Disc brake mount on an older fork?

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Steve B

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May 16, 2011, 3:35:07 PM5/16/11
to Framebuilders
The shop I work for as a mechanic asked me yesterday about a project.
They have a customer who wants to add disc brakes to what's probably a
lower-mid range mid 70's Raleigh fork. They didn't mention the frame.
Brazing on a disc mount is something I'd feel comfortable doing, but
before I say I'll do it I wanted to get an idea if it's wise on an old
fork.

I think the likely mild steel won't be a problem, it's similar to
stuff I've worked with before. I would probably decline on 531 for a
few reasons, and would certainly decline on anything fancier.
My concern is that the fork blades near the ends wouldn't be strong
enough to handle having a disc mount there. I'm mostly looking for one
of three cartegories, 1 )It's done regularly go for it 2)It can be
done, but it's a job for experienced builders only. or 3 )Don't do it,
just don't.

Based on the advice, I'm totally ok with either passing entirely,
refering, or telling him to either have a new fork made or forget it.

Steve Birmingham
Lowell, Ma

Pete Ruckelshaus

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May 16, 2011, 3:45:53 PM5/16/11
to Steve B, Framebuilders
My vote is 3.  70's forks just weren't designed for those kinds of stresses.

Brent Steelman

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May 16, 2011, 3:57:50 PM5/16/11
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Hi Steve

I would not do it. I have had curved fork blades straighten from the
leverage generated by disc braking. I have even had it happen on blades
that were 1.6mm thick at the dropout end. Bigger diameter straight blade
forks seem to hold up just fine.

best,
Brent

Steelman Cycles
3600 Haven Ave #5
Redwood City, CA 94063
tel:650-364-3939
fax:650-364-4029
in...@steelmancycles.com
www.steelmancycles.com

Alex Wetmore

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May 16, 2011, 4:04:10 PM5/16/11
to Pete Ruckelshaus, Steve B, Framebuilders
I wouldn't do it even on modern blades that have the same curve and profile.  I'm building a porteur frame sometime soon that will have disk brakes (because I got a screaming deal on an Alfine wheelset and disk calipers) and have been trying to guess what the lightest disk-safe fork blades that are available right now.  All of the offerings today designed for fork crowns appear to have 12.5-13mm ends that would be very flexible for disk brake use.

What fork blades do other builders use for this purpose?  For instance are the heavier blades that Rawlands used on the dSogn fork (I think they were 16mm at the ends) available through one of the suppliers?  I've checked Nova, Henry James, Bringheli, and Ceeway with no luck.

When Reynolds lists the EB475 fork blade as "rake 15x45mm" does that mean the dropout end of the blade is 15mm in diameter and the blade is pre-raked to 45mm, or does it mean something else?  I can't find drawings of the Reynolds blades.

alex
 

From: frameb...@googlegroups.com [frameb...@googlegroups.com] on behalf of Pete Ruckelshaus [prucke...@gmail.com]
Sent: Monday, May 16, 2011 12:45 PM
To: Steve B
Cc: Framebuilders
Subject: Re: [Frame] Disc brake mount on an older fork?

Mark Bulgier

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May 16, 2011, 4:05:31 PM5/16/11
to Pete Ruckelshaus, Steve B, Framebuilders

I’m curious, how are 21st-century steel forks made any better in that regard than 1970s forks?

 

The braking forces at the top of the blades, crown and steerer should be similar between rim brakes and discs, right?  The limiting factor being tire traction with the ground and I don’t think that’s changed much since the ‘70s.

 

The difference with discs is the localized application of force down where the diameter is smaller.  But modern forks are still made that way, no improvement that I can see.

 

The main difference between cheap fork blades and expensive ones is the expensive ones are made thinner/lighter down at the bottom by butting before tapering, for light weight and for shock attenuation.  Since this old cheap bike is likely to have cheap unbutted blades (which thicken up considerably at the bottom due to the way they are tapered), I would think this fork is more likely to work with discs than a modern fork made with quality blades.

 

Mark Bulgier

 

From: Pete Ruckelshaus [mailto:prucke...@gmail.com]
Sent: Monday, May 16, 2011 12:46 PM
To: Steve B
Cc: Framebuilders
Subject: Re: [Frame] Disc brake mount on an older fork?

 

My vote is 3.  70's forks just weren't designed for those kinds of stresses.

M-gineering

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May 16, 2011, 4:15:40 PM5/16/11
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#3, and before you think of replacing the fork, the downtube might not
be up to it either

mvg

Marten Gerritsen
Kiel Windeweer
Netherlands

Mark Bulgier

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May 16, 2011, 4:20:08 PM5/16/11
to Pete Ruckelshaus, Steve B, Framebuilders
Pete Ruckelshaus recommended against putting a disc on the old Raleigh fork and explained, "70's forks just weren't designed for those kinds of stresses."

I quibbled with that and said "I would think this fork is more likely to work with discs than a modern fork made with quality blades."

That was bad judgment on my part to post that, because it sounds like I'm endorsing discs on old raked blades. I'm not. I've seen disc brakes unrake the blade, even once on a heavy tandem blade. What I meant to say is the fact that fork is old is not a good reason to not put discs on it. But there are other good reasons not to.

I'm sorry I screwed up with that previous email, didn't think before sending.

Mark Bulgier

M-gineering

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May 16, 2011, 4:48:24 PM5/16/11
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On 5/16/2011 10:04 PM, Alex Wetmore wrote:

>
> When Reynolds lists the EB475 fork blade as "rake 15x45mm" does that
> mean the dropout end of the blade is 15mm in diameter and the blade is
> pre-raked to 45mm, or does it mean something else? I can't find drawings
> of the Reynolds blades.
>

I've got a drawing and the 15 is still a mistery ;)

Tip is 14, and i wouldn't be happy with a diskbrake mount on these blades.

They might not break but the one with the caliper will flex enough in
respect to the other blade that the skewer will unwind


--

Brent Steelman

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May 16, 2011, 4:47:17 PM5/16/11
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Hi Mark

There is much more force at the hub with a disc caliper. I build a lot of forks, both disc and rim brake. So far even 1.4 to 1.6 (at the dropout) thickness unwinds when the brake is applied under touring loads. It may work for city riding however.


Brent
Steelman Cycles
3600 Haven Ave #5
Redwood City, CA 94063
tel:650-364-3939
fax:650-364-4029
in...@steelmancycles.com
www.steelmancycles.com

Andy Hillier

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May 16, 2011, 4:51:43 PM5/16/11
to Frameb...@googlegroups.com, in...@m-gineering.nl
"the downtube might not be up to it either"

Could you go into more detail here? I understand how the forces play in on the frame and fork itself; however, for what reason would a 70s or 80s downtube not be able to handle those forces?

Andrew

dave bohm

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May 16, 2011, 4:55:07 PM5/16/11
to Alex Wetmore, Pete Ruckelshaus, Steve B, Framebuilders

Hi Alex and Mark,

 

Indeed blades can unrake themselves with the forces of disk brakes.   I had it happen to me.   A fork came back with a significant amount of rake undone on the disk side.    I have been wondering if I am the only one to experience this and my thought is that I couldn’t be.

 

So far, I fixed this problem by using the “dagger” or the long disk brake mount designed by Wes Willits and sold by Paragon.  I bend the piece to match the bent rake of the fork blade and it distributes the forces over a larger area and seems to solve the problem.  It is a PITA to do and I am kind of anti disk brake on rigids until we get a fork blades that are engineered for this purpose.   Add in the extra weight, setup issues and such and I don’t know what problem we are solving.   Although Alex, Alfine is very cool and the brakes are awesome.

 

Dave

 



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M-gineering

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May 16, 2011, 5:04:17 PM5/16/11
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On 5/16/2011 10:51 PM, Andy Hillier wrote:
> "the downtube might not be up to it either"
>
> Could you go into more detail here? I understand how the forces play in
> on the frame and fork itself; however, for what reason would a 70s or
> 80s downtube not be able to handle those forces?

The frontbrake puts the downtube under compression, a shitty 70's brake
will be much softer on the downtube than a much more effective disk.

Apart from the fact that the average American has gained a few pounds in
40 years ;)

Alex Wetmore

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May 16, 2011, 5:07:34 PM5/16/11
to Mark Bulgier, Pete Ruckelshaus, Steve B, Framebuilders
There are few examples of raked forks for disk applications, but the ones that I've owned have much larger diameter blades.  In my office I have a Novara Fusion commuter bike with a steel unicrown fork, and the tips of the blades are about 20mm in diameter.  It's one of the ugliest forks that I've ridden (the taper is just hideous, with noticeable ugly changes in shape) and not one that I'm looking to copy.

The Rawlands dSogn used blades that appear to be 16mm at the tip:
http://farm4.static.flickr.com/3245/3141696261_fed8c78f63.jpg

Normally I'd avoid this whole thing by using centerpulls or cantis, but I bought a bike (the Novara above) for a killer price with good components and a lousy frameset.  I'm trying to take a bit of a challenge and build around the parts that I have, instead of the ones that I'm comfortable building around.  Disk brakes also wouldn't be a bad thing for the occasional snow commute that we have here in Seattle.

Dave Bohm wrote:
"
So far, I fixed this problem by using the “dagger” or the long disk brake mount designed by Wes Willits and sold by Paragon."

Good to know about, I was wondering if such a thing would be a good idea or a bad one.

thanks,
alex
 

From: frameb...@googlegroups.com [frameb...@googlegroups.com] on behalf of Mark Bulgier [Ma...@bulgier.net]
Sent: Monday, May 16, 2011 1:05 PM
To: Pete Ruckelshaus; Steve B
Cc: Framebuilders
Subject: RE: [Frame] Disc brake mount on an older fork?

Steve B

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May 16, 2011, 5:16:42 PM5/16/11
to Framebuilders
Thanks everyone.

I'll definetly pass on doing it as it sounds like a bad idea all
around. I hadn't thought of unraking, but I was concerned about that
much braking force out on the thin ends. Maybe the several decades of
experience can convince him it's a bad idea. If it gets done, at least
it won't be me doing something silly.

We carry Redline, and the Conquest Classic has fairly traditional
blades with discs. Gentle rake, and a bit more diameter. They feel ok
riding, but it's a 30 Lb bike that's always felt a bit front heavy.
They must be using some heavy blades to make it durable, which would
explain the heavy front.

Steve Birmingham
Lowell, Ma.

Michael Catano

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May 16, 2011, 5:17:09 PM5/16/11
to Alex Wetmore, Mark Bulgier, Pete Ruckelshaus, Steve B, Framebuilders
I'd go with a straight-blade, segmented-style fork out of 4130 with a
wall thickness sufficient to manage the job. Ceeway also offers the
LC12 crown - it's a faux-unicrown designed to work with 28.6mm round
tubes and a 28.6mm steerer. 29er unicrown blades (ie, fork blades
designed to handle the task at hand) are also readily available.

As you said, you're "designing around the components" - traditional
fork materials seem very much a square peg to the round hole of your
braking system.

best
m

Colin Bryant

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May 16, 2011, 7:55:28 PM5/16/11
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I've just upgraded two recumbent bikes to disk front brake, but they both had short (451 wheel) raked unicrown forks with a 2.5mm wall, at the dropout.  I managed to measure the wall thickness, at the drain/vent hole.  I wouldn't feel comfortable doing it on a thinner wall or longer fork.
 
--
Colin Bryant
Vancouver, Canada



From: Steve B <sbirm...@mindspring.com>
To: Framebuilders <frameb...@googlegroups.com>
Sent: Mon, May 16, 2011 12:35:07 PM
Subject: [Frame] Disc brake mount on an older fork?

jon norstog

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May 16, 2011, 8:06:35 PM5/16/11
to Steve B, Framebuilders
Steve, list:

I've put tabs on MTB and BMX forks. I wouldn't touch this job with a
10-foot pole.

jn

"Thursday"

jim g

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May 16, 2011, 8:36:17 PM5/16/11
to frameb...@googlegroups.com
Read the comments on this photo. I'm jumping the gun on Marc, apologies!

http://www.flickr.com/photos/drwelby/5632096652/in/photostream

-Jim G

--
jimg at yojimg dot net

Alex Wetmore

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May 16, 2011, 9:13:43 PM5/16/11
to jim g, frameb...@googlegroups.com
From: jim g [ji...@yojimg.net]
> http://www.flickr.com/photos/drwelby/5632096652/in/photostream

The FB2 blades that he used are still 12.5mm at the tips.

I think I'll use a sloping crown (like the LC22), trim the legs from the bottom (a good idea from that thread), and use the thickest blades that I can find. I'm just surprised that I can't find beefier legs, since most production forks use them (for instance a Soma fork that I have lying around is 15mm at the dropouts). The tops of unicrown blades with a 28x20 profile (Nova and Henry James) seem like maybe they could be cut down and used with a crown.

I haven't heard of the Rawlands forks (which are raked and disk forks) having problems and believe that they are using blades that are 14mm or 16mm at the tips, but I can measure a friend's fork to be sure.

Someone else suggested using a segmented fork, but I'm not a big fan of the design or convinced that they are as strong as a good crown. I could also just build a unicrown fork, and may do that in the end.

alex

Alistair Spence

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May 16, 2011, 10:10:55 PM5/16/11
to Alex Wetmore, jim g, frameb...@googlegroups.com
On Mon, May 16, 2011 at 6:13 PM, Alex Wetmore <al...@phred.org> wrote:

> Someone else suggested using a segmented fork, but I'm not a big fan of the design or convinced that they are as strong as a good crown.  I could also just build a unicrown fork, and may do that in the end.
>
> alex


Alex,

I'm thinking that a segmented fork is actually quite ideal for a disk
application. It lets you conveniently put material where it is needed
by selecting tubing wall thicknesses, something that you don't really
get to choose when using a cast crown. Similarly, you can tune the
blade wall thickness and diameter too, more so than you can with
pre-made blades. This all makes sense to me, considering the location
and the magnitude of the loads that a disc caliper puts on a fork
blade.

Using structural elements that have evolved over time to work with rim
brakes, and trying to make them work with disks, just seems all wrong
to me (I'll qualify that by saying that I've got about 10 minutes
total, ever, of riding a bike with disk brakes and I've never built
with them so I'm clearly no expert).

I can understand not liking the aesthetics of segmented forks. I like
the functional look myself. They look like they're all business, and I
prefer the aesthetic to a unicrown.

They seem to be quite well proven in the MTB world so it would seem
that the design is pretty well refined and understood.

It seems to me the tricky thing would be the mitering and making sure
that both side of the fork were symmetrical, but you have a mill so
that wouldn't be a problem.

Anyway, that's my two cents.

Alistair.

ji...@yojimg.net

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May 16, 2011, 10:16:14 PM5/16/11
to Alistair Spence, Alex Wetmore, frameb...@googlegroups.com
Alistair,

Couldn't you achieve the same results with a twin or triple plate fork
crown?

-Jim G


On May 16, 2011, at 7:10 PM, Alistair Spence <alspe...@gmail.com>
wrote:

Alistair Spence

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May 16, 2011, 10:25:02 PM5/16/11
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Jim,

ultimately, yes.
It would have to be heavier though. A plate crown isn't using the
material as efficiently (a lot of material near the neutral bending
axis that isn't really contributing much to the fore/aft stiffness of
the crown).

Alistair.

Marc Pfister

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May 18, 2011, 12:08:26 PM5/18/11
to Framebuilders
On May 16, 5:36 pm, jim g <j...@yojimg.net> wrote:
> Read the comments on this photo.  I'm jumping the gun on Marc, apologies!
>
> http://www.flickr.com/photos/drwelby/5632096652/in/photostream

I'm not subscribed to the list but I saw all the inbound traffic to my
Flickr page so I thought I'd jump in.

The FB-2 legs have a classic taper, they're about .7 wall at the tips.
For a 29er, I have to use almost the full length so the diameter of
the tube just above the disc brake is about 13.5 mm.

On my other fork, the FB-4 tubes seem like the tapered section is
shorter, so they get larger in diameter faster. Or since that fork is
sleeved, I trimmed it to length at the bottom so I was further up in
the taper. In this case the outside diameter of the leg just above the
brake mount is around 17mm.

Doing a quick calculation, the area moment of inertia for my FB-4 fork
versus my FB-2 is about 2.5 times greater. I probably should have done
the math before I built that fork...

I'm pretty sure I've seen other disc brake forks built with the FB-4,
Waltworks comes to mind. Mine hasn't given me any problems.

Using a long disc brake mount like the Paragon/Willits mount or adding
an extension is probably a good idea. I'd also look at what Jeff Jones
is using on his small-diameter strut forks. I'll try adding something
to my fork to see if I can get the legs to work, if not it will get
canti bosses. But in the meantime I'm using a segmented fork I built
from 853 seat tubes.

Marc



Waltworks

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May 21, 2011, 3:43:06 PM5/21/11
to Framebuilders
The FB-4 works great for disc brake forks. I have probably built 250
of them with that blade in the last 6 or 7 years, and have never run
into a problem, even for fairly big and aggressive riders on mountain
bikes.

As an aside, even ignoring the dropout orientation and blade strength
issues, most old road forks do not have space for the actual disc (the
blades angle in to the crown too sharply and usually interfere) so the
OP's question is probably moot. I do a lot of disc-tab addition jobs
for various types of bikes and the first question I always ask is
"Have you put a disc wheel in the dropouts and checked the
clearance?"

-Walt
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