A thought on disc brakes...

[No Judge]

This is something that has been bugging me for a while now. Ive a bit of an intersest in motorbikes, and notice that some of the now have dual discs on the front wheel. One on each side, two rotors, two callipers, one lever...

So why, in the high end world of MTB performance, dont top end DH bikes have dual discs on the front also? I have asked a few people, with no avail, so i thought id throw it over to the (almost) infinate wisdom of the forum collective...

Double the stopping power, with minimal weight gain. Whats not to love?

Just a thought....!?!

 

[dcrofty]

It has been done, I've seen at least one bike kicking around that had a dual disc setup.

Perhaps because bikes are much heavier and need the extra stopping power?

 

[scblack]

It has been done, I've seen at least one bike kicking around that had a dual disc setup.

Perhaps because bikes are much heavier and need the extra stopping power?

If it is the one I am thinking of, it was a vented set-up. One caliper, but essentially two discs.

Not much use on Australian mole-hills, but as some argued, quite useful (necessary) on long European and Canadian mountains.

 

[whitey89]

Double the stopping power, with minimal weight gain. Whats not to love?

Just a thought....!?!

The weight all adds up but i think the main reson is there no need for stronger brakes. Bikes weigh like 1/10th of a moto so i recon thats why they dont do it.

 

[shake_rattle_and_ roll]

Greater speed and greater weight= greater need for stopping power.

The issue of braking forces on performance road motorbikes means that not only does heat dissipation become a huge problem, but torsional flex also becomes a major issue that influences the bike's steering characteristics under heavy braking believe it or not. A twin caliper front end reduces and eliminates these issues.

Twin caliper setups on mtbs is complete overkill and totally unnecessary. For mtbs, the future of braking lies in the refinement of caliper and rotor design to address the problem of over-heating/brake fade.

Look here to see a pic of a twin setup on a mtb.
http://farkin.net/forums/showthread.php?t=135806

 

[John U]

I saw the twin caliper set up on a custom road bike somewhere. I think it was built up for show purposes. the disc diameters were quite small. the main reason for the dual disc in situation was for one of the discs to offset the torsional forces created by the other (road bike forks being quite lightweight). When the brakes were applied the net result was meant to be zero twisting on the forks.

 

[No Judge]

yeah good point shake...

I was thinking more along the seriously long european/canadian descents (you know, proper mountains), and the idea was strenghtened watching the tour this week, with their crazy downhill sections...

fairy nuff.... cheers for the replys

 

[Tazed]

The other major issue is traction (on top of those mentioned above).
You can easily put more force through a dual piston brake than you need to stop the bike.
You'll lock up the front wheel before you slow down faster, though.
Dual brakes and rotors just aren't necessary.

Sidenote:
F1 cars expend double the power they output under brakes (some 1300kW and up to 5.5g!).
They could use larger brakes, if the wheels were made larger to accommodate them, but they still couldn't brake harder, as they'd overcome the traction they can produce.

 

[shake_rattle_and_ roll]

^^^^^^
This is true, though in the realms of performance braking this is where the issue of greater brake modulation comes into play, and trying find the magic balance that allows the user to broaden that fine line created by the laws of physics.

Speaking of, have we a got a resident physicist out there that can create a formula explaining braking forces, modulation, dissipation and braking power?

 

[Sean]

Because if they're like Sam Hill, they can just drift around corners without the brakes.

But seriously, the amount of innovation and technology that has been put into disc brakes is huge, take the new Codes for example. VERY powerful brakes, and if you were to put one on each side of the front wheel (for example) the wheel would lock up, sliding/washing out. There is simply no need for the added weight on the bike.

 

[Tazed]

...
...breaking power?

lol! Easy typo!
And to add to the post above, as well, the modulation in the new Avid Elixir range (taper bore piston) is supposed to improve the weak point of most Avids' on/off initial bite, increasing modulation.
They mention nothing of increased power, as it's not necessary, but better control of that power is.

 

[s73v30]

Does anyone actually no of a company that makes a rhs caliper? and where I could get one?. Don't worry this isn't for my bike it for a little project I'm working on (Involves 3 wheels so dual calipers are required).

 

[John U]

Does anyone actually no of a company that makes a rhs caliper? and where I could get one?. Don't worry this isn't for my bike it for a little project I'm working on (Involves 3 wheels so dual calipers are required).

Check out the people that make the mountain buggy bikes for paraplegics. They might use something like what you're after.

 

[dcrofty]

If it is the one I am thinking of, it was a vented set-up. One caliper, but essentially two discs.

Yeah I heard abour the vented ones. I think shimano made them many years ago (I also heard that riders put WD-40 on the rotors to calm them down) and I think the new Hope Moto's are the same.

But the one I saw had two calipers and discs, one on each side of the hub with some sort of hydro splitter leading from them up to one lever. The bloke was also running a 24 inch front wheel so perhaps he was a bit odd.

 

[indica]

But the one I saw had two calipers and discs, one on each side of the hub with some sort of hydro splitter leading from them up to one lever. The bloke was also running a 24 inch front wheel so perhaps he was a bit odd.

http://forums.farkin.net/showthread.php?t=135806

4th post has a pic of dual rotor set-up.

 

[Tazed]

...I also heard that riders put WD-40 on the rotors to calm them down...

Yeah, that'll stop them working at all, so they'll be much less grabby...
DO NOT put WD40 on your discs. Ever. You'll contaminate your pads and reduce your braking to nothing pretty quickly.
In fact, put nothing on your discs (not even your fingers) except disc brake cleaner or alcohol to clean them.

Some things to note: Multiple (4, 6 and 8 pot/piston) callipers don't necessarily give more stopping power.
Notice how top downhillers run dual piston callipers for the most part?
What multiple pistons do is create better modulation.
The pistons are different sizes and move a different distance in the calliper under brake lever pressure - effectively, the smaller pistons move farther, first, and push part of the pad onto the rotor.
The other pistons, under more force, push the pads onto the rotor later in the lever action.
So you get modulation from putting the pads onto the rotor unevenly for a little power early on, then max power towards the end of the lever stroke - viola, modulation.
No matter how many pistons you have, friction coefficient of the pads and the ratio of lever to calliper combined with hand force applied dictate how much braking force you can produce.
With two callipers activated off one line, you'll get the same the braking force, albeit applied to each side at the calliper for the force applied at the lever. Won't hep you though if traction at the tyre get that transferred to the ground...
That's why you don't see them in the real world. There is no advantage, but there is a weight/complexity penalty.

 

[dcrofty]

Yeah, that'll stop them working at all, so they'll be much less grabby...
DO NOT put WD40 on your discs. Ever. You'll contaminate your pads and reduce your braking to nothing pretty quickly.
In fact, put nothing on your discs (not even your fingers) except disc brake cleaner or alcohol to clean them.

Yeah I know, I wasn't advocating that everyone goes and drenches their rotors in the stuff. Just relaying a (possibly untrue) amusing story about pro racers who would have known what they were doing.