Testing the fork un-raking theory

There’s a theory, more like a hypothesis, that if you put a disc brake on a raked (curved blades) fork that the blade will ‘un-rake’ itself over time due to the disc brake force on the left leg.  This would result in a lesser amount of rake (less than the 44mm you bend the blades to be) and the fork twisting out of alignment.

I’ve not seen any evidence of this online, but it may be out there and I just haven’t run across it.  It could be like a lot of things framebuilding and otherwise, people develop opinions that become fact without much evidence; just because it seems like it should be true.  That doesn’t mean it’s not true, it’s just possible that this idea has never been tested very thoroughly.  It does seem like disc brakes on some thinner walled road fork blades would not be a good idea, and I have seen a photo or two of fork failures right above where the disc mount was welded to the fork leg. But would some beefing up of the disc brake mount on the fork leg limit any un-raking of the fork leg and prevent fork failures such as using Paragon’s Willit’s style caliper mount?

I wrote about this fork awhile ago and rode the fork as a all-rounder MTB/Cross/dirt road cruiser for over a year.  The fork blades I used were the only available  for that ‘retro’ style fork crown from Nova Cycles Supply (0.9/0.6mm wall i think – pretty thin).  I rode it on the same trails I rode my mountain bike and I was not easy on it.  I linked to another blog that provided the inspiration for using the “Willit’s style” disc tab on a raked fork blade.  Here’s a photo and explanation of the tubing that was used in the fork compared to the newer one I made to replace it.

Anyways, since i made a replacement fork for my 1st lugged MTB/All-rounder fork, I wanted to check how it fared after being ridden hard – to see if there had been any un-raking of the left fork blade from the force of braking.  The gallery below shows the results.  Sample size of 1 doesn’t say much but even with my newbie fabrication skills when I made this fork…it did not move one millimeter — and I rode it hard.  Maybe over more rides and more time it would bend back? I’ll be checking changes in alignment over time for my other forks.

16 Responses

  1. Interesting stuff. I’ve heard a couple or three people talk about their forks un-raking due to disc brake forces. This was on one some bike/framebuilding related online forum or other so take that for what it’s worth.

    I’ve only seen the un-raking phenomenon once in person, on the commute bike of a local, very experienced builder. He had built the fork with light duty blades (I can’t recall the dimensions now) and once he rebuilt the fork with stouter blades the problem went away I’m pretty sure.

    People do seem to have good luck using the Willit’s brace, which makes sense. It does bug me though that in using one, you end up with blades that have different flexure characteristics. In practice, that probably doesn’t mean a hill of beans to the rider, but from an engineering perspective it seems somehow inelegant. Never ridden one though so I really should try one first, before casting judgement.

  2. People who think that forces below the limit of elastic deformation of a steel fork blade will deform it over time are either crazy or have a very different definition of “time” (heat death of the universe?) than I do.

    Now, if you go past that and into inelastic deformation, you will of course bend things. But generally that type of event results in some form of pretty obvious failure (ie, buckled tube, cracks, etc).

    Builders who are still afraid of/unable to cope with disc brakes are just freaking out because they’re starting to show up on road and CX bikes so they can’t keep their heads in the sand much longer even if they never build mountain bikes.

    • Walt, I’ve definitely seen a fork un-rake, yield, however you want to put it. It was a subtle thing, not obvious at first, until the fork was measured. Not a catastrophic thing at all, but it did affect the tracking of the bike.
      If you get into the mechanics of how metal yields ie. slip planes, dislocations etc. (and I’m no expert in this) you can argue that the force required to un-rake the blade is less than the force that it took to rake it. It’s going to depend on a bunch of variables, but it can happen. It’s not just a hypothetical scenario, in my opinion, although not necessarily common.

      • I have built ~1000 disc brake forks of all types, as well as seen lots more that came in to be looked at after a crash or garage/roofrack incident. I have NEVER seen a fork “unrake” without some kind of unusual event/crash. I say BS unless there’s a precipitating event of some kind.

      • Walt, that’s a ton of experience, and I give it a lot of weight. I’m always interested in learning more from those with more experience than me.

        I can’t ignore what I’ve seen though, so I’m going to have to disagree with your conclusion.

        Just to be sure we’re on the same page, are you talking about road bike style forks raked with a curve, or straight bladed MTB style forks?

        I do take issue with your use of the term BS, as it implies that I’ve got some ulterior motive in all this. I don’t. I think disc brakes make a lot of sense in many applications. I’d like to find a way to incorporate them safely on curved road bike style forks ie. light gauge blades that give the ride characteristics that I’m looking for.

  3. Thanks for the great comments you guys!

    I will have to admit my ignorance in flexure characteristics and deformation limits, but intuitively I tend to agree with Walt that unless there’s a force that is very close to or higher than the force one puts into bending the blades…then I don’t understand how a fork blade can physically un-rake from the normal braking force of disc brakes. However, it appears that it has happened at least a few times from reputable builders and that completely confuses me. Maybe it’s as simple as they used what they normally use for road blades and they were too thin-walled and narrow profile? Or maybe they didn’t use a Willit’s style caliper mount to help support the raked leg? Or maybe they stomped on the brakes enough that the force was close enough to the force it took to bend the blades in the first place and as a result they moved back a bit. The last one especially would honestly confuse me. I know that when I try to cold-set frames and forks, I’m pretty surprised at how difficult it is to get them to move even 1mm. I still cold-set forks (not frames though) since none of my forks have come out perfectly straight but I think that’s pretty common. So, in knowing the brute force I put into cold-setting a fork…and that happening on the bike while riding and just using the brakes…well crap…i’d be in the ditch from me going over the bars from braking that hard! But I must be underestimating the force a disc brake places on the one fork leg.

    I never thought about the different ride quality possibility from one side having such an extensive brake mount like the Willit’s. I’m not sure I’d realize the difference myself but it’d be worth trying with two identical forks – one is disc and one is canti. If such a difference in flexure occurs, there would be possible twisting in the blades and some self-steering while braking I’d imagine? I don’t notice any of that with the two forks I have ridden but it may be subtle.

    The new fork I’m now riding has thicker blades and it’s noticeably stiffer in ride and while braking than the last fork (the one I’m writing about in this blog). With the Pacenti crown and True Temper blades, I’m pretty confident in using them with discs. I’m sure there are many other builders doing it out there and it’d be cool to see the data compiled on failure rates and what tubes and caliper mounts were used in each circumstance. Or if someone wanted to dork out and do some computer modeling that’d be cool as well!

    • Yes to your first two suppositions, in the cases that I’ve seen anyway. Light gauge blades were used with no reinforcing torque arm. At the time those forks were built, the Willits style adapter wasn’t available. To my knowledge anyway.

      Regarding forces due to cold setting I agree. The forces required to cold set a fork or frame always shock me too, however…
      have you ever flipped a fork around in your bender to take some rake off it? It’s surprisingly easy and that, in my opinion, is what’s happening with a disc brake.

      The center of the disc brake pad acts as an instantaneous center of rotation, like a fulcrum, and the friction between the road and the tire is pulling backwards and down, trying to eject the axle from the dropouts. That force is what can un-bend the blade, in my opinion. All the action is happening between the center of the DO’s and the caliper mount. That’s why I think it can be hard to relate to, since it’s not how a fork blade would be stressed when you are cold setting a fork.

      As for the difference in ride quality between blades, I think that would only really come into play on really light road gauge blades, where you’d be hoping to get some flex in the lower, curved part of the blade.
      The addition of a relatively massive reinforcing member there would make a difference.

      I think for most fork blades though, the extra stiffness added by a Willits style brace would be lost in the noise of the flexing of all the other things involved. It would definitely be there, by definition it would have to be, but whether the rider could feel the difference? I rather doubt it.

      Anyway, not sure if this helps, but it’s interesting to think about this stuff plus, I’d like to know more about it myself.

  4. Well, every time I hear about this, it’s “well, this one time, this friend of mine” kind of stuff. I don’t see pictures and well documented first-person information. I tend to distrust a lot of the internet-based chatter about this topic from people who mostly don’t build disc brake bikes and/or dislike the whole idea.

    In my direct experience building both curved blade and straightblade disc forks, you get 2 kinds of failures:
    -Type 1 is when the disc tab breaks the fork blade at the highest attachment point. This spot is a huge stress riser so if you use a Willits type tab or just weld on an extension to a normal tab (or do a postmount setup) you can mitigate the risk and spread that load/reduce leverage.
    -Type 2 is when you hit something hard or land flat of a big drop or whatever. Forks (of all types) break where the blades meet the crown in this scenario. In some cases they buckle, in some cases (larger diameter blades) they crack.

    You NEVER get failures or problems in the middle of the fork blade, nor have I ever seen a smooth curve introduced/removed (ie deraking) by braking forces or any other scenario. If point loads made fork blades bend smoothly, we wouldn’t need mandrels to bend them over, right? You could just clamp the tip and pull on the other end.

    -Walt

    • “You NEVER get failures or problems in the middle of the fork blade, nor have I ever seen a smooth curve introduced/removed (ie deraking) by braking forces or any other scenario. If point loads made fork blades bend smoothly, we wouldn’t need mandrels to bend them over, right? You could just clamp the tip and pull on the other end.
      -Walt”

      (re-posting this, the first one got away from me before I was finished, sorry about that).

      The phenomenon I’m referencing is not happening in the middle of the blade. It’s happening towards the end of it, in the curved section.

      If you need to take some rake off a curved fork blade you can do so by flipping it around in the bender, locking the blade tip in place, and pulling on the steerer. In this scenario the blade is only being subjected to point loads, since it is touching the mandrel tangentially (curve against curve). The blade un-rakes by increasing it’s radius of curvature, not by introducing a kink in the blade.

      Obviously, if you take it far enough you’ll eventually subject it to a point load that buckles the blade at some point, but for taking a few mm of rake off the blade it works fine and the blade remains uniformly curved.

      If you like, I can put you in touch with a very experienced local builder here in Seattle who can relate to you directly his experience with disc brakes un-raking forks.

      Alistair.

      • Let’s just say we agree to disagree, and that if you do a good job building your fork, it won’t break OR derake. 🙂

        -W

      • Agreed!
        If the forces acting are understood, and enough material is put in the right places to deal with those forces, all will be good.

        Alistair.

  5. The infamous “fork that unraked” on the internet was mine: http://www.flickr.com/photos/drwelby/5632096652/.

    Note the short disk mount.

    • Marc,
      Very interesting! Thanks for posting this. Kinda completes this thread knowing that it’s not a myth and actually has happened. Maybe the Willits style mount is keeping my forks from un-raking? I’m riding it a lot on dirt roads, 4×4 and other, with singletrack thrown in but not on super technical over the back of the seat rocky trails. I’ll be sure to update if something tragic happens…

      • Yeah, you just need a little more thickness front-to-back than just the road fork blade. The FB4s work fine, but they have a short taper so they’re already somewhere around 17mm at the top of the mount.
        I’ve been wanting to make an aluminum disc mount that bolts to the fork leg near the dropout and then higher up maybe 1/3 of the way up the leg, and have it either slotted or on a small linkage so the for can flex independent of the brake mount.

      • A disc mount with pivots to allow flex..that’s a cool idea! I wonder if Paragon would take up something like this. I would like to ride both types side by side to see how much I notice the different in flex.

        Discs on raked blades isn’t going away IMO. For example, I just got a blog hit from a search phrase “suspension corrected 29er fork monster crossed lug“. Having a better solution would be great! What about Post-Mount…? Or an ISO that is attached to the dropout (like Alex Wetmore’s custom mount:http://alexwetmore.org/?p=1173) but then only to a spot above the bend in the blade? That way between the two points the leg is allowed to flex?

      • I was going to try something waterjet cut that would ride on a brass bushing at the bottom. At the top the brass bushing could ride in a slot, or for a short link I was thinking you might be able to make something work using a half link of chain to bridge the gap.

        Or we just need somebody to make a blade that’s 28 x 20 at the top and 14 x 10 at the bottom. Wouldn’t be a ton of fun to rake but I think it would look good – a little bit aero and retro at the same time. You actually might be able to get those dimensions out of a current road fork blade – might have to experiment a bit and run some numbers.

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