bbundy

Um no you are incorrect the sycro collar hooks the input and output shaft together for the 1:1 ratio no torque is transferred through gear teeth in the transmission at all in that gear. all the rest of the ratios go through two sets of gears and send torque through the countershaft.

99Racer

Rennkafer & Leafy. Sorry but your understanding of how the transmission works is a little flawed.

The basics...

A front engine rear drive manual transmission in "most" car and trucks has the following basic design.

The shaft passing through the clutch we will call the "1st motion shaft". It turns at engine speed when ever the clutch is engaged.

The first motion shaft has a gear on it that drives a gear on a parallel shaft we will call the "Counter Shaft" (or 2nd motion shaft) it turns the opposite direction (or counter direction). If you have a car from over there, the Brits call this the "lay shaft".

The first motion shaft and counter shaft are always engaged and spin at a fixed ratio determined by the pair of gears on the 2 shafts (pair of gears 1). So far we have 2 shafts spinning at a fixed ratio and no power out of the transmission... Its in Neutral!!!

Multiple gear pairs exist between the counter shaft and the "3rd motion shaft". The 3rd motion shaft is the output or tail shaft of the transmission... For a typical 5 speed (like the Mazda Miata), we have a pair of gears between the counter shaft and the 3rd motion shaft for 1st, 2nd, 3rd, and 5th. BUT not 4th gear...

To engage a gear such as 1, 2, 3 or 5 (not 4th) we cause the pair of gears between the 3rd motion shaft and the counter shaft to be engaged. So in total we now have 2 pair of gears (the 1st motion to counter shaft drive pair and the selected counter shaft gear to 3rd motion gear engaged) providing the power path through the transmission.

To get 4th, we couple the 1st motion shaft directly to the 3rd motion shaft. This is possible because they are directly in line with each other or "End to End". Connecting them together gives a 1:1 ratio. 1st motion shaft and 3rd motion shaft are EXACTLY the same RPM... Note NO GEARS are involved in this power path... that all the other gear pairs (1, 2, 3, 5) are spinning freely on their bearings BUT do not participate in the power path.

So, an equal number of teeth can give you a 1:1 ratio BUT that is not how it works in your car... AND 4th is NOT 1.1:1 using the design of your transmission... physically not possible.

Sorry for the long windedness... I was very tempted to flame instead.


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What makes the Richmond and G-Force transmissions interesting is that the indirect ratios are 1, 2,3, 4. Top gear of 5th is 1:1. So we have almost NO Power loss through the transmission in 5th as it is a direct connection between the 1st motion shaft and the 3rd motion shaft.

Minimal transmission power loss in top gear so, maximum power available to attain the top speed (and overcome aerodynamic drag).

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bbundy - definitely! if there is a solution that costs less, requires no custom fab, etc. than that is best option available. Can the Nismo solution hold your 350 HP successfully? If not it seems a little costly experiment based on the quick search I did ($6500 for a complete transmission). re:

Nismo 32010-RRS50 6 Speed Transmission SR20DET S13 S14 S15

Options? I suspect there are others. The Quaife 5 speed gear set should be a little less costly and may be less work overall.

thoughts?

Rennkafer

iTrader: (3)

My apologies Peter, I made the assumption that the Miata boxes were like others I'd worked on and that turns out to not be the case. I should have spent more time looking for my PDF of the 6 speed manual before I commented.

Thanks for resisting the temptation to flame...

bbundy

According to NISMO the stock S15 transmission design torque was 217 ft-lbs I suspect the Miata box is significantly less like around 180 ft-lbs based on some rough calculations comparing gear teeth numbers and sizes and the lower torque in the secondary shaft due to the primary ratio difference. The RX8 is a tiny bit more maybe 190-200 based on the ratio change in the weak 4th gear. The Miata box maybe will last a year of track driving at my power level. The NISMO box design torque is 289 ft lbs (369Hp) which is closer to what I make with the boost turned down to run in NASA TT2. I suspect the gears would last for a many seasons.

I assume design torque is defined as something approaching infinite fatigue life at that torque. and for what its worth Quaife makes no claims whatsoever about their gear set for the 5 speed starting with a weaker box its likely not as strong as the NISMO 6.

NISMO | Silvia•180SX Reinforced Cross 6-Speed Transmission

99Racer

Based on the data, the Nismo transmission sounds like it will work for you at ~ 350HP. I doubt infinite life will be possible or needed (below for more info).

Is it possible to just change the Nissan bell housing from the transmission and substitute a Mazda BP one? This may be the best solution if there are other less obvious improvements in this higher torque capacity transmission.

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The Nismo transmission designers seem to be addressing the strength on multiple fronts: Better metal alloy, gear pairs with fewer but stronger (thicker) teeth (the Module change), reduced helix angle 33 to 26 degrees for 4th (but still a long way from straight cut), ...

Also, 4th from the counter shaft to 3rd motion shaft is 1.034:1 (for an overall 4th ratio of 1.2179). this is great for optimizing 4th gear's strength. But likely reduces the strength of the lower gears (1st & 2nd).

Counter 1.178 Overall
1st 2.468 2.9071
2nd 1.688 1.9888
3rd 1.305 1.5373
4th 1.034 1.2179
5th 1
6th 0.732 0.8624

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Some thoughts on transmission design and how to improve strength have been tossed around in this thread previously. I'll add my own opinions here... No real analysis just some armchair engineering and rambling.

Gear "tooth" strength - Gear teeth are effectively little cantilever beams in bending. Each tooth experiences one load cycle (no load to peak load to no load) for each revolution of the gear.

Fatigue strength - the lowest fatigue cycle is a tensile test for the material strength - you pull the sample, it breaks, you measure the peak force reached for a given sample cross section area (published data for "ultimate tensile strength" exists for this test for almost all metal alloys).

Infinite fatigue strength is at a much much lower force cycling at something around 10 million cycles or more (plus or minus 50% or so depending on the material). That would be about 24 hr at peak power at 7000 rpm in the gear that failed. Clearly not what you are demanding from the transmission. I'm guessing your failures are occurring at higher load levels and at much less than infinite fatigue cycles.

Increasing the strength by 2x will make your transmission last 2x longer (or more) but not indefinitely. Based on your 1 year transmission life span experience and usage levels, you might get 2-3 years from a Nismo gear box or more (but I suspect it will not be infinite).

Gear profile - The engine can only turn the transmission input shaft. The resulting gear force angles are entirely dependent on the gear design.

Helix angle - Helix is added to transmission gears to eliminate the whine in straight cut gears. The problem is the gear teeth can only transmit force perpendicular to the tooth. Any helix angle causes the gear tooth to experience a higher perpendicular load (than engine torque & tangential load would indicate). Helix also causes an end thrust on the shaft the gear is mounted on this can cause housing flex in some cars (unknown to me if this is an issue for the Miata). ---- Reducing helix angle makes the gear tooth experience less load at a given torque level.

Gear tooth profile - When looking at a gear tooth from the end, it has sort of rounded triangle like shape. In power transmission gears, the "pressure angle" is not tangential to the gear but at say 20 degrees. This force vector has a tangential component (power transmission) and a perpendicular component that causes the gear shafts to be pushed apart potentially bending the shaft they are mounted on. In some cases, the shaft bending can cause the gears to spread damaging the teeth by shearing off the tips (followed by the remaining portion of the tooth).

It may be coincidental that 4th fails (in a 6 speed) and it is located about 1/2 way between the bearing support (at 1st & 6th) and the bearing between the 1st motion shaft and 3rd motion shaft... You may also have also noticed the Richmond Gear transmission has an additional bearing in the area of 2nd gear and 3rd gear (in the road race 5 speed).

Increasing pressure angle improves the tooth strength (wider at the base of the tooth) but also increases the forces spreading the shafts apart - a compromise trade off for the transmission designer.

The above is a little long winded... I hope it provides some value to understanding transmission and gear design.

bbundy

Interesting the NISMO price list says 350,000 Yen roughly $3,400.00 that’s quaife gearset price range. Wonder if it could actually be had in the US for close to that.
http://www.nismo.co.jp/en/products/c...lvia_price.pdf
I sort of think these ratios would be freeking awesome for performance competition driving track or autocross. Very Useful first and second gear for autocross with 4 gears tightly spaced in the usable range for track driving 3rd through 6th pretty much perfectly placed for use on track. I will have a standard 6 speed S15 box in a few weeks to make sure if the parts can have interracial offspring as I think they can to produce a Miata box with Nissan internals.

mr_hyde

iTrader: (4)

Props to you for excellent contribution. This comment jumped off the page as a possible contributor to the 4th gear failure. Interesting stuff.

bbundy

I would also like to point out of the three 6 speeds I have broken two of them stripped the teeth off 4th gear and one of them was a thrust failure. The thrust on 4th gear broke the teeth on the reverse gear on the countershaft where it buts up against the thrust retainer for the 4th gear causing 4th gear to walk down the shaft. It also thrust seized the front end bearing on the secondary shaft welding the inner race to the shaft.

concealer404

iTrader: (3)

Sorry to keep this off topic, but...

If the AWR 70a are too rough for a street car, and i find the Mazda Competition mounts to suck seriously bad, what else is there to try in the middle?

thenuge26

iTrader: (2)

That's why I love this site. You guys don't just tell me which parts to buy, but the theory behind why. Keep it up, and don't worry about being long winded.

Leafy

iTrader: (1)

Mod the stock mounts to have a through bolt so they stop tearing apart, which is what I assume your issue is.

concealer404

iTrader: (3)

They sucked on the test drive after i installed brand new Mazda Competition mounts. They weren't/aren't torn. They're just soft as ****.

Leafy

iTrader: (1)

Then get the awr 70 duro and deal with the noise. The softest awr are still going to transmit less vibes than a stock style mount with stiffness that you'll find satisfactory.

bbundy

$3,277.37 + $256.86 shipping from Japan. for complete NISMO 6 speed transmission. Quaife price by comparison

NISMO Silvia Reinforced Cross 6-Speed Transmission - Assembly - RHDJapan

http://quaife.co.uk/shop/products/qke3k

bbundy

That I dont know.

One thing I haven't tried is placing stay rods up high on the engine. I can see in theory how that might get some or most of the benifit of stiff engine mounts and mayby not have as much vibration issue.

never tried it myself though.

Bob

Savington

iTrader: (31)

I would do some more reading on the Nismo box, specifically in S15 circles. It's been over a year since I did my reading, but I recall the Nismo box not being that much stronger than the OE 6-speeds. I may be wrong, YMMV.

bbundy

I find that hard to believe. I havn't found a reported failure of the NISMO 6 yet. lots of stock failurs but I can't seem to descern what they are failing.

I do believe it is true that it is Not any stronger than the 5 speed out of the S15 but the Nissan 5 speed has crummy gear ratios and it would be a heavily fabricated install to put the 5 speed in a miata.

bbundy

OK how about a beefed up version of the NISMO Box same ratios. $4000
Specifications:

Confirmed Torque Compatibility: 50kgfm (490.3Nm) ---------361.6 ft-lbs
Confirmed HP Compatibility: 430ps (316kw)---------------------423.8 HP

HPI 6-Speed Cross Transmission S13 S14 S15 Silvia 180SX - RHDJapan

locomonkeyboricua

After looking at the 6 speed 3.3 combo on the gear calculator i wonder if this combo will hurt the 6 speed due to the added stress. i have blown the 5 speed when shifting to second and once in 3rd. I wonder since we have just turned the 6 speed ratios much the same as the 5 speed if there will be a reduction in the ability for the 6 speed to hold the power. any thoughts??? for those blowing 6 speeds did you have the 3.6 combo ?? i have personally seen a 6 speed get abused with flat shifting and the boost at 20 psi 300whp and it held up fines for many drag races. they had a 4.10 rear ratio tho. anyone here about the retainer spring upgrades that some spec miata builders use??
here a video on you tube.

JasonC SBB

Interesting thought. However, if this were true then slamming into 4th or 5th would more likely break things than slamming into 2nd or 3rd.