750cc Norton Commando Salt Flats Build

If you want to test the theory about the heavy crank, put your road-going commando into 3rd gear, rev the motor to 6000 RPM and ride the clutch out quickly. But make sure you have a long way clear in front of you. You will probably be surprised at the acceleration rate. A normal motorcycle would immediately stall. When the heavy crank is really spinning, nothing stops it. Learning how to use it is a different experience. The low balance factor is not good. Bert Hopwood was probably not stupid, the Commando/Atlas engine is an alternative concept. But it probably was not him who lowered the balance factor, to stop the shake at low revs.

acotrel said:

My message is that with the heavy crank, you do not know how fast your Commando engine will accelerate the bike until you raise the gearing.

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Hmmm… raise the gearing you say Al?

Maybe check out the first post in this thread…

When you are trying to set a land speed record, what affect does the acceleration rate have ? If the bike is continually meeting resistance in pulses, they might have less effect when you have a heavy crank. Once it is spinning, it would motor through. You are not using the bike in short bursts as you might on a racetrack.

Fast Eddie said:

Hmmm… raise the gearing you say Al?

Maybe check out the first post in this thread…

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I read that about the gearing but I saw the picture of the lightened crank. The two things work together. Personally, I always thought the Commando motor was bullshit but it proved me wrong. It is just a different concept. I always thought a light flywheel was better. If you raise the gearing and the motor has a light flywheel, you tend to go backwards. It depends on the shape of the power band and the torque. If salt flats are very smooth and there are no bumps in the air, a light crank might be better. Any time the bike loses speed the revs must be recovered, with a light crank it is probably easier to lose revs.

Are land speeds higher if you accelerate quickly at first then work up to a higher speed as you pass the speed trap. Or is it better to accelerate at a continual rate from start to finish and through the speed trap ?

acotrel said:

Are land speeds higher if you accelerate quickly at first then work up to a higher speed as you pass the speed trap. Or is it better to accelerate at a continual rate from start to finish and through the speed trap ?

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Land speed records for SCTA are set by highest average speed over the measured mile between two runs. First run that breaks the record means you have to make a "return run" to back up the first one. For the record I am aiming for I would have 1 mile acceleration, 1 mile measured, and 1 mile deceleration.
So to answer your question, I'd say it's best to reach top speed before hitting the measured mile and hold/accelerate if possible through the mile before decelerating.

Zachary Brown said:

Land speed records for SCTA are set by highest average speed over the measured mile between two runs. First run that breaks the record means you have to make a "return run" to back up the first one. For the record I am aiming for I would have 1 mile acceleration, 1 mile measured, and 1 mile deceleration.
So to answer your question, I'd say it's best to reach top speed before hitting the measured mile and hold/accelerate if possible through the mile before decelerating.

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That’s a good test of an engine !

You have to either be good at watching course markers, or know your bike's "Time To Speed", and time your run to be going as fast as possible JUST BEFORE you hit the timing trap; then HOLD Max Speed all the way thru the trap.

Fast Eddie said:

That’s a good test of an engine !

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Absolutely.

Since I was running low(ish) gearing, and since I still had four races left in '08 (Sandia & Barber's), I just tooled it up till I felt I was doing a decent speed without risking blowing up.

What I DIDN'T know was that I was running the wrong speedo gearbox and was NOT going 70-something, but instead ALMOST 100. Had I known that, I would have wicked it up enough to do the Ton. I was only running around 6K RPM...

To get better acceleration - close ratio gears are better than wide ratio. You might think it would make no difference - but with close ratio gears, you lose less revs on every up-change, so crank inertia has much less detrimental effect. With wide ratios there is a tendency to use more throttle, so carburation is done using the more tapered part of the needles. The slightest richness beyond the optimum will slow the motor. With close ratio gears, you might be able to pull higher gearing.
With my Seeley 850 on a race track, that set-up made the difference between being able to use the bike effectively and impossibility. The usual Norton box is hopeless. My bike is almost as quick down the straights as any 70s superbike, but quicker everywhere else. What makes me uncomfortable is the speed differential in corners. I use full blast where most other guys cannot. These days historic races are not usually run in capacity classes, and a two-stroke can easily change line. I have not raced in the last 10 years. I would like to do it again, but my wife saw what I did the last time I raced and got nervous. She had been to Winton with us once previously, but had never seen me race - she was worried for no reason. My mate should have put her mind at ease. It was the easiest couple of days' racing I ever had.

Fast Eddie said:

That’s a good test of an engine !

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Might be even a better test if the overall gearing is too low. With close ratio gears top speed is reached quicker with higher gearing.

acotrel said:

Might be even a better test if the overall gearing is too low. With close ratio gears top speed is reached quicker with higher gearing.

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With a 31 tooth front sprocket, I don’t think Zach is gonna have an issue with being under-geared Al. Actually, I’d suggest the opposite, but then I have zero LSR experience.

And, as you already know, he has a 6 speed TTI in this build.

Fast Eddie said:

With a 31 tooth front sprocket, I don’t think Zach is gonna have an issue with being under-geared Al. Actually, I’d suggest the opposite, but then I have zero LSR experience.

And, as you already know, he has a 6 speed TTI in this build.

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Also depends on his sprocket. We played with a 16-tooth, but we also have different rear sprockets since we are using mag wheels

City Garage said:

Also depends on his sprocket. We played with a 16-tooth, but we also have different rear sprockets since we are using mag wheels

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Zach is planning to use a 31 tooth sprocket …

Fast Eddie said:

Zach is planning to use a 31 tooth sprocket …

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I was referring to the rear sprocket not the countershaft which a 16 was the biggest we tried

I must have read that he has a 6 speed TTI and forgot. I bought one of those and do not look like ever getting around to using it. I was watching that old movie 'The Right Line' , which shows the McIntyre Manx on Oulton Park. It is interesting to listen to it. With the 4 speed racing gearbox, it is slow at first, then it really gets going. When I bought my 6 speed TTI box, I was not really thinking straight. With very high gearing, the 4 speed Manx gearbox is perfect everywhere, except in clutch starts. In races on our local short circuit, after the bike is doing about 70 KPH, I never use first gear, even the slowest corner is not too slow for second gear. I do not know what the gear ratios are in the TTI box - if they are wider ratios than they are in the Manx gearbox, my bike might be slower once it gets moving, but it should get to the first corner quicker - and that is important in a road race. But a Commando first gear could have saved me $730 Australian. Bruce Verdon could not tell me the ratios in the 6 speed TTI box - he said 'start counting'.

When I posted previously I had not watched all of the video of Kevin Cameron talking about motorcycle engines. A long way towards the end of the video they discuss motorcycles on the salt, and the problem of slip. To me, getting a record might depend on getting the optimum balance between torque, tyre grip and gearing. -

"If salt flats are very smooth and there are no bumps "

It was when I went in '82.

concours said:

"If salt flats are very smooth and there are no bumps "

It was when I went in '82.

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There are always bumps in the air. When Peter Williams built the Monocoque he was very particular about aero dynamics. Virtually nothing ever slows the heavy crank. If you ride a two stroke which has a light crank, every slight gust of wind slows it. It has loads of horsepower but the crank has almost zero inertia. Overall , I suspect that with the correct gearing the heavy crank is better. The total effect of many small set-backs need more horsepower to recover. Once the heavy crank is spinning high, it should be kept up there. Losing too many revs on a gear up-change is bad. But loss of traction might be worse on the salt.