JPN Monocoque Specs

acotrel · Dec 13, 2016

That photo of the Aermacchi being used as a letterbox seems to indicate that somebody has a mental problem.

Rohan · Dec 13, 2016

You don't get out much ?

Waaay! off topic, but mailbox art is a whole unrecognised genre.
Only, books have been published about it.

BTW, thats not an Aermacchi, but a HD Sprint.
The difference may be only technical, but a difference nethertheless...

SteveA · Dec 14, 2016

lcrken said:
acotrel said:

I noticed in what you have posted, that 33mm carbs are used. I wonder if the inlet ports were opened out to that size ? I use 34mm carbs, however the inlet ports are tapered back to standard within 25mm of the flange - NOT parallel.

Click to expand...

Probably not. In the factory race heads I have seen, the intake ports at the manifold surface are 32 mm, and the manifolds are internally tapered to match the carbs at one end and the head at the other, i.e. 33, 34, or 36 mm at the carb, depending on carb used, tapering to 32 mm at the head. The 33 mm carbs at the time were just standard 932 Amals bored out to 33 mm. The factory tuners didn't generally go crazy on opening up the intake port diameters. Even the heavily modified big valve head that I have from one of the factory flat track bikes still has the intake port at 32 mm, even though it was used with 36 mm carbs (1036 Amals). All the tapering was done in the intake manifold.

Ken

The ex factory head I had was worked by John Baker (Baker Rawlins drag race team tuner). It had been taken out to 34mm right to the inlet face.....

It worked with 36mm carbs, not so well when I tried 34mm carbs so I went back to 36.

By contrast Tony Smith (Thruxton Motorcycles and one time JPN Cosworth rider) preferred the port down at 30mm, with 36mm carbs and Cosworth style wide trumpets, run with a 2S cam.

acotrel · Dec 14, 2016

One thing I've noticed when reading up about ignition systems is that the Dynatek unit which is used on Harleys has a purple wire which is usually left unearthed in other applications. On a Harley it is attached to the vacuum switch in the inlet port, so that if the motor loses vacuum at full throttle, the unit switches over to a less aggressive advance curve. My question is whether when you port the commando inlet ports larger, is the motor more likely to lose vacuum at large throttle openings ?

Dances with Shrapnel · Dec 14, 2016

acotrel said:
One thing I've noticed when reading up about ignition systems is that the Dynatek unit which is used on Harleys has a purple wire which is usually left unearthed in other applications. On a Harley it is attached to the vacuum switch in the inlet port, so that if the motor loses vacuum at full throttle, the unit switches over to a less aggressive advance curve. My question is whether when you port the commando inlet ports larger, is the motor more likely to lose vacuum at large throttle openings ?

And how does this relate to the current thread titled "JPN Monocoque Specs"?

Drifting Alan, drifting. Fair question but perhaps a new thread is warranted?

acotrel · Dec 15, 2016

I was continuing from the comment about the inlet port size on the JPN. I find the way the bike was set up interesting. It was obviously tailored to suit the IOM, where there is not so much tight stuff to contend with. It doesn't mean it would be a good short circuit bike. Once you have ported a commando head bigger than standard, there is no easy way back.

Bernhard · Dec 16, 2016

acotrel said:
I was continuing from the comment about the inlet port size on the JPN. I find the way the bike was set up interesting. It was obviously tailored to suit the IOM, where there is not so much tight stuff to contend with. It doesn't mean it would be a good short circuit bike. Once you have ported a commando head bigger than standard, there is no easy way back.

Re; “It was obviously tailored to suit the IOM, where there is not so much tight stuff to contend with.”

You quite obliviously have never been to the Island :?:

acotrel · Dec 16, 2016

When I was a kid, a mate of mine suggested I join the company he worked for and we could both transfer to the UK and go road racing. Because he was a bit of a rat-bag, I did not listen to him. If I had gone there, racing on the IOM would have been an option. I would probably have gone home in an urn. I now regret that I didn't go there - what is important in life ?
When I started racing in the 60s, from what I read about the UK and the imported racing bikes I rode, I gained the distinct impression that UK race circuits must have been much bigger and free-flowing than most of our Australian circuits. Where do you make the most time up when you ride a Commando on the IOM ?

acotrel · Dec 16, 2016

Bernhard, what I am asking you is whether the JPN Monocoque is set up for big open circuits or short tight circuits. That port size and steering geometry would seem to indicate the theory was to have the JPN behaving like a bevel Ducati where you take the high line in corners and ride around the opposition, rather than go under them on the tight line. The behaviour in the race at Silverstone would seem to confirm this.

pommie john · Dec 16, 2016

Bernhard said:
acotrel said:

I was continuing from the comment about the inlet port size on the JPN. I find the way the bike was set up interesting. It was obviously tailored to suit the IOM, where there is not so much tight stuff to contend with. It doesn't mean it would be a good short circuit bike. Once you have ported a commando head bigger than standard, there is no easy way back.

Click to expand...

Re; “It was obviously tailored to suit the IOM, where there is not so much tight stuff to contend with.”

You quite obliviously have never been to the Island :?:

To be fair, on the IoM there are tight bits, ludicrously tight bits like Governors bridge but the lap time is made on the fast bits, and the fast bits are ludicrously fast.

Last time I was there i was chatting to a classic rider and he had a map of the circuit with highlighter pen on certain sections. A long section through Crosby, large parts of the mountain, about a third of the circuit was highlighted with red pen. The highlighter pen marked sections where he was flat in top gear.

Now he was riding ( I think ) a 350 Ducati which will clearly be flat in top a lot longer than a Commando, but it gives an idea of how fast that circuit is, and why PW spent so much effort on aerodynamics.

pommie john · Dec 16, 2016

acotrel said:
I gained the distinct impression that UK race circuits must have been much bigger and free-flowing than most of our Australian circuits. Where do you make the most time up when you ride a Commando on the IOM ?

They are just as varied as the Aussie ones. Silverstone and Thruxton were very fast, Cadwell quite tight. Brands Hatch a bit of both.

wakeup · Dec 16, 2016

I remember reading an article in the old "Motorcycle Sport" magazine, where former Norton works rider and Australian, Jack Ahern, said that on a 500 Manx, the IOM was flat out all the way, with a couple of obvious exceptions. Not necessarily flat in top, but flat in a gear waiting to change up.
Still frighteningly fast!!
cheers
wakeup

Bernhard · Dec 17, 2016

pommie john said:
acotrel said:

I gained the distinct impression that UK race circuits must have been much bigger and free-flowing than most of our Australian circuits. Where do you make the most time up when you ride a Commando on the IOM ?

Click to expand...

They are just as varied as the Aussie ones. Silverstone and Thruxton were very fast, Cadwell quite tight. Brands Hatch a bit of both.

When riding flat out with the lamp posts, kerbs and other street furniture in the IOM on a big capacity bike ( or even a small bike) it’s called bottle :!:

acotrel · Dec 18, 2016

When I road race, I have the theory that however fast you go, that is the speed at which you must be prepared to get off at. Phillip Island is fast and the kids love it, however any crash there is always a biggie even though there is virtually nothing to hit. When you race an underpowered bike such as a commando, riding around the outside of the opposition is an entirely different proposition to turning under them and getting on the gas earlier . Bearing in mind that the JPN Monocoque have the gearbox, leading the field at Silverstone was probably quite a feat.
When I was a kid, I purposely stayed away from trying to get sponsorship, manly because I did not want to be obliged to race at Bathurst. Going there to race by choice is a different situation. I feel similar about the IOM. I would go there for the experience, in every other aspect it is silly stuff. Road racing should not be about 'bottle' - if you are COMPETENT. If you are not competent, perhaps you should not be there ?

nortonmews · Dec 23, 2016

Rohan said:
I don't believe I've ever seen a pic of it in that format though.
Anyone ?

Isn't there mention too that it has recently been transformed back into a bike. ?
Not surprising, given the rarity of them...

Hi

Just as an update, the January 2017 copy of Classic Bike has an article on the later space frame bikes and replicas that have been made by P&M. The guy who commissioned these replicas (he also has an original and a replica of the earlier pannier tanked version) is the owner of the restored lamp standard bike ............. and my wife says I have too many!

nickguzzi · Dec 23, 2016

Bob MacIntyre, when asked how he managed to tide the TT Mountain section so fast - "... as fast as I can up, and as fast as I dare down..."
I don't recall any questions about his competence, but it was probably on his Matchless framed special.

acotrel · Dec 25, 2016

I met a friend of mine in the 70s whom I had not seen for a while. He'd been away racing on the IOM. He was a pretty fast guy with a Manx. When I asked him how he liked it, he said 'when I first went there, I did an 84 MPH lap. But if you keep going there you get quicker'. What I don't like about what I've heard, is the thought of losing track of where you are on the circuit. I know when Maurice Quincey rode there in the 50s, he spent a couple of months on the ship learning the circuit from the Ordinance maps.

Rohan · Dec 25, 2016

The older magazines used to quote riders that said it took them 7 years to get to properly know the IoM circuit well enough to be seriously fast.

They only race there once a year don't forget, a couple of practice laps and 5 or 6 or 7 laps in a race. So it takes some years to always know where you are - important, as you peel off for many of the faster corners before you can see it.

acotrel · Dec 25, 2016

Perhaps that is the reason the guy on this 750 Rob North Trident seems to be getting on the gas slightly late when coming out of corners ? :
https://www.youtube.com/watch?v=q4ARn3LzuJE&t=412s

Matt Spencer · Dec 25, 2016

Maybe He's late for work .

JPN Monocoque Specs

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