How much torque can the Syncro transmission really handle?

[SyncroGhia]

I've thought long and hard about this for a long for obvious reasons... I keep having to rebuild mine but not for the same reasons each time.

That said, if I wasn't running a 2.5TDi but a standard engine it wouldn't have broken at all!!

So from my personal findings and some others that I know of, the weak points which actually break are:

1) 3rd and 4th gears with over 170ftlbs
2) G/Reverse gear cluster (note this may be more TDi based charateristics than torque)

The parts which wear a lot faster are:

1) Pinion bearing
2) Mainshaft bearing
3) Mainshaft bearing moving forwards in the case

If you replace 3rd and 4th gears with aftermarket gears then you can up the amount of torque going through the gearbox without actually breaking gears bringing up the arguement that we haven't actually reached the max torque yet. Yes parts wear quicker and things start to move but nothing has actually broken.

I've personally put 275ftlbs through my syncro gearbox (4.57 final drive ratio) on a daily basis for the last 4 years. Nothings has sheered when under direct load.

BUT........

Having spent a lot of time thinking about my latest project and how much torque I'll be putting through the gearbox with different final drive ratios and gear ratios I started thinking about what the effective torque on the parts will be when you change the ratios.

If we look at Limeys gearbox, and the figures I'm putting through that and then look at what happens to the torque figures when you change the final drive ratio, you can see that the torque figure goes up when you go for a higher final drive ratio (by higher, I mean the figure).

I'm looking at using 6.17 ring/pinion in the Lightweight. Ok so if I was to bolt on the 2.5TDi with 275ftlbs... this would effectively mean I'd be putting 371ftlbs through the gearbox!

I'm basing this on the fact that if you half the speed, you double the torque. If this is true then a 4.57 transmission can handle the most due to the gearing.

I have been looking at using the Subaru Hi-Lo setup on my 6.17. BUT... the best ratio setup I can find is a 1.59:1 with a 1:1. With this in mind, I'd be running with a 9.81 final drive ratio which would be fantastic for offroading but again looking at the torque figures, I'd be putting an effective torque figure of 590ftlbs through it. That's not going to last!!!

Yes I'll be running a Subaru 2.5 engine which has around 170ftlbs so it would only be 364ftlbs in relation to if I was running it through a 4.57 transmission.

I have no base data to go off for this other than my own vans.

Please comment and tell me I'm talking rubbish!!

MG

P.S. I am still working on an oiling system for the transmission to make the bearings last longer!
_________________
T3 Syncro 16 S6 Westfalia Limey SOLD
T3 Syncro 6x6 SOLD
T3 RS6 Bluestar
T3 Tristar Syncro 16 SOLD
T3 Tristar Syncro RHD SOLD

[gears]

My 2 cents: Most failures of rebuilt Syncro transmissions occur because:

1) Main case and intermediate housing bearing bores are no longer round and tight. The main case should be re-sleeved and intermediate housing replaced with all-aluminum version

2) Low gear housing is worn. This housing ALWAYS requires resurfacing (to hold the mainshaft bearing from moving forward), the gasket should be eliminated, and a thinner shim used to compensate.

3) 3rd & 4th idler gear sets are worn out. These often require replacement.

The pinion bearing is stout and rarely the cause of pinion failure, but it should always be replaced. Same with diff bearings.

The mainshaft bearing is somewhat starved for oil, and should get an oiling plate. This bearing ALWAYS requires replacement, even if it "looks good".

The above is obviously just a short list of the most important rebuild components/failure points.

Being smart about how you drive is critical. With my SVX van, max torque is only applied during 2nd gear acceleration (on-ramps), with a more sympathetic application of torque in 3rd. I never chirp the tires or speed-shift. Why drive like a madman .. especially in 1st or low? That's asking for trouble. Normal street (and even off-road) driving of our Syncro vans is rarely at full throttle, so max torque numbers only come into play maybe 1-2% of the time

A low ratio (high number) R&P is going to be considerably weaker than a 4.57 or 4.86. That's a fact (small pinion head) that you have to live with.

[insyncro]

I run 4.86 gearing with all my converted Syncros and have had amazing longivity and driveability.
All of these conversions are 6 cylinder Subarus and the transmissions have not been rebuilt.
Driving style is the biggest destroyer of transmissions IMHO.

[Vango Conversions]

Correct me if I'm wrong but I thought that going to a lower (numerically higher) ring and pinion, like going from 4.57 6.17 wouldn't actually increase the load on the transmission gears, but it actually may lower it? I don't think making the R&P change will effect the wear or breakage on your 3rd and 4th gears. However, the lower ring and pinions themselves may be a bit weaker due to their construction and smaller pinion gears as mentioned. Also, the lower the gearing, obviously the higher the load on the axles and cvs.

Another thing to consider is that diesels have much harder power pulses than a gas engine. The more cylinders you have the smoother the power pulses are. An 8 cylinder 200ft-lb engine will be much kinder to a transmission than a 200 ft-lb 4 cylinder.

[SyncroGhia]

[Vango Conversions]

I believe that the torque load goes through the transmission gears before the ring and pinion, so the ring and pinion would have no effect on the load of your transmission gears. Having the super low ring and pinion may actually lower the load on the transmission gears because you wouldn't have to give the engine as much throttle for the same acceleration, however if you're using max power frequently, it seems to me that the load on the transmission would be the same. The only additional wear on the gears from the lower R&P would just be because you'd be shifting a bit more.

I'm no transmission expert but this seems to be the way the power goes through these things and I don't really see how the R&P could effect the transmission gears? I could be wrong though.

[SyncroGhia]

Hmmm, I get where you're coming from but surely the effects of torque don't just go one way? whatever your van is pushing against (say an immovable object) impacts all the way back to the crankshaft of the engine and on every component inbetween?

I've just found this small explanation on another website which puts it down on paper much better than I can.

http://www.datsuns.com/Tech/tech_gearing.htm

"If you have 4x4 truck with 44" tall tires, you will want different gearing than an economy car with 24" tall tires, as the taller tires reduce your torque multiplication just like a "taller" (lower numerically) final drive (top gear * axle ratio) would.
Here's an example. the 44" tire will rotate about 458 times in a mile, whereas the 24" will turn 840 or so. With the same axle ratio and transmission ratios, (we'll assume a 3.5:1 axle ratio and 1:1 (let's call it forth) gear, the truck (we'll call it a Nissan Hardbody) will turn 1600rpm at 60mph, and the car (we'll call it an Altima) will turn about 3000.
"

MG
_________________
T3 Syncro 16 S6 Westfalia Limey SOLD
T3 Syncro 6x6 SOLD
T3 RS6 Bluestar
T3 Tristar Syncro 16 SOLD
T3 Tristar Syncro RHD SOLD

[snowsyncro]

This might be complete BS, but I think I recall reading somewhere that the Syncro gearbox was designed for 200 n-m. That would be 147.5 ft-lb.

That would be some kind of number for sustained long-term reliability, i.e. a reasonable lifetime with proper lubrication etc. Obviously it can withstand much higher levels, for shorter durations, so that would not be a breaking torque figure.

That was some time ago I would have seen that, so I can't say that is a reliable information source, or even not just a figment of my imagination. But if you look at what happens when you go beyond that, it does seem an at least plausible number. I always mean to calculate my own estimate, but I never seem to get motivated enough.

RonC

[SyncroGhia]

[snowsyncro]

I think you are right, Mike, and that also reminds me that I have seen photos of a test transmission that was fitted with acrylic viewports, and at the time I concluded that was part of the engineering study that resulted on those oiling plates.

I have the required background, I just need to get my s$&% together and do it.

RonC

[GrindGarage]

I like that fellows gearbox setup with the oil squirters. He has the intercooler radiator up in a extended cargo box on his westy. I wonder if he has put it to the test yet?
_________________
-cliff

91 Vanagon AUTO
97 Single Port EJ22 all smallcar.com

[syncrogreg]

Herman with the 5 banger.

As a general rule the gearbox that have internal components reving fast will have less load applied to it than one that rev slow. I'm wondering how my AAN will react with the boxer diesel coupled by DMF flywheel.


GSB
_________________
conversions: www.boxeer.com Common rail TDI and NOW PDK transmissions!!

travel pictures:
http://instagram.com/crepeattack/

- the Pastis build: http://www.thesamba.com/vw/forum/viewtopic.php?t=434656&highlight=subaru+diesel

[ALIKA T3]

I'm watching
_________________
Silicone Steering Boots and 930 Cv boots for sale in the classifieds.
Syncro transmission upgrade parts in the Classifieds.
Subaru EJ22+UN1 5 speed transmission
http://www.thesamba.com/vw/forum/viewtopic.php?t=416343
Syncro http://www.thesamba.com/vw/forum/viewtopic.php?t=4...num+gadget

[snowsyncro]

[SyncroGhia]

Thanks Ron,

I have been discussing this subject with a couple of my engineering buddies and one of them emailed me last night with a fairly long and very thorough explanation.

"In general when parts such as gearboxes and their internals fail through being overloaded, they do not do so the first time they see the maximum load. Instead what tends to happen is that as you increase the load they are subjected to, their life decreases. There are two main causes of this – insufficient lubrication, and pushing the parts beyond what they were designed for in terms of the stresses they are subjected to.

Lubrication:

I’m going to ignore the lubrication issues other than to say that the higher the loads on gears and bearings, etc, the harder the metal components are pressed together, and the more they squeeze the lubricant out from between them. This results in an increase in the rate of wear, and the resulting extra friction causes an increase in heat. If the extra heat increases the lubricant temperature to above it’s intended maximum, the lubricity will decrease too, so the problem is compounded. Higher spec oils, oil additives, pumped oil systems and oil coolers can all help, but the design of such changes is a bit of a ‘black art’ which few people have a good understanding of. I certainly don’t.

Stresses on the load carrying components:

The way the life of gearboxes decreases as the load they are subjected to increases, but they rarely fail when subjected to the maximum load for the first time are classic symptoms of metal fatigue. It is not possible to understand the way increasing the loading on a gearbox decreases it’s life unless you have a basic understanding of metal fatigue. If fact, thinking about or discussing loads in gearboxes with no understanding of fatigue is pointless. Think of breaking a steel paper clip with only your hands. You won’t be able to break it with one application of load. You can easily bend it 90 degrees in your fingers without it breaking, yet if you repeatedly bend it 90 degrees then straighten it again, it will fail in a relatively small amount of load cycles. If you bend it only 10 degrees then straighten it again you will be able to do this many more times before the clip breaks. However, it will still break in the end if you bend it enough times. This is metal fatigue. The number of load cycles it takes before a part fails decreases as the stress the part is subjected to increases. The relationship is not linear. It is different for each material spec, and is described by a S – N curve on a graph. The loads required to cause fatigue failures can be significantly lower than load which would cause the part to fail in a single application.

Steel is unusual in its fatigue behaviour, as it has a fatigue limit. This is a stress level that has to be reached before the part will ever fail in fatigue. If an item is designed to only ever see stresses below this limit, in theory it will never fail through fatigue. Most or possibly all other metals do not have a fatigue limit. No matter how low the loads they see are, they will eventually fail in fatigue, although in reality the number of load cycles it takes before they fail through fatigue is often so high that it is never reached within the useful lifetime of the product. The difference is often seen in applications where equivalent items are made from both steel and aluminium, where weight is critical, but where occasional failures are acceptable. The most obvious example I can think of is bike frames. A well designed steel one, which does not have its design compromised for low weight will last indefinitely under normal use, because it is not stressed above its fatigue limit This is typical of the sort of endurance bikes people use for biking around the world, etc. In comparison an ultra-light aluminium mountain bike frame will see high stresses both because of its ‘abusive’ application, and because it has been designed using minimal aluminium to keep the weight low. This results in high stresses, and a corresponding low fatigue life. The number of load cycles it can stand before fatigue failure has effectively been compromised by it’d designers to save weight.

Fatigue failure are always catastrophic (i.e. they go form apparently being OK to total failure instantly), and always result in cracks being the failure mode (never bending, distortion, etc). Anything which raises the stresses locally in a component, such as welds, manufacturing defects, bad design, accidental damage, etc, is a potential fatigue crack initiator, especially if located in a highly stressed area of the component. The welds in an aluminium bike frame is a very good example. The lack of welds in a steel bike frame constructed using brazed lap joints is an equally good example (if largely un-noticed, because they don’t fail). The size of a defect is not important, right down to microscopic levels. All it needs to do is concentrate the stresses more than anything else does in a critically stressed area, and a defect becomes a likely fatigue crack initiator. Perfect components are impossible, only ones with less or smaller defects, or with special properties which reduce the likelihood of fatigue failure. Also, fatigue is cumulative, so failure can be made up of any combination of low cycle, high stress and high cycle, low stress use.

See http://en.wikipedia.org/wiki/Fatigue_(material) for more info on metal fatigue.

Loads on Gearbox Parts under Steady State Driving:

Right, back to loads in gearboxes. Loads in all the gear train components are caused by the torque which that part of the gearbox sees. The torque in each part of a gearbox is determined by engine torque, the gear ratio’s the final drive ratio, and the wheel radius. Using a simple 4 speed manual transaxle as an example, such as a VW beetle or post split screen bus, with no overdrive, and no other ratio’s (such as drop boxes or reduction hubs), here is what happens in terms of shaft speeds, as many people understand speeds better than torques:

All 4 gear ratios change the engine speed, by varying amounts. 1st gives a big decrease, 4th gives a small increase in many VW gearbox specs.
The engine speed and the selected gear ratio determine the speed of the pinion shaft. E.g. engine rpm x gear ratio = pinion shaft rpm 4000 engine rpm / 0.82 = 4878 pinion shaft rpm
The final drive ratio decrease the pinion shaft speed to give the axle speed. E.g. Pinion shaft rpm x final drive ratio = axle speed 3280 pinion shaft rpm / 4.86 = 1004 axle rpm
Wheel radius then determines the road speed associated with the axle speed. E.g. axle rpm x 2 x Pi x wheel radius (m) = road speed (m per minute) 1004 x 2 x Pi 0.35 = 2207 m per minute (132 kph, or 82 mph)

Now think of it in terms of torques. Gear ratios have the inverse effect on torques compared to speeds. Ignoring losses (friction, heat, etc), a gear set which decreases speed increases torque by the same ratio, and vice versa. Repeating the above example from a torque point of view:

Engine torque and the selected gear ratio determine the torque applied to the pinion shaft assembly. E.g. engine torque / gear ratio = pinion shaft torque 100 Nm engine torque x 0.82 = 82 Nm pinion shaft torque
The final drive ratio increases the pinion shaft torque to give the axle torque. E.g. Pinion shaft torque / final drive ratio – axle torque 82 Nm pinion shaft torque x 4.86 = 399 Nm axle torque
Wheel radius then determines how much tractive force the axle torque then gives at the road. E.g. axle torque (Nm) / wheel radius (m) = tractive force (N) 399Nm axle torque / 0.35m = 1139 N

If you play around with the numbers in the above calculations you will see that:
1. By changing down a gear you can convert the same engine torque into more tractive force, at the expense of road speed, and vice versa
2. Tyre radius has the same effect as another gear ratio between the axle and the ground (assuming your wheels are not spinning)
3. Increasing the tyre radius will mean you need more torque (but less speed) from the engine to drive at the same road speed, and your maximum (theoretical) speed will increase, and vice versa.
4. Changing to a (numerically) higher final drive ratio, to increase the tractive force (or to go slower), will mean you need more engine speed, but less torque, to drive at the same road speed. Your maximum (theoretical) speed will decrease, but you will be able to make use of the engines peak torque at lower road speeds

Other points to note are:
· Torque is constant along any shaft assembly
· Torque in the gearbox input shaft assembly is determined entirely by the engine only
· Torque in the pinion shaft assembly and axle assemblies are determined by the engine torque, final drive ratio
· When you are driving at a constant speed, the tractive force exactly equals the aerodynamic drag (which acts in the opposite direction), again ignoring friction and heat losses, etc.
· Because the shafts in a gearbox rotate at relatively high speeds the gear teeth soon clock up high numbers of load cycles.

Abusive Use and Dynamic Loading:

If you use your vehicle for something abusive such as drag racing or serious off roading, both are very hard on gearboxes for the same reasons. Do the gearboxes fail while going at a relatively constant speed , while the engine is at its peak torque? Almost never. They fail as you engage the clutch as you accelerate up through the gears at full throttle. Therefore, there must be something else going on too. The explanation above has so far assumed that all the components involved have zero mass, so don’t require any forces to accelerate them. This is effectively true under steady state driving. However, this is not the case in reality. There is much more to the loads involved under such circumstances than just the shaft torques. The masses of the parts involved in a moving vehicle give them inertia. Inertia must be overcome to change the rate at which something moves, whether linear (getting a stationary vehicle moving forwards) or rotary (getting stationary gearbox and wheels to start rotating). The higher the rate at which you want to accelerate these masses, the more force it takes. If you accelerate gradually, like when driving on the road, relatively small forces are needed to do so. If you try to accelerate instantaneously, like in both of these abusive applications, you are effectively trying to overcome all of their inertia instantaneously too. Actual instantaneous acceleration of any mass is impossible, as it require infinite force. To get the vehicle to do what you want, you are overcoming the inertia of all the moving parts in the background first, before the vehicle actually does anything.

For linear movements, like the vehicle going forwards, the inertia is only related to mass. The higher the mass, the more inertia you have to overcome. For rotating components, it is not that simple. Rotary inertia depends on where the mass is concentrated in relation to distance from the rotation axis (i.e. radius). It increases with the square of the radius. Consider an oversize off-roading tyre compared to a stock one. It will be a bigger radius, and will have far more mass concentrated towards the outside (as mist of the mass id the tyre). It could easily have many times the moment of inertia of a stock wheel. For example, if you fitted 2m diameter earthmover wheels and tyres to a VW power train, it is very likely that if you revved the engine to its peak torque speed, and side stepped the clutch, you could break the gearbox without the wheels turning at all, even with them jacked off the ground. The inertia load alone may be enough to break the gear teeth in one go – i.e. not fatigue at all, but an outright failure die to stressing them beyond their ultimate tensile strength.

The effect of trying to overcome inertia very quickly is not to be under estimated. The dynamic loads it places on all the components can be enormous, and they are in addition to the torque required to move at a steady rate, as discussed above. You are trying to make use of the full torque of the engine under circumstances which the gearbox was not designed for In both cases the real gearbox killer is using peak torque at effectively zero speed, either pulling away from the line or trying to get over an obstacle by side stepping the clutch with the engine at peak torque. Both of these applications are likely to have artificially high amounts of tyre grip compared to what the gearbox was designed for, and in both cases the vehicle is not moving and all the rotating parts in the powertrain (except the engine) are not already spinning.

· The effect of fitting much larger wheels and tyres on inertia which must be overcome to accelerate will be enormous. The faster you try to accelerate, the bigger the effect will be.

· People’s obsession with ‘light weight’ – i.e. low mass flywheels in race cars 100% wrong. What they actually want is a low inertia flywheel. Depending in design, such a flywheel could weigh more than a high inertia one, but still have less rotational inertia.

· I imagine that some off roaders thing they will have an advantage if their vehicle is heavier, because they will have more traction, which is true. They will also have more inertia to overcome, which will give their power train a harder time.

In the case of a vehicle modified for off roading, it is very likely that the following modifications have been carried out:
A large increase in tyre diameter
A large increase in engine torque
An increase in the (numerical) final drive ratio, e.g. from 4.86 to 6.17:1. This is largely to try to overcome the effect of speed and tractive force of the large tyres.

All of these increase the loading on between a few and all power train components. Then also consider that the owners probably also expect their vehicle to perform normally on the road too. This is where the fatigue comes in. To drive at normal speeds they will have to use far more engine torque to overcome their off roading mods, all the time.

Summary:

Ignoring lubrication, the causes of gearbox failures are a complex interaction between the dynamic loads caused by steady start torques being transmitted, and the dynamic effects of heavy duty or abusive applications, and modifications done to achieve one thing, which have an unavoidable but very undesirable effect on something else. For such heavy duty / abusive applications these all effectively have the same effect. Your power train sees far more fatigue cycles at high stress levels than would ever have been the case in its intended application. I.e. you effectively ‘use up’ the fatigue life of the components at a higher rate resulting in a shorter life. You are also far more likely to push components which were not stressed above their fatigue limit in their intended application beyond it in such applications, so it may be possible to break things which almost never fail in the normal application. However, it is far more likely that the weakest link of the components which are usually relatively highly stressed will fail first, because you have pushed it too close to it’s limits. Probably not soon after the mods were done, but far sooner than would be likely in a gearbox used for its intended purpose. Obviously such components which tend to break first are the things which tend to get uprated first (gear sets with fewer but bigger teeth, and made from higher spec materials to better the stock standards, etc.).
Understanding how the loads are distributed between the components, and what effects modifications have would involve either an awful lot of specialised calculations, or far more likely these days, computer simulation. You either do it that way, or by years of empirical development (think of something like top fuel dragsters – the engine were originally based on the Chrysler Hemi, and still are to an extent, but I bet that 90% of the development was done with no calculations in sight – just many years of modify it, repeatedly test it, find where it’s limit is, then move on to something else).
Some heavy duty applications such as drag racing, some of the modifications done help to reduce the load on the power train (very low weight vehicles, wheels, etc). For others, such as off roading, most if not all the modifications give the power train a harder time."

I take my hat off to this kind of understanding and knowledge. Although I tinker with VW Syncros, I'm no engineer.

MG
_________________
T3 Syncro 16 S6 Westfalia Limey SOLD
T3 Syncro 6x6 SOLD
T3 RS6 Bluestar
T3 Tristar Syncro 16 SOLD
T3 Tristar Syncro RHD SOLD

[snowsyncro]

[remraf]

Does anyone know where I could buy the oiling plates discussed for a syncro transmission? How much they cost? I'm in the US.

[Christopher Schimke]

[westyventures]

[remraf]