Author Topic: Vector diagram for disc/rim front brake fork loads.  (Read 995 times)

LittleWheelsandBig

  • Whimsy Rider
Re: Vector diagram for disc/rim front brake fork loads.
« Reply #25 on: September 30, 2020, 10:25:45 am »
Through axles came about for a number of reasons. Better security without lawyer lips and repeatable alignment for disc brakes when swapping wheels. Stiffer, lighter axles and fork blades that stay in alignment better under braking (torsional stiffness). Much less damage to carbon fibre contact points than QRs.
Wheel meet again, don't know where, don't know when...

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #26 on: September 30, 2020, 10:36:50 am »
Although mine are suspension forks, and the caliper mounting flange is cast into the fork, the bolt direction is this:-
http://www.bobjacksoncycles.co.uk/product/reynolds-853-disc-brake-front-fork/

I agree that alignment is a fiddle. Mine has a selection of washers that pack out around 1 mm between the caliper and the flange.

Mine are quite old, and I think that many have moved on to flat mount.

However, on flat mount, the bolts should still be strong enough to take the braking force. Bolts are supposed to be tightened to the point where there is always tension in the bolt, to reduce the likelihood of the bolts coming loose and fatigue failures.

It's not a big deal. The worst grade of M5 steel bolts is good for about 1/2 tonne in tension (https://www.engineeringtoolbox.com/metric-bolts-minimum-ultimate-tensile-proof-loads-d_2026.html) so a couple of those will hold the caliper on fine.

Actually, I think that the flat mount calipers can still put the bolts in tension when braking is applied. The mounting face of the caliper is a couple of cm to the left of the disk, so there is a torque trying to twist the caliper around a vertical axis. That is being resisted by the quite narrow mounting face of the caliper, which will apply more pressure nearer the disk and less pressure further away from the disk. The tension in the bolts will stop it turning in that direction.

I don't think it's a problem. Larger bosses could easily be used if it was an issue. But I also don't believe that it would be a problem to put the caliper in front of the disk, where the twisting forces would be exactly the same, so the strain on the bosses would be very similar to what they are now. I think that the calipers are behind the forks to keep them out of the way.
Those are ISO mounts (which are what I'm running on my trike thanks to the adapters). ISO mounts got superseded by post mounts, and in turn they got replaced by flat mount.
I don't think any of this is a problem with the hardware external to the fork, I think it's a problem related to the manufacture of the fork. If you have a metal fork, you can weld a bolt or a plate onto the back or the front of the fork, and provided the welding is OK and the heat treatment process is OK, then it will be pretty sturdy. If you're trying to create a nice light carbon fork, then sticking a plate onto the back, or putting bolts inside the fork with enough structural strength to withstand an emergency stop trying to remove them is far more tricky.  Your argument about putting them out of the way (and presumably having less crash damage potential is also valid - they put the rear brake calipers on the top of the chainstay for similar reasons (and while that has pulling type forces, they are much smaller).

In terms of a pulling force on the bolts, for post mount on forks I don't think there is any. You can basically run them with the bolts done up finger tight and it all works fine (DAHIKT).

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #27 on: September 30, 2020, 10:51:13 am »

my guess is that the skewer has not been done up properly, or a poor quality skewer or some other similar "fault" (or a combination of). if that was a real issue, we would hear about it more often...

IMHO the main reason why it is not a daily occurrence with QR/disc brake wheels  is that forks are fitted with lawyers lips.  Folk often rely on these without even knowing it.

cheers

Or maybe in most day to day circumstances most folk weren’t and aren’t yanking on their disc brakes so hard that it was / is enough to overcome the clamping force. There are not masses of reports about QRs being loose every day are there? Surely if just the lawyer lips were holding the wheel, the QR would still be loose?

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #28 on: September 30, 2020, 11:43:58 am »
Isn't this problem is why closed dropouts and through axles were invented?
That seems like a safe bet. I'm sure Brucey will be along in a moment to confirm.
Rust never sleeps

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #29 on: September 30, 2020, 03:24:49 pm »

my guess is that the skewer has not been done up properly, or a poor quality skewer or some other similar "fault" (or a combination of). if that was a real issue, we would hear about it more often...

IMHO the main reason why it is not a daily occurrence with QR/disc brake wheels  is that forks are fitted with lawyers lips.  Folk often rely on these without even knowing it.

cheers

Or maybe in most day to day circumstances most folk weren’t and aren’t yanking on their disc brakes so hard that it was / is enough to overcome the clamping force. There are not masses of reports about QRs being loose every day are there? Surely if just the lawyer lips were holding the wheel, the QR would still be loose?

there is a difference between 'loose' and 'not tight enough'.    A typical cheap QR with an external cam might exert 300-500kgf (the force is oten limited by the creep strength of the cam seating). If the coefficient of friction x the clamping force  is smaller than the applied load during braking the wheel will start to move, even though the QR is 'tight'.  After the wheel has moved a little during braking, the usual thing is that the first time you go over a bump it gets pushed back more or less where it came from. You can tell that this has probably been happening by inspecting the LH dropout faces and comparing them with the RH dropout faces.  A small amount of movement can occur without slippage between the QR and the dropout, because the skewer is a slack fit in the hollow axle.  This small movement only requires slippage between the LH locknut and the inside face of the LH dropout.

So in most cases  the inside face of the LH dropout is most worn, followed by the outside face of the LH dropout, and both will be more worn than the RH dropout faces. If you see this pattern you can be fairly confident that the wheel has been moving about, at least some of the time. If you only brake gently under normal circumstances, the axle may hardly ever move.

Through axles were developed in good part to overcome this problem (in which they were only partially successful BTW), but have all kinds of other pros and cons of course.

cheers