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- TL;DR Summary
- Does the rated torque capacity of gearbox changes if used in lighter vehicle?

Hi,

This has long been in my head, I've seen it work out in reality, but I have no idea about the forces involved in this. As I don't have an engineering degree or mathematical knowledge in automotive engineering, I've come here for help. Sorry if I've used some terms where they do not fit, as it is as best as I can describe.

As the title states - Does vehicle weight has relation to torque capacity of a gearbox?

Paramters -

Gearbox: 5 speed, manual gearbox, which, by manufacturer, is rated at 400 NM of torque capacity. Let's assume that this is correct, and not undercut/overcut rating, as they usually are. The gearbox, originally, comes from a car that weighs 1500 KG's.

Engine: The engine produces 400 NM of peak torque

Clutch: Let's assume clutch is not an issue in this case.

New car: The gearbox, together with the mentioned engine, is installed in a car, that weighs 800 KG's.

Where I fall short is -

1. How big of an effect does the weight of a car has on the torque capacity of the transmission?

2. Is there no relation at all - no "reverse" force applied to transmission components, increasing "strain"?

3. Is there any strain to begin with?

4. What are the forces the engine and transmission have to overcome to move the car? Tires, internal component resistance...?

I've seen it over and over where "problematic" transmissions live (relatively) long and happy lives being abused when installed in lighter cars, where they fail relatively soon when used in the original application with the same engine. How come?

Granted, the observation is not perfect - all components are used and not new, so multiple factors come into play, but the initial question still stands, I think.

I understand that for automatic transmissions and tow ratings, heat is the main issue, but what about manuals?

Thank you!

This has long been in my head, I've seen it work out in reality, but I have no idea about the forces involved in this. As I don't have an engineering degree or mathematical knowledge in automotive engineering, I've come here for help. Sorry if I've used some terms where they do not fit, as it is as best as I can describe.

As the title states - Does vehicle weight has relation to torque capacity of a gearbox?

Paramters -

Gearbox: 5 speed, manual gearbox, which, by manufacturer, is rated at 400 NM of torque capacity. Let's assume that this is correct, and not undercut/overcut rating, as they usually are. The gearbox, originally, comes from a car that weighs 1500 KG's.

Engine: The engine produces 400 NM of peak torque

Clutch: Let's assume clutch is not an issue in this case.

New car: The gearbox, together with the mentioned engine, is installed in a car, that weighs 800 KG's.

Where I fall short is -

1. How big of an effect does the weight of a car has on the torque capacity of the transmission?

2. Is there no relation at all - no "reverse" force applied to transmission components, increasing "strain"?

3. Is there any strain to begin with?

4. What are the forces the engine and transmission have to overcome to move the car? Tires, internal component resistance...?

I've seen it over and over where "problematic" transmissions live (relatively) long and happy lives being abused when installed in lighter cars, where they fail relatively soon when used in the original application with the same engine. How come?

Granted, the observation is not perfect - all components are used and not new, so multiple factors come into play, but the initial question still stands, I think.

I understand that for automatic transmissions and tow ratings, heat is the main issue, but what about manuals?

Thank you!