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Discussion Starter #1
As many of you know, Blackwatch Racing has done alot of transmission development and builds for the S2 Elise and Exige. We've built World Challenge gearboxes, NASA and SCCA Championship Gearboxes. I thought it would be useful to talk about things that break or wear, why, and what we do about it in the gearbox.

By and large the C64 Gearbox in the S2 Elige is a pretty solid unit. However, mid-engine RWD puts more stress on it than than the front wheel drive Celica GTS did.

There are 4 things that cause by themselves or in combination, gearbox failure: Mechanical Over-stress, Poor design, Operator Error, and heat.

Generally we focus on the first 3 or combinations there-in. Heat can be a problem with back to back long track sessions or low fluid, but it is rare.
 

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Discussion Starter #2
Mechanical over-stress can come from adding power, adding grip, and shock loading. Power is largely separate from the other 2 but can play a part. If you add grip to the car i.e. with our XP Lotus pictured, you will make it harder to slip the tire/pavement interface so any power or shock is transmitted throughout the driveline. Those 12.5" wide "meats" play holy hell with transmissions but we've been able to work around/through it to deliver a bunch of SCCA wins.

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This grip can lead to either too much power or shock loading being transmitted to a mechanical part which starts a fracture and eventually breaks. Having some slip or fuse in the system is very helpful to keep the breakage to a minimum. As you can see from the picture, we don't have much in the way of fuses.
 

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Discussion Starter #3
Power is the smallest problem, grip which makes shock loading easier, is the second issue, and shock loading is the primary problem. You may have heard about racing engines rated in terms of hours. So a 9 hour Miata engine has a high level of reliability provided it is removed and torn apart every 9 hours of race time. This is because the RPMs and load on those engines are known. Failure mechanisms have been identified, and parts are given a change out interval. Nearly all racing engines have a time-based refresh/replacement schedule.

This doesn't work with transmissions. The reason is the transmission takes impacts and forces from the road. In 2012, I was competing at solo nationals. I was coming around/out of a sweeper rolling into full power. About 2/3 of the way through the pavement had a 1" drop from one piece of concrete to another. The car was loaded up, power being stuffed into the driveline, hit the bump and 1 wheel had drive and the other didn't. My excellent differential did what it was supposed to and applied power to the inside wheel in the air. This increased the speed of that wheel massively in a split second. When that fast tire came down on the other side of the bump and gripped instantly, the forces that pushed back through the transmission were immense! With a BANG, the input shaft in the transmission snapped in half! My day and in fact that nationals was over.

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If tires had slipped a bit, or the differential, it might not have happened. However, because the magnitude of a shock load is unknowable, we can't guarantee something is truly idiot-proof. As I am fond of saying, "When you think something is idiot-proof, the world will build a bigger idiot" I say that knowing it is largely autobiographical!
 

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So with that in mind, we start looking at ways to make our cars and drive line more tolerant of use and abuse. If you can stay off of slick tires, it will help. SCCA and NASA have many classes that use 200 treadware tires. Thus the development of these tires has improved grip, wear, and cost. These will have a bit less grip than your Hoosier slick i.e. the Purple Crack, but still provide a great tire for track days.

Another thing is that if you change the clutch, stick with a sprung hub organic. The springs in the hub help dampen some of the instantaeous force and the organic disc can slip a bit more than a racy ceramic disc. We've used ACT HD-SS for everything including TVS400 SC cars. Our XP car runs the XT-SS which is the same organic disc with a heavy pressure plate.

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Discussion Starter #5
When looking at gears, shafts, etc there are 3 things that determine the strength of a particular component. They are: Design, Heat-Treatment, and Material. In that order. For our purposes we'll focus on Design and Material. Let's compare the BWR custom input shaft and the stock one. The BWR input shaft was based on my learnings about redesigning the Quaife sequential input shaft that I broke pictured above. We had to do a few iterations, but all those features made it into the BWR gearset input shaft.
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The first thing to understand is that each of these shafts is supported below the picture (below 1st gear) and above 4 gear by bearings that hold it in place. When the engine is driving the gears hard this shaft(input) and its mate(output) want to push away from each other. This can cause flexing in the shafts. The break in the XP car pictured above happened because there is a very high concentration of stress any place there is an abrupt change in material thickness. One major difference with the BWR design vs stock(pictured) or the Quaife above, is that we reduced the stress risers by putting generous fillets in place at every gear interface. This spreads the load over a larger area which minimizes the chance of a crack forming. We also put them on the reverse idler and below 1st gear. This also stiffens the shaft which reduces flex in the first place.

The other things we did have to do with the gear design. You notice that the teeth are considerably thicker. This reduces flex in the root of the tooth which can cause breakage. Additionally, we reduces the Helix angle on the gears. Higher helix angle is quieter but the force engagement is not consistent across the tooth. The force starts at either top or bottom and moves to the other side across the face of the gear as it rotates. Reducing the helix angle spreads that load over a broader area. The higher helix is quieter, but it is not linear. You may hear some gear noise under load with our gear set, but at cruise it is generally as quiet and underload only a bit more noise. A good trade off.

The other other thing we did was thicken critical gears to add strength..
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As you can see we also made 2nd and 3rd gears considerably thicker than stock. 3rd is by far the most broken gear in the gearbox. It is a perfect storm of too small with too much relative torque. So we made it about 15% thicker AND with lower helix AND thicker teeth.

Additionally, 2nd is as thick as we could make it without interference!
 

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Metallurgy & Materials

Many books could be written on this subject and well, many have been. I'll focus on the medium to high level stuff as relevant here. As a warning to the Uber-nerds out there, yes, this will be edited for clarity. You want to ask legit questions fine. If you want to derail this thread to show everyone how smart you are, go somewhere else.

The reason this warning is necessary is because this is a complicated and fascinating topic.

What do we care about?

1. Strength. How much force can the steel endure without deforming? Steel is unique in that if you stay below its plastic deformation point(point where you bend it and it doesn't bounce back) it can be resilient for millions of cycles. This is usually expressed as Ultimate Tensile strength(pull apart) and Yield strength(the point where plastic deformation occurs).

2. Toughness. How much force a steel can endure before developing a crack. Once a tiny crack starts, the fuse is lit. It is only a matter of time before failure.

3. Elongation. This is how much the steel can stretch before it breaks. A measure of ductility or brittleness.

4. Hardness. How much the steel resists deformation.

There are 2 basic categories of steels we care about: Case-Hardened and through-hardened. You may ask, why not just start with a hard steel and be done with it? A couple reasons:

1. If we post-process the hardening we have control over how much hardness vs elongation we get. Additionally, we can control strength as well.
2. Steel is VASTLY faster and and easier to machine prior to hardening which goes to cost and yield.

In general, case-hardened steels are used in OEM transmission components. They tend to have a higher surface hardness expressed in Rockwell C but are not as strong. OEMs care about very good longevity and wear for 100s of thousands of miles at relatively light loading.

Through-hardened steels are generally quite a bit stronger but have a lower surface hardness. The big differentiation is that these steels are more expensive than their case hardened cousins.
 

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Discussion Starter #7
For example our stock Toyota gears are made from a case hardened Japanese equivalent of 8620. Properties below.

AISI 8620 Steel, single quenched and tempered (230°C (450°F)), carburized
No code has to be inserted here.

So ultimate strength is 168ksi and yield at 121ksi for a standard 1" sample. Elongation is 14.3% This is how much the material stretches before it breaks.

Whereas 4340 tool steel has ultimate strength of 276ksi and yield 222ksi. Elongation is ~11% It can be hardened to low 50s Rockwell C. Clearly far stronger than OEM.

Last, but not least is 300M. 300M is a "super steel" developed for aircraft landing gear and tailhooks for fighter jets. Its closest cousin is 4340.

300M has UTS of 288ksi and YTS of 244ksi. Effective working strength is DOUBLE that of 8620 and at only 4 times the price! Hardness improves a bit as well to RC55-58.
 

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In our gearset we use 4340 for the gears and 300M for the input shaft. The shaft has to do triple duty. It has molded gears that have to function like gears. However is also has to have strength to resist twisting from the Engine AND Bending as it and the output shaft try to push away from each other under load.
 

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I'll have more over the coming days and weeks, but now is a good time to pause for questions.
 

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Fred are your gear-sets close ratio? The stock gearing is not good to say the least :-(
Yes they are much closer 1-4 and he has tables of the resulting speeds on his website. They work great with 250 whp and short or tall tires.
 

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I wanted to add my knowledge on the gearbox that I gained breaking a few.

1. The carbon synchros are absolute trash. Just the worst. It is embarassing that anyone sells them. Mine lasted 14 sessions and I know 2 other cars whos lasted less.
2. I stopped breaking ANYTHING once I put in a clutch delay valve. I used the tilton one from BOE but there is OEM and you could piece together your own
3. When rebuilding the gearbox the one that I had the best lifespan from I changed the center bearing holder. I was told by someone in Australia who rebuilds these a lot in the toyota world that there gets to be a tiny bit of play in the center plate and that leads to some of the synchro wear. Good $100 investment.
4. You should replace your circlip on the drivers side axle.
 

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I wanted to add my knowledge on the gearbox that I gained breaking a few.

1. The carbon synchros are absolute trash. Just the worst. It is embarassing that anyone sells them. Mine lasted 14 sessions and I know 2 other cars whos lasted less.
2. I stopped breaking ANYTHING once I put in a clutch delay valve. I used the tilton one from BOE but there is OEM and you could piece together your own
3. When rebuilding the gearbox the one that I had the best lifespan from I changed the center bearing holder. I was told by someone in Australia who rebuilds these a lot in the toyota world that there gets to be a tiny bit of play in the center plate and that leads to some of the synchro wear. Good $100 investment.
4. You should replace your circlip on the drivers side axle.
08 and later s240 and s260 came with the oem delay valve in the clutch line. Not sure if the quality is comparable to Tilton one you have, Kevin. In theory, delay valve might shorten the life of clutch a tiny bit, but I’d rather blow up/wear out clutches than the transmission itself. Kevin and Fred could chime in here on that theory.

Could you please elaborate a little more on 3rd line for those of us who are mechanically challenged? Any pointers on how/where $100 investment is made? I really appreciate your contribution here Fred and Kevin. Thank you.
 

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Part # 48. Its obvioulsy only $100 to change if you're already in there though!
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I think the bearing holder, which is a steel plate should be changed if there is indication of wear, which is possible.
 
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