Custom fork- wheel ejection risk?
Ben Micklem
I've been following the discussion on wheel ejection for a while.
I have just specified to a frame builder my new frame and fork. This will be
a touring/cyclocross/commuter 700c bike with disc brakes. The frame will be
lugged Reynolds 631 and 725 steel, the fork will have a cast crown and
curved blades made from Tandem-spec tubes. The fork drop-outs will be
straight (i.e. in line with the fork blade at its tip)- so they will be at
an angle of approximately 20 degrees from vertical, facing slightly towards
the front. These are used on tourers to allow easy wheel removal when using
close-fitting mudguards/fenders. The drop-out is designed for road bikes, so
has no lawyer lips.
When I suggested fitting the calliper IS mount on the front of the right
fork, they said it would be very unwise to do this- due to the tensile
forces exerted. I think the mount would be brazed on using sliver, as that
is what the rest of the frame is brazed with- so I guess would not be as
strong as a tig welded mount (as I guess the Cotic Roadrat's fork is?). At a
guess, from the touring forks with disc mounts I saw at their shop, the
calliper would be at around the 2 o'clock to 3 o'clock position. 160mm rotor
and Avid BB7 mechanical road callipers would be used.
Would people be worried about this configuration? Would a silver brazed
mount be strong enough to be mounted on the front of the right fork blade?
They said they have 20 years experience of disc mounts on their custom
forks, and have not heard any problems with ejections.
Ben
Ben C
> Hi,
>
> I've been following the discussion on wheel ejection for a while.
>
> I have just specified to a frame builder my new frame and fork. This will be
> a touring/cyclocross/commuter 700c bike with disc brakes. The frame will be
> lugged Reynolds 631 and 725 steel, the fork will have a cast crown and
> curved blades made from Tandem-spec tubes. The fork drop-outs will be
> straight (i.e. in line with the fork blade at its tip)- so they will be at
> an angle of approximately 20 degrees from vertical, facing slightly towards
> the front. These are used on tourers to allow easy wheel removal when using
> close-fitting mudguards/fenders. The drop-out is designed for road bikes, so
> has no lawyer lips.
>
> When I suggested fitting the calliper IS mount on the front of the right
> fork, they said it would be very unwise to do this- due to the tensile
> forces exerted.
That's interesting, and confirms jim beam's point that that would be a
problem with a front mounted caliper.
> I think the mount would be brazed on using sliver, as that
> is what the rest of the frame is brazed with- so I guess would not be as
> strong as a tig welded mount (as I guess the Cotic Roadrat's fork is?). At a
> guess, from the touring forks with disc mounts I saw at their shop, the
> calliper would be at around the 2 o'clock to 3 o'clock position. 160mm rotor
> and Avid BB7 mechanical road callipers would be used.
With a wheel diameter of 675.14mm (what my 700C wheels and tyres come
to, based on the settings on my bike computer), and with the caliper at
2:30, I make your ejection force direction 27 degrees from vertical
pointing backwards. With your dropout at 20 degrees forwards, that's
a difference of 47 degrees between ejection force angle and possible
ejection path.
Since this angle is just greater than 45 degrees, I don't think the
wheel would eject even if the skewer were undone completely.
The magnitude of ejection force in this system is up to 4.2 times that
of ground reaction force (assuming hard braking with friction
coefficient of 1.0) so there is probably a component in the dropout
direction big enough to lift the bike off the wheel, but there's a
bigger component pressing the axle into the side of the dropout (since
the angle is >45 degrees). I doubt you can pull a wheel out of a dropout
unless the angle between force and exit direction is 45 degrees or less.
It might get to 45 degrees if you hit a rock while braking, but I'd be
very surprised if you'd have ejection problems, rocks or no rocks, with
the skewer done up. If the frame builder is suggesting a front mounted
caliper is a bad idea I'd be inclined to believe him and rate that as a
much higher priority concern.
Don't count on my math or reasoning being right though, hopefully
someone will confirm it.
jim beam
the lack of lawyer lips. i would ask for those. they can weld nubs
onto the dropout tips if necessary.
Mike Causer
> When I suggested fitting the calliper IS mount on the front of the right
> fork, they said it would be very unwise to do this- due to the tensile
> forces exerted. I think the mount would be brazed on using sliver, as that
> is what the rest of the frame is brazed with- so I guess would not be as
> strong as a tig welded mount (as I guess the Cotic Roadrat's fork is?).
A properly done silver-solder braze should be as strong as a TIG weld,
but unless the fork is reinforced at that point I'd be very nervous
about either method. If there were a reinforcement wrapped around the
fork, made from steel strip, it would be a lot better, but shaping that
strip would not be easy because it needs to fit the fork very snugly.
I can't tell from the photos how the Cotic is attached, but properly
welded they should be OK. Those forks are a lot chunkier than yours I
would think, which would make quite a difference in ease (and therefore
quality) of welding.
> At a guess, from the touring forks with disc mounts I saw at their shop,
> the calliper would be at around the 2 o'clock to 3 o'clock position.
Get the caliper absolutely as close as possible to the fork and you are in
the best geometric condition you can achieve. Anything that tips the
drop-out slots further toward the horizontal will also help. And make
sure the QR is tight ;-)
> They said they have 20 years experience of disc mounts on their custom
> forks, and have not heard any problems with ejections.
This could well be because their market is with people who know how to do
up the QRs.
Mike
jobst....@stanfordalumni.org
I'm not familiar with these fork tubes but when you mention fork end
curl, I visualize a conventional steel fork that is tapered toward the
dropouts. The reason such forks are tapered is that there is no
bending moment at the tips so they can be extremely light weight. The
reason road forks have oval tubes oriented, long axis fore and aft at
the fork crown, is that braking torque can be supported safely. Track
bicycles generally use round fork blades, not having brakes.
When a disk caliper (or drum brake) is used, the fork has the same
torque at the dropout that rim brakes cause at the fork crown, only
that it is twice as great, the load being on only one leg.
Therefore, do not use tapered fork blades if you want to avoid fork
failure. You'll notice that bicycles with disk brakes have large tube
forks that have roughly uniform diameter. There are good reasons why
disk brakes are not found on road racing bicycles. They require a
heavier fork and weigh more than caliper brakes.
Jobst Brandt
Michael Press
Ben Micklem <benmi...@hotmail.com> wrote:
Tell them to make the joint strong enough to carry the
load; and not to overheat the fork blade. They are
professional builders.
You may never have a problem. Or your new rear mounted
caliper will have you retightening the quick release
every day.
--
Michael Press
Ben C
>
> I've been following the discussion on wheel ejection for a while.
>
> I have just specified to a frame builder my new frame and fork. This will be
> a touring/cyclocross/commuter 700c bike with disc brakes. The frame will be
> lugged Reynolds 631 and 725 steel, the fork will have a cast crown and
> curved blades made from Tandem-spec tubes. The fork drop-outs will be
> straight (i.e. in line with the fork blade at its tip)- so they will be at
> an angle of approximately 20 degrees from vertical, facing slightly towards
> the front. These are used on tourers to allow easy wheel removal when using
> close-fitting mudguards/fenders. The drop-out is designed for road bikes, so
> has no lawyer lips.
>
> When I suggested fitting the calliper IS mount on the front of the right
> fork, they said it would be very unwise to do this- due to the tensile
> forces exerted. I think the mount would be brazed on using sliver, as that
> is what the rest of the frame is brazed with- so I guess would not be as
> strong as a tig welded mount (as I guess the Cotic Roadrat's fork is?). At a
> guess, from the touring forks with disc mounts I saw at their shop, the
> calliper would be at around the 2 o'clock to 3 o'clock position. 160mm rotor
> and Avid BB7 mechanical road callipers would be used.
A revision to my calculations, described in a post I just made in the
original thread. This time I get a 44 degree difference in angle between
ejection force and dropout exit direction (it was 47). Still close to 45
degrees and pretty safe I'd say (but be careful, disclaimer etc...)
jobst....@stanfordalumni.org
http://www.ne.jp/asahi/julesandjames/home/fork/
When I saw this picture, I was surprised that the fork did not fail at
the upper attachment point of the brake caliper but then attachment to
the fork crown is apparently an inside lug, the kind that leads to
fork crown separation anyway. Note that only one blade failed. That
is because all the braking torque + bending force of retardation work
on that side.
Jobst Brandt
almos...@yahoo.com
>
> - Show quoted text -
James' fork lookes to have a unicrown to me.
jobst....@stanfordalumni.org
http://www.ne.jp/asahi/julesandjames/home/fork/
>> When I saw this picture, I was surprised that the fork did not fail
>> at the upper attachment point of the brake caliper but then
>> attachment to the fork crown is apparently an inside lug, the kind
>> that leads to fork crown separation anyway. Note that only one
>> blade failed. That is because all the braking torque + bending
>> force of retardation work on that side.
> James' fork looks to have a unicrown to me.
You are correct. I didn't look closely. What struck me is the
spreading of the right fork blade and the failure of the left one at
the crown. The sequence of events is unclear but I suppose the report
is reasonable. Paint on the brake side dropout does not show any
indentation from QR force. The asymmetric dimples from the QR nut on
the right dropout are asymmetric because they were not made by
tightening but by the wheel being skewed as it was forced from the
dropout.
In any event, I'll retract that above because when looking at it
carefully it seems the fork was broken by the empty fork striking the
road with that side. That blow was the one that closed th open end of
the dropout.
I would like to have seen the bicycle right after the incident. It
has been my experience that reconstruction of what occurred is often
easier than first indications. That has been so, in every case in
which I was called to testify. That is to say, the event did not
occur as plaintiff described. However, I am fairly sure that in this
one, the QR was not properly tight, but that does not change the cause
of separation, which is ultimately the disk brake position and dropout
slot orientation.
As I mentioned in another thread, hub disk or drum brakes do not
belong on tapered fork blades.
Jobst Brandt