Wheelchair Geometry Explained: Wheelbase and the 60/40 Load Rule

Two chairs with identical motors and batteries can behave completely differently on ramps and wet floors. The difference is geometry: where the drive wheels sit under the loaded chassis. The single most useful mental model is the load distribution between drive wheels and casters.

The load split, measured with a user aboard

Put the occupied chair on four scales. Rear/mid-drive folding power chairs typically show 60-75% of total mass on the drive wheels; front-drive show 45-55%; rear-drive with rear battery packs can exceed 80% rear-biased. That number is not trivia — it is the chair's personality. More drive-wheel load means more tractive force before slip (traction is proportional to load on the driven tire), better hill climbing, and harsher caster behavior over bumps. Less means easier castering, smoother ride, earlier loss of climbing traction.

Why 60/40 is the design sweet spot

Mid-drive layouts target roughly 60-70% on the drive axle. That preserves climbing traction on 10-degree ramps with a heavy user, while keeping 30-40% on the casters so they grip for steering and resist caster flutter at speed. Push past ~75% rearward and three costs appear: front lift under acceleration (the classic wheelie-on-a-ramp event), caster shimmy above 6 km/h, and radically uneven tire wear — drive tires scrubbing, casters gliding.

What shifts the split

• User mass and position: a 130 kg user leaning back shifts 5-10 percentage points rearward versus the 100 kg test dummy.
• Batteries: 2 x 12Ah SLA (6-7 kg) versus 2 x 12Ah lithium (3-4 kg) under the seat changes front load by several points — one reason lithium conversions of SLA chairs sometimes climb worse.
• Accessories: rear bags, oxygen holders, rain covers on the backrest — every kilogram hung behind the rear axle multiplies through the lever arm. A 5 kg bag on a luggage rack can cost 2-3 degrees of rearward tip angle.
• Recline and tilt: 20 degrees of backrest recline with an occupied seat moves the combined center of gravity back 4-6 cm.

Reading the geometry from a spec sheet

Three dimensions tell you more than marketing paragraphs: wheelbase length (drive axle to caster axle — 420-520 mm typical for folding power chairs), drive wheel diameter, and battery location. Short wheelbase plus rear battery equals agile indoors and tippy on ramps. Long wheelbase plus mid battery equals stable outdoors and a wider turning circle. There is no universally better answer — there is the answer matched to your ramps, doorways and trunk.

The two-minute field check

On flat ground with the user seated, press down firmly on the backrest top. If the casters lift with moderate force (roughly under 15 kg of downward push), the rear bias is high — fit anti-tippers before any ramp work. And watch tire wear every month: casters wearing far faster than drives signals a nose-heavy setup; drives wearing to the cords while casters look new confirms rear bias. Geometry always confesses, eventually, through the tires.