Rumblings

P.'umblings BOAUERGES

Four-wheelers on the cinders.

AN interesting innovation at the Wembley Speedway was introduced on the first of last month, when H. J. Aldington, Mrs. T. H. Wisdom, and R. J. G. Nash, turned out on FrazerNoshes to set up a car lap tiine for future reference—so to speak. ' H.J." and Mrs. Wisdom had normal sports models—with blowers, while Nash had the " Terror." The first two set up equal times with quite a nice demonstration, but they were leaving it to the " Terror " to stir things up fully. Unfortunately this vehicle chose to be fractious and seized up the blower, so Nash took Mrs. Wisdom's car and proceeded to imagine he was still in the "Terror," with the result that he turned the motor over, with considerable eclat but little damage.

The fact that a normally non-upsettable motor car can be turned over quite easily on cinders may not be obvious at first sight, but things which are obscure are always more interesting, and this little episode leads one to consider the mechanics of cinder tracking, which are rather intriguing.

Some thoughts on cornering.

In the first place there is a very wide difference between the cornering methods and the forces on the car in dirt-tracking, and the same car on a normal corner.

On concrete or tarmac, the sole forces act-'ng on the car, provided that it is rounding the corner at a uniform speed, are (a) centrifugal acting outwards through the centre of gravity of the car, and (b) the frictional acting at the tyres. in exactly the opposite direction to the centrifugal force, and (provided the car IS not going too fast for the corner) equal to the centrifugal force.

Assuming for the sake of argument that on a particular surface the coefficient of friction is 0.5, then the limiting speed of a car on a given corner is that at which the centrifugal force becomes equal to half the weight of the car. This force is proportional toVT2 X w where V is the I Speed and r the radius of the corner, and w the weight of the car. The limiting frictional force is w times P. where p is the coefficient (in this case 0-5)Thus on any corner the limiting 2 Speed is reached when, tvV =pw, or since gr te_iis=o; both sides of the equation, when V2 (V must be in feet per

gr second and r in feet). From this it is obvious that the coefficient of friction—or in ordinary terms the sort of road surface employed—is the deciding factor, and that the greater the friction the greater the speed. Taking an even curve of 100 ft. radius the limiting

speed will be = V.16000 =-40ft. per sec. i.e., just under 30 m.p.h.

All this assumes that the surface is even, as bumps will tend to throw the wheels off the ground, and it is here that springing and shock absorbers become important. When the car is going slightly faster than the limit, if the weight distribution is correct the rear wheels will start to skid, and therefore the car will be moving sideways. This has an immediate braking effect which will bring the speed within the limit, and the car will (we hope) continue normally. If it is going much too fast it will merely slide off the road

Cinder track requirements. It is when we start to apply these principles to cinder tracks that everything goes wrong. It is generally accepted that skidding badly on a corner slows the car down, and yet the only Ray to get round a dirt track at any speed is to skid con

tinuously. I he two sorts of skid are quite different, however, and any attempt to skid the car round without the wheels spinning continuously will only result in the dirt piling up against the outside near wheel and usually overturning the car.

Driving round the corner at a uniform speed in normal road fashion is useless, as the very loose top surface has practically no grip under such conditions and the car immediately starts to slide and consequently to slow down. It is therefore necessary to develop an entirely new method of cornering, and overcome-centrifugal force by some other method. The actual method is to employ engine power to keep the car cornering as well as driving it along. This can only be done by keeping the back wheels spinning absolutely continuously, and keeping the car in such an angle of skid that it will progress round the curve evenly. Aswfar as the angle of skid is concerned this is largely a matter for the driver's skill in controlling the front wheels which, if the car is behaving correctly, will have no sideways force on them, but will be entirely steering wheels and used solely for controlling the skid. The angle also depends on the speed round the corner. In the matter of keeping the wheels spinning, the two factors are engine power and gear ratio. The gear ratio must be

low enough to provide sufficient torque at the rear wheels to spin them continuously. (A solid rear axle is assumed and would be essential.) The ratio must also be sufficiently high to drive the car round the track fast enough. This means that the engine must give a good power output for the whole range, and the maximum revs, must be very high. The actual ratio will be governed at one end by the size of the track, and at the other by the " heaviness" of the track surface. This means that a large track should have a lighter surface, that is one in which it is easier to maintain wheelspin, than a small track, as on the large track the gear ratio must he comparatively high to maintain the required speed, and there will consequently be less torque available at the rear wheels. On a small track the gear ratio can be lower, and the surface can therefore be heavier without " killing" the motor.

I have already said that centrifugal force has to be counteracted by engine p iwer, and the diagram shows how this is done.

G is the centre of gravity of the car and centrifugal force acts through this along the line OX, being the centre of the curve of the track.

The other external force on the car is the general resistance to motion round the track, due to rolling friction, wind resistance, and the general stirring up of cinders in its progress. This force acts on the line AB which is a tangent to the curve, and of course is opposite in direction to the motion of the car.

If the car is to be in a state of equilibrium, in other words if it is to continue steadily as indicated, there must be forces equal and opposite to those given above. This is supplied by the engine—or rather by the rear wheels—and this force acts along the line CD. This can be considered as two component forces at right angles, one acting

towards the centre of the circle, opposing the centrifugal force, and one acting along AB in the direction of motion.

The triangle of forces a, b, c, with a parallel to CD, b parallel to OX and c parallel to AB, shows how these components act and the amount is of course proportional to the length of the sides Thus the sides a and b give the ratio of the propelling force to the anti-centrifugal force for the angle of skid given in the diagram. If the speed round the corner is greater the centrifugal force will be greater and therefore a bigger angle of skid will be required to produce a greater component of the driving force towards the centre. This will mean less force driving the car on its course unless the revs and power of the engine are increased Hence, the fact that a really highly tuned engine is essential for dirt track racing, and that merely going fast into the corner is of no avail, and merely results in stopping or upsetting the car.

The "500."

Having used such a disproportionate amount of space for such a minor subject, I had better return to more normal matters. The 500 miles race was a bigger success than ever this year and the Bentley win was deservedly popular. 118 m.p.h. for 500 miles iequires no further comment It is simply terrific. Let us hope that something will be fixed up so that this firm will be able to come into action again. [Negotiations with Napiers have been completed.—ED .1 They have certainly done too much for British motoring ever to be forgotten. Many of us would have liked to see a Talbot victory, as their wonderfully consistent record deserves to be capped with an outright victory However, second place under the circumstances was a far better performance than many a win we have witnessed. The M.G. speed of over 92 m.p.h. was another amazing show in a wonderful race.

The Rileys had hard luck, but on the other hand they learnt some useful things. They have now got over any crankshaft trouble they used to have, and developed a most amazing speed in the process. They looked almost certain winners till a hitherto unfound clutch defect rut them out of it. The trouble was appzrently due to breaking of the clutch bolts in the light fly wheel, a thing which would never happen in years of normal use, but now that they have found it out will be altered so as never to happen at all. One more instance of the value of racing ! Poor Humphrey's had hard luck in breaking a stub axle with the finish almost in sight.

Another M.G. Record.

A good finish to the "750 " season was the breaking of yet another world's record in a " baby " car which has been achieved, this time, by E. A. D. Eldridge, once the holder of the world's land speed record, and the last to seclre it on ordinary roads.

Driving an M.G. Midget at Montlhery on 17th of last month, he broke the international five kilometre record for cars of under 750 c.c. at 110.28 m.p.h., the highest speed ever yet attained in a baby car.

This new record, incidentally, is identical to two places of decimals with the existing world's record on water secured by Kaye Don in Lord Wakefield's "Miss England II" on Lake Garda.

G. E. T. Eyston was to have gone out in the M.G., but he is still suffering from the injuries he received in his accident at Montlhery some time ago. Eldridge very sportingly took his friend's place. At Montlhery and also at Brooklands many international records have been secured during the past nine months

in a Midget. It was, for instance, the first baby car to exceed 100 m.p.h. and just recently was the first to average 100 m.p.h. for the space of one hour.