Disc brakes on curved fork blades

[Beardman Bicycles]

I have a customer looking to do a curved bladed fork with disc brakes - any advice / thoughts? Doable?

I should mention its a touring frame - I would imagine a straight blade work wouldn't be a great ride -

Thanks everyone.

[M-gineering]

my take on this is that by the time you've beefed up the forkblades so
that the caliper won't straighten the bend and the skewer won't unscrew,
there is little to choose between curved and straight blades. Get a
decent high volume tyre (compass etc) and all will be good. Here if they
want disks, the customer gets a unicrown fork with a thinwall 28.6
steerer (and a beefier downtube). No complaints so far

--
mvg

Marten Gerritsen
Kiel Windeweer
Netherlands

[Andrew R Stewart]

Many might say that there is no difference between a straight and a curved
blade WRT flex. Certainly some very well respected builders and
manufacturers feel this is true. But part of getting a custom frame is about
the look so if the customer wants a curved blade that is what the builder
should make, providing the fork meets all the other requirements that both
the customer and builder place on a fork.

I wonder if the customer understands that the bracing that usually goes with
a fork disk mount will significantly change/reduce that blade's flex? At
some point one of the jobs of the builder is too translate in understandable
terms what the frame/fork will feel like, then suggest designs to achieve
the intended handling goals. It's always a slippery slope when the customer
controls the design. Andy.



Andrew R Stewart
Rochester, NY USA

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[Alex Wetmore]

I don't think that there has been good enough published testing to say what is actually safe in the long term for lugged crown steel disk brake forks.

For building one today I'd use the Nova disk brake fork blades (which are large diameter and thick and heavy, so they aren't going to provide any compliance) with a stout fork crown and a tab that extends well up the blade. I'd still curve them because it looks nice and lets you fine tune the rake, but it isn't something that you can do for ride quality on a disk brake fork unless you build a very unconventional design.

I'd like to see some real testing and research into a safe reference design and have been talking with some friends about this recently.

alex

________________________________________
From: frameb...@googlegroups.com <frameb...@googlegroups.com> on behalf of Beardman Bicycles <beardman...@gmail.com>
Sent: Wednesday, May 18, 2016 10:21:45 AM

To: Framebuilders
Subject: [Frame] Disc brakes on curved fork blades

I have a customer looking to do a curved bladed fork with disc brakes - any advice / thoughts? Doable?

I should mention its a touring frame - I would imagine a straight blade work wouldn't be a great ride -

Thanks everyone.

[Guy Washburn]

http://www.velocipedesalon.com/forum/f15/friday-picfest-no-354-a-43255.html

Scroll down to post #3.

Joel told me in a PM that he built this fork with a pretty standard blade and a Paragon Wilts mount bent to his will with a form and a hammer...

It is high on my list of things to try...

[Alex Wetmore]

I've been riding this fork for a few years (it has thousands but not 10s of thousands of miles on it):

http://photos.alexwetmore.org/Bicycles/Framebuilding/Porteur-Disk-Fork/i-r76XGVF/A

But it is my bike, not a customers (I don't have any customers since I'm an amateur builder).  I'm also aware of failures on forks that have built the same way as my fork or Joel's fork.  My fork could last 20 years of every day use without breaking, but the percentage chance of failure is high enough that I'd be concerned about selling it.

The biggest concern is having the tip of the disk mount act as a can opener and crack the fork blade at the top of the mount.  I tried to minimize that by feathering the tip of the mount into the blade so that there isn't a stress riser there.  Keeping the tab off center may also help with that.  Thicker and larger diameter fork blades (I used the thickest ones that I had on hand) also help.  I got the diameter to be larger by trimming these fork blades from the bottom and using a sloping crown to keep the blade length shorter.

alex

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From: frameb...@googlegroups.com <frameb...@googlegroups.com> on behalf of Guy Washburn <guido...@gmail.com>
Sent: Wednesday, May 18, 2016 2:54:12 PM
To: Framebuilders
Subject: [Frame] Re: Disc brakes on curved fork blades

 

[Andrew R Stewart]

Alex- A VERY good post you shared! My take away has nothing to do with disk brakes and forks but everything to do with professional responsibility. The moment that one builds for another (and exchange of $ is not the issue) the builder’s choices can impact many people. Note that Alex mentions he builds only for his self.  Andy.

 

Andrew R Stewart
Rochester, NY USA

[Alex Meade]

In addition to the very good posts regarding reliability and responsibility, it's worth noting that fitting a disk mount to a curved fork blade is a real time-consuming PITA.  

Alex Meade

http://www.alexmeade.com

[Alex Wetmore]

Alex: That is a place where you (as a fork bender die maker) could put together a nice system.  It's pretty easy if the same person designs the disk tab and the fork bender and makes sure that they have compatible curves.  

I have a small CNC machine (and access to a much larger one) and cut my own disk brake tabs that pretty closely match my fork bend.  Fitment still takes a little bit of work.  If I were doing this on a production level I'd spend a lot of time refining the cut of the disk tab to very closely match my normal fork blades.

alex

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From: Alex Meade <alexd...@gmail.com>
Sent: Thursday, May 19, 2016 5:14:52 AM
To: Framebuilders
Cc: Alex Wetmore; guido...@gmail.com

[Eric Keller]

A builder's best bet in this case is to talk the customer out of their
magical thinking.

I'm convinced that curved blades have somewhat more compliance than
straight blades, but I'm not convinced that you can't make up for it
with tires. It makes sense to worry about compliance on a rando bike,
but I'm not sure it makes sense to worry about it on a touring bike.
I've seen too many examples of de-raked and otherwies failed disc
forks to make me want to take chances. The Nova disc blade or some
other disc specific blade just makes too much sense.

[Andrew R Stewart]

As is building a frame as opposed to just whipping out one’s credit card and paying some one else to do the dirty work. But like the tag line of a credit card ad “building your own frame then riding with your buddies on it, priceless!” Andy.

 

Andrew R Stewart
Rochester, NY USA

 

From: Alex Meade

Sent: Thursday, May 19, 2016 8:14 AM

To: Framebuilders

Cc: al...@phred.org ; guido...@gmail.com

Subject: Re: [Frame] Re: Disc brakes on curved fork blades