What im thinking is some tech to help people understand why I value used aftermarket gears so lowly, and to maybe help save a lot of those with aftermarket gears from preventable catastrophic failure.
This is currently a hypothesis, based on failure analysis, and wear and tear analysis of a few xfers, and simple physics.
I will look for more supporting evidence as time progresses, and possibly further calculate things too. But currently ive got a lot of car to make
If someone installs gears and has a dial indicator and can set the gears backlash correctly, can you please take a measurement.
Firmly locate the output, and then turn the input, using the dial indicator to measure how much combined backlash in the case we have on new gears.
Id like to investigate a method that we can use to judge rebuild times.
If we can get the angles of the helical gears itd be snazzy too.
I cant find my protractor atm to measure the gears on the stockies or 6.4s.
It was bugging for me a while thinking about the gears and the loading in the case.
Due to the nature of the helical gear, they cause rotation, and a degree of thrust on the gears.
If the gear interface has similar angles to stock, we should typically downrate the maintenance cycle based on the ratios we choose.
The biggest loading factor in the case is what happens to the counter. (blue is opposing forces for equilibrium)
In high hange, it experiences thrust (arrow pictured), and a moment because the one gear on the counter is loaded, one pushing arrow direction, the other is opposite.
In low range, it experiences thrust and torque to an even higher degree. Due to the moment arm being increased and load being much higher.
This picture is the resultant forces to keep it in equilibrium.
From tearing down a few stock cases, and my 6.4s, I'll best describe the loadings based on wear and tear etc.
The load at a, seems very small. There is very little damage done to the counter shaft, the or the counter. There is slight wear on the very outer edge.
This is very much what is to be expected as it is the moment of the counter which is loading the bearings and surface hardening to failure. The damage/wear and tear is minimal because the moment arm is the greatest.
On B, the wear and tear is very significant.
The counter shaft can wear away quite significantly, if I remember where it is on the 6.4s Ill throw a micrometer over it.
There's a deep lip worn in on the edge of the 6.4s and somewhat "normal" wear on stock counter gears at this location. On the 6.4s the bearings has experienced excessive loss in thickness.
As per the loading, well this is expected, as the moment arm in high range is near zero here. Theres a small amount of needle bearings maybe 1-2 on the outside of the gear.
Now add aftermarket gears, we are torque loading the box say 12% -20% in high, and 80-180% in low, but due to the design, we are loading the bearings in this spot a way beyond their life cycle capability. We are loading an extremely small spot a horrendous amount more than intially designed for, because the moment arm is almost zero.
Once a bit of failure occurs, it allows the counter clearance for angular displacement. At this stage gear noise may prevent itself, but it will generally develop for a while before the gear noise means its damaging the gear interface.
To further complicate this once the angular displacement occurs, the thrust washers start contacting the counter less. This means the thrust washer now wears down faster, and allows the counter more angular displacement, and some backlash may develop (counter starts twisting/slapping around inside the box).
Probably when noise develops.
When you can hear it, you've probably fucked the gears already.
Once the thrust washer starts to wear down, it allows angular displacement of the countershaft (it can move)
If i can get a few different measurements of the angles of the helical gears I'll do a spread sheet and calculate the loads on the bearings and shaft etc at point c.
Using this info, Ill then try to get some bearing data with estimated time to failure based on these loads, so that we can get a pretty reliable guesstimate on how long till a case goes bang.
Backlash will grow at a stupid rate, but for most audible backlash (clunky sound when checking by hand) will be the easiest way to check it. Id like to get some info and how much backlash is fine and when its on the way out.
Referring to the factory manual, once excessive noise is heard, its time to replace. (lots of gears are noisey anyway)
Haynes Manual advises if using the car in severe conditions (dusty or offroad often) oil should be changed roughly every 10,000kms.
Now due to the ratios, we should be looking at changing oil more often.
If you have 6.5s and wheel often, it should be closer to 5,000 km for xfer oil. Just based on increased torque loading etc.
If we inspect the oil on oil changes the biggest wear indicators we will see in the diff oil, will be fine particles of brass, and particles of the surface hardening wearing away.
Do some oil analysis.
Drain the oil into a clean container and let it settle. You can work out a way to observe how much metallic compounds are floating around.
The 1st change will often have bits of metal etc as its all seating in, but it should be pretty clean oil after, until its starts to die.
What this all means.
Revised service intervals, unless you want to do you cash and buy another set of gears before really needed, we need to work out how to maintain them properly, and as estimated time to failure would be splendid for this.
If you're buying used gears, unless they are cheap, be really careful.
IMO people want stupid money for used gears and its hard to justify it.
Scenario 1You buy some 31" tyres for $1100 fitted, use them for on and off 4 years, then want to sell them at 75% tread.
Who in the right mind would pay near new prices incl what you paid for fitting? You would be lucky to get back 50% of what you paid.
Scenario 2Some nice 31" tyres, $1100 fitted, used them heavily lots of scrapes nicks and cuts etc, worn down to 20% tread.
Value? Not fucking much.
Gears are relatively cheap, and maintaining them properly is important, except they are a disposable wear and tear item!Once they start to wear down, they are pretty much fucked.
Even if you try to rebuild them, It will work for a while, but its sure as hell not value for money.
Ideally to make it worth while, you should replace the counter shaft and resurface and reharden the inside of the counter (which if someone can be found to bore it out and sleeve it, itd probably be more expensive than new gears)
Stock gears last longer because their loading is lower (suzuki engineered it this way) - but stil exhibit signs of failure, its the rate of failure that is important
The input shaft/gear on stock gears is also used in some aftermarket ratios.
So stock cases last longer and are more useful (versatility wise) than used aftermarket gears.
To some, perhaps stock gears represent decent value, I would suggest perhaps they are under valued?
Once the counter starts to wear too much, the gears are junk and your gambling how long until they break.
