I have a thing for Alfas, just how suitable are they for our roads?

Hi Gavin,

Thanks for your piece on Alfas. I have driven the Alfasud and Alfetta and I am keen on the Giulietta. Any advice regarding their suitability for our not-so-good roads?

 Regards,

Larry M Kīmani

Alfas are no less suitable for our roads than any other make of car designed primarily for tarmac use. They are noted above all for their driveability, based on relatively high performance and handling balance – both of which need a competent suspension and a robust platform/construction. So, although performance cars must be weight conscious and avoid extra heavy-duty components, the good ones are not flimsy.

Most cars are built primarily for highway use, but because local conditions and motoring patterns are sure to include some dirt road driving, two things that might benefit from a little customisation would be ground clearance and tyre profiles.

 Very low-profile tyres are an anomaly in Kenya – we have almost no roads or traffic conditions to fully exploit their potential advantages, and in almost all our conditions they are a disadvantage for comfort, clearance and sidewall and pinch punctures.

You might want to check whether the wheel-size options and wheel-arch space and suspension travel (the amount of up and down movement), allow at least 65 and preferably higher profiles, and that the suspension system is easily amenable to a slight lifting.

Sizing up the pros and cons of bigger tyres

Hi,

I have a Mazda Demio tyre size 175/65/14, I would like to change to 185/70/14. Is this advisable?

Regards,

Ben

Your existing wheel rims should accommodate that size-change in tread width and sidewall height quite safely, as long as there is enough room in the wheel arch and the bigger tyre does not restrict the steering at full lock.

If you can tick those boxes, the change will slightly increase clearance, give you a more cushioned ride over bumps and holes. The tyres will also give better traction on soft ground and will last longer.

They will be slightly heavier (marginally more work for the engine and suspension), give less traction in shallow mud, make a small difference to your odometer/speedometer readings, and give you fractionally higher gearing - you will need a few more revs when moving off from a standing start, but in top gear your car will have ‘longer legs’.

There is a big, big difference between a puncture and a tyre blow-out

Gavin,

Quite a number of accidents reported in the media are attributed to “tyre burst”. Please add to our knowledge on this to help keep us stay safer on the road.

Morwabe J

The most important thing to understand is that “tyre bursts” are very different from “punctures”.

 Both cause the tyre to lose all its air pressure, but with a puncture (where the tyre casing has been pierced by a sharp object) the process usually takes a minute or more so there are usually advance warning signs, and although the air is going-going-gone, the casing is still complete and remains on the rim long enough to bring the car to a reasonably controlled stop.

With a tyre burst, there is a catastrophic failure of the casing itself (it explodes and partially disintegrates, usually with a very loud bang), with total loss of pressure…in a split second! There may be no advance warning and severe loss of control because the casing is no longer complete and its remnants could immediately peel off the rim.

Both punctures and bursts are a hazard to be steadfastly avoided through good tyre condition and care. But punctures are usually just a momentary alarm and a nuisance; an incident. Bursts can readily lead to a fatal accident as the car lurches out of control, perhaps over an embankment on one side or into oncoming traffic on the other; a crash or a roll or both.

Cause and effect

 Tyre casings are extremely tough, made of thick rubber and synthetic compounds reinforced with several plies of an internal webbing made of fibres and/or steel strands. To combine strength and flexible comfort, the tread that is in contact with the road surface is very much thicker, stiffer and stronger than the sidewall and has more plies in its web. The strongest part of all is the beading, which holds the casing to the wheel rim.

The reinforcement of that inner ring is usually a hefty steel cable. Punctures are mostly caused by a sharp and stiff object (a nail, a thorn) piercing a hole through that airtight casing. There are many such objects on almost every road, but the tread is tough enough to resist most of them… until it hits something strong enough and sharp enough at just the right angle for the spike to penetrate the rubber and webbing.

Such a spike will usually fill the hole it has made, so air pressure will escape only slowly and well before the tyre goes completely flat, the driver will notice a change in the car’s handling and take prompt action to slow and stop (at a safe position) while control is still manageable. If the tyre has completely deflated before that action, there will be a flapping noise as the edges of the wheel rim squash the sidewalls onto the road surface.

 Tyres can also suffer gash punctures usually from a piece of metal embedded in the road and projecting like a chisel which can cut even the thick tread, or if the thinner sidewall brushes against a sharp projecting rock.

In these instances, the object which pieced the casing does not stay in the hole it has made, so loss of air can be more rapid…though not usually explosively sudden. There can also be what are called “pinch” punctures, where the tyre passes over a large rock with enough speed and force to squash the sidewall between the rock and the rim so powerfully that the sidewall is split.

 All these types of punctures are less likely if the tread is thick, the sidewall is healthy and the air pressure is at the recommended level or higher. All are more likely if the tread has worn thin and either the pressure is too low or the load is too heavy, causing the vulnerable sidewall to bulge outwards (beyond the reinforced tread shoulder) where it is more exposed to direct assault from the road surface.

For a tyre to burst, explosively, requires more than any of that. It requires a ruinous failure of the casing itself, where the web of fibres built into the rubber are so damaged or weakened that – through a combination of pressure, heat, load, centrifugal force and/or shock – they finally rupture with the violence of all those forces combined.

As I say, the structure of a new tyre casing is extremely strong so, in one way or another, for a tyre to burst, the designed integrity of the casing must have been compromised either by excessive wear or by incremental damage.

The tread of the tyres might be worn completely bald and even down to the fibres. The multiple “ply” of the webbing might have been separated, or torn, by the hammer blows of striking a kerb, sideways or square on. The time has eventually come when the casing is no longer strong enough to contain the pressure, load, heat and other forces. In less than a second, it goes from just managing to giving up. Bang. The weakening process has been progressive. The final moment is instantaneous.

With reputable brands of tyre, with good tread and the proper pressure, the chances of a blow-out are nearly nil. That’s why there are laws about minimum tread depth, and why any bulges or mis-shaping of the tyre walls (sign of ply separation) should be remedied by immediately replacement.

 It is why gaiter repairs to gashes too big for a patch are banned in many countries, not recommended in any, and should be considered only as brief “get you home” remedies and combined with extreme caution.

It is why used tyres with any inner splits should not be bought or sold. It is why new tyres should never be “stuffed” inside each other in ways which distort them beyond their design shape. It is why all tyres have maximum pressure and speed rating limits.

With this budget, pick the car with fewer faults…

Hello,

I am really impressed with the good work you are doing advising people regarding cars. I wish to buy my first car, my budget is Sh600, 000. Between the VW Golf and the Subaru Impreza, which you recently talked about, which one would you advise that I pick?

Regards, Jasper

Because both are sufficiently similar in most respects, check them both out and go for whichever one is available in better condition (age, mileage, appearance, test-drive sound and feel, provenance…) within your budget. If that, too, is similar, then which one do you prefer? Buying at that price isn’t about comparing technical specifications, it is about comparing lack of faults.

If you can, also check out the same models that are a couple of years older; as a forecast of your resale value, which one has depreciated less?

Is this maxim a myth or is it meaningful?

Gavin,

On Jan 4th you explored the relationship between rpm and kph, could you also confirm the myth that any time your rpm is beyond 2000 your fuel consumption increases exponentially?

Ogillo

That maxim is fair general guide to more economical motoring, but in practice, the cause and effect is a lot more complicated than that and has quite a lot to do with what gear you are in, what speed you are travelling at, whether you are going uphill or downhill, whether you are accelerating or cruising (or labouring the engine against load), the shape of your vehicle, the prevailing rolling resistance (surfaces, loads, corners, tyre pressures) and stuff like that.

 True, if the engine is running while the car is standing still in neutral and you want to increase the revs, you press the accelerator (aka increase the amount of fuel delivered) but consumption is probably rising in a straight line rather an exponential curve.

 In practice, leapfrogging over all the technical variables that can make the maxim a myth, the benchmark should perhaps be 80kph rather than 2000 revs, because a primary factor is your speed, hence wind resistance.

 Up to about 60kph (or about 80kph if the vehicle is especially aerodynamic) the wind resistance is low enough not to have a significant effect on fuel consumption. At any speed higher than that, wind resistance does increase, exponentially, to the extent that it becomes a bigger consumer of power (usually gravity, but also an engine) than any other factor. For more science, search references to “terminal velocity”.

Do you have a Motoring question? Email [email protected]