Tires
Tires are the suspension system's report card — every alignment error, worn damper, and balance problem eventually writes itself into the tread.
- Inner-edge wear on both sides is the classic pattern for excessive negative camber or toe-out. Toe issues wear one edge more than the other; camber wears the same edge on both.
- Cupping happens when the tire hops as it rolls — bad dampers or balance are the top causes. Fix the underlying issue before replacing the tire.
- Directional tires must stay on the same side. Rotate front-to-back on the same side only, unless the tires are dismounted and reversed.
Learn to read wear patterns and you can diagnose half the chassis without putting the car on the rack.
Reading Wear Patterns
Inner-edge wear on both front tires is the classic signature of excessive negative camber or toe-out. The detail that sharpens the diagnosis: camber problems wear the same edge on both sides (both inners, or both outers), because each wheel is leaned the same way relative to its own contact patch. Toe problems scrub the tread and often wear one edge more than the other, frequently with a feathered, saw-tooth texture you can feel by running your palm across the tread. Both tires worn on the inside edge, smooth wear — think camber. Feathering, or one side markedly worse — think toe. Either way, the tires are telling you the alignment was out long before the customer noticed.
Cupping — a scalloped, wavy dish pattern around the tread — is a different animal entirely. Cupping is caused by the tire hopping as it rolls, and the two top causes are worn shocks or struts and an out-of-balance wheel. The tire leaves the road slightly, lands, scuffs, and repeats, machining little scoops into the tread. Cupping is not a pressure pattern and not an alignment pattern. The critical repair rule: fix the underlying cause — dampers or balance — before installing new tires, or the new set gets cupped exactly the same way.
Rotation Rules for Directional Tires
Directional tires have a tread pattern designed to roll one way, marked by an arrow on the sidewall. That constraint dictates the rotation pattern: directional tires stay on the same side of the vehicle, rotating front-to-back only — including on a rear-wheel-drive vehicle where you might otherwise use a cross pattern. Cross a directional tire to the other side and it rolls backward, which hurts wet traction and noise and can void the tire warranty. The only way a directional tire legitimately changes sides is if it's dismounted from the wheel and remounted facing the correct direction. Non-directional tires follow the traditional patterns, but always check the sidewall before you start pulling wheels — assuming a rotation pattern without looking is how directional sets end up running backward.
| Item | Spec | Notes |
|---|---|---|
| Legal minimum tread | 2/32 in. | Legal is not the same as safe |
| Wet-performance threshold | 4/32 in. | Wet braking degrades sharply below this — recommend here |
| Winter/snow threshold | 6/32 in. | Snow traction needs tread depth to bite |
| Rotation interval | 5,000-8,000 mi | Directional tires rotate front-to-back same side only |
| Inflation pressure | Door placard, set cold | Sidewall number is the tire's max, not the spec |
| Road force variation | Under roughly 15 lb preferred on passenger cars | Verify your balancer's guidance; high readings cause speed-dependent shake |
Tread thresholds are industry-standard guidance; state inspection minimums and placard pressures govern the individual vehicle.
- Run your palm across every tread in both directions. A smooth feel one way and a saw-tooth catch the other is feathering — the toe is scrubbing the tire sideways down the road.
- Compare the edges on both front tires. Both inner edges worn smooth points at camber or toe-out across the axle; one tire wearing one edge points at that corner's geometry or a worn component on that side.
- Feel for cupping — scalloped dished spots around the tread. That is the tire hopping from worn dampers or imbalance, and it is neither a pressure nor an alignment pattern.
- Check center versus shoulder wear: center-worn means overinflation, both-shoulders-worn means underinflation — set pressures to the placard, cold.
- Check the DOT date code and sidewall condition, then quote the tires together with the cause — alignment for toe and camber wear, dampers or balance for cupping — because tires without the cause fixed are a subscription, not a repair.
- Replacing tires without fixing the wear cause — the new set inherits the same alignment or damper problem and wears out the same way, and the customer blames the tires or the shop.
- Crossing directional tires to the other side of the vehicle during rotation — the tread now runs backward, hurting wet traction and noise. Directional tires go front-to-back on the same side only.
- Inflating to the sidewall number. That is the tire's maximum, not the vehicle's spec — the door placard is the spec, set cold.
- Skipping the wear-pattern read entirely and quoting off tread depth alone — the pattern is a free chassis diagnosis, and ignoring it sends a customer out with new tires and an old problem.
Both front tires are worn on the inner edge with a smooth, even wear face. Camber or toe?
Camber — or steady toe-out — but the smooth texture is the discriminator. Camber wear is smooth because the tire simply leans on that shoulder; toe wear scrubs, leaving a feathered saw-tooth you can feel with your palm. Smooth inner wear both sides says check camber first; feathering says toe.
A customer's fronts measure 3/32 and they point out that 2/32 is the legal limit. What is your professional guidance?
Recommend replacement now, with the reason: wet braking and hydroplaning resistance degrade drastically below 4/32, so at 3/32 the tires are legal but already compromised in rain. 'Still legal' and 'still safe' are different statements, and the customer deserves the distinction before the wet season, not after the skid.
The rear tires on a front-wheel-drive car are cupped. What is the likely story?
Worn rear dampers or imbalance letting the lightly loaded rear tires hop, usually with a missed rotation schedule letting the pattern establish. Fix the dampers or balance first and rotate on interval — new tires installed onto the same worn dampers will cup identically.
Both front tires are worn on the inner edge with a smooth, even wear face. Camber or toe?
Missed one? The reasoning above comes straight from the SUS exam bank, so this is the standard you will be held to.