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  • May 23, 2022, 09:42:13 AM

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Author Topic: Hi-Zoot crank for Standard Fox P/L or ABC set?  (Read 950 times)

Offline Dennis Toth

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Hi-Zoot crank for Standard Fox P/L or ABC set?
« on: August 31, 2021, 04:27:37 PM »
Just a quick question about using the Hi-Zoot crank with either a Standard Fox P/L or ABC set? Seems that since the Hi-Zoot has a higher counter balance then the standard Fox crank so is it better to use the HZ with a steel/iron and use the ABC P/L with the standard Fox crank? Anyone try this?

Best,   DennisT

Offline C.T. Schaefer

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Re: Hi-Zoot crank for Standard Fox P/L or ABC set?
« Reply #1 on: September 01, 2021, 05:13:42 AM »
Al Ferraro uses the HZ in his  ABC  Fox speed motor. Works fine!

Offline BillP

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Re: Hi-Zoot crank for Standard Fox P/L or ABC set?
« Reply #2 on: September 01, 2021, 07:50:46 AM »
Why would it work better with a steel P/L?
« Last Edit: September 02, 2021, 12:30:16 PM by BillP »
Bill P.

Offline Dennis Toth

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Re: Hi-Zoot crank for Standard Fox P/L or ABC set?
« Reply #3 on: September 02, 2021, 07:02:24 AM »
BillP,
I assume you were asking Why would it work better with the steel P/L. My thought when posting the question was, since the Hi-Zoot was developed before the ABC era it was designed to add more counter weight to smooth out the Fox 35 run. With the ABC options that came out from GMA then the Fox factory the original crank ran very smooth. If one used the Hi-Zoot you would now have more counter weight then needed and things would go back to the way the old original crank with steel P/L set ran, just that instead of the having the heavy piston you have the heavy crank.

It seems to work with the ABC and has the advantage of high strength, but maybe more vibration then optimal but  we all handled that with the original setup. Point for discussion is with the lighter AL piston does the original crank hold up better and still give less vibration or is the crank still subject to failure when pushed with higher nitro fuel or load?

Best,   DennisT

Offline Brett Buck

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Re: Hi-Zoot crank for Standard Fox P/L or ABC set?
« Reply #4 on: September 02, 2021, 11:30:12 AM »
What would it work better with a steel P/L?

  As noted I think he was considering the added counterweight. But there were other aspects to it - like the Randy crank is *straight*, and *concentric*. I am told by some of the more expert Fox reworkers that that makes a lot more difference in the vibration that the counterweight by itself, compared to the average Fox crank. Not breaking in half running high nitro levels  is just a bonus

     Brett

Offline Allen Eshleman

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Re: Hi-Zoot crank for Standard Fox P/L or ABC set?
« Reply #5 on: October 16, 2021, 09:53:23 AM »
So does it cause more or less vibration?  The "high zoot" crank.


Offline Dave Hull

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Re: Hi-Zoot crank for Standard Fox P/L or ABC set?
« Reply #6 on: October 17, 2021, 01:41:29 AM »
"It will at least put the vibration rpm way out of the useful range."

Hmmm. Not sure what you are trying to say, here. Since the mass of the piston is not altered (reduced) by the use of maximum Zoot cranks, and for discussion purposes, neither is the mass of the rod, fundamentally the primary and secondary shaking forces retain the same minima. You can improve the static balance of the crank/rod combo, which is what I assume was done to get some Zoot into it. That's certainly to the good.

Without a modal analysis of the crank, it is hard to be sure, but if the single-cylinder primary dynamic imbalance is say, at 150 Hz (corresponding to 9,000 rpm) I'm not sure you are going to see flexible crank modes below 150Hz with the stock crank, and not sure the amplitude of any of these higher modes would be noticeable vibration-wise when it is swamped by the 150Hz. Ditto for the 2x secondary shaking force at 300 Hz. So I can't make sense of the original statement.

Where the Zoot should really pay off is in regard to crank stresses in torsion, which is what kills Fox Stunt cranks at the induction port. Of course, they also fail with annoying regularity across the crank web from fatigue due to bending stress. If the material is higher strength, based on a better alloy and better heat treating, and if it reduces the stress concentrations inherent in low cost machine work that characterized the factory part, then the alternating stresses relative to the total allowables should be much more favorable--and the cranks much less likely to depart in flight or shed important pieces inside the case. Both of these are admirable qualities resulting from Zootness.

At 9,000 rpm, you should be concerned about the fatigue limit (reversed loading in torsion) at say, 2x10^6 cycles, which is only 230 minutes or so of operation. You'd need to know the exact stress ratios (alternating stress and constant stress) for all of the torsion and moment loads to improve the approximation.

You can reduce the alternating stresses on your stock crank by using a lighter prop (wood) and lighter spinner. Reducing the alternating stresses will push the fatigue cycle count to the right, getting maybe 2 or 3 times the operating minutes to failure.

Dave

Offline C.T. Schaefer

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Re: Hi-Zoot crank for Standard Fox P/L or ABC set?
« Reply #7 on: October 17, 2021, 05:44:28 AM »
Whew, that's a mouthfull ;D.

Offline Dennis Toth

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Re: Hi-Zoot crank for Standard Fox P/L or ABC set?
« Reply #8 on: October 17, 2021, 06:36:41 AM »
So in the best situation with the standard crank, wood prop, light plastic electric spinner at around 144 ish flights (assume 4 min per flight) trouble could occur? I think there are many Fox's out there that are just getting broken-in at that point. Probably many that have many times that number of flights and are OK. Seems as long as you don't push the nitro above 10% ish they hold up pretty good. I would think that with an ABC setup it would be even better provided there is not too much pinch at the top. Does the pinch offset the extra balance of the shaft counter-weight?

Question is for a 4-2-4 run on an ABC setup how much pinch is correct? Or is it better to run the ABC's in a rich2 - 2 - rich2 with a lower pitch but higher rpm?


Best,   DennisT


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