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  1. #1
    Deported by koffee allgoo19's Avatar
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    Disc brakes, make sense?

    Maybe I'm ignorant.
    Why you need additional small disc to stop the wheel from turning? I thought regular bicycle brakes use rims as a big disc for that purpose. Since the bigger disc is more efficient of stopping power, why you need to put another small less efficient disc to do the same thing? Is there any logical reason for this, or it's just another fad created by component companies trying to make more money?
    Last edited by allgoo19; 10-18-03 at 08:59 PM.

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    Senior Member wyobiker's Avatar
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    There is no comparison between the v-brake and disc brake systems. I have disc on my mountain bike and v style on my road. The disc in my opinion is the only way to go and I look for the manufactures to start offering disc brakes on the rear for road bikes soon.

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    It's not totally black and white. As the OP said, the rim is actually a very large disc and rim braking is the most efficient "disc" brake you could have. Besides, there are other important advantages with rim brakes:
    - no twisting/bending forces in the forks and no pulling of the spokes;
    - the rim should overheat less than a disk (but see below);
    - few conflicts between brakes, and racks + panniers.
    These factors make rim brakes an attractive proposition on touring and road bikes. Besides, not only the rim brake is lighter than the average disc brake, but the bike should also be lighter (thinner fork, for instance).

    On the other hand, disc brakes have their advantages:
    - away from the elements, so they are as effective in the rain as they are in the sun; no idea how they stand through Winter, however;
    - mud and road grit don't affect them;
    - the braking surface is far away from the tire. It's still possible to overheat the disc, but you won't have a blow out.

    So they have a distinct advantage on all-terrain bikes where mud plays an important role, but on road and touring bikes, there still are a few issues to solve. Besides, if you are touring with disk brakes, how hard would it be to find pads or other parts in the middle of Newfoundland?
    Michel Gagnon
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  4. #4
    Kev
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    I don't think it is quite that simple allgoo, you are not taking into account alot of factors. You have a larger surface area to clamp down onto with disc brakes, you use pistons to do the compression (you can buy hydraulic v-brakes but they are EXPENSIVE). You can replace a disc alot cheaper then a wheel when it wears out also. Disc brakes down have problems with mud and water etc like v-brakes do, and because of the pistons and other things you have better modulation.

    Basicaly larger surface area to for bakes to hit at one time, more power because of the design of the way the pads press on the surface and better modulation because of way they are operated.
    NOw if you got a rim that has a wide of brakeing area as a disc, and used hydraulic v-brakes you would probably have better stopping and modulation then disc.

  5. #5
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    rim brakes wear out the rim. disc brakes also stop better. disc brakes are heavy, though, and disc wheels are also heavy. for certain applications like DH or trials, discs may be the way to go. for me, i'm not sure if my extra stopping power was worth the weight penalty, as well as the pricetag =/.
    i won't deny it i'm a straight ridah

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    Senior Member BAC5.2's Avatar
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    There really are no advantages to V-brakes other than weight.

    In a mountain bike application, a disc brake is actually more efficient than a V brake (Hayes Hydro's put out 3,200 PSI at the pistons, clamped on an 8" rotor, it takes almost zero time to stop). Heat build up doesn't make a difference. Disc brakes are fairly uneffected by high heat situations, most systems dissipate heat quickly due to rotor design and caliper design. V-Brakes, however, can glaze over and fade rather quickly due to the soft compound of the pads. Disc brakes have significantly more modulation because of the constant lever feel, significant amounts of power, and the reduction of effort. Disc brakes reduce hand-pump. They are uneffected by mud, rain, snow, slush, ice, anything you'll ever see on a trail will not effect the brakes. You replace the pads once every season or two, depending on the compound you are running. With V's you'll have to replace them very often (due to softer compounds.) You never worry about pitting out your rims. You never have to worry about your wheels becoming out of true and losing the effectiveness of the brakes. You don't need to worry about cable stretch (unless you are on mech Disc's). Nor housing compression. Maintainence is no more.

    There are millions of reasons to use discs, and almost zero to use V's.

    The front of touring bikes are good to have a disc on if you carry high loads on panniers. Avid makes levers for drop-bars on cyclocross frames
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  7. #7
    d_D
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    The one big advantage to disks is the brakeing surface can be expected to remain a lot straighter and more parallel during normal use than a rim. This means the pads can sit a lot closer to the brakeing surface than rim brakes allowing you to exploit this with levers that take more pull to move the pad a smaller ammount allowing you to put more force behind the pads with the same ammount of effort.

    Another advantage is when you are off-road, they are exposed to a lot less mud and water than rim brakes as they are higher up and hydro versions are well sealed against mud and water. Mud doesn't wear disk pads out in hours like some mud can to rim brake pads and generally the performance comes back to disks quicker after a soaking.

    If you spend a lot of time in mud then disks are by far the best way to stop the bike.

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    Senior Member BAC5.2's Avatar
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    Even in the dry, Disc's and V's are night and day. The only V's I will ever use would be the 04 XTR V's, and only if I was being a huge weight weenie. Otherwise, I'll take my disc's any day.

    It's like the difference between Drum brakes and Disc brakes on a car.
    2003 Banshee Scream. Banshee Pride!

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    i have never used discs but i don't see the need really.i use xt v's and they are all the power and then some i think i would ever need,but i don't ride d.h. or race so i'm not a good example really.my hew forks have disc tabs so maybe i'll see what all the hype is about and go disc, but then what about the rear?

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    Got Avid BBDB on my MTB road compo hybrid. Stops on a sixpence (a Dime to you Yanks) in all conditions and the 'all up weight' is around 125 kgs (281 pounds) crusing at 30k. 'Nuff said!
    "After a certain point, all dangers are equal'

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    Quote Originally Posted by BAC5.2
    Disc brakes reduce hand-pump. They are uneffected by mud, rain, snow, slush, ice, anything you'll ever see on a trail will not effect the brakes.

    Lots of things affect disc brake power. Snow will kill braking power, along with slush, ice, and rain, mud dosent too much. Atleast thats my experience with hayes brakes. They are very fussy sometimes.

  12. #12
    Recovering Retro-grouch CRUM's Avatar
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    As a reformed retro grouch, I never thought much of V brakes. Stuck with my canti's. Then I got a bike with disk brakes on it. I will not willingly go back to any rim brake in the future. I am faster with disk brakes. They allow me to carry a higher speed further into corners. Scrubbing speed is almost instantaneous, leaving you in a better position to re-accelerate.

    But certainly, disk brakes are being pushed so that more money can be made. It's the way of business. Innovation without a profit motive usually ends up going nowhere.
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  13. #13
    feros ferio John E's Avatar
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    A front disk brake is fine, as long as one does not try to use radial spokes with it. I like the simplicity of conventional rim brakes and find the SunTour RollerCam front / Shimano U-brake rear combination on my 1988 mountain bike to be more than adequate.

  14. #14
    Jubalayo Unogwaja! Bokkie's Avatar
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    Discs have a greater contact area where the pads meet them so have potentially better braking efficiency. Discs usually run much hotter so riding in rain can be less of a problem as the pad/disc contact area can skim water off more efficiently and the heat generated during braking can dry the disc much quicker than rim brakes which because of the rims proximity to the road/trail will usually be wetter more of the time. Generally disc pads cannot misalign like brake blocks can do. If a brake block wears down you can generally see it before it becomes a problem. A disc pad can still brake to some extent when metal meets metal but by then the damage is generally done and is very expensive to repair, hence the need to keep an eye on the pad material remaining.

    Less obvious is that the choice of pad can make a difference for there are different grades of pad and these might need to be matched to your riding style. After you've ridden a disc brake bike you'll convert over to them very fast. Where disc brakes used to be found on only the most expensive bikes you now see them on mid range models so they are becoming more affordable.

    With discs you need to look at hydraulic versus cable operated. I have hydraulics so maintenance is more fiddly with bleeding but snag the brake line suddenly and you can end up with a non-repairable leak whereas a cable could be jury-rigged to get you home.

    Personally speaking, I can see little advantage (apart from cost) that would prevent anyone not switching to disc brakes.
    If your bollocks ain't sore, yer ain't on yer boike!

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    Quote Originally Posted by allgoo19
    Maybe I'm ignorant.
    Why you need additional small disc to stop the wheel from turning? I thought regular bicycle brakes use rims as a big disc for that purpose. Since the bigger disc is more efficient of stopping power, why you need to put another small less efficient disc to do the same thing? Is there any logical reason for this, or it's just another fad created by component companies trying to make more money?
    First, it's not an "additional disc", it's instead of". Second, this could be a good time to forget thinking too much and figuring too much and just go ride a disc equipped bike with broken-in pads on a good downhill and see for your self. Personally, I didn't realize how much I disliked rim brakes until I wound up with mountain bikes for me and the wife that happened to have mechanical (vice hydraulic) discs. They are so much more practical, effective and easier to use I wish I could put a set on my road bike.

    I just bought a new mountain bike and got the dealer to buy back the Shimano hydraulic discs so I could put on my old Avid mechanical pull discs. Avids just work well forever with no maintenance.

    If this is a fad, I love it. I love it especially for riding in the mountains. I also like the ease of removing/replacing the front wheel: I have to do that to carry the bike in the truck.

    Al

  16. #16
    ld-cyclist prestonjb's Avatar
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    Avid makes mechanical disks that can be stopped by STI/road-bike levers.

    Airborne makes a cyclocross/touring frame called the carpe-diem that has rear disk mounts and they sell a touring fork (carbon) that has disk mounts.

    I'm a bit concerned about the heat on the fork from the caliper. Disks run a lot hotter esp around the caliper than "traditional" brakes do.

    My plans are to build a USA/EURO touring bike using the airborne carpe-diem frame as the base and start with regular CANTIs for brakes at first. Mainly because I don't want to deal with the clearence problems the rear disk has with a rear-rack. Later I will mount Avid road-disks on it.

    Be careful about long descents. I've heard stories where disks have warped to the point so that the brakes are "always on" at least until the disk cools.

    Also don't expect a disk to provide more long desecent stopping power than rims brakes. Esp known on hydralics are mealting fittings on the calipers. Some say mechanicals can suffer from heat problems and boiling out the grease in the bearings of the caliper.

    Overall the best reason for disks is for moutain bikes due to mud... Gives more clearence and causes less wear and tear on rims...

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    Interesting info on Airborne and Avid. I don't think there'll be a problem with heat from a front disc and the fork. You have to heat that hub first which should be a good heat sink. You can also brake the way mtb racers do. You stay off the brakes until you get to the max speed you can accept then you hit the brakes hard, scrub off the speed you need too as quickly as possible, then release the brakes and keep repeating the process. This saves on hand fatigue and raises the heat at the disc very high which maximizes the heat difference with the air which leads to better heat dissipation. In other words, you transfer more BTU to the air because of higher "delta T" so there is less heat to flow to the hub/axle/fork on average than a more constant application of the brakes.

    This technique was used in auto racing before disc brakes just to maintain braking during road races. I still do it today out in the western mountains as my travel trailer has drums.

    This of course would heat the caliper more and exasperate any problems there.

    As far as mechanicals boiling out the grease, sounds like they might have not had a high enough temperature grease. My own experience is limited to a 6 mile decent on the back half of Winding Stairs trail out of the Nantahalla Valley in NC. It's a paved road decent and you can easily hit 38+ on a knobby tired mtn bike. I've done it three times braking as per above before the sharp turns or intersections and they just worked fine. There's another 4 or 5 mile decent I do up in N Georgia on a forest service road and again, no sign of over heating. Of course, this is on an unloaded mtn bike, total weight including bike and hydration pack is about 223 lbs.

    I had a long discussion with a very competent/experienced bike mechanic/mtn bike rider (on really rough trails) and he says that hydraulics are prone to get air in the system. Even turning your bike upside down to fix chain-suck for example sometimes gets air mixed in the fluid. You heat aerated fluid and the brake calipers will close on the disc on their own and that can cause lock up. Same if any moisture is in the system: the moisture vaporizes and applies the brakes. I flush my automobile break systems every three years because of the moisture issue and I keep my vehicles a while.

    Al

  18. #18
    Senior Member gazedrop's Avatar
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    I don't have any experience with bicycle disc brakes, but plenty experience with them on racing motorcycles.

    One point that I saw raised a lot so far was the issue of heat. Although heat does raise other issues that need to be dealt with, it is also how brakes in general do their magic. A brake converts kinetic energy in to thermal energy (that whole energy conservation thing that Einstein touched upon once...) Sooooo, we are stuck with some degree of heat (no pun!).

    While there is more leverage for the brakes at the rims, a mechanical actuator (i.e. brake lever) can only generate a relatively low leverage ratio (mechanical advantage) or clamping force when powered by a human hand. That is, of course, assuming that the design is practical for use on a bicycle! (Sure, that 18" brake lever has a lot of power, but isn't that 6" lever travel a bit much? )

    A mechanical disc brake should have a greater leverage ratio since for the same amount of lever length/travel, as the disc pads are being moved a much shorter distance. (NOTE: I am making an asumption on this point! Don't shoot me if I am wrong on this one, just put me straight! )

    A hydraulic system, on the other hand, by very nature of hydraulics, can generate rather enormous amounts of mechanical advantage for quite small power inputs... And do so with little mechanical loss in a compact, tidy package.

    Hydraulic mechanical advantages are determined by the piston sizes. But, braking power, modulation, and, well, feel are determined by disc diameter, master cylinder piston diameter and travel, caliper piston diameter, brake lever length, and brake lever pivot length. Get the combination wrong and you either have ineffective brakes, or super-powerful brakes that you can't modulate.

    DOT Brake Fluid:

    Brake fluid is generally non-compressable (sp?). It's designed to be. But it does have problems of its own. Its first problem is that it's hygroscopic, meaning that it absorbs water and moisture super fast and incredibly well. (My favorite way to prove this: Fill a small a small jar 1/4" below the top with fresh fluid. Place this jar in an open bucket. Wait a few hours or overnight. Thank yourself for using the bucket to catch all of the overflow! That's from moisture in the air.)

    That's also why you MUST wear rubber or nitrile gloves to protect your skin.

    Brake fluid is also more resistant (than water) to boiling. But it will do so. And it will do so at a lower temperature if it has had a chance to absorb water (look for the "Wet" and "Dry" boiling points on the bottle.) When it boils, little gas bubbles form in the fluid. Unlike the fluid, those bubbles readily compress, resulting in a brake lever that comes to the bar (until you "pump it up").

    If it boils enough, it's possible for the expanded fluid to "Self-engage" the brakes, even with the lever released (rare, but occasionally happens in motor racing, usually precipitated by another problem like dragging pads heating the fluid...)

    Brake fluid also oxidizes, and breaks-down from repeated heating. It will also, given enough time, draw moisture past the rubber caliper piston seals. That's why you should replace your fluid yearly.

    If you don't want to undertake this yourself, then pay a trusted shop to do it. These are your brakes that we're talking about here! Not a time to cheap out!

    Al noted in his post that air can get in the system from turning the bike upside (He also wrote a great tip on braking technique). This would be true. It's not a flaw of design, but due to the fact that the fluid resevoir has a little bit of air in it. That bit of air isn't a part of the pressurized system, but can get there when inverted. (We've been known to remove the brake systems from our race bikes, hang 'em upside down overnight, then finish bleeding them upside-down in the morning. Now we just use a Mity-Vac.)

    I also saw posted an issue with warping. I'm still waiting for the manufacturers to come out with floating discs... This is where the disc rotors are "Flexibly" mounted to a separate carrier. That way, the swept area (where the heating occurs) can expand and contract independantly, reducing the effects of warping and coning. There's other advantages, too, like reduced drag, lighter weight...

    Downhillers might do well to look for thicker, heavier discs. Since outright weight isn't as big of an issue as road or x-country, you can take advantage of the heat capabilities of a heavier disc; more material = longer heat-up time. Worth a look if you compete...

    Do it yourself? Here's a couple of tips I always follow that may not be in the manual (Note: Applies to DOT fluid only; that's all that I know! ):
    1) Use fresh fluid. Don't know how old that opened bottle is? Get a new one.
    2) When getting that new bottle, unscrew the cap and check that the "Freshness Seal" is intact.
    3) Buy fluid that comes in a plastic----not metal----bottle. That way you can squeeze the excess air out of it before capping. This will keep the fluid drier.
    4) Religiously put the caps on both the resevoir and bottle after you top-off the resevoir, even if you know you'll be topping it off again in just 10 seconds while you're bleeding the system. The entire brake system only holds a small amount of fluid, and therefore cannot tolerate much moisture contamination.
    5) Think you got all the air out? Bleed it one more time... It'll only take another 10 seconds.

    Did I miss anything?

    Another long winded post by:
    -Erik

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    Only that you can get the same amount of leverage in a v set up as with a disc set up, it depends both upon the length of the lever, the travel, the size of the rims and the length of the actuating arms. Otherwise, you covered pretty much everything.
    But, for all the disc fanboys out their, may I just remind you that some motocyles are switching to rim mounted brakes because they offer better braking/lighter weight/better handling.
    I can see a really easy way of incorperating the two - merely extend a thin, replacable, "disc" horizontally from the rim, either as part of the original extrusion or as a bolt on item, add a radially mounted calliper, and voila, benifits of both systems, penalties of none (as far as I can see).

  20. #20
    Senior Member Retro Grouch's Avatar
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    I'd like to comment on a couple of issues that have been mentioned.

    1. Front disc brakes with quick release axles have issues. If you apply too much brake power, the hub tries to rotate around the caliper and can actually pop the axle out of the fork. That problem has been recently been discussed in detail on the Tandem and Hobbs forum.

    2. Most bicycle hydraulic brake systems are closed. If the brake fluid heats up too much, it will expand and lock up the brake until it has a chance to cool again. This doesn't seem to be much of a real life problem for mountain bikes because their brake usage is so intermittant, but it can be a serious problem for tandem bicycles on long descents. Santana tandems use an open hydraulic system to allow the brake fluid to expand, but they have a reputation for being touchy to keep adjusted.

    3. How much brakeing power do you need? I realize that I'm a research study of just one, but, the only situation that I remember wishing I had more breaking power was riding my tandem on a steep downhill. In every other situation that I can think of at the minute, my brakeing power was limited by tire traction.

  21. #21
    Senior Member gazedrop's Avatar
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    But, for all the disc fanboys out their, may I just remind you that some motocyles are switching to rim mounted brakes because they offer better braking/lighter weight/better handling.
    The rim mounted motorcycle brakes have very little use. Currently, they only have applications for Buell Motorcycles (the sportbike division of Harley Davidson). Other than that, their only appearances have been on concept motorcycles, most notably, Honda.

    But if memory serves, they were originally developed by Braketech, an aftermarket supplier (targeting, btw, Buell applications!)

    History lesson aside, they actually have some significant drawbacks. These are greater gyroscopic moment than smaller twin discs, and lending to fork binding (increasing fork stiction since it's bending only one fork under braking).

    So why does it work on the Buell XB9S? 1) It's got really short and steep chassis geometry, even by racebike standards. The big disc returns some of the lost stability. 2) It's not a racebike, so they're not as concerned with a little extra fork stiction.

    I'm not saying that these are major problems for a streetbike; it's more a question of fashion... but you won't see them on a racebike.

  22. #22
    Bike Dork prevail24's Avatar
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    I am putting disks on my surly cross check. A frame builder is retrofitting it. Dont worry...he has done it well on a few bikes already. There is a disk specific cross fork that I will put on it. It is my commuter, so when there is bad weather, I need to be able to stop in anything. This solution will solve all of my problems

  23. #23
    Recovering Retro-grouch CRUM's Avatar
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    As a former retro grouch when concerning Disc brakes, I can see where the Conventional brake people are coming from. But IMO, the Linear Pull brake (V-Brake) was a solution looking for a problem. Decent quality Cantilever brakes, properly set up, can stop as well as Linear pulls. I have a pair of Joe's Brakes that can pucker a rim.

    Linear pulls have the advantage of easy adjustment, powerful stopping if set up right. On the other hand, they are fussier to keep adjusted and must be set close to the rim to be effective. That means wheel trueness is of paramount importance. With Canti's, they are more forgiving of out of true wheels as they can be set up with more room between the pads and the rim. This affords more mud and crud clearance. They also do not go out of adjustment as easily. Then there is the wear and tear on the rim with both Cantis and V's. Under hard usage, it is not unusual for some of the riders in my area to go through a pair of rims a year. And all because of braking surface wear.

    That said, IMO, Disc brakes stop better in all conditions than any other brake out there. And, no, we do not need them. But for me, I have become faster as a result of using them. I can enter corners faster and count on them to slow me down with more predictable results. With Canti's or V brakes, trail conditions dictate how much I can rely on them in dicy conditions. On my mountain bike, they are what I will be using from now on. For my beater and road bikes, whatever is on them works just fine.
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  24. #24
    Ride the Road Daily Commute's Avatar
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    Contrarian Viewpoint

    I have never had a problem with lack of stopping power with canti breaks. Inadequate friction of tire against road, not of pads against rims, has always limited my stopping power.

    The other bike commuter in my office has disc breaks. He reports they are far more finicky than canti's, especially when he has to take the wheel off and put it back on. As a result, he has delayed putting his summer tires on. I have canti's. I have had road slicks on for weeks (except for a few days when I switched back to the Nokians to deal with a late snow storm). The canti's made it easy to switch tires back and forth. I like the simplicity of canti's.

    Questions for disc-break users: Is my colleague right that disc breaks are more finicky than canti's? Are disc breaks as easy to adjust and repair as canti's.
    Last edited by Daily Commute; 03-23-04 at 10:09 AM.

  25. #25
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    Rim wear can be a problem on foul-weather bikes. A worn braking surface can suddenly detach from the spokes; its rare, but does happen. Modern rims should have a wear indicator.
    With disks, can you get rims which have no braking surface? Presumably they would be lighter, but plenty strong enough.
    The disk-compatable touring forks I have seen are fairly massive, and seem to have very little flex. These can be brutal on your wrists. Most MTB disk setups are on suspension forks, where this is not an issue. You can end up adding more and more stuff (which breaks down and needs maintainance).
    The dropout "problem" with disks can be solved by mounting the calipers at the front, not the rear of the fork.

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