LSR racing - lightening engine components

I have a question for you land speed racers........
Last night at the bar we were talking about high rpm "go fast" engines and the
question came up about reducing rotational mass.

Everyone knows that lighter engine components will help an engine spin up quicker, which is an asset in some racing situations, but for WFO land speed racing,
(running an engine at max rpm), is there much to be gained?

Not considering the durability of the engine (rods stretching, ect) with heavy parts,
would a lightened flywheel / crank help or hurt the outcome?
What about reducing drag by removing the charging system, oil pump, etc.?

Will a lightened engine go faster on the salt?
Thanks!

Will a "lightened" engine go faster on the salt?

Everything is relative. How "light" is "light"?

There is a bell curve that will describe the increasing, then decreasing, benefit to removing mass from parts.

The people who are single-minded in land speed records, and SUCCESSFUL at it, are the ones that take more time to RESEARCH removing mass from parts, than the time it takes to REMOVE the mass; and they stop removing material before the point that it will begin to possibly affect nominal longevity.

Mass is a friend of a smooth, vibration free, sustained high rpm spinner.

If you want less rotating mass, lighten the flywheel until you have the rotating mass you want.

Why mess around drilling pinions?

I'm a fan of heavy crankshafts for my landspeed Nortons. They might not spin up quite as fast, but the extra inertia will reduce rpm variation during each cycle, which gives a smoother feel at high rpm. I think that also helps reduce stress on the valve train, but that's just my opinion. Once you've reached the max rpm and are no longer accelerating, I don't see where there would be any benefit to lighter rotating parts. The higher rotating inertia of a heavy crank might also help with traction, which is the limiting factor in going fast once you get enough power to spin the wheel. It's particularly significant when running on dirt or salt, both of which have more traction problems than when running on pavement. Traction is really only an issue when you get up past the power levels most of our Nortons produce. I didn't run into it at Bonneville with 750 engines, but did have problems with it when I went to the 920 engine on nitrous. On the other hand, if traction is good, I don't see crankshaft weight having a significant effect on top speed for bikes running in the 120 mph - 150 mph, which covers most of our Nortons. At the speeds we typically run, all the courses are long enough to get up to top speed well before the speed traps, so I don't see any benefit from less rotating weight, even if it does help acceleration slightly.

In a landspeed engine, what I'd really like to reduce wherever possible is reciprocating mass. That's what shakes things apart.

Ken

lcrken said:

....Once you've reached the max rpm and are no longer accelerating, I don't see where there would be any benefit to lighter rotating parts.

In a landspeed engine, what I'd really like to reduce wherever possible is reciprocating mass. That's what shakes things apart.

Ken

Click to expand...

Thank you for your informative reply Ken. Your thoughts are exactly my position in last nights conversation.
(It feels pretty good to get some confirmation from someone who actually knows what they are talking about )

We were actually talking about removing the counterbalancer and lightening / re balancing the crank in a Hayabusa engine not Nortons, but the principles are the same.

Mark said:

I have a question for you land speed racers........
Last night at the bar we were talking about high rpm "go fast" engines and the
question came up about reducing rotational mass.

Everyone knows that lighter engine components will help an engine spin up quicker, which is an asset in some racing situations, but for WFO land speed racing,
(running an engine at max rpm), is there much to be gained?

Not considering the durability of the engine (rods stretching, ect) with heavy parts,
would a lightened flywheel / crank help or hurt the outcome?
What about reducing drag by removing the charging system, oil pump, etc.?

Will a lightened engine go faster on the salt?
Thanks!

Click to expand...

Once the crank is spinning, the mass is used to keep the momentum of the bike going. If you use a heavy crank with a close ratio gearbox, the increase in ratio as you go up through the box is such that the revs don't drop much. With a heavy crank, it usually takes longer to get the bike up to speed. So on the salt, you need enough run-up before you go through the speed trap. In the end it is wind-resistance you have to defeat. A heavy crank will smooth out the bumps in the air. With the close box, you climb in speed using torque. So once the crank is spinning fast, the idea is to keep it there. I use the heavy crank in my road racer - it is perfect for the job as long as you don't expect it to accelerate fast if you let the revs drop too far. An $700 investment in a close ratio gear set might bring more benefit than spending $2000 on a lighter crank.

With a Hayabusa engine, the gearbox should already be close ratio. But it might be worth checking what is available. The light crank in race bikes is more about acceleration than top speed. What is the use of it, if the first gust of wind knocks you back 10 MPH ?

Gearing and aerodynamics are obviously two other pieces of the LSR puzzle. However,
The question being presented at this time is aimed at the weight of the guts in the engine.

Without real specific knowledge, data and experience of the topic, common sense has
led me to believe that a heavier crank/flywheel would be an asset in land speed racing.
lcrken's views appear to confirm what I have thought.

Once the engine is spinning at it's max, the rotating mass of the crank can help smooth
the pulses and actually make the engine more efficient but not necessarily increase top
end speed.

If I recall correctly, Triumph fitted heavier than stock flywheels in their cigar shaped record bikes of the 50’s.

In the 1960s 650cc Triumph Saints had a lightweight one piece crank. These days those cranks are cheap because they are mostly unwanted. The bikes actually went slower. Crank mass has a big effect on the way bikes perform. In a normal Commando the crank mass does not suit the standard ratio gearbox. If you ride a near-standard Commando fast, it is impossible to be really smooth on either up-changes or down-changes. It is OK as a commuter, but not as a race bike. With a close ratio box, the heavy crank is superb, however with most 4-speed close boxes, first gear is usually too high. For a land speed Commando, an option might be to use a 4 speed close box with a low first gear and live with the gap between first and second.

Since this thread is about was about lightweight engine components and high rpm's.........please tell me about transmission gear ratios.

The two work together. If you don't have the right gear ratios reaching higher revs becomes more difficult. If the crank is very light, it has less inertia, so a worse response to air resistance. A bike with a heavy crank tends to keep going where a bike with a light crank tends to stall. But avoiding the situation where getting the heavy crank has to spin up due to throttle response is important. Once you get it spinning high, you need to keep it spinning high. A big step between the gears causes the revs to drop too much with a heavy crank. So overall progress becomes slower. A very close ratio box in a Hyabusa would be interesting. It might make the bike unrideable. They are bad enough as standard.