75 Mark III refurb/mod project

The latest Born Again Bikes client project is a refurb/mod of a '75 Norton roadster to a monoshock repli-racer. A bit different than anything I've done before in several ways:

1. I'm designing & building my own swingarm & rising-rate linkage for the monoshock

2. It will not be a total restoration, just a chassis modification & refurb (no powdercoat)

3. It will not be a total engine overhaul, just a beefed up top end job (bottom end is in great shape)

4. It will not get new paint, chrom, etc., just a detailed polish, clean & wax.

It will have an inverted fork front end, nearly identical to my 880 project, built from a spare ZX9 front end that I've got standing by. More of the existing Norton OEM brake master cylinder parts will be retained than on the 880 project, as the client wants to keep as much of the "flavor" of the original Commando as possible, down to the stickers on the tank & sidecovers. Basically, just the suspension & top end will be upgraded.

Here's the starting point:

My design uses a second standard Norton swingarm grafted on to the base unit with 2 unrights at the crossover tubes for what I believe to be extremely excess rigidity



I've also designed a rising-rate linkage that will pivot on the upper swingarm's original pivot point, and take an adjustable "turnbuckle" link mounted on a through bolt just between the swingarm pivot and the rear isolastic mount on the tranny cradle. Very minimal welding, and very simple bushing arrangements with grease fittings.



The bottom line figures show a better shock compression ratio and slightly greater rear wheel travel than the Dreer unit on my 880, so I'm thinking it's a good solid design. I think a dual-rate spring on the shock will give me even better results than the donor mule I'm working with at the moment (stock 2000 Triumph Legend triple).

I'm going to wire up the Blue-tipped wrenches after lunch...

Don't like how the rear brake light is crooked, but your design is interesting. Looking forward to seeing more.

Very interesting...

That's a Nortonda rear brake light, VERY rare. I'm thinking the big bucks I get for it on e-bay might pay for the whole project.

It's getting a leftover original Mark III tail light assembly (big rectangle lens) that I have on the shelf.

I've already liberated the axle/shock mount plates from the second swingarm and started mocking up the layout-



Looks like the left side shock arm will need to be re-fabricated to allow proper chain clearance. Might even try to build it so that can still accept the OEM chainguard...

Interesting project GP, are you going to brace the frame and steering head ?
Maybe the flex that's taken out of the forks and swing arm will transfer to the frame ?

Interesting, can't wait to see more.

Jean

The upper shock mount is just behind the web of the rear downtubes, so it's the perfect anchor point with simple gussetting on the front side of the web.

As far as the forks / steering head, I'm considering adding a plate on either side of the steering head. Although the client says he doesn't intend to race it or flog it, there's always the chance that someone else on a borrowed ride might be tempted to demontrate some stoppies...

grandpaul said:

Looks like the left side shock arm will need to be re-fabricated to allow proper chain clearance. Might even try to build it so that can still accept the OEM chainguard...

Click to expand...

Won't you have to completely replace that arm with an oval section or something to clear the chain?

GP,
Did you encounter any change in the vibration characteristics going to the monoshock on the 880? I'm thinking about the vector loads at the swing arm pivot from any shock that isn't vertical, even the stock layout. In a laydown shock it's going to have a pulling component that's a function of the angle of the shock, and that could amount to a couple hundred pounds on the stock setup. The issue is the isolastics. If it was a rigidly mounted swingarm it's directly transferred to the frame, but iso's get into the act and I'm thinking about the loading of them. Probably moot, but still. On a monoshock is could be more. As always though, the proof is in the results, any change in vibes? BTW, totally cool idea, it solves some of the problems of dual shocks, which is perfectly balanced pairs which are hard to come by, and getting any rising rate you want, and it's probably much stiffer in the swingarm department and the load path to the headstock, and it is a totally cool idea.

I like the fact that the head of the shock seems to be in a natural triangle that you could gusset.

I talked to my older brother, a structural engineer, and all he said was, "that's a Finite Element Analysis question, and not my cup of tea.

All I've done as far as "engineering" the design, is to study the direction of forces imposed and the construction of the supporting/receiving structure on the swingarm (way over-designed), and the frame (should be AT LEAST adequate, probably more than adequate with gusseting).

The swingarm pivot forces aren't going to change much at all.

The load being received from the shock is being imposed at the second strongest point on the frame, maybe the strongest; as I'm adding gusseting "just in case", I think it should be quite acceptable. One point of improvement on this prototype over the layout used with Kenny's swingarm is that I have the shock being compressed in a very linear motion instead of a slightly arced path with the last bit of travel (of the rear wheel) being nearly zero compression at the shock, so a very low rate.

I still haven't been able to run my bike through the wringer and get a feel for the handling differences, but then again, this prototype is being done on a bike that will be for sale, not the client's actual swingarm/frame (yet). if any problems arise, it will be easy enough to "go back to the drawing board" and get it right.

Any of you egg-heads want to get involved at a much higher level? I would appreciate some scientific backup!

Yep,

The more knowledge you can get on it the better. You don't want to be where Norton was with the early Commando neck brace.

At this point, my concern is with a satisfactory solution for the offset upper (shock) leg to clear the drive chain on the Left side. I'd like to use the original section, beefed up, but is oval cross-section really necessary?

My rusty brain struggles with stuff I learned in school long ago, but isn't the oval stronger in one plane? I think this was why BMW used it in frames.

It may be, but in this application, I wonder if the standard tube is acceptable with corner bracing where it jogs out and back in?

I usually weld something in and watch it for a while. I tend to go more on the truck side than aircraft so it does not usually break. Hopefully somebody who is more of an engineer will respond.

Does Ferrari make a truck?

grandpaul said:

At this point, my concern is with a satisfactory solution for the offset upper (shock) leg to clear the drive chain on the Left side. I'd like to use the original section, beefed up, but is oval cross-section really necessary?

Click to expand...

The oval section was just suggested to give you additional clearance. Do you know exactly how much room you'll have for the chain on the upper run yet?

By the way,

I've found that old bicycles can be an excellent source of hard to find tubing. I've bought good quality stripped frames in the wrecking yards for a fiver and gooten some nicely engineered tubes out of them, from oval to round stuff.
I usually go to Allen Steel in Redwood City from more common stock but bike stuff makes good brackets, fender braces, and a nice neck brace when I changed the angle on one.