eliminating isolastics ?

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Dec 6, 2005
hey yahl , I have a '69 750 commando in a ridgid chopper frame and i'm interested in solid mounting the motor , more like a Triumph . I'm pretty sure I'll have to have the motor reballanced to do so , question is , has anyone out there done this conversion before ? Any info or pictures would be greatly appreaciated .
A lot more info is required but the first thing that comes to mind is why not just go backwards to the Atlas engine mounting system. Than re-tap the three holes in the drive side case and plug the old. This will stand the motor backup and relocate the primary. Enigne plates should be a breeze to find with all those Triumph motor conversions out there. If I were dumping the isos I would dump the Commando frame as well and go with an Atlas frame. Commando frames bare go 20 lbs and won't stand up to rigid mounting, Atlas frames go 36 lbs bare. norbsa
It shouldn't be necessary to re-balance for a rigid installation. The Isolastics were designed to isolate the engine vibrations from the frame, and I don't think any changes were made to the 750 going from the Atlas to the Commando, except maybe a flexible thick gasket for the carbs, so the gasoline in the float bowls didn't froth.
It shouldn't be necessary to re-balance for a rigid installation.

I'm not sure that is correct Frank.

The balance factor of a Commando engine is apparently around 52-54% and for rigid mounting needs altering to somewhere between 70-80% but the actual figure depends on the type of frame used? As Luke is referring to a rigid chopper frame and not a featherbed type then the actual amount may only be found by trial and error?

Further information on rigid mounting can be found here: http://www.nortonownersclub.org/>Technical>Commando>Crankcase-Crankshaft Balancing.
Although this only deals with fitting to the featherbed frame?
Note that locking up the iso's in a commando frame breaks the frame due to vibrations, if you chopper frame is not stronger than a commando frame you risk the same.
OK, Guys - I was going from memory. We put so much engineering effort into the frame structural design and the Isolastic system, I didn't think any changes had been made to the engine and transmission.

I do remember the prototypes starting out with a final drive chain only 0.25". We were adjusting the chain every two hours when we did the high speed testing at MIRA. We broke one at 110 mph on the MIRA track. The eventual production chain was 0.375", and I even thought that was marginal. I wanted shaft drive, but as a lowly newcomer, didn't get much credibility.

What a bike we could have had - 1200 cc, 4-cylinder, DOHC, shaft drive, integrated fairing, touring panniers. Dream on - didn't have enough development money for that kind of thing. Anyway, it didn't fit with management's vision of motorcycles as a cheap transportation alternative to a car. They never understood bikes as high-tech toys.

hello again & thanks for all the info , really some interesting points. this commando was put together by My dad in the mid 70's , he rode it for awhile then built a 54 Harley Pan head , the old Norton pretty much sat in the corner of the barn for my entire childhood , my dad describing it to me as "cantankerous" . Growing up I just loved the bike ,a total 70s chopper , hard tail frame , 20 over springer front end , later on I bought an 82 550 Suzuki GS & rode that around , then last year I finally talked the old man out of the Norton . first thing I noticed about it was how bad it vibrated , Me & dad talked it over & he explained how the original Commando was set up with a swing arm & theres a preload onthe chain , he originally set up the hardtail chopper with the chain tight & the motor being rubber mounted was pulling on the chain pretty bad , so he loosened up the chain & put a dirt bike chain tensioner to remedy this , now though the bike has a vibration , dad said his plans were to solid mount the motor re-ballance the bottom end & tighten the chain back up as a possible solution , but he built the Pan head & never got back to the Norton , now that I have her I thought why not try & see what I could come up with ....

It should be feasible to incorporate the Isolastic mounts in a hardtail frame. A high chain pretension load was not intrinsic to the Isolastic set-up.

There's no way to balance that engine for low vibration levels. It really should never have been made bigger than a 600. Even the 650SS had vibration problems and the Atlas was a rolling massage parlor. I rode a works 650SS about 50 miles each way commuting to work when I first joined Norton, and was very pleased when we started testing the Commando.

I'm going from memory here, so my ideas might not work. I would think that the entire engine/transmission, in the aluminum side plates and on the Iso mounts, could be installed in a chopper hardtail frame, essentially copying the way Norton did it. The total movement of the engine/transmission is about 3/8 of an inch in an orbital motion. I think the rear chain would be ok with a tensioner sprocket to take up that variation.

If the clearances on the thin polyurethane side "washers" are properly adjusted, the dynamics of chain tension variations with power changes shouldn't cause alignment problems.

In the Commando set-up, the rear suspension acted as a limiter of fore and aft movement, so maybe an additional horizontal Isolastic unit between the propulsion module and the frame would be needed for a hardtail.
If you want to eliminate the vibrations and there forever eliminate the isolastics to run on a rigid frame, you should look at changing the the engine characteristics. My local Norton supplier has been developing a 270 degree crank that is based on an old Norton develepement drawing, he has tested this crank at up to 11000 rpm. The 270 crank lessons the vibrations significantly, since both both pistons do not move at the same time, the rocking motion is eliminated.

I would not run a standard commando engine in a rigid frame (unless it the frame was beefed up) as the vibrations would crack the frame in no time.
Actually, the predominant vibration in the parallel twin configuration is that caused by both pistons moving together, one on the compression stroke and one on the exhaust stroke. This results in a vibration in the axis of the cylinders. There is a secondary vibration at 90 degrees to the cylinder axis.

In a vertical twin-cylinder engine with pistons on opposite strokes, the primary vibration is a rocking couple. The Honda 305 Dream is a classic example. Unfortunately, this configuration results in uneven firing impulses. This can be worse that the out-of balance vibration of a parallel twin, depending on overall capacity.

The only "vibrationless" configuration is an in-line 6-cyliner engine, though a 90-degree v-8 comes close. A 60-degree V6 is also not bad.

While I was at N-V, I tried to raise interest in a Commando with a horizontal 1200 cc in-line 6. With DOHC and a high rpm limit we could've got 100 horsepower or more. With shaft drive, it would have been smooth enough not to disturtb the skin on a pot of cream. Think Honda Gold Wing 10 years earlier.

Ariel had similar ideas and tried to develop its "Leader" 250 into a touring bike with a horizontal 600cc 4-cylinder engine, but the prototype engine only developed 30 horsepower and the project was killed shortly before Ariel went out of business. If only Val Paige and his development folks had done a better job of developing the engine, who knows where Ariel might have gone.

As a Leader owner, I was very pleased with the performance a 250 gave, and the handling was very impressive (brakes less so) . A scaled up 4-cylinder, 4-stroke 600, with DOHC would've been a world-beater. Why it had such abysmal power output, I don't know.

I remeber getting in touch with Ariel about "decoking" the exhaust system with caustic soda. It had aluminum inserts in the tail-pipes and I didn't want to dissolve anything inside the silencers. They replied that there were no aluminum components inside the silencers, so using caustic soda would not be a problem.

They then added that development testing had shown that the 16:1 ratio for petrol to oil was very conservative, and that 32:1 with regular mix 2-stroke oil or even 48:1 with Bardahl would be fine and would reduce the oil build-up in the exhaust system and the distinctive plume of blue smoke at full throttle..

Switching to Bardahl eliminated the "Old Smokey" moniker.

I finally traded the Leader for a 1955 BSA A7, a "real" motorcycle rather than an overgrown scooter. In retrospect, I wish I still had the Leader as a commuter bike. Maybe I'd have replaced that 2-stroke twin with something better and put better brakes on it, but the enclosed engine compartment and integrated winsdscreen, fairing, luggage grid and panniers were pretty impressive.

Lost chances foir the UK industry?

Apologies for rabbiting on out of context. Maybe it will be of interest.
Frankdam, thanks for the info, no apologies neccesary your insight and knowledge of this and other subjects have always been of interest.
very interesting

Thanks , Frank & Jhajjar for two possible solutions . I think the Idea of an additional iso to compensate for the lack of the swing arm might just be a winner , Where the old swing arm mounting hole is can probably be cut out and maybe I could fit a front iso unit there & tie into the frame , or make it mirror the back motor mount by welding in a pipe where another back iso would fit , That and I just wonder how it would perform with new iso's , possibly the adjustable mkIII style . could work .
On the other side of things that 270 crank sounds cool , is that designed to run in a ridgid mount set-up without the iso's ? could be interesting .
Luke, the 270 crank was not there to eliminate the iso's on a Norton frame, but was developed from what I have been told to reduce some of the vibrations and increase the reliability of the motor. Frankdam has set me straight on the vibration issue, as I mistakenly wrote that it would eliminate the vibrations in my previous post.

As well this is quite an expensive venture, as not only the crank is replaced but you would need a new camshaft to go with that crank as well.
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