Gear reduction in bike engines

[nvpF1crazy]

I came across two types of gear reductions in bike specs: primary and secondary. I read that the primary is the gearbox and secondary is the chain drive. The secondary gear reduction ratio is the ratio of rear sprocket rpm to front sprocket rpm. What is the primary ratio?

Eg: For a CBR250R the primary/secondary is 2.808 / 2.714

[nvpF1crazy]

I came across two types of gear reductions in bike specs: primary and secondary. I read that the primary is the gearbox and secondary is the chain drive. The secondary gear reduction ratio is the ratio of rear sprocket rpm to front sprocket rpm. What is the primary ratio?

Eg: For a CBR250R the primary/secondary is 2.808 / 2.714

[Kirk Feldkamp]

The power generated at the crank is transmitted to the clutch basket through a gearset. The gear on the crank is smaller than the one on the basket. This is the "primary" ratio.

-Kirk

[nvpF1crazy]

Oh! Never heard of it http://fsae.com/groupee_common/emoticons/icon_frown.gif So this gearset is there on all bikes?
So while providing gear ratios do bike manufacturers include the primary ratio or is it to be multiplied separately?

[Kirk Feldkamp]

I don't know if it's on *all* bikes, but it's on everything that you'll find on this forum. The primary reduction is always quoted separately from the gear ratios in the transmission. You use the primary reduction just the same way you would use any other gear ratio within the system. You can come up with one overall ratio for the entire system simply by multiplying all of the ratios together.

Ratio = Output Teeth / Input Teeth
Torque Output = Torque Input * Ratio
RPM Output = RPM Input / Ratio

-Kirk

[Spetsnazos]

I forgot the primary reduction in my lap sim and was getting all sorts of cooky results. Primary reduction is a sneaky bastard!