Changing forks

[Jdsk]

...
... however if the force of braking (newtons=mass (kg) x speed (metric) over comes the tensile strength of the fork blade
...

That isn't an equation of force.

Jonathan

[PH]

The photos are scary ,further convincing me that a front disc for road bikes just doesn,t feel right.Not scientific ,not technical.

For clarity - The photos are of forks not designed for disk brakes, I don't know of that happening to any that were.

[belgiangoth]

Be aware,a front disc will exert a alot more force through the beefed up fork which then transmits that force where? The frame, which, crucially, has not been engineered for it.

Errrm, you're going to have to explain how, assuming both types of brake are capable of locking the front wheel, the force going into the head tube is any different between the two systems? (Internally, the fork will, of course, see some interesting forces applied at the disc caliper mounts.)

So Chris Juden took some time out to explain to me the exact same thing,...

Well, the question is the application for torque, which is Force x distance from the pivot. If you apply the force nearer to the hub (disk brake) then you will need more force and hence the fork experiences a higher force.
This is just physics and I totally agree with you (other than your implication that it's a tricky concept).

However I still don't see how cyclop post's point can be correct, namely that this will unduly affect the frame. After all the force of braking will push the fork which will then push the headtube, but it will be moments of force vs moments of force, which will boil down to force from braking vs force applied to headtube - irrespective of type of brake.

[squeaker]

However I still don't see how cyclop post's point can be correct, namely that this will unduly affect the frame. After all the force of braking will push the fork which will then push the headtube, but it will be moments of force vs moments of force, which will boil down to force from braking vs force applied to headtube - irrespective of type of brake.

Quite: the forks see all different sorts of internal forces (as seen in the photos above), but ultimately it's the steerer which transmits the retardation force from the tyre contact patch into the head tube.

[mattsccm]

Going back to the OP. see if ou can find some Pompetamine forks. Planet X later, disc braked, version of the Pompino. Cotic did several bikes that had disc forks. Failing that ebay or Aliexpress.

[cycle tramp]

...
... however if the force of braking (newtons=mass (kg) x speed (metric) over comes the tensile strength of the fork blade
...

That isn't an equation of force.

Jonathan

Google seems to think it is 1 newton=kg x by every 1 metre per second travelled

[Jdsk]

...
... however if the force of braking (newtons=mass (kg) x speed (metric) over comes the tensile strength of the fork blade
...

That isn't an equation of force.

Google seems to think it is 1 newton=kg x by every 1 metre per second travelled

The dimensions are wrong. For mechanical force it's force = mass x acceleration, not force = mass x velocity.

Jonathan

PS: In appropriate settings momentum = mass x velocity.

[fastpedaller]

Without a doubt, if the forks fold due to the braking forces involved, there will be less force through the headtube
Not a good outcome though!

[rogerzilla]

The cynic in me says that SJSC have either a lot of non-disc forks or V-brakes/cantis they want to use up. They were still trying to shift the SunTour SE cantilevers years after they went out of production.

I have a lot of time for SJSC - they are one of very few designers that can make a loaded tourer handle properly - but they buy components in bulk.

[Steve O'C]

The cynic in me says that SJSC have either a lot of non-disc forks or V-brakes/cantis they want to use up. They were still trying to shift the SunTour SE cantilevers years after they went out of production.

They still have them for sale at a ridiculously low price. I seriously have these in mind next time I buy enough bits from SJS to avoid having to pay the postage price. Does anyone have any personal experience of them?
Steve

[AndyK]

The cynic in me says that SJSC have either a lot of non-disc forks or V-brakes/cantis they want to use up. They were still trying to shift the SunTour SE cantilevers years after they went out of production.

They still have them for sale at a ridiculously low price. I seriously have these in mind next time I buy enough bits from SJS to avoid having to pay the postage price. Does anyone have any personal experience of them?
Steve

I do - would you like to buy them?

I bought them more than 20 years ago because (a) my wife complained that she couldn't exert enough force on the back brake on her tourer and (b) SJSC were trying to shift their remaining stock of them at bargain prices. (Some things never change.) So I thought "right, let's get the most powerful canti brakes I can find." And they were. BUT... their design means that when you pull the lever you get little or no braking force for a split second, rapidly followed by too much braking force. Essentially as the pads hit the wheel, the wheel drags them forward - at which point the "self-energising" mechanism ensures they also start to move inward, clamping the rim without firther effort from the rider. All very clever and probably fine on a tandem or heavily-loaded tourer where the full SE effect won't lock out the wheel too easily, but on lightly-laden solo tourer it was too disconcerting - especially that tiny delay at the start. So off they came.

(Of course the real answer to the problem was to throw away the ghastly "suicide levers" her bike had and replace them with inline CX-style secondary levers.)

[rjb]

The Suntour self energiser's were only meant for rear braking to avoid locking up the front wheel. This is why SJSC had their tandem forks made with the brake post on the back of the blade and entailed the use of a Tektro brake booster to work them.
The introduction of V brakes rendered the SE brake to history and the sell off of their stock.

[AndyK]

The Suntour self energiser's were only meant for rear braking to avoid locking up the front wheel. This is why SJSC had their tandem forks made with the brake post on the back of the blade and entailed the use of a Tektro brake booster to work them.
The introduction of V brakes rendered the SE brake to history and the sell off of their stock.

Are you saying SJSC used to sell tandems with these things fitted to the front? That was - er - brave of them...

[PH]

The cynic in me says that SJSC have either a lot of non-disc forks or V-brakes/cantis they want to use up.

In the context of this thread, that's entirely cynical. Their forks have been in and out of stock over the last couple of years, with design and colour changes, the V brakes offered are current models, I doubt they hold a huge stock. They do stock stuff that's gone out of production, they'll buy it up, or in some cases persuade the manufacturer to do them an extra run, or in at least one case take over the company to continue supply. I'm sure they do get their fingers burnt sometimes, though I'm equally sure the motivation is a belief in the product, I wish more retailers had that attitude.
Back to forks - I bought a Thorn Mercury five years ago, went for the ludicrously expensive 853 V brake fork, I wasn't convinced it'd make much difference but it allowed me to re-use an existing wheel, SON and brake (I'd have gone V brake on the frame, for the same reason, if that had been an option.) It is a comfortable fork (And frame), having since had their Reynolds ST canti fork and the standard crown disc forks, there is a noticeable difference. Whether it's enough to justify the hype is subjective, it's enough that I'd make the same choice again.

[belgiangoth]

...
... however if the force of braking (newtons=mass (kg) x speed (metric) over comes the tensile strength of the fork blade
...

That isn't an equation of force.

Jonathan

Google seems to think it is 1 newton=kg x by every 1 metre per second travelled

I suspect there is an omission of the squared. 1N =kgms^-2
(And F= rate if change of momentum)