story by paul van valkenburgh • photos as credited
Sometimes the simplest, most innocuous question requires a complex and demanding explanation. Or not. For example, say a 7-year old asks, “Dad, where did I come from?” And after enduring a long, strained explanation about the birds and the bees, the child replies, “No, I mean what country!” Recently, a racer’s question regarding the selection of gear ratios led to this analysis of the engineering considerations. The answer really varies a lot, depending on every individual’s specific needs and resources, from the casual amateur autocrosser to professional road racers—or from stock front-drive sedans with few gearing options to V8-powered racers with quick-change transmissions.
If you run the same course a lot, and your car is just the way you bought it, you’ve probably found that the gears don’t match the turn exits or the straights very well. Maybe second gear is too low and you have to shift right away into third, or perhaps that gear is too high and you don’t have enough torque. Or maybe right at the end of the straight you either have to over-rev the engine or upshift for just an instant before braking.
You have three options to improve the situation, although your paths can be limited by budget, available equipment, or the rules dictated by the sanctioning body. You can change tire diameter, but this usually has a minor effect on gear ratio and a major effect on handling and ride height. (Don’t forget that raising the car will also affect aerodynamics.) Your second option is to equip your car with a quick-change transmission with many ratios available in every gear.
As a bonus, this might make it possible for you to get by with just one differential ratio for most tracks. Finally, if there are few or no options for the transmission, then hopefully at least the final drive can be changed to suit different tracks. However, that will change the ratio in every…
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