The Peritrochoid

Welcome To The Peritrochoid!

"Photo of a router housing"

This site is a repository of technical articles regarding the Mazda Rx-7 sports car, with an emphasis on first-generation Rx-7's (1979 to 1985). Most of the words here were sourced from various Rx-7-focused Internet mailing lists that have existed over the years. Many are as old as the late 1990s. I've included e-mail addresses for the original authors of these articles, when available. (Don't be surprised if many of them are no longer valid.) If any of you authors object to this, tell me and I'll promptly make any requested adjustments.

Standard Disclaimer: Neither myself nor the authors take responsibility for any Bad ThingsĀ® that might happen to you or your car, etc. should you choose to follow any of the friendly advice contained in these pages.


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"Geometric construction of a peritrochoid"

What Is A Peritrochoid?

I'm glad you asked! The peritrochoid is the basic curve that forms the inner surface of a Wankel engine's rotor housing. In his book Rotary Engine, author Kenichi Yamamoto, godfather of the Mazda Rx-7, describes it thusly:

It is the actual base for the geometric construction of the rotary engine. The peritrochoid is the locus of the tip point P of an arm fixed on the revolving circle B in Radius q when it rolls along the periphery of the base circle A in Radius p as inscribed.

Got that?


Comments, suggestions, questions, answers? Constructive feedback is always welcomed. Send e-mail to feedback at nellump dot net.

NB: I am not a rotary guru! If you have any specific technical questions, you're better off joining one of the various mailing lists or web forums dedicated to our cars. First gen owners should subscribe to the SA22C list at