Flex shaft myth?

[sundog]

There's something I don't quite understand - maybe someone can explain it. It is suggested that you leave a gap between the drive dog and the strut/stinger because with use (torque) the cable will twist and shorten. But if you unwrap one, there are counter-wound layers underneath that should counteract the effect. Is this just a holdover from past days when flex cables were of lesser quality? I run mini's and micro's so don't really experience this effect (as much as in larger boats)? Sdg

[LarrysDrifter]

I've wondered the same thing.

[siberianhusky]

I think there is less of an effect if you're using a teflon liner as opposed to plain brass because the cable has less play in the teflon.
When the cable tries to twist up is assumes as S shape using as much free space as it has. This causes the cable to " shorten" I don't think the cable twists that much around its wraps to " shrink" that way. It would a little bit, now add the effect of the cable assuming that S bend. This is how the shrinkage adds up. IMO
In a plain brass tube the cable doesn't actually contact the tube for its whole length, mostly just where it makes it's S bend. Thats why IMO a plain brass tube has less drag than a tighter fitting teflon liner.
Hold one end of the cable tight and twist the other, see what shape it tries to assume? it does that in the stuffing tube as well.

[ray schrauwen]

Cool.

[Fluid]

Don't try to re-invent the wheel here, flex cables have been around for decades and are well understood in the hobby. The fact is that flex cables shorten under load. Not only do they twist, but they are under compression as the thrust from the prop is transferred to the motor. How much thrust? Enough to push your boat to 30, 40, 50, 60 mph. This pushes the cable tightly into the stuffing tube, shortening it further. If no gap is left, the cable will be in tension with a lot of drag between the drive dog and the strut, and excess stress on the cable. Too small a gap gives the same thing. The rule of thumb for many years has been a gap equal to one cable diameter. In some applications this is more than needed, in others it is about right. A small excess gap has zero negatives, while a too small gap that causes friction is always bad. One cause of high motor/ESC temperatures and short run times is insufficient gap between the strut and the drive dog. Seen it too many times.


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[Alexgar]

Ive found lack of gap can also pull the flex out of the stub shaft

[siberianhusky]

I was actually running boats when the change over began, prior to that we were all running solid 3/16 shafts with a couple of u-joints! All sub surface drive on monos back then! I was there when we were trying to figure out how to use them.
To be clear nowhere did I advocate no gap, I explained why you need a gap! with a simple experiment you can try yourself.


Over thinking? lol you're kidding right?

[JackBlack26]

My MG still has the reversed cable and when I hit the throttle, unloaded, the cable grows. The cables definitely give under torque, even when it just blowing air on the bench.

[FRED]

There's something else too, the bigger the prop is ,the more the tightness of cable will be especially at the HOLE SHOT.

[sundog]

Thanks for the explanations. So what I'm reading is the flex can kink in the tube which can shorten it some, and it can compress under load - which shortens it more. Makes sense. And I can see why it would happen less with smaller loads/motors/lengths.

So the drive dog only serves to transfer (forward energy) to the shaft/cable, it does not transfer torque to the strut/transom? Then would a square end cable (which slips into the square ended shaft) be more efficient? - it would allow for the shrinkage and transfer energy directly to the strut/transom, instead of through the cable and thrust bearing to the motor end cap bearing/ motor mount. I don't mean to ruffle feathers here - just clearing things up in my tiny brain!

[DV8ion]

Don't try to re-invent the wheel here, flex cables have been around for decades and are well understood in the hobby. The fact is that flex cables shorten under load. Not only do they twist, but they are under compression as the thrust from the prop is transferred to the motor. How much thrust? Enough to push your boat to 30, 40, 50, 60 mph. This pushes the cable tightly into the stuffing tube, shortening it further. If no gap is left, the cable will be in tension with a lot of drag between the drive dog and the strut, and excess stress on the cable. Too small a gap gives the same thing. The rule of thumb for many years has been a gap equal to one cable diameter. In some applications this is more than needed, in others it is about right. A small excess gap has zero negatives, while a too small gap that causes friction is always bad. One cause of high motor/ESC temperatures and short run times is insufficient gap between the strut and the drive dog. Seen it too many times.


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Great explaination but a lighter touch goes a long way. I don't see anybody trying to re-invent the "wheel", just a person asking a simple question. Not trying to offend, just offering a new comers (to OSE) perspective, thats all :)

[SweetZ28]

I leave only a gap about the thickness of a dime..... These cables hardly wind up and shrink at all... Just do this test hold your boats motor from spinning and with your hand wind up the prop the way it rotates you will feel a little sponginess a little wind up and then from there nothing at all the cable wont even wind up enought to take out the little gap you left for it.

[keithbradley]

There's something I don't quite understand - maybe someone can explain it. It is suggested that you leave a gap between the drive dog and the strut/stinger because with use (torque) the cable will twist and shorten. But if you unwrap one, there are counter-wound layers underneath that should counteract the effect. Is this just a holdover from past days when flex cables were of lesser quality? I run mini's and micro's so don't really experience this effect (as much as in larger boats)? Sdg

I applaud your post. If it weren't for people challenging what everyone else believes to be true, we would never move forward. The fact of the matter is, SOME OF THE STUFF WE BELIEVE IS WRONG. That's just the way it is, and the way it will always be. Not just in FE boating, but in every part of life. There is no point in history we can look back at and say "they had it all right". Many of the things we believed in the past were wrong, at any point in time, and now is no exception. It its foolish to think that everything we believe today is correct, and that we will never improve from this point. 10 years from now we will laugh at how pathetic our brushless motors and lipos were. EVERYTHING EVOLVES, unless people become complacent and stop asking themselves if there is a better way. I dont know about you, but I dont use wagon wheels on my car, I prefer to use the ones that were re-invented. I'm glad someone took an objective look at the wooden wheel and refused to just keep continuing to accept it as the only way.

With flex cables, if they were actually un-winding from the thrust of the motor, it would make sense to NOT run a gap, and prevent ruining the cable. I think the way they are made prevents un-winding beyond a small degree, but they still compress somewhat.
I run a gap between my drive dog and strut (unless the cable is set up to thrust from the strut), but I don't go by the method of gapping it equal to the diameter of the cable. This method makes no sense to me. I have to believe that if my cable is 14" long in one boat, and 4" long in another boat, those cables will not compress equally. As mentioned earlier, the size of the prop (and the weight/size/drag of the boat) will have an effect as well. If you want to use this method as a starting point and go from there, thats fine, but I don't believe my sons little rio will compess an 1/8" cable an eight of an inch, but compress a 1/4" cable a quarter of an inch. Does that make sense to anybody?

If you use marking compound on the back of your strut, you can see if the drive dog touches it or not. If it doesnt, you have plenty of gap. If it does, you may need to add a little gap. If you dont have marking compound available, you can always use white paint (or pink if that's how you roll) and apply it with a brush to the same area. You will still see if the drive dog hits under power.

[Fluid]

Just do this test hold your boats motor from spinning and with your hand wind up the prop the way it rotates you will feel a little sponginess a little wind up and then from there nothing at all the cable wont even wind up enought to take out the little gap you left for it.

Like I said above, don't try to re-invent the wheel. Your suggestion will get someone into trouble. I tried your "test" on three of my boats, measuring the actual amount of cable shrinkage. One was 1/16", one was 3/32" and one was 1/8". Had I used your method I (or worse a newbie who took what you said as gospel) would have overheated a setup.

Bottom line is that each setup is different. It depends on the length of cable, the brand of cable, the stuffing tube diameter, number and severity of bends, etc. Far better to avoid a problem by having a bit too much gap than having too little. Remember there is no negative to a bit too much gap. If you are scared of having too much gap then follow the suggestion above and mark the strut with paint. Of course, that is like a "too late light" because the damage is already done by the time you find out you have a problem...

BTW, you need to wind the cable the opposite direction it rotates. It can make a big difference.


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[sundog]

Ha! yeah, you don't learn much from a "'cause that's how it is" answer do you? If you don't ask, you don't know why....so you either gotta make the mistakes or dig a little more (ow!) to find the real answers. I don't mind digging a little! There's a wealth of info out there and I'm glad to be able to glean from it! Thx, Sdg.