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Looking at the above title, you could be forgiven for wondering, ‘What on Earth is a CRF?’ .As it happens, a CRF is one of those Michael Caine things: not a lot of people know what one is. To put you out of your misery, a CRF is a car tyre science value. It stands for the Coefficient of Rolling Friction. Still mystified? Don’t be – all will become clear!

A CRF is a figure that relates to how much force it takes for a car tyre to be driven along the road. The CRF is used to calculate the amount of drag, otherwise known as rolling resistance, that the tyre has in use. If you need a still simpler example of what rolling resistance is all about, imagine that you’re trying to ride a bicycle with flat tyres. Is it easy? No it isn’t, because the tyres have a high CRF.

We can use our new knowledge practically, to calculate how much force a typical car uses to push its tyres along a road. The force is needed to overcome friction and we can calculate it simply: if we know the tyre’s CRF and the weight it is carrying, we can calculate the force used.

In this example, our car weighs 4,000 pounds and the tyres have a CRF of 0.015. If we now multiply one figure by the other, we can find out the force in pounds. So, 4,000 x 0.015 equals 60 pounds. Does this car tyre science figure mean anything yet? Not really, because we’ve one more sum to do.

The next calculation uses an equation that can tell us how much power the car uses to push its tyres along. The equation is simple enough and we can use it with some figures that are, again, typical. So, Power equals Force multiplied by Speed. Adding figures into this equation shows that the power used is dependent on the speed at which the car is travelling.

Our theoretical car tyres, therefore, use up more power as the car’s speed increases. At 55 miles per hour, they use 8.8 horsepower. At 75 miles per hour, they consume 12 horsepower. Moreover, like with our flat-tyred bicycle, the lower the air pressure in the tyres, the higher the CRF.

Apart from using more power and therefore burning more fuel, what does this friction do? As you can neither create nor destroy energy, it changes its form. In fact, it turns into heat energy. Should the road surface be soft, more heat is transmitted to the ground and less builds up in the tyres.

This car tyre theory is interesting but asking the question, ‘what is it for’ brings an equally interesting answer. Tyre manufacturers sometimes publish CRF figures for their various kinds of tyre. The lower the CRF the less the rolling resistance or, to put it simply, drag. The less the drag, the better the economy. This also shows what is costly about running your car tyres at too low a pressure. However, remember that wasted fuel isn’t the only cost of under inflation.

Lastly, here are the CRF figures for some typical wheel and tyre (and just wheel) configurations.

A low rolling resistance car tyre’s CRF is 0.006 to 0.01

A conventional car tyre’s CRF is 0.015

A lorry tyre’s CRF is 0.006 to 0.01

A train wheel has a CRF of 0.001

Meaningless figures? Look at the one for the all-iron train wheel and imagine how much engine power is used to keep it rolling. As the Americans say, go figure!

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Appearances can sometimes be deceptive. Like the first part of this title, for example. In this instance, the article isn’t about money, or the consumer price index. It can, however, be about saving you money. Buying a car tyre inflator can be a costly business. On the other hand, since a good one makes inflating your car tyres easier and quicker, it will tend to make you carry out this simple chore more regularly. This in turn means your tyres will remain at the correct pressure; you’ll save money by not burning extra fuel to push under inflated tyres along, your tyres will last longer and will be less likely to fail or suffer damage from overheating.

What are car tyre inflators about? They’re a simple electric pump for blowing up your tyres. Yet, a quick look through the available inflators reveals a lot. Some inflators are powered by your car’s battery, usually via the cigar lighter socket. Some inflators are rechargeable. Some are claimed to be quiet. Some are billed as being rapid inflators – one example inflating a tyre from totally flat to full pressure in four minutes. Others aren’t so rapid – one on test took 15 minutes to do its work.

The car tyre inflator’s abilities don’t stop there. Many have a number of LED lights, so you can utilise their inflating facility at night, which could be useful in an emergency. Most are digital, in that they have a digital readout for the pressure of the tyre to which they’re connected. This readout can usually be set to give a figure in the desired scale, that is pounds per square inch, Bar or kPa (Kilopascals).

Another benefit of a good car tyre inflator is that it can do some of the work for you. True, you have to connect it to the electricity supply (or make sure it’s fully charged). You also have to connect it to the tyre you want to inflate and you ought to oversee what it’s up to. However, the majority of good car tyre inflators feature an auto shut off facility. When the tyre is at the right pressure, the inflator stops inflating – it’s that simple.

All this begs a rather obvious question. Which one do you buy? Well, this is not a buyer’s guide. However, we all know that the Internet is laden with user reviews as well as advertisements. You can spend a happy hour doing your research, on which car tyre inflator has the best reviews.

There are some aspects of car tyre inflators that you may want to consider before buying. A quick look through some of the web tests of these devices reveals much. One point relates to a very old, well-worn adage: you get what you pay for. This is scarcely surprising and you won’t get a lot for very little money. You will have the choice between quiet inflators, fast ones, rugged ones and so forth. However, one overarching drawback seems to apply to even some of the more expensive car tyre inflators. More than one test website offers the advice to add a top-quality tyre pressure gauge to your shopping list. Quite a number of the tests discovered that the on-board digital pressure gauges were not necessarily accurate. Comparison will out, of course, during your research. On the other hand, you shouldn’t really need to buy an additional pressure gauge. As a responsible motorist, mindful of your own safety, that of others and the way in which incorrect pressure shorten tyre life, you already have a quality pressure gauge. Don’t you.

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