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Bolt torque: What's all the fuss about?

Billy Wight, President, Luxon MX

July 14, 2022

Updated November 9, 2022

Torque Wrench
Tightening a bolt to torque with a digital torque wrench.

It seems to be everywhere now: test rider comments, magazine articles, vlogs, forum posts, and more... People are talking about bolt torque, and often talking about tuning the torque level to change the feel of the bike. But how does changing bolt torque a small amount affect the ride of the bike? In order to answer that question, we need to understand what torque actually means, what torque does to a bolted joint, and why we use torque in the first place. Armed with that knowledge, we can understand the limitations of using torque as a reference for what it's really doing, and whether or not changing it slightly can affect the ride of the bike. SPOILER ALERT! It doesn't.

The Bolted Joint

Before we can talk about the effects of changing bolt torque, we need to understand what bolts do. This may seem obvious, but from what I've witnessed, it's often not well understood!

Bolts are used to hold two (or more) things together. This forms a bolted joint. Perhaps the simplest example is a bolt holding two plates together. Bolts can also be used to clamp one part around another; like the triple clamp bolts that hold your fork tubes. Here are a few generic examples of bolted joints:

bolted joint 1
bolted joint 1
bolted joint 1

The images above show three types of bolted joints; from left to right we have: two plates bolted together with a bolt and nut (some engine mount setups), a stud holding a plate down with a nut (your cylinder head studs and nuts), and a bolt holding a plate down to another component that the bolt threads in to (many of the fasteners on the motorcycle).

In all cases, though, the job of the bolted joint is to hold the parts together and ensure they don't move relative to one another. The bolt holds the engine hanger against the frame tightly so it doesn't move. And the bolt squeezes the triple clamp tightly around the fork tube so it doesn't slip. The key here is that the bolt's tension is holding the assembly together tightly so it doesn't move. If it's too loose and the parts move relative to one another, the bolted joint has failed. You should never have this scenario anywhere on the motorcycle (or most anywhere else for that matter).

So, What About Torque?

Torque doesn't matter! Well, sort of… What matters is the tension in the bolted joint that holds it all together. The tension of the bolt clamps the parts against each other, and the friction between the parts keeps them from moving relative to one another. If we know the bolt tension, then we know how the bolted joint will perform. We can calculate the correct bolt tension knowing the strength of the bolt, the friction between the parts being bolted, and the loading the bolted joint sees (from riding the bike, etc.). Too much tension and you could break the bolt. Too little tension and the parts move relative to each other. Both of those scenarios are undesirable, so it's important to get the bolt tension right!

Torque Tension Bolt
A force at the end of the wrench creates a torque that tightens a bolt to tension, clamping parts together.

So, what about torque? Torque is just force multiplied by distance, so in our case it's the amount of force required to turn a wrench of a particular length. If you're trying to torque a bolt to 20 Nm, and your wrench is 1 meter long, then you need to apply a force of 20 Newtons at the end of the wrench. That's a really long wrench, though! So, you're probably using a wrench about 100 mm long, in which case you're applying a force of 200 Newtons and you end up with the same torque of 20 Nm (for the metric challenged, that's about 45 lbs. of force over 4 inches, or 15 ft*lbs. of torque). Generally, torque wrenches are longer than 100 mm, but you get the idea.

But wait, torque doesn't matter, tension is what matters, right? Correct! Unfortunately, though, it's rather difficult to measure bolt tension directly; but it's pretty easy to measure bolt torque. And torque is related to bolt tension, so in that sense, torque does matter. This is why you hear so many people talking about torque. Because it's an easy way to approximate the bolt tension (the thing that really matters).

The Torque-Tension Relationship (Friction)

As I wrote above, torque is related to bolt tension, which is the only reason why we care about torque. The relationship between torque and tension is “simple”. Bolt tension force is just bolt torque divided by the diameter of the bolt and the friction coefficient in the bolted joint:

Tension = Torque ÷ (Diameter × Friction)

This is a simplified version, but it's accurate enough for this discussion. Should you want a more detailed equation, you can download a much more involved bolt torque spreadsheet from our resources page.

Tension is what we're after, torque is fairly easy to measure with a torque wrench, and bolt diameter is obvious. The problem we have is friction; and it's a big problem! The actual bolt tension is directly related to the friction in the bolted joint - typically the friction between the threads of the nut and bolt and the friction between the underside of the bolt head and the part being bolted (or the washer in some cases).

And that friction can be massive. Most of the torque applied to a bolt doesn't actually tension the bolt, it's eaten up by friction. In a typical bolted joint on the motorcycle, nearly 94% of the tension you apply with the wrench goes to overcoming the friction, leaving only 6% of it actually tensioning the bolt.

Friction Split
The vast majority of torque goes into overcomming friction rather than tensioning the bolt.

If we accurately know the friction values, then we can accurately calculate the bolt tension. The problem is that the friction values can vary significantly. It varies from bolt to bolt, and it even varies along the length of the same bolt. New bolts, used bolts, bolts with different coatings, bolts made of different materials, thread locking devices, clean or dirty bolts, lubricated bolts, etc., all have a different friction level. Friction is drastically different between dry bolts and lubricated bolts. And all this applies to the nut side of things as well. And if the friction varies, then so does the bolt tension if you're using torque as a way to measure it.

Torque Tension Bolt
Bolt tension reduction with each re-use. Dry on the left, lubricated on the right. (Image from Nord-Lock Group)

Even if the exact same bolt is reused multiple times (e.g., more than once), it will increase the friction level, resulting in an under-tensioned joint, especially if it's dry and not lubricated. Check out the chart above of experiments run of this exact scenario. The same bolt is re-used multiple times and re-torqued to the same value each time. But the resulting clamp load (bolt tension) drops significantly for the dry bolt (left) after the first use. The difference is still present in the luricated bolt (right), but not nearly as bad. Note that most torque specs on a motorcycle are specified as dry and not lubricated; so if you re-use a bolt multiple times, this is likely happening!

Bolt Tension Uncertainty

Friction isn't the only issue, though. Nothing is perfect in reality, and this leads to more uncertainty. For example, a torque wrench is typically accurate to plus or minus five percent. So that 5% variance leads to uncertainty. Even the very best SnapOn wrenches are rated at ± 2% accuracy. Most of the SnapOn wrenches are rated at 4-6% accuracy. There are other issues of uncertainty in the wrench itself too:

  • Is the torque wrench really nice, middle of the road, or a Harbor Freight special? Typically, the higher end wrenches will be more accurate.
  • How has the wrench been stored? Is it stored at the lowest torque setting? Is it corroded? Has it been dropped? In any of these cases, the wrench can come out of calibration, leading to more uncertainty.
  • How old is the wrench? SnapOn instructions state: “Periodic recalibration is necessary to maintain accuracy”. Has the wrench been calibrated recently?
  • Where in the torque range are you using the wrench? If the wrench is good between 15-200 Nm, it will be more accurate in the middle of that range than at the ends. Trying to torque a bolt to 20 Nm (end of the range) will have a far less accurate outcome than torquing a bolt to 105 Nm (middle of the range) with that same wrench.

Beam Torque Wrench
A simple, beam style, torque wrench

User error comes into play too:

  • Is the user tightening the bolts at the same speed consistently? Torquing things faster or slower will change the effective friction and the resulting torque-tension relationship.
  • Oftentimes, digital torque wrenches don't click when the right torque is achieved. They beep and/or vibrate instead. The odds of someone stopping exactly on the right torque are pretty much zero. You'll end up either over-torquing or under-torquing things by some amount.

Click Torque Wrench
The common, click style torque wrench, matters where you hold the handle!

  • For a double pivot "click" style torque wrench (the most common type of non-digital torque wrench), it actually matters where you hold the wrench when you use it. That may seem counter-intuitive as "torque is torque", however, because of the dual pivot, where you hold the wrench actually does change the torque applied to the fastener! Holding the wrench closer to the bolt will increase the actual torque applied and holding the wrench further away will decrease the actual torque applied, despite the wrench clicking off at the same torque value.
    • Here's a great video about this if you want to get into the math: why grip position on torque wrenches matters
    • Moving your grip around just 50 mm (2 inches) from center can change the actual torque applied by ± 2-6% (depending on the wrench geometry) alone. And even if you're perfectly consistent (you're not), but not on center with where the wrench was calibrated, you'll be applying a torque different than what you intend.
    • The guy in the photo below is way off from where he should be grabbing the wrench. In this case, his torque will be off by nearly 30 percent!
  • In a beam style torque wrench, parallax error can come into play as well.

Click Torque Wrench
This is a screengrab from a Facebook ad selling a series of videos on riding and bike setup tips. Ironically, the use of the torque wrench as shown is very wrong!

You can see that the wrench and user can be a source for error, but as you've read earlier, a bigger uncertainty is the amount of friction in the bolted joint. It's pretty much guaranteed that there's plus or minus 10% uncertainty in the friction. And it wouldn't be unlikely for there to be ± 20% or more depending on the conditions.

So, if you add all those uncertainties up, it's massive! 5% possible error in the torque wrench (or more if it's out of calibration, abused, low quality, etc.), ~5% in user error (possibly more), and 20% or more in friction. That's ± 30%! And that's not just me making up numbers; have a quick Google search for “torque tension relationship uncertainty” and see what you get. Most references state an uncertainty range of resulting tension as ± 20% to ± 35% when using a torque wrench.

For the sake of argument, though, let's be conservative and go with half of my 30% uncertainty: ± 15%. Meaning the for a given torque the resulting tension in the bolt can actually be 15% lower or 15% higher than you intended. For example, if you torque a bolt to 100 Nm with that uncertainty, the resulting tension could fall within the range of 85 Nm - 115 Nm equivalent torque. That's a big range!

Two Misconceptions Involving Bolt Torque

OK, now that we're armed with a little knowledge, let's take a look at two claims that are very popular right now:

1) We can tune a bike's feel by adjusting torque on various bolts.

This one has been popular lately, especially with the rise in popularity of aftermarket engine hangers. There are a lot of people suggesting a small change in torque will make for a noticeable change in feel when riding the bike. The recommended torque changes are usually pretty small. For example, a rather prominent test rider has published a list of “optional” torque specs for a bike. These optional values range from 5% to 18% less than what the manual calls for (interestingly they're also all lower torque values than the OEM specifications).

Let's say you torque an engine hanger on your bike to a particular value using your torque wrench. And your buddy does the same on his bike to the “same” value using his torque wrench. Do you think the two bolts ended up with the same tension (remember, tension is all that matters)? Not likely! It's very possible that the bolt tension in his engine hanger is 15% or more different from yours. Remember the uncertainty section above?

Minor Torque Change
Minor changes in torque are lost in the noise of uncertainty.

Or let's say you've torqued your engine mounts down with the recommended torque, but read that reducing the torque by 5% really helps the feel of the chassis. If the uncertainty in the torque wrench accuracy plus the user error uncertainty adds up to ± 10%, how are you going to know you even made a 5% difference (in bolt tension)? Throw in friction uncertainty (different bikes, different bolts, etc.), and that 5% torque change is meaningless and completely lost in the noise of the uncertainty.

But I know some people won't be convinced by the uncertainty discussion... So, let's pretend for a moment that your torque wrench is 100% accurate, you have zero user errors, and there's no friction variation in the bolts (completely impossible, but for the sake of argument…). With this imaginary scenario, you actually could get your bolt 5% less tensioned than what it was before, but would it change anything? No, it would not!

Remember, the job of the bolt is to hold all the pieces together so they don't move relative to one another. If they're all still stuck together with 5% less torque than they were previously, how has the stiffness (and resulting “flex character”) changed at all? It hasn't!

2) Fixed vs. “Floating” Rear Axle Blocks

Here's another fun one. An axle is just a big bolt with a nut, so everything we've discussed here directly applies. KTM,Husqvarna, and GasGas all use a “fixed” axle block setup while most everyone else uses what has been called a “floating” axle block setup. Really, the only difference between the two is that on the non-nut side, the axle either slips into the block (“floating”) or it's threaded into the block (“fixed”). This is directly analogous to the difference between a regular bolt with a washer under the head and a flange bolt. See the image below, I've colored each part of the two axles types to match up with the equivalent part on the bolts.

Axles are bolts
Axles sets are really just big bolts, washers, and nuts.

KTM calls out a torque of 80 Nm for the axle nut. With this being an M25 x 1.5 thread, that results in about 16,000 N of clamping force on the axle (about 3,600 pounds of force). That's A LOT of force holding everything together! At that level of force, everything is very much stuck together and there's simply no floating involved. The supposed floating block just can't move relative to the axle and it's just as fixed as the “fixed” axle that has the block threaded directly to it. So, there's no change to be felt between the two as they're functionally identical.

There's always an exception...

Exception
Lower triple clamp bolts are an excepton as they're a special case.

OK, things aren't always cut and dry; there is a notable exception to Misconception #1 above: Lower triple clamp bolts. This is a bit of a special case as the lower triple clamp bolts are "squeezing" the triple clamps around the fork tube. If you have a lot of torque (tension) on these bolts, the fork tube becomes distorted. And when this happens, the fork bushngs bind up when passing through this area. This causes a harsh feel in the suspension.

In this case, you only want to tighten the bolts enough so the forks don't slip in the triple clamp. Any tighter and the fork tube will distort and the suspension will bind up. Some people say that you can “tune” the feel here, and that's true, but it's only for the worse! You can get the same effect by increasing your compression damping a few clicks and it will be much more consistent than over-torquing the triple clamp bolts.

Other Things to Note

Galling and the Case for Anti-Seize

Galling is a combination of friction and adhesion. It's a situation where parts that slide against each other (like bolt threads in a nut) essentially weld themselves together. This typically happens with titanium fasteners, aluminum fasteners, and stainless steel fasteners. On a motorcycle, we're usually concerned with titanium bolts threaded into aluminum threads. This is the scenario for a lot of Luxon parts as our bar mounts and triple clamps all come with titanium bolts. But that's why they also come with anti-seize, which helps prevent galling. It's important to realize, though, that anti-seize is a lubricant...

Anti-seize
Loctite C5-A anti-seize is used to prevent galling on our titanium bolts when threaded into aluminum threads.

Speaking of lubricants

Some people like to use grease or oil on bolts instead of installing them dry. This creates a more consistent torque-tension relatonship, especially with multiple uses of the same bolt. In general, it's a good idea, with the only real downside being that lubricated bolts are more likely to come loose than dry bolts, so they should be checked more often.

Lubricating a bolt with oil, grease, or anti-seize (or any lubricant for that matter) reduces the friction in the threads and also under the head of the bolt if it is used there. Remember that torque is related to bolt tension by friction, so a reduction in friction means an increase in bolt tension for the same torque. It is critical that the torque is adjusted when lubricant is used on bolts that have a torque spoecified as dry. The resulting tension can increase by 40% or more going from a dry bolt and a lubricated one for the same torque. People often question the low torque specs on our triple clamps, particularly the bottom clamp that's specified at only 8 Nm. The reason for this is because we use titanium bolts with anti-seize. The 8 Nm spec with anti-seize results in the same bolt tension as a 12 Nm spec using a dry bolt.

In Conclusion

Tension is incredibly important, and torque is a convenient way to measure tension. Always use a quality torque wrench on critical bolts (there are many on your motorcycle), and follow the torque specs and conditions (lubricated, Loctite, etc.) recommended by the manufacturer to be sure you're getting as close to the design tension as possible. Too loose and the parts can slip. Too tight and you may damage the parts, the bolt, or put things into a state of stress that's dangerous and could break while riding! All Luxon triple clamps and bar mounts come with torque specs engraved next to the bolt for easy reference. If you have any questions about torque on our products, please contact us and we'll get you sorted out!

Engraved Torque Spec
We engrave our torque specs directly into our clamps so you don't have to remember. This one even calls out the correct threadlocker.

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Comments (2)

Ivica Zivkovic:

After reading this iwant yourclamps!

paul g breda:

from a CNC process , programmer , shop manager andmachinist with 40 years in the trade , spot on

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