While crawling under my 4WD truck today I got curious how a single u-joint could handle the steering angles encountered for a front 4WD axle. I’m referring to part 47 in the diagram. It seems like the physics would dictate you’d have to double up a pair of u-joints. The rear driveshaft has a pair of u-joints on the transfer case end, and the angle there isn’t nearly what the front axle has to handle. What am I missing?
Compare the turning circle diameter of that 4WD with the 2 WD version @George_San_Jose1. My best guess is that it’s quite a bit tighter making a U turn in the 4WD model.
That’s how all front solid axles are with 4wd. The stub axles are just changing angle. Two u joints are needed when there’s also vertical movement, like between the rear axle and the transfer case. There, the only way to maintain angles is with 2.
I understand what you said, but I don’t understand what you mean @texases
@Rod_Knox … There’s no u-joints in the front of a 2WD truck , b/c the front wheels just free-wheel on the spindle.
Do you need two U joints to hook up your spark plug socket to your wrench? Nope. FWD cars need two because with independent suspension the wheels move relative to the transmission. This doesn’t happen with a solid axle.
I’m still not getting it. Ok, I admit I’m a little auto-design dense … lol … The wrench’s turning axis is directly in line with the spark plug, so there’s no need for any u-joints for that application, or at most one. The turning axis coming out of the differential is definitely not in line with the turning axis that turns the wheel when the steering wheel is turned at wide angle.
When you do need a u joint for a spark plug the geometry is identical to the solid axle.
Read here- https://en.m.wikipedia.org/wiki/Beam_axle
1/4 of the torque (1 of 4 wheels) If the steering wheel is turned quite far, the speed is limited so the speed pulses caused by the joint is minimal. Turning radius is much greater than 2WD to reduce the maximum angle. Lastly, serious 4WD folks know not to hammer the gas in 4 wheel low with the wheels turned tight or the joints will fail!
Ok, that makes sense. The front axle u-joints at the wheels at extreme steering angles do indeed present a design problem, but as a practical matter the driver tends to prevent the damage by using common sense. Like you say, I don’t hammer the gas in 4WD with the steering at the limit, but I didn’t realize I was doing that to prevent the u-joints from breaking. I limit the power b/c the truck seems to be complaining by the squirrely way it handles at extreme steering angles in 4WD, and especially 4WD low. I think it would be possible actually to roll the truck by applying too much power at extreme steering angles, even starting from a pretty slow speed. I don’t have the sense the turning radius is more limited in 4WD than 2WD, but I’ve never done the side by side comparison.
I’m not sure how much the spark plug ratchet extension/u-joint-at-the-socket analogy works. You don’t have to battle the u-joint speed impulses in that situation b/c you are stopping and starting the ratchet all the time. Plus you can usually change the angle of the extension to the u-joint a little if the motion gets awkward. Often in fact when using that tool the u-joint is at 90 degrees, and that presents no problem at all as long as the extension has room to rotate fully around the socket axis. In other words when the u-joint is at 90 degrees you remove the ratchet & use the extension like a breaker bar. That’s not possible to do with the axle configuration.
And it’s likewise not possible to steer so that the front wheel is any where near 90 degrees with the axle.
It’s an analogy to get you thinking about how it works not an identical example. He said geometry is identical not that it was an exact replica to be compared under all possible conditions. Dang…
That’s the exactly what I was trying to ask about. A single u-joint works perfectly at 0 degrees steering angle, and won’t work at all at 90 degrees. So at some steering angle between 0 and 90 degrees, a single u-joint just won’t work. Just by observation I’d say the front wheels can be steered to 20-30 degrees. Maybe more than that. This same angle problem occurs in the rear driveshaft, which has to accommodate what appears to be a considerably smaller angle between the transfer case and the rear axle. And the rear driveshaft is designed w/ double u-joints. So it’s an obvious question (to me anyway) why double u-joints are required for the rear driveshaft, but single u-joints work ok for the front axle. @Mustangman above I believe has provided the plausible explanation.
The steering angle does not change when operating in 4WD vs 2WD. The 4WD models have limiters built into the steering system.
Would you agree the maximum steering angle – whatever limits it – is greater than the maximum angle the rear driveshaft has to accommodate?
I would agree. Steer angles are usually in the 30-35 degree range.
To get that in a drive axle, the lift kit on a short wheelbase vehicle would be rather high! Jeep CJ-5 maybe. But you can rotate the pinion upwards to reduce those angles.
They need two because the distance between the axle and the transmission varies, and because the most efficient way to transfer power in that situation is for the transmission output to be parallel to the axle input. I guess you could have a giant swing axle with one u joint at the transmission, but that would not work with leaf springs.
Good info above, thanks. There’s one other detail I got to thinking about that might help the single u-joint design make more sense. The manual locking hubs have a clutch mechanism apparently. Refer to the part called the “inner clutch ring” in the diagram below. Perhaps that part is there to prevent the u-joint from breaking at extreme steering angles? It looks like it allows the axle shaft sleeve to slip before bind-up occurs.
While they call that piece a clutch I’m sure that there’s no way it can slip @George_San_Jose1. It might chatter and the lands skip out of the grooves if well worn. They call that mechanism a locking hub and it should be locked up when engaged.
Maybe it is a “clutch” in the sense that it makes it easier to manually turn the actuator knob. Consistant w/that theory, in the hub diagram that has no free running selection option, there’s no parts identified as a clutch.