Bicycle torque wrench from a fisherman's scale

Occasionally when working on your bicycle it is advisable to use a torque wrench suitable for measuring inch pounds. The infrequent user does not particularly want to pay out twenty to forty dollars for a tool that seldom gets used. You can do your torquing jobs with an investment of only six dollars for a dial indicator fisherman's scale. You probably have the other things you will need.

Step 1 Calibrating the scale

Torque is force over distance. Pulling the end of a ten inch bar with a force of four and one-half pounds will put a torque of 45 inch pounds on a hex key.

Get a dial indicator fisherman's scale at a sporting goods department. These run about six dollars and read up to 50 pounds in one-half pound increments.

Your scale may be accurate at some weights, but may vary at others. Calibrating the scale is easy. My carbon fiber seat post has a recommended torque of 45 inch pounds. I want to place a weight of 4.5 pounds at the end of a ten inch bar.

Water weighs 8.33 pounds per gallon. There are 8 pints to a gallon and 2 cups to a pint. The weight of a plastic milk jug is negligible. Use a kitchen measuring cup to pour four pints and one cup of water into the milk jug.

Comments

[shrpshtr325]

just a comment on this, the torque is equal to the force (applied at a 90degree angle to the handle of the wrench) times the length of the handle, if you pull on the scale at an angle other than 90 (approximately, im sure a little bit off wont hurt too much) you will be exerting a different than intended torque on the part (lower, which means that it could come loose on you while riding)

[sharlston]

great ible is that a specialized mountain bike?

[Phil B]

 Thanks.  It is a Specialized Allez Sport road bike.

[sharlston]

nice i ave a falt mystic pro 2009 bmx

[kearney]

what if you were to take say a foot long piece of pipe and weld a ring to the end for the scale? when you wanted to use it all you'd have to do is slide the pipe over the shaft of the wrench and attach the scale. then you wouldn't need a long handled ratchet, and you could better judge the distance of the shaft.

[Phil B]

Sure. That is a good idea. The main object of this Instructable was to help folks who do not want to spend $20 to $40 for a torque wrench they will use infrequently, so that they can use a few common items costing much less money and still get the job done well. Feel free to adapt the basic principle according to what is available to you.

[gritz]

If you've ever snapped off a bolt or stripped the threads of a nut because you didn't know your own strength you'll appreciate this advice and torque wrench setup. A little ingenuity goes a long ways in being more precise in making in-lbs adjustments .... plus, need I say, saving some bucks in not having to purchase a store-bought' tool! Good idea ..........

[Phil B]

Thanks, gritz. Discussions of torque wrenches reading in inch pounds as per cycling forums frequently say you really ought have two torque wrenches reading in inch pounds: one for lower numbers and one for higher numbers. This wrench setup will handle both. Discussions on cycling forums also say any torque wrench will have its range of greater accuracy, outside of which accuracy falls off. But, with this setup you can calibrate for the new range by weighing the right amount of water and still be just as accurate. After posting this the idea came to me that an individual hex key could be held between the jaws of the adjustable wrench just as easily as a tool with multiple hex keys. Just point the short end of the hex key toward the outer end of the jaws.

[reddeth]

Wow, crude but extremely ingenious. Very nice!

[Phil B]

Thank you. It has worked well for me. Originally I used a bar from scrap steel and worked out a semi-satisfactory way of attaching individual hex keys. But, the holder for the hex keys was welded. Not everyone has access to a welder, so I decided to find a more simple way, which resulted in holding a hex key tool with an adjustable wrench. Another means of calibrating the scale would be to weigh packages of meat from the grocery store. Meat comes with a label that tells the exact weight as weighed on very precise scales certified by state regulatory agencies. Use a plastic grocery back to hold the meat packages.