Breakover angle -- how do they figure it?

So on the Toyota website they list the 2010 DoubleCab shortbed with stock 30" tires as having a breakover angle of 21 degrees.

But I took the wheelbase (128"), ground clearance (under chassis at center of wheelbase - 14")), and tire diameter (30") and drew it all up super accurately in CAD and measured the resulting breakover angle to be 26 degrees.

I've heard that Toyota's claims of breakover angles for the FJ Cruiser are also kooky compared to real life, too -- but in the opposite direction (they claim a greater breakover than reality suggests).

Why such a disparity?

 

Not sure... maybe they used the lowest point between axels instead of the center of wheelbase?

 

Sorry....I wasn't very clear in my OP. I know how breakover is *supposed* to be measured, I just don't know where Toyota got 21 degrees for their breakover for the '10 Taco DC shortbed. Doesn't compute by my calculations.


FWIW, here's the discussion I refered to with the FJ Cruiser -- some interesting thoughts:

http://www.fjcruiserforums.com/forums/suspension-tech/55547-all-4x4-math-gurus.html

 

Hmm... I guess that could be how they did it. Seems kinda kooky though. The way I see it, breakover angle is 100% useless in the field and is only useful as a benchmark for comparing the potential offroad capability of different vehichles when they are sitting in the lot. And if the manufacturers don't all figure the angle using the same convention (i.e., ground clearance at midpoint of wheelbase) then what's the point of figuring it at all?

I guess it probably is more of a marketing thing than anything... but seems like if you are going to use the number for marketing, wouldn't they try to cite the highest angle they could get away with rather than low-ball their estimate by 5 degrees?

 

There's only one angle you can use for it. And I'm sure they all use the same formula that the federal gov. uses.

"Breakover angle means the supplement of the largest angle, in the plan side view of an automobile, that can be formed by two lines tangent to the front and rear static loaded radii arcs and intersecting at a point on the underside of the automobile."

Like someone pointed out somewhere, it doesn't say "midpoint" in that definition. It doesn't need to though, because it's implied and is the only way you can get the correct angle. On that yotatech link for instance in the guy's "B" scenario; that' isn't the correct angle because if the truck tries to move forward it won't be able to and would get high centered...


Also if anyone is interested, the smallest sphere your truck (stock DC SB) can drive on has a radius of about 13'5"

 

Exactly! Which gets back to my original question: how did Toyota come up with 21 degrees if I come up with 26+ degrees? Not that it really matters that much ... I'm just curious.

Yes, I am interested, actually. I was comparing my stock truck's dimensions to my buddy's LR Discovery because he was bragging to me that his Disco is "way more capable offroad" than my truck -- but it is interesting that my breakover is 2 degrees better than the Disco's but the Disco's minimum sphere radius is like 12 feet. Which illustrates that breakover angle isn't necessarily more important than wheelbase afterall.

Either way, 26 degrees for the Taco vs. 24 degrees for the Disco, and 13.5' radius for the Taco vs. 12' radius for the Disco comes down to negligible in the field.

 

You can't argue with gravity I guess......

 

Well only thing I can think of is because of the path of the breakover angle. Now the only place we're calculating is at about 14 inches which is right in the center of the truck at the bottom of the frame. Technically that's fine and dandy if we're just setting the truck up onto something, but not if we're driving over it. In front of that is the tranny cross member which hangs lower than the rest of the frame. As you drive over it the tip of the breakover angle travels in I want to say an elliptical (or circular? some type of rounded) path between the wheels. At the path using 14 inches that cross member might interfere as the tip of the angle comes around requiring the angle to get smaller. Not sure if I explained that well, I could be wrong, I've been wrong before.

 

I think you are right. It doesnt make sense to use 14" when there is a part that's lower. It makes more sense to use the lowest point between the axles.

 

Just something to ponder but maybe they figured it out at max load capacity?

 

No, you explained it great and it's a good thought. Maybe if I get bored enough tomorrow I'll draw in the frame crossmember and exhaust and other parts that hang lower onto my CAD thing and model the vehicle going over the 26 degree break to see what happens. Yes, I am that much of a geek.


Kyouto - that is a good thought I wonder how much ground clearance is lost at max gross.

 

I'd imagine a pretty fair amount, I think the bed comes down almost 3-4" with non tsb leaf pack...

 

I guess try building a ramp on your CAD software and see what the biggest one it can go over without hitting anything.