Copyright 2003 Craig Seko

When to shift, and why, has always bothered me. None of the explanations made sense to me. Then I read a section on ‘Ratio Theory,’ in Paul Van Valkenburgh’s "Race Car Engineering and Mechanics." One spreadsheet later, and I had my answer:

Gears are shifted when there is more thrust available in the next gear. Thus, for maximum thrust, one shifts: 1->2 at redline, 2->3 at almost redline, and 3->4 and 4->5 a couple hundred rpm below redline! Finally!

Another mystery (to me, anyway) that was solved was one facet of tire selection – gearing. There are two common 16" tire sizes available to me: 245/45x16 and 255/50x16. Let’s look at the thrust they produce:

Within each gear, the smaller 245’s (black lines) show a little over 5% higher thrust than the 255’s (red line) over most of the curve. However, this is misleading. The grey areas show where the smaller tire has the actual advantage, and the green areas, the reverse. Depending on where the revs fall on a particular circuit, the "slower" tire’d car may be faster! (holding all other quantities, e.g., ride height, constant). For example, with the larger tires, I can avoid a time-consuming 4->5->4->3 shift sequence at the end of Mosport’s back straight.

With some astute video, speeds in corners can be recorded. Apex to corner exit speed ranges can be added to the above graph to see where time can be lost or gained.

This doesn’t mean the larger size is necessarily the hot ticket – there are other considerations, e.g., rolling resistance. However, it does make for interesting bench racing.

thrust[lbs] = 0.85 x torque[ft-lbs] x gear ratio x final drive / tire rolling radius[ft]

speed[mph] = 0.072 x engine rpm x tire rolling radius / ( gear ratio x final drive)

tire rolling radius[ft] = 840.34 / tire revs per mile (Tire specs usually have revs per mile.)

I’ve put these equations in a spreadsheet. I don’t claim the spreadsheet is user-friendly. You can’t complain at the price. The data come from the owner’s manual for the Porsche 944 S2, and the specs from the ToyoRacing.com website.