Brake failure - shold you change down the gears ?

[Horse]

Keeping with the motorcycle comparisons . . . the perceived wisdom for bike crashing is 'slide, don't roll'. So back to cars, an attempted handbrake turn's out . . .

[Renny]

I've suffered brake failure at least 3 times.

Total brake failure in a Ford D-Series, at low speed (<5mph), resulted in me crashing through the workshop door as I appraoched it and found the brake fluid had leaked from a failed wheel cylinder. Fortunately, noe of my workmates were behind the door. I did have time to apply the handbrake, which had little effect as one drum was soaked in fluid, and kill the engine, which did slow the engine slightly. Learning Outcome - Static brake test is useful

Significant brake fade having driven my rally prepared Sunbeam through Clashindarroch stage (I was Stage Commander) as I was making my way (rapidly) back to the start. Fortunately uphill, and not total failure, so recovered the situation.

Loss of brakes when driving a 38T artic, during sub-zero conditions, noticed as I check-braked on a long descent: Cause, air relay valve frozen closed, resulting in no brakes. I was able to allow the speed to fall using the exhaust brake and down-changes, and stopped safely in a lay-by using the secondary brakes (parking brake). The cure was a kettle of hot water over the valve and some methanol down the air lines back to the trailer.

None of these were pleasant, and none resulted in serious consequences.

[Astraist]

So, worst case you managed to change to and fully engage a 'too low' gear, will the engine go bang? If so, will it stop you? I suppose the options could be the top end turning to shrapnel, a seize (which would lock the driving wheels). Presumably a disel over-revving has differnet outcomes from a petrol engine?

Also, albeit you'll [most of your] lose steering control, would turning off the ignition help or hinder engine braking?

Turning off the ignition results in some retardation, but much less than under a lower gear with a running motor. Steeering control isn't so easily hindered by turning off the ignition - the steering does not lock with the key in the ignition and when at speed - the lack of power steering isn't too dramatic either. Nevertheless, this step is not recommended.

Overreving usually results in valve to-piston contact and failure at the valve lifters, prior to any other kind of damage. Obviously a diesel engine which runs at lower revs and is usually sturdier overall will be a bit more tolerant to slight overrevving, although damage to the crank happens in such motors earlier during over-revving.

I seem to remember Tom Topper recommending grazing walls or barriers

This is a last resort if both engine braking and manual braking (i.e. the handbrake) don't provide good enough a result and it might not slow down the car as quickly just as you might think (especially at motorway speeds). Also, you must scrub very carefully at a sloped angle, particularly if you try it against the divider (which can roll the car).