I would think a heavy flywheel would be the best for Bonneville
for short tracks like El mirage and Maxton maybe something lighter.
Turbo Rick
http://www.turborick.com
----- Original Message -----
From: <ardunbill@webtv.net>
To: <land-speed@autox.team.net>
Sent: Thursday, April 12, 2001 7:11 AM
Subject: Effect of Flywheel Weight for Bikes and Cars
> George raised this question the other day, and it is a good one, with
> considerable practical and theoretical interest. Tom made an
> interesting observation about his experience with it.
>
> As we all know, objects at rest and in motion both resist changes to
> their speed.
>
> It seems impossible that flywheel weight could have any effect on
> horsepower and torque that any engine could produce, but it can change
> certain characteristics of a bike or car, for better or worse.
>
> Looking at one and two-cylinder bikes, fairly heavy flywheels are
> necessary on them to get acceptable smoothness of low and mid-range
> pulling. The selection of flywheel weight is something of an art with
> these. The Vincent 1000cc twin had its flywheel weight selected by the
> great designer, Irving, and as you would expect, it is perfect. In the
> sense that you are never conscious when riding one of either "heavy"
> flywheels that give a sluggish feeling, nor "light" flywheels that give
> a lumpy feel to the drive. A couple of years after the twins came into
> being in the late '40s, the factory came out with a 500cc single, using
> many of the twin parts, to expand their range of market offerings, AND
> used the twin flywheels (altered slightly for the different balance
> factor, natch). So forever afterwards the 500cc model suffered from a
> flywheel weight that was slightly too heavy, and gave a slightly
> sluggish feel to the bike. On the other hand, should the 500cc model
> chance to have a sidecar fitted, which was not unusual in the period,
> the extra flywheel effect would be beneficial in getting the whole works
> moving.
>
> In riding a new Harley Sportster several hundred miles not long ago, it
> was clear that Harley likes heavy flywheels on their road bikes. This
> is good for two reasons, first it improves smoothness of low and
> mid-range pulling, which is what riders are using almost all the time,
> second, every time the rider starts off from rest in first gear, the
> bike wants to jump away and the rider thinks "Wow!"
>
> No doubt flywheels on the heavy side help a racing bike or car start
> harder from rest because of the stored-up inertia of the flywheel, but
> the trade-off is that the engine will have to work harder to build rpms
> further down the course since the flywheel will be resisting speed
> increase. 40 years ago we were told that high-gear-only dragsters liked
> heavy steel flywheels (maybe 60-70 lbs)which helped get the car weight
> off the line. I imagine the driver would be holding 4000-5000 revs, and
> start by applying throttle and slipping the clutch to get the amount of
> wheelspin he wanted. But further down the course the engine had to work
> harder to get the flywheel to rev up to max, subtracting power available
> to accelerate the car. With a car engine, of course, the crankshaft,
> clutch or torque converter and the whole rotating assembly is part of
> the flywheel weight.
>
> I think with racing bikes where there are only one or two cylinders, the
> feel of the engine could easily be ruined by taking too much flywheel
> weight off. Needless to say, the "correct" balance factor has to be
> retained (whatever it is, varies by engine) to minimize vibration.
> However, when converting road bikes for racing, probably some weight
> could be removed, but in my mind the main benefit would be just weight
> reduction of the whole machine, rather than any performance improvement
> to the engine.
>
> The whole history of the Japanese multi-cylinder motorcycle movement has
> been "light flywheels". With the fours particularly, the crank itself
> is most of the flywheel effect, but the engines are so smooth-pulling
> that the rider quickly learns to buzz it and give it a little throttle
> for smooth starting.
>
> My Ardun has a light aluminum flywheel and this works fine for speed
> trials, giving no particular feel one way or the other. On the other
> hand, Elmo Gillette prefers a 60 lb steel flywheel on his Arduns at
> Bonneville, to ease push starts in high gear. He did tell me that the
> downside of the heavy wheel is that you have to be careful not to snap
> the throttle shut at the end of a run because it tends to keep the car
> running fast longer, and this is the time when you are most likely to
> break con-rods. So you ease the throttle shut to slow the car and save
> your rods. The reason being, of course, that the gas pressure atop your
> pistons with the throttle open tends to offset the weight of the pistons
> which wants to break the rods.
>
> Another detail with Arduns is that with his heavy steel flywheel Elmo
> can start the engine at full advance of 45 degrees on a fixed magneto
> without kicking-back and bending the starter shaft, whereas with my
> light flywheel I would (I found this out using an H/C mag with fixed
> timing at first) get kick-back and a bent starter. So I must use a
> Vertex with an advancer and start my Ardun at 20 degrees only. But this
> is no handicap.
>
> One more consideration would be that using a gearbox and starting hard
> from rest with a heavy flywheel, would be more punishing to the box and
> more likely to break its internals.
>
> Cheers Bill
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