Formula for number of bolts/bolt-holes required on a wheel

[sprashanth]

How do you determine the optimum number of bolt holes or bolts required on an automotive wheel (like the wheels of cars or trucks), when you only have data on the tyre size, the corresponding wheel size and the load rating for the wheel? And for instance, why is it that tractor wheels have usually 6 or 8 bolt holes - while those for buses have 10 or 12 bolt holes?

I'm guessing this involves calculating of Tensile and Shear stresses on the bolts based on the wheel load, but what are the exact formulae I need to use in this regard & what are the types of forces to be considered? Is it due to tyre friction or cornering effect? Can someone help me out on this, since I haven't really worked on this type of problem before.

 

[ubrben]

So for a racing wheel with drive pegs and a centre lock hub the pegs are just for radial location and all the forces are reacted through friction?

If that's the case then you should only need three pegs seeing as the clamping force is coming from the nut. Most cars have 6 pegs - is this just redundency or a (poor?) assumption that they're transmitting the torque?

I have seen drive pegs ratchet back and forth and lock the wheel on. I assume this is probably due to insufficient tightening torque due to poor air pressure in the wheel guns.

Ben

 

[Tmoose]

"a (poor?) assumption that they're transmitting the torque/"

The folks with early VW sometimes go to great lengths to augment their cranks' 4 dowels with stouter gland nuts permitting greater torque, then to 8 snugly fitting line reamed dowels as used in pre-914 4 cylindrical Porsceae. Racers then go to 8 plus a wedge fit (Gene Berg) or even to a proper crank flange (SCAT).

flywheel attachment history in pictures.
early British (MGT?)

http://www.mgcars.org.uk/mgtd/Pictures/Reports/tm_crankshaft.jpg

Jaguar XKE

http://www.limora.com/clickable/gross/Bilder/2608.gif

Chevrolet 1955 265 CID V8 or 1968 450 HP 454 CID V8

http://images.1aauto.com/CLF/1ACLF00024.jpg

(yes there is one dowel to ensure consistent positioning, but some crankshafts come with that hole empty)

Chrysler 426 Hemi

http://stores.casselent.com/catalog/130-10231_thumb.jpg

lexus V8 flywheel?

http://www.thor-racing.co.uk/images/1UZ-FE-to-r154_Flywheel.jpg

Civic SI/ del sol flywheel

http://www.slowboyracing.com/sbs/images/T/ACT 600105 Flywheel-01.jpg

If there's enough clamping force to keep the dowels or bolts from any real torqueage loading, there's enough. If not, there's not, and the situation gets real bad real quick.

 

[IceStationZebra]

A pure guess - but they could be using 6 pins as insurance if the tire man doesn't torque the nut quite enough during a tire change. Plus, if the pins were line tight with the holes it would be more difficult to install the wheel. (And you have to deal with heat expansion - hot hubs and cold wheels)

A third possibility is that with only 3 pins the tire man might have to rotate the wheel 120° to hit a pin - but that doesn't prevent one from having only 3 pins but 6 holes in the wheel!? In reality the first person to do this probably figured that if the road going car needs 5 studs the race car will need at least that!

ISZ

 

[Bribyk]

My FSAE outboard suspension design used a single aluminum center lock nut (torqued to just 60 ft-lbs) and was hub centered (I don't like using a bolt pattern to center anything). We included four 1/4" dowels to prevent the wheel from freewheeling on the hub, should someone forget to torque it, allowing the car to be stopped and preventing excessive hub/wheel damage. We used four dowels purely because most of the available conventional wheels use four studs/nuts and it would allow us to adapt the hub to these wheels if custom wheels were not available. No issues were found with the design but the dowels haven't been tested yet. FSAE cars don't have to make timed pit stops so this did not figure into the design.

 

[Tmoose]

"My FSAE outboard suspension design used a single aluminum center lock nut (torqued to just 60 ft-lbs)"

what size thread is the center lock nut?

Dan T

 

[Bribyk]

It was about 1"-1.25" fine thread, from memory. We used a similar approach to the napkin calcs above with our anticipated wheel loads. Keep in mind this is a 500lb asphalt racecar (low safety factor) with a short design life. Using the above approach gives ~1600 lbs to break the static friction on each wheel hub and our design bump load was 3.5g (or about 500lbs per wheel). We didn't want to torque the nut down more than required as the threads were aluminum and could gall (they were anodized to help prevent this though).