rigid crank versus swingarm crank

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Vincent.vanGinneke
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rigid crank versus swingarm crank

Postby Vincent.vanGinneke » Thu Jan 23, 2020 10:06 pm

I have a WNG crank and a NH '55 crank.
The '55 crank is heavier (wider flywheels) compared to the WNG crank.
question; is it o.k. to mount a swingarm crank in a rigid frame?
Any balancing issues to expect? , or -as they where only statically balanced- does it not matter ?
Ofcourse pick up will be slower, I got that part... :)
Any thoughts on this ?

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Re: rigid crank versus swingarm crank

Postby JohnnyBeckett » Thu Jan 23, 2020 11:00 pm

HI i have got it the other way round i have a 1939 350 in my 1953/4 nh red hunter with a swinging arm and it goes very well :?:

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Re: rigid crank versus swingarm crank

Postby will_curry » Fri Jan 24, 2020 12:09 am

I too have 'mixed and matched' over the years and not noticed any difference, in either
vibration or acceleration.

The flywheels were changed in 1954 but whether it was to suit the different mechanical
properties of the new PRF frame I know not.

The death of flywheels comes when the mainshafts or crankpin come loose. The W/NG or for
that matter the NG or VG flywheels never struck me as being that robust.

If it were mine I'd put the later flywheels in, reasonably confident that when I broke it it wouldn't
be the flywheels.

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paul.jameson
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Re: rigid crank versus swingarm crank

Postby paul.jameson » Fri Jan 24, 2020 8:31 pm

Wiser men than me - Bob Brassington for one - have suggested that the balance factor is different for rigid / plunger and swinging arm frames. That said, my original 1957 NH used to destroy cycle parts if ridden at 55mph. An extreme illustration of this was the rear section of the rear mudguard which used to develop vertical cracks from the bottom edge. These were welded up - and appeared again. Welded up again, with drilled holes at the tops of the cracks. Cracking resumed until I fitted a 500cc engine.

I have never had such problems with an appropriate period 350cc engine in a rigid frame. Maybe it was the BSA influence which led to the later problems.
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Vincent.vanGinneke
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Re: rigid crank versus swingarm crank

Postby Vincent.vanGinneke » Fri Jan 24, 2020 9:24 pm

Hmmmm food for thought that is...
As I need to fit a new bigend to the '55 NH crank anyway, I am going to weigh each cheek separate.
And compare them to the dismantled WNG crank (mainshafts still in place) that I have.
Maybe -I still have to think about this- I trim the outside of the NH cheeks down to get the weight per side near the WNG cheek.... :?


Ah yes, it must have been BSA... :lol:

Will; 1954 is indeed the year of the swingarm...

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Re: rigid crank versus swingarm crank

Postby JohnnyBeckett » Fri Jan 24, 2020 11:01 pm

you can buy a swinging arm ariel 350 NH in 1953 I have got one :!:

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Re: rigid crank versus swingarm crank

Postby nevhunter » Sat Jan 25, 2020 4:10 am

Taking flywheel effect off (reducing it) can make a bike much less pleasant to ride and require more use of the gearbox at lower road speeds. The Balance factor may change with the type of frame as frames have their own natural FREQUENCIES. . only the NH before that late type wheel came along is steel. all the others are cast iron.. The only flywheels I've heard of giving trouble are post around 53 VH/VCH engines. BSA maybe didn't want the ARIEL to be too good, perhaps? People racing the pre 50's, 350's pretty much always fitted the forged steel wheels when racing. I think they have less flywheel mass than the LATER Iron ones.. For ordinary riding as far as I know the post 53 350 iron wheels are OK. A lot of those machines were sold and did good miles. if a little underpowered 2 up. Nev

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Re: rigid crank versus swingarm crank

Postby brenton.roy » Sun Jan 26, 2020 8:43 am

There is a letter about this topic somewhere in the library or forum. I found it a couple of months ago and have now lost it..
It says that Hartley suggested 65% for rigid singles and 70 something percent for swinging arm.
If anyone finds it, please let me know.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.


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