Tyre Temperature versus Tyre Pressure: Which gauge to use?

Adam Fellows - Monday, December 19, 2016

I am often asked about tyre temperature. In response, my direct question is “What is the temperature you are attempting to measure?” “Well tyre temperature of course”. When asked “what section of the tyre are you thinking of measuring” is usually met with blank looks. Tyres are composite construction, they are made from various and quite different rubbers, fabrics and steel in varying configurations. A race car tyre has a very lightweight construction compared to an OTR (mining) tyre and looks for traction as the ultimate property whereas the OTR tyre seeks load and life as prime qualities. The tread face of a racing slick may exceed 400˚C during hard braking. If an OTR tyre exceeds 150˚C at the tread belt interface it will more than likely suffer a heat separation and potentially fail catastrophically. A passenger car tyre can be sitting in the sun and rise tens of degrees without actually doing any work.

Many non‐tyre people relate to the ideal gas equation that relates exactly for rigid pressure vessels,we know the P1V1/T1 equation and I suggest it seems that this equation doesn’t exactly relate to composite flexible pressure vessels also known as tyres. The variance in a tyre’s volume during operation throws the calculation into chaos. Yes, a tyre is elastic, stretching and constantly being reshaped in response to load, speed, the interaction with the pavement, add friction, and the calculation becomes horrendous. Major manufacturers have all attempted to establish the correlation but it has continued to allude those who seek this Holy Grail.

The concept that a tyre’s cold pressure can be calculated from a hot pressure of fraught with unknowns and external influences such as wheel hub heat, brake radiant heat, being on the sunny side of a vehicle. The list of variables grows not to mention air quality but that’s another story for later.

At a point the person asking for temperature realises they have walked into a Tardis like situation where what appeared to be a simple external appearance takes on complexities of ever increasing magnitudes. Eyes widen and expressions indicate realisation that the Rumsfeld principle had hit home, they don’t know what they don’t know.

A tyre of any size is a container for the inflation medium. Whilst there may be macro variances within a tyre’s air chamber resulting from fluid dynamics and thermodynamic activity the pressure acting on the valve stem remains consistent, of course it varies as the tyre works but the applied pressure on the valve stem remains consistent (agreed not static!).

Think of a pimped up car with bling spinner wheel covers, you know the ones that keep spinning after the vehicle has stopped. Now, think about the air inside the tyre that has its own inertia and mass and will keep on moving even when the tyre is at rest until it consumes the inertial energy imparted by the motion of the vehicle. This motion causes eddies and vortexes that influencetemperatures and macro pressures, there are situations of partial pressures. When a giant OTR tyre stops turning the hot air will rise to the top of the air chamber as the laws of convection apply. Even in a TBR tyre, this is easily measurable.

Rubber is an extremely poor conductor of heat, with a heat transfer rate magnitudes lower than metals. This phenomenon is what permits the racecar tyre to survive the tortuous nature of having rubber applied to the pavement chasing traction. Rubber will commence reversion in a small temperature range about 105˚C when measured at the critical point between the belts and tread.Tread face and air chamber temperatures may be related as is the tread/belt interface but measuring it is a physical impossibility with current technology.

The inflation medium on the other hand imparts a consistent pressure on the valve stem from where it is trying to escape. The whole body of air is pushing on the valve core and valve cap as fluid dynamics dictate and as fluids move from a high pressure to a low pressure situation it makes logical sense to measure the pressure at the valve stem. Of course, this is how the industry has been measuring a tyres pressure since, well Mr Dunlop and Goodyear developed rubber‐impregnated cotton that formed the first pneumatic tyres.

So the question then arises how is a tyres performance measured when it is in use? It’s not possibly to attach a traditional inflation gauge to the valve stem but with the advent of microelectronics it isnow possible to attach a sensor and miniature radio transmitter to the valve stem relaying the tyre’s pressure to a monitor so that real time operating pressures can be observed.

With Nano‐technology developing rapidly, it is almost certain that tiny sensors can be manufactured to measure the temperature at different parts of a tyre. With carbon tube and piezo technology a particle that generates its own power when in motion and then transmits the data its internal sensors collect is no longer the thoughts of sci‐fi, it’s around the corner but for the moment we have to measure what we can, as we can.

Pressure is the prime metric for measuring the performance of a tyre, the consistency and accuracy of pressure to a tyres work capacity is well established. Road race teams as well as mining technicians utilise pressure as an indicator of the work a tyre has experienced. Having personally undertaken hundreds of tyre temperature tests for various manufacturers of OTR tyres I am yet to find any correlation between internal structure temperatures and surface temperatures even though the locations maybe only tens of millimetres apart. Using pressure / temperature devices that bleed air over a sensor, I am yet to find any direct correlation between the bled air temperatures to the critical temperature at the tread/belt interface.

When someone asks for tyre temperature my response is to simply ask, “Which temperature would you like?”, and then use the Tardis analogy, the complexities of the composite nature of a tyre are hidden from external gaze. On initial thought, a tyre is simple, as we industry members know a tyre is far from being a simple machine.

If we can’t save you money on your fleet operations, we’ll refund our costs at the 12 months mark,when we save your operation money we’ll accept a 25% share of those savings.

Look after your tyres and in turn, they’ll look after you, ignore them at your own peril.

Adam Gosling
TyreSafe Australia Pty Ltd