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Engineering 101 : The Nuts & Bolts of Torque

They are small bits of metal in a relatively simple machine, but the nuts and bolts on a bike keep the whole thing together. Treat these little guys properly and you are much less likely to come to grief, and you’ll keep your bike running smoothly, efficiently and quietly. 

THIS ARTICLE LOOKS at different aspects of fasteners on bikes, gives relevant background technical information and basic advice on how to approach and tighten these underappreciated items.

WHY DO FASTENERS COME LOOSE?

Riding a bike creates vibrations that are transmitted into the frame and its components. The components are secured using fasteners like nuts, bolts and screws which can work themselves loose during operation. They are manufactured to certain tolerances, with a gap between the threads allowing them to turn in one another.

When we tighten a bolt, we apply torque to the head of it. When the underside of the head contacts the parent material (or washer), friction is generated between these mating surfaces. As we continue tightening the bolt, this friction increases and so too does the friction between the threads due to the fact the bolt starts to stretch. This stretch and resultant friction is what stops the bolt from loosening during operation.

The most common cause of loosening is due to side sliding of the nut or bolt head relative to the joint, resulting in relative motion occurring in the threads. To stop this from occurring, fasteners are installed to a certain tension called preload.

When engineers design components they calculate the required clamping force for safe operation (eg stem face plate holding handlebars). To achieve this clamping force, bolts are carefully selected with bolt material, tensile strength and diameter all playing a part. This will determine the bolt body’s tensile failure point. Two other factors that determine a bolted joint strength are the shear failure point of the bolt’s thread and the internal thread.

WHAT IS TORQUE?

Torque is defined as the rotational force applied to turn an object about its axis, a force that tends to cause rotation, based on Newton’s second law of motion.

Torque is a measurement of energy in the form of inch-pounds, foot-pounds or Newton-meters. The English system uses foot-pounds (ft. lbs). The metric system uses Newton-meters (Nm).

In general terms there are several potential methods to ensure a bolt is tightened correctly, for example: angle-controlled tightening methods are used in the automotive industry for oil filters. Bolt stretch method is used during engine rebuilding for example measuring conrod big end bearings.

The only practical way to ensure bolt tension is correct in the bicycle industry is the use of ‘torque control tightening’ via a torque wrench.

HOW DOES FRICTION AFFECT TORQUE

Whenever a bolt or nut is tightened, the friction between the threads and also the mating surfaces of the bolt head or nut must be overcome. This is where things start to get a little more technical but understanding how friction affects the effective tightening of the bolt is important.

Torque applied to bolt/nut can be broken down into three main components:

1. torque to stretch the bolt is 10%

2. torque required to overcome friction in bolt and nut threads is 40%

3. torque required to overcome friction at fastener head or nut is 50%.

This effectively means that only 1/10th of the applied torque ends up acting as clamping force. The condition of bolts will affect these frictional losses; dirty/ damaged threads and mating surfaces will increase friction. This increase in friction will lead to a lower than expected torque applied to the bolt and an incorrect preload. Ensure threads and surfaces are clean and not corroded etc before assembly.

To ensure accurate torque readings, lubricant is used to reduce frictional losses and also to decrease the failure rate of the bolt and ensure safe riding. This lubricant can be in the form of a liquid, grease or anti-seize compound. Smaller threads (eg stem bolts) require liquid and larger threads (eg pedal axles) require grease or anti-seize compound. It is recommended that fastener threads and mating surfaces are lubricated before assembly unless stated otherwise by the bicycle and/ or component manufacturer but always ensure that any sensitive components in the surrounding area are not contaminated (eg brake pads during caliper bolt tightening).

GALLING AND CORROSION

Galling is caused by adhesion of two sliding surfaces in contact with each other. As the surfaces slide against each other, material is lifted from one and deposited on to the other. This is caused by lack of lubrication.

Galvanic corrosion begins where two dissimilar metals come into contact with an electrolyte. This can occur where the bolt material differs to the parent material it’s screwing into and comes into contact with water. For example, if a stainless steel bolt is fastened with a cad-zinc nut. The corrosion can bond two surfaces together or substantially deform the faces and increase friction, thereby affecting torque values.

JOINT PASTES AND THREAD LOCKERS

A common way to prevent fasteners from working loose is using a thread locking compound. These are commonly referred to by the brand name ‘Loctite’ and are primarily used on brake disc rotor bolts, calliper mounting bolts, suspension mounting bolts and adjustment screws. Threads should be cleaned of grease etc with alcohol based solvent beforehand and depending on the application thread locker is applied to either the bolt or threads prior to assembly.

Retaining compounds are similar to thread lockers but have a higher viscosity. They are sometimes used in applications such as press fit bottom brackets to help eliminate creaking and also on poorly fitting headset cups.

These reduce friction in threaded and press fit applications and also help to prevent corrosion; they are intended for use on steel, aluminium and titanium parts (not carbon).

All metal and carbon surfaces should be coated with assembly paste prior to installation to help prevent creaking, for example, where handlebars contact the stem (never on stem to steerer), headset bearings in head tube, seat post in seat tube.

Metal to metal contact points should have grease applied but on any joint with carbon you should use specially designed carbon paste. This paste contains grit which increases the friction between components to minimise creaking.

WASHERS, NUTS AND BOLTS

Another way to stop fasteners from loosening is by using a lock washer under the bolt head or nut. There are several types but the two most commonly found on bicycles are spring washers or star washers that have serrated edges which bite into the surface of the bolt head or nut and also the parent material.

Another way is by using lock nuts, which are commonly referred to by the trademarked name of Nyloc. They have a captive, undersized nylon insert in the collar which damps vibrations between the nut and bolt and also grips the bolt threads to help prevent untightening.

Bolt material selection factors include strength, brittleness, corrosion resistance, galvanic corrosion properties and cost. With bicycles being subjected to all weather conditions, and rider’s sweat, corrosion resistance is highly desirable. Steel is the most common and cost effective. Stainless steel is highly resistant to corrosion and marking, slightly stronger than un-hardened steel but weaker than hardened. Titanium is a very soft material on its own but can be as strong as mild steel, but not as strong as high-tensile steel bolts. Titanium bolts are very resistant to corrosion, stretch at a much different rate to steel bolts and are around 50% lighter.

Carbon fibre bolts for the bicycle industry are becoming available from limited manufacturers.

CRITICAL AREAS

Due to certain frame and component designs there are a few areas where bolts should always be torqued to manufacturer’s specifications. These include seatposts, for example, where sufficient torque is applied to stop the seatpost from slipping down. On conventional seatpost collar type setups, 5Nm is normally required. Over tightening can lead to cracking the frame!

SUMMARY:

The tensioning provided by bolts and fasteners on a bike is not as simple as it may first appear; friction, lubrication and materials all affect how the applied torque converts to clamping force and whether the bolt remains tight over time.

Anyone working on their own bike should really invest in a small torque wrench kit and be guided by the manufacturer’s recommendations for each part; these recommendations are based on ensuring adequate clamping without compromising component life. Purchase good quality grease, lubricants and carbon compounds and use as recommended by the manufacturer for each component. If in doubt, don’t over tighten. Failure due to over tightening has the potential to cause sudden breakage/failure whereas a fitting coming loose will likely cause a slip or a creak, which while annoying, is less likely to cause a crash or sudden failure. In short, take care of your nuts and bolts and they will take care of you.

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Roe at the Tour of Tasmania, Stage 6 Devonport Criterium.

Timothy Roe

Roe at the Tour of Tasmania, Stage 6 Devonport Criterium.

Battling Bugs in the Bunch