Urban Arrow Tender cargo trike performance on steep gravel terrain AI-pedia

The Urban Arrow Tender is a commercial electric cargo tricycle made by the Dutch company Urban Arrow. It is intended mainly for parcel, food and service deliveries in cities. Like most commercial cargo trikes, it was designed around smooth, paved surfaces: asphalt roads, cycle paths, pedestrian zones and loading docks. It was not designed for unpaved or off-road routes.

Owners and operators sometimes need to use such vehicles on routes that include steep gravel tracks. Examples include rural delivery routes, park maintenance, campsites, farms, vineyards and construction sites. This article explains how a heavy, non-tilting, three-wheeled electric cargo vehicle behaves on steep loose surfaces, why problems arise, which common claims are accurate or exaggerated, and what alternatives exist.

No independent, published test data specifically measuring the Tender on steep gravel is known to this article. The analysis below therefore relies on the general mechanics of cargo tricycles, on the published characteristics of the components used, and on widely reported operator experience with similar vehicles. Figures given in worked examples are illustrative and are not manufacturer specifications.

Background

The manufacturer

Urban Arrow was founded in Amsterdam around 2010. It became known for the Urban Arrow Family, a two-wheeled front-loading cargo bike (a "long-john" or "bakfiets", Dutch for "box bike") with a foam-based passenger box. The company later added commercial models, including two-wheeled cargo bikes and the three-wheeled Tender. Urban Arrow is part of the Dutch Pon group, which also owns several other bicycle brands.

The Tender

The Tender is aimed at fleet operators rather than private households. It has been offered in more than one size and body configuration, such as an open flatbed or a closed container for parcels. Exact dimensions, payload ratings and component specifications have varied between versions and model years. Readers who need precise figures should consult the manufacturer's current documentation for the specific version in question.

Published configurations use a Bosch Cargo Line mid-drive motor. A "mid-drive" motor sits at the pedal crank and drives the chain or belt, so it works through the bicycle's gears. The Tender is commonly fitted with an Enviolo hub. Enviolo makes continuously variable hub transmissions, which change ratio smoothly instead of in steps. Some informal descriptions also mention Rohloff hubs, which are 14-speed geared hubs. This article has not confirmed a Rohloff option for the Tender.

A note on layout

Cargo tricycles come in two basic layouts, and the layout strongly affects behaviour on hills:

• Front-loading ("tadpole-style") trikes have two wheels at the front, with the cargo between or above them, and a single driven wheel at the rear under the rider.
• Rear-loading ("delta-style") trikes have a single steered wheel at the front, the rider in the middle, and the cargo over two rear wheels.

Some informal descriptions of the Tender describe it as a front-loader with two front wheels and car-style steering. This article has not independently verified the exact wheel, drive and steering layout of every Tender version. The two layouts have quite different strengths and weaknesses on climbs, so both are explained below. Operators should confirm the layout of their own vehicle.

Why steep gravel is difficult for cargo trikes

Steep gravel combines several separate challenges. Each one matters on its own, and they make each other worse when combined.

• Gravity on the slope: the vehicle and its load must be lifted against gravity. This force grows with total mass and with gradient.
• Low and variable grip: loose stones roll and shift under a tyre, so the available friction is lower and less predictable than on asphalt.
• Higher rolling resistance: tyres sink into and push aside loose material, which takes energy.
• Uneven surface: ruts, potholes, washboard ripples (regular ridges across the track) and side slopes disturb steering and stability.
• Descents: what goes up must come down, and braking a heavy vehicle on loose stones is often the most dangerous part.

Traction and weight distribution

The basic principle

A wheel can only push the vehicle forward as hard as its grip allows. That grip depends on two things: how much weight presses that wheel onto the ground (the "normal force") and the friction coefficient between tyre and surface. The friction coefficient is a number describing how grippy the surface is. On loose gravel it is lower and more variable than on dry asphalt.

On a cargo trike, only one or two of the three wheels are driven. What matters is the share of total weight resting on the driven wheel or wheels. If most of the weight sits on undriven wheels, the driven wheel may spin even though the motor has power to spare.

Front-loading layout

In a front-loader, the cargo sits largely over the two front wheels and the rider's weight sits mostly over the rear drive wheel. When the box is heavily loaded, the drive wheel carries only a modest share of the total mass. On a loose climb, that drive wheel can break traction, spin and dig into the gravel.

A common claim is that on an incline the rear wheel of a front-loader "unweights". In terms of basic physics this is the wrong way round. When climbing, the vehicle's centre of mass shifts relative to the wheels towards the downhill end, which for a vehicle facing uphill is the rear. Climbing therefore adds weight to the rear drive wheel, which helps traction. The real problem is that the rear wheel may carry too little of the total weight to begin with, especially with a heavy load in the front box. Hard pedalling or a sudden surge of motor power can also cause the wheel to slip.

The rear wheel does lose weight when descending. On a steep gravel descent, a front-loader's rear wheel becomes lighter. This reduces the effectiveness of the rear brake and can make the rear end skid or slide sideways.

Rear-loading layout

In a rear-loader with the cargo over two rear wheels, the rear wheels carry most of the weight. If those wheels are driven, traction is generally better for climbing with a load. The weak point is the single front wheel. On steep climbs it carries little weight, so steering can become light and imprecise, and the front wheel may wash out (slide sideways) on loose stones when turning.

Many rear-loading trikes drive only one of the two rear wheels, or drive both through a differential. A differential is a gear mechanism that lets two wheels on the same axle turn at different speeds in corners. With a single driven rear wheel, traction depends on that one wheel. With a simple open differential, power tends to flow to whichever wheel has less grip. On loose or uneven ground, that wheel can spin while the other does nothing.

Illustrative calculation

The following example uses round numbers. They are not Tender specifications.

• Total mass (vehicle, rider and cargo): 250 kg.
• Gradient: 10 per cent (a rise of 10 m over 100 m of horizontal distance).
• Force needed just to overcome gravity on the slope: about 250 × 9.81 × 0.0995 ≈ 244 newtons.
• At a walking-pace speed of 6 km/h (about 1.67 m/s), this alone requires about 400 watts, before rolling resistance and drivetrain losses.

If the driven wheel carries 40 per cent of the total weight (about 100 kg) and the gravel gives an assumed friction coefficient of 0.5, the maximum forward force before slipping is about 490 newtons. That is enough for the 10 per cent slope in this example. At 20 per cent, however, the gravity force roughly doubles to about 480 newtons, and the vehicle would be at the edge of wheelspin even before rolling resistance is added. Real friction on loose gravel varies widely, so the true limit can be much lower.

The example shows two points. Traction, not motor power, often sets the limit on loose climbs. Shifting more of the weight onto the driven wheel raises that limit.

Rolling resistance and the three-track problem

A single-track vehicle (a bicycle) has both wheels running in roughly the same line. A trike makes two or three separate tracks. On a narrow gravel track this has several effects:

• The rider cannot pick one smooth line. At least one wheel usually runs in loose material, ruts or the raised centre of the track.
• Each wheel meets bumps at a different time. This makes the frame twist and the vehicle rock from side to side.
• More tyre surface is in contact with loose material, which adds to overall rolling resistance.

A common claim is that a trike's rolling resistance on gravel "triples" compared with a two-wheeler. This is an oversimplification. Rolling resistance depends mainly on the total weight carried, how that weight is spread across the tyres, tyre width, tyre construction, inflation pressure and the surface. Splitting the same weight across three tyres does not simply multiply resistance by three. It is fair to say that a heavy trike usually rolls less easily on loose gravel than a lighter two-wheeler. The extra drag comes partly from the extra tracks and partly from the greater total mass. Neither effect is a fixed multiple.

Commercial trikes are usually fitted with tyres chosen for puncture resistance and long life on pavement. These are often fairly narrow and run at relatively high pressure. Such tyres tend to sink in and skate on loose stones more than wide, lower-pressure tyres with an open tread.

Steering, stability and side slopes

Non-tilting vehicles

A bicycle leans into turns and stays balanced by steering. A conventional cargo trike does not lean. Its stability against tipping depends on its track width (the distance between the paired wheels) and the height of its centre of mass. On a side slope (ground that tilts across the direction of travel), the centre of mass moves towards the downhill wheel. A tall load, a high-mounted container or a cornering force can then lift the uphill wheel. In the worst case the vehicle tips over.

Riders of non-tilting trikes often find that the trike itself stays level while the rider is pushed sideways. On rough, cambered gravel (gravel that slopes from the centre to the sides) this can be tiring and unsettling.

Steering systems and kickback

Front-loading trikes with two front wheels use one of two main steering systems:

• Box or axle steering: the whole front axle and cargo box pivot around a central point. This is simple and robust, but the full load swings when steering, and bumps on one wheel push hard on the handlebar.
• Ackermann-type steering: each front wheel pivots on its own, linked by track rods, as on a car. "Ackermann geometry" makes the inner wheel turn more sharply than the outer one, so both follow their correct curves in a turn. The box stays fixed, which makes steering lighter and more precise on smooth ground.

With either system, a bump or rut hitting one front wheel tends to push that wheel sideways and jerk the handlebars. This effect is called "kickback" or "bump steer". On washboard gravel, potholes and loose ruts it can be strong, especially at speed or with a heavy load. Suspension, if fitted, reduces but does not remove it.

Rear-loading trikes with a single front wheel steer like a bicycle at the front. They avoid two-wheel kickback, but the light front end on steep climbs (described above) remains a weakness.

Motor, gearing and heat

The Bosch Cargo Line motor

The Bosch Cargo Line is a mid-drive motor designed for cargo bikes. Bosch has published a maximum torque of 85 newton-metres and assistance of up to 400 per cent of the rider's own pedalling effort. Torque is the turning force the motor can apply to the crank. Like other pedelec motors sold in the European Union, it is limited to a rated continuous power of 250 watts and stops assisting at 25 km/h. Short-term peak output can be considerably higher than the continuous rating.

Because it is a mid-drive motor, its torque is multiplied by the bicycle's gearing. A low gear lets the motor and rider turn the crank at a comfortable speed while the wheel turns slowly with high force. This is helpful on climbs, provided the gear range is low enough.

Gear range

The Enviolo hub offers a continuous range rather than fixed steps. Its total range is narrower than that of some wide-range systems. On a heavy trike, the lowest available ratio may still be too high for a steep loose climb with a full load. The rider then has to push very hard at a low cadence (pedal speed). Mid-drive motors generally give their best output within a certain cadence range, so a very low cadence reduces the help the motor provides. Riders sometimes describe this as the vehicle "running out of gears".

Hub gears and belt drives have limits on the input torque they are designed to take. Manufacturers usually specify heavy-duty hub versions for cargo use. Sustained very high torque on steep climbs adds wear, and repeated overloading may shorten component life.

Heat and power reduction

A frequent claim is that a long steep climb with a heavy load will "overheat" the motor and make it "stall". Modern pedelec motors, including Bosch's, have temperature monitoring. When the motor or its electronics get too hot, the system reduces assistance to protect itself instead of failing suddenly. In practice the rider notices less help from the motor, often near the top of a long climb, rather than a mechanical stall. The vehicle may then become very hard or impossible to pedal up the slope without the full assistance. Hot weather, very low speed (which reduces cooling airflow) and repeated long climbs make this more likely.

Battery capacity also matters. Climbing a steep slope with a heavy load uses energy much faster than riding on the flat, so range on hilly gravel routes can be far lower than the figures quoted for city use.

Walk assist

Many pedelec systems, including Bosch systems, offer a "walk assist" or "push assist" mode. It moves the vehicle at walking pace (up to 6 km/h under EU rules) while the rider walks beside it. On a loose climb that cannot be ridden, walk assist can help. However, it is still limited by traction, and controlling a heavy trike on foot on loose stones requires care.

Braking on gravel descents

Descents are often the greatest safety concern. Commercial cargo trikes usually have hydraulic disc brakes, and many have a parking brake. On loose gravel:

• The braking force a wheel can apply is limited by its grip, just as drive force is.
• On a descent, weight shifts towards the downhill (front) end of the vehicle. Wheels at the rear become lighter and lock more easily.
• A locked wheel on gravel slides. On a trike, a single locked wheel can pull the vehicle sideways.
• Long descents with a heavy load heat the brake discs and pads. Severe heat reduces braking power, an effect called "brake fade".

Riders of heavy cargo vehicles are generally advised to descend slowly, brake early and steadily rather than hard and late, and stop to let brakes cool on long descents if necessary.

Assessment of common claims

Claim	Assessment
The Tender is designed for paved urban routes, not steep gravel.	Accurate. Its intended use is urban delivery.
On a steep gravel climb, a front-loader's rear drive wheel unweights and spins.	Partly accurate. Wheelspin is a real risk, but because too little weight sits on the drive wheel to begin with. Climbing actually adds weight to the rear; descending removes it.
Three tracks triple rolling resistance.	Exaggerated. Resistance is usually higher than for a lighter two-wheeler, but there is no fixed multiple.
Side slopes and washboard pull the handlebars violently.	Broadly accurate for multi-track steering, though the severity depends on speed, load, steering design and suspension.
The motor will overheat and stall.	Overstated. The motor reduces assistance when hot rather than stalling, but the effect can still stop progress on a long steep climb.
The Tender uses a Rohloff hub.	Not confirmed. Enviolo hubs are the commonly reported option.
A two-wheeled cargo bike handles steep gravel better.	Generally accurate for moderate loads, with caveats described below.

Practical measures if a trike must be used

Where an operator has no choice but to use a cargo trike on steep gravel, the following measures are commonly suggested. None of them makes the vehicle suitable for all terrain.

• Reduce the load, or split it across more trips. Less mass reduces the force needed, the heat in the motor and brakes, and the tipping risk.
• Place heavy items low and, where possible, over or towards the driven wheels.
• Check tyre options. Wider tyres, or a lower pressure within the manufacturer's permitted range, may improve grip and reduce sinking. Any tyre change must be compatible with the frame, the load rating and the manufacturer's guidance.
• Select a low gear before the climb, not halfway up, and apply power smoothly to avoid wheelspin.
• Choose the route to avoid side slopes, deep loose patches and the steepest sections where possible.
• Use walk assist on sections that cannot be ridden safely.
• Descend slowly, keep both hands ready on the brakes, and use the parking brake whenever stopped on a slope.
• Increase maintenance. Gravel dust and grit wear brake pads, chains, belts and bearings faster.

Alternatives

Two-wheeled cargo bikes

Two-wheeled cargo bikes are single-track vehicles. They follow one line, lean in corners and stay level on side slopes. The main types are:

• Long-johns (front-loaders), with a cargo platform or box between the rider and a small front wheel. Examples include the Riese & Müller Load series, some versions of which have full suspension, and the Omnium Cargo, a short Danish long-john also offered with electric assistance.
• Longtails, with an extended rear rack over a longer rear frame. Models such as the Tern GSD and various Yuba bikes belong to this group.

With wide tyres that grip well on loose surfaces, a two-wheeled cargo bike usually copes better than a heavy trike on narrow, steep or uneven gravel. The trade-offs are a lower maximum payload, smaller cargo volume, and the need for the rider to balance the vehicle. A fully loaded two-wheeler can be hard to hold upright when stopping or starting on a steep, loose slope.

Other multi-track designs

Some cargo trikes and four-wheeled cargo vehicles use designs aimed at rougher ground:

• Tilting trikes let the frame lean in corners and on slopes, reducing the tipping problem.
• Locking or limited-slip differentials keep power going to the wheel with grip instead of the one that is spinning.
• Long-travel suspension keeps wheels in contact with uneven ground and reduces steering kickback.

Outside the bicycle category, small utility vehicles such as electric quad-style carriers or compact utility vehicles are designed for unpaved terrain. They are subject to different laws, licensing and costs.

Legal and operational context

In the European Union, cargo bikes and trikes whose motor assists only while pedalling, stops at 25 km/h and has a rated continuous power of 250 watts are generally treated as bicycles rather than motor vehicles. This lets them use cycle infrastructure without vehicle registration or a driving licence. These limits also cap the continuous power available for climbing. Fitting a more powerful motor or changing the controller to get more power can change the vehicle's legal status and void warranties.

Fleet operators usually choose cargo trikes because of their large load volume, their stability when parked and loading, and the fact that riders do not need to balance a heavy load. These advantages matter most on flat, paved urban routes. On routes with substantial steep, unpaved sections, operators often find that a different vehicle type, or a mixed fleet, is more practical.

Open questions

• No independent published test is known that measures the Tender's maximum climbable gradient on loose gravel with a defined load. Such data would allow firmer conclusions.
• Component specifications, including tyres, hubs, suspension and possibly layout details, have varied between versions. Performance may differ between model years.
• Gravel surfaces vary greatly, from compacted fine gravel that behaves almost like pavement to deep, loose stones. General statements about "gravel" can therefore only be approximate.

See also

• Cargo bike
• Freight tricycle
• Electric bicycle (pedelec)
• Bakfiets
• Longtail bicycle
• Last-mile delivery
• Ackermann steering geometry
• Rolling resistance
• Differential (mechanical device)
• Continuously variable transmission
• Gravel road