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So today when I was taking a taxi and looked at the vehicles, I observed that the majority of the light vehicles (i.e. 16-person minibus/ 4-6 person cars) use 5 (or less frequently, 6) wheels studs for each wheel.

Intuitively, 6 evenly distributed components are more stable as it has 3 axes of symmetry, as compared to 5.

Is there any mechanical reason for that?

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    $\begingroup$ @AndyT - I think he means the studs that hold the wheel onto the hub with lug nuts. $\endgroup$ – Mark Apr 4 '17 at 11:38
  • $\begingroup$ 4 is a common count too. $\endgroup$ – agentp Apr 4 '17 at 11:45
  • $\begingroup$ @Mark Yes Mark that is what I mean. And agentp I did see a car with 4, but cars of many brands like Toyota, BMW use 5. $\endgroup$ – Mythomorphic Apr 4 '17 at 11:48
  • $\begingroup$ Your intuition about stability is wrong. For any number of "distributed components" bigger than 2, the stability is exactly the same about every possible axis through the center - even if such an axis doesn't pass through any of the components. $\endgroup$ – alephzero Apr 4 '17 at 14:46
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On reason is that die cast (alloy wheels) generally have an odd number of spokes. This is because having directly opposed spokes causes problems with residual stress distribution as the casting cools and shrinks (this is also why cast iron hand wheels often have S shaped spokes). So 5 studs tends to be a more convenient number, if only for aesthetic reasons.

Wheels with an even number of spokes do exist but these are generally machined from a solid billet in several parts and are substantially more expensive than die cast ones.

Nowadays most vehicles are designed on a common platform which is used across a wide range and even things like vans often have alloys wheels as an option so there is little point having different hub designs for steel and alloy wheels.

There is also the issue of redundancy. If one of five or six studs fails or is loose then you still have a reasonable distribution of functional nuts ie covering more than 180 degrees of the hub. If you only have 3 out of 4 present then it is entirely possible for the wheel to not sit flat on the hub.

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To some extent, the number of bolt studs depends on the anticipated torque on the wheel. That's why small cars may have 4, larger cars 6, and semitrailer trucks have 11 or 13.

There's certainly a cosmetic contribution when considering sporty or luxury cars, but "form follows function" here as well. The designer might add a bolt or two for looks but would never under-design the fitting.

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  • $\begingroup$ Another consideration is safety, if one wheel nut is not tightened correctly or a stud cracks and fails. The safety margin for "3 out of 4" studs is worse than "4 out of 5", and with "3 out of 4", you have half the wheel unsupported, so when the vehicle is cornering the stud opposite the failed one will experience a large variation of load during each wheel rotation. If that is the next stud to fail, you only have two studs opposite each other! These considerations would almost certainly rule out a 3-stud design, even for a very light vehicle. $\endgroup$ – alephzero Apr 4 '17 at 14:53
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Most probably cost optimization. 6 might be stronger, but 5 is strong enough and when you make thousands of vehicles a year those 4 nuts, bolts and assembly time add up to a significant enough value. 4 lug wheel mounting systems are more common in Europe where the cars are smaller and 10 inch rims are common in 'mini' style cars.

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