Speed, rpm & acceleration explained

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Speed to rpm relationship

Speed is set by rpm. The relationship between speed and rpm is a fixed, linear relationship. It does not change based on rider weight, wind resistance, or anything else. Each time your pistons cycle through one full (four stroke) cycle the crankshaft turns twice. The clutch separates the crankshaft from the transmission's input shaft. When the clutch is engaged, the input shaft gear is turned by the crankshaft gear. As gears are solid things, that ratio is fixed. It will not change as things go faster or if more load is imparted.

The ratios inside the transmission are fixed. You have 6 set ratios - one for each gear. They are metal parts and do not bend, flex or twist; when you turn the input side at one speed, the output side will spin at a fixed speed, matching the ratio of the gears used. If you had a 1:2 (.5) ratio, then for every 2 turns of the input shaft the output shaft would turn once.

The chain is also solid. If it weren't (if it did things like jump teeth, flex, bend, or in any way not be solid) it would be a terribly inefficient way to put power to the ground. So, if the front sprocket turns at one speed, the rear will turn at a speed set by the ratio of the front to the rear sprocket. With a 14 tooth front sprocket and a 45 tooth rear sprocket, it will take exactly 3.214286 turns of the front sprocket to make the rear sprocket turn one full revolution.

The rear sprocket is bolted to the rear wheel. If the rear sprocket turns once, the rear wheel will also turn once.

So, no matter if you're Valentino Rossi or Meatloaf, if you're going the same rpm you'll be doing the same speed. Weight has nothing to do with rpm or speed.

Acceleration

This is where weight matters. Acceleration is the change in speed over time and is determined by the amount of force over the mass trying to speed up (a=F/m). If we consider the force to be strictly the same (see above), then the more we increase the mass, the smaller the ratio becomes, and the less acceleration we have. In other words, the more you weigh, the slower you accelerate.

Along with mass, acceleration is limited by resistance. A larger person will have a larger drag profile. For example, Ru Paul is a much larger man than Chris Rock and also weighs more. Therefore, Chris Rock will accelerate much faster than Ru Paul. Once our demonstrators hit 11,000 rpm in 6th gear, though, they will both just be a blur. Two blurs going the same speed. The difference is, Chris will get there faster.