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Old 01-21-2009, 01:51 PM   #251
Dave_RalliSpec
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Good points! No, I never completely dismissed case flex either but decided that it must be just one small component of many in the whole picture rather than the smoking gun of why these gearboxes fail. You are right on about the case changes...obviously they were looking for ways to increase rigidity and they wouldn't be spending more money on bolts and machining if there was no reason to do so. And if you look at the 6-speed you see a complete revolution in the case design, one part of which is the elimination of the longitudinally split case and the other is the introduction of a thick steel shaft support plate. I think also the bellhousing casting thickness is increased in some places as well.

Good overview on the shockloading aspect also! It gives you some idea of how much affect a very aggressive clutch design (one with very little slip) can have on the gears in a hard launch. We've always tried to steer people in the direction of an organic clutch with the 5-speeds whenever the torque levels and application didn't demand going with something more robust.

A side point I'd like to make....very frequently people tell us they were not doing anything crazy when their gears let go. However most often the failures originate at some earlier point during a hard launch or agressive shift. Small cracks form at the roots of the teeth engaged at the time of the shockload. These cracks continue to propogate over time until finally the teeth let go. So just because the gears failed during normal driving doesn't mean they were not compromised at some earlier point during some more aggressive driving.
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Old 01-21-2009, 05:59 PM   #252
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Dave,

Great points as always. Actually the idea of using additional dowel pins or something along those lines is certainly intriguing. I would think that movement between the case halves would be in more of a shearing motion (halves sliding past each other) rather than expansion. The potential miss I see here when using strain gauges is that only a tiny percentage of the gauge's surface would be placed over a point where it would see distortion. Since only maybe .001"-.002" of the gauge's overall legnth would experience any deformation directly over the seam in the case halves, the gauge would show very little resistance change on an ohm meter... Also depends on the gauge itself, they come in lots of different sizes and different resistor film layouts, and rates of resistance change per unit of expansion. The larger the gauge, the less deformation it would register.

As a side note, 7/16" SAE bolts are almost a press-fit in the holes for 10mm bolts, and 3/8" would nearly fit in the holes for 8mm bolts. So they'll go in with just a little bit of drilling using a really long drill bit. While it's sort of bad form to use SAE bolts on a metric car, it does work nicely. In fact, I've been playing with the idea of offering these as a full bolt kit in grade-8 as well, not only would they be a slighty larger diameter than stock on top of being of a higher grade, they would also act like dowel pins to keep the case halves better aligned.

I agree with you that too much emphasis was put on case flex early on, I really don't think it's the main factor here... However, there is one bit of evidence that kept me from dismissing case flex as a complete myth and this drove me to do a little testing of my own: That is, Subaru themselves beefed up the case design in 1998. The changes they made strengthened the case a bit over the old 4-bolt models, but their changes were obviously limited by cost. These changes were within a fairly cost effective scope, rather than making radical changes to the case that would require major re-tooling in the manufacturing process, they added material where it would be fairly inexpesive to modify the castings. I think Subaru wouldn't have put any effort into changing this design at all if there would be no benefit in doing so. Because of this, my guess was there could still could be some level of gain with further reinforcement.



http://www.botlanta.org/converters/d.../flywheel.html

I used this online calculator as a quick reference to show just how much energy is stored in your flywheel @ 6,000rpm.

The figures are kind of off the top of my head and only approximate a stock subaru flywheel. I used 21 pounds (336 ounces) over a 12" diameter @ 6,000rpm... I don't have a flywheel in front of me, but the clutch disc diameter is 230mm (about 10"), and another inch or so on either side sounds about right. Now this calculation is assuming a uniform spread of weight throughout the fylwheel, so these figures will only be a ballpark estimate.

At this speed the energy stored in the flywheel is 21,846 Joules. The conversion factor is roughly 0.73 ft-lb to every joule... So you're looking at about 15,946 ft-lbs stored up in that flywheel... In other words, if you could stop the flywheel instantly, it would exert almost 16,000 ft-lbs into whatever is stopping it.

Now, when a clutch engages, a lot of this energy is lost to friction/heat, and some of it is spread out over the period it takes for the clutch to fully engage (slippage). The car is also able to roll, so the gears aren't facing infinite resistance from the driveline like they would be in both my test and the test Dave took part in. However, I would still expect to see at least few thousand ft-lbs of torque finding its way through your gears for a split second during a really violent launch, and that's not even including the torque that your engine is also applying to the driveline along with the inertia in all of the engine's moving parts (crank, pulley, rods, etc.).

This sounds like a lot, but think about it... Even a small hand-held pneumatic impact gun can be rated at 250-300 ft-lbs with the tiny weight it has spinning inside, so it's not so hard to believe that a big 21 pound flywheel can generate so much gear carnage.

When all is said and done, it's the gears that have to take the abuse, so they're still going to be a point of focus. Dave's test seems to prove that the gears themselves are moving around in there, and the shafts on which they spin are flexing beyond what the gear teeth are engineered to handle. No surprize to me really, you'd really have to have been there to see just how much that input shaft twists at 300+ ft-lbs, it was quite amazing to me. I do agree with Dave that the failure rate we see is due to gear seperation caused by shaft flex, with lots of other little contirbuting factors. So when it comes down to it, to build a really strong box, it's going to take really strong gears. Again, the way I see it is that better hardware and some reinforcement to the case can keep those bearings just a little more stable, and that at least is a starting point. It doesn't solve the root cause of many 5mt failures, but it does address one of the many contributing factors.
I didn't realize how much abuse the trans actually sees. It's nice to see some numbers to actually compare everything, but by no means am I on the same level as you or Dave (as I have no engineering background). I have heard of people breaking the input/main shafts before, and I know that it always will be a problem until it's replaced. What makes it one of the weak points though? Also - how much of shaft deflection can be accounted for the gear failures?
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Old 01-22-2009, 01:22 PM   #253
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I didn't realize how much abuse the trans actually sees. It's nice to see some numbers to actually compare everything, but by no means am I on the same level as you or Dave (as I have no engineering background). I have heard of people breaking the input/main shafts before, and I know that it always will be a problem until it's replaced. What makes it one of the weak points though? Also - how much of shaft deflection can be accounted for the gear failures?
The input shaft is kind of narrow and because of the front diff arrangement it has a pretty long span over the diff that is not supported by a bearing. The bearings on which the shafts spin are a bit small for the application (especially the front input shaft bearing). The longitudinally split case that holds the bearings in place aperantly is susceptable to torsional flex, which likely further accomodates misalignment of the gears. Centerline spacing or diameter of the gears is pretty short which limits the size of the gears. That's the short version though... As Dave mentioned, the list of related factors that contibute to failure are complex. Gear seperation is not the cause, but the final result from the sum of all of these factors, and I suspect that it's what eventually kills the transmission in most cases.

A couple thousandths of an inch in gear lash can be the difference between a transmission that lasts 200,000 miles and one that wears down and fails in say 30,000 miles or less... So when all of these small factors are stacked together, it doesn't take much to put undue strain on the gears.

In whatever gear, when the clutch is engaged violently, failure can either be immediate or a small microscopic crack can appear which over time propogates/grows from fatigue (repeated back and forth flex in metal) and causes the gear to fail at some point in the future.

The logic of course, is that since there are multiple factors involved, anything we can do to patch up one weak spot may help overall by contibuting less to gear seperation. Even very strong aftermarket gears are subjected to the same problems that the stock gearset has to deal with... The big difference is that PPGs, for example, are allowed a rather aggresive/large tooth profile at the cost of increased noise, so the gear teeth are just so big that they can cope with a larger amount of seperation than the stock gears can handle and the customers that buy them don't really tend to mind the noise. Could Subaru have made gears similar to PPG's product? Sure they could, but even PPG's helical tooth profile would be far too loud to put in a production car, and smaller more numerous gear teeth for any given centerline spacing tend to be slightly more efficient at transfering power.

Last edited by jhargis; 01-22-2009 at 01:31 PM.
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Old 01-25-2009, 04:18 PM   #254
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good info in this thread. keep up the awesome work.
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Old 01-28-2009, 06:57 AM   #255
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I saw that the website was updated. Everything looks like it's coming along nicely. I'm looking forward to seeing what the diff brace will look like.
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Old 01-28-2009, 11:07 AM   #256
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I saw that the website was updated. Everything looks like it's coming along nicely. I'm looking forward to seeing what the diff brace will look like.
Yup

I finally got around to getting the Nighthawk website up to par... That site has kinda been in limbo for a couple of months while I've been working on the production side of things.

Due to recent happenings, I'm pretty much the only person running the factory floor at the day job. And on top of it, I'm still trying to keep up with everything for Nighthawk, so life has been all kinds of busy lately.

Here's the brace:


I haven't talked to Dylan about it yet, but I think I'll be dropping the idea of a passenger's side diff brace. I don't think I can come up with anything that will be effective and affordable at the same time. There's no super convenient pre-drilled boss to bolt the thing into like there is on the driver's side. No matter really... A passenger's side brace would be nice for the really high horsepower guys as an added reinforcement, but the driver's side brace covers the part of the diff housing that flexes really badly.

Last edited by jhargis; 01-28-2009 at 11:22 AM.
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Old 01-28-2009, 06:39 PM   #257
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Yea, I could see that as well. Especially with the downpipe being there anyway - it may be hard to make a universal brace that works with all downpipes.
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Old 01-28-2009, 09:34 PM   #258
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^^^ That's exactly what I was having problems with, I'd have to keep it really snug up against the transmission to clear the pipe, and the only place to bolt anything to is the threaded hole where the stock downpipe support bracket bolts to, which is actually into a fairly thick piece of of the case structure that doesn't need reinforcement as badly as the other side anyway.
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Old 01-30-2009, 12:01 AM   #259
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Jhargis, what about building a two piece frame that encases the transmission housing? One half goes on from the top and the other from the bottom. You could use steel tubing and put a ring plate around the outside of the front diff adjusters, pressed against the transmission. This type of brace would have to be installed with the trans out of the car of course. Maybe using several smaller steel tubes with integrated "x" patterns here and there would allow more attachment points. Would you be able to make one unit and form a jig around the test unit to produce the others? Just a thought.
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Old 01-30-2009, 12:29 AM   #260
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That was one thought that I kicked around a bit earlier on until I took some measurements and rough calculations of cost. In fact, I had originally envisioned having the frame structure extend back to the center diff section of the case the strengthen that area as well.

Alas, it would be very difficult to get a cage structure to fit inside the trans tunnel and clear the downpipe, the plate only allows just enough clearance as it is and it gets prety tight back there toward the rear of the transmission. Also, who wants to spend $600 or more dollars on a case cage short of maybe the rare cost-is-no-object 5mt project guy? I'd have a hard time selling something so expensive and wouldn't be able to make very many of them because it would be pretty labor intensive.

Thanks for the input though. It would be pretty cool, but I'm not sure if all the effort and cost would be worth it when 6mts are getting more affordable and easier to find.
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Old 01-30-2009, 12:55 AM   #261
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Yeah, exactly what I was talking about. Extending back to the rear housing would give maximum bolt locations as well. I think you could do it, and am sure you would get substantially less deflection if the two reinforcement halves overlapped eachother and shared several of the side bolt locations. C'mon.. you can do eet! Besides, the 6mt's aren't perfect either.


Quote:
Originally Posted by jhargis View Post
That was one thought that I kicked around a bit earlier on until I took some measurements and rough calculations of cost. In fact, I had originally envisioned having the frame structure extend back to the center diff section of the case the strengthen that area as well.

Alas, it would be very difficult to get a cage structure to fit inside the trans tunnel and clear the downpipe, the plate only allows just enough clearance as it is and it gets prety tight back there toward the rear of the transmission. Also, who wants to spend $600 or more dollars on a case cage short of maybe the rare cost-is-no-object 5mt project guy? I'd have a hard time selling something so expensive and wouldn't be able to make very many of them because it would be pretty labor intensive.

Thanks for the input though. It would be pretty cool, but I'm not sure if all the effort and cost would be worth it when 6mts are getting more affordable and easier to find.
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Old 01-30-2009, 02:02 AM   #262
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pm me when this is finalized and I will be more then happy to install on my 06wrx with ppg's and post my reviews.
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Old 02-04-2009, 10:15 AM   #263
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so when would the finished product will be availabe for sale???

how much is it gonna cost???
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Old 02-04-2009, 01:20 PM   #264
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so when would the finished product will be availabe for sale???

how much is it gonna cost???
-I shot some pictures over to DS1 a couple days ago so Dylan can set up a feeler to get an exact idea of demand. The first run is complete, once that thread is set up, we should have a few available, and I'll know how many more I need to make.

-http://www.nighthawkautomotive.com/ <- look in the products section. The reinforcement plate is listed, and I'll have a page for the diff brace up probably in the next few days.

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Old 02-04-2009, 09:17 PM   #265
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How about machining a plate that goes where they crack and weld it to a open case, then reassmble it? Wouldnt that be the ultimate solution?
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Old 02-04-2009, 09:47 PM   #266
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How about machining a plate that goes where they crack and weld it to a open case, then reassmble it? Wouldnt that be the ultimate solution?
That thought did come up. However, Dylan over at DS1 has tried welding to the case in the past and said the results were not very good. From what I understand, the cast aluminum that the case is made from is quite porous and doesn't take to welding very well. That factory-drilled boss (when threaded using a proper tap) is going to be stronger than a bad/porous weld any day.

You also have to figure that the average mechanic or DIYer can install the diff brace with the transmission in the car in about 30 minutes using a ratchet, some sockets and a torque wrench. Dropping the transmission, taking the case apart and welding a plate in is something the average buyer is not going to want to do... In this case, I'm trying to keep things as simplified as possible for customers.
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Old 02-04-2009, 10:21 PM   #267
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keep us posted on price and availability I would like these for my 07 when i get my clutch and PPG's done
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Old 02-05-2009, 07:19 AM   #268
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What about a possible transmission cooler? I know it's hard to weld to the stock aluminum case as well, but what about just drilling and tapping so that another fitting can be added as a return/feed line?
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Old 02-05-2009, 10:42 AM   #269
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Yup

I finally got around to getting the Nighthawk website up to par... That site has kinda been in limbo for a couple of months while I've been working on the production side of things.

Due to recent happenings, I'm pretty much the only person running the factory floor at the day job. And on top of it, I'm still trying to keep up with everything for Nighthawk, so life has been all kinds of busy lately.

Here's the brace:


I haven't talked to Dylan about it yet, but I think I'll be dropping the idea of a passenger's side diff brace. I don't think I can come up with anything that will be effective and affordable at the same time. There's no super convenient pre-drilled boss to bolt the thing into like there is on the driver's side. No matter really... A passenger's side brace would be nice for the really high horsepower guys as an added reinforcement, but the driver's side brace covers the part of the diff housing that flexes really badly.
call me a sceptic but doesn't the blue metal brace looks rather insignificant compared to that entire chunk of gearbox casing?
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Old 02-05-2009, 11:24 AM   #270
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call me a sceptic but doesn't the blue metal brace looks rather insignificant compared to that entire chunk of gearbox casing?
You'd be surprized how hollow that case is when you crack it open... The part of the case that the center bolt location fastens to is pretty thin in some spots actually. So flex (in this case, expansion) at that fairly weak point in the case exerts a load on that solid steel brace, and that load is spread to stronger parts of the case... At the front, the brace is botled to both the bellhousing and the engine block, and at the rear it ties into a large 10mm bolt that holds the bearings in place.

True, compared to the size of the case as a whole, it doesn't seem like a gigantic change... But take for example, that the webbing on the case (the thin vertical structures in the case that spread out from the axle stubs) look fairly insignifcant as well, but they add a lot of stregnth to a case because they are located strategically at just the right spots. Notice that my brace bolts to a location that does not have much external webbing, and just to left of where the brace fastens in that picture is where the case tends to crack

Last edited by jhargis; 02-05-2009 at 11:29 AM.
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Old 02-05-2009, 12:10 PM   #271
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I am ready to buy, just say when.[06/wrx]
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Old 02-05-2009, 01:39 PM   #272
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I am ready to buy, just say when.[06/wrx]
+1 I Even got the visa on my hand!!

02 wrx with 04 tranny lgt 1st and 2nd gears... act clutch :-S
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Old 02-05-2009, 02:02 PM   #273
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Very interested in buying this when they come out
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Old 02-05-2009, 02:43 PM   #274
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yes i also want it
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Old 02-21-2009, 11:40 AM   #275
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Where is this product now?
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