Budget Torque Tool

For many years I worked in bike workshops and had a lot of assorted tools developed for specific purposes, one of this tools is the Torque wrench, an important tool to ensure the integrity of parts, especially those made of carbon fiber or other "exotic" materials (example images for Bikeradar)

But, in a recently days I want to built my own carbon bike with my standard tools and I missed this tool, without an expensive budget I want to find a solution to avoid damage to carbon parts and avoid some risk with my future bike

With some useful tools that I have I teach you in a creative way to make a torque wrench with low budget

Let's start with some basic tools:

1. Ruler or tape meter
2. Hook weight scale or Luggage Scale, I think we can't exceed 15kg
3. Assorted tools

Torque is the rotational analogue of linear force (Torque - Wikipedia), we can convert a linear force to torque with a radius fixed in one of its ends, and a bit more difficult convert torque in a linear force with a mechanism like pinion-rack, screw, cranks... Always use the relation : Torque = Force x Radius

But, Force is measured with special tool called Force gauge ("dinamometro" in spanish, for Dyna units) we can found a cheap alternative for this measure, a Hook weight scale, but the units of this tool are kg or lbs (gr or oz), we need to convert Force Units to Mass units

Due Force = mass x acceleration, and accel. is 9.81m/s2 (gravity accel.) we could operate this magnitudes to find a mass equivalent to torque. and.... the radius? the tools are the keys!!! they have a fixed distance from center to bolt/nut to hand attachment, and we could fix the end hook of weight scale to end of tool

With the physical principles we can operate this "low budget torque meter", bolt or nut always be the center of rotation and we take this point of reference, with a fixed and measured radius we could put the hook in this point and pull in the other extreme of weight scale as show as the pic.

We can use as a torque measure or torque limit. As a measure tool you can obtain the torque with formula Torque = mass*gravity*radius, you could pull the weight scale with progressive force to measure how much torque is needed to loosen a nut or bolt, but I thought this budget tool to use as a torque limit tool, using the final formula of step 1, I calculate the "Maximum mass" to have the maximum measurement that can be displayed in the weight scale, this is very useful to avoid an undesirable damage in mechanical parts

For accuracy in measure i think about 5 causes:

1. Weight Scale Precision: - is about 1~2% error, more the cheaper scale more the loss in accuracy, but It does not exceed 5%
2. Weight Scale Display, or the number displayed always change until "autohold" function
3. Radius: in a regular measure we could take 1~2mm extra or less, in a 10cm tool this implies 2% error
4. Angle (as image): Is difficult without a reference to mantain 90° to the radius, So, there will always be a loss of force... 30° of angle (a lot) is a loss of 15% of force
5. Tool and attachment flexibility... It's hard to explain but it exists

But, If I think in a "Torque Limit", I would avoid the maximum measure with a proper "Engineering Safety Factor"

A first example is easy, I'm assembling my bike and some pieces need a special treatment because torque limits, the less problematic is the "thru axle" of the wheels... Could the tighten torque I applied work?

First I measure the tool, a 6mm allen key, this tool measure 138mm or 0.138 mt. with the weight scale I applied a little force and measure 1.51 kg, operating with formula of torque (Torque=mass*gravity*radius) T=1.51 x 9.81 x 0.138 =2.044Nm. this is the tighten torque of this stud/bolt

If you look at the head of the bolt, they mark "6Nm-H6", then I could apply more force to thru axle to tight. but, I can't exceed the 6Nm mark and apply only 2.36kg (2.36 x 9.81 x 0.138 = 3.195 Nm)

For a Heavy application one of heaviest torque bolt in a bike is the cassette lock-ring, various brands recommended a 40Nm Torque for tighten this lockring

As a previous example I measured the ratchet tool from center of radius to hook attachment, they measured 20cm (0.2m), I try to apply almost 30Nm to proper tighten. Using the Torque limit formula (step 1) I obtained mass=Torque*(1/(radius*gravity))=30*(1/(0.2*9.81))=15.29kg

In a first measure I measure 6.770kg, or 13.28Nm (Torque = mass*radius*gravity), and need more torque to proper tighten. Sorry for the last photo, It´s difficult to make a proper force and take the photo twice, I mark a little more of 15kg but in the photo only mark 12kg

For last example I show you the most critical part, a carbon integrated handlebar with titanium bolts... the manufacturer sets a maximum torque of 5.2 Nm, and If I have exceed this torque probably I will wreck this attachment and lose the bike part

First I measured the tool (a 4mm allen key) and set 108mm (0.108m) and using the formula in step 1 I obtain 4.91 kg to limit measure. Curiously in this case, there is not much difference between the torque measurement and the measurement that the device should display.

for tighten this bolts I applied a progressive force without reach the limit measure (4.91kg), I set in this case 2.03kg (2.15Nm) because I need to adjust later the handlebar spacers in fork tube. I recommend, in my experience, reach only 80% of maximum torque in this carbon parts

With the same principle you could use this budget torque meter in a lot of applications, I show you a various examples in my bikes because this parts don't need a heavy torque

BUT!!! If you need a heavier application, as a nut/bolt in a car wheel (90 Nm or more) with this method I suggest you to use a longer tool and make a proper attachment.

In all cases I recommend you check the tools used (weight scale + tool), check the nut+tool fixation and don't exceed 15kg to force applied

Without these recommendations, the tools can come loose, jump and cause an accident.