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  1. #1
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    Brake choice for a rigid fork.

    I didn't know where to post this, so apologies in advance if this isn't the correct forum.

    Spring is here, which means looking for a potential custom frame build. This build will be a singlespeed, rigid-specific frame. I am starting to go down the path of full on retro-gression. This will be my 3rd SS and 2nd rigid bike. This build, I was thinking of going with canti or V-brakes. Main reason being, I want a good balance of compliant fork without brake flutter issues. I have *heard* and read that discs will cause much unwanted flutter on a compliant fork. With rim brakes, the fork can be built stout up top, where the brake is, and tapered down substantially down low for the compliancy.

    So I was thinking going with rim brakes. The climate I live in is very dry and seldom rains (and when it does, I don't ride). I have been happy with rim brakes in the past (XTR, SD7), although my current BB7's and XT's do brake better. But I would definitely give up some brake performance for a more compliant fork. Does this make sense? Is this something worth pursuing a custom build with rim brakes? Or am I off my rocker (literally)?

    Other details--still debating on a 1" threaded fork and/or dropbar set up....

    Thanks for any input.

  2. #2
    Randomhead
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    brake shudder will come with a compliant fork. V-brakes are probably better for this than cantis since cantis can have steerer-induced force fluctuations.

  3. #3
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    Hey;

    I don't think you are nuts. I am also dealing with brake shudder or vibration on my Fatbikes. Because of my size and riding style, I cannot get by with less than an 8" front rotor, and it is quite possibly causing some issues. I have not decided whether it is the fork exacerbating the situation, or simply a bad mix of brake bits. This happens on two different bikes with both Hayes Nine and Stroker systems, both using the same fork.

    Your premise is a sound one in my opinion. What to do from there I cannot say for sure, but it seems that rim brakes might be helpful if our hypothesis hold water.
    Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
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  4. #4
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    Speaking generally, trying to get compliance from small diameter fork tips is not compatible with having enough rigidity at those tips to take the force of a disk brake. If you want retro and fork compliance, then V brakes with a raked fork and a Pacenti or Bob Brown crown would fit your bill.

  5. #5
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    I am going to be contrarian here and just say that:
    1: I've seen this issue many times on all kinds of forks and it's *always* been traceable to the brake system itself rather than the fork (excepting cases where a loose headset is responsible). Slightly warped rotor, ding in the rim, long canti brake cable doing weird stuff, etc.
    2: I have built probably 300 (at least, I am losing count at this point) forks with 14mm blade tips (the KITFB4 from True Temper, usually) and disc brakes and I have never had anyone complain of this problem except when it turned out to be a brake issue.

    That said, if v-brakes will work for your riding area/conditions/style, they are still darn good brakes. You don't "need" discs. Avoid cantis, they have shudder issues in many cases unless you build a stop into the fork crown for the brake cable and let's face it - v-brakes really are better in basically all ways.

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
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    waltworks.blogspot.com

  6. #6
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    Here's my take on the retro singlespeed - This bike has the KITFB4 legs sleeved into 28.6 tubing and clamped with an old Manitou crown. I don't mind the slight loss in retro-ness to not have to deal with rim brake pads. I don't think the fork is terribly compliant, though. I find a fat front tire does a better job.


  7. #7
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    I don't have actual data (someone probably does), but I don't think fork tips bend all that much anyway. There's just not that much torque down there. Forks mostly bend at the crown/steerer. You can actually watch this happen on some flexier forks while your riding. Just much more leverage.

  8. #8
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    Thanks for all the inputs. I guess my assumptions were wrong. I still think v-brakes would look better on this "retro" build, but after this morning's ride (Hogle Zoo to Dry Creek, which Walt is probably familiar with), I just like the 1-finger braking of my XT hydros way too much. Plus rim choice is the other big factor as well. Maybe I will end up sticking to discs for this build. Thanks again for the thoughts. Dr. Welby - That bike looks awesome.

  9. #9
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    Dude, you should have called me up, we could have argued about it while we rode!
    Waltworks Custom Bicycles
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  10. #10
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    Not to derail too much (yes, on this essentially settled topic), but I will let you know next time. Surprising number of people that ride/run/walk on a weekday morning on that trail.

  11. #11
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    Quote Originally Posted by Feldybikes View Post
    I don't have actual data (someone probably does), but I don't think fork tips bend all that much anyway. There's just not that much torque down there. Forks mostly bend at the crown/steerer. You can actually watch this happen on some flexier forks while your riding. Just much more leverage.
    I did some quick FEA modeling few years back (disclaimer: I am not an expert in FEA by any means) and in a raked, tapered fork the forks tips do provide a good deal more vertical displacement to vertical loads - about 50% more. Which isn't that surprising if you simplify the raked fork down to two segments. Since the tips are closer to horizontal they offer more vertical movement. They may have about half the torque, but given their smaller cross section they're also much more flexible - somewhere around 16X more!



  12. #12
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    Quote Originally Posted by dr.welby View Post

    I've been under the impression that raked and straight blades have the same displacement under a given load, guess thats incorrect? I can't tell from the graphic if the vertical displacement at the tip of the blades is equal (but occurs along different "paths"). Does the graph show what would occur with an identical blade, one raked and one straight (with offset/rake at the crown)?

    thanks, Brian
    Last edited by calstar; 04-12-2013 at 06:45 PM.

  13. #13
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    Yes, the blades have the same dimensions and same offset (raked blade versus cranked crown). The graph is of vertical displacement ("Displ in Y"). The top of the blade is constrained, the load is applied to the tip in the vertical direction only.

  14. #14
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    What are the units you're using here?

    I've ridden a lot of both curved and not and could never tell the difference, but that's just me (also I never ride road bikes so I'm always on big fat cushy tires, maybe it's easier to tell on 21c clinchers at 150psi).

    -Walt
    Waltworks Custom Bicycles
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  15. #15
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    They are "units", meant for comparison. Same load on each model, which I don't remember. Like I said upthread, I think fat tires are more effective. I just did it to resolve RPOTI* arguments I had heard over and over.

    *Random People On The Internet, of which I am technically one but I have rainbow graphs so there.

  16. #16
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    Brake choice for a rigid fork.

    Dude, you have DOUBLE rainbow graphs!

    But seriously thanks for posting all that. A cool lesson for me. I usually look down while I'm riding and toggle my front brake and go...yep...that flexes nicely! Or not. Right before I almost run into a tree.

    To the OP, just go with small diameter and/or tapered blades. Raked and a Pacenti crown if you're a real retro grouch. But either way definitely go disc, if for no other reason that you have better rim & wheel choices.
    Last edited by Meriwether; 04-13-2013 at 12:18 AM.

  17. #17
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    Quote Originally Posted by Meriwether View Post
    I usually look down while I'm riding and toggle my front brake and go...yep...that flexes nicely!
    Yeah, though in that case the force is in a different direction than what I used in the FEA. One problem about when we talk about fork flexibility and what we observe is that we're usually talking about a bunch of different situations.

    Interestingly enough, because the center of gravity of a cyclist is pretty close to the wheelbase, because of weight transfer the force on the fork tip is about 45 degrees from the vertical, which is pretty close to straight up the fork tip. So it seems like a raked fork tip is well designed to flex to vertical loads from surface irregularities but not react as much to brake forces.

    Quote Originally Posted by Meriwether View Post
    To the OP, just go with small diameter and/or tapered blades. Raked and a Pacenti crown if you're a real retro grouch. But either way definitely go disc, if for no other reason that you have better rim & wheel choices.
    The only catch is that none of the blades that fit the Pacenti crown will hold up to disk brake use off-road, unless you use a disk mount that extends further up the leg, like the Paragon/Willits design, only you would need to make/modify one to fit the curved leg. Then your legs are two different stiffnesses which at minimum defeats your design goal, and might be noticeable in certain situations.
    Last edited by dr.welby; 04-13-2013 at 01:32 PM.

  18. #18
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    Thanks Welby, I was more joking than comparing our methods. But your point is well taken. I will have to mount a GoPro on the side of the fork while I ride to see the "vertical compliance" of my Pacenti fork.

    Brake choice for a rigid fork.-mc10-newcolor-riverview.jpg

    I've made two raked forks for off-road use and both performed very well. The Pacenti crown one I'm using right now is a bit stiffer (via my very unscientific method) but it's noticeable compared to a Nova crowned fork I just retired. The new fork has has thicker blades (28x20mm OD with 1.2/0.7mm wall). I bent the Willits ISO mount for both pretty easily and I'm sure it does not offer the same flex as the other leg, it's much more flexy a ride than any of the segmented forks I've made or ridden. I'm riding it as a MTB/All-rounder so we'll see how long it holds up. Here's a similar conversation on Flickr I had awhile back.
    The all-rounder | Flickr - Photo Sharing!

  19. #19
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    Awesome! Reduction to practice! Data points! Drinks all around!

  20. #20
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    Nice graphs. As mention above, I think my assertion still fits with your data in that flex and compliance isn't necessarily vertical.

    The thing I wonder about with the multiple directions of forces is that the rakes tip want to flex upwards (curve more) due to a vertical force but also wants to flex backwards due to a horizontal force (curve less). I wonder who wins? I suppose both *could* happen if you consider the whole system of the rest of the blade and the crown/steerer.

  21. #21
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    Quote Originally Posted by Feldybikes View Post
    The thing I wonder about with the multiple directions of forces is that the rakes tip want to flex upwards (curve more) due to a vertical force but also wants to flex backwards due to a horizontal force (curve less). I wonder who wins?
    One of these days (he says!) I'll run simulations at varying angles. I'd like to graph the displacements versus the angle and see if the two fork designs cross curves at some point.


    I suppose both *could* happen if you consider the whole system of the rest of the blade and the crown/steerer.
    I've avoided looking at the crown and steerer since they are common to both models and should react the same. Though it would be fun to see what influence something like steerer stiffness has on fork tip deflection, especially with the tapered steerers that are popular now.

  22. #22
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    I think the other point of including the top half of the fork is that the displacement there might be enough that it trumps any movement at the tips anyway. I.e. if a given impact displaces the wheel by a cm due to the crown flex and a mm due to the tip flex (just making up numbers), then you wouldn't really care much about varying the tips.

    And seriously, Welby, get your priorities straight. Why wouldn't running simulations for random ppl on teh internetz be your first priority?

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  24. #24
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    Nice!

  25. #25
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    Quote Originally Posted by Meriwether View Post

    I've made two raked forks for off-road use and both performed very well.
    The all-rounder | Flickr - Photo Sharing!
    From the looks of your frame you're not a small rider, i'm wondering from my perspective as a 245lb rider what wall thickness(assuming a straight gauge tube is used, if they are even used anymore) you use for the steerer? HJ has a 25.4 2.3/1.6 and a few 28.6, the stoutest a 2.1/1.6. Other than weight, any disavantage using st gauge? Any others want to give their $.02?

    thanks, Brian
    Last edited by calstar; 04-15-2013 at 06:07 PM.

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