will a disc brake rip the QR from front fork and kill me?

scottgeniusltd2005

2alexcoo wrote:

Horton wrote:

keep the disc mount on the rear - there's a reason every other fork manafacturer keeps it there...

Except Cotic with their Roadhog fork...

isn't this a straight fork? i.e., the v brakes pins are removed and put to the back so then the disk mount is at the back? that way the axle slots would face forward as per tradition?

scottgeniusltd2005

but then again it has mudguard eyelets so maybe not. i wouldn't use it, just looks wrong

eh

Here is a quote from Cy who designed the Cotic forks, and it is one that the original poster needs to seriously consider, especially as his material choice is more susceptible to fatigue than steel (italics are my addition):

It is a different fatigue environment (than the traditonal rear of fork mounting) , but in terms of the mount it's simply a matter of understanding that and designing appropriately.

andrew_s

I reckon the only problem with a front right disc mount is that it means you can't use generator hubs for lighting (if you care). The centre section of the hub unscrews to get the electrical gubbins in, and mounted the wrong way round will tend to come undone.
The default rear left siting is probably just to keep things further out of the way of possible impact damage.

The disc mount being in tension doesn't matter. There are bolts and welds in tension all over the place in much higher stress applications than a bicycle disc brake. It may not be a good idea to use lightweight aluminium bolts, but that about all.

A disc brake will pull the wheel out - I've seen it happen, fortunately with no damage beyond bent forks. That was the result of no lawyer lips and using a lightweight QR rather than a proper Shimano one.

tarquin_foxglove

Wasn't there a recent review of a road bike with disc brakes on this site?

I seem to remember it was quite expensive & it claimed it had got round the fork deflection by having 2 small discs, rather than 1 normal sized one.

I had thought it was this one http://www.factor001.com/ but it isn't.

tarquin_foxglove

TimB34 wrote:

I was reminded of this : http://www.canyon.com/_en/technology/project68.html

Where they have an interesting discussion of braking forces applicable for more traditional road forks - including the amount of deformation (you really don't want to be able to make the front wheel touch the frame...) and the amount of twist that having one brake creates (you really don't want to turn left every time you hit the brakes!)

:oops: I knew I should have read the thread properly before posting.

AidanR

I'm less than convinced about this dual rotor front brake business. Their rationale appears to be "we wanted to keep the front fork flexy for comfort" therefore we'll come up with a complicated and heavy design to get around the problem. And as for 4x lacing... really? For a 32h hub? If mountain bikes don't need it, I can't imagine why road bikes would. And as for more even spoke loading, I can see their point but it's not like a hub can't transmit torque from one side of it to another.

Disc brakes haven't seen the light of day on road race bikes because they aren't allowed by the UCI and they won't provide racers with any real benefit. That's not to say more recreational riders might not benefit from disc brakes, but the majority of road bike technology trickles down from racing.

As far as a titanium disc 'cross fork goes, I'm sure it'll be fine as long as it's beefy enough. There are carbon alternatives but they're expensive and difficult to source here in the UK (Winwood Muddy X springs to mind). As for the QR ripping from the fork, if you're worried about it put lawyer tabs on.

motomd

Okay, so I am a new member and just found this thread by searching google, hence the 1 post. But, I think I have a useful input. I made some drawings to illustrate my point (which is in agreement with the assessment that front mounting the caliper is best for keeping the axle in the dropout.)

So, here we go with the rear mount disc. This is the setup.

Rear Mount Disc


Imagine the ground is moving beneath the wheel, as if it is on a treadmill. Now, in an instant, imagine we remove the nuts holding the axle on, so that it is NOT CONNECTED at all at the red dot. At the same time we APPLY the brakes so hard that we LOCK them up. This creates a pivot point around which the wheel will now rotate (rotate around the blue dot). In this setup, the wheel will pop out of the dropout, no question. The ground will pull the wheel underneath, popping it out.

Now, reset the whole situation, and instead of just removing the nuts on the axle, let's just barely tighten them on; then lock up the brakes. What happens? Well, the wheel pops out still.



Now, front disc mount.

Front Mount Disc


Do the same thing as above, remove the nuts, jam on the brakes..what happens? The axle will be forced in. Question answered.



Now why would the bike manufacturers put it on the back then??? Well, my guess is that they copied from a sector that was already using disc brakes - motorcycles. Motorcycles have them on the back and so that should work for bikes right? Well, first of all, motorcycles don't have dropouts, so there is no risk of pulling an axle out. Also, motorcycles probably just chose to put them in the back for aesthetics or in the case of off-road motorcycles, protection. So, there ya go, it was merely conventional (and still is), and the convention should change if they want to be sensible about it.

Of course, if you have a high enough clamping force then it doesn't matter either way, but the real concern is if something starts getting loose.

ride_whenever

AidanR wrote:

As far as a titanium disc 'cross fork goes, I'm sure it'll be fine as long as it's beefy enough. There are carbon alternatives but they're expensive and difficult to source here in the UK (Winwood Muddy X springs to mind). As for the QR ripping from the fork, if you're worried about it put lawyer tabs on.

TBH i'd just go for one of these they're rebadged as most of the straight legged carbon forks you see and are more than strong enough to take discs and a qr...

balthazar

@motomd: nice diagrams. They should make clear to those who cannot visualise it, that this is a very real problem.

There is a case of dropout clamping failure here:

http://www.ne.jp/asahi/julesandjames/home/fork/

with details here:

http://www.ne.jp/asahi/julesandjames/ho ... ml#support

and a good general analysis in this archived discussion:

http://yarchive.net/bike/disk_brake_qr.html

for those who are interested in yet another example of woeful product design in bicycles.

AidanR

ride_whenever wrote:

AidanR wrote:

As far as a titanium disc 'cross fork goes, I'm sure it'll be fine as long as it's beefy enough. There are carbon alternatives but they're expensive and difficult to source here in the UK (Winwood Muddy X springs to mind). As for the QR ripping from the fork, if you're worried about it put lawyer tabs on.

TBH i'd just go for one of these they're rebadged as most of the straight legged carbon forks you see and are more than strong enough to take discs and a qr...

I wouldn't - it'd f*ck up the geometry no end. You'd be better off with one of these at 415mm axle to crown: