Jumat, 09 Mei 2014

The Physics of a Front Wheel Drive Lawn Mower

Image: Rhett Allain

You can consider this to be Part II of my review of the Craftsman Quiet Front Wheel Drive Mower. In my previous post, I looked at the quiet part. Now, I will look at the front wheel drive aspect of this mower. Just to be clear, the Craftsman mower was provided by Craftsman for this review.

I have had a Toro walk behind mower for quite some time (this one is maybe 5-6 years old). It’s a fine mower, but there is a big difference between this and the Craftsman mower. Both mowers have a type of self propelled wheels, but the Craftsman drives the front wheels and the Toro uses the back wheels. Does this matter? Let’s look at the physics.

Although both mowers are “self propelled”, you still have to push sometimes. This is especially true in thicker grass or really soft ground. Here in Louisiana, the ground is VERY soft when it’s been raining. Sometimes the wheels don’t have enough traction to get the mower moving. So, would it be better to have rear wheel drive or front wheel drive?

In terms of slipping, the key is friction. For most materials, you get a greater frictional force when the two surfaces are pressed against each other more. Let’s start with the Toro. Here is a diagram showing the forces on the mower while being pushed (and propelled).

This is a diagram right at the instant that the mower starts moving. This means that the friction force accelerates the mower for just a little bit. The key is that more friction means better. There are two things that must be true for these other forces. First, the vertical forces have to add up to zero Newtons otherwise it would accelerate in the up or down. Second, the sum of the torques about any point must also be zero. If not, the mower would have a rotational acceleration instead of staying level.

The torque is the important thing to consider here. If you look at torques about the center of mass, the force from the person would create a clockwise torque. There has to be some counterclockwise torque in order for the total torque to be zero. This means that the force from the ground on the front wheel has to be greater than the back wheel. That’s not good. The harder you push on the mower (horizontally), the more the front wheels “dig in” the ground without providing any traction.

Now imagine the mower had front wheel drive like the Craftsman. In this case when you push harder on the mower, you get MORE traction. Remember, more traction equals more better. Plus, there is another benefit of front wheel drive. When you want to turn the mower, you push down on the handle and lift the front wheels off the ground. Now it doesn’t matter if they are turning or not because they are off the ground.
Mower Tests

Ok, let’s get to some tests. First, the speed. I used both my Toro and the Craftsman to mow a straight line of 76 feet (23.2 m). The Toro did this with an average speed of 1.65 m/s and the Craftsman had an average speed of 1.28 m/s (both averaged over two runs). It was clear in using both that the Toro was faster than the Craftsman. In fact, I kept trying to push the Craftsman a little too much because I wanted it to go faster.

This speed difference might not seem like that big of a deal, but just imagine that a yard takes an hour to mow with the Toro. If the Craftsman traveled the same distance it would take it 17 minutes longer (assuming there is no stoppage time for turns). Ok, there will be turns – really this time difference would be even smaller. I guess I should stop complaining.

This brings me to my second test – pushing force. I attached a spring scale to the mower handle and then pulled the mower. The Craftsman only needed about 5 Newtons to get the thing moving at a constant speed but the Toro required 25 Newtons. Now, I guess I should make two points about this. First, I’m not sure I needed to push on the Craftsman mower at all. It really does drive nicely. However, I pushed on it as though I were actually mowing and 5 Newtons is the value I recorded. Second, the Toro has this pacing bar on the back. The harder you push the bar, the more the driving wheels are engaged. So, you kind of have to push hard. I still don’t think this Toro could drive by itself but the Craftsman could.

There are a few other things to point out about this Craftsman mower. It has this garden hose port on the top.

Attach the hose, turn on the water and then run the mower. It’s a quick way to clean the blades. Does it work? I’m not too sure. I didn’t really get the underneath part all too dirty so it’s difficult to test. Seems like a good idea though.

My other notes:
I think the front of the mower might be too low. I didn’t have the wheels on the lowest setting, but still the front lip of the mower would get caught on any tree roots that stuck up. A couple of times this would cause a collision between my hip and the push handle when the mower came to a sudden stop. I suppose the low clearance is there to make sure that the blades don’t hit a root, but it’s still annoying.
The back bar (where you push) isn’t very comfortable. It seems like it’s too thin and it makes my hands hurt after using it for a while. This is probably mostly my fault for pushing at all – but I get impatient.
My 12 year old son really likes the Craftsman more than Toro. He said it’s much quieter and much easier to push. He gives it two thumbs up.
This is probably true of all new mowers, but it is very easy to start. I found it odd that there wasn’t a gas prime button.

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