Gearbox position

Charkmandler · Oct 2, 2024

I'm putting a Jawa engine in my racing Norton to give some time to develop a 500 engine out of my racing 350. Using std length rear shocks I've had to run the chain quite slack so that it's not tight at full compression. With this chain tension set and the shock unloaded the chain is just about touching the top of the swing arm. On the existing setup the gearbox AMC/Norton is about as far back as will allow full adjustment and the gearbox mainshaft centre is just under an inch under the centre of the swingarm spindle.
Making up new engine plates is a good opportunity to correct at least one of the issues but I'm aware that changing gearbox height could alter the handling when under power. Is there a mainshaft height relationship with the swingarm that is recommended? Thanks

Mike40M · Oct 3, 2024

Is there any marks on the swingarm proving that the chain had actually rubbed the swingarm? If not, don´t worry. Only two JAWA engined Norton racers I've seen had gearbox in standard position.
A small change in gearbox height will not affect handling noticeably. Something like 5 mm should be more than enough.
On a pair of 2-stroke racers we had fitted plastic strips on the swingarm to prevent rubbing. On one of them my son crashed due to a too tight chain limiting wheel movement.
I could theorize making geometrical and force vectors drawings, but it is about conflicting factors which has to be optimized.
I think Norton not only did tests on centre of gravity when designing the featherbed Manx but also on gearbox position. There is good reasons for the approx. 1" drop of the gearbox.

Charkmandler · Oct 3, 2024

Mike40M said:
Is there any marks on the swingarm proving that the chain had actually rubbed the swingarm? If not, don´t worry. Only two JAWA engined Norton racers I've seen had gearbox in standard position.
A small change in gearbox height will not affect handling noticeably. Something like 5 mm should be more than enough.
On a pair of 2-stroke racers we had fitted plastic strips on the swingarm to prevent rubbing. On one of them my son crashed due to a too tight chain limiting wheel movement.
I could theorize making geometrical and force vectors drawings, but it is about conflicting factors which has to be optimized.
I think Norton not only did tests on centre of gravity when designing the featherbed Manx but also on gearbox position. There is good reasons for the approx. 1" drop of the gearbox.

I'm not sure that the gearbox is / was in the correct position with the 350 engine in as someone made the plates and gave them to me years ago. The chain has just marked the swingarm so raising the gearbox about 5mm when I make new engine plates for the Jawa should cure it.

acotrel · Oct 5, 2024

Raising the gearbox should give slightly less anti-squat. If you are running wide in a corner and trail brake to lose speed, there is a point at which you must accelerate and rely on the bike's self-steering and tyre grip to get you around the rest of the corner without running off. When I have not ridden for a while, I sometimes intentionally create that situation during a practise session, usually when I am riding slow enough to avoid a real problem. When a neutral handling bike is made to squat when on a lean, it will usually self-steer in the correct direction. The amount of squat affects the amount of self steering. I like a lot of oversteer and to stay more vertical.

acotrel · Oct 5, 2024

If you are ever in the situation where you get into a corner miles too hot, and are going to crash regardless of what you do - GAS IT ! Somebody else has always gone around there much faster than you. There are not many ways you can end up dead.
I don't know why my wife won't let me race again.

Mike40M · Oct 6, 2024

Charkmandler said:
I'm not sure that the gearbox is / was in the correct position with the 350 engine in as someone made the plates and gave them to me years ago. The chain has just marked the swingarm so raising the gearbox about 5mm when I make new engine plates for the Jawa should cure it.

Finally walked down to the shed. My Manx has mainshaft 15 mm under swingarm centre. The slimline 650SS has 10 mm under, possibly a bit more as the engine is out. Measured with a folding ruler so not very accurate.
Hopefully not to late answer.

Charkmandler · Oct 7, 2024

Mike40M said:
Finally walked down to the shed. My Manx has mainshaft 15 mm under swingarm centre. The slimline 650SS has 10 mm under, possibly a bit more as the engine is out. Measured with a folding ruler so not very accurate.
Hopefully not to late answer.

Thanks

acotrel · Oct 8, 2024

I think that when I made engine plates. I did not consider the relationship of the mainshaft to the pivot. It was the relationship of the gearbox mainshaft to the line from the crankshaft centre to the rear axle. I was 5mm above it. On my bike, the rear caliper hangs on a pivoted plate which has a rod with rose joints secured to the engine plate below the pivot. When I brake the caliper causes the rear wheel to rise in relation to the seat - pulls the frame down. You need less rake while braking and more while accelerating. The rear brake steadies the bike. Sprocket sizes affect handling. My gearbox sprocket is larger and my rear sprocket is smaller than on a lot of bikes. - the rate of squat affects the steering.

Matchless · Monday at 3:02 AM

I spoke to Richard Peckett about this subject a few years ago. He told me the best starting point was to have the gearbox mainshaft, swingarm spindle, & rear wheel spindle in line when the rear suspension was at one third compression.

Gearbox position

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