TLDR Version:
1) This is not a common issue, but still worth mentioning since I have seen it fail one engine and jump timing on 2 others.
2) Extreme change in engine speed in a very short period of time (extreme negative acceleration of the crankshaft) is the key factor, not starting and stopping the engine or wheels, which we do all the time.
3) Brake lockup can be achieved in a controlled fashion without causing harm, but threshold braking is usually ideal. If you haven’t yet mastered threshold braking, braking a little less aggressively, or clutching in, both avoid accidentally rapidly locking up drive wheels.
Full version:
While we usually think of knock, loss of oil or fuel pressure causing engine failure, there’s another potential cause worth noting so you can avoid it.
I actually work with all my rally customers on avoiding this, and it seems worth sharing publicly because it seems little known or understood. If you’re wondering what braking can have to do with engine failure, you’re in the right place.
With ABS (anti-lock braking system), you can slow the vehicle and the engine relatively quickly. Because ABS avoids lockups, there’s a predictable limit to vehicle and engine deceleration rates because you only have so much grip between the tire and the road. Engines are designed to handle deceleration at that rate without issue, and it stays running.
Without ABS, however, it’s possible to lock the drive wheels by applying more braking force than the available grip can support. The drive wheels are mechanically coupled to the engine through the hubs, axles, differentials, transmission, and clutch. If the clutch is engaged and all drive wheels stop, the entire rotating assembly stops, including the engine.
Stopping a running engine isn’t by itself concerning. We start engines up and turn them off all the time. Doing it with the brakes isn’t actually the concern either. The concern is the potential to negatively accelerate a running engine at an extreme rate, and that can be damaging.
If an engine at 4,000 RPM is forced to zero in 1 second, that’s an angular deceleration rate of 4,000 RPM per second. If it happens in 0.05 seconds, that’s 80,000 RPM per second. That 20x faster change in engine speed applies 20x more torque on the crankshaft.
Instead of progressive combustion related forces rotating the crankshaft, during an extreme over braking event, the crankshaft experiences a sudden shock load that can exceed 1,000 ft-lb and cause timing to jump, damage belts or chains, shear teeth or crack cam gears, bend or break rods, damage bearings, and harm engine driven accessories. Failure may be immediate, or fatigue parts which fail later.
This issue is more common in rally because the cars often have great braking systems, no ABS, and very little grip due to driving on snow, ice, mud, and other low friction surfaces. If you’ve ever test driven a car on a lift to listen for a noise and hit the brakes like you would on the road, it’s shocking and startling how the wheels stop in what seems an instant. That happens because there’s zero grip. In rally with tires on the ground, downward force applied, there’s always some grip. However, in the heat of rally you probably hit the brakes more aggressively than you would on a lift, and sometimes on a road surface with little grip allowing the tires to resist the slowing of the wheels, and the negative acceleration of the wheels can be extreme while the vehicle barely slows down.
The simple solution I teach my drivers is to clutch in any time they’re not confident they won’t lock the wheels up rapidly in a braking zone. Low to medium speed braking zones approaching hairpins or virtual chicanes are where drivers are most likely to accidentally lock things up, and possibly lock them up too quickly.
It’s true that disconnecting the drivetrain isn’t ideal for platform control when grip is available, but if the drive wheels are fully locked, engine braking and differential action aren’t happening anyway. Advanced drivers may only disengage the clutch during specific phases of braking, or may threshold brake without ever clutching in, but that requires precision, and carries additional risk. I suggest working up to those techniques carefully, and if you’re going to lock wheels, try to do it progressively, not all of them all at once.
If you’ve had an engine failure and you’re curious if this was a factor, or simply want to check if you might be fatiguing your engine in this way, check your datalogs for brake force combined with GPS showing the vehicle is still in motion when engine speed and drive wheel speed drop to zero together. Then see how rapidly the engine speed dropped because this is key. If the engine was slowed at an extreme negative acceleration rate, which I’ve shown how to calculate, this was likely a key factor in the failure.
Beyond providing driver feedback and best practices, I can also provide visual wheel lockup indicators using wheel speed sensors and a display. This alerts the driver when they’ve made a mistake to hopefully help avoid repeating it, and in some cases seeing one wheel has locked up gives the driver time to avoid locking all drive wheels, and avoid stopping the engine.