Originally Posted by SkyDog75

Generally correct.


Tolerance varies by brand and model. Decent ones will have a documented rating for accuracy and it should be a lot better than 20%. Maybe the "20%" you're referring to is in reference to the wrench's scale. It's generally accepted that a torque wrench will be more inaccurate toward the ends of its scale (particularly the low end), so readings shouldn't be trusted below the 20% threshold. If you've got a torque wrench that measures up to 100 foot pounds, don't use it for a torque below 20.



Installing a rack, especially on a metal frame, isn't a job that requires precision torque values; just some common sense. Don't leave it so loose it'll slip and don't tighten it so much that it'll strip or break. Of course, if you're unsure or don't have much mechanical experience, then go ahead and use a torque wrench to be on the safe side.

There are a few jobs where torque wrenches are particularly handy, like tightening fasteners on carbon fiber components where too much torque will crack a part. Or for higher-torque installations like bottom brackets and cassette lockrings, where "tight but not tight enough" and "too tight" don't feel all that different when turning a wrench.