KTM upgraded parallel twin on way

[Richard Simpson Mark II]

Apparently, the 270 parallel twin was chosen to emulate the sound and vibration qualities of a harley? Bikers were complaining that the Japanese in-line 4 was Too smooth and quiet, and therefore bland.

People like Honda must have experimented with the offset setup and timing for ages, to get the right sort of vibration that resonated with bikers' bums lol.

My NC750 is such a 270 twin, and honda have certainly got the depth of exhaust note and inner rumble just right. Strangely, they say they left in the over-rumble when the engine is labouring, with some sort of plausible explanation, but I think it is more to do with overall compromise between fuel efficiency, sound, vibrations, and power.

I think it's to replicate a 90 degree because the firing order on a 270 matches the 90. Always thought it was properly kicked off by Yamaha with the TRX850 wanting it to mimic a Ducati!

IIRC the 270 crank was developed initially for the Yamaha works Dakar twin to give a 'rougher' power output, making them more like a single and better able to find traction...it then found its way into the road range via the TRX850.
Gray from SWTRF had a Super Tenere special, which IIRC, had an 850cc 270 degree engine, plus uprated motocross suspension.

[Toe]

Apparently, the 270 parallel twin was chosen to emulate the sound and vibration qualities of a harley? Bikers were complaining that the Japanese in-line 4 was Too smooth and quiet, and therefore bland.

People like Honda must have experimented with the offset setup and timing for ages, to get the right sort of vibration that resonated with bikers' bums lol.

My NC750 is such a 270 twin, and honda have certainly got the depth of exhaust note and inner rumble just right. Strangely, they say they left in the over-rumble when the engine is labouring, with some sort of plausible explanation, but I think it is more to do with overall compromise between fuel efficiency, sound, vibrations, and power.

I think it's to replicate a 90 degree because the firing order on a 270 matches the 90. Always thought it was properly kicked off by Yamaha with the TRX850 wanting it to mimic a Ducati!

IIRC the 270 crank was developed initially for the Yamaha works Dakar twin to give a 'rougher' power output, making them more like a single and better able to find traction...it then found its way into the road range via the TRX850.
Gray from SWTRF had a Super Tenere special, which IIRC, had an 850cc 270 degree engine, plus uprated motocross suspension.

The power delivery reason would make sense then. It was always commentated when Ducati still used their 90 v-twin in racing that the power delivery was smoother or less likely to break traction. The firing order and intervals are the same on the 270 parallel - the second cylinder fires 270 degrees after the first, followed by a gap of 450 degrees until the first cylinder fires again.
And a link for garyboy to visualise https://en.wikipedia.org/wiki/Straight- ... angles.gif

[Richard Simpson Mark II]

same principle as the 'big bang' engines in 500cc GP bikes.

Get the cylinders firing in close succession and you then have a period in which the crank is freewheeling and traction can be recovered.

Back in the day, my employer sponsored a rider on a 580 CCM. After it did a full-on practice racing start you could actually see where the back wheel had momentarily spun up on the firing stroke by a series of indentations in ground. It only had 3 gears, and after the start it would usually be in second gear for most of a given track...changing into third if there was a long straight.

[Scott_rider]

same principle as the 'big bang' engines in 500cc GP bikes.

Get the cylinders firing in close succession and you then have a period in which the crank is freewheeling and traction can be recovered.

…or spit the rider off the bike…which they seemed to do quite frequently

I seem to remember that one rider said one of the bikes was almost unrideable…might have been Doohan, maybe?

[Richard Simpson Mark II]

Yes, that was pre BB.

I think it was Doohan who realised that the way to stay on was to always rev the bike beyond peak torque unless it was in a straight line, that way if the revs went up (as the rear wheel span) the torque fell off and traction improved.
At less than peak torque, when the bike lent over the decrease in the rolling circumference of the tyre forced the engine revs, and hence the torque, up and the wheel would spin up on an already reduced contact patch. If the rider didn't 'correct' by throttling off then the back wheel would slide out and dump him on the floor, if he did throttle off too abruptly, the back wheel would suddenly 'bite' the tarmac, the bike would lift upright and throw the rider up in the air.
So, yes, once the bike was in that situation it was pretty much doomed!
Amazing that they could find anyone to ride them, really!

[garyboy]

Thanks, Toe .. the piston firing animation clearly shows what I was not comprehending.. That in All cases, each piston only fires when at TDC and when the cranked bit (of the crankshaft) is in direct line with the con rod. .. its a straight line. in all cases. Its like the top of the piston is pull-stretched to extremity point, straightening the line of conrod and crank. just as in a single cylinder. .. .. it is the position of attachment on the crankshaft that varies with each conrod.

The other thing is .. the firing order , after the first, the nearest firebang is at the furthest connection, say the 270, not the 90, and then an even longer delay (540) for the next pair of firebangs.

[garyboy]

Torque .. I've been trying, lol, to figure out how different torque characteristics apply to use on the trails, say between the DR350 and the CRF250LRally.

DR350 is a phenomenal mud plugger with loads of really usable trail torque, while the CRF250Rally has a beautiful rolling movement at low speeds that can ramp up when needed and is very usable on backroad tarmac.

The DR engine is slightly undersquare 3"×3" while the crf is well undersquare at 3"×2.5". They both have a 3" bore but the Dr has a longer stroke and so more torque, but the crf is gonna rev higher and smoother?

Power to weight ratio is going to favour the DR but the crf,s charm is in its easy power take up?

Tractability ?? dunno

[minkyhead]

same principle as the 'big bang' engines in 500cc GP bikes.

Get the cylinders firing in close succession and you then have a period in which the crank is freewheeling and traction can be recovered.

…or spit the rider off the bike…which they seemed to do quite frequently

I seem to remember that one rider said one of the bikes was almost unrideable…might have been Doohan, maybe?

wonder how they would be now with traction control