Helical gearbox failure modes
Turbo Mini Forums

I've seen of a few failed helical boxes on the forums recently and broken a few myself, which has gotten me thinking... If certain parts fail at certain power / torque / RPM levels, can you work around these problems?

1) The standard diff can be killed at relatively low outputs (75ish BHP). Easily fixed with a 4 pinion diff.

2) Some cases of destroyed laygear + 1st motion teeth in 2nd / 3rd gear (130ish BHP). This seems to be the power output at which people decide to move to straight-cuts.


If point 2 is down to transmitting too much power through the 1st motion and laygear, could you limit torque in 1st, 2nd, 3rd then turn the boost right up in 4th gear when you're not putting any power through the laygear? What would be the next failure point, and at what output - Is 200 BHP possible with a helical box???

In my opinion, the first motion and thid motion shafts ae being pushed away from the laygear under load. the input gear support beaing reduces the first motion shaft movement. However, the third motion shaft is only supported at one end. The torque though third gear pushes the shaft away fom the lay gear, and is compounded by the pinion pushing away fom the crownwheel. the little nose beaing in the end of the first motion shaft, and the support bearing are trying their best to prevent any movement.

In my opinion, the KAD pinion support and the larger turbo support bearing should help reduce any movement of the third motion shaft.

Then of course there all the clearancies. I tend to set them all to bottom limit, since under strain, things only get wider.

There is also the layshaft to consider, I have the swiftune shaft in mine, but then I also have the KAD support beaing and the turbo support beaing.

I did build a helical box with all this in mind, and it was a selection of parts from three geaboxes to set the clearancies on the gears on the third motion shaft, thee is a pic on the forum somewhere.

I still am not convinced by the KAD setup personally... We have seen very few (if any?) failures of the mainshaft (3rd motion if you wish) at the pinion in front-wheel drive minis, which the support seems to suggest it helps solve?

The force trying to seperate the "second" or "third" gear set is not that great in the big picture, as the ratio's are not a million miles apart from each other- witness again the lack of failures where the pilot nose of the mainshaft seperates from the mainshaft, or the bearing fails (as the prime contributing, not secondary) failure...

Now, If the FD pinion was spaced 1" away from the bearing, sure, there could - and would - be a very valid argument for better support, but it is no worse of a cantilever than a pinion on a conventional rear axle (in fact, the load is way closer to a very substantial bearing in the mini than in a hypoid- although admitedly the hypoid pinions are a one-piece forging in most cases... The closest you can get your CW to the mainshaft bearing, is less bending moment, but again - I don't believe we see enough failures to warrant this as the answer.

My biggest issue though, is the KAD thing is so robust and ridgid, in plain comparisio to the case it is attached to one end of. If the crownwheel wants to seperate, it is clear from when this has happened that it cracks the rib above the bearing, and pushes the diff out the back of the casing. Attaching this "rigid" support to a third point, out in space, may actually increase the load on the mainshaft and bearing as the case flexes - which it clearly does witness the failures...

I believe a better idea would be to tie the front and rear of the cases together with an additional rib, and add a pilot bearing in that. Of course, you can't then market this to the masses as being the ultimate accessory to "bullet proof" your gearbox...

If someone REALLY wanted a strong gearbox no matter what rhe type, or ratio used, the best option would be to overdrive it at the drops.
Going to a 10%" overdrive would open up the main box ratio spread (2.54 becomes 2.8) and reduce the torque in the box by 10%. It'd be like running a 2,75 instead of a 3,11 FD - except you'd be doing it at the input, not the output; but the numerically lower ratio between the FD also reduces the loadings on the highest loaded bearings in the transmission (mainshaft double-roller and CWP), as well as all other bearings, so is a win-win...
The only REAL downsides I can envision are these:
1) A small increase in drag (but the difference between your gears turning at 7000rpm vs 6000rpm is not like the difference between 650 rpm and 7000 rpm; it might not even be measurable in the big picture...
2) Reffered inertia on the synchro's will increase, but even this this would be a lot less load on the syncro's than the step-up from an organic to a sintered clutch plate, so they will probably be just fine - 2nd gear syncro is still the weakest of course, as this has the two biggest steps in the trans (ignoring 2>1 as that actually doesn't happen in anger "that often"...
3) Making a 10% overdrive gearset happen.... That is definately the biggest challenge ...

And to answer the OP's question: