The Commando Frame - NOC Roadholder May 2018

With a road race bike, you are always braking and accelerating while banked over. If you can do that on a race track, you should be able to do it on public roads. With an old British bike, if the weight is well forward the handling usually inspires confidence, especially if the steering geometry is right. No Commando ever hi-sided anyone unless the drive train seized.

acotrel said:

No Commando ever hi-sided anyone unless the drive train seized.

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Certainly not true! My friend high sided his Commando race bike going into turn 2 at Daytona and spent a couple weeks in the hospital with broken pelvis, etc. I'm sure he's not the only one to ever do so.

Ken

baz said:

There are all sorts of theories being bandied around on here !
Do people really think that Norton got the iso mounting plates perfectly aligned when the frames were welded up ? and any extra ones can't be aligned properly?
So going round a bend pulls the frame out alingment and causes vibration?
Ect ect ect ect I tend to just do what I want with my bike ,I don't just theorise about what may or may not happen i just go on what I have actually done every person that has ridden my bike has been impressed with it
I guess it's down to what you want from the bike, each to their own I guess

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A few incorrect assumptions here. The subjects being explained are why a fourth isolastic is no necessary and, relevant to the correct working of isolastics, alignment of components to horizontal and vertical planes (far from all sorts of theories) which you have possibly done yourself to some extent as you previously mentioned a surface plate. The majority of posts suggest people are fully aware of the variability of manufacturing of Commandos and therefore the variability in handling, no two bikes being the same. Subsequently leading to owners looking for ways to rectify these problems. Any frame will have its limits in maintaining wheel alignment but it does not follow that cornering a Commando will pull the frame out of alignment. I agree the main problem with a Commandos handling (including mine) is factory manufacturing. So unless you corrected the existing misalignment when adding an extra isolastic yours may be a case of, to quote parallel engineering, "partially address the symptoms - not the cause". I had read the worlds straightest Commando several years ago but both websites offer useful info in my opinion. I think I will see about rectifying the problems this winter.

Chris ton a bike said:

But we are not considering three 'points', we are concerned with the surface areas of 6 pairs of flat thrust faces (2 pairs per isolastic), ..

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Well.. it took a while , but I was expecting this to turn up .
The “ 3 points , one plane” is of course a simplification .
In reality it is 4 parallel planes , or 6 , depending on type of headsteady .
(a tie rod type headsteady is supposed to have no lateral play at all .)
But because of the relative small diameter of the mounting collars and a 0.2 mm gap , it would take an awfull amount of deformation for them to bind up .
While in a 4 point setup , a tiny movement of one of the points ( you suggested 0.001 “ ? ..) all 4 would bind and transmit vibration .
BTW ,the thrust faces of the collars are often not parallel , so I believe it is better to use a dial gauge instead of feelers to set the isos .
On the same page of this forum , there is a thread about cracked frame tubes .
Suggestion is that it might be caused by too tight isos .
I “ heared “ that bolting the engine solid in a Commando frame will cause the frame to crack .
Don’t know if this is a myth or a real risk .
If true , it would be a compelling argument against a 4th iso..

ludwig said:

Well.. it took a while , but I was expecting this to turn up .
The “ 3 points , one plane” is of course a simplification .
In reality it is 4 parallel planes , or 6 , depending on type of headsteady .
(a tie rod type headsteady is supposed to have no lateral play at all .)
But because of the relative small diameter of the mounting collars and a 0.2 mm gap , it would take an awfull amount of deformation for them to bind up .
While in a 4 point setup , a tiny movement of one of the points ( you suggested 0.001 “ ? ..) all 4 would bind and transmit vibration .
BTW ,the thrust faces of the collars are often not parallel in the first place , so I believe it is better to use a dial gauge instead of feelers to set the isos .
On the same page of this forum , there is a thread about cracked frame tubes .
Suggestion is that it might be caused by too tight isos .
I “ heared “ that bolting the engine solid in a Commando frame will cause the frame to crack .
Don’t know if this is a myth or a real risk .
If true , it would be a compelling argument against a 4th iso..

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Let's be clear about this ,if you set the ISOs to zero IE locked solid it can sometimes cause the frame to crack
In my opinion if you want your commando to handle the best it can be is to align the frame/gearbox cradle swing arm
ream engine bolts to fit, address the sloppy swing arm pivot decent head steady first
Then brace the swing arm (especially if it's a 750) only after these have been done go for extra ISOs modern forks /light wheels etc etc as I have done,
If I hadn't achieved what I wanted with my bike I would have considered rigidly mounting the swing arm or a complete new frame

If anyone wants an ISO experience try riding a Harley rubber mount

Chris ton a bike said:

But we are not considering three 'points', we are concerned with the surface areas of 6 pairs of flat thrust faces (2 pairs per isolastic), assuming a isolastic head steady is used, which all need to be parallel to each other and the vertical plane and perpendicular to the horizontal plane for them to work as the designers intended. Any frame distortion will result in the surfaces no longer being parallel thus transmitted vibration is increased. The main problem is not frame distortion but getting the components which make up the chassis assembly parallel and perpendicular to the horizontal and vertical planes as required.

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ludwig said:

Well.. it took a while , but I was expecting this to turn up .

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Yes, you appear to be very wise....that is after I have explained why banging on about 3 points is useless in a 3 D application.

ludwig said:

In reality it is 4 parallel planes , or 6 , depending on type of headsteady .

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Sounds familiar?......

ludwig said:

While in a 4 point setup , a tiny movement of one of the points ( you suggested 0.001 “ ? ..) all 4 would bind and transmit vibration .

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There you go again about 3 points make a plane, 4 points don't. We are concerned with surface areas, 6 pairs with 3 isos fitted. Note that is three 3D isolastic mounts with thrust face surfaces.

ludwig said:

BTW ,the thrust faces of the collars are often not parallel.

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No shit Sherlock!

ludwig said:

so I believe it is better to use a dial gauge instead of feelers to set the isos.

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????????????

lcrken said:

Certainly not true! My friend high sided his Commando race bike going into turn 2 at Daytona and spent a couple weeks in the hospital with broken pelvis, etc. I'm sure he's not the only one to ever do so.

Ken

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A supercharged road race Commando, or was it oil all over the rear tyre ?

The balance factor of a Commando crank is probably designed to give smooth running at about 3000 RPM. If you use the bike for highway riding and keep it buzzing, the isolastics probably have to do a lot of damping. In most old bikes the balance factor is set to suit the most common mode of use. If you are racing or doing high speed highway, it is normal to raise the balance factor to get smooth running. Just because you have isolastics, that does not mean destructive forces are never there. To my mind, the hole in the Commando crank is a belt and braces answer to the vibration problem.

acotrel said:

A supercharged road race Commando, or was it oil all over the rear tyre ?

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Neither. The bike is the silver and black one (number 407) in this picture from Daytona in 1990. Commando frame with short stroke 750 engine running in AHRMA Formula 750. He got into the left hander leading to the international horseshoe really hot and grabbed a big handful of throttle. The rear slid out, then caught, high sided, and threw him up in the air. When he hit the ground he had a broken pelvis and other injuries. It was the end of his racing days.

I don't make this stuff up, Alan. If you have trouble believing it, that's your problem.



For those interested, the bike on the left is my heavily modified 920 Production Racer, next to it is my ex-Jim Schmidt monoshock Norton, also with a 920 engine, and the bike on the right (182) is Rob Tuluie's 750 Commando race bike.

Ken

A nice line up Ken.

I can hear them start up in my head!

Those were the days eh?

Chris ton a bike said:

No shit Sherlock!
ludwig said: ↑
so I believe it is better to use a dial gauge instead of feelers to set the isos.

????????????

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Elementary my dear Watson :