Why the 270 Degree “Crossplane Twin” is Suddenly So Popular

Summary

This retrospective examines the evolution of twin-cylinder motorcycle engines over the past century, explaining how the predominant designs shifted from 360 to 180 to 270-degree crankshafts. It delves into the engineering considerations of vibration, power, emissions regulations, and manufacturing that influenced these changes. The narrator argues 270-degree 'crossplane' twins are a compromise, as the theoretically optimal angle is actually 285 degrees, which only KTM builds despite higher costs. Overall, an in-depth analysis revealing it took 100 years to fully optimize twin motorcycle engines, though they still aren't quite perfect.

Takeaways

• 😀 Early twins were 360° for smooth power delivery from imprecise carburetors
• 😮 180° twins enabled higher RPMs and power from lighter crankshafts
• 🤔 270° twins balance vibration for bigger, faster pistons
• 😠 Emissions regulations drove adoption of smoother, cleaner twins
• 😡 Total crevice length causes more unburnt hydrocarbons in 4-cylinders
• 😱 Secondary vibration worsens with bigger, faster twins
• 🧐 270° 'crossplane' twins offset this vibration through crank timing
• 🤨 Perfect crossplane is really 285° for secondary force balancing
• 😒 Manufacturing costs have limited adoption of true 285° twins
• 😫 KTM builds proper 285° twins despite higher machine costs

Q & A

Why were early twin-cylinder engines made as 360° twins?

Early twin-cylinder engines were made as 360° twins because that configuration allowed the use of a single carburetor to supply fuel evenly. The paired pistons moving together provided balanced power delivery that worked well with less precise carburetors.

What are the advantages of a 180° twin cylinder layout?

A 180° twin has a lighter crankshaft that can rev higher than a 360° twin. The lighter reciprocating mass allows higher rpm operation. Also, the constant crankcase volume improves efficiency.

Why have manufacturers recently switched to 270° twins?

To meet tighter emissions regulations, manufacturers have switched to 270° twins to reduce unburned hydrocarbons. The 270° layout balances secondary vibrations that become more significant in larger, higher performance twins.

Who first proposed the 270° twin cylinder layout?

Phil Irving first proposed an offset twin cylinder firing order in 1962, but not specifically 270°. He envisioned a 285° firing order to balance secondary vibrations.

Why wasn't the 270° twin adopted earlier?

The manufacturing process for an offset crankshaft was more expensive. It requires an additional twisting operation compared to an inline crankshaft.

What company first produced a 270° twin?

In the 1990s, Dick Cookson built the first 270° twin by modifying a Triumph Bonneville using parts from a nuclear submarine.

When did manufacturers begin adopting 270° twins?

Manufacturers only recently began adopting 270° twins, with Honda releasing their Africa Twin in 2016. Improved and affordable machining made it practical.

What is the optimal offset angle for a crossplane twin?

The optimal angle is actually 285°. This balances secondary vibrations while keeping maximum piston velocity centered between TDC and BDC.

Why are manufacturers using 270° over 285° twins?

The 285° manufacturing process remains more expensive. 270° crankshafts can be forged without an additional twisting operation to create the offset.

Who is producing 285° twins?

Currently, KTM is the only manufacturer producing 285° twins. They have invested in the more complex manufacturing to optimize performance.

Outlines

00:00

😀 Early history of twin cylinder engine designs

The first paragraph discusses the early history of twin cylinder engine designs. It explains how 360° twins were common for feeling powerful due to synchronized pistons and heavy counterweights. 180° twins were better for actual power due to lighter crankshafts that could rev higher. This was the standard for about a century until recently.

05:02

😮 The switch to 270° twins and the benefits

The second paragraph explains the recent switch to 270° twin cylinder engines. This design balances smoothness and power better. The change was driven by new emissions regulations that favor fewer cylinders. 270° twins meet the regulations while avoiding the vibration issues of 360° and 180° designs.

Keywords

💡Inline twin engine

An inline twin engine is a type of internal combustion engine with two cylinders arranged in a straight line. In the video, the narrator discusses the evolution of inline twin engines over the past century, from 360 degree twins to 180 degree twins and finally to 270 degree crossplane twins. He explains the engineering reasons for the different configurations and their tradeoffs between smoothness, vibration, power delivery, etc.

💡360 degree twin

A 360 degree twin engine has two pistons moving together in sync, with crank pins arranged 360 degrees apart. As explained in the video, 360 degree twins have more balanced power delivery and feel smoother due to extra crankshaft counterweights. They were common in early motorcycles when carburetor technology required evenly spaced ignition pulses.

💡180 degree twin

A 180 degree twin has two pistons moving opposite each other, with crank pins arranged 180 degrees apart. As the video explains, 180 degree twins have lighter crankshafts that can rev faster than 360 degree twins. They sacrifice some smoothness for more peak power output.

💡270 degree crossplane twin

A 270 degree crossplane twin staggers the crank pins by 270 degrees instead of 180 or 360. As explained in the video, this provides better secondary balance and smoothness compared to traditional inline twins. It became popular in the 21st century as emissions regulations forced adoption of cleaner-burning twins.

💡Secondary vibration

Secondary vibration refers to the vertical forces exerted on the crankshaft by the pistons and connecting rods. The video explains how secondary vibration becomes more prominent with larger, high performance twins, and 270 degree twins help balance these forces.

💡Emissions regulations

Stringent emissions regulations, especially Euro 5, forced motorcycle manufacturers to adopt cleaner burning twins instead of inline 4 cylinder engines. As explained in the video, twins have lower total crevice volume hence lower unburnt hydrocarbons.

💡Crevice volume

Crevice volume refers to the narrow gaps between the piston and cylinder wall where a small amount of unburnt fuel can hide from combustion. Twins have lower total crevice volume compared to inline 4s, resulting in lower emissions.

💡Smoothness

Smoothness refers to an engine's freedom from vibration, allowing it to operate seamlessly across its power band. As discussed in the video, different twin configurations trade off smoothness against other factors like power delivery.

💡Power delivery

Power delivery refers to an engine's output characteristics - how much power and torque it produces at different engine speeds. As explained in the video, twins optimized for smoothness tend to have more linear power delivery.

💡Fueling

Fueling refers to getting the proper air/fuel mixture into the engine's combustion chambers. As noted in the video, smooth power delivery was important historically when motorcycle carburetors provided inconsistent fueling.

Highlights

360° twins make power every 360° of crankshaft rotation for regular output to allow equal fueling from a single carburetor

360° twins feel heavier than they are due to double the counterweights making the crankshaft heavy

180° twins have lighter crankshafts that can rev faster with no power lost to compressing air in the crankcase

Emissions regulations forced a 40% reduction in unburned hydrocarbons, limiting performance twins to 270° crossplane for balance

Larger, faster pistons in twins caused previously negligible secondary vibration forces to become problematic

Crossplane 270° twins offset the pistons to balance secondary vibration forces

Crossplane concept originated 60 years ago but wasn't feasible until recent affordable tooling

First crossplane twin was built in a shed with parts from a nuclear submarine 30 years ago

Phil Irving actually proposed an optimal 285° firing order, not 270°, for secondary force balance

285° is optimal because piston velocity peaks 75° past top dead center, not 90°

Only KTM builds proper 285° high-performance twins, other brands cite cost barriers

We finally got crossplane twins after 100 years but still didn't fully optimize the design

Manufacturers may someday switch from 270° to optimal 285° twins in the future

Crossplane concept took a century for adoption due to past manufacturing expense

Emissions rules forced adoption of crossplane twins despite higher costs

Transcripts

hello for the past Century everyone made

inline twins like this with 360° cranks

or 180° cranks 100 years do recipes but

just within the last decade every

manufacturer has switched to making

270° twins why the sudden change of

taste if the 270 twin is so great why

didn't anyone build it sooner or maybe

it isn't that great at

all

our earliest Twins were all 360s two

Pistons moving together taking turns

firing see 360 twins make Power every

360° of crankshaft rotation like your

nan on all brand the output is regular

that used to be important because with

equal ye Olden time between firing

hither and firing thither you can get

equal fueling from a single carburetor

and back in the day carbs fueled with

all the Precision of a shotgunned beer

so Engineers wanted to avoid the feudal

task of trying to sink two imprecise

canisters this is why old Brit bikes are

invariably 360 twins but so is my modern

F800 and so is the fat 500 twin air see

when Pistons move together you get

double the counterweights that makes the

crankshaft abnormally heavy so 360 twins

feel Meier than they really are if you

want a small engine to carry its

momentum around like a big one a 360

twin is the way to

go of course Japan was not content with

the mere illusion of power so they

perfected 180°

twins Pistons moving in Balance to each

other with no need for counterweights

these engines feel less substantial but

their lighter crankshafts can rev up

faster and because the crank case volume

stays constant none of that power is

lost to squishing air if you want a

small twin to be as fast as possible

180° is still the right

answer that's how the rc116 hit 21,000

RPM and 16 horsepower with the same

displacement as a shot

[Music]

[Applause]

[Music]

glass okay so a twin that wants to feel

powerful should be a 360 and a twin that

wants to be powerful should be a

180 hm well that about covered us for a

century then what happened we are

obliterating we are

shattering euro5 emissions cut carbon

monoxides from 1.14 G per kilometer to

1.0 nitrous oxides from 009 to 06 and

unburned hydrocarbons from .17 to .100

this is a 12% reduction this is a 33%

reduction but this is a 40 1% reduction

today unburned hydrocarbons are the

limiting factor in making a motorcycle

legal of course our intentions are pure

we want all of our hydrocarbons to burn

that's the boom boom that makes Mother

Nature's panties hit the floor but Dem

government doesn't see our intentions

all they see is the tiny bit of gasoline

that hides in the Piston Ring crevice

and later escapes unburnt now the total

crevice length is just the circumference

of our cylinder High Times bore

multiplied by the number of cylinders

I'll take the Honda Fireblade versus the

Africa twin both precisely 998 CC

engines but4 and one twin the blade has

942 mm of unburned hydrocarbon heidy

places where the Africa twin by having

the number of

cylinders

578 there is our 40% reduction that eur5

requires and there is the reason our

four-cylinder super sports are being

replaced by super twins but super twins

pose a problem of their own see twins

are lovable when they're little but grow

them up and they always turn out

weird and the same is true of engines

it's because of secondary vibration this

upward force that exists whenever a

conrod has to straighten itself so it

exists at both top and bottom dead

center so it's unsolvable because

whatever you do to balance it TDC will

only double it BDC secondary forces are

small in magnitude so they never

mattered for light duty engines when

your Pistons are slow or tiny or both

you can just ignore the problem however

when we started replacing inline fours

with inline twins suddenly we were

asking for very large very fast pistons

and suddenly secondary forces become

significant so suddenly our twins became

270s with this crossplane offset one

piston tops out while the other is

halfway down excellent secondary balance

the smoothness of a 270 twin should

really be felt next to a 360 or 180 so

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Modern Performance twin should be

crossplane soall because a 360 crank has

two crank pins at the top a 180 has one

top and one bottom but a 270 has one top

and one at 90° crossplane crankshaft

this was always the right answer

uncoincidentally it's my favorite engine

on the market so why did it it take so

long to

arrive Phil Irving first proled the idea

60 years ago the drafter of the Vincent

raped was bound to realize a vwin firing

order could be applied to an

inline Phil brought the Revelation to

Triumph in 1962 and was asked to bug her

off simple reason being it's cheaper to

smush metal in One Direction so forged

crankshafts remained flat

plane Phil resolved to build a

crossplane himself but never did

probably because he got busy tuning a

Holden engine into the Repco bra F1 car

that won World Championships in ' 66 and

67 dick cookson picked up the idea from

one of Irving's articles and hacka his

Triumph matis in half shrink fitting the

ends into a central phasing disc stolen

from the Barrow and Furness nuclear

submarine plant so the first road-going

crossplane twin arrived 30 years late

and was built in a

shed but it would be another 30 years

before manufacturers signed on seriously

Honda kicking it off in 2016 with the

Africa twin Yamaha deploying their cp2

to the t7 mto7 xsr 7 R7 G7 Suzuki's new

GSX 8 ailia RS

660 270 twins are all we make now

because in 2023 it's finally affordable

to close die forage in two axis and that

is why it took a 100 years for us to

finally get the best inline

twins except we

didn't not quite read the fine print and

you'll discover Phil Irving never

proposed a 270° firing order in the

first place he proposed

285 see if a piston is stopped at top

and bottom then it's moving fastest

halfway down right so to balance

secondary vibration we'd want the offset

between our cylinders to be half a

stroke right and if TDC is 0° and BDC is

180° half stroke is

90° right wrong and call our stroke X so

this diameter is also X meaning the

cosine of this angle is X over 4X and

the inverse cosine of a quar is

75° huh so a piston moves fastest when

it's 75° after top dead center not 90

meaning a perfect crossplane twin in

terms of secondary imbalance is not a

270 it's a

285 and that still puts us shy of peak

piston velocity after top dead center

but now our vibration is only on one

side of the crankshaft so it's

balanceable 285 is the answer that is

what Phil Irving envisioned 60 years ago

that's what dick cookson built 30 years

ago but it's not what we're being given

today for the same old excuse

manufacturing an oblique offset requires

a twisting stage after for Ing and that

machine is

expensive the only manufacturer building

proper 285° twins is KTM a company that

isn't afraid to spend money and charge

money in the pursuit of absolute

[Music]

performance so 100 years to get a proper

crossplane firing order and we still

cocked it up come back next century when

my grandson will be happy to explain why

manufact suddenly abandoned 270° for

285s

thanks