Engine Power requirements
The main constraints on high speed performance are the bikes restraining loads, ( eg aerodynamic, inertial, and rolling resistance). Against these loads (which all change during the course of the run), the power from the engine has to be transferred to the drive wheel and hence ground. Of these factors, two of the most sensitive to good performance are the low coefficient of friction (slipperyness) between tyre and ground, and the lowered power availability from the engine at Bonneville caused by the low pressure, high temperature conditions.
The initial concept was to use the engine naturally aspirated in the lower and middle gears, and then introduce massive amounts of additional power, (up to 850hp) from the engine by bursts of nitrous injection in the top two gears. This concept has evolved through the realisation that by better management of the loading effects upon the rear wheel more tractive effort can be used to propel the bike forward in the lower gears.
Below is a graph which shows the expected speed versus distance profile of the bike, gear change points are indicated by gaps.
So in summary, for the existing design, the low speed section of the run, power is limited entirely by wheel spin caused by lack of grip, and at the top end by what is available from the engine. This is shown in the graph below which plots the amount of power that is used to propel the bike forward in each gear. The straight lines of the power in 1st and 2nd gear are limited just by traction, the levels of power used in top gears are minimised to achieve the target average speed of 400mph, without overloading the engine. (Note these are true Power at the Wheel, bhp values).
Drive Train Selection
The initial design intention was to use a toothed belt drive rather than a conventional chain drive system typically used by other record breaking bikes. Running on the salt at Bonneville, the un-lubricated non metallic, low power loss belt option looked attractive, and with new drive belt technology developments the team was, (and still is), eager to exploit any advantage over our rivals. Considerable time and effort was spent in pursuing this design path, selecting belts, and optimising power and gear and pulley details to make this option feasible. Promising belt and pulley selections and exotic layouts with intermediate layshafts however all resulted in drive train layouts that exceeded the permissible space envelope of the bike. The design layout below shows the two rear belt gears that would clash, and exceed the allowed width at the rear axle of the bike where a slender taper to the outside body of the bike is needed to reduce aerodynamic loss.
With a review of the power requirements and available gearbox and gear selections currently underway, chains have made a comeback.
Gearbox gear and differential ratios have been selected from standard available gear train sets. The one exception however is the final gear ratio that with the chain ratio has been selected to obtain a track speed over 400mph.
Selection criteria included looking at the unboosted predicted performance, comparisons between energy that could be transferred to the ground for different gear selection but with the same engine loads, followed by comparisons with Nitrous boost, and how much N2O boosting was needed to obtain the 400mph goal, (while not inducing too much wheel spin and with a view to keep the peak power levels from the engine as low as possible).