Why

[Salonika]

Just got done doing rear wheel bearings on a car. The hub and bearing assembly is all one unit. This assembly is bolted to the vehicle suspension with (4) 3/8 » bolts torqued to 37 ft lbs. The wheel hub side is a five stud, standard diameter (1/2 inch probably) studs which are torqued to 100+ ft lbs.
If it requires (5) half inch studs torqued to 100+ ft lbs to hold the wheel on, why is it that only 4 smaller bolts torqued way less is enough to hold the hub to the vehicle? Anyone want to take a shot at this one?

 

[NatersXJ6]

Lawyers.


Actually, mostly a guess, but the loads on the wheel lugs are much higher and less predictable as it moves and hits stuff and whatnot. The loads on the other side of the bearing unit are more predictable and as you can see, there is contact between the bearing unit and the axle housing to hold it… look at all the machined surface area and think about how hard they are to remove when rusted.

 

[Peter_n_Margaret]

The inner ones are handled very rarely and by mechanics.
The wheel ones are messed about with often and by ... ... ... ... ... (insert suitable expletive).
Cheers,
Peter
OKA196 motorhome

 

[nwoods]

One word, and its not Laywers, its Physics. The cantilever force at the outer end of the hub bears exponentially more force than the inside flange does.

 

[Michael Brown]

The smaller bolts also do not hold the entire force of the vehicle. Their surrounding surfaces and bores hold the weight. The bolts just provide a clamping force. Modern hubs on some SUVs now only use 3 bolts for the hub attachment.

 

[Salonika]

The smaller bolts also do not hold the entire force of the vehicle. Their surrounding surfaces and bores hold the weight. The bolts just provide a clamping force. Modern hubs on some SUVs now only use 3 bolts for the hub attachment.

I think you nailed it. I was looking at the setup on the car last night and thought the same thing about the bore. Same goes for holding the wheel on the hub, the bore is what takes the load not the studs. Wheel studs are probably bigger because they are used dozens and dozens of times in their life cycle. I was also thinking that those inner bolts really don’t hold that much, since the tires will slide on the ground surface before ever stressing those mounting bolts. But in a crash, or sliding into a curb, etc, I’ll bet they shear off pretty easily.

 

[MontySquareo]

Dodge trucks have had 4 bolts holding the front bearings in for a long time. I think the torque is like 100 ft-lb though, not just 37

 

[billiebob]

Might be different grades if steel.

AND the wheel studs are built to hold the industry standard wheels installed by the industry standard techs.... you really do not want to see special rims for EVERY vehicle..... Look at it the other way.... aftermarket rims are built to a pretty common standard.... everything inboard of that is a dedicated part and therefore much more finely engineered.

ps,,, Quite right....
the load on the wheel studs is less than the load on the 4 studs holding the assembly to the suspension.