Robthebrit said:
The rolling circumference of the new tire is around 9% bigger so you'll be going the same speed with 9% lower RPMs, you'll also have 9% less torque at the same setting.
Rob
Just keep in mind that the 9%
less torque Rob is referring to is 9% less
available torque. The
actual torque on the drive-line will be 9%
more, in almost any given circumstances.
Reason: the torque on any component is the "twisting force" it undergoes. It's a result of the engine/transmission applying a force, and the ground resisting it. The more the ground resists (i.e. a steep hill, or if you chain the vehicle to a tree), the higher the torque will be. The maximum torque experienced will be reached when
either the wheel starts to turn (whether the vehicle moves or the wheel spins),
or the engine reaches its maximum torque capacity (in the lowest possible gear). *
It is easier to overcome the ground's resistance with a small diameter wheel than a large one (it's a gearing/leverage issue), so a wheel with a 9% increase in radius will require 9% more torque to turn it against any given ground resistance.
It therefore follows that, other things all being equal, you
need 9% more torque at all times, if you have 9% bigger tyres. But your engine hasn't changed, so at the extreme limit, you are 9% worse off. Hence Rob saying you have 9% less.
The implication for vehicle stress isn't as bad as you might think, though. Yes, at all times, there is 9% more torque on your components, but to compensate for that, everything turns 9% more slowly. Wear-and-tear is therefore probably not changed drastically either for the better or for the worse. And at the extreme, the limiting factor on torque is what the engine can produce. If the vehicle is designed to to put, say 500N.m of torque through overall gearing of 50:1, it means the side shafts should be able to withstand at least 25000Nm. No matter how big the tyres are, you can't put
more sustained torque on the shafts than the engine can produce.
(Having said that, it is almost always shock-loads rather than sustained torques that break things, and you will be more susceptible to shock-loads with bigger tyres. The extra "leverage" and the extra grip from big diameter tyres does pose significant extra risk of breakage, especially if you have locking diffs as well).
*Footnote: Torque is not only required to get the vehicle moving initially, but also to continue accelerating it. The torque peaks when the engine reaches its maximum torque capacity, at which time no further acceleration can take place. Again, 9% bigger diameter tyres will mean that any given acceleration will require 9% more torque. (But the same power, so you don't
necessarily reduce your acceleration ability).