lets go racing

Hi Acadian I have a 750 Maney engine one that Steve took as a spare to Daytona. It has the welded plate on the drive side & runs standard Commando rods, steel flywheel. Enough go & revs quickly.
Alloy barrels are a massive weight saving, however you have already gone to the next stage with Molnars crank & Jim's rods & pistons. I would protect that investment with the Molnar crank cases first.
Your not going to stop lol the barrels will come later.

Tough call and the obvious answer is both !

But if you really gotta prioritise one… then I’m with Chris.

good advice, makes sense... much easier to swap out to a better barrel down the road than a set of cases!

If I was building a race bike using a Commando frame, I would try to extend the swing arm spindle and pick up the ends with modified Z-plates. That would connect the handle bars to the rear tyre contact patch. A Commando motor should be fast enough in relatively unmodified condition. When you begin to work with the bike, you will tune the motor and gearing, and probably get more out of it than you might expect. With petrol as fuel, it should still be fast enough to be competitive. The crank balance factor and gearing determine the revs you use. High gearing with close ratios use torque better. Petrol as fuel is more difficult to tune than methanol - but not impossible. Carburetors are a British thing - like cricket. The rev range I use with my 850 motor is from 5,500 RPM to 7000 RPM. It is relatively unmodified.

I am unsure how connecting the isolastic mounted spindle to the rigidly mounted Z-plates would result in anything beneficial? Wouldn't that just result in the Z plates getting ripped off the bike... but maybe I'm missing something in the logic here

On a normal Commando, the rear wheel is connected through the swing arm to the engine cradle, which moves independently to the handle bars. That movement detaches your feel of the rear wheel contact patch on the road. If you are riding fast, your safety depends on feeling any sideways slip. By firming the spindle to the frame, you should get better feel - the loads should not be great enough to destroy the mounts - because you shim the isolastics. The modification would tend to stop the swing arm from flexing the cradle. With Manx Nortons - the replacement if Silentbloc bearings with brass bushes in the swing arm makes a difference to the feel.

acotrel said:

On a normal Commando, the rear wheel is connected through the swing arm to the engine cradle, which moves independently to the handle bars. That movement detaches your feel of the rear wheel contact patch on the road. If you are riding fast, your safety depends on feeling any sideways slip. By firming the spindle to the frame, you should get better feel - the loads should not be great enough to destroy the mounts - because you shim the isolastics. The modification would tend to stop the swing arm from flexing the cradle. With Manx Nortons - the replacement if Silentbloc bearings with brass bushes in the swing arm makes a difference to the feel.

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It seems that you don't understand how isolastics work. The shims restrict left right movement which is negligible and the clearance is solely to allow free vertical plane movement constrained by distortion of the rubber donuts within the isolastics. You can't just bolt up the area near the rear isolastic since it would defeat the whole purpose of the frame's design. It would eventually either destroy the Z-plates or fatigue the fasteners and they would shear.

I would not modify the Commando engine until the bike has been ridden on a race circuit, and a need for more torque has been detected. The Commando engine is different from many. With most race bikes, if you lower the gearing - the bike will accelerate faster. The Commando engine has the heavy crank, it tends to spin up at the same rate regardless of the gearing. Smooth power delivery is more important than more power. I use a 2 into 1 exhaust system, high gearing and a close ratio gearbox. It means I can be much faster in corners - there is no power bump. I fly into corners while braking on a lean while changing down, then immediately start accelerating full blast while changing up. My bike oversteers - both going into and coming out of corners, and it stays more vertical.

Al.... You are hijacking Acadian's build thread talking about yourself,.. again.

hhh said:

It seems that you don't understand how isolastics work. The shims restrict left right movement which is negligible and the clearance is solely to allow free vertical plane movement constrained by distortion of the rubber donuts within the isolastics. You can't just bolt up the area near the rear isolastic since it would defeat the whole purpose of the frame's design. It would eventually either destroy the Z-plates or fatigue the fasteners and they would shear.

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Doesn't the spindle pass through the rear isolastic mount ? Where does the cradle push against the frame ? All you would be doing would be providing a connection of the bars to the rear wheel - a couple of rubber bushes in the Z-plates should be enough to stop destruction.
That assembly where the swingarm is attached to the engine cradle is the same problem which pre-unit Triumphs and Matchleses had - the mount is too narrow, so gets twisted. Also clearances have a greater effect. BSA Twins had a spindle which was mounted much wider - at it's ends. And so did the Featherbed.
What I am suggesting would cause the engine cradle to rotate around the swing arm spindle.

I don't really know how it all works. When I ride, my brain is glued to the rear tyre contact patch. But I know if the bike is light in the front and if it is understeering or oversteering. I also know when I am losing traction when the bike is accelerating, and whether it is accelerating faster or slower. The riders' only contact with the bike is through hands, feet and backside. Your eyes are used to judge the corners and your ears hear the revs.

I appreciate that you're trying to offer advice Al, but the commando frame is just not designed that way, there are alternative options for rigidly mounting the motor and swingarm, but that's a different bike altogether

acotrel said:

I don't really know how it all works.

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Perhaps best to refrain from suggesting changes to the design then.

acadian said:

I appreciate that you're trying to offer advice Al, but the commando frame is just not designed that way, there are alternative options for rigidly mounting the motor and swingarm, but that's a different bike altogether

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Herb Becker did something to the Isolastics on Doug MacRae's Commando which improved it's handling. I am probably different from many other who race. If I can help other guys to race faster, I am happy to do it. When I was young and started, only two older guys ever told me anything which helped. I do not believe in secret racing intelligence. There is only one other guy here who has a Seeley Commando, and I will never get to race against him. My mate out-rode him at Broadford with a Yamaha RZ350, and was proud of it. An RZ350 should be almost as fast as a TD3. It is all bullshit anyway. The only bike I would really like to beat with my Seeley 850 is a good 900cc Bevel Ducati.

acotrel said:

Herb Becker did something to the Isolastics on Doug MacRae's Commando which improved it's handling. I am probably different from many other who race. If I can help other guys to race faster, I am happy to do it. When I was young and started, only two older guys ever told me anything which helped. I do not believe in secret racing intelligence. There is only one other guy here who has a Seeley Commando, and I will never get to race against him. My mate out-rode him at Broadford with a Yamaha RZ350, and was proud of it. An RZ350 should be almost as fast as a TD3. It is all bullshit anyway. The only bike I would really like to beat with my Seeley 850 is a good 900cc Bevel Ducati.

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If you actually read this thread, you’ll see that Acadian has already executed his own iso / cradle mods.

Fast Eddie said:

If you actually read this thread, you’ll see that Acadian has already executed his own iso / cradle mods.

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That is probably a very good idea. The long and short of it is, that I would not attempt to race with that frame, particularly as a beginner. I know you guys have brand loyalty - but so do I. The rider adjusts to the bike. If you begin racing with a bike which does everything properly, it creates an expectation about what a good bike should do. Mike Hailwood began racing on a World Championship winning Mondial 250 - not some bitsa. His feel for a motorcycle was conditioned by that, and the rest is easy. If you start with old crap, you are beginning from square one. To get one bike to handle, I had to move the motor forward, and fit a two into one exhaust. It worked -but it was mainly arse beats class. My Seeley 850 is an excellent and easy ride, but I knew how to fix it because of my previous mistakes.
It is very noticeable that Australians who race internationally, are usually miles ahead when they return and race in Australia - if compared with what they were like when they left. One of my mates was 2nd in 250cc MotoGP one year. He was really funny at Sandown - the length of the front straight ahead of the A grade field on a borrowed TZ250. He had brought two tyres and done the chamber mod.