Casebook

In his final Casebook before taking a sabbatical, Garageman dispenses some knowledge, quiets noisy tyres and corrects formulas

SOME FREE WISDOM

A reader recently asked me to explain why we often manage to solve problems that other workshops cannot. I think the main reason is I have not fallen into the same trap as so many other garage owners. This is best explained by what I call the stages of workshop deterioration.

A workshop is usually started by a single mechanic who no longer wants to work for a boss. If he is experienced and honest, he will make lots of money and soon progress to the stage where he can afford to get an untrained assistant to help with the jobs where extra hands are required to do the dirty work.

If the business is successful, he will eventually employ more mechanics until he can sit in an office all day and keep his hands clean. Eventually, the workshop may get a franchise to sell new cars and the boss will stay at home and get a manager to run the business. At each stage, the quality of workmanship will tend to drop unless the organisation is extremely well managed.

I run our workshop on a profit-sharing basis and conduct regular training sessions with the three mechanics, using the unbelievable material available on the Internet. Hannes is our oldest mechanic and is an expert on vintage Mercs. Syd was our other senior mechanic but he retired some years ago and moved back to England. August, who is 20 years younger than Hannes, then joined us from a Mazda agency. He’s more familiar with computer-diagnostic procedures. Japie is in his early 20s. He used to be our apprentice but is now qualified and usually gets those routine assignments.

NOISY TYRES

Megan Farquhar is not called “Nutmeg” because she’s a little bit nuts but because she runs the local store that sells dried fruit, nuts, spices and other natural products. She also runs an everlasting first-series Hyundai Getz with more than 200 000 km on the odometer.

When Nutmeg recently brought in the car for its usual service, she complained about an annoying cyclic hum at the rear. August’s initial thought was that it was a wheel bearing but, when he took the car for a spin, he discovered the level of noise depended on the road surface. It was completely absent on a dirt road and most prominent on a very smooth tar road. A worn wheel bearing would not behave like that and would be more prominent during cornering.

August ran his hands over the rear tyre

In the interests of protecting him from unwanted fame, stalkers and the paparazzi, CAR cannot reveal the true identity of the resourceful Garageman. Megan Farquhar is not called “Nutmeg” because she’s a little bit nuts

tread but could not feel any unusual wear pattern. He was nevertheless convinced the tyres were to blame, so he “borrowed” two wheels from another customer’s Getz and drove back along his previous route. The noise was now completely absent.

August suspects the tread was slightly damaged by incorrect wheel alignment at the rear wheels. If this is not checked regularly, there is a chance the slowly changing alignment can result in tread wear which increases the noise that a tyre usually makes. This effect would have been masked if the wheels were rotated regularly but very few motorists insist on this procedure. We informed Nutmeg she had to buy two new tyres and arranged to have the wheel alignment reset on all four wheels. It was an expensive solution but the Getz was soon on the road again.

THEORETICAL ERROR

The other day somebody asked me whether young Nick Botha from the farm Mooimeisiesfontein has a nickname. I replied that he hasn’t done anything stupid or noteworthy enough to warrant a descriptive name. Nick is his name but that’s not a nickname … is that a conundrum?

Nick is a third-year mechanical engineering student who recently inherited his grandfather’s old Datsun 1200 GX. He scored so well in a thermodynamics exam that his newfound knowledge goaded him into improving the car’s performance by raising the compression ratio. He removed the cylinder head and measured the combustion chamber volume on one cylinder by laying the head upside down on a bench so that the gasket face was horizontal and he dripped paraffin into the chamber using a burette. He then calculated the amount of material to remove from the gasket face to change the compression ratio from 7:1 to 8:1 and sent the head to a machine shop to perform the modification. When he eventually got the engine together again and ran it, the pinking was so severe it was undriveable.

Nick came to me for advice and I asked to see the formula he used to calculate the compression ratio. He had calculated CR = V/v where V = cylinder-displacement volume and v = combustion-chamber volume. I gently pointed out the correct formula was CR = (V+v)/v. In other words, the CR is equal to the total volume above the piston at bottom dead centre divided by the total volume above the piston at top dead centre.

That small mistake meant Nick tried to run the Datsun on a compression ratio of 12:1!