A matter of weight...

Paul Slootheer
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Re: A matter of weight...

Postby Paul Slootheer » Thu Jul 25, 2019 7:51 pm

Perhaps the lighter flywheels are better for high revs racing but the heavier ones are better for low revs normal use.


Sounds good Paul, so this is benefit number one! better for low revs and normal use! (what I'am aiming for...) So The Works where right in this case...
Curious for te rest... :D

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Re: A matter of weight...

Postby PeterW » Thu Jul 25, 2019 9:32 pm

Paul Slootheer wrote:
PeterW wrote:Spooky, at lunchtime I was reading a 1945 edition of The Motor Cycle and the letters page had one from some chap called Hartley listing all the benefits of heavier flywheels.

Makes me curious, what are the benefits of heavier flywheels according to this ''chap''?


WP_20190725_21_14_09_Pro.jpg
Fly wheels by LW Hartley


Sorry about the poor quality pic.
Essentially, less vibration, better pulling away, less crank whip & wear, smoother torque.
I guess one reason the perceived advantage of lighter wheels might be small is that reducing the inertia on the power stoke (180%) also reduces the momentum for the rest of the cycle (540%). Probably less of an issue on a 4 cylinder engine.
Interesting stuff.

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Re: A matter of weight...

Postby Paul Slootheer » Thu Jul 25, 2019 9:39 pm

Thanks Peter! Lovely article! I assume the ''guy'' knew what he's talking about... :D

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Re: A matter of weight...

Postby Simon.Gardiner » Thu Jul 25, 2019 10:37 pm

BMW lightened the flywheel on post-1980 airhead twins, some reckon this gives a more responsive engine (I reckon that makes sense, with b.all flywheel my 'modern' Trumpy is extremely responsive even though it's the 'soft' tourer version), some reckon it makes the gear change much better although that could be something to do with the transmission getting quite a few changes as well.
I've also heard that it was to update the machine to keep it more in the same space as contemporary Italian and Japanese products, so I assume these were 'benefitting' from lighter, rather than heavier, flywheels.
In the Rover V8 world there's also view from respected practitioners that you'll get a better driving experience by lightening the standard production flywheel (unless maybe you're using it in a big old saloon) .
I like my BM with it's old-style 'heavy' flywheel. I like my '56 VH with its CI flywheels (which are bog standard but significantly lighter than a spare set of parallel-pin steel wheels that I've got), and I love the Trumpy which seems to have absolutely no flywheel at all.
So IMHO I think heavy flywheel versus light flywheel is mainly down to what sort of driving experience you prefer.

SG (getting in the bunker... :D )
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)

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Re: A matter of weight...

Postby nevhunter » Thu Jul 25, 2019 11:32 pm

Pistons give torque in pulses which the flywheels smooth out. With singles particularly, they have a lot of work to do to let the engine work at the lower RPM of the range. It also takes the stress off engine shock absorbers, chains and the gearbox. Lightening of flywheels was common in racing where the engine will accelerate through the gears faster and feel more responsive but certainly won't slog up a hill very nice. or want to idle slow, in traffic.. You also have to get higher revs before going into the next higher gear
I test rode the very BMW model you mention RS something or other, and it was noticeably harsh in clutch action and idled with a lot of shake and was a high compression motor. BMW had gone too far to try to get the performance of the Japanese competitors multi's and had produced a DOG in my view I certainly could not live with it especially in traffic and the "gears" are part of having an engine speed clutch, inherent in the design so you have more rotational inertia in the clutch components and first motion shaft, contributing to the clunky changes.
A V8 has enough cylinders to get away with a lighter flywheel and unless they are tuned too enthusiastically excel at being OK at low(er) revs a V12 even more so. In multi's piston weight contribute to flywheel effect, in singles it's from max to nothing twice per revolution and helps CAUSE the problem. I don't think you would sell too many single cylindered CARS these days no matter how you dressed them up. Nev
Last edited by nevhunter on Fri Jul 26, 2019 7:17 am, edited 1 time in total.

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Re: A matter of weight...

Postby brenton.roy » Fri Jul 26, 2019 12:28 am

Any room in that bunker Simon?
An issue is that the heavier the flywheel, the more momentum it has and the more it resists change in speed or direction.
Nev, I can't agree re Beemer's, but they are a good example. The post war to '70's R models have slow clunky gearboxes and a lot of sideways torque reaction. They also have sidecar gearing - a use for that extra flywheel mass.
My 80's Guzzi is similar. You tend to ride 'economically' to maintain speed.

https://www.youtube.com/watch?v=gDkjF6WIqe4

What I was trying to say, but not very well, was that adding flywheel weight to a KH may have helped sort out engine balance issues, but probably at the expense of engine versatility.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.

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Re: A matter of weight...

Postby paul.jameson » Fri Jul 26, 2019 9:04 pm

LWE Hartley, the author of the article, was the Hartley in Carter & Hartley stamped (C&H) on my flywheels. So he didn't think too much of the light flywheels either and perhaps that is why they ended up in my engine. But they do go well in a light frame at high revs, as their owner, one John Nelson, an occasional contributor to this Forum will confirm.

One of the reasons for the redesign of the OHC Four into the pushrod Four is given as lack of flywheel effect in the OHC Four. Having owned and ridden both for many miles I can confirm the truth of this. The OHC four thrives on revs. The pushrod four is far more flexible at low revs. In 1997 I rode up the Mull of Kintyre on the 1936 600cc OHC four with Lester Grant on a 1946 1000cc pushrod four. He tried to leave me behind, using the nominally higher power of his bike. He failed in this and the overall performance of the two machines on this occasion can only be described as near identical.

In summary, in my view, the pushrod four is more flexible but slower in response. The OHC four is less powerful but you can open up to full power very easily and then use it all day long.

I sold the OHC four because I preferred the Red Hunter single and so the OHC four saw little use when I had to choose between them. Subsequently, I changed the 350cc engine in the Red Hunter for the current 500cc version which has improved it no end and made it even more relaxing to ride.

So, as an old fart, you can't beat the flexibility of heavy flywheels. As a young hooligan, you can't beat the sheer performance of light flywheels - in my humble opinion. (For avoidance of doubt, I write this as an old fart.)
Paul Jameson
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.

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Re: A matter of weight...

Postby nevhunter » Fri Jul 26, 2019 11:38 pm

Another side effect of light flywheels is to kick back in traffic lights when idling when you don't get the manual advance back a bit from what you were using at cruising speed quickly enough or keep blipping the throttle. They are also inclined to kick back when starting especially if the motor is high compression. A heavy flywheel doesn't absorb power (torque) in a nett way. It SMOOTHES the peaks and lows. HP is NOT affected at all.. A dyno tells you that. Most of my life I've been a bit of a thrasher but never ever blown an engine. YET. I don't kill it with revs . A good engine will be tractable from quite low revs. Some modern bikes are useless below 3500 revs. so if you get caught in traffic lanes at freeway speeds a change through two gears is often needed or a bit of nasty clutch action to hurry it up (stop it "chugging" if you get baulked by a dwardler).
Nortons have produced some very tractable and pleasant "single" 500 and 588 OHV motors over the years and if I'm not mistaken they have adequate flywheel mass to achieve this. The flywheel mass in the mid 30's 1200 Harleys was 38 Lbs (the drive side wheel is much heavier) and that was one of the factors making them a very good flexible and tractable sidecar engine, and cope with a longish chain drive often without a compensator cush drive on the engine shaft. Nev

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Re: A matter of weight...

Postby Leejm » Fri Aug 30, 2019 5:29 pm

My new 51 kh engine has the heavier flywheel! I guess this is not the oringal item. But a later crank by the sounds of it this may be a good thing.
1948 NH, BSA D10 SPORTS. 1953 VHA, 1951 KH rigid project.

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Re: A matter of weight...

Postby Paul Slootheer » Fri Aug 30, 2019 7:53 pm

Lee, engine modifications for 1951 where weight increase of the crankshaft with 20%, barrel and head increase finning depth, different rocker box covers (round type). So all in all your heavier crankshaft is correct if it's a 1951 engine! :D


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