If there is one spec-reading habit that separates contractors who make money on clearing work from those who do not, it is this: they never buy on the biggest number on the data sheet. A published maximum diameter tells you roughly where an attachment stops. It does not tell you how fast it works at that size, what species and moisture produced the rating, whether repeated cutting at that size is intended, whether the carrier can control what results, or what it costs in blades and teeth. This guide explains why the maximum is the wrong number, what the right number is, and how to measure it on your own site before you sign.
Buying rule
Select every cutting attachment, from a hedge trimmer to a 550 mm shear, on the diameter class that makes up most of your work, confirmed by sampling. Treat the published maximum as a limit you touch occasionally, and plan a separate method for the small share of material above your production diameter.Why the maximum misleads
A maximum diameter is a single-event figure. A Trevi Benne WR20 with a nominal 550 mm cutting class can close on a 550 mm stem under the right conditions. Whether the cylinder pushes the blades through it in one stroke, in one slow stroke with a stall at the end, or not at all depends on species, green versus dead timber, fibre structure, trunk shape, blade sharpness and the pressure and flow the carrier actually delivers. None of that is on the data sheet, and a 300 mm pine and a 300 mm ironbark are not the same job.
The same distinction runs through every family on this site. A Powerhand EX36 grapple saw is listed with a 650 mm maximum cut, but the safe length of a 650 mm section is set by the lift chart rather than the bar (see lift charts and attachment mass). An OMEF CS400H will sever a 420 mm limb, but at that size the piece has to be able to fall safely because the CS has no holding grapple. A Dipperfox SC850 Pro is rated for 850 mm stumps, but an 850 mm hardwood stump with a heavy butt flare in rocky ground is nothing like an 850 mm pine stump in sand.
Wood volume scales with area, not diameter
The reason production falls away so fast near the maximum is arithmetic. The cross-sectional area of a stem is proportional to the square of its diameter, and the volume of wood the blade or rotor has to sever is proportional to that area.
Doubling the diameter quadruples the area. A 400 mm stem has four times the wood of a 200 mm stem, not twice.
For a shear, that means the cylinder has to drive the blade through four times as much fibre when the diameter doubles, and the time and pressure required climb accordingly. For a mulcher, a 400 mm stem feeds four times the volume of a 200 mm stem into the rotor per metre of height, so a head that clears a hectare of 200 mm regrowth in a given time will not clear a hectare of 400 mm timber in twice that time; it may take far longer, or a different method may be cheaper. For a stump grinder, drilling depth and root mass add to the area effect, which is why a modest change in average stump size moves a quote more than buyers expect.
Shearex’s honest metric
Most manufacturers publish only a maximum. Shearex publishes an optimal target material diameter for every VM and HM mulcher, and it is the most useful production figure in the whole catalogue. The VM-35SK is listed at 15 cm, the VM-50SR at 25 cm, the VM-60SR at 40 cm and the VM-65SV at 50 cm, with the front-mount HM-100SV2 at 60 cm. None of these is a maximum. Each is the size of material the head is designed to process continuously at a commercial rate on a suitably powered carrier, and each will process larger stems slowly when it has to.
That is exactly the number you want from every supplier, whatever the family. When a shear or saw supplier gives only a maximum, ask for a recommended continuous cutting diameter in your dominant species. When a stump grinder supplier quotes stumps per hour, ask what diameter and species produced the rate. The willingness of a supplier to answer is itself a signal.
Sample the diameter distribution
Production diameter is a property of your site, not the attachment. To find it, walk the block and measure. A practical method that works for felling, mulching, pruning and stump work alike is to lay out several straight transects across the area, sized so that between fifty and a hundred stems fall within a fixed distance either side of the line, and to record the diameter class of every stem within that band. Use a diameter tape or a calliper at a consistent height, note the species, and mark stems that are multi-stemmed, leaning or dead, because each of those behaves differently under the blade.
The result is a distribution rather than a number. An illustrative block might look like this.
| Diameter class | Share of stems | Share of wood volume (approx.) |
|---|---|---|
| 0–100 mm | 35% | 3% |
| 100–200 mm | 40% | 22% |
| 200–300 mm | 20% | 32% |
| 300–400 mm | 4% | 15% |
| 400 mm and over | 1% | 28% |
Two lessons sit in that table. First, three-quarters of the stems are below 200 mm, so the attachment should be chosen for fast cycles at that size, which favours a lighter WL Series shear with an accumulator over a heavy WR head. Second, the one per cent of stems over 400 mm hold more than a quarter of the wood, so they will take a disproportionate share of the time on any machine and are best planned as a separate task: a chainsaw crew, a grapple saw, or a subcontracted heavier carrier. Pricing the whole job on the average stem, or buying the whole fleet for the largest stem, are the two symmetrical mistakes.
Build a trial block
Sampling tells you what the site contains. A trial tells you what the attachment does with it. Before committing to a purchase, or before quoting a large contract on a head you already own, mark out a representative block that contains the same diameter distribution, species mix, slope and ground conditions as the contract, and work it with the exact carrier and attachment configuration you intend to use. Record productive hours separately from repositioning, refuelling and travel; count stems or measure area; note fuel; photograph blades or teeth before and after; and log every stem that needed a second cut, a chainsaw or another method. The forestry mulchers guide sets out the measurement list for area work and the stump grinders guide does the same for stump counts.
Twenty to fifty representative stems or stumps is usually enough to produce a mean and a median cycle time by diameter class, and the median is the more honest number for tendering because one oversized tree can distort the mean. The cost per unit calculator converts the trial into a cost per stem, per stump or per hectare. Insist that any supplier demonstration takes place in similar material; a demonstration in light regrowth tells you nothing about a contract in mature hardwood, and the Australian hardwood guide explains what to record when the demonstration is in the right timber.
Stump difficulty classes
Stumps deserve a further refinement, because two 500 mm stumps can differ by a factor of several in grinding time. Height above ground, butt flare, species, age, decay, root density, soil and rock all matter, and none of them is captured by diameter. For trial and tender purposes, classify each stump into a difficulty class and record production by class rather than by diameter alone. A four-class scheme works well in practice.
- Class A: clean softwood, light roots, sandy or loam soil, low stump
- Class B: normal mixed conditions, moderate root flare, average soil
- Class C: dense hardwood, heavy butt flare or tall stump, dense root mass
- Class D: rocky, contaminated or stony ground, buried debris, old fence lines
A Dipperfox SC600 that clears a Class A stump quickly will spend much longer on a Class C hardwood stump of the same diameter and will consume blades faster on a Class D site. A quote built on the diameter count alone will be wrong in both directions. The same logic applies to a Trevi Benne WE extraction job, where root architecture and soil decide how much the excavator has to fight before the stump comes free.
Applying the rule across the families
For linear pruning, the production dimension is often width rather than diameter. The three OMEF TRI bars all cut the same 100 mm class and differ only in working width, so on a highway contract measured in kilometres the wider bar is the production choice, provided the carrier can position it. For grapples, capacity in square metres and safe payload matter more than jaw opening. For augers, torque and ground conditions matter more than bit diameter. In every case the discipline is the same: identify the dimension that governs production on your work, measure it on your site, and buy for it.
Confirm before ordering
- Recommended continuous production diameter in your dominant species, in writing, not just the maximum
- For mulchers, the optimal target material diameter of the model quoted and the carrier power it assumes
- A diameter distribution from your own transect sample, with species and multi-stem notes
- A plan and a price for the small share of stems above the production diameter
- For stumps, a difficulty class breakdown rather than a diameter count alone
- A demonstration or trial block in material that matches your distribution, slope and soil
- Median cycle time by diameter class from the trial, used for the tender rather than the mean
- Blade, tooth or chain consumption recorded during the trial and priced per unit of production
Frequently asked questions
If the shear is rated to 500 mm, why should I not plan on cutting 500 mm trees all day?
Because the rating marks a limit, not a rate. Cutting force, cycle time, blade wear and the mass the carrier has to control all climb steeply with diameter. Ask the supplier for a recommended continuous diameter in your species and plan production around your dominant stem size.
What is the Shearex optimal target material diameter?
It is the material size a given VM or HM head is designed to process continuously at a commercial rate on a suitably powered carrier. It is deliberately not a maximum. It is the closest thing in the catalogue to a production diameter and the figure to compare between models.
How many stems do I need to measure?
Fifty to a hundred stems across several transects usually gives a stable distribution for a uniform block. Stratify a mixed site into zones and sample each. For stump work, twenty to fifty stumps classified by difficulty is a workable minimum for a trial.
Why does a small change in average diameter move the quote so much?
Wood volume rises with the square of diameter. Moving the typical stem from 200 mm to 300 mm more than doubles the wood per stem, and the largest few per cent of stems can hold a quarter or more of the total volume on a site.
Should I buy the bigger attachment to be safe?
Usually not. A heavier head is slower on the small stems that make up most of the work, consumes more lift capacity and hydraulic power, and costs more in wear parts. Buy for the dominant class and handle the oversize material with a separate method.
Get a recommendation, not just a price
Send us your excavator model, hydraulic figures, tree species and typical diameters. We will come back with a carrier-matched configuration and a written specification you can compare against any other quote.