Twin Talk Torque

[Steve Bass]

Interesting piece about the genesis of the twin and why we see so many 270 degree cross plane twins today..

https://www.youtube.com/watch?v=Qn9JrN1JERI

[sclayto2]

I find most of the F9 videos to be some of the best scripted and produced motorcycle stuff on YT.

it is well worth a trip into their archive.

[PT1984]

Yep! I try, but can rarely keep up with the math.

Interesting that only KTM kept with the original design. A triumph twin does sound lovely though.

Edited by PT1984 on Thursday 1st February 08:59

[gareth_r]

I'm not sure that KTM went for a 75° crank offset (giving 285°/435° firing intervals) out of loyalty to Phil Irving's original concept, which was actually 76°, not 75°.

The "285" parallel twin crank mimics the sound of tne 75° KTM V-twin, just as the "270" crank sounds like a 90° V-twin and the old Husqvarna Nuda sounds like a Harley because it has a "315" crank.

The summary of the history missed out the air-cooled Triumph Scrambler and the Yamaha TDM/TRX, and I think there were special 90° and 76°cranks for BSA, Norton, and XS650 twins before the production Yamahas.

[OutInTheShed]

A 270 degree twin has one piston moving fastest when the other is stopped at TDC or BDC.
The energy content is roughly constant, so the engine naturally runs close to a constant angular speed, whereas a single or 180 or 360 tends to slow down the crank while the pistons speed up.
A 90/270 twin can have a lighter flywheel so pick up quicker for the same level of smoothness.

Obviously for smoothness, triples (except 180 Jota) , sixes and eights are better still...

In the old days, people liked even firing intervals for the sound, and also so a single carb worked better on a twin.

[KTMsm]

sclayto2 said:

I find most of the F9 videos to be some of the best scripted and produced motorcycle stuff on YT.

it is well worth a trip into their archive.

Agreed - a shame that Ryan seems to be taking a step back though

[Steve_H80]

Another good F9 video.

[ChocolateFrog]

Steve Bass said:

Interesting piece about the genesis of the twin and why we see so many 270 degree cross plane twins today..

https://www.youtube.com/watch?v=Qn9JrN1JERI

Is the TLDR because they're cheap?

[black-k1]

As ever from F9, an interesting video based mostly on science/fact.

[Steve Bass]

ChocolateFrog said:

Steve Bass said:

Interesting piece about the genesis of the twin and why we see so many 270 degree cross plane twins today..

https://www.youtube.com/watch?v=Qn9JrN1JERI

Is the TLDR because they're cheap?

No. Have a watch. The emissions related info is very cool and makes perfect sense when compared to a 4 cylinder engine....

[Krikkit]

Yep excellent video, for a bit more info on their character the "driving 4 answers" has quite a good video on parallel twin crank angles:

https://www.youtube.com/watch?v=a9ZFZABaLbg

[iidentifyaswoke]

OutInTheShed said:

Obviously for smoothness, triples (except 180 Jota) , sixes and eights are better still...

I drove a V8 (car) for the first time in about 8 years last month. I'd forgotten how smooth they are

[carinaman]

A response to that Fortnine video. I've not watched so cannot offer an opinion:

https://www.youtube.com/watch?v=w0PBlc1b0vo