eli5 How are torque specs determined for various parts on a car? And what would happen if you increased them by something like 25% across the board?
In: Engineering
Torque spec is normally determined by the fasteners size, material of the fasteners, and what you’re threading in to. 25% on most fasteners aren’t much until you get into the high torque range (like where 25% might be another 50 ft/lb), threading into a soft metal like aluminum, or working with tiny fasteners.
At a simplified level, the torque spec is determined by how much static friction is required to keep the fastener from loosening itself over time, and how much torque the fastener and threads can take before failing. Over tightening a fastener doesn’t make the connection any stronger, and can cause damage to the materials.
The required strength of the mechanical joint versus the strength of the fastener and the materials being joined. For areas with significant dynamic loads, vibrations and fatigue are taken in to account.
Increasing the torque beyond the spec for a fastener can cause the fastener or surrounding materials to fail. For example, if you over-torque a bolt, it will permanently deform, weakening it because it becomes thinner. You might also crack surrounding materials.
Usually, there’s a large safety margin between the spec torque and the point at which the joint will fail. After all, torque is a proxy for the holding power (tension) of the bolt. Various factors, such as the friction between the threads and the accuracy of the torque wrench can affect the actual load applied when installing a fastener. Whether there will be an issue with the car if you increase all torque by 25% is hard to say. Usually the margins (if given) are around 10% of the specified torque, but the actual point to failure is likely much greater than that. For a lot of publications, such as engine repair manuals, there isn’t a margin given, but only a single specified torque.
Vehicle OEM engineer here. To add to the good answers already here, many fastener torques are based on prior experience.
For example a new subframe with a similar duty cycle to a previous one would start with a common mounting fastener torque. Testing would look for movement or shuffling in the joint and correct any issues.
New designs would base initial torques and fastener sizing on calculation, competitor benchmarking, existing availability of parts and previous experience.
Critical fasteners will usually be calculated and FEA tested before tooling, and once prototyped typically checked with a load cell measurement for clamp load and variance.
As for overtightening, depends on the joint. Cylinder head bolts would probably fail, wheel bolts will probably be fine. Probably is the important term here as they’re all unique.
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