Advantages of longer rods in Nortons

Found this brit bike rod info on the web.

http://b50.org/WP/?page_id=2930

According to the article - longer rods (usually with short pistons) show some definite advantages (reducing stress and giving more useful RPM). By coincidence - the motor in the article has nearly the same improvement in rod to stroke ratio as a Commando -stock is 1.68 to 1 and the JS long rod is 1.83 to 1

The Stock Commando rod is 5.875 long
The JS Commando long rod is 6.4"

The B50 stock rod referred to in the article is 6"long.
The B50 improved long rod referred to in the article is 6.5" long.

There is a long discussion about longer vs shorter rods in B50s here:
http://www.b50.org/phpBB2/viewtopic.php?f=3&t=547


Here's one of the posters (sixth article down):

"There are , imo, several advantages to the 1/2 inch longer connecting rod in a B50. It gives you the same rod ratio as the latest DBD Gold Stars, it stops breaking things, ie the crankpin assy, piston pin, and the rod itsself. It also broadens the power band quite a bit. On my flattrack race bike, I concentrated all the power at the top of the rpm band to try to compete with the Rotaxs. If the least little thing went wrong in the corners, the rider had to shift down to get going again. Gearing was also a problem as max HP occured only about 2 rpm under max possible rpm and the engine would go from max power to hitting a wall in a flash. With the 1/2 inch longer rod, it added nearly a thousand rpm on either side of the former power band and even to my amazement the Rotaxs were no longer a problem. In a street bike the biggest advantage is the no longer breaking things I'd say. For MX it makes it possible to not have to shift as many times per lap and for RRing it would be the best of all worlds I believe. The biggest strain on the crank assy comes not under power BUT, at top dead center of the waste stroke as the piston wants to keep going up and a short con rod violently yanks it back the other way. A longer con rod slows the piston speed ATDC and speeds it up ABDC where the parts are under compression and not trying to fly apart. If you are racing the engine there is another problem with a short rod. The thing that makes HP in your engine is the heat from the combustion process burning and the heat expanding to push the piston down. This process is basic chemistry and can happen only just so fast. The problem with a short con rod is that at higher rpms, the crankshaft yanks the piston away from TDC faster than the heat front can expand and at that point it makes no difference how well the head ports flow, how well you have the ignition setup, or anything else,,,,, You just stop making HP ! The proper length con rod is one of the few "free lunches" I can think of mechanically as the only drawback is that the engine takes up more space in the chassis. Sorry for getting so long winded but it is a rather complicated subject... Stan"

jseng1 said:

Found this brit bike rod info on the web.

http://b50.org/WP/?page_id=2930

According to the article - longer rods and short pistons show some definite advantages (reducing stress and giving more useful RPM). By coincidence - the motor in the article has nearly the same improvement in rod to stroke ratio as a Commando -stock is 1.68 to 1 and the JS long rod is 1.83 to 1

Click to expand...

I read the reference article and no there was mention of the "short pistons". The article does state "The longer rod also lessens the side loading of the piston to cylinder at bottom dead
center" Are short pistons less tolerant of side loading? Is that what you are referring to?

Jim,

Thanks for bringing this to the attention of everyone here. You referenced the wrong link, though, for the forum discussion regarding the subject, so I'm including it here:

http://www.b50.org/phpBB2/viewtopic.php?f=3&t=547


Intriguing that the article and conversation are from 9 years ago, back in 2005.

---------------------------


Shrapnel dancer,

Thanks for clarifying the article doesn't discuss pistons, just rods, but neither does it mention short pistons having less side load tolerances. Since you race Nortons, and are, I believe, well connected with other persons with a very high level of Norton knowledge, I assume your question to Jim is rhetorical in nature, which would make your post less helpful than it could have been, and may actually confuse other readers that may have missed the recent thread regarding bore coatings (and whatever else was discussed in that thread).



In the forum discussion about the article Jim cites in his original post, there are forum posters mentioning a "high pin" piston, which I'm guessing means the wrist pin is located higher in the piston. It's not clear whether or not the pistons are shorter than the regular or stock alternative.






.

I think that with the long rods the rock-over time around TDC is longer, so torque is better.

The old Side thrust on short rods due to the Angularity under the power stroke is a major source of bore wear & wasted power , on any injun .

There has been lots of debate on this topic for decades.
My friend Dan Smith remade his Vincent Black Shadow as two BSA Goldstars, or rather he duplicated the crank angles of the Gold Star. So he shortrodded the Vincent to do this. This moved the head closer to the crankcase and caused a change in Valve train geometry. The new pushrod position meant that the pushrod tubes no longer went square into their recesses, so Dan made new tubes with a curved jog in the centre to accommodate the change.
Change one thing and it changes everything. I'm not sure it was worth the effort. Phil Irving looked at what Dan had in mind and said "there is no point, we already had it right". So Dan had a T shirt made that said "who the hell is Phil Irving?" He wore this quite proudly for years.
He will tell you that shortrods are the way to go and give many good reasons why.
With the modifications done, and much more than just the shortrodding, he did set the fastest time for any Vincent roadbike at the Ramsey Sprint in 1999 . No one has touched his record since.

Ed Iskenderian had this to say on the short rod vs long rod question

http://www.iskycams.com/techtips.php#2005

Glen

Re: Advantages of longer rods

Dances

When I refer to short pistons I'm saying that with a given motor the (easy) way to go to a longer rod is to shorten the piston. Then you also get the advantage of a lighter piston.

Worn torn.
There is theory and there is results. Both are valuable but the results of a longer rod in the B50 are pretty obvious according to the link http://b50.org/WP/?page_id=2930. The same thing happens in a Norton. When Dave Watson went to the longer rods for his 1000cc racer - the first thing they noticed was an increase of approx 1300RPM. That's difficult to argue with.

The Norton factory went to the short stroke (with better rod length/stroke ratio) for more power & RPM so they could win. Then they solved the bore wear problem (caused by higher RPM with same pistons) by hardening the cylinder bore. The photo below is of factory short stroke cylinders with hardened bores - see the yellow point on the fins which identify the hardened cylinders.



You can pour over the web and find pro/con info about rod length. But overall its the longer rod motors that put out more power. Just look at the highest power F1 motors - with their rod ratios of about 2.5 to 1 Those motors are the most advanced. If you like theory then visit this site and note the rod length comparisons of Nascar V8- to F1 engines.
http://www.epi-eng.com/piston_engine_te ... _to_f1.htm

They sound OK Jim but what happens to all your customer support and the source of parts when you keel over? This would keep me from buying. In the future there will be plenty of Norton rods I think but no replacement jseng rods.

batrider said:

They sound OK Jim but what happens to all your customer support and the source of parts when you keel over? This would keep me from buying. In the future there will be plenty of Norton rods I think but no replacement jseng rods.

Click to expand...

When that day approaches I've made plans to make the job reproducing numbers available to everyone (on the web) and give permission for anyone to get them directly from the supplier. Presently Carrillo and JE protect my products and don't sell them to others (others have tried). Preferably someone else will take over for me (and hopefully buy me out). In either case - I could be raising hell for another 30 years.

Robert_Norton said:

Shrapnel dancer,

Thanks for clarifying the article doesn't discuss pistons, just rods, but neither does it mention short pistons having less side load tolerances. Since you race Nortons, and are, I believe, well connected with other persons with a very high level of Norton knowledge, I assume your question to Jim is rhetorical in nature, which would make your post less helpful than it could have been, and may actually confuse other readers that may have missed the recent thread regarding bore coatings (and whatever else was discussed in that thread).

Click to expand...

Rhetoric? I see three imbedded assumptions in your assertion above. Thanks for providing the link to the forum but Jim had already provided that in his initial post. Interesting reading and seems aligned with my understanding of the various aspects of rod length as it relates to stroke but really it does not touch on my question.

It was a simple question and I do not see the context link to bore coating. Furthermore, my question still stands as I don't know all nor do I claim to. I was the fellow who enlightened jseng1 on the use of BoreTech as a good remedy to premature cylinder liner wear in two of my Norton race engines which do not run jseng1 pistons and rods. Another 1,007cc build with next to no time on the engine/bike does run jseng1 steel rods and pistons. I used the BoreTech process on this build as a standard course of action from lessons learned on liner wear from other builds.

jseng1 said:

Dances

When I refer to short pistons I'm saying that with a given motor the (easy) way to go to a longer rod is to shorten the piston. Then you also get the advantage of a lighter piston.

Click to expand...

Thanks Jim. Raising hell for another 30 years - I like that.

Re: Advantages of longer rods

jseng1 said:

Dances

When I refer to short pistons I'm saying that with a given motor the (easy) way to go to a longer rod is to shorten the piston. Then you also get the advantage of a lighter piston.

Worn torn.
There is theory and there is results. Both are valuable but the results of a longer rod in the B50 are pretty obvious according to the link http://b50.org/WP/?page_id=2930. The same thing happens in a Norton. When Dave Watson went to the longer rods for his 1000cc racer - the first thing they noticed was an increase of approx 1300RPM. That's difficult to argue with.

The Norton factory went to the short stroke (with better rod length/stroke ratio) for more power & RPM so they could win. Then they solved the bore wear problem (caused by higher RPM with same pistons) by hardening the cylinder bore. The photo below is of factory short stroke cylinders with hardened bores - see the yellow point on the fins which identify the hardened cylinders.

You can pour over the web and find pro/con info about rod length.

Click to expand...

I did not pour over the web to find the Iskenderian article, I read it some time ago. I mentioned it to my friend Dan, but he is not about to change his views on shortrodding as being the hot ticket, anymore than you will change your views on long rodding. As you mention, results are all that matter, and his result was very good.


Was the change in rod length the only change which brought about a 1300 rpm change or were other factors changed at the same time, ie lighter pistons, high comp etc?

Glen

My priority this season is grass cutting tech but realize that these clunker engines ungoverned in 30-ish cubic inch, 600-ish cc can turn 7000+/70+ hp.
Turn key pulling engines cost $4-5000 unless getting full serious then double it.
Right now I'm piston and new bore shopping. Kohlter piston clearance with cast piston in cast iron cylinder starts @ .007" if feeling lucky to .010" nominal and .0140" for forged pistons run at hi rpm, ie: over 3600-4000.

Note: Longer rods tend to increase detonation proneness d/t the TDC delay and stifle intake mixture pull in some what too. Spoons on rod big ends oil the cam and notch jets oil under piston crowns.
------------------------ hobot -------------------------------------------------

http://gardentractorpullingtips.com/engine.htm
Ever wondered why there's longer-than-stock length custom-made connecting rods?

The reason many high-performance engine builders prefer to use a longer, custom-made, billet [heavy duty] connecting rod is because there's less pressure from the piston skirt placed against the cylinder wall during high rpm. At high rpm, a long rod moves or "swings" side to side with less force, while a stock length rod moves more rapidly. Longer rods operate at less of an angle than shorter stock rods do. Therefore, piston skirt "drag" or scraping and force against the cylinder wall is greatly reduced and an engine will produce more rpm with less friction resulting in more power. In order for an engine to accommodate a longer rod, a special made light-weight piston with its wrist pin located closer toward the top (wrist pin location is known as "compression height") must be used. Many engine builders prefer to pop the piston out of the cylinder a few thousands of an inch to help increase compression. To do this, either offset bearing inserts are installed in a stock rod, or a longer rod/custom piston combination is used.

Because billet connecting rods are wider than OEM ones, each lower side of the cylinder wall must be ground away with a small disc grinder so the rod will clear it. The minimum clearance between the rod and cylinder wall is .050".

The billet connecting rod bolts should be torqued to 18 to 20 ft. lb. Overtorquing them could result in distortion of the big end of the rod.

NOTE - When used in a high rpm application, aluminum connecting rods develop "rod stretch" over time, which could lead to rod failure. Even if it's a custom-made one. So to be safe, measure a used rod against a new or a known good one, or replace it after several years of use. And the connecting rod in ordinary lawn and garden engines will outlast the rod in high-performance engines because there's less strain on them at 3,600 rpm.

And if you've ever wondered about this: the bolts and studs in Kohler connecting rods can be reused over and over. They're very durable. Besides, nobody makes replacement bolts or studs for Kohler rods.

I've mentioned my 500cc short stroke Triumph on this forum previously. It was a 650 stroked down from 82mm to 63mm stroke. It still used the long 650 rods and 12 to 1 650 pistons. It would rev easily to 10,500, and on a big circuit pulling high gearing it was scary. On a short circuit it was useless. I tried many things to try to get a bit more torque out of the motor, and ended up with a two into one exhaust which lopped 1500 RPM off the top however at least made the bike rideable. The pistons were in my opinion always too heavy. I've used lighter pistons in 650 Triumph motors and they add wings to the bike - acceleration is much better. Getting higher compression in the 500 was always a problem. What the bike really needed was the Weslake 4 valve head. I can believe that a Commando engine fitted with lighter pistons would immediately pick up 1300 revs, and the longer rods would give a better combustion situation around TDC because of the greater rock-over time. Comparing my 500cc Triumph with the Triumph factory Daytona - they had it right with their shorter rods, and I had it wrong. My problem was in the end, that I needed shorter barrels - mine already had 12mm off the top, and the Weslake head to get the compression and lighter pistons. It all became a futile exercise, so I quit the bike.
I've looked at the Jim Schmidt rods and pistons on his site, and I believe they are a great way to go. In a Norton twin the pistons speeds at mid-stroke are extremely high, and at TDC and BDC - zero, the accelerations and resultant loads must be immense compared with most other motors, so piston weight is critical to performance.
I've always had the opinion that I should not seek a performance increase from my 850 by increasing the rev limit, with the long rods and light pistons I would feel braver. With my Seeley 850 on methanol and a close box, it spins up extremely rapidly and even though I always try hard not to exceed 7,000 RPM when racing, I always see 7,500 RPM. It is the 'big bang' waiting to happen.
Ed Iskenderian's opinion is interesting - the rod length affects the characteristics of the cam you can use.
I never believed in commando engines, however these days I believe they are so right in many ways. I cannot imagine why anyone would reduce the stroke in one. If you have a cylinder wear problem, you simply change the materials.

When you have an engine that already has longish stroke and therefore suffers from that RPM limitation, you can squeeze a bit more out of it with longer rods.

My only personal experience on this subject comes from 350 Ducati singles, the early narrow case engines had short rods while the later wide case engines had long rods, other than that they were identical bore and stroke, heck IIRC they even shared the same pistons, the difference being made up with deck height. The later engines also were built to handle the Desmo heads that offered zero valve float higher RPM. Anyway, the long rod/wide case engines would spin all day, they were happy at 9000; with the right cam they would make the most power. That admitted, I preferred the short rod engines; their power hit harder, sooner, snappier, while the long rod engines were kind of lazy and just sort of rolled up there, making their power with a flatter rising curve. Both engines had the same size crankpins and as said, same stroke, and I had replaced more narrow case/short rod big ends than I did wide case/long rod big ends but that could have been that they were just plain older too . . . . Anyway, the long rod engines seemed to be more durable but then their whole lower end was more beefy too.

When the internals of my Commando's last saw light it was 42 years ago in England. When her internals see light again, it will be to install, among othr things, Jim's lifters, rods and pistons.

worntorn said:

There has been lots of debate on this topic for decades.
My friend Dan Smith remade his Vincent Black Shadow as two BSA Goldstars, or rather he duplicated the crank angles of the Gold Star.....

Glen

Click to expand...

Glen (Worntorn)

When you're talking about BSA 500 Goldstars your talking about a very long rod - 7-3/8" (187mm) or 6-7/8" (175mm) depending on year. With a stroke of 88m that gives you a rod stroke ratio of about 2.1 to 1 or 1.99 to 1 depending on year. Thats a lot longer than the 1.86 to 1 JS rod stroke ratio with the 6.4" long rods. So if your friend Dan Smith is copying a Goldstar on this Vincent then he does not have a short rod motor but a (very) long rod motor. By comparison, a Commando with JS long rods would be a short rod motor.

The article was by Stan Malard, he indeed did have a very fast B50. I used the same set up in a B50 I used for MX. The rod came from Carrillo, I got the piston from Stan, I believe It was made by Wisco. The piston wasn't any shorter but had the wrist pin located 1/2" higher and much thinner modern rings.

The engine did have a wider power band and was much smoother running. I liked the set up enough to do the same thing to a B44 Shooting Star.

Every engine I've installed longer rods in has run smoother with less stress.

worntorn said:

Ed Iskenderian had this to say on the short rod vs long rod question

Click to expand...

There's one big thing that the article neglects and that is the effect rod length has on mass forces. Longer rod means shorter compression height and that - at least potentially - means less piston weight. Plus there is a small side effect mainly on secondary mass forces as the longer rod pivots less. Less mass forces means less stress on the engine or more margin for rpm and thus power.

Plus the fact that there is room for improvement on the Norton crank train - which is the case because engine technology has improved since this engine was designed and not because the designers back then didn't know what they were doing - this doesn't mean there's room for improvement on all other engines ....



Tim

I think the Norton engine works quite well as designed, that is as a low rpm high torque engine. With the updated bearings it makes a great road engine. If the desire is to turn it into something it is not - a high rpm race type engine, then it needs a major revamp, or as Steve Maney and Kenny Deer both decided, a complete redesign.

It was mentioned that a long rod ratio is used in Formula one race engines. There does seem to be a consensus that the longer rod ratio works best with very high rpm engines of this type.
I don't see the Norton engines as having much in common with that type of engine, nor would I want to attempt to remake my engines into high rpm Screamers, invariably at loss of low and midrange punch.
Best to start again for that.
Or buy a litre Sport bike and watch the tach go effortlessly to 12 or 14 grand without vibration or risk to engine.

Glen

Tintin said:

Longer rod means shorter compression height.

Click to expand...

I am not following you on this. Would you please explain?

Dances with Shrapnel said:

Tintin said:

Longer rod means shorter compression height.

Click to expand...

I am not following you on this. Would you please explain?

Click to expand...

Compression height is the distance from the center of the pin to the top surface of the piston. Jim