Crank diameter and weight
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Member
Joined: 01 Dec 2017
Posts: 9

Wed Oct 30, 2019 6:45 am quote
Does anyone know what the benefit of larger crank web diameter is.

Is it just reduction in likelihood of twisting because there is more web on the perimeter beyond the crank pin/

Or does it affect the rotating mass characteristics too?

Also what are the benefits of a heavier crank over a lighter one?

Reason I ask is for a small frame. I can either run a Fabbri Crank - web diameter 83mm and weight 1.6kgs or a Malossi one - web diameter 85mm and weight 1.4kgs.

thanks
Hooked
79p200e 66smallstate 85pk50xl 84p125ets
Joined: 24 Nov 2016
Posts: 102
Location: Flatness, Tx
Wed Oct 30, 2019 7:14 am quote
In a nutshell, you can run a stroker (54mm stroke) crank.

Stock small frame cranks (with 51mm stroke) have 2 crankweb diameters, the flywheel side, and the rotary valve side.

Stock cranks are 87mm rotary valve side, and 82mm flywheel side. Full circle, stroker 54mm race cranks are 87mm both sides. When the fly side goes bigger than 83mm or so, I believe, the crank case must be machined or opened up to fit this larger crankweb.

What type of crank are you thinking about? 51mm? 54mm? Rotary valve? Bell? Full circle? Are you prepared and able to machine the crank cases to accept the larger crankweb diameter?

Just asking some questions as to see what type of motor you planning to build and hopefully help with the crank decision.

Hope this helps.
Member
Joined: 01 Dec 2017
Posts: 9

Wed Oct 30, 2019 7:47 am quote
Thanks for the reply. I understand very well the dimensions of the case and need to mill in the case of the 85mm webbed version

Both cranks I have are 51 stroke full circle.

The issue is whether to go with the lighter but bigger diameter one or the smaller diameter but heavier one.
Hooked
79p200e 66smallstate 85pk50xl 84p125ets
Joined: 24 Nov 2016
Posts: 102
Location: Flatness, Tx
Wed Oct 30, 2019 9:14 am quote
This is a tricky one. Reason is the lighter crank has the larger diameter and the heavier crank has the smaller diameter. All things being equal, lighter rotating mass will rev quicker for stronger acceleration and a heavier rotating mass will promote tractability, smoother power delivery, less stalling with low rev take-off. This is the reason for lightened flywheels (race) and added flywheel weights (mostly off-road).
Your situation is not all equal. The rotating mass qualities also differ with how far the weight is from the center of rotation. The farther from center, the bigger the effect. Rotational inertia is I=mr squared, where m is mass and r is radius of that mass from the center. It is hard to say without knowing where the mass is distributed around the crank. Near the center or near the periphery. I believe in your case, the difference would be negligible as the weight is pretty small and so close to the center of rotation. The smaller diameter would be my choice. Diameter has a greater negative effect on Rotational inertia than overall weight (less acceleration) and less chance of having to machine the cases (or machine as much) to fit. Just my two cents.

Hope this helps.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7101
Location: Victoria, Australia
Wed Oct 30, 2019 12:49 pm quote
You're probably right about the ability to withstand twist vs the amount of meat around the pin... a mate of mine had an early M200 and the balance holes were closer to the pin than in this current picture. It twisted.

Seems unlikely you'd notice the 200 grams difference, with the weight difference as Poidog says very close to the centre of rotation. Although it would definitely have an affect on the balance factor, good or bad I don't know.

The Fabbri crank being heavier, might indicate it has a denser grade of steel and therefore greater holding power?
I'd also go for the smaller one and avoid the hassle of machining the case. I did it to mine (for a BGM full circle crank, yet untested!) and had no issues, but my friend went through the side on his 50 case. That was a sad moment...

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Member
Joined: 01 Dec 2017
Posts: 9

Wed Oct 30, 2019 3:26 pm quote
Thanks for the replies.

very helpful.

the other confounding factor is that the Malossi one has a 105mm rod which I want to use with the MHR barrel and an 8mm packer, as I have seen stories of shorter rods causing the inlet bridge piece on the barrel to collapse, but the Fabbri crank has a 97mm rod. I would use the Fabbri one if I could get someone to change the rod, maybe Fabbri himself would do it?

Machining the cases is no issue, apart from the added cost, as I am using Sip Evo cases.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7101
Location: Victoria, Australia
Wed Oct 30, 2019 6:17 pm quote
Might be an option, you can only ask I guess.
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