Total Motorcycle Community Forums

Hi. New member here. This is probably a question for the more technically-minded motorcycle pilots out there, so I guess I'm directing it more at them. But, if you can help me out, please chip in.

I'm in the process of building a kit car, and I plan on using a mid-mounted Honda CBR1100XX Super Blackbird engine. I'm currently attempting to work out a suitable final drive ratio for the car, so I can keep the top end speed low...well, by low I mean "less than 160mph (260kph)", since the tyres I'll be using aren't rated to go any faster than that. Also, -I- don't want to go any faster than that!

I'm using the standard 6-speed gearbox from the bike, and I was given the following info concerning the gear ratios:

Primary Reduction - 1.571 (88/56)
Gear Ratios - 1st 2.769 (36/13); 2nd 2.000 (32/16); 3rd 1.579 (30/19); 4th 1.333 (28/21); 5th 1.167 (28/24); 6th 1.042 (25/24)
Final Reduction - 2.588 (44/17)

What I want to know is, what does 'primary reduction' and 'final reduction' mean and how do they affect the final drive ratio? I need to know so I can work out what axle ratio I need to use and go from there. Any assistance is very much appreciated.

Incidentally, I'm not a motorcycle virgin. I started with a Honda CB250, graduated to a CBR250RR (in Western Australia, the law is you must spend at least one year restricted to motorcycles with an engine capacity of 250cc or less), then moved on to the bigger boys: a CBR400RR (love my Hondas!), Kawasaki ZX-6R Ninja, and now a ZX-9R Ninja. So I'm not new to bikes, I'm just new to the whole how-they-work side of things!

well the primary reduction happens inside the gearbox. final is the sprocket ratio or hypoid in a shafty.
so to maintain the bikes original gearing you'd want a 2.588 in the dif.
i assume the car will be heavier than a motorcycle though, so you should probly go a bit lower.

Primary reduction is the gearset on the clutch cage to the tranny. After that, gear sets are for each gear. Final reduction is the driveshaft gearset.

Just keep dividing the RPM of the motor by each ratio to get the output RPM.

Figure out the RPM of the tire size you want at the MPH speed you want to go.

The RPM of the tire / RPM of the transmission uotput shaft ration will tell you the gear ration you need.

Make sure you check all the gears. A high top speed may be nice, but not if first gear is so tall you can't accelerate.

I would design for first gear and see where the other gears fall.

Thanks very much you two.

Unfortunately I'm stuck with 205/50R15 tyres, since the suspension and bodywork is based around this size, otherwise I'd be using tyres with a lower overall diameter to keep the top end speed down.

I'm expecting the completed car to weigh in at about 450kgs (roughly 1000lbs for my American friends), which is pretty much double what the original Super Blackbird would have weighed in at, so I'm going to need the drop the final ratio quite severely to avoid labouring the engine. It's going to be a lot of work, but it's going to be worth it.

The biggest regret of the whole thing is having a Super Blackbird's running gear and no Super Blackbird to run...

Thanks again.