Old 03-04-2008, 08:16 AM
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Foxy
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Default FAQ: HPI Baja Top Speed and Gearing info...

I was wondering about 5b gearing, and ratios etc, as a full set of steel gears is not in the budget right now, so I got to calculating the ratios using only the available hop up pinions, and the stock plastic spur. Assuming a gearbox reduction of 2.4, a tire diameter of 170mm, and a top engine speed of 19,000rpm, the ratios and top speeds came out as follows.

Pinion (T) / Spur (T) / Final Ratio / Rollout (mm) / Top Speed (mph)

16 / 57 / 8.550 / 62.46 / 44.2mph
17 / 57 / 8.047 / 66.37 / 47.0mph (stock setup)
18 / 57 / 7.600 / 70.27 / 49.8mph

For comparison, here are the calcs from the popular RR steel gearsets...

16 / 58 / 8.700 / 61.39 / 43.5mph (Torque set)
18 / 56 / 7.467 / 71.53 / 50.7mph (Speed set)
18 / 55 / 7.333 / 72.83 / 51.6mph (Top Speed set)
19 / 55 / 6.947 / 76.87 / 54.5mph (Super Speed set)

Although rpm/torque loss caused by drivetrain friction and engine/pipe choice, may lower these figures a little, the swelling of the tires at speed will cause an increase in final drive, balancing out somewhat. For example, if the tire diameter expands to 180mm, for an increase in total tire diameter by 10mm, the theoretical top speed of the 'Super Speed' set increases by 3.2mph.