why don't my wheels fall off?

Hello, I just repacked the bearings on my 93 ford ranger since I had to replace the disc brake rotors anyway. One thing really kind of disturbs me. The wheel seems to be held on by only the hand-ish tightened nut and the cotter pin. Are you trying to tell me that when I'm going around a turn with a load of crap in the back, the only thing keeping my wheels from flying off is that skinny little cotter pin and that not very tight nut? I mean, you have these five big lugs attaching the wheel to the hub, but only that nut and cotter pin attaching the hub to the axle. Am I missing something about how the tapered roller bearings work? Any light would be appreciated. At least my brakes don't pulse anymore.

thanks, andrew

The cotter pin doesn't play any part in handling the loads. It just stops the nut undoing. The nut takes the load and is actually well over specified for the task.

Assume a screw thread of 20mm diameter for the spindle and nut. That's means a cross section about half a square inch. For decent quality steel (nothing as fancy as a high tensile caphead bolt though) we can assume about 50 tons per square inch tensile strength. So that nut will hold maybe 25 tons. Maximum cornering force of an average road car is under 0.8g so the load on the wheel will be no more than 0.8 x the weight on that wheel. Take even a 2 ton car cornering at its limits and each corner will only be seeing about 0.4 tons lateral load. The spindle is operating at less than 1/50th of its ultimate tensile strength. It needs plenty of reserve to cope with long term metal fatigue and shock loadings but even so it has more than it needs.

So that's why the wheels rarely fall off a road car :) When they do it's usually because a bearing broke up because someone stupidly ignored the rumbling noises for too long and not because of the nut or spindle failing. In fact if you changed it every few weeks to avoid metal fatigue and didn't bump any kerbs you could theoretically hold a wheel on quite happily with a single

8mm high tensile bolt. I don't suggest trying it though.

Dave Baker - Puma Race Engines

You find somebody to love in this world you'd better hang on tooth and nail - the wolf is always at the door. (Don Henley - In A New York Minute)

The tapered roller bearings take almost all the load, there is hardly any load on that nut. It really just holds the bearings in place in the thrust direction. The tapered bearings if I remember correctly take loads in both the trust and radial directions. Of course for the outer bearing the thrust load is transfered to the nut, but it's not anything it can't handle.

Hi, Dave. Been spinning wrenches for 55 years--and found your explanation quite interesting. I was familiar with the more-than-adequate strength of the axle and nut, but had never attempted to see just how much safety factor was involved. Keep up the good info. sdlomi

Why would one replace the washer? It doesnt wear out nor is replacement called for. Just reuse it. The pin gets replaced of course.

The ones on my 30 year old car don't show any wear what-so-ever.

It *shouldn't* turn - that not only wears the washer but the spindle as well. If it's repacked regularly it should be easier to spin the balls or rollers than the inner race, so the inner race won't turn.

That said, I've seen the effect you describe on the rears of VWs semi-frequently. Those outer bearings seem to be undersized for some reason and are a semi-regular maintenance item. In fact I need new stub axles for my Scirocco because of the inner race wearing the stubs.

nate

I was driving down the expressway a couple years ago and smoke started pouring from the right rear of a Ford Focus. Soon it was obvious that the wheel was coming off the hub. Then I heard about the recall for that problem for the bearing failure.

replacement

Yes it should turn. The spindle is dimensioned so that both inner races can turn. Don't the bearings slip onto the spindle easily? What keeps the inner race from turning? Other wise the race, especially the smaller outer one will wear in one spot. Sometimes spindles do wear if not made of quality material. There should be no need to regularly re-pack a properly greased and assembled hub.

Chas

That would be the -outer- race.

When the whole enchilada is considered as an assembly (which it is) it is.

That would be the -inner- race.

True and correct.

Yes I know what a race is. There is an outer race, sometimes refered to as the cone. And the inner race, which also has the caged rollers. The washer provides no "pressure" against the bearing because there is a clearance between the two. Some of the more anal repair manuals will specify that clearance. If you examine the spindle on a properly functoning assembly, the spindle will show signs of the inner race turning. If it doesn't turn the bearing is subject to a flat spot forming. Now go play your word games with someone else.

Chas Hurst wrote in article ...

Apparently, you do *not* know what a race is....

The outer race is refered to as a "cup" while the assembly composed of the rollers, cage, and inner race is trefered to as the "cone."

These names come from the approximate shapes and relationship of these items.

Inner race or cone spinning = instant burned up spindle.

Outer race or cup spinning = instant burned up hub.

That is how my Jeep for one example worked....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Chas Hurst wrote:

Thats exactly why I use the term "walk" instead of "turn" or "spin."

The inner race doesn't spin unless something seizes, but it SHOULD *very slowly* rotate on the spindle as the rollers repeatedly roll around it. One rotation of the race per many THOUSAND turns of the wheel is normal. If it doesn't do this, it will fail in short order because the full weight of the vehicle is always carried on one small spot of the race. It is this very slow "walk" or rotation which cause the very mild "polished looking" wear pattern on the spindle and on the retaining washer where it contacts the race.