Gear Load: Paper Limit & Breaking Load?

Hello,

When it comes to TDI tuning, there is often a focus on the durability of the transmissions, for example, a torque limit of 350 Nm for 6-speed transmissions.

What do these 350 Nm mean?

I can imagine that even with a standard "cavalier start" (a quick getaway), even with a 75 horsepower 3-cylinder engine, significantly higher values (likely referring to engine speed or power output) would be achieved if the clutch is released "quickly" – and this is due to the stored kinetic energy resulting from pressing the accelerator pedal while the car is still stationary.

If you try to induce a burnout (by flooring the accelerator and letting the clutch slip around ~3000 rpm), the input torque to the transmission will likely be limited only by the clutch. As long as the clutch is overloaded, it will slip momentarily instead of following the pedal and fully disengaging.
(Based on my gut feeling, I would estimate the peak torque to be around 500 Nm or even higher.)

For me, these are the maximum loads that a transmission must be able to withstand - it would be embarrassing if every new car scattered its transmission onto the road after just ~3 such aggressive or panic starts...

That the 350 Nm at 6th gear only indicates a long-term acceptable value does not, in my opinion, contradict this. Rather, the material will wear out faster (initially) if the 350 Nm are exceeded.

Question: By how much higher than such paper limits is the torque at which spontaneous material fractures must be expected?

Hi Ulf,

Think about the Gremlins scene with the broken drive shaft... so, shattering is possible.

I would rather think that it's about stresses that *could* leave damage, and a "caviar start" might certainly be one of those things if it's done frequently.

"Nothing is going to break down immediately, because if it did, the Ibiza-ASZ faction from the Ibi-Forum would definitely have already destroyed some gearboxes, because I wouldn't put it past them to drive like that."

My last attempt at doing something similar was with the AEX (60 horsepower) in a Polo... I wouldn't even think of doing that with the Ibiza, just to protect the tires and the drive shafts. It normally pulls away nicely, so you don't need to resort to such tricks.

Best regards,

Jan.

My thought is as follows:

I can subject the transmission to a maximum load test by using the engine with the highest possible torque (350 Nm), which, according to your description, means suddenly applying the maximum torque to the transmission. And I can do this continuously! Based on a 20-30% manufacturer's tolerance, this would mean it could still handle up to 420 Nm without issues.
But?
Can I cause a gearbox failure with a 500Nm engine using a 'caviar start' (sudden, rapid acceleration)? I doubt it. I suspect the clutch will slip first, or that the lack of grip from the tires won't allow enough force to reach the gearbox gears to cause a breakage.
What kind of stress would be placed on the system if, while driving downhill at 120 km/h in third gear with the clutch depressed and a vehicle weight of approximately 2 tons, I suddenly release the clutch? This wouldn't be related to the engine's power, but it would still represent an abnormal stress until the engine reaches its operating speed.

To do that, you would need to determine the moments of inertia, and then you could calculate the loads.
Anyone with the relevant information is hereby invited to submit it...

CU Gremlin.

According to your description, at maximum torque, the drive

ObenbeiMutti wrote:

suddenly forces itself onto the transmission. And that permanently! In my opinion, that's not possible. Applying sudden force to the transmission means that the clutch was disengaged beforehand. Then the engine is running in a loaded idle state (similar to the AU test), but in that state, the engine's RPM increases faster than the turbocharger can keep up. If at all, the boost pressure required to achieve the Nm maximum is only reached well above 1900 rpm. I think that the total rotating mass on the engine side of the clutch can release such high forces during a

sudden engagement (braking) at approximately 3000 rpm (depending on the clutch's grip) that the proportion of the "active" engine torque is almost negligible.
But this power surge can, in principle, not be generated continuously.

Approximate analogy:
Let's see a 150 HP TDI engine, with a measly 20 km/h (approximately 12 mph) speed, disengage from its transmission and roll into a wall without brakes.
Then, you place a similar, undamaged car against the wall and try to push the front of the car without any momentum in first gear, pressing it in as far as possible (with full throttle at 1900 rpm, causing the clutch or the wheels to smoke) -> there's no chance, even though the engine can accelerate the car to over 200 km/h.

What kind of stress is actually involved if I, with the clutch depressed and a vehicle weight of approximately 2 tons, am driving downhill at 120 km/h in third gear and suddenly release the clutch?

Quote:

This would have nothing to do with the power of the engine, and yet it would still represent an abnormal load until the engine reaches its operating speed.

That is probably essentially the same as releasing the clutch quickly when starting, but "in reverse" .
"The 'natural Nm limit,' in my opinion, is likely also the maximum grip of the clutch in this case."

Hello Ulf!
Torque specifications for transmissions in the automotive industry do not indicate either a breaking load or the maximum continuous torque.
Components in the automotive sector are typically designed for operational strength, meaning they must withstand a specific range of loads for a defined period without failure.
Regarding automotive transmissions, I imagine that each automotive and transmission manufacturer has its own specific approach when it comes to defining the load spectrum.
However, it is certain that a vehicle transmission will never reach its design lifespan (e.g., 2000 hours) when subjected to continuous stress from the specified "rated torque."
In addition, there are likely statistical values available regarding which gear is used with what frequency, so differences between gear pairs in the design could also be considered. A simple example: try to achieve the specified rated torque continuously in reverse gear .

Regards,
Alex.

donalexo wrote:

When it comes to automotive transmissions, I imagine that each automotive/transmission manufacturer has its own specific approach regarding the definition of the load spectrum. It is certain, however, that a vehicle transmission will never reach its design lifespan (e.g., 2000 hours) under continuous stress from the specified "rated torque."

Oh my goodness - that means the specified input torques can be interpreted almost arbitrarily by outsiders...
As a "constant" load, it can handle even less, but surprisingly, nothing breaks immediately above that point.

So, the best thing to do is not worry about it at all, but rather tune your TDI as much as you like (of course, everything within the legal limits ), and if the transmission fails...
. . . the tuning data of the engine, its driving profile, and the lifespan of the transmission under the tuning load are entered into a yet-to-exist experience database, so that future tuning customers can roughly estimate what they can expect when they add a certain amount of Nm and horsepower.

But what do you think about the Nm safety valve called "Kupplung"?

Some 74 kW PD models already put too much strain on their clutch, even without any modifications.
Some people reportedly tune their cars to produce well over 300 Nm of torque without the stock clutch slipping.
How is it possible that clutches, with their "holding torque" (what is the correct technical term for this?), exhibit such a wide range of values?

Are there any practical guidelines for how much Nm (or rather, what percentage) the holding torque of a clutch should exceed the torque of the engine?

In particular, some 74 kW PD models already put a strain on their clutch, even without any tuning.
'It is reportedly possible to tune some vehicles to produce well over 300 Nm of torque without the stock clutch slipping.'
How is it possible that clutches, with their 'holding torque' (what is the technical term for that?), exhibit such a wide range of values?


It's enough if the installer has just left their greasy fingerprints on it.
or the simmering pot is sweating...

CU Gremlin.

Gremlin wrote:

ulf wrote: In particular, some 74 kW PD models already put too much strain on their clutch, even without any tuning. "It is reportedly possible to tune some vehicles to produce well over 300 Nm of torque without the stock clutch slipping." How is it possible that clutches, with their "holding torque" (what is the technical term for that?), exhibit such a wide range of values? It's enough if the installer has just left their greasy fingerprints on it... or the simmering pot is sweating...

Follow-up question:

A leaking oil seal provides a constant supply of lubricant, which means the clutch should theoretically be constantly slipping.

With Glibberpratzen ( ), on the other hand, no additional lubricant is added, so the lubricant should gradually evaporate along with the wear, and the clutch should eventually "grip" fully again.

Does this roughly correspond to actual practice?

Hi,
"During full load, the first gear would only last for 10 hours," we were told during our studies...

'By the way, my mother wrote the following:'
According to your description, suddenly engaging the drive at maximum torque and sending it directly to the transmission, and doing so continuously!

In my opinion, that's not possible.


Sure, no problem! Please provide the text you would like me to translate from German to English. I will only provide the translation, without any explanations.

My statement should be more in the direction of DonAlexos' 'Lastkollektiv.'
This means that the worst-case scenario is occurring. For example, the 21-year-old Mirtl from Hinterdupfing, who subjects the vehicle to the maximum possible stress at every possible traffic light.
Over time, this likely causes the bearings to wear out to the point where damage becomes unavoidable, but I think a sudden failure only occurs with severe material defects and errors. And manufacturers are not responsible for that, which is probably why they provide cautious maximum load specifications.

In Hintedupfing, there's probably only one pedestrian traffic light, and recently, they added a roundabout.

So...
Since I once had to do a rather silly gearbox design in a machine element course (though it wasn't a multi-speed gearbox), I'll chime in with my opinion now.
When designing a new gearbox, engineers initially rely on experience and general rules of thumb. A predetermined load is specified, often statistically determined, that the gearbox must withstand. During the design process, various shafts and notches inevitably arise, which represent weak points and are tested based on a material fatigue life curve. Of course, this is primarily done through calculations. Ultimately, the resulting gearbox, with its safety margin applied to the specified load, is likely to exceed its intended lifespan with a reasonable probability. In a simple, two-stage gearbox, so many assumptions and estimations are involved that even the designer might not be able to accurately predict how long it will last. This is especially true if a different load is applied. Typically, the safety factor against breakage is higher than the safety factor against yielding (deformation). Under normal operating conditions, there is usually no yielding. If the material's yield strength is reached, it will deform slightly (very slightly) but then solidify, preventing immediate failure.
Of course, new transmissions are only truly tested in practice, and one also has a certain amount of experience that a student might lack. This makes the whole process more predictable, and allows one to better manage the weaknesses. However, there are also material and manufacturing tolerances to consider.
I once read that VW stated a lifespan of 500,000 km for the Golf 4. They probably based that on a transmission survival rate of somewhere between 90% and 100%. However, increasing the load definitely reduces the probability of survival. In this case, the higher peak loads are much more damaging than an increased average load.

Conclusion:
If you treat your transmission properly, it should last a long time even with normal tuning.

PS:
I'm currently studying for my materials science and component strength exam.

@ulf

You should try driving our Passat. When you engage the clutch quickly, nothing slips. It immediately transfers the power to the transmission. It's a very aggressive clutch, possibly derived from racing technology. It's quite difficult to depress and becomes very stiff in the last quarter of its travel.

Best regards, WarLord.