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Old 08-18-2011, 07:54 AM   #286
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Dude! Yer doin the DAkar, too!! I'm taking off all of January, the first half just to watch and the second to recover. Good hunting, Ned.
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Old 08-18-2011, 09:26 AM   #287
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In! Looking forward to January even more now!
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Old 08-18-2011, 11:49 PM   #288
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Quote:
Originally Posted by neduro View Post
Thanks for keeping me honest!

I do plan to run cush hubs, but, I'm not convinced they are super critical. To me, supermoto is the ultimate test of strength in a drivetrain- sticky tires, grippy pavement = huge loads. Sometimes they land from a jump on pavement, so they've got a spinning tire that suddenly hooks up- and the bikes seem to hold up fine in that application with non-cush wheels. I hate to take an unnecessary risk, though, so I'm working on getting wheels.

The goal is three sets- one in the bike + two spare, and 7 rears/ 6 fronts for spare tires.
Good lad...run a cush drive

The issue is how things break, and with metal bits that is rarely because they are grossly overloaded. Generally, there are three failure modes:
  • Turbo-Busa (UTS Failure if the bits are grossly overloaded)

  • Fatigue

  • It was already broken when we installed it (manufacturing flaws) or we broke it putting it in (assembly flaw)

OK, given that Turbo-Busas are a bit thin on the ground, Dakar-wise, we can eliminate "the 450 BHP ripped all of the teeth off the sprocket" failure mode.

So we have fatigue, both steel (gearbox cogs, shaft splines, chains, etc) and aluminum (wheel hubs, clutch hubs, some sprockets, and engine cases).

And each of these metals has a unique fatigue failure mode:



"Oh, fuck me, there he goes with the bloody S-N curves again"

Well, sorry about that. Might I suggest propping your eyelids open with a few toothpicks while we muddle through this?

Yes, the bloody S-N curves (so named because it graphs Cycles (N) against Strain (S)).

Steel is pretty neat stuff. If you keep the peak loads below a certain level (endurance limit), there is no fatigue failure mode. Steel lasts forever as long as you keep the peak loadings down to a reasonable sum. What is that number? Well, about 50% of the UTS (Ultimate Tensile Strength) is a pretty good starting point, and the good news is that the fine people at KTM have already worked out these sums. Mostly.

Aluminum has no such magic endurance limit number. All aluminum will eventually fail given a certain load and enough cycles. But what the graph tells us is that aluminum parts will last infinitely longer if the peak loads are also controlled and kept below a certain level. This is why the wings do not snap off your airplane every third landing. But this is also why high cycle aircraft (50K take-offs and landings) need to be looked at closely, and why even structural aluminum parts have a service life. Smart people keep good records and replace bits on a schedule. Dumb ones say "oh, fuck me running" when the wings snap off their airplane.

How tough can fatigue be on aluminum parts? Ducati WSBK engine cases have service life measured in hours you can count...and still leave your boots buckled while doing the counting

So, how do we control peak loads? Well, we can make parts big and strong and heavy as battleship anchors so the peak loads are kept very low as a percentage of UTS. It works. No DC3 has ever suffered a structural failure. Tractor gears rarely fail. But neither would make a good basis for designing a racing motorcycle.

The other way to happiness is to reduce peak loads. So where do these peak loads come from?

Well, issue number one is you are about 11 cylinders short of a smooth running engine. By this we mean you have about 60 degrees of adding energy to the rotating crank, and 660 degrees of taking it away.


And actually, that is just us being polite. Because while we may have 60 degrees of useable pressure, rod angularity ensures that some of that pressure is doing pretty much fuck-all for work.

(PS - Rod angularity is what causes side-thrust on the piston, and piston side-thust is the only reason engines without 17-foot diameter propellers attached have a torque reaction when you open the throttle (most notable in fore & aft crankshafts like BMW's boxer). Oh, and "counter-rotating" anything has no affect on this torque reaction...unless there is also another set of connecting rods side-loading pistons the opposite way as well...but that is another set of eyelid toothpicks entirely).
Can a flywheel damp out this jitterbugging crank? You bet. Install a set big enough to gyro-stabilize he QE2 and no problemo. But you may not like the throttle response.

Now, add to our stew pot the cyclic forces of your cam drive, which also get fed from...and then back into...the crankshaft. We take the engineering behind this for granted now, but it was actually one of the big challenges for Keith Duckworth when developing the Cosworth DFV (from which all modern engines descend). He finally solved it by installing a quill drive. Clever lad. Oh, and a quill drive functions very much like a cush drive.
(As an aside, Bonneville racers almost always benefit from very heavy flywheels. The entire engine, especially the valve-train, is more stable with biggish flywheels.

Oh, and counter-balancers do not eliminate loads, they simply keep them confined. As an experiment, have a mate smack you on the left ear with a plank of wood. Your head will move to the right. As you regain consciousness your head will move back to the left. If your mate keeps whacking you, and in the interest of science I see no other course of action, your head will now be vibrating right and left.

Now, again in the interest of science, have another mate simultaneously smack you in the right ear with the same size plank. Your head will not vibrate, as the forces are now completely balanced (within your noggin) by your L&R ears being smacked at the same time. But you will no longer think there are no forces at work, even though your head, vibration-wise, is as smooth as a Packard V-12)
So we have a somewhat spiky power delivery going back to the gearbox. And when that power delivery reaches the countershaft sprocket (after the internal cogs do their sums)...it gets worse.

Because of the dread Polygon affect

See, sprockets aren't round. They are polygons. And polygons are nasty things when it comes to transmitting torque, because the torque values keep changing (though, in fairness, a 50-sided polygon makes a reasonable approximation of a circle )



"R" does not equal "r"...so chain velocity keeps changing even when sprocket speed (RPM) is constant

PS - Does the same thing happen with gear teeth? Well, several billion hours of engineering design have gone into making sure they pretty much don't. Which is why gear teeth have such interesting shapes. But all straight-cut spur gears have a (very itty-bitty tiny) bit of cycling as well. And all straight-cut spur gears have backlash.


So why do we put up with this? Because chains and straight-cut gears are efficient in transmitting power

And none of the above matters one iota if the bike is on a center-stand with the rear tire off the ground. Because in that case there is no resistance to the cyclic loadings (except the inertia of the parts themselves) and they will happily sort themselves out. But once that big Dunlop bites into mother earth (and, it goes without saying, in the most loving and Gaea-friendly way possible), then it does matter. And we need only note that a loaded tire is many things, but smooth & round are not among them. In fact the shape of a loaded tire at low pressure makes the whole thing seem like the Devil's Limb when it comes to cyclic load variation.

So, the million dollar question? Do you need a cush drive? Well, Googling "Dakar" and "Gearbox failure" doesn't exactly come up empty. OK, lot's of these are dead trucks, but some ain't. We know that peak loads are not our friend, and that a cush drive can damp and reduce this peak loading (by storing the excess energy in compression, and then releasing it as peak loads drop).

I would not run a racing bike without a cush drive if one could be had that:
  • Weighed less than a small but very fat hamster

  • Cost less than a riding jacket made from small fat hamster pelts

In other words, if I could run one, I would. But then, I have a horror of wings falling off airplanes. Cheers

Oh, there is the other option: A twelve cylinder 450cc engine consisting of two sixes with counter-rotating crankshafts (with quill-drive cams) transmitting the power through a hydraulic coupling to a gearbox with herringbone gears and both sprockets being a minimum of 50 teeth driving a two inch wide tire inflated to 80 PSIG and a racing surface consisting of greased BB's. In which case I would sod the cush drive

PPS - Oh yeah, third failure mode. Pre-broken parts. More later. I ran out of toothpicks.
Quote:
Originally Posted by JMo (& piglet) View Post
Hi Ned!

I'd certainly run with a cush-drive rear hub in your main set of wheels (even if you don't spring for one in your second/spare set) - as I'm sure you're aware (so really for the benefit of anyone reading this thread), having a cush-drive not only helps prolong tyre and drive chain/sprocket life, but also helps prevent backlash through the transmission potentially damaging the ouput-shaft and/or gearbox itself.

Obviously in a long distance rally raid event, and particularly in the Dakar where daily mileages are more than a whole season of supermoto circuit racing (for example), then the constant strain on the final drive and backwards into the gearbox itself can easily lead to a failure of shaft or seals, particularly at high speed on long and choppy washboard piste type terrain?

The Talon rear cush-drive hub is a very nicely engineered thing, and not really that much bigger or heavier than their stock HD hubs...


Jx


Short version...what he said
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Old 08-19-2011, 12:20 AM   #289
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Sheesh! 3:20 in the morning and I read that entire post. Interesting stuff. Thanks for the write-up. Pass the toothpicks.

Go Ned !

Quote:
Originally Posted by Jinx View Post
Good lad...run a cush drive

.....



Short version...what he said
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Old 08-19-2011, 04:14 AM   #290
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Okay, so Jinx is officially a hero. Can a moderator make that post a sticky somewhere so we can put the cush drive debate to bed? I think that was the first time I saw something beside anecdotal evidence as to why the cush drive was needed.

The question becomes, if you're not running one how often are you going to be checking your drive components for failure...or are you just going to be waiting for the wings to fall off?
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Old 08-19-2011, 05:13 AM   #291
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There's some funny shit in there....

Great post Jink!
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Old 08-19-2011, 05:39 AM   #292
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For God's sake, someone buy that man a drink! (Better make it soon though ... I see from his location that it might not be possible for very long. Make the drink top shelf as I think he's about to have a ready and unlimited supply of ice).
Excellent post Jinx
Ned, with guys like this behind you, you can't go wrong!
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Old 08-19-2011, 06:14 AM   #293
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Quote:
Originally Posted by Jinx View Post
Oh, and counter-balancers do not eliminate loads, they simply keep them confined. As an experiment, have a mate smack you on the left ear with a plank of wood. Your head will move to the right. As you regain consciousness your head will move back to the left. If your mate keeps whacking you, and in the interest of science I see no other course of action, your head will now be vibrating right and left.

Now, again in the interest of science, have another mate smack you in the right ear with the same size plank simultaneously. Your head will not vibrate, as the forces are now completely balanced (within your noggin). But you will no longer think there are no forces at work, even though your head, vibration-wise, is as smooth as a Packard V-12)


Thanks for the morning laugh!!!

BTW great write up.
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Old 08-19-2011, 06:28 AM   #294
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Quote:
Originally Posted by Jinx View Post
I would not run a racing bike without a cush drive if one could be had that:
  • Weighed less than a small but very fat hamster

  • Cost less than a riding jacket made from small fat hamster pelts

In other words, if I could run one, I would.
So, that's settled!

Does it make me sound like a stalker that one of my favorite ways to read adv is Jinx --> Find more posts by this user?

In other news, the bike has experienced a bit of entropy since the last post:



Swingarm pivot was dry! Glad I checked that. When it goes back together this time, the goal is that everything is right for a long test in the desert.

Brown Santa brought some gifts from Motion Pro yesterday. Here I am acquainting myself with them...



I covet tools, if I'm honest, and these are some damn nice ones. They sent some stuff I'd never pulled the trigger on buying, and now I regret it- after a day of using the shorty T-handles, I can tell these are going to be favorites. Thank you Motion Pro!

And finally, a failure mode I'm quite concerned about, even more than non-cush hubs, is keeping the air the engine breathes clean, and the air filter from being clogged. I've got a bunch of ideas for how to address this, one that I've been experimenting with is sealing the airbox so that the only place air can get in is under the seat, and covering that with a curtain of filter skin material. I figured, if it keeps even some dirt from ever getting to the filter it's done a worthy job.



It seems to be working pretty well. The filter came out after 200 dusty miles cleaner than expected (and much cleaner than the other bike that shared the ride) and there seems to be no impact on engine performance.
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Old 08-19-2011, 07:57 AM   #295
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Ned, On the prefilter.. 3 yrs ago at the 3 day vegas to reno on top of my lc4 airbox I used a filter media that was intended for air conditoning return vents on top of the normal filterskin and filter. it was a little thicker than yours but both ideas seemed to do the job good on the pre ran silty ass course.. and on the motion pro I am keen on the t-handle that is a male 1/4 socket and keep the 8/10/12 sockets close by. Hope the foot is healing well!
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Old 08-19-2011, 09:42 AM   #296
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+1 Jinx rocks it again.


Quote:
Originally Posted by tofire409 View Post


Thanks for the morning laugh!!!

BTW great write up.
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Old 08-19-2011, 09:51 AM   #297
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Quote:
Originally Posted by Jinx View Post
Because of the dread Polygon affect
Damn! Why did I choose "Dread Pendragon"?! I should have been "Dread Poloygon!" I would have been able to strut around in my hamster pelt riding jacket yelling "I am not a circle! Don't underestimate me!"
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Old 08-19-2011, 11:00 AM   #298
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Quote:
Originally Posted by Dread Pendragon View Post
Damn! Why did I choose "Dread Pendragon"?! I should have been "Dread Poloygon!" I would have been able to strut around in my hamster pelt riding jacket yelling "I am not a circle! Don't underestimate me!"




Good Stuff Jinx. - Go Ned!!!
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Old 08-19-2011, 11:08 AM   #299
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Running a pre-filter might make your engine burn more fuel... Test it first.. Could be an issue over greater distances (ie Dakar)...
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Old 08-19-2011, 12:06 PM   #300
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Quote:
Originally Posted by Dread Pendragon View Post
Damn! Why did I choose "Dread Pendragon"?! I should have been "Dread Poloygon!" I would have been able to strut around in my hamster pelt riding jacket yelling "I am not a circle! Don't underestimate me!"
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