UK Slip Resistance Group | Frequently Asked Questions

If you have a question which is not answered below please email 'enquiries@ukslipresistance.org.uk'.

Why Do People Slip Over?
When we walk we need friction between the shoe/sole and the ground to enable us to do so. If that friction is not sufficient then we slip.

Why Is A Wet Surface More Slippery Than A Dry One?
The water on the surface acts as a lubricating film in the same way as oil in an engine. It is often known as the "hydrodynamic lubricating film" or "squeeze film".

How Is The Coefficient Of Friction Defined?
If a body of weight W is resting on a horizontal surface and requires a horizontal force F to move it, then the coefficient of friction numerically is F/W and is referred to as µ.

Is There Just One Coefficient?
No, there are two. The first is called the Static Coefficient of Friction. This is related to the force required to cause the body to start moving from rest. The second is the Dynamic Coefficient of Friction. This is related to the force required to keep the body moving at a constant velocity over the surface.

Does A Material Have A Unique Coefficient Of Friction?
No. Friction is dependent on both surfaces. Even if they are the same material, variations can occur depending on the micro-roughness of the two surfaces and the presence of contamination.

Does The Weight Of The Body And The Contact Area Make A Difference To The Coefficient Of Friction?
In certain groups of materials, eg. metals, ceramics, etc., which are hard and non-resilient, µ is to all intents and purposes independent of the weight of the body and the contact area, in other words, the pressure exerted by the body on the sliding surface. This is not however true where one or both of the surfaces is made of a resilient material such as leather, rubber, or plastic; this is critical when it comes to measuring the coefficient of friction in pedestrian slipping since shoe heels are normally made of such materials

Does The Weight Of The Body And The Contact Area Make A Difference To The Coefficient Of Friction?

In certain groups of materials, eg. metals, ceramics, etc., which are hard and non-resilient, µ is to all intents and purposes independent of the weight of the body and the contact area, in other words, the pressure exerted by the body on the sliding surface.

This is not however true where one or both of the surfaces is made of a resilient material such as leather, rubber, or plastic; this is critical when it comes to measuring the coefficient of friction in pedestrian slipping since shoe heels are normally made of such materials

What Is Slip Resistance?
Slip resistance is essentially the coefficient of dynamic friction as related to a pedestrian. It should always be quoted in relation to the contaminant, e.g. dry, water wet, oil wet, and the shoe heel material, the device used to carry out the test and the rubber or other slider used with the device to measure the friction.

Do All Test Machines Measuring Slip Resistance Give The Same Result?
Devices generally work on the principle of measuring the friction between the surface and a resilient slider as it is dragged across the surface. However the speed of the slider, the contact pressure, the resilience of the slider materials and the presence of any lubricants all affect the result. In the dry there is generally not a great deal of difference between different portable devices. In the wet the differences can be significant.

Do All Test Machines Measuring Slip Resistance Give The Same Result?

Devices generally work on the principle of measuring the friction between the surface and a resilient slider as it is dragged across the surface. However the speed of the slider, the contact pressure, the resilience of the slider materials and the presence of any lubricants all affect the result.

In the dry there is generally not a great deal of difference between different portable devices. In the wet the differences can be significant.

Do Any Machines Give The Right Answer?
The right answer is one which correlates with the risk of slipping accidents. Studies over many years and experience in UK has shown that the TRL Pendulum, as described in British Standard BS 7976, does react correctly.

Why Are There Different Slider Materials In The Guideline?
Four S rubber is an abbreviation for Standard Simulated Shoe Sole rubber. It was developed to simulate the performance of shoe soles of the period. It is now also known as Standard Pedestrian Hard Rubber or Slider 96. It has the advantage of being relatively insensitive to temperature. It is the most common material for pedestrian slip evaluation purposes. TRL/ Standard Pedestrian Soft Rubber was initially developed to simulate the rubber used in vehicle tyres. However as shoe sole materials have become softer it may give a better indication of slip resistance in some circumstances. It is particularly suited to measurements in areas where people are barefoot. It should preferably be used at 20ºC but a temperature correction can be made. (a TRL rubber slider is also known as a Slider 55)

Why Are There Different Slider Materials In The Guideline?

Four S rubber is an abbreviation for Standard Simulated Shoe Sole rubber. It was developed to simulate the performance of shoe soles of the period. It is now also known as Standard Pedestrian Hard Rubber or Slider 96. It has the advantage of being relatively insensitive to temperature. It is the most common material for pedestrian slip evaluation purposes.

TRL/ Standard Pedestrian Soft Rubber was initially developed to simulate the rubber used in vehicle tyres. However as shoe sole materials have become softer it may give a better indication of slip resistance in some circumstances. It is particularly suited to measurements in areas where people are barefoot. It should preferably be used at 20ºC but a temperature correction can be made. (a TRL rubber slider is also known as a Slider 55)

How Easy Is It To Use The Test Machines?
The Pendulum requires an experienced operator - it is not an easy machine to handle. There is a procedure set out in the UK Slip Resistance Group Guidelines explaining in detail how to do it. It is particularly important that the machine is set up strictly in accordance with the Guidelines/standard and that the relevant slider is correctly prepared and rejected when too worn.

How Easy Is It To Use The Test Machines?

The Pendulum requires an experienced operator - it is not an easy machine to handle. There is a procedure set out in the UK Slip Resistance Group Guidelines explaining in detail how to do it.

It is particularly important that the machine is set up strictly in accordance with the Guidelines/standard and that the relevant slider is correctly prepared and rejected when too worn.

Where Can I Get Details Of These Machines?
There are two manufacturers of the Pendulum in UK. Their web sites are found via the "Links" page on this website. Munro Group 406 Roding Lane South, Woodford Green, Essex, IG8 8EY. Wessex Engineering . Unit 17 Knightcott Ind. Estate, Banwell,Weston super Mare BS29 6JN.

Where Can I Get Details Of These Machines?

There are two manufacturers of the Pendulum in UK. Their web sites are found via the "Links" page on this website.

Munro Group 406 Roding Lane South, Woodford Green, Essex, IG8 8EY.

Wessex Engineering . Unit 17 Knightcott Ind. Estate, Banwell,Weston super Mare BS29 6JN.

What About Roughness?
Roughness, or more precisely micro-roughness, is particularly important in the wet as it provides drainage paths for the lubricant to escape from beneath the contact area so that the sole is in contact with a dry surface. For a viscous liquid such as oil more roughness is needed than for a less viscous liquid such as water. Roughness of the surface can be a guide to its potential slip resistance. However, it should never be used as a stand alone measurement. Unfortunately, the way roughness affects the slip resistance is very complex and it is not a simple proportional relationship.

What About Roughness?

Roughness, or more precisely micro-roughness, is particularly important in the wet as it provides drainage paths for the lubricant to escape from beneath the contact area so that the sole is in contact with a dry surface. For a viscous liquid such as oil more roughness is needed than for a less viscous liquid such as water.

Roughness of the surface can be a guide to its potential slip resistance. However, it should never be used as a stand alone measurement. Unfortunately, the way roughness affects the slip resistance is very complex and it is not a simple proportional relationship.

How Does One Measure Roughness?
We suggest using a suitable Rz Surface Roughness Meter (links can be found on this website). There are a wide range of roughness parameters. Rz is the parameter recommended to be measured by the UK Slip Resistance Group Guidelines for floor slip resistance. Rz is the maximum peak to valley height of the profile within a sampling length.

Where Can I Find Out More About Slip Resistance And Measurement?
We suggest you look at the "Bibliography" page on this web site.