Axle ratios and final drive: single reduction, hub reduction and portal

Volvo Trucks UK & Ireland’s specification page for the FMX gives each rear axle two things: a type, and a set of numbers. The type is a short phrase — single reduction, hub reduction. Each number is a bare decimal with no unit attached: 2.31, or 3.70, or 5.41. The number is the reduction between the propeller shaft and the wheels; the type says how that reduction is made. And those types, with the portal axle beside them, are defined in EU law, in the annex of a European regulation that certifies the torque losses of driven axles.

The regulation that names the parts

Regulation (EU) 2017/2400 determines the CO2 emissions and fuel consumption of heavy-duty vehicles by simulation rather than by driving them. Article 2(1) applies it, subject to the second paragraph of Article 4, to medium lorries, heavy lorries and heavy buses; that second paragraph disapplies Articles 5 to 23 — the block that reaches the axle provisions of Annex VII — to heavy lorries of vehicle groups 6, 7, 8, 13, 14, 15, 17, 18 and 19, to medium lorries of groups 51, 52, 55 and 56, and to any vehicle with a driven front axle in groups 11, 12 and 16. Article 2(3) excludes off-road vehicles, special purpose vehicles and off-road special purpose vehicles as defined in Annex I, Part A, points 2.1 to 2.3, of Regulation (EU) 2018/858. Article 9(1) puts the duty on the vehicle manufacturer, which shall determine the CO2 emissions and fuel consumption of each new vehicle, with the exception of new vehicles using vehicle technologies listed in Appendix 1 to Annex III, to be sold, registered or put into service in the Union using the latest available version of the simulation tool; with regard to heavy buses the vehicle manufacturer or interim manufacturer shall use the method set out in Annex I, point (2). For the excepted technologies only the input parameters specified in Table 5 of Annex III are determined, not CO2 emissions and fuel consumption.

The simulation takes the axle’s torque losses as input data — standard values under Article 13(5), or certified values under Article 14(7) — so the Regulation had to describe the axles first. Annex VII, headed “Verifying axle data”, does that for driven axles. Its paragraph 2 opens by stating that the definitions which follow apply for the purposes of that Annex — they are the vocabulary of one certification procedure, not a general legal definition of an axle.

What the number counts

The ratio is a multiplier between the shaft and the wheel, and the Regulation uses it as one. Point 4.4.4 of Annex VII calculates an axle’s torque loss at the input side as the input torque less the output torque divided by i_gear, the axle gear ratio; point 6.2.3 calculates the efficiency of each point measured in a conformity check from output torque, input torque and i_axle, the axle ratio; Appendix 3 to the same Annex calculates the basic drag torque on the wheel side as T_d0 = T₀ + T₁ × i_gear. In those formulas torque reaches the wheels multiplied by the ratio, less the losses. The other half — shaft speed divided by it — the Regulation does not write as a formula; Volvo’s fact sheets put it in words, the RSS1370A’s saying that in a single reduction rear axle rotational speed is reduced in one single step in the final drive.

Appendix 2 shows how the ratio is declared. The axle information document asks the manufacturer for the axle line — SR, HR, SP, SRT or HRT — and the axle gear ratio, and among the gear specifications for the crown wheel diameter, the vertical offset between pinion and crown wheel, the teeth number of pinion and the teeth number of crown gear; point 1.13 asks for the gear ratio of the hub reduction.

Volvo publishes the practical consequence. Its RSS1370A, a hypoid single-reduction solo axle rated to 13 tonnes on the axle and 70 tonnes gross combination weight, is listed on its fact sheet at 2.06, 2.17, 2.31, 2.47, 2.64, 2.85 and 3.08:1. Between the 2.31 and the 2.85 there is a 23 percent difference in propeller shaft speed at any given wheel speed — that percentage is our arithmetic from the two published ratios, not a Volvo figure. What the sheet does not give is an engine: it describes the axle as designed for an electric drive unit torque of 3550 Nm, its specification line reads a maximum electric drive unit torque of 3500 Nm, and no engine torque appears anywhere on it. The ratio is a shaft-to-wheel figure, and that is as far as the document licenses it to be read. Two vehicle makers publish the same tall end of that list from the other direction: IVECO’s February 2024 brochure for the S-Way lists new rear axle ratios of 2.17 and 2.06 in its “Best performer in TCO” paragraph, the same two numbers the Volvo fact sheet carries, and Ford Trucks’ English July 2026 data sheet for the F-MAX gives 2.31 in its specification row for the F-MAX and the F-MAX L and 2.17 for the F-MAX LL; its tyre and fifth-wheel table lists 2.47 on every F-MAX and F-MAX L row, with 2.31 as an alternative on the 315/70 R22.5 rows, and 2.17 and 2.31 on the F-MAX LL.

Where the reduction happens, and what it does not settle

Put two Volvo axles of the same axle load and combination weight side by side and their fact sheets differ in ratio range and in crown wheel size. The single-reduction RSS1370A runs 2.06 to 3.08:1 with a 485 mm crown wheel. The RSH1370F, a solo axle with hub reduction rated to the same 13 tonnes on the axle and 70 tonnes gross combination weight, carries a crown wheel of 295 mm and is listed ratio by model: 3.46, 3.61, 3.76, 4.12 and 4.55:1 on the FE, FH and FM; 5.41:1 on the FH and FM; and 4.67, 5.46, 6.18 and 7.21:1 on the FE alone. On those two fact sheets the hub-reduction axle’s lowest ratio sits above the single-reduction axle’s highest. Only the weights are shared, though: the RSH1370F sheet gives a maximum engine torque of 3550 Nm, where the RSS1370A’s rates the axle against an electric drive unit. Volvo’s own explanation of the centre gear is that its spiral bevel design is used mainly in rear axles where the major part of the reduction gearing takes place in the hub gears and only a small portion in the centre gears; the diameter difference between pinion and crown wheel is small, and that, Volvo says, results in a service-friendly centre gear with low drive losses. Reading the 485 mm and the 295 mm as the visible consequence of that is ours; Volvo prints the two diameters on separate fact sheets and draws no comparison between them.

Widen the frame past those two sheets and the pattern stops holding. Volvo’s fact sheet for the RSS1370B, version B of the same single-reduction axle at the same 13 tonnes on the axle and 70 tonnes gross combination weight, lists it at 3.40, 3.67 and 4.11:1, and the top two of those sit above the RSH1370F’s lowest of 3.46. Volvo Trucks UK & Ireland’s specification page for the Volvo FMX puts the two families side by side in one table: it sorts its rear axles into a single-reduction group and a hub-reduction group, with approved combination weights running from 44 tonnes on the RSS1344C/E and the RSS1244B to 150 tonnes on the RTH3815. Its rear axle ratio table lists the two versions under the combined designation RSS1370A/B, at 2.47, 2.64, 2.85, 3.08, 3.40, 3.67 and 4.11:1 — the A sheet’s ratios from 2.47 up and the B sheet’s three, without the A sheet’s 2.06, 2.17 and 2.31; that matching is our reading of the three documents, not a line Volvo prints. The same table lists the single-reduction RSS1344C/E as high as 5.29:1 and the single-reduction tandem RTS2310A as high as 4.63:1, both above every hub-reduction ratio the FMX page offers on the RSH1365. Where the reduction happens is not what fixes where the ratio lands; the published ranges overlap. Where the documents do differ is torque: the FMX page gives 3800 Nm for the RSS1370A/B and for the RSH1370F, the RSS1370B and RSH1370F fact sheets give a maximum engine torque of 3550 Nm, and the RSS1370A’s rates that axle against an electric drive unit torque of 3550 Nm. The two documents give different figures, and neither explains the difference.

The Regulation sorts the lines by ratio as well, though not in order to describe them. Where a conformity-of-production check is done by measuring drag torque — point 6.1(c) of Annex VII, one of three methods that apply “upon prior agreement between the approval authority and the applicant” — point 6.4.1(b) requires that the drag torque of the tested axle “shall be lower than the corresponding drag torque of the type approved axle or within the tolerance indicated in Table 2”. In its columns for axles measured after run-in, Table 2 bands that tolerance, in Nm, by gear ratio, with the boundary set by axle line: i ≤ 3 or > 3 for SR and SRT, ≤ 6 or > 6 for SP, ≤ 7 or > 7 for HR and HRT. The boundary is a step in a tolerance table rather than the edge of a line’s ratio range, and it is set higher for the hub-reduction lines than for the single-reduction ones. Paragraph 3 of the Annex adds a testing consequence: different axle ratios of hub reduction, hub reduction tandem and single portal axles — HR, HRT and SP — may be measured by exchanging the hub reduction only.

The portal axle exists for ground clearance or a low floor

Paragraph 2(2) is the definition to quote, and it is the only one of the five that ties a mechanical layout to a vehicle concept. A single portal axle means an axle that has typically a vertical offset between the rotating axis of the crown gear and the rotating axis of the wheel “due to the demand of a higher ground clearance or a lowered floor to allow a low floor concept for inner city buses”. The definition goes on to describe the construction: typically the first reduction is a bevel gear set, the second a spur gear set with vertical offset close to the wheels. Regulation (EU) 2022/1379 added “or helical gear set” to that sentence.

Two things follow. The offset is the purpose of the axle rather than a side effect of it — the Regulation names higher ground clearance or a lowered floor as the demand that produces the layout, and it ties the second of those, in its own words, to “a low floor concept for inner city buses” — the Regulation itself defines low floor as vehicle codes CE, CF, CG and CH of Regulation (EU) 2018/858 (Annex I, Table 3, footnote 2). And the Regulation treats portal axles as a special case in Annex I and in the appendices to Annex VII. Annex I uses the axle line to classify complete and completed heavy buses: under point 1.2.3(d) and its Figure 1, a low-floor bus of vehicle code CE or CG is “low entry” when its axle type is other than single portal; on a single portal axle it is low entry only with a step in the central gangway, and the default is no. Appendix 2 has a line used for portal axles only, the angle between the pinion axle and the crown wheel axle in degrees. Appendix 4 carries two portal-only family criteria — pinion angle to the horizontal within ± 5°, angle between pinion and crown wheel axles within ± 3.5° — and exempts single portal axles from the axle-housing geometry criterion that binds every other line.

Solaris specifies the drive axle of the Urbino 12 electric with modular drive, a battery-electric city bus, as a portal axle with stabiliser, alongside a central 240 kW electric motor, on a bus of 19,500 kg gross vehicle weight — up to 20,000 kg depending on country and tyres. The data sheet names the axle; it does not say which of the two demands in the definition it answers.

A tandem is two components, not one

The last two definitions describe a drive-through. An SRT is a single-reduction axle that also transfers torque from its input flange over an output flange to a further axle; an HRT is a hub-reduction axle with the same ability. On a tandem-drive bogie such as Volvo’s RTH3312, the drive-through is how the second axle is driven: the transfer gear’s output shaft operates the second axle’s final drive via a propeller shaft. The axle configuration notation records only that two axles are driven, never how the torque gets to the second one.

The Regulation accounts for the pair rather than the unit. For a measured tandem, point 4.4.8.5 of Annex VII adds the loss maps of the two single axles into one combined map; for standard values, Appendix 3 states that tandem axles shall be calculated using a combined efficiency for an axle including drive-through, SRT or HRT, plus the matching single axle, SR or HR. Volvo’s RTH3312 shows the hardware: two spiral-bevel single-gear units with hub reduction, a distribution gearbox of cylindrical spur type in the forward rear axle with a ratio of 1:1, three differential locks, ratios from 3.61 to 7.21:1, a maximum bogie loading of 33,000 kg and a maximum combination weight of 120,000 kg. The distribution gearbox does no reduction at all; the number on the specification sheet is still the hub-reduction ratio. The 120,000 kg is a rating of the axle, not a permitted weight: legal limits and the vehicle’s own plate are separate ceilings.

What the law assumes when nobody measures the axle

An axle family can be certified by measurement, or the manufacturer can accept the standard values in Appendix 3 to Annex VII. Those values are a generic efficiency for the load-dependent losses plus a generic basic drag torque, and they are set per axle line.

Two readings are worth taking from that table. The first is the cost of not being one of the five named lines: an axle that fits none of them is assumed to be 0.90 efficient with a basic drag torque of 150 + 50 × i Nm, against 0.98 and 70 + 20 × i Nm for a plain single-reduction axle. Neither pair of numbers is itself a drag torque — they are the coefficients T₀ and T₁, and the drag torque is what the formula returns from them. The second reading follows from that: the drag figure grows with the ratio, and nothing else in the table does. Applying the Regulation’s own formula to the ratios above, a single-reduction axle at 2.31 gets a basic drag torque of 116.2 Nm and a hub-reduction axle at 5.41 gets 178.2 Nm — our arithmetic from Table 1 and the published ratios, not numbers printed in the Regulation. It is worth being exact about what that comparison moves, because it moves two things at once. The efficiency column is keyed to the axle line and not to the ratio: every SR axle is scored 0.98 whatever number it carries, and the drop to 0.97, 0.96 or 0.95 comes from being an HR, an SRT or SP, or an HRT. Only T_d0 answers to i_gear. So on the standard values a numerically higher ratio is penalised on drag alone; the second penalty in the example belongs to the change of line, not to the change of number.

A separate appendix sets how the parent axle of a family is chosen. Under point 4.1 of Appendix 4, the parent axle is the one with the highest axle ratio; in case of more than two axles having the same axle ratio, the manufacturer “shall provide an analysis in order to determine the worst-case axle as parent axle”. Features which “may be considered to affect the torque losses” shall also be identified and taken into account in the selection (point 4.3), and the approval authority “may conclude” that the worst-case torque loss of the family can best be characterised by testing additional axles (point 4.2).

Where the number surfaces on paper

Annex IV sets out the files the simulation produces, each to contain the listed information “if applicable”. In the manufacturer’s records file, point 1.7.4 is the axle type — the model gives “single reduction axle” as its own example — and point 1.7.5 is the axle ratio. In the customer information file, the short document that has to accompany lorries and complete or completed heavy buses when they are registered, sold or enter into service, the transmission and axle are reduced to a handful of entries: transmission values measured or standard, transmission type, number of gears, retarder yes or no, and at point 1.2.8, axle ratio. Of the six axle entries in the records file, points 1.7.1 to 1.7.6, only the ratio is repeated in the customer information file; the axle type stays in the manufacturer’s records file. For vehicles using the technologies listed in Appendix 1 to Annex III, Article 9(1) requires only the input parameters of Table 5 of Annex III, and that list has no axle ratio or axle type.

It is a figure to read against the gearbox rather than alone. Annex IV spells the relationship out where it describes an integrated electric powertrain: point 1.14.6 of the manufacturer’s records file is the lowest total transmission ratio, “highest gear times axle ratio if applicable”. Multiply it out and a direct-drive top gear and an overdrive top gear reach the same total ratio on different axle numbers — our arithmetic, not a line the Regulation prints; the gearbox side of that choice is in automated manual transmissions. Volvo’s RSS1344C makes the point about availability rather than theory: of the two models the sheet covers, 3.70:1 is offered on both the FM and the FH, 4.11 and 4.63:1 on the FM but not the FH, and 5.29:1 on the FM in 4×2 only. That sheet, like the RSS1370A’s, is written around an electric drive unit torque — 2600 Nm — and names no engine. A ratio an axle is made in is not always one offered on every model.

What an axle is for, Volvo states on each fact sheet in its own words. The RSS1370A “has fast ratios and is intended for high speed heavy transports”; the RSS1344C “is intended for high-speed transport on good roads”; the RSH1370F “is designed to be used for heavy transports on demanding and hilly roads”; the RTH3312 “is designed for heavy, demanding haulage and train weights of up to 120 ton”.