Rod/stroke ratio effects explained

" Engines with the cranks offset from the cylinder’s center line are called "desaxe" engines. Desaxe, the French word for "unbalanced," describes an engine in which the cylinder is positioned with its exact center (the bore axis) slightly offset from the centerline of the crankshaft. And, while offset cranks are rarely discussed in motocross and are relatively new to motorcycles (mostly within the last 15 years), there is nothing truly new about desaxe engines. Offset cranks and cylinders have been around as long as internal combustion engines—even Henry Ford recognized the Desaxe philosophy—and implemented it in the Ford Flathead V8 engine in the 1930s. "

Matt Spencer said:

" Engines with the cranks offset from the cylinder’s center line are called "desaxe" engines. Desaxe, the French word for "unbalanced," describes an engine in which the cylinder is positioned with its exact center (the bore axis) slightly offset from the centerline of the crankshaft. And, while offset cranks are rarely discussed in motocross and are relatively new to motorcycles (mostly within the last 15 years), there is nothing truly new about desaxe engines. Offset cranks and cylinders have been around as long as internal combustion engines—even Henry Ford recognized the Desaxe philosophy—and implemented it in the Ford Flathead V8 engine in the 1930s. "

Desaxe Philosophy And Crank Offset

In most old-school engines the piston tracks the center line of the cylinder’s bore. It achieves this by being mounted directly in line with the center of

www.mxa-digital.com

Cylinder axis in 750/850 Norton?

When Norton originally made the 750cc engine out of the 500-650cc Dominator, they did it by machining the Dominator crankcase and cylinder head castings a bit differently and adding a new cylinder casting. They had no room to keep the cylinder bore axis where it was, so they moved it...

www.accessnorton.com

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The desaxe discussion is mildly interesting but not remotely relevant to this discussion.
What Beng doesn't mention is the reason for the movement of bore centreline was the camshaft location.
First happened on the Atlas (you'll note that the timed breather on these has a hose at the end of the camshaft - no room for the gallery to the rear either) - so... only 750s & 850s.
The amount of desaxe movement was not enough to warrant longer conrods, hence interchangeability with 650s (no desaxe).

This video came up on my You Tube feed last week and had meant to go back and watch it.

I had edited this picture with the rod to stroke ratio's but it must be on my expired laptop, this one was in my PC.
It might be a surprise which one had the highest ratio.



The connecting rod length has some input on the exhaust cycle evacuation going over TDC long and short rod.
You can add any rod but the piston quality will hold you back at some stage which is no doubt why JS Motorsport put so much time and effort into that and not just rods (quality parts)

Time Warp said:

This video came up on my You Tube feed last week and had meant to go back and watch it.

I had edited this picture with the rod to stroke ratio's but it must be on my expired laptop, this one was in my PC.
It might be a surprise which one had the highest ratio.

View attachment 84804

The connecting rod length has some input on the exhaust cycle evacuation going over TDC long and short rod.
You can add any rod but the piston quality will hold you back at some stage which is no doubt why JS Motorsport put so much time and effort into that and not just rods (quality parts)

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That's very strange!
If factual it means that, when Triumph unitised, they went backward in both big-end journal size and rod length!

robs ss said:

That's very strange!
If factual it means that, when Triumph unitised, they went backward in both big-end journal size and rod length!

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The short Triumph rod shown above is the 500 version Rob, hence the smaller journal.

Unit 650 rods were also 6 1/2” same as pre unit.

Triumph went to the 6” ‘short rod’ format when they went oil in frame. Some claim it was the frame that was the reason as the long rod motor was too tall for easy assembly etc.

Many claim that a Morgo 750 converted 650 is sweeter than a stock 750… and a primary difference is the rod length.

When I discussed rod length with Dave Nourish, he was pretty adamant that the long stroke Nourish motors (ie 89mm) needed 6 1/2” rods.

I built one with 5 7/8” rods, it worked fine, but Dave didn’t think it was ideal.

It does not matter much where the ignition advance is or what the comp, ratio is. Usually they are both constant and you jet to suit them and your fuel. My 500cc Triumph had 650 conrods and 63mm stroke. It seemed the never stop accelerating,.. On a big circuit, it was scary. It used to rev to 10,500 RPM until I fitted a two into one exhaust to get decent lap times. Then it only revved to 9,500 RPM. But it stopped trying to kill me.

acotrel said:

It does not matter much where the ignition advance is or what the comp, ratio is. Usually they are both constant and you jet to suit them and your fuel. My 500cc Triumph had 650 conrods and 63mm stroke. It seemed the never stop accelerating,.. On a big circuit, it was scary. It used to rev to 10,500 RPM until I fitted a two into one exhaust to get decent lap times. Then it only revved to 9,500 RPM. But it stopped trying to kill me.

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I tend to disagree Al - if an engine needs to be significantly advanced to run well it is an indication that the propagation of burning is slower than other engines with lower advance. That slower "flame front" will, inevitably, mean less Mean Effective Pressure (Power, Torque, whatever) resulting in less performance.
Stack Norton twin serious race results against Triumph twins and you may begin to see the pattern.

Must say I noticed the big end size on all those rods. Norton out did the Triumph but the 'beam out did them all.

robs ss said:

That's very strange!
If factual it means that, when Triumph unitised, they went backward in both big-end journal size and rod length!

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The stroke is quite different at 65.5 mm on the 500 cc and 82 mm on the 650 cc but forget what the actual ratio's were.

That short all alloy Sunbeam rod (114 mm centres / 63.5 mm) has a 1.795 ratio being a 'racing engine.
Coupled with the original cross flow head it was deemed to powerful extinguishing the test riders pipe at speed.

The Sunbeam S7 has a racing engine?
Aren't they a 20ish HP 75 mph plugger?

Glen

worntorn said:

The Sunbeam S7 has a racing engine?
Aren't they a 20ish HP 75 mph plugger?

Glen

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… “it was deemed to powerful extinguishing the test riders pipe at speed” …

Missed that!

worntorn said:

The Sunbeam S7 has a racing engine?
Aren't they a 20ish HP 75 mph plugger?

Glen

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...Bob Smith was a Lancashire lad who started racing on a Norton 750 in a copy of the Dunstall Drainpipe frame, he became British champion in 1981 on a TZ750 Yamaha and rode RG500s against, and beat, the likes of Barry Sheene, Wayne Gardner, Mick Grant, Roger Marshall to name a few....and rode Daytona BoTT on Gary Bryan's RGB Weslake....and he did the Suzuka 8 hour....

Sadly he lost his life at Scarborough in '83, but he is well remembered, by many and various people who knew him, as did I....

Anyway, according to his widow, Bob Smith cut his teeth on a Sunbeem S7...that both he and she loved...I suspect that means their courting was conducted on it...so, this is a piece of text written about him apparently riding that S7, by someone local to his home who knew him before he went racing....

'Bob Smith, we are not worthy........
......suddenly out of the darkness of another boring sodium lit night on the streets of Parr, sparks would ignite as the bike grounded its pegs around the sharp and dangerously rutted "Oddfellows" bend. The rider wrestled the ill handling machine, teasing out more and more performance than was ever intended, in a split second blur of soundtrack and speed he had battered the corner into submission....he now owned it... and off he sped...to his next task. Nailing it round the "horse shoe". "Aye, that will be Bob then"... said the teenager looking on in awe. "Aye, that was Bob"......'

But yes, a 20ish HP and possibly sub 75mph plugger.....

worntorn said:

The Sunbeam S7 has a racing engine?
Aren't they a 20ish HP 75 mph plugger?

Glen

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The prototype was allegedly good for 90mph...

Just read another article on rod ratio. The view was that any ratio between 1.5 and 2.0 will do with sub 10k rpm engines.
The author mentions that the 473 bhp BMW M3 uses a 1.48 rod ratio and makes 125 bhp per litre.

worntorn said:

Just read another article on rod ratio. The view was that any ratio between 1.5 and 2.0 will do with sub 10k rpm engines.
The author mentions that the 473 bhp BMW M3 uses a 1.48 rod ratio and makes 125 bhp per litre.

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That's a twin turbo I-6. Hardly comparable to the average N/A engine.

The rods and pistons still have to do their job and survive. For output it compares with a seriously hotrodded standard engine, however the hotrodder in this case is the factory.
That's the ratio they went with ( or ended up with)

Dan1950 said:

That's a twin turbo I-6. Hardly comparable to the average N/A engine.

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worntorn said:

The rods and pistons still have to do their job and survive. For output it compares with a seriously hotrodded standard engine, however the hotrodder in this case is the factory.
That's the ratio they went with ( or ended up with)

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The point I was trying to make is that the BMW is making HP/TQ via boost rather than basic engine geometry.

The point he made was that high performance engines do not require a high rod ratio to survive.
There is an almost cult like belief that high rod ratios go with high performance.
He used the BMW engine as an extreme example to show how relatively unimportant rod ratio is as a factor in engine design/performance. The high ratios are only needed for extreme high rpm F1 race engines or modern Sportbike engines that rev two or three times as high as our Nortons.

The highest HP per cc two valve motorcycle engine I know of is the HD XR750 with claims up to 100 HP. It is not an extreme RPM motor and has a broad enough power band for dirt track. It has a rod ratio of 2:1 by choice.

The other thing is that Nortons twins have a vibration problem - so bad that they must be rubber mounted. The vibes can destroy the crank and cases and shake everything loose. The shorter rods with the lower rod stroke ratios have the worst vibration. Changing to a longer rod reduces vibration noticably as well as reducing stress.

Going to an even shorter rod and lower rod stroke ratio would just make things worse. BMW boxers don't have such a bad vibration problem because their pistons move in opposition to each other.