Workshop: Why torque wrenches are invaluable

By James Huang | Wednesday, March 17, 2010 9.00am

Torque wrenches were once considered an optional luxury, even in better bike shops. But advances in frame and componentry technology mean even the most finely calibrated 'mechanic's feel' can no longer substitute for the real thing. Consider yourself warned, says BikeRadar's technical editor, James Huang.

Today's high-end parts are lighter, stiffer, often stronger, and definitely more expensive, and yet simultaneously more delicate as they’re increasingly engineered to fulfill very specific functional requirements and little more. 

As riders continually demand more efficient uses of material, designers must prescribe more narrowly defined requirements when it comes to how those bits are installed and serviced. Whereas back in the day we used to see snapped bolts and stripped threads, it's become increasingly common to see cracked parts and broken bikes instead – the fasteners are no longer the weak link like they used to be.

"Products of the past were limited in their design by the manufacturing technology that was available to make them rather than the loads the products would see in use," says Garrett Smith, engineer manager for SRAM's California development centre and former stress analyst for Lockheed Martin Astronautics (in other words, a bona fide rocket scientist).

"Engineers could only make tubes so thin, or forgings so complex, because of manufacturing constraints. So you could say that the strength of the part was driven by manufacturing rather than use. But modern manufacturing technology is no longer the constraint that limits how thin a tube can be; the limit is now being driven by the loads the product actually experiences in the field."

"The design of our frames and components is always oriented towards light weight," says Scott engineer Benoit Grelier. "Thanks to our experience and FEM analysis, we can optimise precisely all wall thicknesses and we can use different materials depending on the forces involved. But this also implies that we have less margin for the tightening torques."

The proliferation of carbon fibre in cycling means the proper torque is even more vital. "While torque specifications are important with all type of materials, the growing number of carbon fibre components has led to an increasing focus on torque specifications," says Pedro's engineer Jay Seiter. "While carbon fibre allows for more optimal design and provides a far higher strength-to-weight ratio compared to steel and aluminium, it's also more susceptible to crushing and cracking when improperly set up."

Torque is the most readily accessible indicator of the tension in the fastener, and subsequently the amount of force applied to the parts that are clamped or otherwise held together. Knowledge of what those forces are supposed to be is exactly what is allowing the industry to push the envelope on equipment weights.

Alberto contador's personal mechanic, faustino munoz, uses his torque wrench on virtually every bolt on the 2009 tour de france champion's rig – including on the brake pad holder as seen here:

Alberto Contador's mechanic, Faustino Munoz, uses his torque wrench on virtually every bolt

See those little torque markings on your parts? Use them

Car mechanics generally have to resort to extensive reference manuals to determine the proper torque spec on a particular fastener but we're much luckier – most critical parts now have the specifications etched or marked right on them so there's little chance for error. That is, assuming you pay them heed.

A spec of 5Nm doesn't mean 'snug' and 15Nm doesn't translate as 'tight', and regardless of what you may believe, so-called 'mechanic's feel' can at best only provide a close approximation of reality. Even the most finely calibrated hands are still both imprecise and inconsistent, varying by the day, mood and routine of their owner.

"Having and developing a correct 'feel' when tightening fasteners is still important to the professional mechanic; there are many cases where the part can't be fitted with a socket or a torque wrench is simply not available," says Park Tool's resident guru Calvin Jones. "[But] I've seen people be off a torque spec by as much as 50 percent, either too loose or too tight. As a rule, untrained people tend to under-tighten the higher torque values and then over-tighten the lower torques. 

"Given a limited number of steel fasteners, a mechanic might learn by trial and error, by failure, how much to load a thread. Now throw in aluminium threads, carbon parts, magnesium and titanium, where the feel and even the desired loads are different, and you're asking for trouble."

Studies conducted by torque industry specialists JH Williams Tool Group/CDI Torque Products (a division of Snap-On) yield similar conclusions, using a more high-tech test apparatus that accurately measures actual applied torque on a load cell in relation to what the subject thinks he's applying. 

"At Interbike [2008], we had a transducer set-up to a 0.25in breaker bar and hex bit socket, and we asked for volunteers to tighten the bolts at the stem/steerer to the recommended torque value [4.5Nm – a common spec for those types of parts]," said marketing product manager Glenn Kalnins. "Of the nine volunteers, only one person was within spec (+/- four percent) of the 4.5Nm value [and] some people pulled five and six times the value that was set on our clicker.

"Imagine your stem faceplate needs to be tightened to 5Nm to hold your brand new carbon bar in place and you or your local mechanic tightens it to 25Nm or 30Nm. This variability is totally unacceptable in terms of safety and liability in the bicycle industry."

Given that quoted safety factors in some high-end parts can be as low as 1.1 (meaning that exceeding the spec by just 10 percent can result in failure), it doesn't take much of a ham-fisted mechanic to do some real damage. Conversely, the narrow window also means it's easy to leave a bolt too loose, resulting in unwanted movement or slip.

JH williams tool group/cdi torque products' torque measuring apparatus readily demonstrates how much variance there can be in 'mechanic's feel' when it comes to torque: jh williams tool group/cdi torque products' torque measuring apparatus readily demonstrates how much variance there can be in 'mechanic's feel' when it comes to torque

JH Williams Tool Group/CDI Torque Products' torque measuring apparatus

Torque wrench types

Torque wrenches remove the guesswork from tightening fasteners and their wide range of available types and costs mean that integrating one into your home toolbox is now easier than ever – and strongly recommended, if you couldn't tell by now.

So-called 'clicker' wrenches such as the Park TW-5 and TW-6, Pedro's Demi-Torque, Effetto Mariposa Giustaforza and Syntace Torque Tool are the most common and readily available, operating via an internal spring mechanism that gives way when the desired torque is achieved. Both fixed-torque and adjustable models are currently on the market, and Kalnins stresses the 'sweet spot' for adjustable models is 30-80 percent of the calibrated range. For more specific applications, small (and cheaper) fixed-setting tools such as Ritchey's Torque Key can be had.

Proper usage and tool care is essential, though. According to Kalnins, the most common errors are continuing to apply force after the 'click' or applying torque too quickly (which can lead to overshooting the target), holding the wrench incorrectly (he says to always use the designated handle) or putting the tool away in anything other than its lowest torque setting (which can affect the calibration). Used correctly and adequately maintained, though, clickers' claimed accuracy of around +/- five percent should suffice for most applications.

Home mechanics on a budget can still find reasonable accuracy in a simple 'bending beam' type of torque wrench such as Park's TW-1 and TW-2. Devoid of mechanical joints, these operate on the principle of calibrated flex: simply insert the tool bit into the fastener, hold the handle and apply torque until the indicator needle moves to the desired value. Bending beam wrenches are prone to damage if improperly stored (the needle is often exposed) and improper readings can result if the user isn't looking straight on at the gauge. In addition, they're often quite big and cumbersome, and ill suited for portable use.

For well-heeled mechanics, the ultimate torque wrench is a digital type such as CDI's Computorq3, which offers a +/- two percent claimed accuracy by foregoing mechanical devices for a solid-state electronic force transducer. Most models include an easy-to-read (and interpret) LCD display that readily converts between various units, too, and in the case of the Computorq3, a three-stage LED and audible tone also warns users when the preset torque is near. Even more useful, however, is that it stores the applied torque, which also serves double-duty to train mechanics on proper usage. That extra functionality comes at a high price, though. Suggested retail price on the Computorq3 is over US$500 (approx £330).

CDI's high-zoot computorq3 torque wrench is expensive and cumbersome but extremely accurate and coaches its user on proper technique: cdi's high-zoot computorq3 torque wrench is expensive and cumbersome but extremely accurate and coaches its user on proper technique

CDI's Computorq3 torque wrench is expensive and cumbersome but extremely accurate

Other notes

Fastener and part conditions can drastically affect torque readings – and the more important actual bolt tension – so be sure to follow indications provided by the component, not the tool, manufacturer. Should the bolt be greased or should threadlock be applied? What if the bolt is rusty? All of these factors can affect the reading and not always in the direction of prudence.

Likewise, clicker-type wrenches should be periodically calibrated for optimum accuracy as values can drift over time. Kalnins also recommends that buyers stick to wrenches that include a calibration certificate from a qualified third party, such as the National Institute of Standards and Technology (NIST) in the US or equivalent.  However, even a slightly out-of-calibration torque wrench will still almost certainly boast better accuracy than your bare hands.

The consequences for failure

The cost of a torque wrench may be high but it can be a small price to pay compared to the cost of failure. Take BMC's Pro Machine SLC01, for example, which uses a carbon fibre seatpost clamp integrated into the frame structure. The torque spec on the lower bolt is just 8Nm and 5Nm on the upper – exceed either of those values by too much and you've basically got yourself a £3,500 trainer bike. That's an extreme example, though one we've witnessed first-hand (a little too first-hand, in fact).

Other failures can be far less expensive but more catastrophic: carbon bars can break during a sprint, steerer tubes can crack, cranks can come undone. In more fortunate situations, improperly torqued parts will fail during servicing instead of while on a ride, or items will simply loosen up without causing a crash. In any event, the negative effects can range from merely inconvenient to much, much worse.

From a technology standpoint, more rigorous adherence to torque standards may allow designers to build ever lighter and higher-performance bits in the future. "If stricter torque values are adhered to, products can be made lighter and lighter yet keep all the required strength," says FSA technical support manager Dae Oh.  "Currently, a lot of the overbuilding of products is to survive improper installation."  

In other words, proper torque can mean safer and more hassle-free riding, cost savings in the long run and lighter and more reliable parts for all of us. Sounds like a win-win to us.

Over or under torquing a part can lead to a minor inconvenience at best or a very expensive mistake as seen in this cracked integrated seatpost collar on a bmc pro machine slc01. at worst, it can also lead to a catastrophic component failure during a ride:

Over or under torquing a part can be an expensive mistake, as on this Pro Machine SLC01

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