physics

Undercut crankshaft

Undercut crankshaft is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Undercut crankshaft rather than just read about it. In short: An undercut crankshaft is a form of crankshaft for piston engines, where the overall length of the crankshaft is shortened by overlapping the main bearings of the crankshaft with the big end bearings of the connecting rods. Although this seems impossible at first glance, the geometry of the engine does permit this overlap.

Key takeaways

Undercut crankshaft belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Undercut crankshaft to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Undercut crankshaft from memory before moving on to harder problems.

Reference excerpt

An undercut crankshaft is a form of crankshaft for piston engines, where the overall length of the crankshaft is shortened by overlapping the main bearings of the crankshaft with the big end bearings of the connecting rods.

Although this seems impossible at first glance, the geometry of the engine does permit this overlap. The big-end bearings of connecting rods are wider (along the crank axis) than the thickness of the rod itself. By making the crank webs with an offset to them, it is possible to place the main bearings closer together, closely sandwiching the rod passing between them, where this is now a narrower gap than the bearing size. The main bearings are similarly confined and each main bearing can be supported by only a thin wall across the crankcase. The difficulty with these engines is in their manufacture. The overall shape of the crankshaft and its webs is not a problem, but finishing the bearing surfaces by grinding is. There is no longer access for the large abrasive discs of a crankshaft grinder to reach each bearing, the offset crankshaft webs now being in the way. As a result, each bearing must be made and finish-ground individually; then assembled into a built-up crankshaft, usually by shrink-fitting. One of the few engines to use undercut crankshafts was the Bugatti U-16 aircraft engine, a typical Bugatti approach, where only their unlimited budget and attention to detail could afford such complexities. The U-16 engine also had each 8-cylinder crankshaft built as two 4-cylinder crankshafts, assembled by a bolted taper joint. This joint may be seen at the left-hand side of the close-up drawing. The French Ariès narrow-angle V4 of 1907 had a built-up crankshaft with disc webs. The journals were held into the webs by nuts on their ends, the nuts protruding past the webs and having the effect of an undercut crank. The Chaise, also French, 100 hp engine of 1930 used a built-up crankshaft with a 'quincunx' (staggered) cylinder layout to give a short, compact engine.

See also Tunnel crankshaft, another crankshaft design that makes engines shorter overall by combining the crank web and main bearing

References

Illustrations

Undercut crankshaft: Crankshaft close-upNote the tight spacing of the connecting rod between the crankshaft webs in the 3rd cylinder from the left. — Crankshaft close-upNote the tight spacing of the connecting rod between the crankshaft webs in the 3rd cylinder from the left.

Worked examples

Example 1 — a first encounter with Undercut crankshaft

Start with the simplest possible case. Write down what Undercut crankshaft claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Undercut crankshaft before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Undercut crankshaft ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Undercut crankshaft

In research: Undercut crankshaft appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Undercut crankshaft in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Undercut crankshaft is common in secondary-school and first-year university syllabi. It links to neighbouring topics Crankshafts, Mechanical engineering stubs, so understanding it makes those chapters shorter.
In everyday life: Look for Undercut crankshaft outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Undercut crankshaft” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Undercut crankshaft in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Undercut crankshaft means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Undercut crankshaft out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Undercut crankshaft in simple terms?

Why does Undercut crankshaft matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Undercut crankshaft?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Undercut crankshaft.