The Commando Frame - NOC Roadholder May 2018

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Sounds good Baz.

Isn’t it a bit of a stretch by Norton claiming to have 3 iso mounts though, the stock head steady is only silent block mounts after all.

I have the Comnoz tie rod head steady, cotter pin swinging arm spindle, and run my iso’s ‘just backed off’. Tight iso’s also increase vibration a little, but it’s by no means unpleasant or an issue. Falcon shocks and Lansdowne dampers were the icing on the cake.

I do kinda wish I’d put the extra effort into ensuring perfect alignment whilst it was all stripped down. But I didn’t. Maybe next time?
 
Does the tie rod type head steady transmit any vibration?
I assume from reading other posts that they clamp to the frame?
 
The Comnoz style one bolts to the stock hole in the frame.

I don’t think it transmits any vibration. It’s running the front and rear iso’s quite tight that does that.

To my mind, it gives just the right amount of ‘mechanical feel’ to the bike (read: mild vibration) and certainly tightens up the handling a lot.
 
I do not doubt that adding an iso below the gearbox can improve handling ,
If you were to mount two levers, one longer than the other, perpendicular to a shaft which isable to rotate but with high resistance due to partially seized bearings, to rotate the shaft at a certain angular velocity (rpm) less force would be required at the end of the longer lever than the short lever.

When a Norton Commando is leaned into a corner the rear wheel gyroscopic force and inertia resists any leaning force and therefore applies a torque acting on an axis between the front and rear isolastic mounting points because the rear wheel is connected to the front isolastic via crankcases via gearbox cradle(inc.rear isolastic) via swinging arm. To overcome the torque Norton engineers fitted a 3rd isolastic mounting point at the cylinder head because this provides a longer lever than fitting an isolastic on the gearbox cradle in the ratio of approx. 3:1. So the advantage of fitting a fourth isolastic can only be psychological as there is no mechanical advantage.

The goal of smoothing out all vibration above 3000rpm can only be acieved with 3 points , and rather loose settings of the isos .
Four isolastics with thrust faces mounted parallel to each other and perpendicular to the horizontal plane will work as intended even with minimal clearance (0.001") as the assembly will still be able to move in the vertical plane. The difficulty is setting the thrust faces exactly in the vertical plane. The closer to the plane the less clearance required to achieve mimimum transmitted vibration.

adding a 4th iso goes against the concept of the Commando frame , and can only result in increased vibration .
4th isolastic is superfluous to requirements, see above.
 
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Four isolastics with thrust faces mounted parallel to each other and perpendicular to the horizontal plane will work as intended even with minimal clearance (0.001") as the assembly will still be able to move in the vertical plane..

Yes , but only static .
The ( 4 ) thrust faces will not stay in the same plane , when the bike is rolling .

Kenny Dreer placed an Iso below the gearbox , but omitted the headsteady ( so still 3) on his VR 880 . Which I found a little odd , because even rigid mounted Norton twins have a headsteady .

Comstock's headsteady is good , but mine is better .. :)
 
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Yes , but only static .
The ( 4 ) thrust faces will not stay in the same plane , when the bike is rolling .

Kenny Dreer placed an Iso below the gearbox , but omitted the headsteady ( so still 3) on his VR 880 . Which I found a little odd , because even rigid mounted Norton twins have a headsteady .

Comstock's headsteady is good , but mine is better .. :)
The parallel thrust faces of three isolastics would still be subjected to the same temporary frame distortion as four. Omitting the cylinder head isolastic makes no sense at all.
 
My frame, a new one from AN, as my original ended up pretzel like due to a prang, has none of the fancy mods that a lot of you guys have. However, it handles extremely well, to the point where I would call it the best handling bike I have ever owned. With a carefully chosen list of additions, it is, I would call, outstanding. Prod racer front end with Lansdowne conversion, with the basic Ikons fitted at the rear, and, importantly, a Jim Comstock/CNW head steady which pulls it all together well. I did these mods one at a time so the impact could be assessed in isolation. Jim's head steady is outstanding, tightening the whole deal up considerably. My Norton has none of the Commando vices that owners have found over the years. The handling tells me that Norton got the geometry exactly right for a road bike and it was only cost cutting that was it's downfall. The icing on the cake is definitely the 2.5 inch Morad rims (19 inch) at both ends with 100/90 Avon Roadriders. The steering is light and precise, but, importantly, it is still light and precise at even higher speeds. There is a certain series of curves that I attack at nearly 60mph and I cannot believe the speed and precision with which it can be flicked from side to side. As you go harder, it is astounding how well it responds. I'm no racer, but I like to move along on the road (with the speeding tickets to show for it). This is to the point where the guy who built it, is finally building a bike for himself with the same set up as mine. He loves the way it handles. In fact, while road testing a T150, he ended up sideways in a driveway trying to take a corner at the same speed as my Commando.

I see so many posts on wheels and tyres, asking what rim/tyre combination works best, and then they end up going for a compromised combo. I simply looked at Avon's recommended rim size for the tyres and went with that. It was the rim size used to give the tyres the profile that they were designed for. To me it was a no brainer. Everybody else thought I was mad. The builder questioned my choice, as did the wheel builder. I insisted. The proof was in the pudding. Generally, people tend to make a compromised choice in rims and tyres and then make mods to make their choices handle better. I laugh when I see steering dampers fitted to Commandos and shake my head (unlike my Norton, which doesn't). It's like putting a Band-Aid on a fractured leg! Owners do all sorts of frame mods which they think will make it handle better when their basic choices are flawed. Form over function wins again.

I have a Commando that really handles, with no compromises. Call me an idiot. That's fine. But, like I say to people, "Being an idiot is like masturbation, it's cheap and satisfying" !
 
Sticking with the tyre manufacturers recommended rim widths is the way to go and sticking with Norton recommended 19" diameter rims is also the best choice as you have less tyre width than 18" rims, which improves steering. Only downside is faster wear rate. In theory possibly better to go with a 90/90 front with matching rim although only pukka road racers will really benefit, mere mortals like us probably wont notice the difference.
 
Sticking with the tyre manufacturers recommended rim widths is the way to go and sticking with Norton recommended 19" diameter rims is also the best choice as you have less tyre width than 18" rims, which improves steering. Only downside is faster wear rate. In theory possibly better to go with a 90/90 front with matching rim although only pukka road racers will really benefit, mere mortals like us probably wont notice the difference.

My point is that people use standard rims with a 100/90 on the rear and a 90/90 on the front. Two tyre sizes on the same rim width? Compromised right from the start.
 
The parallel thrust faces of three isolastics would still be subjected to the same temporary frame distortion as four.

The three points although changed by the frame distortion still define a flat plane so movement is still possible and the vibration reduction is still effective. The same frame distortion on 4 points will move one of the points off the plane so reducing the effective clearances in the points and increase vibration.
 
For me the slight increase in vibration is worth the effort of fitting the extra ISO s ,
The vibration that I get is through the rear sets that are aluminium and bolted to the Z plates it's possible that rubber footrests would help here
But standard footrests are too far forward and way too heavy so I will never know
 
The three points although changed by the frame distortion still define a flat plane so movement is still possible and the vibration reduction is still effective. The same frame distortion on 4 points will move one of the points off the plane so reducing the effective clearances in the points and increase vibration.
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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Are people having problems with Commandos vibrating in corners?
My thoughts exactly. This really comes down to problem definition - exactly what is the problem and in this case is it real world (in practice) or purely theoretical. Good points are being made and I go back to the article about “the world straightest Norton”
Makes one wonder what three double him joints would be like as direct replacements of the two ISO’s and rubber thingies as head steadies.
 
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
 
http://www.nortonclub.com/docs/Straightest_Commando_Frame.pdf
The Problem With Production Tolerances

If my frame is typical of production tolerances, the fore and aft discrepancy will keep the isos out of parallel planes and the vertical discrepancy will keep the rear wheel out of plane with the steering head. Even if your wheels appear to align, the paths they travel through don't. Adding to this all the other things that may be out of alignment and/or adjustment on a 20-year-old motorcycle, it's easy to see why most Commandos have a weaving feel when going in a straight line and don't have the high speed stability of a Featherbed
.

http://www.parallelengineering.co.uk

http://www.parallelengineering.co.uk/index.php?id=2
The poor handling of a Commando is typically blamed on the isolastic design and/or Commando frame design but these assumptions are incorrect. During the 1972 Production 750cc TT Peter Williams lapped the Isle of Man at an average speed of 96.56mph on the 750cc production racer using a correctly aligned standard Commando isolastic frame. During the 1973 F750 TT he lapped at a record average speed of 105.47mph on the isolastic equipped monocoque - the best handling bike he'd ever ridden. A motorcycle with handling faults and especially one with a relatively underpowered engine is not going to achieve these speeds around the TT course.
 
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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

Baz I would imagine your setup works very well . What caught my eye was your comment on Standard riding positions .. I cannot say that I have ever noticed on mine any of the familiar criticism of Command handling , further you can shut the throttle and brake when banked over . Both a no no. And you alweays get a sense of what the wheels are up to, and I think in my case possibly because almost from new the bike has had rearsets . The weight of the rider and how he or she sits on the bike and how much is on the front wheel has to be a factor in all this.
 
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