FJ60 front axle on rear. Why not? (1 Viewer)

[rickyrockrat]

So I keep hearing about how you've got to have a really strong axle in the back...But don't know why.

The front axle has more weight, and has to withstand more stress (i.e. E-brake on highway) than the rear axle.

As to axle shaft, they don't look any smaller than the full-float axles (and full-float axles are stronger anyway).

The last thing I can see is that there's no way I'll get much steering angle on the rear...maybe half, but more like 1/4 the angle of the front, which means the rear birfield will never be anywhere near the angle of the front.

I'm going to have to look at a birfield angle vs torque efficiency table, but I suspect the more you turn, the more force per angle it takes to rotate the same amount of torque.

Application is a SOA FJ60.

Thoughts, anyone?

 

[StevoJonesFJ60]

Sounds like a good idea, the only thing I would worry about is the loading that it would experience while going uphill. But if you got some longs for it I don't see that being a big issue.

The application is a little vague, is it going to be a crawler? what size tires do you want to run?

 

[rickyrockrat]

So everyone keeps talking about loading...but again, how is this somehow more than what the front experiences - given the turning restrictions? We all know full-float axles are stronger than semi-float, and the front is most certainly a full float, especially if there's no turning.

Yes, it's somewhat of a crawler. I've got 33's, and might one day do 35's. I'm planning on 4.56 gear ratios (it currently has stock 3.70). 4.56 would put me a little lower geared w/ 33's than stock tires/gears.

I've already busted a front birfield, and replaced the front with lifetime chromalloy shafts/birfs - but didn't intend to go chrome on the back. Again, I think my limited turning angle will put a lot less stress on the back....But I need to find torque curves to see if I'm right.

 

[Tapage]

We all know full-float axles are stronger than semi-float,

not necesary .. FF are designed to carry more load .. but not necesary to have strongest shafts ?

then FF fron in the rear you have birf variable but also knuckle ( king pin bearings ) point of failure .. and the weight transfer crawling any decent hill will be interesting to measure in the rear axle compared to the front ..

 

[StevoJonesFJ60]

Good point about the bearings Tapage. In order to prevent blowing bearings all of the time, I would run the trail gear trunnion bearing eliminator kit.

 

[StevoJonesFJ60]

And about loading, I made a quick doodle.

The first picture shows the truck sitting or riding on a flat surface, the blue arrow represents the weight of the truck, whereas the red arrow is the normal force.

The second drawing represents a truck on a hill. You can see how the weight is facing down, while the normal force is related to the angle of the incline. The steeper the angle, the more weight is transferred to the rear axle.

 

[rusty_tlc]

Save your money and buy a dual case, or selectable lockers.

 

[incogrhino]

what if it don't cost nuttin.well that kit would cost.I got parts. I did just read a write up about doing it, and the guy said he'd never do it again.Mini truck setup i think.I read lots.

 

[mookie]

I've read heard or read about a few people who tried it and it didn't hold up for any of them

 

[Fast Eddy]

The diff is on the wrong side. Not sure if it's possible to just clearance for the ring gear and flip it over. You'd have to deal with the breather hole and C&T the ends.

 

[rusty_tlc]

what if it don't cost nuttin.well that kit would cost.I got parts. I did just read a write up about doing it, and the guy said he'd never do it again.Mini truck setup i think.I read lots.

There is no such thing as a free mod.

 

['74 UA FJ]

You would have to put chromolys in it for it to have any chance of holding up in real wheeling situations. There is a LOT more weight/torque on the rear axle when climbing hills than there is on the front. If you wheel on flat ground all of the time (who does?), I don't see it being an issue.

 

[leftarmtan]

Take it from someone who has rear steer,Its a lot of force not only on axle shafts but steering parts and knuckles.light weight would help.

 

[rickyrockrat]

I see the drawing on the force going up hill. What about the force in an emergency braking situation on the high way - say we hit the brakes doing 75 on dry asphalt. Are you trying to say there's less weight on the axle than in a vehicle going up hill?- I don't think so - there's just no way to put that much force on the back unless you're backing up and hit the brakes...

The axle is not on the wrong side - it's a 180 cut and turn, and it's the same pig both front and back (i.e. front and rear are swapable on a stock FJ60 - I know, I did it.

 

[rickyrockrat]

Take it from someone who has rear steer,Its a lot of force not only on axle shafts but steering parts and knuckles.light weight would help.

How much turning do you have on your back steer compared to the front? In other words, does your back go through the same angle as the front?

let's say for the sake of the argument that you decide to put a front axle on the back but just weld the link to the axle so it's always straight. In that configuration, are you breaking axles? Is the front axle just weaker than the back?

As to the trunnion weight, I still go back to an e-brake at speed on the highway. 90-95% of your braking force is on the front, plus the weight of the vehicle which shifts forward (just ask anyone like myself who has endoed a street bike) and is multiplied with your deceleration force should surely exceed anything the trunnions would see just going up hill.

I'm sure I'm missing something, but I don't know what.
Here's a link to a physics problem. It shows that the entire weight of the car is on the front axle at e-brake time - according to this, a 11kN (2473 lbs) car weighs 15.5kN (3485 lbs) while braking, and that is a car low to the ground. The forces forward will be even worse on a stock LC, since it has a higher center of mass.
http://www.physicsforums.com/showthread.php?t=551553
Another:
http://phors.locost7.info/phors01.htm
I need to run these numbers for a stock LC. I wish I knew the center of gravity height.

 

[Mace]

I see the drawing on the force going up hill. What about the force in an emergency braking situation on the high way - say we hit the brakes doing 75 on dry asphalt. Are you trying to say there's less weight on the axle than in a vehicle going up hill?- I don't think so - there's just no way to put that much force on the back unless you're backing up and hit the brakes...

The axle is not on the wrong side - it's a 180 cut and turn, and it's the same pig both front and back (i.e. front and rear are swapable on a stock FJ60 - I know, I did it.

In an emergency braking situation you are putting all of the forces on the spindle and knuckle. nothing on the axle shafts to speak of. Going up a hill and possibly bouncing, you are putting all of the stress on the axle shafts. Also, during a panic stop, you cannot exceed the traction limits of your tires. When going up a hill, you have potentially thousands of ft lbs worth of torque trying to spin the rear tires.

Apples to oranges.

The 3rd member is the same, the axle housing if you put it in the rear would be offset to the wrong side. If you flip the housing, you will either have to do some work to the housing to leave the pumpkin in it's standard low pinion orientation, or get used to having 4 backwards gears and one forward gear.


To put any sort of useful rear steer on a 60, you'll have to modify the body significantly. If not, you are simply not going to get any turning radius.

And the birfields won't hold up.

 

[rickyrockrat]

Actually, it's an SOA, so the body is not the issue - I've got lots of clearance on the 33s. The spring, frame, and gas tank are the problem. I never intended to get full steering out of it - at most I had planned on moving the spring mounts to go inside the frame, then I just have about 4" of steer on the rear.

I understand the torque on the axle, but since I'm only maybe going to get 5-10 degrees angle out of it, I didn't figure it would matter a lot?

What I don't understand is why people keep talking about trunnion bearing holding up. It seems to me they should be plenty strong - in that case it's apples and apples, and the e-brake analogy holds, I think.

I'm not sure I understand...the pigs on front and rear are already low pinion...so that's a wash- or did I miss something? Cut & turn the axle (which I had to do with my front SOA already), only now it's 180 instead of 15 degrees.

 

[Mace]

The forces on the rear axle during 4wheeling are significantly higher than on a front axle during a high speed braking situation. That is why the trunions fail quickly.

Make a popsicle stick set of axles and move them around to see why the third location matters.

I doubt you can even get 5 to 10 deg of steering before it binds even with the so.

What are you planning for steering?

 

[leftarmtan]

How much turning do you have on your back steer compared to the front? In other words, does your back go through the same angle as the front?

Mine steers much sharper in the front,48 degrees in the front maybe 32 degrees in the rear.To be honest I spent alot of time and a fair amount of money getting the front to steer that sharp and most of the time its just not necessary.A little bit of rear steer make a huge difference in maneuverability.

What your saying makes sense,but all I can say is wheeling puts stress on things in different ways than they were designed for.I say go for it but consider longfields and possibly fj80 outers
[URL="http://inlinethumb46.webshots.com/41517/2685273240100142503S500x500Q85.jpg"] [/URL]

 

[rickyrockrat]

What are you planning for steering?

It's real simple - Hydralic cylinder with lever style control, with a potentiometer feedback to tell me where the thing is.

I also plan on having a pin to lock it straight for highway use, and truth fully I don't know that I'll actually steer with the rear much - only in tight corners.