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Twin Cylinder Design Theory #727759
03/06/18 6:11 pm
03/06/18 6:11 pm
Joined: Feb 2018
Posts: 101
Oklahoma
T
Tracey Spear Offline OP
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Posts: 101
Oklahoma
Here's a question for the true engine mechanics out there. This is something probably elementary I should know, but don't.

In the vertical twins, both pistons rise and fall at the same time, sometime firing both at the same time, sometimes alternating. But this design has inherent vibration that is difficult to balance out.

Why isn't the crank designed so that the pistons alternate? Wouldn't that balance out better?

Obviously there is something fundamental I'm missing.

Tracey

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Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727760
03/06/18 6:15 pm
03/06/18 6:15 pm
Joined: Aug 2001
Posts: 7,672
scotland
triton thrasher Online content
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No vertical twins fire together.


Amateur Loctite enthusiast.
Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727761
03/06/18 6:25 pm
03/06/18 6:25 pm
Joined: Aug 2001
Posts: 7,672
scotland
triton thrasher Online content
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One up-one down (180 degree) four stroke twins have been sold in large numbers by Honda. Their old CD range of single carburettor twins in the 1960s were 360 degree, like a Brit twin, but the CB twin carburettor twins were 180 degree, for better balance. It's supposed to be hard to make a single carburettor work well on an uneven-firing twin, such as a 180 degree.

As well as wanting to sell single carburettor twins, the British factories believed that an even exhaust note was a selling point. The 250 Norton Jubilee is supposed to have been tried as a 180 degree twin, as a prototype, but reached the market as a 360.

A 180 degree twin evens out most of the up and down imbalance of the pistons changing direction, but balance is not perfect. The engine is rocked from side to side as its pistons change direction at top and bottom of their stroke. Also, two pistons changing direction at the same time in either type of twin takes a bit of energy out of the flywheel and gives the whole engine unit a kick in the opposite direction of rotation from that of the crankshaft.


Amateur Loctite enthusiast.
Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727767
03/06/18 6:46 pm
03/06/18 6:46 pm
Joined: Feb 2018
Posts: 101
Oklahoma
T
Tracey Spear Offline OP
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T

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Posts: 101
Oklahoma
Thanks Triton, nice reply

Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727768
03/06/18 6:58 pm
03/06/18 6:58 pm
Joined: Feb 2018
Posts: 101
Oklahoma
T
Tracey Spear Offline OP
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T

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Posts: 101
Oklahoma
Hey Triton! Since your name brought it to mind. What the hell's going on with three cylinders? Are they spaced 120 degrees?

Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727771
03/06/18 7:20 pm
03/06/18 7:20 pm
Joined: Aug 2015
Posts: 1,301
Bolton Lancs UK
A
Andy Higham Offline
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Bolton Lancs UK
The smoothest twin cylinder configuration is a 90 deg V twin, a "boxer" flat twin is also quite well balanced
As far as parallel twins go the 360 degree version is about as bad as it gets, the 180 degree twin has less primary vibration but secondary high frequency vibration can be a problem. Some people have had good results with crank pins spaced 90 degrees apart


BSA B31 500cc "Stargazer"
Greeves 200cc "Blue Meanie"
Greeves 350
Greeves 360 Challenger
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500cc sprint/LSR bike "Deofol"
Jawa 500cc "Llareggub"
2003 Aprilia RSV Mille "Lo Stregone"
'35 OK Supreme
'36 OK Supreme
Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727772
03/06/18 7:30 pm
03/06/18 7:30 pm
Joined: Aug 2001
Posts: 7,672
scotland
triton thrasher Online content
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Originally Posted by Tracey Spear
Hey Triton! Since your name brought it to mind. What the hell's going on with three cylinders? Are they spaced 120 degrees?


BSA/Triumph triples were 120 degree.

Some Laverda triples were 180 degree: one up, two down.


Amateur Loctite enthusiast.
Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727773
03/06/18 7:34 pm
03/06/18 7:34 pm
Joined: Aug 2001
Posts: 7,672
scotland
triton thrasher Online content
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My own 650 Triton doesn't feel bad until it gets well past 6,000 rpm and I don't do those revs often.

It "doesn't feel bad" but I did literally lose fillings from my teeth when I first put it on the road.

That's a 360 degree twin.


Amateur Loctite enthusiast.
Re: Twin Cylinder Design Theory [Re: Andy Higham] #727785
03/06/18 9:02 pm
03/06/18 9:02 pm
Joined: Aug 2001
Posts: 2,803
New Hampshier USA
MikeG Offline

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Originally Posted by Andy Higham
The smoothest twin cylinder configuration is a 90 deg V twin, a "boxer" flat twin is also quite well balanced
As far as parallel twins go the 360 degree version is about as bad as it gets, the 180 degree twin has less primary vibration but secondary high frequency vibration can be a problem. Some people have had good results with crank pins spaced 90 degrees apart


My Hinckley Bonneville is called a 270 degree twin, any different than a 90 degree in terms of operation, or just a matter of reference between left and right cylinder? The bike is pretty smooth, but the balance shafts probably have a lot to do with that as well.


1960 BSA A10
2007 Suzuki Bandit
1957 A10
(Used to be a Triumph here)
71 Norton Commando
17 Triumph Bonneville

Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727787
03/06/18 9:03 pm
03/06/18 9:03 pm
Joined: Aug 2012
Posts: 309
Hampshire, England
DoubleDiamond Offline
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Hampshire, England
I have a 280/80 crank for my A65. Unfortunately I haven't yet run it with the correct timing and camshaft as I bought it as a 270/90 crank!!

It has a 50% balance factor and sounds a bit like a V twin. It also had the Nova 5-speed gearbox fitted which was a dream in use.


BSA: '71 B175; '68 B25; '71 A65; '71 A75
Triumph: '87 T140; '72 T150v
Re: Twin Cylinder Design Theory [Re: MikeG] #727792
03/06/18 9:34 pm
03/06/18 9:34 pm
Joined: Aug 2001
Posts: 7,672
scotland
triton thrasher Online content
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Originally Posted by MikeG

My Hinckley Bonneville is called a 270 degree twin, any different than a 90 degree in terms of operation, or just a matter of reference between left and right cylinder? .


They must think 270 degrees sounds more intriguing than 90.


Amateur Loctite enthusiast.
Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727814
03/07/18 12:40 am
03/07/18 12:40 am
Joined: Nov 2012
Posts: 751
Isle of Wight, UK
K
koan58 Offline
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Posts: 751
Isle of Wight, UK
TT,

"One up-one down (180 degree) four stroke twins have been sold in large numbers by Honda. Their old CD range of single carburettor twins in the 1960s were 360 degree, like a Brit twin, but the CB twin carburettor twins were 180 degree, for better balance. It's supposed to be hard to make a single carburettor work well on an uneven-firing twin, such as a 180 degree."

As far as I know, all the early twins (C, CA, CB this and that) were 360 cranks. I got my initiation on a CD175, which was such. I suspect the CB175 was the earliest (Honda) experiment with a 180 crank.
My mate's CB175 certainly made me envious (schoolboys) due to its prettier styling, but almost identical in performance (my CD had long megas, can you imagine the noise at 10,000rpm?).

When they went to the CB200, it was back to 360, as were the others in the range.

For reasons only known to Honda, they played with 180's on their 450 and 500 twins around this time, I find this amusing:

"While not receiving much praise from critics and riders alike, Cycle World took note of its comfortable and roomy seat in its 1975 test, writing, "Unusual in that it is brown in color, it is long enough to carry a briefcase or passenger without crowding the rider. And the padding is soft enough for comfort. Believe us, without this seat you couldn’t ride a 500T very far and get off smiling.""

Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727817
03/07/18 12:52 am
03/07/18 12:52 am
Joined: Nov 2012
Posts: 751
Isle of Wight, UK
K
koan58 Offline
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koan58  Offline
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K

Joined: Nov 2012
Posts: 751
Isle of Wight, UK
"It's supposed to be hard to make a single carburettor work well on an uneven-firing twin, such as a 180 degree."

I don't know how real this is, many V-twins share a carb?

Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727821
03/07/18 1:28 am
03/07/18 1:28 am
Joined: Nov 2012
Posts: 751
Isle of Wight, UK
K
koan58 Offline
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K

Joined: Nov 2012
Posts: 751
Isle of Wight, UK
" Tractors like hardleys' are probably ok."

Yes, I know where you're coming from. The first time I messed with one, was a homemade lowrider shovel, running dreadfully.

I'd never met such a system where its wasted spark could actually fire and damage the engine if the timing were way out.

I've yet to meet a Hardon Colluder who does his tappets and timing.

Re: Twin Cylinder Design Theory [Re: Les P] #727822
03/07/18 1:30 am
03/07/18 1:30 am
Joined: Aug 2001
Posts: 5,135
Stone Creek OH USA
R
Rich B Offline

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Posts: 5,135
Stone Creek OH USA
XS650 Yama was a 360 twin just like the Brit twins. It was also 75 X 74 mm stroke. Hands down, best 4 stroke twin from Yamaha in that era. It was also a tooth rattler. The crank was a pressed up crank like a 2 stroke. They could be made to rev.


Never underestimate the human ability to elevate stupid to a whole new level!.
Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727824
03/07/18 1:42 am
03/07/18 1:42 am
Joined: Nov 2012
Posts: 751
Isle of Wight, UK
K
koan58 Offline
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Isle of Wight, UK
Definitely, a good solid bike. Kawasaki have done some similar since (W etc). Never popular with the crowd though.

Re: Twin Cylinder Design Theory [Re: NickL] #727829
03/07/18 2:09 am
03/07/18 2:09 am
Joined: Aug 2001
Posts: 5,135
Stone Creek OH USA
R
Rich B Offline

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Rich B  Offline

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Joined: Aug 2001
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Stone Creek OH USA
Yes, the XS650 could become a bit “fragile” when raced. The crank and rods were definitely a weak link.

Had a 750 version (Wiseco big bore kit) that ended up being the most spectacular blow up I have ever been involved with. You could see the transmission standing in front of the bike eek There was nothing left between the bottom of the cylinder head and top of the sump plate. Both rods were simply gone. Actually tweaked the one down tube where bits exiting the engine caught the front motor mount bolt on the bits road to freedom.

That was the end of my involvement with those engines for flat track.


Never underestimate the human ability to elevate stupid to a whole new level!.
Re: Twin Cylinder Design Theory [Re: Rich B] #727831
03/07/18 2:34 am
03/07/18 2:34 am
Joined: Aug 2001
Posts: 4,601
Owego, NY, USA
Mark Z Offline
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Mark Z  Offline
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Owego, NY, USA
Originally Posted by Rich B
XS650 Yama was a 360 twin just like the Brit twins. It was also 75 X 74 mm stroke. Hands down, best 4 stroke twin from Yamaha in that era. It was also a tooth rattler. The crank was a pressed up crank like a 2 stroke. They could be made to rev.


I saw a lot of successful Yama flattrackers back in the seventies, XS650 engines bored to 750. One advantage they had over the Brit twins was a center bearing on the crank, which necessitated the pressed-up crank and horizontally-split crankcase. Yeah they could rev; I'm sure the OHC helped.




Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727845
03/07/18 3:09 am
03/07/18 3:09 am
Joined: Nov 2012
Posts: 751
Isle of Wight, UK
K
koan58 Offline
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Just because an extra bearing in the middle looks like a good idea doesn't mean it is in practice.

Where in most Brit twins, crank flex is just accepted, the centre bearing in an AJS is trying to play "Kiing Canute" against the tide, in a parallel twin.

Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727847
03/07/18 3:14 am
03/07/18 3:14 am
Joined: Jun 2013
Posts: 97
Texas, USA
R
Ray Elliott Offline
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Texas, USA
After my Brit bike business closed about '76 i went to work in the Honda & Yamaha business. Selling XS650s was a terrible chore & Yamaha forced them on the dealers. It really was easier to sell a Brit twin as the few buyers actually wanted one. The XS650 buyer really wanted a CB750 but hadn't the budget. Warranty claims were low & unlike Triumph/BSA/NVT Yamaha paid the dealer's claims.


Ray Elliott
---
A65, A70, A75, T120, T140, T150, T160
Re: Twin Cylinder Design Theory [Re: NickL] #727849
03/07/18 3:33 am
03/07/18 3:33 am
Joined: Aug 2001
Posts: 4,601
Owego, NY, USA
Mark Z Offline
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Mark Z  Offline
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Originally Posted by NickL
The other advantage was, they had rich owners or wealthy sponsors.



Ha, yes, I guess that's why I never heard about the blowups - they didn't hobnob with the renegades.




Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
Re: Twin Cylinder Design Theory [Re: Mark Z] #727864
03/07/18 11:13 am
03/07/18 11:13 am
Joined: Aug 2001
Posts: 5,135
Stone Creek OH USA
R
Rich B Offline

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Stone Creek OH USA
There were few XS650 based flat track bikes that were consistently competitive. The power delivery was a balance of having enough power and avoiding wheel spin. Usually wheel spin won. It took a unique rider to master the power delivery. There were precious few of those.

The ignition was always a crap shoot. The stock points cam drive was Mickey Mouse, the crank mounted units would work, but not consistently.

Cranks/rods were delicate. It took money to keep the bottom end alive. You had to have the crank welded to keep it from twisting under the stress of racing. Even with the crank pins tack welded to flywheel, the welds would break and the crank would twist. There were special rods, but even those, when available, had a finite life.

Yamaha and Kenny Roberts gave up on the engine. It hit a ceiling of development and there was no more to be had.

The engine had a brief run in flat track around the time of the demise of BSA/Triumph and the intro of the alloy XR750. But the XS run was brief. As the XR become more available and developed, the XS was done.

I will admit, the XS taught me a lot about chassis set up, ignition, timing, gearing, and tires in flat track. I got lots of practice finding a combination that allowed traction out of the corners. When it worked, the line between working and not working was incredibly thin.

An XR or Triumph was simple by comparison.


Never underestimate the human ability to elevate stupid to a whole new level!.
Re: Twin Cylinder Design Theory [Re: NickL] #727869
03/07/18 2:02 pm
03/07/18 2:02 pm
Joined: Aug 2001
Posts: 2,803
New Hampshier USA
MikeG Offline

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[quote=NickL]Another variation is the Twingle, firing one cylinder after the other using small crank offset angles.
Having used various offsets, yes the vibration and power area can be changed using different
offsets, the rod ratio and other factors need to be considered.

Thank you for that memory jog! I remember a few Puch/Sears Twingles around when I was younger, but never paid much attention to them. After a google search I see I missed something unique. I knew that the cylinders shared a common combustion chamber, but never knew they were not synched together.

Last edited by MikeG; 03/07/18 2:04 pm.

1960 BSA A10
2007 Suzuki Bandit
1957 A10
(Used to be a Triumph here)
71 Norton Commando
17 Triumph Bonneville

Re: Twin Cylinder Design Theory [Re: Tracey Spear] #727887
03/07/18 6:28 pm
03/07/18 6:28 pm
Joined: Jan 2004
Posts: 3,909
ca, us
D
DMadigan Offline
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Posts: 3,909
ca, us
It sounds as though you guys are saying this is a waste of time?
[Linked Image]
[Linked Image]
I have not had the time to work on it of late. I have to have a head and cylinder cast up. Yes, I know JHill makes a cylinder but with an 80mm bore it would need 5/16" head bolts which in aluminum with the course thread means a 1/4" core to the bolts. I want through bolts into the cases by the four corner bolts.
I still have to change the rods to CR500. Otherwise the cylinder would have to be shortened by 3/4"!
I found an investment caster that is willing to work with me on the new triple clutch housings and this now seems a possibility.

90 deg. parallel twins change the vibration from vertical/horizontal to less vertical/horizontal with a rocking couple. Years ago I worked out the forces and moments on an A65 with 360, 180 and 90 deg. offsets.
Offset Force Moment
360 9174*w 0
180 4520 *w 690*w
90 3290*w 1159*w

Where w is the reciprocating weight of one cylinder.
On L twins the moment is reduced because the distance between cylinders is far less. On H*rleys (H*rlies?) the rods are coincident so there is no moment but weaker rods due to the fork and wobbly cranks due to the long throw. Ask how often XR750 cranks were rebuilt.
To the original question, a 360 crank is cheaper than 180 (both rod journals can be ground at the same time). Remember, BSA/Triumph were into cheap, not racers. Riders feel less of the horizontal vibration than vertical so balance factors were biased this way. Watch a Brit twin idling on the side stand. The front wheel oscillates fore/aft. A Japanese twin will show little movement.
The H*nda 250/350 six cylinder racers had pressed together cranks but seemed to hold together enough for Hailwood. Race engines always need more maintenance than a street engine.

Re: Twin Cylinder Design Theory [Re: DMadigan] #727892
03/07/18 7:47 pm
03/07/18 7:47 pm
Joined: Aug 2001
Posts: 7,672
scotland
triton thrasher Online content
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scotland
Originally Posted by D Madigan
It sounds as though you guys are saying this is a waste of time?


The man that made time made plenty o' it.


Amateur Loctite enthusiast.
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