I wouldn’t expect to find myself analyzing motorcycle gear positions or engine speeds often, but I do. If you’ve got a motorcycle at final rest with the tach stuck at 7,400 RPM, know the bike was in 3rd gear at impact, or have video/audio of the approach to AOI, a proper analysis can bolster your reconstruction.

The related calculations are straightforward: divide the engine speed by the product of the primary drive ratio, gear ratio, and final drive ratio to determine the rear wheel speed (in RPMs). From there, calculate the circumference of the rear tire to convert that angular speed to a roadway speed. The ratios can be found in almost all owner’s manuals. Check out this prior post to find manuals for most motorcycles.

If you’d like to corroborate your calculations, or create some accompanying graphics, Gearing Commander is an excellent resource. Select the bike of interest and the site will load the appropriate ratios and prepare related tables and graphs. I’ve had conversations with the owner of the site; he acquires all ratios and tire sizing straight from owner’s and service manuals, so it's well-sourced.

The site allows you to create graphs like the one below, quickly relating a specific gear and engine speed with a roadway speed (assuming the bike is approximately upright).

Forensic miracle worker, Jarrod Carter, pointed me to another useful gearing analysis tool, which can be found here. With this site, you’ll have to find the ratios and tire sizing yourself, but it provides some different (and useful) graphs like the one below.

Considering the nature of our field, I always perform the calculations myself, but supplement with a tool like Gearing Commander. Such platforms offer quick iteration, appealing graphics, and just as importantly, peace of mind in the form of a quick confirmation of my calculations.

Thanks for reading, keep exploring!

Lou Peck
​Lightpoint | Axiom

P.S. Me trying to do any serious work on a Friday afternoon.​

P.P.S. I'm off next week hanging with family, but will see you for the December 6th installment!