I recently listened to an interview of a rally competitor describing their tragic engine failure. It was a sad story that I’ve heard before, and had all the hallmarks of a concern I focus on with my customers.
The driver was braking hard, the engine stopped, failed to restart, and was terminally damaged. But what could using the brakes have to do with engine failure? Each time I explain what’s happening, I find the driver is either hearing it for the first time, or perhaps now understanding it..
On the road with an ABS (anti lock braking system), you can slow the vehicle and engine relatively quickly, but still in a controlled fashion with an expected maximum deceleration rate of both the vehicle, and the engine. You can only accelerate or decelerate the engine so quickly when the drivetrain is connected and you haven’t lost grip. Engines are designed to handle those stresses, and no harm is done.
Without ABS, you can lock up the drive wheels, meaning wheels driven by engine power, by applying excess braking force for the current grip level. The drive wheels are mechanically coupled all the way to the engine via the hubs they’re bolted to, the axles, differentials, transmission, clutch etc. That means if the clutch is engaged when all drive wheels stop, that entire rotating assembly stops, including the engine.
While locking up all the drive wheels and stopping the engine isn’t kind to it, it doesn’t necessarily cause engine failure. Things get more dangerous when wheel lockup is rapid. I mostly see this in data from rally cars, but it can happen in any driving situation combining low grip, a capable braking system, and no ABS or an ABS fault. When I review data and give drivers feedback, they usually say they didn’t think they braked very hard, were expecting more grip. Sometimes they don’t realize they stopped the engine, as long as it re-fires as soon as they release the brakes.
That’s how the engine gets stopped, and now I’ll explain the specifics of why this is troubling.
If your engine is at say 4000 RPM, and you hit the brakes in a low grip situation, you can stop those wheels almost instantly. The negative angular acceleration rate is massive, way beyond what’s possible without breaking traction, and a number of components simply aren’t designed to handle that condition.
If the engine goes from 4000 RPM to 0 in 0.1 second during a brake lockup, that’s a deceleration rate of 40,000 RPM per second. If the lockup takes 0.05 seconds, that’s 80,000 RPM per second. Compare that with a max effort ABS stop in a low gear, or flooring the accelerator in first gear. Perhaps you can achieve 4,000 RPM per second. The ABS stop causes a 10-20x slower change in engine speed, and applies 10-20x less force on the engine.
Rapid deceleration of the crank from lockup comes in the form of shock loading rather than progressive pulses from combustion events, and the force can be in excess of 1,000 ft lbs. That sudden massive force applied to the engine’s crankshaft can cause timing to jump, damage a timing belt or chain, shear teeth off or crack cam gears, bend or break rods, damage bearings, and cause all sorts of damage to engine internals, even engine driven accessories.
With my customers, I harp on the simple solution of clutching in for low to medium speed aggressive braking zones like approaching a virtual chicane or hairpin, because that’s where rapid wheel lockup is most likely to occur. On snow and ice with extremely low grip, this is even more critical. Disengaging the engine from the drivetrain allows you to lock wheels up while the engine coasts down safely to idle.
Disconnecting the drivetrain from the engine may not be ideal for platform control and stability in a grip situation, but remember if you’re fully locking drive wheels, the diff(s) and engine braking aren’t helping you anyway. Disengaging the clutch prior to applying the brake avoids the concern of halting the engine entirely. Alternatively, you can also disengage the clutch for part of the braking event rather than all of it to maximize performance. That’s a more advanced technique, certainly not easy, and carries more risk.
When performing failure analysis to avoid repeat failures, see if the datalog shows significant brake force, and the engine stops when drive wheels stop. Fatigue of parts over time can play a roll, and this driving behavior can be the last straw. I tend to see drivers get away with locking the engine up once, twice, sometimes a few more times before killing it. How rapidly crankshaft rotation is halted is a key factor, and that’s determined by the excess of braking force relative to the braking capacity of the current grip level.
In addition to talking drivers through this so they’re aware of the potential risk, I also give them options to visually indicate wheels are locking. This requires wheel speed sensors and electronics that allow me to display that sort of data, which are common to my rally electronics packages.