy]

I agree with the advantages of hard line (lighter, cheaper, stiffer), but I think there are some pretty good arguments for running soft line all the way as well. For example, in the front the distance is often really short between your master cylinders and brake calipers, and the part of that distance that could be hard line is likely near zero (especially if you have an adjustable pedal box).

Even for the rear of the car, running hard line means you are most likely going to have more interfaces between hard and soft lines (again, especially if you have an adjustable pedal box). Making the hard lines is more difficult, as is installation, replacement, etc.

As for a factor of safety, if you have soft line anywhere in the system they will be your weak link, so you system factor of safety won't be improved by using hard lines. Or are you running hard line all the way or weaker/lighter hard line?

Rule B7.1.6 states that "The braking systems must be protected with scatter shields from failure of the drive train (see B.8.13) or from minor collisions." In a quick search I couldn't find anything that distinguished hard and soft lines in the rules. It seems that both hard and soft lines must be protected from the drive line.

I agree that there are advantages to using hard line, but there are also advantages to not using it. We have used hard lines in the past, but in 2010 our cars used only soft line and I think it was a good choice.

[Mikey Antonakakis]

Easier serviceability with soft lines. We've run both ways. The only thing I didn't like about when we ran hard lines were that they were yellow.

[oz_olly]

I haven't done a direct comparison between the Bill of Materials and associated cost for hard line and flex line but when I tried to add it all up in my mind earlier in the year the combination with more interfaces suggested more cost and potentially more mass as the fittings are always the heaviest and most expensive parts.

To me the only real pro is reduced compliance at the pedal and with a combination of hard and flex the lines will still act as springs in series with the much more flexible line still dominating the system stiffness. Has anyone done a direct comparison?

[moose]

Back in '05, when we were a brand new team, we had shown up @ the silverdome with all soft lines, and had major sponginess problems and struggled mightily to pass brakes (but also struggled to accelerate with our un-tuned engine). If I remember correctly, after plenty of attempts to bleed them, we flipped to soft at the fringes but hard line in the middle, and it made an improvement. It was such a mess of a system, hard to say how much of a difference it really made.

If you think of the sponginess of the soft line as a spring, I'd say that the distance has an impact on how spongy it is, so the longer the distance, the softer the spring - so by limiting that, it still helps in the "spring in series" analogy.

[Pico]

In 2009 I decided to run flexible lines throughout the entire car. Although now we have a solid-flex setup I believe that on a FSAE we don't have the pressures encountered on larger cars and flexible lines could be used if done properly. Flex bought already made in the correct length is a plug and play system. We reduced the number of brake fittings by a good 10-15 pieces compared to a solid line setup (less chance for leaks).

With that car we never had a spongy pedal, in fact as all of our drivers told me "it's like hitting a brick wall". Passed brake test on the first attempt as well. It all depends on the type of line you're using and it's rating. Also if you don't route them properly you can have bubbles impossible to get out.

[Adambomb]

If you have a spongy pedal, the lines would be about the last thing I would blame. 90% of the time the problem is terrible routing which makes the lines nearly impossible to bleed properly. Although with soft lines I suppose you could "massage" the bubbles towards the calipers. Aside from that, here's the reason for/against using hard lines.

REASONS TO USE HARD LINES:
1. Lighter
2. Cheaper (I've pretty much always used $1/ft. mild steel stuff with good results, most speed shops also carry Al hard line)
3. Stiffer
4. Looks nicer/cleaner

REASONS NOT TO USE HARD LINES:
1. Some people can't properly double flare the ends, resulting in leaks
2. Routing much trickier

As for having more junctions, in practice, you are generally only adding one more in most applications (soft line from MC to hard line for rear brake line).