Excuse the n00b question, but what difference does that make?
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I don’t think this is at all a ”Noob” question as a lot of experienced travellers probably aren’t totally aware of the gearing of their vehicle. And, many who are aware don’t necessarily have a fulsome understanding of why gear ratios are important.
Carson G already answered you succinctly; If you want a longer answer/explanation here you go; I am not an expert and I’m just drawing on what I’ve experienced and learned over the years. If wiser people can jump in and correct me I’m 100% happy for that as it helps my learning too. What it really boils down to is how much power is getting to the wheels for a given speed; for slow off-road driving we want as much power — the ability to move the weight of the car — as possible, being delivered to the rear wheels at a speed that is appropriate for the conditions (Maintains forward movement without losing traction).
Assumptions:
1) First, we’re going to totally ignore transmissions. All a transmission does is add more ratios. The principles below still apply with the transmission in the equation, but changing ratios in a trans is not something I’ve heard of as common, where as changing ratios in a rear diff is very common and the focus of this discussion, so I’ll stick with that. Realistically, every time the engine turns once, the transmission turns a certain number of times depending on what gear you are in, which is translated to the rear diff that turns a certain number of times depending on it’s gearing, which finally gets put to the wheels to turn a certain number of times. Change any of the components here (wheel size, trans gear, diff gearing, or engine revolutions), and the wheels turn more or less, depending on the change you make. Turning wheels is what this is really all about, so let’s use a simple example of an engine connected directly to a diff, which is connected directly to the wheels, with no transmission.
2) For the sake of making things easy, I won’t use exact numbers from LR. let’s just say our vehicle is 5,000 lbs. In this example we are really lucky actually and we have two IDENTICAL defenders — same weight,shape, colour, and even the same motor. The only difference between the vehicles is one of them is a 300 HP motor with a ratio of 4:1 in the rear differential, and another 300 HP engine in a vehicle with 3.5:1 gearing.
Power and Engines:
The power to move the vehicle obviously comes from the motor, but it’s not static — it’s on a curve; as a motor gets more fuel, it puts out more power. At very low RPMs, there is very little power produced. At really high RPMs, there might be a ton of power being made, but it is probably not very efficient because as engines spin faster and faster, they start to ease off on the extra power produced — Between these points is referred to as the “Power Band” of a given motor, and when it’s graphed it looks like a curve. The curves usually look like this:
When people talk about HP and Torque, they are usually talking about “Peak” HP and ”Peak” torque, not torque or HP all along this power band, because as you can see from the graph, it varies depending on RPM. This is one of the reasons why Diesel engines are so popular off road — they hit “Peak” torque at very low RPMs, and tend to produce a lot more torque for a given displacement, which makes them good in slow, off road environments.
Why is it good? Well imagine you are approaching an obstacle and need to move your 5,000 lbs hulk of a rig up and over a rock. Moving 5000 lbs is not easy — it takes energy. So the goal is to put as much energy from the engine to the wheels, but at as low of a speed as possible/reasonable — the wheels turning is ultimately what moves the vehicle forward, but if they try to do that too quickly, the tire spins and you lose traction.
Gear Ratios:
Let’s say we have a 300 HP engine, connected to our wheels via a rear differential with a 4:1 ratio. In this example, with this ratio, if the 300 HP engine turns 4000 times, the rear wheels will turn 1000 times (4:1). Additionally, let’s say you have a tire with a 5 foot circumference, it means every time your tire makes one full rotation, your car moves forward 5 feet. With this ratio of 4:1, it means that at 4000 Engine revolutions per minute, your tires are rotating 1,000 times in that same minute, and that means your car is moving forward at 5000 feet per minute.
Our second car is identical — The exact same engine and all — but this second Defender has a 3.5 to 1 ratio. That would mean that for every 3500 rotations of the engine, the wheels rotate 1000 times. This means that you can run your engine at 3500 RPM, a full 500 RPM less than your buddy with the 4:1 ratio, and still move forward at 5000 feet per minute, although you are now 500 RPM lower - and if “peak” torque is at, say, 5,000 RPM, you are further away from that power peak than your buddy weigh the 4:1 gears.
What does this mean? It means that on highways, the guy with the 3.5 to 1 ratio is a bit happier - he’s using less fuel for a given distance, after all. The engine rotating 3500 times uses less fuel than the same engine rotating 4000 times for the same distance.
Now I’ll introduce one more assumption here — let’s say this engine produces ‘peak’ HP and torque at 5,000 RPM.
After some highway, our defenders are now on the trails. Now it’s our friend in the 4:1 ratio vehicle is the one grinning. This is because when he’s off-road, his gear ratio allows him to more effectively use the engine — he revs at 4000 RPM to move 5000 feet in one minute, which is much closer to the 5000 RPM “Peak” number for this particular motor. His buddy revs at 3500 RPM to move 5000 feet per minute, but he’s further away from ‘peak’ power and torque — For him to get closer to that peak RPM number, he would have to rev his engine much more, and that means he’d have to travel across the ground faster. So, While 4:1 dude is getting 85% of his engine power by travelling at 5000 feet per minute, 3.5:1 dude might only be getting to use 60% of his total engine power for the same speed. And maybe, to move this 5,000 lbs vehicle up over an obstacle, 60% of the engine’s power just isn’t enough. That means that the guy with the 3.5:1 ratio will have to increased his RPMs to get closer to that 85% peak number, which means he’ll be travelling at a higher speed. Off road, not only is this not always possible - it’s quite dangerous in some situations.
Returning to our base assumption — of an otherwise identical pair of 5000 lbs vehicles. But this time, let’s put a bigger 400 HP motor in the 3.5:1 ratio car, while keeping the 4:1 ratio car With the 300 HP motor. How does this play out?
If both cars are at the same obstacle, the “peak” HP and Torque don’t really matter — instead, the
minimum HP and Torque required to move 5,000 LBS is what matters. For ease of illustration, let’s say that the motor needs to produce 100 HP and 100 Ft-lbs to get our 5,000 lbs vehicle over an obstacle in it’s way. The big motor may hit that 100 HP and 100 Ft-Lbs number at only 25% of it’s capacity — say, 2000 RPM. The smaller motor would have to rev higher to hit that same 100 HP and 100 Ft-Lbs number — it may need 50% of it’s capacity to hit that number, so let’s say 3,000 RPM.
Bringing this back to gear ratios, Imagine yourself in the drivers seat on a typical off-road trail when you look at these numbers — remember, terrain is usually what limits speed off-road. Take an obstacle too fast and you’ll break something or lose control; take it too slow and you won’t get over the obstacle at all; the goal is to hit that 100 HP and 100 Ft-Lbs of energy at an appropriate speed for off-road driving.
| Engine | Gear Ratio | Engine Revolutions Per Minute (RPM) | Distance Travelled per Minute |
|---|
| 300 HP | 3.5:1 | 3,000 RPM | 4300 Feet |
| 300 HP | 4:1 | 3,000 RPM | 3,750 Feet |
| 400 HP | 3.5:1 | 2,000 RPM | 2800 Feet |
| 400 HP | 4:1 | 2,000 RPM | 2500 Feet |
The smaller motor needs to work harder to reach that same 100 HP/Ft-Lbs as the bigger motor, but if they didn’t adjust the gearing, the smaller motor would be covering a lot more ground than the bigger motor, and that’s really the core of the issue - how much ground is covered for a given RPM, which means at a given RPM the wheel is turning faster. A faster turning wheel is hard to place, reduces mechanical sympathy from the driver, and increases the risk of losing traction.
By providing the smaller motor with wider gears (4.xxx:1) and the bigger motor with closer gears (3.xxx:1), it’s getting as much useful power — the ability to move the weight of the car — delivered to the wheels at a speed that is appropriate for the conditions we are driving in to preserve traction and forward movement.
Thank you for reading my gear ratio thesis. I hope I explained it well but if I’ve misunderstood anything (Again, not an expert) I welcome the feedback.
. If that combo doesn't provide you with adequate sidewall, then nothing will.
