[AlanHo]

Whist walking for my Sunday paper this morning I was idly inspecting all the cars on people's drives. For some wierd reason I noticed that out of about 40 cars - only one had an even number of "spokes" in its wheels (A volvo 4x4). All the other car's wheels had either 5, 7, or 11 spokes.

Why do cars not have 4, 6 or 8 spokes then?

[Phil №❶]

Scary Alan, I've done the very same thing and thought likewise. I never mention it because my family already think I'm strange. I mean 7 or 11 don't even divide evenly.   

[Doggie 1]

They are odd numbers, aren't they? 

[AlanHo]

I am guessing that 3,5,7, etc spokes in a cast wheel, will allow it to stress relieve (move) better as the casting cools, than 2,4,6 etc spokes  - because they are not diametrically opposed.

In Victorian times spoked cast iron wheels often had curved spokes to provide some flexibility when the casting cooled. Straight spokes would have snapped. Nowadays people erroneously think that the curved ribs and spokes in old castings were a styling thing – but it was actually for engineering reasons.

Furthermore, an even-numbered spoke pattern will place one open inter-spoke rim area across from another, resulting in alternating weakness and stress. An odd number places a spoke directly across from each open inter-spoke area on the wheel.

A further reason could be harmonics – an odd number is less likely to set up sympathetic frequencies.


Yer pays yer money and yer teks yer pick.

[Alasama]

  That's what I thought half hours ago.  It could be all about stress. Just like we allow "T" weld seams for steel tanks, but "+" type weld seams are not acceptable.

[baroudeur]

Scary Alan, I've done the very same thing and thought likewise. I never mention it because my family already think I'm strange. I mean 7 or 11 don't even divide evenly.   

They are prime numbers i.e. cannot be divided other than by one or itself

 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, etc

I am guessing that 3,5,7, etc spokes in a cast wheel, will allow it to stress relieve (move) better as the casting cools, than 2,4,6 etc spokes  - because they are not diametrically opposed.

In Victorian times spoked cast iron wheels often had curved spokes to provide some flexibility when the casting cooled. Straight spokes would have snapped. Nowadays people erroneously think that the curved ribs and spokes in old castings were a styling thing – but it was actually for engineering reasons.

in which case why don't cast aluminium car wheels  have curved spokes?

[AlanHo]

in which case why don't cast aluminium car wheels  have curved spokes?

It is because aluminium has good tensile strength and ductility - with proper design of the wheel to allow for differential cooling rates in the mould, curved spokes are not necessary.

Conversely - cast iron has a low tensile strength and ductility - it is strong in compression but weak to the point of being brittle in tension. Hence it is necessary to minimise any tension stress in an iron casting - therefore the ploy of using curved spokes.  (Note that I am referring to cast iron - not wrought iron or steel  - which do have good tensile properties.)

[eye30]

Why do cars not have 4, 6 or 8 spokes then?

If they did you would then be asking - Why do cars not have 5, 7 or 9 spokes then?   

[Shambles]

Zero spokes is the way forward, and bikes are leading the way:

[Lakes]

Zero spokes is the way forward, and bikes are leading the way:

LOL Steve, saw that bike years ago, don't think it could take a corner with geometry like that LOL
so did anyone mention the number of spokes on Hy wheels?

[2i30s]

this looks like a job for the count from sesame street,3 spokes,4 spokes. 

[AlanHo]

You might have spoke too soon.

[Dazzler]

My Hybrid is an exception it has 10 spokes per wheel...

The CW has aftermarket alloys with an odd number though (15)

[AlanHo]

My Hybrid is an exception it has 10 spokes per wheel...

The CW has aftermarket alloys with an odd number though (15)

T'was me who spoke too soon................

[Shambles]

You're all "speaking outside the box", to misquote a rhetorical trope


That makes you all "outspoken"

[AlanHo]

And I started this nonesense - so I bags being appointed spokesman................... 

[Surferdude]

And I started this nonesense - so I bags being appointed spokesman................... 

Alan, given all the talk over the last couple of days about racism and political correctness,  you can be "spokesperson"

[Phil №❶]

So, what do they do with the points of a degree that doesn't divide evenly into 360 

[AlanHo]

So, what do they do with the points of a degree that doesn't divide evenly into 360 

They use decimals. Hence 7 spokes would be set at 51.4286° angular apart. No problem at all with modern toolmaking machine tools (or with an old fashioned dividing head for that matter).

[Phil №❶]

I thought they might just leave a gap in the circumference