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 Post subject:
PostPosted: Fri May 18, 2007 4:03 pm 
Dirt Disciple

Joined: Mon Apr 30, 2007 10:45 pm
Posts: 95
Russell wrote:
A good Disc outperforms good cantis, end of. Lets not let retro nostalgia get in the way of common sense chaps :)


In what way do they outperform?

Do they allow you to brake harder?
Will they stop your bike faster on any given surface apart from dry tarmac?
Or is it just less lever effort for a given level of decceleration?

It's not retro specs, it's physics. You have a limited amount of grip in your contact patch, that grip is easily overcome by even good cantis, let alone v brakes.

Discs allow lower effort at the lever which subjectively feels like better brakes, in much the same way that servo brakes on a car will.

In lightweight race cars your will find unassisted brakes because although a servo would make the pedal lighter there is a loss of feel and the braking available would be far in excess of the grip available.

It's never 'end of' in the my kit is bigger than your kit debate. :D


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 Post subject:
PostPosted: Fri May 18, 2007 4:22 pm 
Old School Grand Master
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Joined: Sat May 20, 2006 2:56 pm
Posts: 4776
Location: No brakes? Way to commit soldier.
A lightweight racecar may have unassisted braking, but they still use discs. Ask the driver of the car if he'd like to use discs, or 'rim-brakes' and (once he's stopped laughing) he'll tell you that he'll "run discs today please". Point me in the direction of a racecar that uses a rim braking mechanism because its more efficient than a disc based system.

Struggling?

Surely a racecar has a small contact patch with a limited amount of grip aswell? Why don't they use rim brakes? You'll probably find that the answer is pretty close to the reason that discs are better on bikes too.


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 Post subject:
PostPosted: Fri May 18, 2007 5:02 pm 
Dirt Disciple

Joined: Mon Apr 30, 2007 10:45 pm
Posts: 95
Russell wrote:
A lightweight racecar may have unassisted braking, but they still use discs.


Rim brakes haven't been used on a car for almost 100 years. Ask about drums though and he would undoubtedly still come down on the side of disks, although perhaps not on the rear where they can often overcome the available grip. If you asked about assisted or unassisted the answer would be unassisted. We are talking more about the amount of usable braking force available here, not really the method of obtaining that force. None of the cars in, say Formula 600 or the Jedi classes, run assisted brakes. You can have too much force, even for a Jedi. :D

Quote:
Struggling?


Not so far. :D

Quote:
Surely a racecar has a small contact patch with a limited amount of grip aswell? Why don't they use rim brakes? You'll probably find that the answer is pretty close to the reason that discs are better on bikes too.


Not nearly so small as a bike. But yes, it is limited. And they don't use rim brakes because the predecessor to disks was drums. Rim brakes went out with wooden wheels.

On the car, which would never have had rim brakes, the alternative, drums, were not efficient enough to overcome the grip available, especially with the advent of modern tyre compounds. The disk has other advantages over the drum for a car too, they are lighter and less prone to overheating. These are not advantages that disks have over rim brakes.

There is no point having brakes which produce breaking force far in excess of the available grip. This is why on a car, even with unassisted disks, the amount of breaking force available is reduced in times of reduced grip, either by changing brake cylinder ratio, or the leverage available at the pedal.


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 Post subject:
PostPosted: Fri May 18, 2007 10:09 pm 
Old School Grand Master
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Joined: Sat May 20, 2006 2:56 pm
Posts: 4776
Location: No brakes? Way to commit soldier.
You're too wrapped up in theory which is fine, but in the real world I want a brake that doesn't give me arm pump on long descents, I want a brake that I can rely on to stop me when its raining, I want a brake that I can use after I snap a spoke and bend my wheel, I want to be able to rip my wheel out to fix a puncture without having to faff about undoing brake cables with freezing cold fingers, I want to be able to throw my bike in the garage after a long hard cold ride without worrying about lubing my brake cables, I want a brake that isn't situated exactly where all the mud collects at the top of my seatstays, I could go on.

None of the above has anything to do with grip or contact patches or needs backing up with physics theory or automotive analogy, discs work better because they meet the above criteria and many more that most rim based systems don't. Its not marketing hype, its fact.


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 Post subject:
PostPosted: Fri May 18, 2007 10:56 pm 
Dirt Disciple

Joined: Mon Apr 30, 2007 10:45 pm
Posts: 95
Russell wrote:
You're too wrapped up in theory which is fine,


Actually all that 'theory' is practical, real life, not classroom or lab.

Quote:
but in the real world I want a brake that doesn't give me arm pump on long descents...

...I want a brake that isn't situated exactly where all the mud collects at the top of my seatstays, I could go on.


So it's not a issue of actual performance, it's a more subjective stance.

Quote:
None of the above has anything to do with grip or contact patches or needs backing up with physics theory or automotive analogy, discs work better because they meet the above criteria and many more that most rim based systems don't. Its not marketing hype, its fact.


It's not fact, it's opinion.
None of those issues were ever issues to me (except perhaps wet braking) so I don't see disks as any sort of advantage.
And you introduced the automotive analogy, how come you don't like it anymore?

Struggling?

Anyway, belt drive for bikes?

A good thing or absurd marketing overkill?


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 Post subject:
PostPosted: Fri May 18, 2007 11:05 pm 
Gold Trader
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Joined: Sat Jul 29, 2006 11:14 pm
Posts: 3835
Location: Somerset
Easy way to sort this.

Use rim brakes and just go out when it is sunny :D




Sorry :wink:


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 Post subject: way over rated
PostPosted: Fri May 18, 2007 11:06 pm 
BoTM Winner / PoTM Winner
BoTM Winner / PoTM Winner
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Joined: Sat Jan 13, 2007 10:44 pm
Posts: 1184
Location: norcal
all brakes do is slow you down


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 Post subject:
PostPosted: Fri May 18, 2007 11:19 pm 
Old School Grand Master
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Joined: Sat May 20, 2006 2:56 pm
Posts: 4776
Location: No brakes? Way to commit soldier.
How can not getting arm pump, or having brakes that work in the wet (surely a major factor for most MTBers), or wanting brakes that don't clog with mud not be 'real', 'practical' issues?

The reasons I stated are facts, disc brakes don't suffer from those things, most other rim systems do. Its not my opinion that a disc brake isn't situated at the top of the seatstay where the mud collects, its a fact, unless somethings gone horribly wrong with your rear end!

You lost me saying that I introduced the automotive analogy though. Read through this thread, YOU introduced the automotive thing with the "In lightweight race cars your will find unassisted....." comment and I never really agreed with you in the first place did I so no, not struggling, thanks.


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 Post subject:
PostPosted: Sat May 19, 2007 12:54 am 
Dirt Disciple

Joined: Sun May 06, 2007 12:31 am
Posts: 12
Location: Great Britain
Worm. Open. Can.

Part of the reason now that so much more braking is available is that the traction from the tyres and the quality of suspension travel have now moved on a lot. If you have a rigid bike with old 1.9" tyres and cantilevers, because the tyres will constantly be out of contact with the ground (or at least not supplying max pressure, the brakes will easily be squandered due to the lack of grip available. If you compare that to a full sus xc bike with 4-5" front and rear, with the suspension set up to resist fork dive but still absorb most things without being too stiff then there will be far more grip available as the tyres are able to more closely follow the terrain. Add a set of slightly larger 2.1ish tyres on there which can be run slightly softer etc and there is a noteable increase in the traction available. This will help when cornering etc but also when braking (the same principle of suspension allowing you to climb over rougher tracks easier) as the suspension and tyres prevent the tyre leaving the ground, bcoming so unweighted and then losing grip.

I mean, don't get me wrong, the brakes on my big bike would be completely unecessary on my hardtail - there is simply too much power there and they also weigh a ton. For plain old riding a set of Magura racelines are superb when combined with a set of mavic rims - but they still need looking after. In contrast, the brakes on my xc bike were gracing the dh bike for 2 seasons abuse and aside from a hose damaged in a crash, they went untouched - aside from 3 or so pad changes a year. And you don't even want to see the state of my wheels on that thing. Which neatly brings me onto my next point. Rim brakes are fine when everything is smooth, the wheels are perfectly true, the brakes have no slop in them, the cables aren't gunked up and the rims aren't covered in slop. Mountain biking is, unlike road riding, mainly in the slop on this fair isle and that causes a problem. You get grinding, go through many sets of pads, have rub from when the rim gets a tiny bit out and you dare ride through a puddle (after which the brakes lose performance) and of course you also have the problem of running straight through the rims sidewall (unless they're ceramic). That then necessitates a new wheel. In contrast a rim brake will perhaps occasionally grind but because the disc is steel and the pads rock hard (on most systems), it causes no ill effects. There are only pistons which move and no pivots and therefore the slop is reduced and the element which is difficult to adjust and control on rim brakes. Yes, you do knock rotors but an adjustable and two minutes in the shed easily solves that.

Rim brakes are fine if you are a reasonable gentle rider but I would never consider them to be better than discs for anything other than reasons of retro-ness. A disc can be lighter, more reliable, more powerful and also more controllable.

One overriding memory I have of rim brakes is that there was a period after the pad touched the rim where bugger all happened (XTR V's on Ceramic rims) before things suddenly decided to grip and you would start to slow down (ok, exaggerated a little but you get my point). Discs give power and quite linear control from the moment the pad hits the disc - very much in line with car brakes.

I love this site, the retro kit is awesome and it really takes me back to when i started out as a kid but things have moved on at the front line for a very good reason - modern kit just works soo much better. No reason not to have the retro stuff mind - who said that sense had to come into the equation anywhere?!


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 Post subject:
PostPosted: Tue May 22, 2007 9:37 pm 
Dirt Disciple

Joined: Mon Apr 30, 2007 10:45 pm
Posts: 95
Stick Legs wrote:
Easy way to sort this.

Use rim brakes and just go out when it is sunny :D




Sorry :wink:


It works for me. :D


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