trenace
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posted July 19, 2005 03:38 PM
 Edited By: trenace on 19 Jul 2005 16:57
Welded stroker question
I have a question that may seem dumb, but, well, I don't understand. I am sure some will have better info than I have and can help. Thanks!
I know it is a commonplace belief that "welded stroker cranks aren't as durable" and the blame is placed on being welded.
My belief is, there are two other factors that are working to decrease life:
1) When a factory strokes a motor, almost invariably they reduce the redline so as to keep piston speeds (or really piston accelerations, or probably even more to the point, bearing loads) the same. However, most who stroke their motors do NOT do this. They operate the engine through the exact same rev range as before. So in terms of piston accelerations and speeds, and stress on bearings, they are doing equivalent to the guy who over-revs his engine habitually. A 60 mm stroke at 11,650 gives equivalent stress as a 55.4 mm stroke at a little over 12,600. How can the same life be expected?
2) At least in the case of the 12 where the rod is already very, very short compared to the stroke, the rod/stroke ratio is dropping into dangerous territory when increasing stroke yet further with no increase in rod length. (This, unlike the above, is not necessarily so for all bikes. For example the Busa has long enough rods that there's no drop into a questionable range there.)
So I dunno, maybe one or both of those are the cause of the problems, not the welds per se?
Seems to me like a good weld should not reduce strength??
Anyhow, this is what I'm considering -- while previously ruling out stroking entirely due to not wanting to decrease longevity and concern of that being typical with stroked 12's, it's occurred to me that if I used the same spacer plate (namely one that will compress to 2.3 mm taller than stock) as if stroking by 4.6 mm, then I could stroke by 1.1 mm while running rods 1.75 mm longer, which would keep rod/stroke ratio virtually identical, actually improving it a trace. Since I'm also getting taller 5th and 6th gears, I could deal with reducing the redline for normal riding by the necessary slight percentage.
So I am wondering, since the piston speeds and accelerations would be the same, bearing load would actually be less due to lighter pistons and rods, possibly lighter wrist pin as well if I'm lucky, and rod/stroke ratio would not be worsened, might my durability be just fine?
Or am I missing it, that inherently that would be so if it were a factory crank of that extra stroke, but not with a Falicon-welded-up stroker?
And as a second question: if there should still be doubts about durability, what would be needed? Pulling the crank on some schedule to be sent out for inspection?? That would kind of suck.
Thanks!
  
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Ra12r
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posted July 19, 2005 06:45 PM
The only current problem with stroking the zx12 is 1) Proper crank grinding 2) Bearing clearances 3)Enough Oil in motor. The optimum R/S ratio is 1.85.
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trenace
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posted July 19, 2005 07:59 PM
Edited By: trenace on 20 Jul 2005 00:36
The 12's stock ratio is 1.575 (Which I learned from Entropy.)
I don't myself have confidence that a stroked 12, given proper grinding, bearing clearances, and enough oil will necessarily have the durability of a stock stroke 12 (particularly if operated through the same rev ranges and redline is not reduced.)
The 12 is just way short of having the deck height that would allow the R/S ratio you suggest.
For illustration, for a 350 Chevy to have equally low rod/stroke ratio as the stock Twelve, you'd have to run 5.48" rods in that motor.
For a 350 Chevy to have as low a rod/stroke ratio as a +4.6 mm Twelve with stock rods (1.454 ratio), it would be running 5.06" rods. (YIKES!!!)
(If those illustrations are helpful.)
The stock bike is about as low as one should go in the first place, and worsening that, as with the typical stroking situation, just is not where I want to go, myself. That isn't to say it may not be necessary or efficient to accomplish an objective, with understanding that rebuilds every season may be called for: but in and of itself a rod ratio below stock isn't what one would intentionally design, I don't think.
But that is another matter. Related though, as I would not be getting into a worse R/S ratio, instead keeping it same as stock, if doing the stroking at all. Which I still may not.
I was wondering instead about the above questions in first post, regarding whether welding really inherently reduces durability, or whether perhaps if rod/stroke ratio is kept the same as are piston accelerations and speeds, welding, for example Falicon's work, may not be just as durable as factory (why shouldn't it be, unless it violates the rule of welds being as strong as the material itself?); and then secondly the question about how to operate long-term a crank that is expected to be prone to such failure, if that should be expected.
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NINJA12
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posted July 20, 2005 06:21 AM
Edited By: NINJA12 on 20 Jul 2005 07:28
I ran a stroker for a year street and track.
It died the same as a stocker , #3 rod.
I don't think it's the stroke but a oiling problem in the 2000 motors.
Lots of trafffic and many many track days. ~7k miles and 4-500 passes.
That mileage would last most people 2-3 years.
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bossman12r
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posted July 20, 2005 08:34 AM
All good questions, but the longer rods will be weaker right?
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trenace
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posted July 20, 2005 10:22 AM
No, I don't believe there's an issue there. If anything longer rods take the force closer to being in a straight line rather than offset -- less angularity. (An extremely long rod would point almost straight up from the crank regardless of whether at TDC, 90 degrees ATDC, or wherever; whereas a very short rod goes to a very substantial angle.)
Anyway changes in rod length are often made in other engines and I've never seen a complaint of them being weaker.
I'd certainly expect Falicons or Carillos to not be weaker than stock.
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trenace
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posted July 21, 2005 06:29 PM
Edited By: trenace on 21 Jul 2005 22:53
On the matter of bearing failures, I just read a Kevin Cameron article that while not directly addressing this, raises a point that seems relevant.
Namely, that while already it's a definite fact that when increasing stroke by some percentage yet operating through the same rev range (not decreasing redline as the factories almost always do when stroking a motor), we're increasing g-loadings on the bearings by that percentage, there's more to it than just that.
If the rods were infinitely long, so that they'd be pointing "straight up" from the crankshaft at all times, then the rpm of the crank relative to the big end, in other words the local speed at the bearing, would be the same as the nominal rpm.
But with rods shorter than infinite, that's not so. Rod angularity results in speedings-up and slowings-down of that.
Cameron's figure where the rod ratio is 2.0 to 1, is about 25%.
In other words, at say 12,000 rpm (for convenience) when running that rod ratio, the rpm actually experienced at the bearing, the speed of the surfaces sliding past each other, actually varies between about 9000 and 15,000 rpm.
This is not a trivial effect, and though I have not done the specific math, I can see that while the effect is substantial at a ratio of 2:1, it is going to get rapidly even worse as the ratio gets even shorter than this.
The variation must be quite substantial with the short rod ratio the stock Twelve has of 1.575 -- so while the limiter speed of 11,650 (if I have that correct) sounds like nothing much, actually peak relative rpm between the crank and big ends must be quite a lot beyond that due to this effect.
Reducing it to 1.454, as with a +4.6 mm stroker, has to get that variation even greater. I haven't done the specific math, but perhaps peak relative rpm within the bearings winds up being hundreds of rpm higher with the stroker.
For example if this effect is twice as strong with a rod ratio of 1.5 than with 2.0, and if the change in effect is linear, the peak relative rpm increase on dropping from stock rod ratio to 1.454 would be about 720 rpm. Definitely enough to potentially make a difference, if so.
And so at the same rpm -- and most don't reduce redline when stroking the motor -- not only are the g forces greater with the stroker, but the rotation speeds of the crank relative to the big end, or the speeds the surfaces are sliding past each other, are increased. Probably quite substantially.
In other words these may not all be assembly or oiling failures -- maybe the bearings are just being pushed to the ragged edge and beyond what they were designed for, when stroke is increased without decreasing operating rpm range and on top of this rod ratio is reduced, thus increasing peak speeds at the bearings?
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trenace
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posted July 24, 2005 11:30 PM
Edited By: trenace on 25 Jul 2005 00:31
Ah well. I wrote Falicon, as part of asking about getting an order together, but they did not answer whether the welded cranks may be inherently less durable. Maybe they'll answer eventually, maybe not.
Perhaps they would really rather not say -- one can understand a company wanting to say that the billets are better, while not wanting to admit the extent of any real weaknesses to their welded product they also offer, let alone offer a suggested inspection / maintenance schedule that may be called for. If it's the case they think the crank should be pulled every so many 1/4 mile passes, or every so many street miles, do they really want that info put out? Maybe not. It sure doesn't seem to have been put out there, or at least I haven't seen it, yet you would think they have to have an opinion, to have a database on when cranks come back to them or are reported to have shown cracks.
In the meantime I'm going to assume that the conventional wisdom is correct, that while perhaps theoretically the welds might be or even should be as strong as everything else, in practice this may not be so for example due to stresses not being properly relieved or heat treatment issues -- this possibility being reinforced by some points that Gunner brought out to me.
So, for me personally, not worth it -- for others, I recognize it can be a lot of bang for the buck and/or just downright effective, even if not so durable in some cases, or adequate for some needs in others.
So back to the older plan of running a spacer as if running a 4.6 mm stroker, but instead running a little longer rods -- assuming that I can source these rods at reasonable cost and delivery time.
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MadMike
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posted July 25, 2005 07:21 AM
You can have custom rods made. I know crower will make them and I am going to assume that falicon will make them also.
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trenace
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posted July 25, 2005 10:41 PM
Edited By: trenace on 25 Jul 2005 23:56
Thanks, I am going to try asking Falicon again (maybe if I don't ask about the welded crank durability they'll answer my e-mail  ) as well as checking Carillo and maybe Falicon. It'll be interesting to find out the weights of the Falicon and Crower -- the Carillo is already known, thanks to Entropy weighing them, and represents a nice weight savings. I don't know their speed of delivery or cost of custom lengths though.
If there's no particular reason not to go with the Falicon, I might go with those what with them doing the crank at the same time. I should also check out Crower -- I'd pretty much forgotten about them till your post, thanks!
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MadMike
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posted July 26, 2005 04:38 AM
Crower can make titanium if you got the bucks.....
Mad Mike
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trenace
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posted July 26, 2005 01:52 PM
I'd thought about it.
Pretty much decided against it on account of, from almost every result out there it seems like I'm unlikely to have the peak hp occur past the existing redline or if it does, it won't be much of an increase. I'm expecting my hp curve is going to be fairly flat, all very near peak, from 10K to stock redline and possibly to about 12,250 or whatever a up-clocked ECU will give.
And the 12,250, I should think should be OK just from lighter weight of pistons and conrods, and maybe lighter piston pins as well (not sure what the situation is there yet.)
Also the valvetrain with stock springs doesn't even seem to necessarily like redline that well, and while heavier/better springs and bigger lift seems OK, I am kind of wondering whether more rpm is really called for there, though the above point (where the peak and near peak hp is expected to be) is my main thought.
And, titanium does have shorter life than steel, as it has fatigue issues that steel, short of extreme loadings, does not.
So with the intent being also plenty of durability for the street, I think I will stick with the steel. The titanium is intriguing though.
If I really had reason to expect that power -- with ported heads, Muzzy cams, and Brock Megaphone pipe, but stock throttle bodies and valve size -- might be substantially higher in the 12,000's than in the 10K - 11K range then maybe titanium would be better for that?
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trenace
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posted July 26, 2005 11:25 PM
Edited By: trenace on 27 Jul 2005 17:09
BTW, Falicon has now replied. In case any are interested in what they had to say, it's below. The capital letters are theirs.
A short answer: I can't expect a dissertation, they are in business to make money not to spend time writing e-mails, so I'm happy (plus, update, I've gotten quite helpful followups, so very good e-mail help from Falicon!) One significant aspect though is that the stock ZX-12R rod ratio is 1.575 while strokers typically dip into the 1.4 range, both substantially differing from his general rule below:
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WE BUILD A GOOD NUMBER OF STROKERS FOR THE ZX-12, USUALLY +4MM STROKE BUT
ONLY 1.1 WOULD NOT IN MY OPIONION BE WORTH THE COST AND EFFORT
APPROXIMATELY $900. THE ROD RATIO ON ALL THE NEW BIKES ARE IN THE 1.7 TO
1.8 RANGE. THIS IS OK. THE WELDED STROKER CRANKS ARE IN FACT QUITE
DURABLE, USUALLY THE ONLY FAILURES IN CRANKS ARE BECAUSE OF POOR
LUBRICATION.
The question here is, many say that welded stroker cranks are less
durable than stock cranks. My guess is the comparisons may not be
direct, since they operate them at higher piston speeds and lower
rod/stroke ratios. I won't be doing that.
LEAVING THE BIKE COMPLETELY STOCK IS THE MOST DURABLE ANSWER, BUT NO FUN....
HORSPOWER AND TORQUE RULES.
Should I expect reduced durability if I decide to ask you to increase
stroke by 1.1 mm? If so, then what sort of service schedule should I
follow in terms of removing and sending off for inspection on account of
needing to have the welded crank checked?
WE'VE HAD STROKER CRANKS RUNNING FOR YEARS WITHOUT PROBLEMS, KEEP THE OIL
CHANGED, POSSIBLY A LITTLE OVERFILLED IF YOU LIKE TO WHEELIE.
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