JPN racer isolastic arrangement

Fast Eddie · Aug 9, 2017

Now then chaps, I was reading an article on the JPN racers t' other day and notice something I hadn't before.

The swinging arm spindle appears to be fixed to the frame, in the traditional 'non isolastic' fashion. Yet the front mount has a clearly visible isolastic mount.

Is this correct?

The only way I can see this working is that it allows the engine to 'pivot' around the spindle mount as it vibrates, or, that the swinging arm is not also mounted to the engine cradle (just giving the drive train a hard life).

Does anyone know?

See pic in this older thread:
jpn-750-commando-t14854.html

gripper · Aug 9, 2017

There are quite a few "Featherlastic" Nortons around where the engine is isolastic mounted but the swinging arm is frame mounted. If the Isolastics are set at 0.010" I'm sure the chain could deal with that sort of mis-alignment.

acotrel · Aug 9, 2017

A long time ago, I suggested on this forum that it might be a good idea to force the engine/gearbox assembly to rotate around the rear mount, and have the swing-arm fixed to the frame. When the connection between the handle bars and the rear tyre contact patch is flexible, handling must suffer. If you think back to the 50s' Manx Nortons, the first thing we usually did was replace the silentbloc bushes in the swing-arm with bronze bushes. It DOES make a difference.

SeeleyWeslake · Aug 9, 2017

Fast Eddie
I think your eyes might be deceiving you – caused by the angle the photo was taken at. The spindle you are seeing on the outside of the frame is the isolastic stud. The swingarm pivot is behind the frame gusset and immediately below it, but out of view in that pic.
Norman White gave me a copy of the drawing for the 72 JPN swing arm mount and I copied/adapted it to the commando I was racing. It used needle roller bearings and the inner races consisted of 2 top hats, the small end of which was spigotted in to the holes in the cradle The 2 top hats were drawn into the cradle with a long through bolt. It worked well but in hindsight I think I would have used bronze bushes instead of needle rollers since their use necessitated modifying the swingarm to take the larger OD of the needle rollers.

Fast Eddie · Aug 9, 2017

SeeleyWeslake said:
Fast Eddie
I think your eyes might be deceiving you – caused by the angle the photo was taken at. The spindle you are seeing on the outside of the frame is the isolastic stud. The swingarm pivot is behind the frame gusset and immediately below it, but out of view in that pic.
Norman White gave me a copy of the drawing for the 72 JPN swing arm mount and I copied/adapted it to the commando I was racing. It used needle roller bearings and the inner races consisted of 2 top hats, the small end of which was spigotted in to the holes in the cradle The 2 top hats were drawn into the cradle with a long through bolt. It worked well but in hindsight I think I would have used bronze bushes instead of needle rollers since their use necessitated modifying the swingarm to take the larger OD of the needle rollers.

Top man! Thanks for a very clear answer.

Bit disappointing though, I thought I'd 'discovered' something !!

lcrken · Aug 10, 2017

I'm not totally positive about this, but I think the only JPN racers with the swinging arm attached to the frame were the 750 experimental frame in 1971 and the 750 experimental monoshock bike that was intended to house the Cosworth engine, but ended up with a Commando engine and was tried out in some races in 1974. . As far as I can tell, all the rest (backbone, monocoque, and space frame) had isolastic mounts with the swinging arm mounted to the engine cradle.

Ken

acotrel · Aug 10, 2017

Over the years I have ridden many old bikes. It is amazing how you can feel if there is flex in the frame between the handlebars and the rear tyre contact patch. With modern tyres it probably does not matter much under normal conditions, but on a slippery track it can alter your confidence level.

JPN racer isolastic arrangement

Fast Eddie

gripper

acotrel

SeeleyWeslake

Fast Eddie

lcrken

acotrel

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