Today's idiotic question: torque wrench usage

[Tom]

I decided to monkey with my bars and roll them forward a little. Being cognizant that bike parts and particularly carbon bike parts don't like to be reefed down like a gorilla's been working on them as opposed to the macaque I really am, I decide to break out my torque wrench for the job.

It's one of these 'positive reading' spring-arm types. I get the brilliant idea to use it to loosen the clamp bolts so I can see exactly how tight they were torqued down in the first place. No problem. They jump free right at the same place for both of them. Call the reading "50 units".

I fiddle, decide I like the bar angle, go to tighten up the bolts. The wrench immediately goes to maximum, like 75 units, and the bolts still turn easily. I say "holy crap" and stop. I test the bars. They're tight, but not so tight that I can't move them if I try. Hm. I realign them and tighten again. Now they don't move, but for the purpose of science I decide to loosen them. They spring free at 35 units.

Good grief, I mutter. I tighten them back up, tight but not simian tight. They don't move, I decide to go for a ride and see if I'll kill myself. No problems. I climb steep hills and put some torque into the bars, ride over rough pavement, carve a few high speed corners and even aim for a few small holes with my weight on the bars. No issues.

Is my empirical method of taking the torque reading the wrong way to go about it?

Had to be. If my calculations and recollection are correct, I should have maxed the wrench. It's one of those cheesy Park ones, the 0-60 inch pound version. 6.8 newton meters comes out to about 60 inch-pounds. Ha.

Now you guys get to chastise me for monkeying around on the bike before I figured out what I should have been doing. If I figure out what I'm supposed to be doing ahead of time it sort of takes away the wonder of it all, so I am loathe to go down that road.

[rwl]

Tom,

There are 2 sorts of friction working here - static and dynamic. Static is higher - it takes more 'oomph' to get something started than to keep it going. Try pushing a book along a table - it takes more to get it moving, than to keep it moving.

It's important to work with the actual units (foot lbs or inch lbs, for us imperial measurement types). When it says don't overtighten those bars, they really do mean it. You might be safe at 25% over, you might crack something. If the bolt keeps turning when you reach the torque limit, you're there, stop.

But if you tighten the bolt at 30 in lbs, and do so 'smoothly' stopping at 30, and then want to tighten it again to 35, you might reach 35 long before the bolt turns. This is where most folk go very, very wrong. The want the bolt to turn, the static release point might be 75, and then it keeps moving quickly. And then you've way overtorqued the bolt.

"Positive reading spring type arm" is this a beam type, with a pointer round arm in paralllel, with a scale at one end? Such are pretty good torque wrenches, and unless banged around, remain pretty accurate over their lifetime. They measure the deflection in the arm, when weight is applied at the handle. (Others measure friction or torque in the arm, and are more precise (sensitive) and can be more or less accurate, depending upon maintenance) These latter often have a very clever 'clicking' release, so that the torque wrench makes a click and releases when you've hit a preset torque. For carbon goodies, I like these kind best - the good ones look like a screwdriver with a scale on the shaft, you set the limit you want, and use it just like a screwdriver until it 'clicks'.

Rick

[Tom]

Like you say, when you've stopped at thirty and want to go to thirty-five, the best thing to do is loosen it up first and then re-tighten.

What confuses me is that it seemed to release about 40 inch-pounds, when 60 seems to be the right tightness.

The one key to it all is, like you say, don't overtighten. In this application, if it isn't moving about I am not going to reef down on it just to hit a number.