A review of correspondence from NOC-L
The debate about how Commando wheels are supposed to be centred
Commando wheel offset dimensions
"I sent my wheels out to be rebuilt with stainless steel spokes. The front came back built incorrectly - the rear I'm not sure about. The offset on the front is way off, such that the tyre is contacting the fender bracket. The rear I'm pretty sure is OK, since the tyre is equidistant between the swinging arm - it's not positioned in the middle of the fender, but that might be correct"
Thanks to everyone who responded with measurements of their wheels. While waiting for responses, I was able to find someone who has built a fair number of wheels who gave me copies of his notes about the correct offsets. I also spoke to Ron Fratturelli of British Marketing who gave me quotes from the specs they have there at British Marketing.
It turns out that my rear wheel is within spec. and my front is off by approximately 1/8". The specs are 1/2" from the hub rotor flange to the rim for the front, and 1" to 1-1/16" from the edge of the hub on the cush drive side of the rear wheel to the rim. That puts the rear rim in the centre of the swinging arm.
The fascinating part of this exercise is that several people said their front rims were centred in the middle of their forks (or felt they should be centred). Neither my wheel, nor the specs. have the rim centred in the forks; it has an offset of about 3/16". I don't know if this is a Mk.lll specific offset or if all front disc-braked Commando rims are supposed to be offset.
John S. Morris (firstname.lastname@example.org) on NOC-L 4th. Oct 1997
Commando rear wheel offset
I read someplace that the offset that you find in the isolastics is made up at the rear wheel. The hub is laced so that the wheel isn't centred in the swinging arm but set about ¼" to the right.
Bob Patton (email@example.com) on NOC-L 9th. Oct 1997
Commando rear wheel offset - why should it be that way?
Whereas the offset reported by Bob Patton is likely true, it is still really counter intuitive, and difficult for me to accept. If the rear wheel is centred in the swinging arm, and the swinging arm is centred in the frame, then the front wheel should be as well. Since the steering head by definition is on the frame centre line, then the front wheel should be centred in the forks. Having the front wheel offset to one side makes no sense to me at all.
Greg Kricorissian (firstname.lastname@example.org) on NOC-L 10th. Oct 1997
Commando rear wheel offset - it shouldn't be
I just checked a Mk.lll that has never had its front wheel tinkered with since leaving the factory and it is centred between the forks. Also all pre-Mk.lll Commandos that I have had re-laced were centred before and after I received them back from Buchanan's Frame Shop.
Gary Slabaugh (email@example.com) on NOC-L 11th. Oct 1997
The Commando swinging arm is offset
I agree that the front rim should be in the centre. The swinging arm however is not centred in the frame! The frame mounts are centred but the rear isolastic is offset by 3/16". If the arm itself is symmetrical then the rear rim has to be moved over to put it in the centre. I have straightened many Commando frames but not as many swinging arms. Perhaps on the later ones the arms were made with an offset. I do know that the 750s had a symmetrical arm and the rim offset 3/16" to make up for it.
Vernon Fueston (firstname.lastname@example.org) on NOC-L 11th. Nov 1997
Is the Commando front wheel bracket important to the offset?
Do all Commandos also have the bracket (06-6160) between the fender and the slider where the brake line passes by? My bracket is just about the same thickness as the offset of the factory spec for the rim, which centres the fender over the tyre. Or another way to ask - are all fenders offset over the tyre 3/16" because of the bracket?
Today at European Motorcycle Day I took advantage of the Nortons in attendance, in particular the six Mk.lll Commandos which were entered in the show. All looked to have original spokes and wheels. I chatted with three of the owners and all three said that they believed their wheels to be original. Five of the six front wheels had offsets comparable to the Norton spec. One of the six was centred in the middle of the slider. All six had the same bracket holding the brake pipe so that the fender appeared to be centred on the five with offsets. On the one without the offset, the fender looked slightly off-centre.
As much as I respect the engineering logic of having the wheel centred, the fact that the Norton spec., a reputable and experienced Norton wheel builder and a fair number of examples all include an offset for the Mk.lll front wheel, I'm convinced that the offset spec. is correct but that there are some examples on the road which do not conform to the spec. Apparently they don't suffer any ills because of it.
John S. Morris (email@example.com) on NOC-L 12th. Oct 1997
Do not confuse Commando hub offset with rim offset
I think there is some confusion between front wheel offset as measured in relation to the hub, with offset as measured in relation to the forks.
The front wheel is supposed to be centred in between the forks of a Commando no matter what year or type of brake (i.e. no offset). On disc braked Nortons the wheel is not supposed to be centred on the hub since the hub flanges are not equidistant from the forks. If the wheel is laced up so it's centred on the hub it will be offset to one side of the forks.
On a disc brake equipped Norton, in order to have the wheel centred between the forks, the wheel must be laced with some offset in relation to the hub centre. The easiest way to do this is to have the wheel builder measure the offset before disassembling the wheel. Experienced wheel builders usually do this as a matter of course but unfortunately some assume the wheel must be centred on the hub.
As far as I know the rear wheel on a Commando should be centred in the swinging arm. I have never measured a Mk.lll wheel but I would guess it is conceivable the rear must be offset on the hub like the front.
Raul Biascoechea(firstname.lastname@example.org) on NOC-L 14th. Oct 1997
Commando engine and rear wheel offsets
"When assembling a Mk.lll Commando recently, I found that the engine/gearbox was indeed about 1/8" offset.
If you are lucky enough to have wheels that line up, you still have to deal with the offset of the engine/cradle/swinging arm. The load path isn't straight and cants the cradle and frame against each other which is maybe why you can't set them with the wheels on the ground.
Drum brake Commandos, like their Matchless and Featherbed framed predecessors using the same hub, have their rear rims centred on the hub. I am not familiar with the Mk.lll rear wheel - it has a conical hub, no? and uses that same whopping big thick disc caliper as used on the front; the one that requires a huge offset of the front rim for clearance.
On every Commando I have ever seen, the top frame tube runs dead centre down the middle of the cylinder head, and the head steady, symmetrical in design and construction, bolting up neatly as intended. It is true that the main front and rear isolastic spacers and so forth are not symmetrical, but look at any motorcycle and you will find differing left and right spacers on the motor mounts. Engine cases are never symmetrical due to the different requirements for containing timing gear on one side and primary drive on the other, and I suspect the whole mystery has its origins in that fact, although on the Mk.lll Commando the caliper-required wheel offset also influences the situation.
Ben English (ben.english@DMVMS.mailnet.state.ny.us) on NOC-L 14th. Nov 1997
The Commando rear wheel is offset
First of all, Commando drum brake rear rims are not centred on the hub. My notes (stolen from Buchanan's) show the rim centre three and a half inches off the brake drum outer edge (not the backing plate). If you centre the rim on the hub you will miss the frame centre by about ¼". The isolastic mounts are off centre but the swinging arm is not symmetrical either which confuses the issue.
I now have a 1973 850 Commando frame on the table. Fortunately I am also in possesion of cases, cylinder, head, swinging arm, rear wheel and all the related hardware. I will assemble the pieces on my straightening jig and see how they all interface. The Norvil headsteady is definatly not symetrical so I suspect there is an offset to the head.
Vernon Fueston (email@example.com) on NOC-L 15th. Nov 1997
Commando front disc and engine only are offset
All my records show that the rear drum on a Commando is perfectly centred. The front disc is offset, yes, but any non-symmetry at the rear is new to me. I did a wheel perfectly centred last winter without any ill side effects, and yes, the engine is also mounted offset inside the frame.
Klaus Kaak (firstname.lastname@example.org) on NOC-L 15th. Nov 1997
Commando wheel offset - the definitive answer
I have the definitive answer as to what is offset, how much, and which way!
I set up a 1973 850 on the frame table and verified it was straight. I then put together a dummy engine and installed it with new isolastics, new washers, Hemmings adjusters, and a Norvil head steady. I put both adjusters on the left side, checked the swinging arm in my fixture and then installed it in the frame. I took all the play out of the adjusters and started measuring:-
- The frame is symmetrical
- The engine/gearbox cradle is offset 1/8" to the left.
- The swinging arm is offset 1/8" to the right so the axle pads end up centred in the frame
I then installed the rear wheel:-
- The spoke flanges are offset 1/8" to the left as mounted in the arm, so the rim is laced off 1/8" to the right to put the tyre in the centre
- The centre of the rim is 3.3/8" from a straight edge laid across the brake drum (not the backing plate
With the stock Dunlop rim, if you sight across the rim on the brake drum side you will see about 1/16" of hub when it's in the right place. In my opinion, front and rear adjusters should be on the same side. Otherwise as things wear and you take up the slop you angle the engine and gearbox in the frame. Since the swinging arm mounts to the cradle, a little one way in the front and a bit the other way in the back and the rear wheel is out of line considerably. The chain will still line up but the wheels will not.
Vernon Fueston (email@example.com) on NOC-L 22nd. Nov 1997