Water and gearing

235/85/16. Need to look at gearing I think.

Very definitely need to address the gearing as those are like a 32 inch, which is huge on standard gears.

32" tyres plus at least a 50mm lift gives us a height increase of around 4.5".. This puts the amount of water being pushed at nearer 3 tons. Add to that the 1.5 tons of water inside the car and this whole scenario becomes laughable as the vehicle is now illegal.

Roger

Don't forget the water going under the car also has more room now we know that the car is so much taller than standard.

The greater the height, the deeper the water has to be to flow over the bonnet so there is more water in-front to be moved. The water flowing underneath creates more of a vortex so slowing the Jimny down. Sports cars are designed to be low to the ground to reduce this vortex, not just to reduce the centre of gravity to aid cornering. At 4mph this Jimny is running at approx. 2,500 rpm first gear low range. That' far enough into the torque band and I doubt that it would go much faster.
Imagine hitting a wall of water that is 42" deep and you are doing 4 mph. Now imagine the damage.

Roger

Sports cars are low to increase down force by reducing the air pressure under the vehicle and then have flat floors to induce the venturi effect further reducing the air pressure. The hydrodynamic forces acting on a semi submerged jimny cannot be assumed to effect it in the same manner as a solid block of similar dimensions making contact with the bed of the watercourse. Yes an account can be taken for the turbulence offered by the components under the vehicle but they will still allow a significant flow compared with the previously mentioned solid block.

lf you're needing to drive through that depth of water it might be worth considering something with a higher rated maximum wading depth, such as a Defender

Torque is irrelevant, it's power that does work and at 2,500rpm (actually it will be 25% lower than that due to the 32" tyres) the engine is at less than half of its maximum power output. You can't shift down a gear from 1st low so reduction gearing is the solution to going quicker through the water.
I'm amused by the comment that a vehicle partly submerged in water is "illegal", presumably suggesting it's overloaded due to the mass of the water inside it? I'd like to see VOSA tackle that one!

Busta wrote:

Torque is irrelevant, it's power that does work and at 2,500rpm (actually it will be 25% lower than that due to the 32" tyres) the engine is at less than half of its maximum power output. You can't shift down a gear from 1st low so reduction gearing is the solution to going quicker through the water.
 

There's a contradiction here...

What does "down shifting" a gear do, what does reduction gearing do?  They both multiply the torque, that's what gears are for.

Yes an no. Torque is the amount of work that can be done, power is that work being done over time. So by lowering the gearing either by selecting a lower gear which is not possible if you're already in the lowest one in the gearboxes or by proactively installing an entirely different set of lower gears you are able to have the engine work effectively in a rpm range where the available power is higher. There is no contradiction.

As to using a more suitable vehicle for wading I would suggest that partially submerging any vehicle regularly that is not specifically amphibious is going to cause maintenance issues at which point if you are going to be regularly fixing it anyway it doesn't matter what vehicle it is. Though personally if it were that regularly I might consider a boat or hovercraft.

Torque is how much force there is at the crank, it’s measured in foot pounds.
Power is Torque multiplied by crank speed.

If your engine is providing more torque than is needed to overcome the force then it will accelerate, if not your engine will reach a steady rpm.

To go back to the original question once you reach peak torque the vehicle won’t accelerate any more as the force of the water increases with speed.

However, if the force applied by the water means that you can’t achieve maximum torque (the force of the water as speed increases is greater than the torque that can be generated) then lowering the gearing should help.

No contradiction, unless you know of a way to downshift from 1st low.
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The question is would lower gearing help the car drive quicker through the water and the answer is yes. Anyone who has ever shifted down a gear in order to accelerate faster should understand this.

It's torque at the wheels that matters, and you can increase that with gear reduction. More reduction means that at the same forward speed the engine will be spinning faster, producing more power and therefore more torque at the wheels.