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Please help me read this torque table.

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Please help me read this torque table.

Old 12-06-16, 06:15 AM
  #1  
Robert C
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Please help me read this torque table.

I am in the process of rebuilding my trike. The frame was cracked and the manufacturer replaced it at no charge. However, I want to assemble the replacement correctly, as such, I wrote to the manufacturer and asked for the torque values for the large bolts that attach the main boom to the crossmember.

I can get most of the values from this site: however, as I mentioned, it is the large bolts that I have trouble finding the values for.

I am not bothered by the Chinese writing at the top (it just identifies the table as a torque table). I am concerned with what the numbers on the horizontal and vertical axis mean.





These are the bolts I want the correct values for:

Note: this is after about a year of use; but yes, I clearly overtorqued them.

Last edited by Robert C; 12-06-16 at 06:27 AM.
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Old 12-06-16, 06:32 AM
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The table at this link looks like the same table...with some explainations.


8.8 metric bolt torque chart - conttrapacbu23's soup

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Old 12-06-16, 06:57 AM
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Across the top is the grade of bolt, the first column is the size of the bolt, then the recommended load and torque values.
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Old 12-06-16, 07:05 AM
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You may want to wait until the mfr. gives you the correct torque for those particular bolts since it depends more on what the frame can handle rather than the bolt and threads as the tabs broke before the bolt/threads failed as will many pieces on a trike/bike. The charts only provide bolt strength.
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Old 12-06-16, 07:07 AM
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Originally Posted by Crankycrank View Post
You may want to wait until the mfr. gives you the correct torque for those particular bolts since it depends more on what the frame can handle rather than the bolt and threads as the tabs broke before the bolt/threads failed as will many pieces on a trike/bike. The charts only provide bolt strength.
You do make an excellent point. Unfortunately, I wrote to them and that chart is what they provided.
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Old 12-06-16, 07:26 AM
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In that case some more info would help. Is that clamping a bearing, bushing, rigid tube? What diameter are the bolts and diameter of the shell, brand of bike, Aluminum or steel frame and a few more pics of the clamp and entire bike would also be helpful.
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Old 12-06-16, 07:33 AM
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Originally Posted by Crankycrank View Post
In that case some more info would help. Is that clamping a bearing, bushing, rigid tube? What diameter are the bolts and diameter of the shell, brand of bike, Aluminum or steel frame and a few more pics of the clamp and entire bike would also be helpful.
The picture looks like the clamp bolts for an eccentric bottom bracket. (My tandem has a similar arrangement) These bolt only need to be tight enough to keep the eccentric shell inside from turning. You should start at the very low end of the scale.

If it is doing something else, post a bigger picture.
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Old 12-06-16, 08:32 AM
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The bolts in the image are the bolts that attach toe cross-member (the part holding the front wheels) to the main boom.
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Old 12-06-16, 08:10 PM
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8.8 is the most common grade of metric fastener; bolts in grades below that are uncommon. I would suggest removing one of the bolts and measuring its diameter. There may even be markings on the bolt to indicate its grade. Read down the left side of the table to find the corresponding diameter and then read across to the 8.8 column. You would probably be okay tightening to the torque indicated for that diameter of grade 8.8. If you are not confident doing it that way, order new bolts, from a place like Fastenal, in grade 10.9 and tighten them to the torque indicated for that grade. They might even have grade 12.9, although in my 50 some years of wrenching, I don't think I have ever seen a grade 12.9 bolt.
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Old 12-06-16, 11:00 PM
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There are TWO considerations relating to torque specs.

The first is what this table is about, based on the elastic and strength properties of the bolt, and generally the maximum torque before breakage is an issue.

The second is what the assembly requires to perform as designed, and/or what the assembly tan take before failure by stripping, bending, or breaking.

I point this out because in many applications such as your frame, the maximum load the frame can take is well below the breaking strength of the bolts used, and if you blindly follow the screws' torque specs you'll end up with properly tightened bolts in a broken frame.
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Old 12-06-16, 11:20 PM
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And then there is the fact that it's a chinese document, which could be accurate, or just a likely, something suitable for wiping your a*se with.
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Old 12-06-16, 11:23 PM
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Originally Posted by coominya View Post
And then there is the fact that it's a chinese document, which could be accurate, or just a likely, something suitable for wiping your a*se with.
I doubt it's a Chinese document. More likely, it's simply a Chinese heading of a chart downloaded from an international standards site of some kind. I'll bet a beer that if one searches, he can find the identical chart, with the heading in English, German or French, if not all three.
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Old 12-07-16, 05:54 AM
  #13  
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Originally Posted by FBinNY View Post
There are TWO considerations relating to torque specs.

The first is what this table is about, based on the elastic and strength properties of the bolt, and generally the maximum torque before breakage is an issue.

The second is what the assembly requires to perform as designed, and/or what the assembly tan take before failure by stripping, bending, or breaking.

I point this out because in many applications such as your frame, the maximum load the frame can take is well below the breaking strength of the bolts used, and if you blindly follow the screws' torque specs you'll end up with properly tightened bolts in a broken frame.
This is exactly what I am dealing with on this frame. The frame will break long before the bolt is at any ridk of striping. That is why I wrote to the manufacturer, hoping for a useful number.
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Old 12-07-16, 09:20 AM
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Are you really sure that damage was caused by over-torqued bolts?
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Old 12-07-16, 09:33 AM
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No torque number is going to be adequate to keep the ears from breaking off that clamp. That is just one poorly engineered clamp, and the cantilever from the clamp to the spindle makes it more likely to break. Forget about proper torque. Re-engineer the clamp.
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Old 12-07-16, 09:51 AM
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Originally Posted by reddog3 View Post
No torque number is going to be adequate to keep the ears from breaking off that clamp. That is just one poorly engineered clamp, and the cantilever from the clamp to the spindle makes it more likely to break. Forget about proper torque. Re-engineer the clamp.
Yes, lots of strain cycling and dynamic stress. And the clamp, as is, appears poorly manufactured.

Last edited by AnkleWork; 12-07-16 at 09:56 AM.
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Old 12-07-16, 09:55 AM
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FB's point is a good one to keep in mind, although I'd quibble that about the "maximum torque before breakage" being the issue.

Bolts in critical applications are torqued, not because torque is important, but because the clamping force of the bolt is critical. This force is bolt stress times bolt cross sectional area. Torque is a surrogate for bolt stress. Because the bolt stress is hard to measure, and torque is easy to measure, we use the latter. There's a lot of imprecision though. Was the bolt lubricated with antisieze or Loctite? Is the surface the bolt is turning against rough or smooth? Is that surface perfectly perpendicular to the bolt? In more critical applications (connecting rod main bearing connections), the experts actually measure how much the bolt has stretched (the strain) as a more accurate surrogate for stress. So you put a micrometer on the bolt before tightening, and tighten it until it's stretched by an amount that corresponds to the right amount of strain. In some super-critical cases, special washers are used that crush at a pre-determined stress value. The bolts and washers at this point are astronomically expensive. Fitting, as they usually only end up in aerospace applications.

Why does this matter? Well, it turns out that for most bolt applications, if you don't get the bolt tight enough, the joint is much weaker and susceptible to fatigue failure. "Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook" goes into the details. It's a bit dated, but worth a read if you are interested in practical details of "boltology".

As FB and others have pointed out, though, the torque tables tell you how much torque is required (under specified conditions, by the way - with/without lubricant, anti-seize, or Loctite) to get the maximum clamping force and fatigue performance. This presupposes that (for example) your 12.9 metric bolt is being screwed into a matching nut, or an appropriate forging with proper threads. Threading a 12.9-rated bolt by definition has an ultimate tensile strength of 1200 MPa, or 174,000 psi. This will be a high-strength steel alloy which has been heat treated to be strong and tough.

If this bolt is threaded into an aluminum alloy part, or an alloy steel part that's not high strength, heat treated alloy, and torqued to proper values for a 12.9 bolt, the part will fail. So in my CF stem, the screws that clamp the handlebar are fancy. They look like mil spec titanium screws. In the CF part, there are embedded nuts that these screw into. The CF stem has a specified torque printed on it. I'd be nuts to go to a torque table and haunch on these little screws to full specified torque for a 12.9 bolt, because 1) I don't know the actual grade of the bolt, and 2) the CF part may not tolerate it.

You also don't know for sure what bolt grades you have. You need to verify bolt and nut grade for each joint. And, frankly, since a lot of bike stuff involves light weight alloys or CF, I'd want the mfr to specify the proper torque, which would account for material of construction, design goals, and part design and manufacture.

Sorry to be such a techno wienie, but the point of torque specs is strength and fatigue life, and these goals are dependent on more than just the bolt.
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Old 12-07-16, 10:24 AM
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in auto and other applications there are 2 torque specs for same bolt whether greased or dry threads.

I suggest use less force if thread is into aluminum, than steel..
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Old 12-07-16, 10:58 AM
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Also, FWIW, UI agree with others, the welded-on bosses (the part that is shown broken off) that the bolt goes through for clamping look way undersized for purpose, and (again, in agreement with others) they should bear directly on the mating part. So, in your first picture, proper design would have there be no space between the boss and the piece that the bolt screws into. That is, the bolt should be clamping two parts that already touch, and not stretching the cantilevered boss.

As a possible workaround, if your new frame has a gap there, you might find a spacer to put in so that the bolt clamps the part without bending the bosses.
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Old 12-07-16, 11:39 AM
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Originally Posted by WizardOfBoz View Post

....As a possible workaround, if your new frame has a gap there, you might find a spacer to put in so that the bolt clamps the part without bending the bosses.
Sorry, but no. Any effort to shore the clamps will defeat or undermine their holding ability.

Odds are there's adequate clamping strength to do the job without any extraordinary measures. If operating without guidance from the maker, the OP should tighten the bolts by degrees keeping the load balanced by feel or torque wrench, so no single bolt or ear is taking more than it's share.

He should shoot for what seems reasonable (purely by hunch, if nothing else) and ride the bike, being ready to tighten all 4 a bit more if there's any slippage.

If (and only if) he does need more holding power, he can achieve it by applying lapping compound to the shaft before assembling and tightening. That will multiply the hold significantly. There are other engineering options to improve the design and function, but the OP can cross that bridge later if necessary. Meanwhile, having spent serious dough on the bike, he should be ready to give the maker some credit for knowing if the design is adequate to the task.
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Old 12-07-16, 01:01 PM
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Originally Posted by FBinNY View Post
Sorry, but no. Any effort to shore the clamps will defeat or undermine their holding ability.
If it were a clamp setup, your comment is spot-on: a spacer that limits the travel of the cantilevered bosses also limits clamping force (though there might be a balance that could be reached - enough clamping force, whilst limiting the stress on the bosses).

But I don't think its a clamp. The way I see the pictures is that there is a reinforcing jacket welded to the tube, and that the bosses are welded to that tube. There are corresponding jackets and bosses on the crossbar. If this is correct, the bosses aren't clamping anything, but are merely connecting the two rigid parts. And using an inapt cantilevered boss design to so so.

The assembly instructions for the Performer line of trikes is here. BTW, not a word about torque is mentioned! From the instructions, I infer that the "clamp" halves are welded to the frame and to the wheel boom on top and bottom, as I describe. The space they left is to allow you to adjust the frame to get it perfectly level. One could do this, and then measure the spacing, and then put spacers in that matched the spacing, and then proper torque would be appropriate.

But I could be misinterpreting the pics (and you might even own one of these bikes and so know it in great detail). The OP, or any owners of the bike in question can clear this up. Anyone?

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Old 12-07-16, 01:16 PM
  #22  
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One of the first and most important rules in any mechanical work is "Don't fix what ain't broke". We can criticize the design until the cows come home, but there's a much better than even chance that the design works as it is.

Are there possible issues in the design?, of course. Can it be improved? Maybe.

But, unless there's a reason to "fix" it, the OP is at serious risk of making thing worse by tinkering without knowing all the considerations.

Regardless of what I might do if working on a bike, I always advise strangers to credit the designer with knowing what he's doing, until/unless the design doesn't work, and he has no option but to modify it.

To the OP, cross your bridges one at a time as you come to them. Use reason, guided by experience to tighten the bolts, and see if things work before moving to a possibly unnecessary or problematic modification.
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