Brake Fade and a Worn Tyre on the Mdina Descent
You are driving a fully loaded five-door car down the steep, winding descent from Mdina towards Mtarfa at about 50 km/h. For the last two kilometres you have stayed in third gear but have also been on the footbrake almost continuously. The pedal now feels spongy and travels further before it bites. There is a hot, acrid smell from the wheels and the red brake warning light has come on. The tyre pressure warning light is also lit. Two weeks ago you checked the tyres cold: front left 1.5 mm, front right 3.5 mm, rears 4 mm, all set to the handbook pressures. There is a 125 cc scooter following about 10 metres behind. The next lay-by is 400 metres further down, and there is a space-saver spare in the boot marked with a maximum speed of 80 km/h.
1. What is happening to the brakes, and what do you do in the next 400 metres? Explain why pressing harder is the wrong instinct.
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Diagnosis: brake fade. Continuous use on a descent turns speed into heat faster than the discs and fluid can shed it. The fluid can boil and become compressible, which is exactly why the pedal feels long and spongy, and why the warning light has come on. Pressing harder is the wrong instinct because you are adding heat, not stopping force; the harder and longer you hold the pedal, the worse the fade becomes.
Next 400 metres.
- Hazards on, to warn the scooter sitting 10 metres behind - he has no idea what is happening and needs the earliest possible signal.
- Off the footbrake. Select second gear and let engine braking do the work of holding your speed.
- If you must shed speed, use short, firm brake applications and then release, so air can cool the discs.
- Do not use the handbrake for ordinary slowing; it acts on the rear wheels only and can lock them on a wet or gravelly surface.
- Pull into the lay-by, stop on level ground, leave it in gear, and let everything cool. Never pour water on hot discs. Do not continue until the cause is checked.
2. The front left tyre is at 1.5 mm. Explain the legal and safety position, and what the pressure warning light does and does not tell you.
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Legal position. The minimum tread depth is 1.6 mm across the central three-quarters of the tread width, around the whole circumference. At 1.5 mm the tyre is illegal and must be replaced, not just monitored. A worn tyre also disperses water far less effectively, which matters on a wet limestone road.
Safety position. The 2 mm difference between the front left (1.5 mm) and front right (3.5 mm) matters as much as the absolute figure. Grip and aquaplaning resistance are now mismatched across the axle, so under hard braking the car will pull towards the better tyre and ABS will intervene unevenly.
The warning light. It is usually indirect, comparing wheel speeds through the ABS sensors. It flags a pressure loss relative to the others - a slow puncture, a leaking valve, a tyre hot from braking. It does not measure tread depth and it will not tell you the tyre is illegal or that pressures are correct. Check cold with a gauge, fill to the handbook figures, and recalibrate the system afterwards. Replace the front pair together rather than leaving a 2 mm imbalance.
3. Can you fit the space-saver and carry on at the 80 km/h limit to your destination 30 km away? Justify your answer.
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No, on both counts.
The 80 km/h marking is a ceiling, not a target. A space-saver is a temporary wheel with less rubber, less grip and a different rolling diameter. With a fully loaded car, on a wet descent, and with a braking system that has just overheated, you should be well below that figure - and on the shortest route to a tyre fitter, not a 30 km journey.
Systems. The different circumference can confuse ABS, stability control and the indirect pressure warning, so expect warnings and do not treat them as faults to ignore.
The brakes. Continuing at speed immediately after fade simply recreates the heat problem, and you still have the rest of the descent to manage.
Procedure. Stop on level ground, chock the wheels, loosen the wheel bolts before jacking, fit the space-saver, torque the bolts, check its pressure - space-savers are often found soft because nobody checks them - and replace the full-size wheel as soon as possible. Do not mix a space-saver with a worn tyre on the same axle.