Tim Krantz wrote on 12/31/69 at 16:00:01:


Tim,

Sure.

The basic formula for torque is: T=F x D  (Torque equals force times distance).  Keep in mind that your torque wrench, while it reads in units of torque (ft-lbs) is actually measuring the force (F) you are applying to it.  Since the length (D) of the torque wrench is fixed, this force can be displayed as torque.  Note that the length (D) is often referred to as the "moment arm."

You need two length measurements.

The first measurement you need is the length of the torque wrench's moment arm (huh?).  This is the distance from the point your hand applies force to the wrench to the middle of the wrench's drive square.  Let's call this length "D."  Let's assume that, for the example below, that D is equal to one foot.

The next measurement you will need is the new moment arm you get when you add your six-inch combo wrench on to the end of the torque wrench.  Measure this the same way you did for the first measurement, except you don't measure to the location of the middle of the torque wrench's drive square, you measure instead to the location that corresponds to where the concentric center of the bolt would be on the combo wrench if you were trying to torque the bolt.  Let's say this new distance is D' or, for our example, 1.5 feet.

Example:

Let's assume you are going for 10 ft-lbs of torque, but you can't get your torque wrench on the darned bolt.  You can, however, get your six-inch combo wrench on the bolt.

So you cleverly hook up the combo wrench to the torque wrench (keeping the whole contraption as "straight" as possible [for an accurate torque calculation]).

For the basic torque wrench,

T = F x D, or

10 = F x 1 (for the basic torque wrench)

Therefore, F = 10 / 1, or 10 lbs.  What this is saying is that you need to apply 10 lbs of force to your 1 foot-long (moment arm) torque wrench to get 10 ft-lbs of torque.  Remember from the discussion above that your torque wrench is really measuring the force you are applying to its handle.

Now, with your six-inch combo wrench hooked up to the torque wrench as a "torque adapter," here are the new calculations:

T = F x D', or

10 = F x 1.5

Therefore, F = 10 / 1.5, or about 6.7 lbs.  So, you'll need to apply 6.7 lbs of force to your torque wrench to get 10 ft-lbs of torque on the bolt.

In General:

After you do all of the algebra, you can use the following formula for any torque adapter you care to piece together:

T' = T x (D / D'); where

T = Desired torque on the fastener
D = Moment arm of basic torque wrench
D' = Moment arm of the torque wrench and torque adapter combination
T' = Reading you want to see on the torque wrench to get the proper torque on the fastener while you are using a torque adapter.

FWIW, every time I invent a new torque adapter (I've got about eight ones I use nowadays), I write down, in my maintenance book, a description of the torque adapter and its corresonding torque conversion factor (the value of the "T x (D / D')" term).

I hate to hit you with all of the math, but this is how it works.

Clear as mud?

Or you could set the torque wrench correctly and put it on something accessible, like your axle, just to see what it feels like.  Then use whatever dohickey you have to use to tighten whatever about that tight.

Probably just as accurate.

In the old days we used the simple and infallible torque rule learned on British iron "Tighten till it strips, then back off half a turn"  A more refined version is given by Sheldon Brown, the guru of bicycle mechanics:
http://sheldonbrown.com/tork-grip.html
enjoy  

 Thanks Digger,printed it out.

Savage_Greg wrote on 12/31/69 at 16:00:01:


Greg,

No arguments there.  However, with the eight different torque adapters I've had to invent thus far (for bikes AND cars), I've not had to do anything at a right angle.

Usually a combo wrench placed on the drive square of a torque wrench creates a slight offset from the straight line of the torque wrench.  I've done the calculations with this slight angle included and found that the difference is neglible.

Thanks for the correction!