Check roof compatibility, explore wind drag and prepare a car roof magnetic sign RFQ. No safe speed or holding capacity is inferred.
What you will get
Estimate illustrative wind drag and identify missing mounting evidence. All fields are required; defaults are an example, not a recommendation.
You will get a wind-drag example, material checks and an RFQ brief. Retention stays unverified until the actual assembly is tested.
By CoatedMagnets. Published: . Page reviewed: . Source check dates appear below. Calculated examples are separated from verified product data and unknown assembly performance.
Material first
Aluminum, glass and composite need a different mounting system. Wrap over steel needs its own interface and finish review; it does not change the underlying steel into aluminum.
Evidence and scope4 or 6 mm
Both thicknesses appear in the Coroplast graphics range. Neither carries a roof-sign road-speed approval. Specify the complete face, frame and mount together.
Evidence and scopeVelocity²
At fixed density, area and drag coefficient, increasing relative airflow from 65 to 80 mph raises calculated drag by about 51.5%. This is a calculation, not a measured vehicle result.
Evidence and scopeRelative airflow
For a direct headwind, 65 mph driving plus 15 mph headwind gives 80 mph relative airflow. This is speed, not force. Crosswinds require a vector assessment; neither this example nor the checker determines a safe driving speed.
Evidence and scopePull vs. Shear
A rubber layer can improve grip while increasing the distance between magnet and steel, reducing normal pull. The balance depends on the assembly and surface; verify pull, sliding and peel on the intended wet and dry interface.
Evidence and scopeSeparate gates
NYC TLC Chapter 67 §67-16 is one local example requiring authorization for taxicab rooftop advertising fixtures. Check rules for your own vehicle and jurisdiction.
Evidence and scope
Concept illustration; not a tested assembly or installation instruction.
A sign face, frame, magnetic base, protective interface and vehicle roof form one system. Submit a drawing showing each layer. Adding a pad or film changes the interface and requires verification.
The checker uses q = ½ × rho × V² and D = Cd × q × A. V is relative airflow in m/s (mph × 0.44704); A is width × height in m². We assume air density rho = 1.225 kg/m³ and Cd = 1.25 for illustration. The actual coefficient must be established for the assembly. NASA formula reference.
Identify the roof, finish and contact gaps.
Record geometry, relative airflow and assumptions.
Request installed retention tests and drawings.
Agree an engineering test plan and release criteria.
Illustrations, not field trials. All use density 1.225 kg/m³, Cd 1.25 and frontal area 0.1352 m². Values are rounded; no case establishes a safe operating speed.
| Scenario | Inputs | Result | Next action |
|---|---|---|---|
| Curved steel, 4 mm face | 520 × 260 mm; 65 mph relative airflow; 6 magnets | About 87.4 N illustrative drag; retention unverified | Send curvature photos and installed retention measurements. |
| Same sign, stronger headwind | Same inputs except 80 mph airflow (65 mph road speed + 15 mph headwind) | About 132.4 N illustrative drag; about 51.5% higher | Revise the wind envelope with the engineer; do not infer a safe speed. |
| Aluminum roof | Same dimensions and airflow as the first example | Magnetic-only mounting unsuitable; wind example does not establish attachment | Request a vehicle-approved mechanical mounting option. |
Use your own dimensions to prepare a brief with the calculation assumptions and the evidence your supplier needs.
Build your roof sign RFQ| Topic | Known | Unknown | Minimum next step |
|---|---|---|---|
| Steel roof and protective layers | A magnetic interface requires a suitable substrate and contact. | Actual roof steel thickness, curvature, wrap gap and finish compatibility. | Confirm material and inspect contact on the vehicle; get wrap-maker approval where applicable. |
| Wind and geometry | The checker uses frontal area and relative airflow with explicit assumptions. | Actual Cd, lift, turbulence, gust response and overturning moment. | Ask an engineer to assess the complete assembly; do not treat the example as an upper bound. |
| Pull versus sliding or peel | Normal pull is a different load direction from sliding or edge lift. | Vehicle-specific retention capacity, friction and uneven load sharing. | Require separate installed pull, sliding and peel tests with the intended pads and finish. |
| Face material | Coroplast stocks multiple sign-sheet thicknesses. | Stiffness, flute orientation, frame behavior and complete assembly weight. | Provide the panel grade and drawing; do not assign a safety factor solely from thickness. |
| Heat and exposure | Magnet grade and flexible-sheet care have published limits. | Supplier-specific adhesive, pad, coating, and printed-face limits. | Obtain every component data sheet and test the intended environment. |
| Public-road release and certification | The checker organizes an RFQ; it does not certify an assembly. | A verified operating envelope and applicable approval requirements for your vehicle, sign and jurisdiction. | Request the exact model, vehicle fit, test conditions and limitations behind any certification claim. Require engineering review before a public-road trial. |
Select for the vehicle and use case first. Each option needs its own supplier data and installation approval; these comparisons do not rank safety.
| Option | Useful for | Tradeoff | Action |
|---|---|---|---|
| 4 mm coroplast assembly | Removable messaging where a supported lightweight face is suitable | Needs a frame and validated mount | Quote complete face/frame/base; verify installed behavior. |
| 6 mm coroplast assembly | Projects that need a different panel stiffness or construction | Thickness alone proves neither higher wind area nor safer retention | Compare mass, stiffness and frame details using supplier data. |
| Lighted acrylic roof box | Night visibility or taxi identification | Adds wiring, mass, height and enclosure constraints | Use a purpose-built vehicle mounting review. |
| Vehicle-approved rail or clamp | Non-ferrous roofs or projects lacking magnetic test evidence | Fit and vehicle load limits still apply | Confirm approved attachment points and assembly limits. |
| Door magnetic signs | Side-facing business identity | Does not provide roof-level visibility | Review side-panel magnetic signs |
A direct headwind increases relative airflow. At otherwise fixed inputs, drag rises with the square of that airflow. Neither road speed nor magnet count establishes installed retention.
Action: Ask for assembly-specific wind and retention tests, including gust and crosswind conditions. Follow the assembly maker’s operating and removal limits; keep unverified assemblies off public roads.
A dry stationary fit does not cover automated washing, ice, grit or salt.
Action: Remove before automated washing unless the assembly maker explicitly permits it; inspect before reinstalling.
Debris and trapped moisture can harm the contact surface.
Action: Follow the actual assembly and finish manufacturers. Do not extend flexible-sheet guidance into roof-mount approval.
A magnet-grade temperature figure does not establish the limits of pads, adhesives or printed faces.
Action: Collect supplier-specific service limits and select an assembly validated for the operating environment.
Material boundaries: magnet temperature specifications and flexible-sheet care restrictions. These do not replace complete-assembly instructions.
Prices and failure rates are unknown without a project quote and test data. Compare these cost drivers instead of assuming the cheapest face produces the cheapest fleet program.
| Cost driver | What changes it | Buyer action |
|---|---|---|
| Prototype and test budget | Roof models, base design, fixtures and repeat tests | Price a representative sample/test phase before the production order. |
| Handling labor | Removal, cleaning, inspection and storage frequency | Time the actual operator workflow in a controlled pilot. |
| Replacement and finish repair | Pad wear, trapped grit, adhesive aging and roof finish | Define inspection/replacement criteria and responsibility in the quote. |
| Permit or design changes | Vehicle class, operating location and advertising content | Confirm local requirements before printing inventory. |
Sources establish formulas, product properties and specific regulatory boundaries. None of the sources below establishes a universal magnetic roof-sign failure rate, holding capacity or safe speed.
| Source | Evidence | Scope | Date |
|---|---|---|---|
| NASA Glenn — Dynamic Pressure | q = ½ × rho × V² defines dynamic pressure. Pressure is not total sign force. | Physical relationship; no roof-sign rating | Checked September 20, 2026 |
| NASA Glenn — Drag Equation | D = Cd × q × A; the coefficient depends on geometry and the chosen reference area. | Formula only; our Cd = 1.25 is illustrative and unvalidated | Checked September 20, 2026 |
| Coroplast — Print specifications | Lists 2–10 mm print sheets, including 4 mm and 6 mm graphics stock. | Available face thicknesses, not vehicle retention | Checked September 20, 2026 |
| K&J — Leverage and friction when using magnets | Explains how a high-friction layer can resist sliding while the added separation reduces normal pull. Its wall-mount examples are not vehicle tests. | Qualitative tradeoff only; actual wet/dry roof retention needs testing | Updated August 27, 2025; checked September 20, 2026 |
| NASA Glenn — Relative Velocity | Airspeed is the vector difference between ground velocity and wind velocity. Direct headwind adds to forward speed in the one-dimensional example. | Relative-airflow input only; not a force, gust model or safe-speed rating | Checked September 20, 2026 |
| K&J — Neodymium specifications | Lists 80°C for standard N material and notes that geometry and magnetic circuit affect actual performance. | Magnet-grade boundary, not a whole-assembly heat rating | Checked September 20, 2026 |
| Magnum Magnetics — Clean & Care | Flexible vehicle magnets should be removed and cleaned daily. The manufacturer warns against horizontal direct-sun surfaces and requires full contact with a suitable surface. | Flexible-sheet handling limits; not approval for a rigid roof assembly | Checked September 20, 2026 |
| 49 CFR 390.21T — CMV markings | For covered self-propelled CMVs, required markings generally go on both sides, with contrast and daylight legibility at 50 ft. See the rule for exceptions. | Side-marking requirements, not roof-sign certification | Checked September 20, 2026 |
| NYC TLC Chapter 67 §67-16 | Sets authorization, permit, testing/certification and Safety and Emissions approval requirements for the covered taxicab rooftop advertising fixtures. | NYC taxicab example only; not a universal sign permit rule | PDF updated October 24, 2025; checked September 20, 2026 |
Confirm the roof material, face size, assembly weight, relative airflow and installed mount test evidence. Use the checker to assemble those questions into an RFQ before ordering a prototype.
The first phrase describes the mounting approach; the second describes the use. A coroplast magnetic roof sign still needs the same roof, structure and mounting review, plus any local advertising authorization.
No. It calculates an illustrative drag load and flags missing information. It cannot determine holding capacity, a safe speed, a safety factor or road-use approval.
No. Count is recorded for the RFQ only. Grade, gaps, load directions and spacing also matter. Rubber can improve grip while reducing normal pull through added separation; verify the complete assembly on the intended surface rather than assuming a universal benefit.
Enter airflow relative to the sign. For a direct headwind, 65 mph driving plus 15 mph headwind gives 80 mph relative airflow. That is a speed, not a force; force is reported in N or lbf. Crosswinds, gusts and local roof flow need separate review.
Cd = 1.25 is illustrative, not measured for your sign or a guaranteed conservative bound. Geometry, orientation and reference area affect Cd. A certification label alone supplies neither Cd nor transferable speed approval; ask for the exact assembly test report.
A magnetic-only attachment does not have a steel contact path there. Ask for a vehicle-approved rail, clamp or other mechanical system rather than adding stronger magnets.
The steel may still attract a magnet, but the wrap adds a gap and potential finish damage. Obtain compatibility guidance from the wrap maker and test the complete installed interface.
Thickness alone cannot answer that. Compare the sheet grade, stiffness, flute direction, mass and frame. Neither thickness establishes the capacity of the mount.
Remove it before automated washing unless the manufacturer explicitly permits that exposure for the complete assembly. Follow its removal and reinstallation procedure.
Do not assume so. Magnum warns against horizontal metal surfaces in direct sun for its flexible sheets. A rigid roof sign needs a suitable purpose-built mounting system and separate evidence.
No. Covered CMVs must meet the applicable identification and placement provisions of 49 CFR 390.21T. Roof advertising and required vehicle identity markings are separate decisions; check the current rule and its exceptions for your vehicle.
Attach vehicle make/model, roof photos, material and finish details, face/frame drawings, total weight, mount specification, intended wind/weather exposure and available test reports. The brief can be downloaded or opened in your email app.
Run the checker, download the result, and attach roof photos, vehicle make/model, face and frame drawings, assembly weight, proposed mount details and existing test reports. Review the test plan before a road trial or bulk order.