Fastest Isle of Man Commando?

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Hi Kenny

I hope you make it to Pukekohe. But Im still stuck here in Kazakhstan - another 18 months to go but my bank balance has gone up a bit. I will be 60 next year and have to save a bit for retirement - or more likely racing bikes when I get time to "retire"

Just got some good news.

Chris Swallow got second in the Manx 350 Classic a couple of hours ago.

He is living down in NZ these days (Actually looking after my house while Im away). He rode my Dommie down at the Burt Munro and was pretty polite about it for an old bus :) He and Tony are a pretty good match riding. Way out of my league.

Chris rode the ES2 in Australia and I think got a few wins on it. Its very good but sometimes a bit fragile. The Lodges have got a big engineering place in Auckland so they set the bar pretty high for us wage earners :-0 Petes a good guy. And a pretty full on rider one or two years back !!

Regards

John
 
Here is a shot I took last month of Jamie Waters incredible, almost exact to the original, running Norton Monocoque replica race bike he had Norman White, the original Peter Williams mechanic spend 3 years building. It sounds awesome. He also happens to own the only original, unrestored Monocoque in existence. I had an hour long chat with Peter when he was in town in January 2010, he is a good bloke who still has some new interesting motorcycle frame ideas.

Fastest Isle of Man Commando?
 
By original, that means long stroke?, he wasn`t tempted to mix the best of the J.P.N. F 750 short stroke `74 mills with the `73 monocoque?
 
From some previous threads here, the short stroke 750 engines didn't live up to their promise - tthey didn't rev higher enough to make any more power was the long and the short of it.
And the head for it was a dud, I think the conclusion was.
Correct me if my memory has fuzzed the precise details.

By eck that is a neat bike, I'll take a dozen please, with lights, for road use....
 
Rohan said:
From some previous threads here, the short stroke 750 engines didn't live up to their promise - tthey didn't rev higher enough to make any more power was the long and the short of it.
And the head for it was a dud, I think the conclusion was.
Correct me if my memory has fuzzed the precise details.

By eck that is a neat bike, I'll take a dozen please, with lights, for road use....
Click to expand...

The works ran short strokes, the heads worked with the pistons designed for them. And as I pointed out on another thread my own short stroke head and suitable pistons worked with the 89mm crank to make an 850 version pretty potent. That motor only revved to 6800, long stroke 750s were running easily to 7500.

Building something today might give different conclusions, as you suggest most seem happy to stay with 89mm stroke and a standard squish head with smaller ports and there were plenty of those around going pretty well in the '70s. I am planning to go short stroke and I have an 80.4 from Maney. I have said before that the reason is more nostalgia than a search for power. But I am 58 and have done 5 race meetings since returning after a 28 year break! I ain't going to be running at the front with the likes of Gary Thwaites and his Watson 1007 Seeley. I want to run a 750 to get more rides in a weekend with the UK CRMC by running F750 as well as 1300 twins, and maybe get into Belgium where '72/750 is the max I could use, bore and stroke changes are fine with CRMC.

With the short stroke you are trading some mid range for some revs, but it could be more revs than you think. With JS valve gear I expect higher revs, Holmslice tested a short stroke and said it was like his long stroke but revved 1500 higher! but as you will know has stuck with the long stroke for mid range so far. Getting the compression you want with a short stroke may be harder, but I never was a fan of really high compressions and would prefer to use pump fuels. I would just like to get around 10.25:1 to 10.5:1, I have a set of JS flat top 77mm pistons and am not sure yet what else I need to do to get that high with them.

If I find I need more horsepower and run fewer rides per weekend another option is to use an 81mm barrel and run a square 850 using the short stroke crank?

But one thing at a time. Long project, short pockets!
 
This head of yours, what RH_ is it ? (talking bikes here!)

P.S. Couldn't get facebook page/pics to open, do we need permissions or passwords or something ? Cheers.
 
SteveA said:
With the short stroke you are trading some mid range for......!
Click to expand...

Not so. My expereince is to the contrary. More revs was a simple misconception on my part in the past. Keep in mind that with the shorter stroke comes larger valves. With larger valves comes better volumetric efficiency (ie torque). I am running a 75mm crank and it is like running a very well race tuned long stroke to 7,000rpm and then you have an additional +1,500rpm of good breathing and power.
 
SteveA said:
The works ran short strokes, the heads worked with the pistons designed for them.!
Click to expand...

I don't think i have ever seen quoted that the JPN works Commandos were short strokes - is this written down or discussed anywhere ?

The fact that the production short stroke 750 engine for ~1973 ish wasn't a success makes you think they didn't have experience with making such things fly ??
 
Doug MacRae said:
Here is a shot I took last month of Jamie Waters incredible, almost exact to the original, running Norton Monocoque replica race bike he had Norman White, the original Peter Williams mechanic spend 3 years building. It sounds awesome. He also happens to own the only original, unrestored Monocoque in existence. I had an hour long chat with Peter when he was in town in January 2010, he is a good bloke who still has some new interesting motorcycle frame ideas.

Fastest Isle of Man Commando?
Click to expand...

I was just looking at this bike in the real this past Sunday. Something I never realized about the bike that is now apparent is the clutch. Yes it looks like a standard Commando style but it is significantly smaller in diameter.

Wonderful craftsmanship of the bike. And to think, the heart of it is a Norton Big Twin engine.
 
I think the assumption that with shorter stroke comes larger valves, and thus more torque is incorrect. If the crank is short stroke, and the rods are still long, the angularity has the effect of moving the torque up the rev range, and you find the bike will be faster at the end of long straights. If you enlarge the inlet port to suit larger valves , the gas speed drops - better to taper the port as done in aermacchi singles ? You have a choice - build a high revving stressed motor with a lot of top end, or build a slow revving torqey motor and pull a lot of gearing. With any racing bike, the limitting factor is always the gearbox. If you use the peaky high revving motor, you need t o keep it on the boil. If you use the torqey motor you need a lot of gears to get it up to speed on long circuits. A lot of guys use converted street bikes for racing. I tried to use the standard commando box - a waste of time. Even the close 4 speed box will get the bike accelerating much faster, and make it much more rideable around tight circuits, but first gear will probably be too high for fast clutch starts, if you get your overall gearing right for most of the circuit..
 
Huh, missed the smear but dam well better go faster on next Peel or she's a bust. Brambles are fun first few times, no problemo with full armour on and mirrors turned in, if ya can take the suspense of not seeing where ya going at break neck speed but quit it d/t dam ticks and chiggers getting in for misery later. This shot is where growth low enough to get off and have bike show up. The MX guys wouldn't run it and I got bored on their open paths. My great surprise was off road bike character and pilot reflexes works even better on tarmac twisties.

Fastest Isle of Man Commando?

Fastest Isle of Man Commando?
 
We had better put you in the baja 1000 .P11s won Mohave Twice . said they invented the big off roaders before they knew what they were .Super motad .
 
acotrel said:
I think the assumption that with shorter stroke comes larger valves, and thus more torque is incorrect. If the crank is short stroke, and the rods are still long, the angularity has the effect of moving the torque up the rev range, and you find the bike will be faster at the end of long straights. If you enlarge the inlet port to suit larger valves , the gas speed drops - better to taper the port as done in aermacchi singles ? You have a choice - build a high revving stressed motor with a lot of top end, or build a slow revving torqey motor and pull a lot of gearing. With any racing bike, the limitting factor is always the gearbox. If you use the peaky high revving motor, you need t o keep it on the boil. If you use the torqey motor you need a lot of gears to get it up to speed on long circuits. A lot of guys use converted street bikes for racing. I tried to use the standard commando box - a waste of time. Even the close 4 speed box will get the bike accelerating much faster, and make it much more rideable around tight circuits, but first gear will probably be too high for fast clutch starts, if you get your overall gearing right for most of the circuit..
Click to expand...

Those are your assumptions, not mine. All your assumptions and suppositions above are fine (except the assumption of "peaky high revving motor" - that is a bit of a cliche) but this is my real world experience. As I mentioned above, the bike behaves like a top performance standard stroke race motor (which are not peaky) but has an extra +1,500 rpm to spin up, breath well and continue to increase power. This is with seat of the pants track experience as well as dyno torque/power curves to back it up.

What appears to be overlooked in the above analysis is that a short (80.4mm) stroke 750 has about 10% more piston area than that of a stock (89mm) stroke 750 for the gas to act on so for a given cylinder pressure that is 10% more force. With the ultra short (75mm) stroke the piston area is about 20% greater than that of a stock stroke Norton twin. You do not necessarily need to enlarge the ports much to get very good performance. I have to check but I believe the 75mm stroke Norton has ports at around 35/36mm with 38/40mm carbs and runs like stink from 4,000 rpm upwards. With the larger valves is great valve-time-area; this is one key element which allows the engine to breath at higher rpm where there is less time for inflow. Cam timing/duration is also critical
 
More interesting data, good stuff guys.
In that 'Best of British" Norton doco, Wierd Al Crashcart wobbles about on the `73 & `74 J.P.N.s back-to-back, & comments on the character of the short stroke `74 mill favourably...[if anything he sez can be taken on face value].
 
Hmm, what Dances describes sure matches my po P!! power band , strong off idle reaction that got alot better til 9000. It remains the best power to weight thing I ever tried. it didn't have a speedo just tach so don't know how fast I ever went ahead of others. The 1%'rs I meet said they were at the ton to cruise with me, while my hands swelled up. If it was a short stroke then a stroke of Norton genius to mimic. i never blew it up so never knew how fragile Nortons were to rpm. I sometimes pretend practicing for IOM by passing close to rock faces and trees to find no problemo as long as they stay in their places.
 
Hmm, what Dances describes sure matches my po P!! power band , strong off idle reaction that got alot better til 9000
Click to expand...

curious if you could tell us how your Commando continued to pull all the way until you saw 9000rpm

given the inevitable valve float somewhere over 7500rpm?

perhaps you had installed super heavy duty valve springs to eliminate this?

also if you don't mind, whatever gave you the tremendous confidence to take your motor over 1000rpm past redline?

did you have the motor prepared by one of the world's top rebuilders?
 
I don't mind setting ya straight u/dn. 1968 P!! P!! P!! P!! P!! P!! P!! P!! P!! P!!. Tach was marked at 9000 by race builder, not me who didn't know better than peg it there often as could. I don't think my 920 Maney JS piston rod Norris D cam engine could match it w/o a blower.
No way to build a Commando as light either w/o it no longer being a Commando.
But could not run IOM road course as could not lean much. I had nothing to do with its creation which was Paget's mc shop in Fla. project to show what Brit Iron could do in the 2 stroke hey days. Boy Howdy did it. Good as Axtel and Woods desert sleds. It changed my life but only recently been able to try to get back the insane thrills it delivered in public use. Still have deeply mixed feeling about losing to theft as may of saved my life or spend it more like Munroe. if ya never ridden a Norton that would float its front level in 4 gears then ya missed out on something unforgettable. Peel of course will be faster yet but so safe I'd let ya ride it as so so good to handle front lifing power turns I'd get off on your bug eyed reactions for putting up with my unbelievable reports.
 
Well, if those old school NASCAR pushrod mills can handle being pinned at 9000rpm for long periods, then it shouldn`t be outside the realms of reason to get it from a Norton, what do those race XR 750 short stroke H-Ds rev to?
 
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