How to "Vet" a Sporting Car By Captain Richard Twelvetrees A.M.I.Mech.E

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Some Practical Points for the Prospective Purchaser of Used Vehicles.

For the sporting motorist who decides to purchase a new car with the object of gaining laurels in competition work, the guarantees of the makers will be sufficient to assure him as to the suitability of his purchase to perform satisfactorily and to ensure of safety as understood by the speedman. Certain considerations, however, may prompt the prospective owner to investigate the possibilities of purchasing a second-hand car, in which case the problem is apt to be rather more complicated.

In general terms, it may be said that the owner of a genuine sporting model will be so careful to maintain his ” bus ” in first class mechanical order, that there is far less risk in buying a used car of this type, than when the ordinary touring model is purchased after being in the hands of other owners. The price of speed is made up of other factors than progressive design, high class workmanship and careful tuning.

Enormous wear and tear has to be taken into account, the racing enthusiast ever exercising his knowledge and observation to make quite certain that no hidden defects in his car will spoil his chances of success, or cause him any unpremeditated absences from the scenes of his exploits.

The owner of a fast sporting car learns all there is to know about every single detail of the mechanism, calls each bolt and nut by its Christian name, so to speak, and in consequence runs very little risk of untoward happenings (let us call them nothing worse) by reason of mechanical neglect.

But in acquiring a used car, the new owner cannot expect to enjoy such familiarity at first, and so must be prepared to spend some time in cultivating a close acquaintanceship with his new friend the car. This essential sympathy between the car and driver can best be achieved as the result of carefully conducted inspection work, carried on the broad lines indicated below.

Determining Engine Efficiency.
If the prospective purchaser is given the opportunity of watching the performance of the car he intends to purchase when running on the track, or up some recognised test hill, it will be a fairly easy matter to decide whether the engine possesses the necessary amount of “pep.”

The speedometer and revolution counter will tell their own story as to how the engine is behaving itself, besides which, the experienced speedman will be able to draw his own conclusions in this direction.

But the conditions of a sale cannot be expected to include an indefinite number of trial runs, and after a time the vendor may lose interest in customers who fail to conclude the deal.

As a matter of fact, it is possible to derive a great deal of information about the condition of an engine without submitting the car to a test run at all, providing the following investigations are carried out.

A test for compression is all important. Though advice as to testing the compression of each cylinder separately may appear to be of an elementary character, a prospective purchaser may quite easily be misled as to the exact cause of faulty compression. Weak compression in one or more cylinders may be due to such trivial defects as the failure of valve cap washers, neglected valves or ” blown ” sparking plugs.

But it may, on the other hand, be caused by worn or scored cylinders, burnt out valves, or details of an equally serious nature. The possibility of external leakages can soon be disposed of by surrounding valve cap washers, plugs and valve guides with a film of lubricating oil, when the presence of bubbles will reveal leaks as soon as the starting handle is turned.

Supposing, however, one or more cylinders are weak, and no external leakages account for the defect, it is essential to impose a further test. This should consist of removing the crank case inspection covers, or the entire base chamber if necessary, to listen for escapes of compression past the pistons, whilst the starting handle is being turned.

The character of the kiss of air past the pistons will decide whether the escape is due to weak or gummed rings, or if serious scores exist on the cylinder walls. A test for determining the condition of the engine bearings should then be made.

The condition of the internal parts of an engine can best be ascertained when the engine is running under close observation, but the ordinary difficulties of distinguishing the different sounds are increased in the case of diagnosing sporting cars, on account of the deep note of the exhaust.

For a person with a well educated car, the sounds are not very likely to be confused, and by running each cylinder separately from the minimum to maximum degree of throttle opening, it is possible to recognise the harsh metallic ” clinking ” caused by worn gudgeon pins and bushes, as well as the thumping of a loose big-end and the dull rumbling denoting slackness in the main bearings.

Every sound tells its own story, and the discord produced by wear is as painful to the keen motorist as is a false note to the musician.

Acceleration Tests.
When testing an engine for acceleration care must be taken not to give the full throttle opening, unless some kind of a load is imposed. By carefully observing the time in which the ” revs ” increase when the throttle is partly open, one can judge as to the amount of acceleration obtainable.

A sluggish engine will soon show its weak points, when the throttle is handled by someone who knows what real acceleration should be and yet will not cause any damage by misuse of the test. The acceleration test is also useful in revealing any weak points in the engine suspension, or in the members forming the forward part of the chassis.

By alternative acceleration and deceleration the reversal of stress, due to variations of engine torque, reveals any undue spring in the frame or defects in engine suspension. Loose frame rivets, springy cross members or stretched engine bolts can be shown up very clearly by judicious use of the accelerator.

Should the valve mechanism or overhead camshaft gear be entirely enclosed, there may be some difficulty in discovering the amount of wear existing in vital actuating portions, and in such a case it will be desirable to remove the covers to expose important parts.

Backlash, which may have developed to the extent of interfering with accurate timing, can usually be detected by exerting a temporary braking effect either upon the camshaft or the shaft which drives it. Should the application of .a. retarding effort reduce the noise of the backlash, it may be taken that there will be enough lost motion to need attention before the engine can be relied upon to develop its maximum efficiency, or indeed, to possess any satisfactory degree of reliability.

Some Vital Details.
Though the above-mentioned items are the more important to be investigated with regard to the engine, the inspection should also include examination of the lubricating system, the radiator and circulation system, the condition of the carburettor and fuel supply system, as well as of all details of the ignition.

The condition of the bolts and nuts, as well as the general appearance of the engine, will indicate whether it has received considerate treatment or otherwise ; for bad mechanics invariably leave records of their misdeeds in the form of damaged nuts and similar evidences of barbarous methods.

Testing the Clutch.
To judge by the way some drivers use their clutches on sporting cars, it would appear that they regard excessive fierceness as a highly desired quality. One has only to think for a second to realise that an ultrafierce clutch is very nearly as bad as one that slips excessively when taking up the drive. Real efficiency is to be found midway between these two extremes, and the test for this is carried out in the following manner.

Having seen that both brakes are acting properly, the engine is started and the first speed is engaged. The brakes are then applied to the fullest extent, after which the clutch is allowed to engage gradually. If the movement of the clutch pedal and the pressure on the accelerator are regulated judiciously, the clutch should pull the engine up.

Failure to do so will indicate that an undue amount of slip exists between the members of the clutch, a fact that should be noted for further attention. With very powerful engines one would, of course, exercise special care with the accelerator whilst making this test.

Except in cases where unit construction is adopted, it is possible to detect any slight imperfections in the alignment of the engine and gear box by the amount of vibration observed on the clutch shaft. Irregularities in the movements of flexible joints between the two units should be regarded with suspicion, as indicating possible errors of alignment between the engine and gear box.

Steering and Suspension.
The importance of accurate and sensitive steering needs no emphasis for the driver of fast ears, whose experiences may remind him of occasions where much has depended upon the slightest turn of the wheel. In locating the backlash that may be present in the steering gear, one should pay particular attention to its distribution.

Whereas a normal degree of backlash resulting from slight amounts of wear evenly distributed over all the joints need not constitute a danger, a lesser amount due to the partial failure of a single component becomes a very serious matter.

Each possible point liable to wear should be investigated with the greatest of care, and the examination should be carried out in a systematic way by beginning at the steering wheel and tracing every single movement throughout the entire mechanism until the front wheels are reached. This is a very important point in examining any part of a chassis.

Sometimes inspectors are thrown off the scent of important defects by not adhering to a system for doing their work. For example, when examining a part of the steering gear, one may catch sight of a defect somewhere else, leave the steering to look at another part and forget to complete the part of the job that was engaging one’s attention. One section of the chassis should be examined at a time, and completed thoroughly before another is commenced.

Before a car is offered for sale, the original owner may have been tempted to remove a favourite set of shock absorbers for fitting to his new car, with the result that the suspension of the bereaved car will suffer, and may, indeed, be rendered positively unsafe.

When the suspension is examined this detail should be borne in mind, as well as the fact that binding may cover a multitude of sins, or defective spring leaves.

The balance of the wheels has a very marked influence on the accuracy of steering at high speeds, and both front wheels should be spun when raised clear of the ground with a view to ascertaining if they are correctly balanced.

Transmission “Discoveries.”
When one becomes practiced in the art of inspecting cars, it is really extraordinary how many things can be discovered during the simple transmission test. This test was used by the writer when engaged on inspection work for the Director of Transport during the war, and by continual practice he became rather quick at locating defects, much to the discomfiture sometimes of the M.T. drivers and a few of the officers.

The test for transmission which can be applied with equal effect to all classes of chassis, consists of raising one rear wheel clear of the ground engaging the first speed gear, then rocking the raised wheel to and fro against the engine compression through the medium of the whole transmission.

While the transmission test is in progress and the inspector follows each possible detail of wear, item by item, it will be possible to locate faults that may exist from the rear wheel driving splines, through the differential, final drive, along the cardan shaft, and along to the gear box.

The reversal of stresses imposed on the driving mechanism by the jerking of the rear wheel will show up practically every kind of wear between the gear box and rear wheels, besides which the security of the torque member or spherical coupling for the torque tube can be seen at a glance.

Recording Inspection Results.
However carefully an inspection may be carried out, it will lose a great deal of its value if the findings are not recorded.

Enthusiasts who have studied speed work and carry out their activities on a scientific basis, are most meticulously careful about recording the results of inspections, and show exact measurements relating to such wear as may be found, with the object of deciding as to the steps to be taken to reduce it in other cars from which higher speeds are desired.

The value of an inspection report decreases if it is loosely worded, or expressed in such a way as to convey no actual comparisons. Such phrases as “Slight wear on universal joints” may mean nothing or disguise the fact that the joints may be in need of instant repair.

Whether the inspection is carried out for the purpose of ” vetting ” a second hand car or used as a part of a system of scientific speed work, every detail should be carried out with as much care as repairs or any other vital attention to a sporting or racing car.