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04-For-B7 Transportation of Forest Products · May 2013

Question 3 of 6: Variable Tire Inflation Pressure Systems (VTP)

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Forest Engineering, 04-For-B7 Transportation of Forest Products, May 2013. Closed book; approved Casio/Sharp calculator only. 3 hours. Six question groups in two parts: Part A (Questions 1–4, choose 3 of 4, 60 points) and Part B (Questions 5–6, both compulsory, 60 points).

Reference texts: Heinimann, Forest Operations Engineering (forest transportation systems, vehicle/road interaction); Sessions, Forest Road Engineering Guidebook (road geometric design, stopping sight distance, road-surface traction); FPInnovations/FERIC reports (log-hauling vehicle configurations, tire-inflation systems, machine/vehicle productivity); Transportation Association of Canada (TAC), Geometric Design Guide for Canadian Roads (stopping sight distance formula, Canadian design practice); the exam's own “Required Propulsion Formulae” page (standard Davis-type railway tractive-effort and train-resistance equations, consistent with general railway/traction engineering practice, e.g. Hay, Railroad Engineering).

Check: the printed question text for Question 1 has three sub-parts (1.1, 1.2, 1.3), but the source's own mark-allocation table (page 4 of 4) lists only two 10-mark line items for Question 1 ("1.1 10 marks total", "1.2 10 marks total") -- no separate mark line for 1.3, even though every other Part A question group (2, 3, 4) prints exactly two sub-parts matching its two 10-mark line items. This looks like a table/print omission in the original exam rather than an instruction to skip 1.3 -- all three sub-parts are answered in full below, with the group shown as its 20 marks total per the printed table.

Question 3: Variable Tire Inflation Pressure Systems (VTP) (20 marks total, 3.1–3.2)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

3.1 — Concept and technical requirements

A variable (central) tire-inflation system lets the driver adjust every tire's pressure from the cab, on the move or stopped, matching pressure to the surface being driven: pressure is lowered for soft, loose, or freshly built forest-road surfaces (a lower pressure lets the tire carcass deflect more, spreading the same axle load over a larger contact patch and so reducing ground bearing pressure and wheel slip) and restored to normal highway pressure before the paved public-highway portion of the haul (restoring a compact, low-rolling-resistance contact patch and normal tread wear at highway speed). Delivering air to a rotating wheel while the truck is moving requires specific hardware beyond a standard tire/axle: an onboard compressor and air reservoir, a rotary air union (a sealed rotating joint) at each wheel hub that feeds air through the spinning axle into the tire, a control valve and in-cab display/controller so the driver (or an automated system) can select and monitor pressure per axle group, and hub and valve seals rated for continuous rotation at highway speed without leaking.

3.2 — Advantages and disadvantages for log hauling

Advantages. Better flotation on soft or wet forest-road surfaces (larger footprint, lower ground bearing pressure) reduces rutting and road damage and improves traction on grades and loose material, cutting wheel spin and stuck-truck downtime; this can extend the usable haul season on roads that would otherwise become impassable when wet, reduce road maintenance cost, and improve ride quality (less load shifting on rough sections, less driver fatigue). Correctly matched pressures can also improve fuel economy and extend tire life relative to running one compromise pressure for the whole trip.

Disadvantages. The system adds real capital and maintenance cost (compressor, rotary unions, controller) and mechanical complexity, with the rotary seals being an additional failure/leak point; if the system malfunctions or a driver fails to restore highway pressure before the paved segment, an under-inflated tire running at highway speed can overheat and fail, while an over-inflated tire on a soft forest-road surface loses the flotation/traction benefit entirely. The system also adds unsprung system weight and requires driver training and operational discipline to be used safely and to actually realise the benefits.