Why only five lug nuts?

Who said 6 was better?

You are only holding on a wheel, just enough is all that is required. Trucks have more because of the unsprung weight, etc. ... but after all the effort Ferrari (and Lambo, etc.) have gone to to design nice and light components to keep their unsprung weight down, they are not going to add unnecessary wheel nuts are they?.

Pete

 

Torque is force times moment arm. A 13 inch rim is smaller than a 22 inch rim. A lateral force at the edge of a 22 inch rim will put more torque on the hub than the same force at the edge of a 13 inch rim.

The same kind of relationship applies to longitudinal force at the rim, translated to rotational torque (torque tending to rotate the wheel relative to the rim). The hubs that have a single large lug nut also have positioning/anchor studs about where lugs would be.

More details to consider: Smaller rims tend to have the lugs closer to the axis, where larger rims might have the lugs further out. (For example, the STi has its five lugs closer to the axis than the EVO -- both with 17 inch rims. But the STi has 225/45-17 tires to the EVO's 235/45-17s, FWIW.) Also, if you have a 13 inch rim with four lugs, and a 17 inch rim with five lugs, and a 22 inch rim with six lugs, you wind up with roughly the same distance between adjacent lugs, more or less.

I don't know if car companies do a lot (or any) torque studies on this, or whether they just follow a "rule of thumb".
Or maybe Dodge added a lug after they lost a wheel in testing.

 

well ... perhaps the Viper's engine weights as much as a complete Ferrari! lol

 

The max shear stress that is caused by the the maximum force acting on the lugs as a result of applied torque and weight of the car (i think they use 3x the weight acting on each corner, because 3g's is commonly used in automotive applications) cant be too high. So DGS, they definately look at if the lugs will experience a shear stress that is imposed on them by any forces or couples acting on the wheel that exceeds their max allowable, i think.
Im sure the real reason that the viper uses 6 lugs is because crystler didnt have any car hubs that were strong enough, so they just use ones from one of their pickups instead. Trucks have more probably because they weigh whole lot more, so instead of making 5 really big lugs to handle the shear stress, they just make a whole lot more small ones to increase the cross sectional area that the shear force will act one, which in turn lowers the shear stress. i dont think i worded the last few sentences properly.

 

But the trick here is that Torque Breaks things, HP burn things up. Lug nuts are not subject to being burned up, so it is TQ that controls whether or not more lug nuts are required.

The Viper has 450 lb-ft (now 500 lb-ft), whereas the 360 has 275 lb-ft, the 575 has 425 lb-ft. So the 360 is a long way away from needing 6 lugs, whereas the 575 might be closing in on the range where anothe lug nut would add to the package.

However, if you look at the size of the studs on teh Viper hubs, you will find that they are only 85% as thick as the lug nuts on the F355. Since tensile strength goes with the square of thickness and bending strength goes with the 4th power of thickness, a small increase in the bolt size goes a long way in dealing with the forces involved.

And then there is vibration. Here you really want an odd number of bolts.

 

Tim, you're exactly right on why Chrysler used 6-lug hubs.

You're also right that lug nuts (or bolts) are in shear stress, not tensile stress.

--Matt

 

Im not sure if its the wording you used. You may be trying to simplify what is going on here, which is misleading. The tensile strength does not go up if you increase the thickness. Tensile strength is usually given in the dimensions of force/area like psi or MPa. What happens when you increase the diameter of the lug, you increase the cross sectional area that is subject to any forces you might have (torque from engine, etc.) When you increase this cross sectional area, the shear stress is less.
Im not sure what you mean when you talk about the bending strength going up in the 4th power of thickness. Are you refering to the calculation of the polar moment of inertia?


It is ironic that im taking a break from solving a problem that asks me to find the max shearing stress in a each section of a fixed shaft connected by a flange which has a torque applied to it so i can read this thread. Well, back to work.

 

Actually it's a satanic kind of thing. That's why own a red one. Still looking for Liz Hurley though...

 

Nobody has mentioned the PCD yet...the larger the PCD, the less stress. Renault and Peugeot have both used only 3 wheel bolts in the past, but with very large PCD.

Maserati from the 70's use only 4 bolts on their big heavy cars, but again, a massive PCD. (The Ghibli has the cheesiest fake centrelock system I've seen....a beautiful fake nut with a small screw fitting and a plate which just covers up the 4 bolts!!)

 

I don't think I phrased that well. I expect that they run stress calculations when they design a hub (for a specific vehicle), but whether they re-run the calculations when the hub is used in another vehicle may depend on the budget and manpower available at the time.

I've noticed that most cars use about the same lug torque - the 328 specifies "10 Kgm", the EVO specifies 98 Nt-m - which work out to 73-74 ft-lbs, and most shops use 75 ft-lb as a "rule of thumb". But the Alfa (4 lug 14" rims), Ferrari (5 lug 16" rims), and EVO (5 lug 17" rims) all work with 75 ft-lb lugs (with three different sized lug nuts). Whether this is coincidence, a design parameter, a characteristic of the lug metal, or just a "rule of thumb" isn't, of course, specified.

But then, I've never owned a truck, so it might be different, there.

 

That's an interesting coincidence...

The rule of thumb recommended to me for a center splined hub knockoff is a whopping 275 lb-ft to 325 lb-ft! As I said in an earlier post, that's more torque than my 330's engine puts out through that same hub in fourth (1:1) gear!

 

I thought lug nuts are not supposed to experience sheer stress. They're job is to keep the rim clamped to the hub.

 

Good point. I totally forgot that bolts are generally not designed to see any shear stress, only tensile stress and that its friction that holds together whatever it is you are trying to bolt together. However when used in a car hub im sure there are some significant shear forces present anyway.