Engineers!
who can define and explain hypoid drop? and also what would make one setup have a higher hypoid drop than the other?

I will try to help out. A hypoid drop is the pinion below the ring gear centerline and a Amboid drop is a pinion above the ring gear centerline. The only way to move the pinion up or down is to change the angle on the gear. I hope that helps out some.

To expand on Don's post, a 9" (for example) is a "high hypoid" because the pinion is much lower than a dana axle. The gears are almost a right angles to each other. This is less efficient, but slightly stronger, than a typ. dana arrangement.

Ok so why would it be stronger? I assume because more teeth are engaging at any one time?
would a lower hypoid drop also be more efficient due to the lesser amount of friction due to less of the gears engaging?
any good diagrams that explain this?

Originally posted by Gozuki
To expand on Don's post, a 9" (for example) is a "high hypoid" because the pinion is much lower than a dana axle. The gears are almost a right angles to each other. This is less efficient, but slightly stronger, than a typ. dana arrangement.

I think that the 9" is stronger do to the extra bearing on the pinion and not because of the angle. I am probably wrong but I thinking that is the reason. A lot of the Caterpillar axle in the wheel loaders run the pinion right on centerline.

The 9" gear teeth are stronger because they are less helical, IE almost linear. There is more "wipe" accross the gear face. This adds more friction (less efficient). The 3rd pinion bearing is needed to prevent the pinion from being pushed or flexed from the ring under load.

could I assume that an eaton ho72 rear axle would be sronger than a 14 bolt in the hypoid drop situation?

I wrote this a while back in reply to a post on D60 vs 9", I tried to explain it there....

"On comparison of the pinions one of the first things you will notice is the additional pinion support extending from (in this picture) the lower end of the pinion. This feature helps reduce the level of stress on the pinion. "

They are forgetting something here...

You may have noticed your tube of gear oil saying "hypoid gear oil" A hypoid gear is what separates your standard differential gear from a regular bevel gear. A bevel gear has the centerline of one gear intersecting the centerline of the other. In a hypoid gear they are not. This is why you have "high" or "low" pinions.

The ford 9" was designed for CAR applications, and the lower the pinion (and more "hypoid" the gearset) the lower the driveshaft could be, which meant the tranny tunnel could also be lower, making the interior of the car more comfortable.

This sucks 2 ways:

1) it hits rocks

2) It puts more force on the gearset pushing it away from the ring gear, this. This is why you see so many pinion bearing failures. It is also why a 9" NEEDS a 3rd pinion bearing. It is more "hypoid" than any other common gearset, so its pinion bearings are under more stress than any other gearset out there.

Yeah they are strong, and well designed to overcome their shortcomings, but they are still inherently designed to work in a car, not a truck.

Of course, a currie high-nine kit resolves some of those issues for short wheelbase, or front drive applications.

What the last post said plus:

Bevel gear is all rolling action
Spiral bevel gear is still all rolling action (strongest, most multiple tooth contact)
Hypoid gear has some sliding / some rolling due to the offset

Ken