And I thought you guys were open minded about unusual motors
I've never quite figured out how twin engined machines work. Just how do you link two engines together? Is the final drive chain of the front one hooked up to the crank of the second? Or to the final drive cog of the second? Doesn't that put the second engine under twice as much strain as the first? Do you have a single coil/points setup, with two sets of HT leads? Presumably you'd take the gearbox out of the first & rely on the one in the second?
Morrag’s quite right.
It’s worth noting that when describing these things, terminology is important.
For instance. ‘Engine’ on a current jap bike (‘unit’ construction) tends to include the gearbox and clutch (so the engine and gearbox are a unit). There’s only one chain visible (the final drive chain) which goes from the gearbox sprocket to the drive wheel, which makes this chain gearbox-speed dependant.
‘Engine’ on a pre-unit bike (which is basically all the early British and the current big Harleys), is literally the lumpy bits that bolt onto the crank and make it go round & round.
There is then a chain/belt called the primary drive (this is running at engine speed) that connects the engine to the gearbox/clutch.
There is then a second chain/belt (final drive) which runs at gearbox speed to the drive wheel (same as for the unit/Japanese bikes above).
When folk wanted more power ‘back in the day’ it was a matter of tuning and cubes, however the biggest British bikes around 650/750/850 cc. So once the engine was tuned to the hilt (including superchargers), what was left…….. Answer – add another engine. (Busas, GSXRs Z’s were a few years off).
When the drag boys were coupling up these pre-unit engines in the 60s/70’s, they were using the largest engines that were available to them.
At the time the XS650 was first made (and for a year or so after too), it was Yamahas largest road going motorcycle. So the choice to go and get a bigger lump if you wanted Yamaha basically didn’t exist.
When the pre-unit lumps were connected together, a connecting chain went between the crankshaft sprockets of both engines.
Each engine had its own ignition (magneto way back then) and own carbs, which meant set-up was important to try and balance the power from the two engines so they worked together as opposed to against each other (debate still resides today about this).
This combined power was then fed from an extra sprocket on the crank of the rear engine, via a primary drive chain, to a gearbox bolted somewhere on the frame.
You now have to realise that this gearbox was taking the power from two engines so had to put up with a lot, so a strong gearbox and clutch was definitely order-of-the-day, hence the choices Morrag said.
Kick starts on the older bikes work by going through the gearbox/primary chain to the engine, so when you have two engines, kick starting becomes less-easy, so bikes were either push-started, or put on rollers to turn the engines over and fire ‘em into life.
It looks like this double-excess may have had the gearbox from the front engine removed, which implies its either going to use the rear-engine’s gearbox, or, it’ll have the rear gearbox removed as well and use a separate gearbox.
These motors can be made very strong, the grass track brigade take ‘em out to something like 1100/1200cc (??) on their outfits, so I’m guessing that the gearbox could be made to cope with 1300cc ?? As my mate dslam says, its got two choices!
Hope that helps Mr Manky sir.
