A De Dion suspension joins the wheel hubs with a rigid tube while mounting the differential to the chassis. Articulated halfshafts transmit torque between the fixed differential and moving hubs. The tube preserves the relationship between the hub axes; the differential does not move with it.
De Dion Tube Suspension Interactive Calculator
Compare selected axle-component masses while watching the De Dion tube move independently of the chassis-mounted differential. The estimate includes an assumed 50% of total halfshaft mass.
Equation Used
- Rigid tube holds hub axes parallel and maintains hub-center spacing.
- Differential is fixed relative to the illustrated chassis; articulated halfshafts change angle and plunge.
- Half of total halfshaft mass is allocated to this approximate unsprung subtotal.
- No ride-frequency, damping, tyre-contact or dynamic effective-mass prediction.
The 50% halfshaft allocation is a rough estimate, not a calculated dynamic effective mass. Use matching exclusions in the live-axle comparison.
A moving tube around a fixed differential
The blue tube and both hubs move together. During a one-sided bump the whole assembly rolls, so the wheel axes remain parallel to each other but need not remain horizontal relative to the road. Jointed halfshafts connect the hubs to the differential, which remains attached to the chassis.
The drawing shows the tube behind the separate chassis-mounted differential. Springs and chassis references make the relative movement visible. Fore-aft and lateral locating links are omitted for clarity, so the prescribed motion is not the kinematic path of a particular vehicle.
Compare component mass on a consistent basis
The calculator adds tube, hubs and outboard brakes, then allocates half of total halfshaft mass to an estimated unsprung subtotal. Enter a live-axle subtotal that excludes the same items. Wheels and tyres, springs, dampers and locating links are not included here.
Changing a mass updates the subtotals and comparison bars. It does not change the illustrative bump amplitude. A dynamic suspension model would need the geometry, inertias, spring and damper properties and tyre behavior.
Estimating the component mass subtotal
Let m_DD = m_tube + m_hubs + m_brakes + 0.5m_shafts. Mass saved is m_live − m_DD; percentage reduction is 100(m_live − m_DD)/m_live. Negative saving means the entered De Dion subtotal is heavier.
The halfshaft fraction is an explicit convention, not a universal physical result. Dynamic effective mass follows kinetic energy and depends on the component velocity field and chosen suspension mode. A shaft with one fixed end does not automatically contribute exactly half its mass to every dynamic calculation.
A 40 kg estimated subtotal
A 14 kg tube, 14 kg pair of hubs, 6 kg outboard brakes and 12 kg pair of halfshafts give 14+14+6+6 = 40 kg. Against a comparable 70 kg live-axle subtotal, the difference is 30 kg, or 42.86%.
Moving the brakes inboard would set the outboard-brake entry to zero for this subtotal comparison, but the brake system and all associated components must be assessed consistently in a real comparison.
What the subtotal does not establish
Separating the differential can reduce mass moving with the wheel assembly, but the complete result depends on actual component weights and layout. The tube is still a rigid connection between the two hubs.
Ride behavior, component life and suspension travel depend on the complete vehicle geometry, loads, springing and damping. A mass subtotal alone does not determine them.
De Dion questions
Is the differential inside the tube?
No. It is chassis-mounted and separate from the tube.
Do the wheels remain vertical over every bump?
No. The rigid tube keeps their axes parallel to each other; the complete assembly can roll relative to the road.
Why are there joints on the halfshafts?
The hubs move relative to the fixed differential, changing shaft angle and required joint plunge.
Why does mass not change the motion?
The animation is a geometric demonstration. The calculator compares mass subtotals rather than solving suspension dynamics.
Manufacturer assembly reference
Caterham Sigma Assembly Guide, edition 2.2, differential mounting, De Dion tube installation, rear dampers and driveshaft installation (printed pages 96–113). The illustration is a general De Dion schematic, not a dimensioned Caterham suspension model.
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