Head to head

Optimum diameter versus maximum cutting capacity

Which published number should actually drive the specification?

8 criteria compared

This is the most common and most expensive specification error in Australian forestry. Manufacturers publish three different numbers, and buyers routinely quote only the largest.

The three are: the optimum DBH range, the recommended maximum regular DBH, and the absolute maximum cutting capacity. They are not interchangeable.

Framing

Why this comparison recurs

This is not a comparison between two products. It is a comparison between two numbers on the same specification sheet, and confusing them is the most expensive specification error in Australian forestry.

Manufacturers publish three separate diameter figures: the optimum production range, the recommended maximum regular DBH, and the absolute maximum cutting capacity. Buyers routinely quote only the third, which is the least useful of the three.

The consequence is paying for weight, carrier class, boom payload and component cost that never converts into production — carried on every cycle, for the life of the machine.

Side by side

The comparison

Criterion	Optimum production range	Absolute maximum cut
What it describes	Where the head is most productive	The largest stem it can sever
Frequency of use	Most of every shift	Occasional
Drives cycle time	Yes	No
Drives consumable wear	Yes	Marginally
Appears in sales material	Sometimes	Always
Example — SP 661 LF	~160-510 mm	700 mm (option to ~810 mm)
Example — SP 761 LF	~250-500 mm	800 mm
Example — Waratah HTH624C 4x4	Much smaller than the maximum	815 mm butt saw

The decision

Which one, and when

Choose optimum production range when

Your stand's average and 90th-percentile DBH sit inside the published optimum band
You are comparing two heads on production rather than on capability
Cycle time, feed speed and consumable cost drive your cost per cubic metre

More detail →

Choose absolute maximum cut when

You genuinely have occasional oversize butts that must be handled without a second machine
Stem form is irregular and grip capacity, not speed, is the constraint
You have costed the extra head weight against boom payload and carrier class

More detail →

The decision rule

Obtain all three numbers in writing: optimum DBH range, recommended maximum regular DBH, and absolute maximum cutting capacity. Then specify against your stand data, not against the largest figure on the page.

Increasing head size buys tree capacity and feed force, but costs head weight, carrier requirement, effective boom payload, agility, transport weight and component price.

The money

How the two differ on cost, specifically

Capability comparisons are easy to find. This is where the commercial difference actually sits.

Factor	Optimum production range	Absolute maximum cut
What it describes	Where the head is most productive	The largest stem it can sever
Frequency of relevance	Most of every shift	Occasional
Drives cycle time	Yes	No
Drives consumable wear	Yes	Marginally
Appears in sales material	Sometimes	Always
Should drive the purchase	Yes	No

What would change the answer

The variables that move this decision

Read across the row your own situation matches. Where most rows point the same way, the decision is straightforward; where they split, the comparison is genuinely close and the economics below should settle it.

Variable	Points to optimum production range	Points to absolute maximum cut
Where your volume sits	Mostly inside the lighter head's optimum band	A substantial share above it
Stem form	Straight and clean	Crooked, branchy or heavy
Frequency of oversize stems	Occasional exceptions	A regular part of the work
Carrier capability	Limited — weight and flow at the margin	Ample flow, pressure and lift at reach
Cost of handling exceptions another way	Low — subcontract or second pass	High — no practical alternative

Three situations

What we would actually recommend

Not a balanced summary — a recommendation for each case, with the reasoning.

Stand averaging 280 mm with occasional 450 mm stems

Choose: Specify against the optimum range

The dominant stem class sits comfortably inside a mid-size head's productive band. Occasional oversize stems taking longer is cheaper than carrying extra head weight through thousands of hours.

Genuinely mixed stand with a substantial proportion of large butts

Choose: Step up, but check the 90th percentile first

If the percentile figure genuinely sits near the larger head's optimum band, the step up is justified. If only the maximum does, it is not — the percentile is what tells you which.

Landing processing where stem form is irregular

Choose: Prioritise feed force over cutting capacity

Crooked and irregular stems are a grip and feed problem rather than a diameter one. A head with higher feed force at moderate capacity will out-process one rated larger but with less grip.

Reading the sheet

Published figures that mislead on this particular choice

Each of these is accurate. Each of them answers a different question from the one a buyer is asking when they compare these two.

What is published	What it actually tells you
Maximum cutting diameter	What the saw can sever once. It says nothing about cycle time, grip reliability or consumable wear at that diameter, and it is the figure most likely to sell a head that is wrong for the stand.
Maximum feed speed	Achieved on straight, clean stems. Real throughput on crooked or branchy material is governed by how well the rollers hold the stem, which is a feed force and configuration question.
Feed roller opening	Frequently larger than the delimbing opening. The delimbing opening is the tighter practical bound on routine work and the more honest figure to plan against.
Head weight	Not just a carrier compatibility number. Mass at reach reduces lifting capability throughout the chart and changes the stability envelope on slopes.
Published optimum range	The most useful figure on the sheet and the least advertised. Where a manufacturer publishes one, match it against the diameter distribution of your volume — not against your largest stems.

What goes wrong

Mistakes specific to this decision

How to settle it

The order to work through, for your own case

Each step narrows what the next has to establish. Worked in order, most buyers find the decision resolves before the last one.

Take the diameter distribution of your volume

Not the range of stem sizes present. A stand with a handful of very large stems and most volume mid-range is a mid-range job with exceptions.

Overlay the published optimum band

If most of the volume falls inside it, the head is well matched. If a substantial share sits above it, a heavier head is the honest answer — and if only a few stems do, handle them another way.

Price the exceptions separately

Occasional oversize stems can be felled conventionally, handled by a subcontractor, or left for a second pass. Specifying the whole head around them is paying for capacity on every cycle to use it on very few.

Confirm the carrier supports the head at reach

Weight, continuous flow, working pressure and cooling, plus the lift chart with the head fitted. A head inside its optimum band on a carrier that cannot run it is still the wrong purchase.

Often overlooked

The option that is not on this page

Size for the volume and handle the exceptions separately

The choice is usually framed as one head that must cover everything. It rarely has to. Oversize stems can be felled conventionally, processed on a second pass, or subcontracted, and doing so is almost always cheaper than carrying capacity on every cycle to use it on a few. Establish what the exceptions actually cost to handle another way before specifying the head around them.

Common questions

Frequently asked questions

Which diameter figure should I specify against?

The optimum production range, matched to your stand's average and 90th-percentile DBH. The recommended maximum regular DBH tells you the upper bound for routine work, and the absolute maximum describes an occasional capability that should not drive the purchase.

Do all manufacturers publish an optimum range?

Not all, and those that do are making honest comparison possible. Where only a maximum is published, ask for the optimum range and the recommended maximum regular DBH explicitly — a supplier who cannot provide them has not characterised the head for your application.

What does over-specifying a head actually cost?

Head weight that reduces effective boom payload, a heavier carrier class than the work requires, reduced agility, higher transport weight and higher component and consumable cost. None of it appears as a failure; all of it appears as a slightly worse cost per cubic metre, every hour.

Detail