Intake query

[Paul in NZ]

One of the great myths is that the Japanese are not creative engineers. I personally don’t believe that – I’ve always found them pretty adventurous. Sure – at one stage or other nearly every configuration has been tried but to look at something like an RG500 and say Ariel made a Square 4 years earlier is plain daft – yes of course they did…. Moto Guzzi won with L twins yonks before Ducati was even a company but that’s not the point is it. Engineering is about making things work and hopefully making things (in the case of motorcycles) that people want to buy…

Anyway – most manufacturers are pretty open to any wild engineering idea that works but unfortunately consumers tend to like things they understand. In the case of putting the carbs behind the engine, well that’s not always where they have been but development has shown that in most cases, it’s a good place for em because you can whack a great big airbox around them to smooth the airflow into the intakes.

Whats changed is that tyre and chassis / suspension technology has allowed makers to produce engines with much more power without killing the rider in the 1st 5mins. The weight is now carried higher so that under violent acceleration the weight transfer stops the rear wheel from spinning (I think) which makes the whole bike taller etc. On modern sports bikes with inclined engines that’s an issue because the carbs sit on top of the engine, not behind it making even more height.

The other issue is exhausts exiting from the rear would leave us with a short header pipe which may not be a good thing.

Given the trend to full fairings, you could put any old engine config in there I suppose??

[pete376403]

Japanese manufacturers are most unlikely to deviate from convention.
Also in the 60" & early seventies small trailblikes had a rotary carb tucked in the side of the engine case.

Bridgestone and Kawasaki were the main exponents of rotary valves. Bridgestone were persuaded to go out of the motorcycle business by the other Japanese manufacturers in exchange for agreements to buy tyres.
However it wasnt just little bikes. Both Bridgestone, with a 350 and Kawasaki (250 and 350) made some nice rotary valve twins. And I had a Kawaskai F9, 350 rotary valve single trailbike in 1972.
Suzuki *may* have had rotary valve singles early in the piece, but I don't think Yamaha every did, they were more committed to reed valves

[pete376403]

Exhaust ports were probably placed at the front initially to the hottest part of the motor would get the best shot at cooling, bearing in mind all engines used to be air cooled and exhaust valve steel wasn't too flash. Since then it's probably been more a matter of following tradition and, as Paul in NZ says, getting a decent length of exhaust pipe.

[avgas]

I no like hot pipe near my balls!!!
No but seriously i have thought about this (and ran a few designs), and i don't like hot pipes near my balls. Its prob the main reason why im not a vtwin fan.
There were other conclusions like : too much air is bad and motorbike tuning can be done via appropriate air box etc.
But i no like hot pipe near me nuts

[Coldrider]

Yes Suzuki had rotary valves in the Van Van fat wheel beach bikes, my sister had a 50cc and I knew of another RV90. Not sure of the TS50 and TS100 trail bikes, 125cc plus had conventional carbs.
I remember the 350cc kawa trail bikes, had a fancy tail piece/mud guard like a road bike.

[slowpoke]

The other issue is exhausts exiting from the rear would leave us with a short header pipe which may not be a good thing.

Here's another thing that seems to have a few contradictions. We seem to be seeing more bikes like the R6, ER-6N, GSXR600 etc with extremely short exhaust systems. Even drag cars run very short open primary exhaust systems. Yet the conventional wisdom has it that longer exhausts make better power, so what gives? Note the "pigs tail" to lengthen the exhaust of an RC211V below.

[skidMark]

i used to have a yamaha tzr 250 3MA carbys were on the fron zrost on the back that went under the seat and came out the tail.....

carbys ddnt take up much room at all....

airbox then ran up the front and under the tank....

[Mekk]

All good answers, thanks.

So we understand it could be done, but is there one simple answer as to why it isn't?

Space? Cost? Performance?

Or is it just all of them together?

[slowpoke]

i used to have a yamaha tzr 250 3MA carbys were on the fron zrost on the back that went under the seat and came out the tail.....

carbys ddnt take up much room at all....

airbox then ran up the front and under the tank....

Good point, the compact, low hp nature of a small 2 stroke probably opens up a few more packaging options.
On the other hand a modern litre bike has a frikkin' huge airbox, sometimes with CPU controlled internal flaps or bell mouths etc which severely limit your options. This is a much flasher version of the airbox on my R1. My airbox is actually even bigger with the carbs being completely enclosed.