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!
Merityre are a leading UK independent supplier of car tyres. Why not visit their website at www.merityre.co.uk and see where you can buy your next set of tyres.
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