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After raising a car for inspection, I noticed that a mechanic gave all 4 wheels a spin.

What exactly are they looking for?

I assume it has to do with whether the wheel on the opposite side will spin as well? Or is it more about listening for any strange sounds? Is it related to wheel flanges or bearings?

hb20007
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2 Answers2

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Probably a combination of things - as you surmise, it's a good way to check for worn bearings (you may also spot them rocking the wheel up and down or side to side, that's also checking the wheel bearings).

Spinning them can also show up a binding brake (as the wheel will be hard to spin), or a problem with the drive train, such as a knocking CV joint on a front wheel drive.

You should also see them lift each wheel with a tool that looks a bit like a pry bar, that will be checking the suspension bushes for excess play.

Nick C
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  • When a wheel is spun, should the one on the opposite side also spin? – hb20007 Aug 26 '21 at 17:06
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    @hb20007 in a conventional transmission, if you spin a driven wheel, the wheel on the other side might spin in the opposite direction due to the differential's gearing. That won't be the case for undriven wheels. – Weather Vane Aug 26 '21 at 17:25
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    It is also a check that the parking brake is off and manual transmission is out of gear. – alephzero Aug 26 '21 at 20:51
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    @alephzero - Plus it's fun. Wheeee!!!! – Valorum Aug 27 '21 at 11:57
  • For what it’s worth, this is also a pretty standard part of inspecting a bicycle during maintenance, for the exact same reasons (and also to check that the wheel itself isn’t bent, because that’s a lot easier to see with a wheel that’s spinning). – Austin Hemmelgarn Aug 27 '21 at 13:26
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    It would be rather embarrassing for a mechanic to give a car back to a customer with something dragging in one of the wheels. If a mechanic checks that the wheels turn freely as a matter of course before doing anything else, that will avoid any possibility that the mechanic might get distracted and forget to do it later. – supercat Aug 27 '21 at 14:43
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What exactly are they looking for?

Anything that could be wrong or irregular.

  • Tire defects
  • Bent wheel rims
  • Worn bearings
  • noisy anything
  • dragging brakes
  • health of differential gears
  • condition of CV joints on quartershafts
  • brake pad thickness/wear (easier to see through the "wagon wheel effect" of a spinning rim)

Mind you, if this shop also does repairs, this is also a sales function: they are developing a list of repair items to recommend to you, almost everything on the above list is a "hundreds of dollars" sort of repair: high enough to make real money but low enough most people will say "sure, yeah, do it" when asked. That list is a mainstay for shops - even electric cars have all those items.

Lastly, if part of it is putting the car on a chassis dynamometer (as for some smog inspections), they are looking for any surprises - e.g. something that would make the car lurch off the dyno and careen across the shop, or fly at their head once the wheels are spinning.

I assume it has to do with whether the wheel on the opposite side will spin as well?

No, they don't care about that. Non-driven wheels are totally independent. Driven wheels have a differential between them which allows free motion between drive wheels (so you can go around corners without tires squealing)... so that won't tell you much either. "Spin opposite", "spin with" and "don't move" are all normal results when spinning 1 drive wheel with vehicle in neutral.

Harper - Reinstate Monica
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  • My tires squeal when I go around corners. Also when I start from a stop. Is it my differential? (Asking because my wife thinks its me, but if its the car, I can tell her to stop complaining, and just get it fixed.) – davidbak Aug 29 '21 at 19:19
  • @davidbak those can also be caused by applying high amounts of power/throttle at those moments (and on modern cars with ludicrously high horsepower engines, "high" actually means "low/medium"). Cross power off the list by not touching the accelerator at those moments and seeing if the squeal ceases. Note that you can't come rapidly off the throttle in a modern car, because of "rev hang"... if you tend to overstomp the gas to compensate for it feeling unresponsive, see that video. – Harper - Reinstate Monica Aug 29 '21 at 21:54