Torque wrench conversion formula?

[Josey]

Does anyone remember the torque conversion formula when a crowsfoot extension is used on a torque wrench? I'm installing new Speigler brake lines tomorrow, and the rear line screws into the master cylinder directly - no banjo. I won't be able to use a socket to torque it down, so a crowsfoot is needed.

TIA

[CYoung]

S = (T x L) / (L + A)

S = torque wrench setting
T = desired fastener torque
L = distance from the middle of the grip to the center of the drive square
A = distance from the center of drive square to the center of the fastener when an attachment is used.

The above comes from the instructions for my torque wrench. Hope it makes sense to you.

Also, the crowsfoot must be aligned with the handle.

[Josey]

CYoung:
Makes perfect sense. Thanks much.

[roger albert]

Or if you run at 90 degrees to the handle the formulae comes D = 1 x S
where D is Sesired torque and S is the Setting

[Josey]

Roger:

You're saying that attaching the crowsfoot at 90 degrees eliminates any conversion? D=1xS equates to D=S.

[roger albert]

Yes, that has always been my understanding. (sorry, I was being kind of a smart--ass) Think about it, at 90deg, there is no additional length, and thus no additional torque, and thus no need of correction factors.

Anyone know differently?

[Josey]

Roger:

You funny.

I'll remember this...

[roger albert]

But I was serious in stating the adjustment factor is not needed at 90deg. Of course, there's not always room to use that, but when you can, it saves some time.

roger

[Josey]

Yea, I know - I'm just havin' fun with ya...
The logic seems sound, although it will probably still be slightly off. Think trig: If the extension is at a perfect 90 degrees, your lever arm length (from center of handle to center of fastener in the extension) will be longer than the handle to drive-head center measurement.
I know - I'm spliting hairs. I can't think of any fitting on this bike that requires torque settings THAT precise.

[roger albert]

Cool.

Though I'm having trouble picturing where the differntial is (barring some small offset in your crowsfoot. Don't draw some trig triangle. If you do, resolve the resultant into X and Y components, and note that one of them (depending on frame of reference chosen) is 0. I.e. one term drops out, thus it should be exact. (assuming the crows foot, or extension is straight, and neglecting slop in the fittings which prevent a perfect 90degree alignment etc etc) This is no kludge. It's spot on.

roger


A 15mm boxend (vs crowsfoot) extension is what I have to use to torque ducati heads tucked in all those frame rails. Very tight quarters.

[Josey]

Sorry - I was using trig, which probably isn't appropriate here. Really Rough example - Crowsfoot - 2" from drive center to center of fastener (call this "a"). Torque wrench length from drive head to center of handle - 18" ("b"). Angle of 90 degrees. "c" = center of fastener to center of handle. For right triangles, solve for "c": "a" squared + "b" squared = "c" squared. Comes out to about 18.11"

[roger albert]

True, I know the formula (pythagorean), it's just not applicable here.
That's the whole point of running at 90 degress. You get to neglect that.

[Josey]

Cool...