Brake lines
 

OK, so you've typically got two different brake line setups for twin discs...
1. single line which splits above the mudgaurd to each caliper, or
2. two separate lines from master cylinder to each caliper.

Question is, what's the difference between the two, and what's the technical reasons behind the differences?

Answers on a postcard, or on this thread!

a good question that i'd be interested to see a more in depth answer to. the reason i ask is my 749 has the 1 down and 1 over the mudguard line not the usual twin line setup. under braking the bike feels real strong on the right side,even pulls slightly making the left feel very weak. would this be down to the line set up or something else

First consider what’s the same:

• The volume of hydraulic fluid that it takes to move the pistons from their retracted position to where contact occurs between the brake pad and the rotor disc.

• The pressure in the hydraulic fluid for a given braking force between the pads and rotors.

• The stiffness of a given brake line construction, that is, it’s resistance to expansion both circumferentially and longitudinally.


Now consider what’s different:

• The total length of brake lines that runs between the master cylinder and the calipers. Two lines are longer than a split configuration (which is the key to the answer.)

When you pull the lever to the master cylinder, a small volume of fluid is displaced that moves the caliper pistons. Then the pressure inside the brake lines increases as you squeeze harder - more force is exerted by the master cylinder and the brake pads are in contact with the rotor.

As the pressure rises inside the brake lines they expand and elongate. The volume of fluid that they contain increases and some of the force you apply to the lever is being used to expand and stretch the brake lines. This effect gives the feel at the lever what’s called a “spongy” feel. Said another way, you loose part of the pull harder - brake harder effect needed to modulate the amount of braking.

The longer total length of dual brake lines will undergo a greater increase of internal volume (which is less desireable) than a split configuration.

What’s interesting is that brake line sizes are commonly either -3 (3mm I.D.) or -2 (2mm I.D.). 2mm lines will expand less circumferentially than 3mm lines. So if you run two lines, use the 2mm size to negate the spongy effect of the longer line length. Two 2mm lines will move almost the same amount of fluid as one 3mm line so flow restriction is not a concern.

That statement is not true as the hydraulic system is common so air would be in the system causing loss of feel at the lever.

Now that is something I'd never considered. And maybe why I've never managed to get the brake setup on my 851 to feel as good on track days as my identically padded/disc'd/calipered/master cylindered old 888.....

Just goes to prove that you can still get good info from the DSC site.:devil:

Cheers Larry

Thanks shazaam, that's just what I was thinking.

I've been a bit curious as to why a twin setup is considered by some to be some kind of performance upgrade, when theoretically all it does is make the lever action softer. Thought I might be missing something!

I've got the twin setup on the 620 for just this reason, I've come to prefer a softer lever with a bit more travel between on and off, and lets me fine tune me braking pressure easier.

As mastercylinder/lever ratios can be matched to either setup, the standard setup on bikes from the factory will also be considering costs and possibly weight as the 916 style setup with the short hose T-ing off across the mudguard must weigh less than 2 full length hoses with the extra banjo fitting and double banjo bolt.

Most of my bikes have a T piece just above the bottom yoke so there is only about 200mm of single brake line, so not alot lost on the hose expansion/softening effect over the standard system and only a little bit saved over the two seperate hose setup. But then mostmy brake setups are fitted to either Brembo or AP Radial Mastercylinders, the AP giving the best setup with its adjustable ratio

I may be wrong but I always understood that ACU rules insist on the '1 line for each caliper from the master cylinder' arrangement. The single hose and splitter was road use only!

Dave the ACU rules say the Hose must split above the Bottom yoke

So you have the choice of two seperate Lines coming from the M/cyl or a single line from the M/cyl to a T piece above the top yoke and then a line down to each caliper

With either setup having a cable tie holding the two together below the line of the bottom yoke has been known to fail scrutineering