OPINIONS FROM OTHERS.
Page 26
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Risks in Reboring Cylinders.
The Editor, THE COMMERCIAL MOTOR. [2321] Sir,—I recently had an unfortunate experience with a cylinder block in which two of the cylinders had been badly scored, and in view of the possibility of other readers of your journal having the same trouble, it may be of interest to place the matter on record.
decided to have the whole block rebored and new pistons fitted. The block was eventually returned, apparently a good job. The engine was assembled and run on test for some hours, and the vehicle was on the point qf proceeding to work when a pronounced miss was noted in one of the cylinders. Upon, examination, it was found that water was leaking into this particular cylinder. The block was immediately removed and a further examination has shown that it is useless and will have to be scrapped. Incidentally, the block is all in one piece without detachable head.
Admitting that it was not convenient to ascertain the exact thickness of the cylinder wall before reboring, I consider that it was the business of the repairers to satisfy themselves, before undertaking the job, that it would be successful, and in this connection I should welcome views on the subject. I may mention that the quotation for the work did not include any guarantee, but merely mentioned the price.—Yours faithfully, -Manchester.
[We have written to the author of this letter, pointing out that the boring of a worn cylinder is always a matter of some risk, and this is usually understood. It is almost impossible thoroughly to test the thickness of the cylinder walls, particularly in an en bloc casting without a detachable head, such as that to which the letter refers. Apart from this, even a slight amount removed. from a cylinder wall will sometimes disclose metal sufficiently porous to allow seepage of water into the cylirider. The only point which can be criticized is that the block was apparently not sufficiently tested under water pressure before being returned for service.—En., UAL]
Servo-operated Brakes : Does the Power Increase with Speed ?
The Editor, THE COMMERCIAL MOTOR.
[2322] Sir,—In a recent article dealing with servooperated brakes, which appeared in The Commercial Motor, mention was made of these brakes possessing "the added advantage of proportioning the extra power obtained to the speed of the vehicle."
I cannot quite agree with this remark. The maximum effort on the driver's part, in operating the servo-motor, is approximately constant, and the co-efficient of friction of the iming fitted falls away, if anything, as the speed increases. Row, then, can the effort produced increase with the speed?
It would seem that what you suggest is that the reaction of a disc brake is affected by the speed of the rubbing parts, or, alternatively, that the principles* underlying the measurement of torque by the Prony brake are erroneous.
I respectfully submit that the reaction of a friction brake of any type is directly proportional to one factor only, assuming the co-efficient of friction to be constant, and this factor is the pressure applied to the friction surfaces. Furthermore, the reaction in question is calculated by simply multiplying the pressure applied by the co-efficient of friction.
Now, the friction-disc servo-motor is merely a disc brake in which this reaction is employed to operate further brakes, instead of being absorbed by chassis parts, and there can be no question of its " taking charge" as mentioned on page 18 of your issue dated February 17th, its effect being directly proportional to the driver's pressure on the pedal. In other words, if the effort-were plotted out as a curve, this would show merely a straight line of constant value until the very moment motion
ceases.—Yours faithfully, F.L.H. Buxton.
[This letter contains some extremely ,interesting views on the matter of servo-operated brakes and raises a point regarding the important question as to . whether or not the servo action increases with the speed. This is a matter which deserves' -to be thrashed out, and we shall be glad to learn the views of our readers on the subject, particularly from those who have had practical experi-, mace with servo-assisted braking systems.—En., C.2/1.1.
Should the Use of Solid Tyres be Discouraged?
The Editor, TEE COMMERCIAL MOTOR.
[23231 Sir —We are afraid we cannot always agree with what Sir,—We Edge' says, but on the question of the relative damage done to roads by solid tyres, a.s cornpared with pneumatics, we feel there is no shadow of doubt, as is evidenced by the state of any road over which solid tyres are run consistently. , There-are very few motorists who have not known roads which have, been consistently good for a long' period of time, but have quickly become almost unusable when adopted as bus or char-iiabancs routes. Therefore, surely it is in the interests of the rate and tax payers to do all they can to encourage, either by legislation or otherwise, the use of pneumatic tyres. We ourselves, have always felt that legislation could operate to considerable advantage in this direction by taxing the vehicle in accordance with the damage it does to the roads. Undoubtedly, there are complications which would arise in the administration of such legislation, but we do feel that the results would warrant it.
We also feel that everything should be done to encourage the distribution of the weight of heavy vehicles over as large an area of road surface as possilalé,. either by the adoption of twin wheels or by six wheels instead of four.
. We certainly think that some of the very large vehicles which have adopted six wheels have created a prejudice against their use, but in this case the additional wheels have not been added in consideration of the roads, but rather for economy in the general construction of the vehic]e.—YourS faithfully,
HARVEY FROST AND CO. (1924)3 LTD.,
WILLIAM FROST, Director and General Manager. London.