Rob North Framed Commando

SteveA

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Here we are Ken, hope it is of interest.

Martin.
Yes, very much so....

I am interested in the following statement from the article:

....The isolastic mountings were dispensed with, only a damping rubber was retained for the cylinder head steady......

Any idea what was used? Where are the pictures referenced?

On my Rickman I had the rigid head steady to frame lugs fracture, it happened twice, so after having it TiG welded the first time I wondered how to resolve this.

In the end I used an alloy plate on the head and mounted 2 Commando head steady rubbers vertically then linked them individually to clamps around the central for/aft frame tube that links to the headstock. It seems to be holding up, but this Rob North Commando is the only mention I have seen of a rigid mounted engine with rubber in the head steady.

Chris Tyler's Rickman Commando did have a head steady that resembled an after market Commando one in it's previous incarnation (before Yellow!) That mounting had originally used rubber, but from what I recall when it was in my workshop it didn't have rubber in but was bolted up rigid. Chris may know better?
 

Chris

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Hi Steve
Bit of a basic try & tie up the top end job! I think Graham Buller made it up, I remember him saying he didnt want it ridged.
Seeley is the opposite brick sh*t house solid as you like.
Just went to post a photo! Cant file to big for server again!
Stainless Norvil headsteady home made side plates 2 number 2 piece clamps to hold to frame.
 
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lcrken

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I am interested in the following statement from the article:

....The isolastic mountings were dispensed with, only a damping rubber was retained for the cylinder head steady......

Any idea what was used? Where are the pictures referenced?
Mick originally used Metalastic rubber bushes for engine mounting, but he wasn't happy with them, and modified the mounting points to eliminate them. If I recall correctly, the change was just to add large washers to the bushing tubes. I'll try to get some pictures of the mounting areas. I've forgotten the reason he gave for not liking the bushes.

Ken
 

holtcorseaux

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Hi Steve, the photos are in this linked article.
They don‘t show any detail of the head steady, unfortunately.

Martin
 
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On my Seeley 850, the head steady is two short tubes with rose joints at each end. The ends at the rocker-box are held by shoulder bolts, which are in shear. The only way the top of motor can move when it vibrates is sideways. When that motor pulses, everything goes down the chain and drives the bike. With a 72% balance factor, the motor is dead smooth at 7000 RPM, and acceptably smooth anything above 4000 RPM. When the motor idles, the whole bike rocks backwards and forwards. If your motor can move backwards and forwards, you will crack the frame. The impact when it stops causes metal fatigue.
 
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A friend of mine used to do a lot of work for other road racers. One of the things he did was to rubber mount the motors on TZ Yamaha two-strokes. It stopped the frames from cracking, but the bikes never seemed to perform as well as they did with rigidly mounted motors. Compressing rubber mounts at 10,000 RPM probably consumes energy.
 

SteveA

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On my Seeley 850, the head steady is two short tubes with rose joints at each end. The ends at the rocker-box are held by shoulder bolts, which are in shear. The only way the top of motor can move when it vibrates is sideways. When that motor pulses, everything goes down the chain and drives the bike. With a 72% balance factor, the motor is dead smooth at 7000 RPM, and acceptably smooth anything above 4000 RPM. When the motor idles, the whole bike rocks backwards and forwards. If your motor can move backwards and forwards, you will crack the frame. The impact when it stops causes metal fatigue.
I used a single rose joint and tube (quite large for the application). The forward end of the rose joint on a 11mm alloy plate on the cylinder head attached with allen bolts as original steel set up but longer (retained and reused in the revised set up. The rear end was the same material alloy plate, bolted to the two original Rickman frame tabs used in their head steady design. It was the tabs on the frame than broke, presumeably from vibration and for/aft pulsing! But remember these tabs were over 40 years old and the frame has carried different motors over the years from 750 to 1007!

So the damage could have been done whilst warming the bike up and waiting in holding areas as much as on corner exits or at the end of the straight!

Academic really, those tabs are no longer on the frame, and for now, the rubber seems to help!

I have JSM extra long short stroke rods and pistons, balanced at high 70s rather than low 70s it is smooth throughout the rev range! Vibration is certainly not enough of an issue at the bars or through your feet too even think about changing the balance factor, unlike other rigid mounted Nortons I have ridden!
 
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Vibration cracked the featherbed frame of my Triton. But the motor was short stroke and light crank. I really like the heavy crank in the Commando motor. When it is revving near 7000 RPM, it is super smooth and the power delivery is great, with the close box. With a light crank, there always seems to be more anxiety as you ride. You have to be much more on top of what is happening. These days when I race, my brain is pretty much in neutral. I just sit there like a blob and let the bike do it's thing - minimal input. When I had the Triton, I had to always be 110% on top of it.
 
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... for/aft pulsing...I have JSM extra long short stroke rods and pistons, balanced at high 70s rather than low 70s it is smooth throughout the rev range! Vibration is certainly not enough of an issue at the bars or through your feet too even think about changing the balance factor, unlike other rigid mounted Nortons I have ridden!
A higher balance factor increases horizontal vibes and reduces vertical vibes. A lower balance factor does the opposite. Balance factors in the low 60s (wet) give about the same amount of vertical as well as horizontal vibes. This has been measured. Higher balance factors may seem smooth because the riders gravity is downward on the seat and pegs but this could be BS. Stress on the bike is lowest with balance factors in the low 60s with vertical & horizontal vibes balanced - reciprocating weight being the overwhelming cause of stress. My personal preference is a balance factor between 65 and 68% (wet).
 
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I don't know why people go berserk seeking more power from a Commando engine. Mine has minimal done to it and is still quite quick enough to win. I think Jim's long rods with 12 to 1 pistons might give me a bit more power - but with the methanol fuel, I probably don't need it. The Commando engine loves methanol - buy yourself an ELF drum to carry it and try it. The needle jet size is 0.116 inch and 670 mains.
 

Fast Eddie

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Nice enlargement, thanks.

I’d add some vertical bracing to that. Expansion and vibration are gonna put that under quite some stress.
 
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Stuart Garner sold the intellectual property rights of the Norton 961 engine to Zongshen in China, just before his company went into administration.. I wonder what the motors will cost individually ? With 90 BHP on tap, a Seeley or Rob North would be a real whiz.
 

Fast Eddie

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Stuart Garner sold the intellectual property rights of the Norton 961 engine to Zongshen in China, just before his company went into administration.. I wonder what the motors will cost individually ? With 90 BHP on tap, a Seeley or Rob North would be a real whiz.
There isn’t a 961 in the world with “90 BHP on tap” Al...
 
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There isn’t a 961 in the world with “90 BHP on tap” Al...
Even 80 BHP would be good, and if it is 70 as standard., the rest might not be difficult. At least we would be playing with a new motor. If I lived in the UK, I would be looking for makers of after-market frames. Even an early Commando would probably accommodate that motor.
 
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How many gears does the 961 have and are they cassette ? When you trick them up to be close ratio, the bike is usually much faster. What we need is retro-racing and limit capacity and number of cylinders.
 

Brooking 850

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Al, there is already rules like that for some classes, pushrod motors and two Vv's per cylinder!
 
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Al, there is already rules like that for some classes, pushrod motors and two Vv's per cylinder!

Sounds good. In Australia, if the rules have two syllables we have a major problem.
I have always thought the vertical twin four-stroke motor is a long way from fully developed. I'd even bet the Kenny Dreer motor did not have it's ultimate race cam and exhaust system. It takes a lot of racing to get the best out of any motor. I think the 961 would safely rev higher than the early Commando. More revs = more BHP.
 
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