Design of Lawnmowers

Factual Questions
1

Design of Lawnmowers:

Having to go from a twenty-two inch cutting deck to a nineteen inch cutting deck recently, I started to wonder why standard size for walk behind mowers is roughly two feet. For some it might be getting through a narrow gate or door, but I rarely see a gate that could not accommodate a thirty-two inch—or even a forty inch cutting deck.

Is there some horsepower trade-off that keeps mowers so limited? Would a 32” blade require a significant increase in power? Or in the frame that attaches the motor to the blade? Or even to the wheels? Would the handle need to be beefier, or have a different shape to control the extra weight?

It seems to me that a thirty-two inch blade would be infinitely better unless it requires a great deal more infrastructure to make it work. It might be a little harder to steer around trees or other obstacles, but that could be easily compensated for by an efficient weed whacker.

Related note:
When I lived in central Ohio, most of the lawn services used a huge, self propelled cutting deck that dragged a little cart behind it for the operator to stand on. It seems like the thing was over fifty inches wide (52” or 55”maybe and I remember being told there was a six foot model). They were huge, and they were fast, and they could turn VERY sharply—but they had a huge power plant too. No way two guys could just pick one up and put it into a truck or onto a trailer; they had to be driven in and out of the trailer. They were a fast and efficient machine but I doubt an average homeowner could even operate one on the first try. Plus they had big engines (inside the small engine world), and it would likely be a disproportionately high investment for a guy who just mowed his lawn once a week for six or seven months a year.

But if the engine on my mower would spin a blade that is five or six inches longer on each end - - - well, that would be pretty useful for me, and I bet a lot of others as well. Can it be done and at what expense? Is it just that people are used to seeing mowers that size and they won’t try the new thing due to bias? Riding mowers have bigger cutting decks, but they are a different animal to me – essentially a very small tractor configured almost exclusively to suburban and rural lawn mowing. Why are there not 32” walk behinds in every hardware store? It seems to me they would be much better in most ways. They could still offer the 22” and/or 24” models for smaller yards, or for those who plant their yards like an obstacle course. A 32” walk behind would eliminate one pass for each eight feet of lawn to be cut.

2

A wider blade makes everything cost more. The blade, the mower deck, the engine crankshaft or at least a bearing needs to be heavier, the engine more powerful, the engine speed has to match the blade speed. The small push mower market is very price competitive. People with more money start looking at tractors and reel mowers, or pricier options like zero-radius.

3

With a quick Google search I found a 30" Toro for over $1,000. So yeah, pretty pricey.

4

Where are you getting this 22 inch and being forced down to a 19 inch??
I didn’t see any 22 inch but 21 inch looks very popular?

I do a fair amount of mowing and my 42 inch zero turn, its a 2 blade mower and when i cut along side a 48 inch 3 blade mower there is a remarkable difference in how NICE a 3 blade mower will do in cutting. What i am trying to say is a longer blade will produce poor looking cuts compared to a shorter blade. And that is only my Fudament based opinion.:stuck_out_tongue:

5

Think of how many blades of grass the blade is trying to cut. As the mower is moving forward, all of the cutting action is taking place at the front edge of the blade’s sweep. If you go from 22" to 32", the total length of arc that’s making that initial cut increases by 45%, so the power requirement for that initial cutting action is going to go up by 45%. Except the power requirement will increase even more than that, for a couple of reasons:

-the entire blade is churning the air under the mower deck. The swept area for the 32" blade is over twice what it was for the 22" blade, so the power requirement for this phenomenon will double.

-Likewise for the mulching action, which takes place all over the rotor disc.

So now instead of a 4-hp engine, you need maybe a 7-horsepower engine for everything the blade is doing. The engine is probably the most expensive part of a cheap walk-behind mower, so the cost of your mower will go up a lot. Weight goes up, which means that for a mower with a 32" blade you’re probably going to add driven wheels. And a sturdier chassis. Weight goes up further, and the power requirement for driving the wheels on this burgeoning monster means you’ll probably want an 8- or even 9-horsepower engine. With a big engine like that, you’ll need at least a decompression feature to facilitate pull-starting, or maybe even an electric start - possibly with a battery. Cost and weight go up further.

A very wide blade may also present problems when mowing over uneven terrain. As you roll up/down over the edge of a hill, a 32" blade may scalp the grass in places where a 22" blade wouldn’t. You could reduce or eliminate this problem with extra wheels around the edge of the mower deck, but this also adds cost.

Here’s a 30" walk-behind self-propelled mower. 223 cc engine, $1100.

For comparison, Home Depot has a 20" walk-behind mower for $170. 125 cc engine, so roughly half the power output of the 30" model above.

Ultimately it boils down to cost (possibly also storage space for some folks). At some point the width gets so small that mowing the hard takes too much time, but from there the cost goes up quickly as the width goes up. Just looking at Home Depot’s gasoline-powered mowers, here’s the number offered in each size:

20": 15
21": 64
22": 12
23": 2
24": 2
26": 7
28": 2
30": 7
32": 3

So 20" - 22" is basically the cheap (and very popular) end of the market for walk-behind mowers. 30" is the transition point between expensive walk-behind mowers and cheap riding mowers.

6

Unless you’re cutting an extremely flat area a wider blade gives a poorer cut. Even small dips and rises will cause missed areas or scalping.

7

I’ve got a typical, single family household-sized mower (probably 21") and it’s a hassle getting it in and out of my shed. It clears the door frame by only a few inches either side and then needs a sharp turn to clear the interior wall. I usually end up having to lift it in some way or another.

You also need to be able to get it home after purchase. If it doesn’t fit in a trunk or hatchback, a lot of customers will simply pass.

8

It’s not just getting around trees and obstacles that is harder with a bigger mower, it’s every turn at the end of a line. And unless it has powered wheels (which adds cost), it’s a lot of extra weight to be pushing around.

9

Ever run over a rock or an exposed tree root with a 21" mower? I wonder what the effect would be with a 30" blade, particularly on a not particularly well built mower?

10

A big part of the design of mowers relates to whether they are designed for residential or commercial use. The Obama administration added many safety features that cause residential mowers to, well, just not work as well as mowers used to work.

Some examples:
They limited the speed of the blade – it’s gonna take longer to mow and have lot more stragglers than don’t get cut on the first pass. They made the deck sit lower to the ground to make it harder for the user to stick is foot under the deck – but that messes up the Venturi action that pulls the grass upright allowing it to get a smooth mow, so now you have do lots more ‘go over it again’. They put on those huge side chutes to throw the grass clippings farther (at a slower speed), making the machine more awkward to maneuver.

You can eliminate most of those ‘safety innovations’ by purchasing a commercial mower; doesn’t have to big a one, just one designed for commercial use.

11

Dammit. Missed a few edits on a time-out. You get the idea, though.

12

Cite?

13

Some quick googling shows ANSI/OPEI sets standards for consumer turf care equipment. Unfortunately these standards are only available for purchase from their site.
I did find an older inquiry on-line of max blade speed from 2001 stating blade tip speed should not exceed 19,000 fps. Another on-line inquiry made in 2017 stated the same, 19,000 fps blade tip speed.
I see no change made by the Obama administration.

14

:dubious:

19,000 feet per second? Nearly four miles per second? That seems…fast. The speed of sound is only 0.2 miles per second, so we’re not just talking about supersonic, we’re talking about hypersonic. If that were true, you’d need some good hearing protection, and so would everyone within a half-mile radius.

Typical small-engine speed is a little over 3000 RPM. If we’re running a 22" diameter blade, then the tip speed is about 290 feet per second, which works out to 17,400 feet per minute.

So I’m going to guess the standard is actually 19,000 feet per minute.

15

I think maybe it was from the Clinton days, not Obama.

I recall several years ago out of idle curiosity doing the calculations figuring out the tip speed of various size blades using the (then) new regulation of a maximum blade speed of 19000 feet per minute (about 215 mph).

16

Correct. 19,000 fpm