When turning a corner I've been hearing the tyres squeal. I'd like to have an understanding of what might cause it so I could understand if it's related to a recent service and have sensible discussions when trying to diagnose the issue
What causes tires to squeal when turning a corner?
In the literal sense, tires squeal when they begin to slide against the road surface. The rubber sliding on the hard surface is what produces the squealing noise.
The next logical question is, why are my tires starting to slide on the road?
There could be a number of reasons for that. Considering the context of your question (you're asking about squealing in a turn), there are a few likely scenarios:
- You're driving really fast and exceeding the natural grip of the tires. The tread on the tires is squealing as it loses grip.
- Your tires are underinflated. Tires need the correct air pressure to retain their intended shape and structure. If a tire is underinflated, it will tend to be soft from a structural perspective. When you side-load an underinflated tire, it can start to "roll" such that the tire deforms far enough that the sidewall starts to make contact with the road surface. As the tire deforms, even if the tread itself retains grip, the sidewall typically gets "dragged" against the tarmac, and makes a squealing noise.
- Tires that are severely worn or not correctly matched to your vehicle in terms of load rating or other specifications can deform or lose grip and cause squealing.
You haven't mentioned severe tire wear or grossly incorrect tire size or spec, but if you're unsure of the condition or suitability of your tires with respect to your vehicle, it's good to verify that first.
After that, Since you've indicated in your question that there was recent service, and it's questionable whether or not your tires are inflated correctly (the shop told you 39, and you measured 29), the best next step is to correctly inflate your tires to the pressure recommended by you vehicle's manufacturer. Note that this is not the pressure stamped on the sidewall of the tire. If you don't have your owner's manual, you can look inside the door jambs for a sticker indicating the correct pressures. And make sure your're measuring with a gauge you trust, and that you understand the impact of temperature if you're measuring the tires when they are extremely cold or hot.