Our engineering editor joins the select banc of 'foreign bodies'
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who have been allowed in to see how high-speed, high-stress tyre testing is carried out at Michelin's proving ground at Clermont Ferrand
FOREIGNERS are not allowed into the Michelin proving ground very often, and for ' "foreigners" read non-Michelin ; personnel. If rumours are to be believed, it is difficult enough for people on the Michelin payroll to get past the gates. So it was with more than usual interest that I accepted an invitation from the famous M. Bibendum to find out a little more about the design and development of new tyres.
The process starts with the basic concept being envisaged in conjunction with the industrial chemist. This leads to the stage where about 20 prototype tyres are made to the new design and tested on a machine to see if there are any basic faults.
If this stage is reached with satisfactory results, a further batch of tyres is produced again for testing by machine for such parameters as side force, slip angle and rolling resistance. It is only after this second stage of machine testing that a prototype gets anywhere near a vehicle. Even then, the vehicle used will probably not be of the type for which the tyre is eventually intended; it will be more of a mobile test bed to measure adhesion, rolling resistance and cornering resistance.
If the new tyre gets through this procedure to everyone's satisfaction, then it will finally be fitted to a production vehicle where a team of testers — the "seat of the pants type" — will assess the tyre on a subjective basis. It is at this stage that Ladoux, Almeria and Laurens come into the picture — these are the names of Michelin testing grounds in France, Spain and the USA.
Ladoux is situated near the Michelin headquarters at Clermont Ferrand in central France and covers about 1,200 acres. The extent of the proving ground is shown in the accompanying map.
The Number 1 outside track is 7.8km (4.8 miles) around and is for high-speed testing. The surface is of a composition which is claimed to guarantee freedom from cracking during the winter frosts. This particular track was designed for use by vehicles running at axle weights up to 20 tonnes. The maximum average speed around the circuit is limited by the performance of the lorry, not the track itself. Cars, for example, can run up to 300krn/h (186mph) as verified by the Formula One Renaults which have been tested there.
The curves are banked to achieve "hands off' with no side slip at 100km/h (62mph).
The circuit numbered 3a in the accompanying map is known as the "canard" track which will be recognised by those readers who enjoy their food as the French for "duck". The reason for this peculiar nickname is that when viewed from above the track resembles a duck complete with beak. That's the Michelin story. This particular circuit is used for handling tests mainly, although it is occasionally brought into use for fatigue testing.
Some of the circuits have their direction of use altered from time to time. The canard track, for example, is covered in a clockwise direction for about 90 per cent of time whereas the high-speed circuit is always covered in an anti-clockwise direction with the overtaking being done on the right. The endurance circuit (number 3 on the map) is used for equal times in each direction, changing over on the hour every hour.
The number three circuit is 2.8km (13/4 miles) long with the radii of the corners varying from 245 to 30m (800 to 100ft) They are spaced to allow the highest possible average speed.
The Ladoux complex was begun about 20 years ago with the circuit with the other tracks being added gradually over the years. The round track (No 61 was used originally to test all the off-road tyres, but most of this work is now carried out at Almeria in Spain with only the smaller off-road tyres being tested at Ladoux.
Ladoux also incorporates a low-adhesion section which has its surface controlled to a very high degree for finish and flat ness. For example, it is flat to within one millimetre in 10 metres (equivalent to around 50 thou in 33ft). This compares with the average flatness of a "normal" road of three millimetres in one metre (about 0.12in in three feet).
Adhesion is, however, very difficult to qualify. It is possible to assess the "feel" of the tyre but extremely difficult to put a figure to it.
As one of the Michelin engineers explained — "you do 20 tests and get 20 different answers".
Drivers on endurance testing are not necessarily expert in the context of the word as normally applied to factory test drivers. This is because Michelin does not want the drivers to have any influence whatsoever on the results.
Expert drivers are, however. used for much of the other work, with the unseating test being an excellent example. In this case the lorry is driven around tight corners at lower and lower tyre pressures until the tyre itself comes off the rim. This test is done on a track with an easily repairable road surface!
By the very nature of endurance testing, testing to destruction or the forementioned unseating test, there is a certain degree of risk involved when the Lyre fails. With the overload tests on the trailers, extra wheels with a smaller diameter than that of the test wheels are bolted to the outside of the axles. If the test Lyre actually fails the load is then taken by the smaller wheel with the driver being warned of what has happened by means of a pressure monitoring system.
One of Michelin's more interesting test chassis is a 230kW (310bhp) V10 engined MAN 4x2 rigid which grosses 16 tonnes. This high power-to-weight ratio is used for high-speed handling and endurance testing. The power allows a high degree of acceleration between the corners to permit a high entry speed. Getting the inside wheels off the ground is the object of the excercise as this scuffs the tyres very badly as the wheels come back down again. Watching the Michelin driver setting up the MAN in what can only be described as a six-wheel drift was an impressive sight. This same MAN is used to test the high-speed stability of the tyres under both wet and dry conditions with the power from the big V10 allowing a high speed to be maintained in spite of the tyre scrub. The test driver during "my" test is highly experienced in this sort of trial and was extremely impressive in the way he flicked the vehicle from lock to lock in and out of a series of cones which formed a tight Sbend. This was on a streaming wet road at a speed I would think twice about using on a straight road in the dry.
To test on-site tyres for earthmoving equipment, seven parallel tracks comprising test complex number five are used. Six of these tracks share the same 10 per cent constant gradient while the remaining track has a slope of increasing gradient with a clay surface to test the tyres adhesion. The other tracks have surfaces composed of either loose blocks of flint or rounded metal spikes.
The spikes have a fatiguing action on the casing without wearing out the rubber while the flint surface is used to test the susceptibility of the tyre to cuts. As the rubber is more prone to damage when wet, the test vehicles drives through a water splash at the bottom of the hill.
This part of the extensive Michelin test programme must rank as one of the most boring driving jobs anywhere in the world. Sitting on top of a huge Caterpillar or Terex on a hot day, up the test slope, round the other side and back again — and so on — and so on — and so on ..
The MAN was also used to demonstrate the testing procedures employed on tracks eight and nine. In essence these are concentric circles with radii of
120 and 60m i390 and' ifeift) using two types of surface; a basalt compound in addition to the familiar tarmac. The tracks Cart again be used dry or wet with 8 set of sprinklers provided a constant degree„of "wetness". The water for the sprinklers IS re.. cycled through a large tank to keep the water specification as consistent as possible. AithatUgh to most people water is water, on tests like this it can became contaminated with water and oit so these a re filtered out.
The basalt compound is toted because it combines the extreme properties of high grip when dry with a very low ,E-, cient of friction when wet down to about 0.1 in fact.
On the wet surface, the tett driver managed to slide the MAN, but only just. As he explained: "With these tyres, 1 need more than 310 horsepower to make it slide." No comment.