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.
On 2007-02-12, Ben Micklem <benmicklem@hotmail.com> wrote:
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.
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.
Mike Causer 12 February 2007 18:13:05 [ permanent link ]
On Mon, 12 Feb 2007 12:22:04 +0000, Ben Micklem wrote:
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.
Jobst Brandt 12 February 2007 23:56:42 [ permanent link ]
Ben Micklem writes:
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.
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.
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