A tank bag is the most useful piece of luggage on the bike and the one most likely to be a mistake. Useful, because it puts documents, phone, gloves and snacks where you can reach them without getting off. A mistake when it is too tall, too heavy, or fitted with magnets to a tank that will not tolerate them. The rule that avoids most regret: buy the smallest bag that holds the things you need at a fuel stop, and put nothing dense in it. If you find yourself packing tools into a tank bag, you have bought the wrong one.
Check the tank before you choose the bag
Magnetic bags are the cheapest and quickest to fit, and they are wrong for a large and growing share of bikes. Work through this before spending anything:
- Test whether the tank is steel. Put a fridge magnet on it. Plastic and aluminium tanks are common on modern adventure and sports bikes, and plastic tank covers over a steel tank behave unpredictably — the magnet may hold at rest and let go under braking.
- Look at what is actually under the "tank". On many bikes the panel between your knees is a cover, and the fuel is under the seat. A magnet on a plastic cover holds nothing.
- Check the tank's curvature. A strongly domed or heavily sculpted tank leaves magnets contacting on their edges, which is where they slide.
- Check the filler cap position. If the bag has to come off entirely to refuel, you will resent it by the third fill. Bags with a hinged rear or a quick-release ring solve this; magnetic bags usually do not.
- Sit on the bike and check your knees. A bag wide enough to push your knees out changes your riding position over a long day in a way that is uncomfortable rather than dangerous, but it is still a reason to size down.
- Check what it hides. This is the one that matters most, and it is covered below.
What a tank bag must never block
Motorcycle instrumentation is regulated for a reason. FMVSS 123, at 49 CFR 571.123, specifies requirements for the location, operation, identification and illumination of motorcycle controls and displays, with the stated purpose of minimising "accidents caused by operator error in responding to the motoring environment". Paragraph S5.2.2 requires that a display for a listed item "shall be visible to a seated operator under daylight conditions".
A tall tank bag that hides the top of the instrument cluster undoes that. It is your speedometer, your neutral light, your warning lamps and — on modern bikes — your ABS and traction control indicators. Sit on the bike in riding position, in daylight, with the bag packed to its normal height, and confirm you can read every warning lamp without moving your head. If you cannot, the bag is too tall, and expanding it further for the ride home is not an option.
The same check applies to the steering. Turn the bars lock to lock, seated, with the bag full. On bikes with a short tank and clip-ons, a bag can foul the bars or your hands at full lock, which you discover in a car park manoeuvre at the worst moment.
Mounting types compared
| Mounting | Works on | Strengths | Weaknesses |
|---|---|---|---|
| Magnetic | Steel tanks with a reasonably flat top | Cheapest, fits many bikes, on and off in seconds | Slides under braking when loaded; traps grit against paint; useless on plastic or aluminium |
| Strap harness | Almost any bike | Universal, secure, no paint contact if padded | Straps wear paint at the contact points; slow to remove; hides the filler on some bikes |
| Filler-ring quick release | Bikes with a compatible ring fitted | Most secure, cleanest, fastest on and off, filler stays accessible | Ring is bike-specific and adds cost; ties you to one manufacturer's bags |
| Tank-panel bolted bracket | Bikes with dedicated accessory points | Very secure, carries more weight safely | Least flexible; needs the right bike; bracket usually stays on permanently |
| Hybrid magnet plus straps | Steel tanks | Straps stop the slide the magnets cannot | Inherits the paint problems of both systems |
Whichever system you end up with, the single most common tank bag complaint is a ring of fine scratches around where the bag sits. It is not the magnets that do it — it is the grit that gets between the magnet backing and the paint, then grinds when the bag shifts. The fixes are mundane and they work:
- Wipe the tank and the underside of the bag every time you fit it. Every time, not every week.
- Fit a protective film or a microfibre sheet between bag and tank on bikes you care about.
- Never slide the bag into position. Lift it off and place it down.
- Remove it when the bike is being transported or washed, and do not leave it fitted in the rain for days.
- Check strap contact points on strap-mounted bags — that is where the paint goes first, usually at the tank's rear edge.
- On a bike you intend to sell, fit clear protective film under the bag from day one. Retrofitting it over existing swirl marks does not undo them.
A second, quieter form of damage is fuel. Petrol splashed during a fill runs down the tank and into the underside of the bag, where it softens adhesives and attacks the backing on some magnet sets. Wipe spills immediately, and if the bag has to sit on the ground while you refuel, put it somewhere clean — the grit it picks up in a forecourt is exactly the grit that ends up between magnet and paint on the next leg.
The phone problem
Tank bags almost always have a clear map pocket, and riders put phones in them. Be aware what that does over time. Apple states that it is not recommended to attach an iPhone to motorcycles with high-power or high-volume engines, because the amplitude of vibration in certain frequency ranges can degrade the camera system — long-term exposure may affect the optical image stabilisation and closed-loop autofocus hardware. For small-capacity and electric machines the recommendation is a vibration-damping mount or a gimbal, and to avoid prolonged regular use. The advice is on Apple's page about exposure to vibrations and iPhone cameras.
A tank bag map pocket is a rigid pocket on a rigid tank, so it transmits engine vibration fairly directly. If you use the phone for navigation daily, that pocket is not a neutral place to keep it. The alternatives — and the wider vibration and heat problem — are covered in phone mounts and vibration damage, and the case for not making the phone your primary navigation device at all is set out in the risks of phone navigation. A dedicated riding system with its own display takes the phone out of the map pocket entirely, which is the tidiest fix.
The second problem is heat. A phone in a sealed clear pocket in direct sun, running navigation, will throttle and then shut down. That is not a fault; it is thermal protection working. Nothing you pack around it solves it.
When a tank bag is the wrong answer
Be honest about these cases:
- Sports bikes with a short, steeply sloped tank. There is often nowhere to put a bag that does not interfere with your forearms in a tuck.
- Bikes with the filler under the seat or at the rear. A tank bag here has no natural anchor and no reason to exist.
- Riders who carry a rucksack anyway. Duplicating capacity means neither gets used properly, and a rucksack is the worse of the two for your back over a long day.
- Anyone tempted to load it heavy. Mass ahead of the steering axis dulls the front end. The proper place for dense items is low in the side cases, as set out in pannier and top box load limits.
Used within those limits it is the piece of luggage you will miss most if you take it off. For how it fits the rest of the load, see hard versus soft luggage and the trip packing list.
Frequently asked questions
Will a magnetic tank bag damage my paint?
The magnets themselves will not. Grit trapped underneath will, especially if you slide the bag rather than lifting it. Clean both surfaces every time and use a protective sheet.
Can I use a magnetic bag on a plastic tank?
No. Test with a fridge magnet first — plastic and aluminium tanks are common, and some steel tanks are hidden under plastic covers where a magnet has nothing to grip.
How big should a tank bag be?
Small enough that you can read every warning lamp on the instrument cluster while seated in riding position, and turn the bars lock to lock without contact. Expandable bags need this checked at full height, not at rest.
Is the map pocket a good place for a phone?
Not for daily use. It transmits engine vibration and traps heat, and Apple explicitly advises against attaching an iPhone to high-power motorcycle engines because of camera damage from vibration.
Can a tank bag be waterproof?
The bag body can be, but the map pocket and the zips rarely are. Most come with a rain cover, which is an outer layer rather than a seal — keep anything critical in a small dry bag inside.
