Usually (not always though) the reason why a 6spd box is stronger is due to the closer ratio gears, so less shock load is being applied to each gear

My 5 speed dog box is supposedly rated for far more power than stock 5-speed. However it runs straight cut gears over the helical which would help stop the thrust loads. It also has a single billet input shaft / drive gear and the gears all re-arranged so it's easier to work on. Also the gearing is so close there is another entire gear between the 4th and 5th of a stock 5 speed.

The 5 speed is good because it has far nicer shifting feel and is much easier to rebuild for a novice which is a factor for the DIY miata cheapskate like me. The 6 speed adds another $2k to the total price once you factor in purchase price and that you need to send it to an expert to build.

The issue I see is there really needs to be two gear sets. A close ratio set for the track guys, and a longer street ratio set for the really high powered turbo cars and pretty much most street builds.

Helical gears are actually stronger than Straight Cut gears due to more surface contact area and spreading out the surface load between several teeth rather than just 2 teeth. The only reason why most aftermarket gear companies make straight cut gears is because it is cheaper to do so and lack the knowledge and/or equipment required to machine Helical gears. And, by playing on the fact that most consumers associate "straight cut" with "strength", they can sell them for more also, thus make even more profit. The only advantage (besides cost and ease of design/manufacturing) that straight cut gears have is less friction = less parasitic drivetrain loss. Good for WTCC and F1 cars that have the budget/sponsorships to replace their transmissions every race; not so good for the grassroots racer

In addition, although straight cut gears remove axial forces, they generate radial forces above and beyond what helical gears generate in axial forces. The larger the gear (another advantage of straight cut), the larger the radial force, and the more shaft reinforcement required to prevent the shafts from separating (this is why we use a 5th gear cuff on our 4spd drag transmissions. This removes 5th gear, which isn't required anyway, and prevents the counter and mainshaft from separating)

Another item to look into concerning helical gears with axial forces is upgraded shaft end bearings. These help tremendously in keeping the transmission intact.

First off, I think the gears should be helical not straight. If you build it straight cut there would be a far limited market. The aim here is to build an upgraded street box for a miata, that can also be used for motorsport and survive the 400+ HP cars.

Whilst I agree that a helical _can_ be stronger due to larger surface area of engagement for a given width, you tend to have wider teeth with a straight cut as they are nearly always run with dogs rather than synchros. Also the teeth themselves are generally thicker as there can be less of them so the non-hardened metal volume inside the teeth can be greater. They also have other benefits like less power loss and less loading on the case but of course the noise rules them out for anything bar track cars.

I was more commenting that the thrust load is greater with a helical gear and also the tendency for the gears to want to 'walk' apart which might be why the 5-speed boxes fail. The casing doesn't have as much rigidity as a 6-speed casing which tend to survive longer with more power. I think the gear sizing is similar so unless the gear material changed the only real difference is the layout and housing

Less loading on the case, yes, but far greater loading on the shafts forcing them to separate. But anyway, that's getting off topic. I'm more than happy to discuss the merits and negatives of both Helical and Straight Cut with anyone who is interested. Feel free to PM with any questions :)

But yes, I agree. For the street car/weekend warrior, definitely Helical cut, as not everyone wants their transmission to rattle/whine (like what it does with Straight Cut gears). BTW (not aimed at you, as you already know this), but Straight Cut gears does not automatically mean dog engagement; you can have Helical Gears with dog engagement, just as you can have Straight Cut gears with synchro engagement. As pointed out above, most aftermarket gear manufacturers don't have the available resources to make "complicated" things, so tend to go the easy route, which is straight cut gears with dog engagement. As you are down under, I'm sure you are familiar with Albins. This company has invested a lot into the proper equipment and almost exclusively make helical cut gears as opposed to straight cut (like PPG etc)

As mentioned in my post above, one of the "fixes" that we use in our own high hp transmission builds (1000whp+ Honda drag cars) is to upgrade the shaft end bearings. This helps counter any excess axial/thrust loads, and increases the life of the transmission very noticeably.

We have a very close working relationship with Jacks, so will make it a point to bring up this topic during our next discussion.

My off road transaxle uses both straight cut and helical gears. 1st and 2nd are helical, then 3 and 4 are straight. Again, I think the was due to the gear cutting process being so much less expensive on the straights. 1st and 2nd were like 500 bucks, then 3,4 were only 100 each or something. ..all sychro. My particular case has external "ears" welded in place to accommodate the force from the straight cut gears.

This is not true these boxes aren't braking because of hard shifts or anything like that. they break full throttle as the engine passes through peak torque RPM. They are braking because of torque and each time a gear tooth meshes as its rotating a cyclic load on the root of gear tooth. Up the torque and RPM and the fatigue life starts going down in a hurry weather it is shifted between gears or not. There are some arguments that heavy damped flywheels would help with peak pulse loads due to firing frequency but I think it is a small factor. The factory SR20DET that came with an AZ6 box however had a massive dual mass flywheel that had sprung travel for about 60 degrees of relative input to output rotation via two springs that ech wrap about halfway around the circumference. That might help a little.

Note that I did say "not always though" ;)

Gear teeth are in a constant mesh, regardless whether that specific gear is engaged or not. Gears don't just "break" all of a sudden; they normally wear/crack over a period of time before ultimately failing, and it is "shock" that starts this in motion

That "hard shifting" you are talking about, and it not being a cause of failure? Think of it this way, which would cause more shock:

1) A gear that has marginal load applied to it (i.e it is in a free state of rotation), then boom, all of a sudden, you have 300lb+ (or whatever you cars make) of shock load applied to it

or

2) A gear that has a consistent load applied to it

Or in laymans terms, what would cause more shock:

1) You are lying in bed, then a 300lb person suddenly lands flat on your face

or

2) You are lying in bed with a 300lb person already sitting on your face

Visual representation of axial load on helical gears

You would be very surprised at how many people on this forum would pick number 1.

Especially if it was a man.

Sorry guys, I didn't mean for this to get off-topic, and the last thing I want is to get into a heated debate on a public forum.

So yeah, back to the topic on hand :beer:

I guess in your limited time here you've not taken the time to read just about any thread. We have a serious case of ADHD :giggle:

Many of us quite literally.
Subbing for continued debate.

The teeth never have a constant load on them they have a load profile applied once per revolution going from zero to some maximum value and the load peak value is highest when in gear passing through peak torque rpm completely unrelated to when it was shifted at some engine operating point off peak torque rpm.

On a track 4th gear is a good gear for repeatedly doing asphalt dyno run with a smooth pass through the power band because on just about every track there are several straights where you get an optimal and easy shift point into 4th and have it floored flat all the way till you need to go to 5th several times a lap.

My 2 cents...

Make a gear set that is as cheap as possible for total installed cost for the end user, and is stronger than the factory 6 speed. My guess would be stronger gears for a 5 speed trans since 5 speeds are cheap and simple compared to a 6 speed. Make one ratio that is very similar to stock 5 speed gear ratio, but stronger. Perhaps make the overdrive a bit steeper, that would be a selling point to most folks who don't like spinning 4K going down the highway.

That's what quaife did. And don't do anymore. They weren't as strong as advertised, and we're expensive

Right, so just make it cheaper and stronger.

Chop-chop. What are you waiting for? :hahano:

If I recall the Quaife was/is not a drop in fitment?

It still shows as an active part on their website, is it no longer available?

Quaife didnt claim to be stronger, just different ratios. We just assumed aftermarket gears = stronger.