# Does vehicle weight have an effect on torque capacity of the gearbox?

• Automotive
• RalphZ
In summary, vehicle weight does not directly affect the torque capacity of a gearbox. The only factor that matters is the engine torque, which must be able to overcome the weight of the car and any resistance from internal components in order to move the vehicle. This strain can be increased if the engine must operate at a higher rpm to produce the necessary torque, so it is important for cars with heavier weights to have engine-transmission combinations designed for that specific service.

#### RalphZ

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!

Welcome to PF.

RalphZ said:
Gearbox: 5 speed, manual gearbox, which, by manufacturer, is rated at 400 NM of torque capacity.
I assume that 400 Nm is specified as input torque.
The spinning wheels will set the maximum output torque. A heavy vehicle will require more torque before the wheels spin. A light vehicle will lower the maximum torque employed, so it will be easier on the gearbox.

russ_watters
As I see it, the transmission is a passive device to transfer or transmit, as the name states, torque produced by the engine to the drive wheel. It is similar to a wrench where the torque produced by your arm is transmitted to a bolt. If the torque is not large enough to turn the bolt the wrench stops turning without damage unless you exert more torque than the wrench can handle. The transmission must be able to handle the engine torque. If the torque cannot move the drive wheel the engine will stall. The problem with a large load is that the engine must run a higher rpm to produce the maximum torque. For cars, it is a relatively high rpm as opposed to trucks which are designed to move heavy loads. The continual higher rpm will put a greater strain on the engine and the transmission because of the higher rpm. Heavier cars should have engine-transmission combos designed for this service.

RalphZ said:
Does vehicle weight has relation to torque capacity of a gearbox?
Nope. Only the engine torque matters. Even if you had an engine with, says, 800 N.m and put a 2:1 gearbox reducer between the engine and the gearbox, the 400 N.m of the gearbox reducer would still be OK. (Although, there might be a limit too on power or RPM that must be respected).
RalphZ said:
1. How big of an effect does the weight of a car has on the torque capacity of the transmission?
The weight of the car will influence how much torque is required. But it will never exceed what the engine can produce (times the gear ratio, that is).
RalphZ said:
2. Is there no relation at all - no "reverse" force applied to transmission components, increasing "strain"?
3. Is there any strain to begin with?
Imagine yourself pushing on a block sitting on the ground, with a stick that can withstand 500 N of force before breaking. Say you can push with a maximum force of 500 N. Say your block can move when you apply a 200 N force. If you apply a 100 N force, the block doesn't move and the block pushes you back with 100 N. You reach 200 N and the block starts to move, and the block is pushing you with 200 N as well while moving.

Say now that another, heavier, block requires 1000 N to be moved. Now you apply your whole 500 N of force and the block doesn't move, and it pushes you back with a force of 500 N. Because you cannot produce more than 500 N, the block will never move and the stick will never break.

This is known as Newton's third law.
RalphZ said:
4. What are the forces the engine and transmission have to overcome to move the car? Tires, internal component resistance...
Aerodynamic drag, rolling resistance, inertia due to acceleration (F= ma), and the internal friction losses.

Lnewqban
The torque rating of a vehicle gearbox assumes a certain duty cycle: So many hours in first gear, so many hours is second gear, etc. A powerful engine in a light vehicle spends little time in the lower gears, while a engine of similar power in a heavy vehicle will spend much more time in the lower gears. Imagine a 450 hp Corvette weighing 4000 lbs vs a 450 hp semi truck pulling 80,000 lbs gross weight. There is a large difference in the amount of time in the lower gears, and the larger vehicle runs at a higher average power. The torque in the lighter vehicle is also limited by wheel slippage in the lower gears.

If you put a Corvette engine and transmission in a semi truck with appropriate rear end ratio, it would not last very long. The transmission would fail soon.

jack action, berkeman and russ_watters
RalphZ said:
Summary:: Does the rated torque capacity of gearbox changes if used in lighter vehicle?

Does vehicle weight has relation to torque capacity of a gearbox?
I would say that what really counts is the Output Power of the Engine. That is what ultimately governs the amount of Work per second you can do in accelerating the car and/or pushing it against the air etc.. A properly designed gearbox will allow you to run the engine at a suitable torque (for it) that delivers maximum rate of work.

Both power and torque are relevant in engine and transmission design BUT it's torque times speed that wins races.