why is centering important on axles?

[superj]

i just read the post about shortening the axle to fit a kit car and it makes me wonder why is it important to center the diff for somethings when they are not aligned to begin with? they have u-joints or cv joints to allow for off sets and differences so why do people say its important when doing stuff to center it exactly?

some vehicles came with off set diffs, of course because the trans wasn't dead center or the front t-case, etc, but if the axle already is going to be lower then the output of the transmission, why does it matter if the diff is over to one side or the other?

the joints won't know its off set, they just see the plane they are working in and as long as the two joints are within the acceptable range to avoid vibration, shouldn't it not matter if the diff is 0 degrees behind the output, 10 degrees to one side, or off set and down?

i am asking because i cannot figure out why its so important? planes are not just up/down or left/right

does someone know why?

 

[ericbphoto]

I don’t know the exact answer to your question. But from my mechanical experience, every time you increase the offset or angles you increase the amount of movement the u-joints must go through as they perform. This increases vibration, noise and wear. Ideally, we would make all that rigid and have a solid, rigid shaft from transmission to differential with no u-joints. But that is not practical. Does it work with some lateral offset? Yes. But it should be minimal.

there are some excellent videos on YouTube that show the dynamics of a driveshaft with u-joints. With too much angularity, the speed of the shaft is not constant throughout a revolution. This even robs you of power.

 

[superj]

i know if its off set too much than it exceeds the limits of hte joints and wears them and you get vibes but how does it matter if its offset to the side instead of up and down? thats what messes me up. people lift jeeps a bunch and hten have these extreme angles and have to buy cv joint driveshafts so the angles are way off versus normal but when people built hotrod cars, they want to center the diff? the off set diff is probably offset way less then the angle on jeep drivelines so wouldn't it be ok to be off set a bit when using a different rear axle?



it probably doesn't really effect it that much but maybe its more of an aesthetics thing?

 

[Shran]

Sometimes it's done to provide clearance for other things... gas tank, etc. Jeep CJ applications, some old Toyotas, etc are pretty extreme.

If it can be centered that could make things easier.... I'm sure it's sort of a matter of building the vehicle frame and body and then building an axle that fits under it.

 

[snoranger]

Couple of things here:

Any amount of angle to the u-joint puts more load on it. A centered diff and trans will help it handle more torque. That could be part of the reason hot rods have centered diffs.

When U-joints that are offset in 2 different planes (both verticals and horizontal) it promotes needle bearing rotation. That means it won’t wear out some of the needle bearings, but not the others... they all wear evenly. This leads to a longer life.

U-joints that are at an angle speed up and slow down as they rotate... this is why they have to be phased the same. If there not, then the speeds don’t match causing uneven loads. This leads to vibration.

 

[rusty ol ranger]

Itll vibrate itself into oblivion

 

[scotts90ranger]

On the Ranger for example since most people here have them... the diff is offset to the passenger side to clear the gas tank, I think the Explorer axle might be more centered since my rear driveshaft hits the gas tank skid plate on the '90 at times... It's all about clearance, the old Land Cruisers had both diffs offset to the passenger side for whatever reason...

Gotta keep the U joint angles reasonable, unless the driveshaft is really short then the angles don't change too much as the suspension cycles.

The shorter the axle shaft the more likely you are to break that shaft as opposed to the longer one, the longer shaft can twist and rebound more before breaking...

 

[gw33gp]

I know that FWD vehicles that have an offset differential have a problem with torque steer. This is felt under hard acceleration. Having one axle shaft longer than the other will result in more twist ( as indicated before) in the longer one. This ends up inducing torque steer initially as hard acceleration starts.

I am guessing this would not be as noticeable in the rear axle, but in a high hp situation could be dangerous.

 

[gaz]

superj,

I had 2 version of the same car. One was turbo charged the other was naturally aspirated (na). The engines, transmission and drive lines were all in the same locations though the right drive shaft was almost 2x the length of the left. In the na version this presented nearly no issue but in the significantly more powerful turbo version the longer drive shaft of the right produce what is know as torque steer, it made the car want to drive in the direction of the longer drive line. The manufacturer resolved 98% of the torque steer by adding CV in the middle of the right drive line and supporting it in a psuedo bearing/bracket. This allowed for the 2 outer sections to be the same length while the right side had the torque producing intermediate section extra torque nearly nullified.

If I am trying tighten a fastner to a high torque value or remove that same faster, at say 580 foot pounds if torque, some interesting forces come into play. For example, if all I have is a 12" torque wrench, the isn't getting done; when a 6 foot ¾" Dr torque wrench is introduced, even a small person can make enough leverage to accomplish the work.

In the case of our beloved drive lines being different lengths, the longer line, just like the 6' wrench make greater torque. In a race application or in a situation where the driving surface is loose, unequal length drive lines are going to create different levels of torque, which will make the vehicle pull to one side more than the other.

 

[ericbphoto]

but how does it matter if its offset to the side instead of up and down?

It doesn't. Offset is offset, whether it's horizontal, vertical or diagonal. The U-joint just knows that there is some angle between it's input and output. Any angle will cause wear, even angles that are below the max design spec of the joint. We just try to minimize it as best we can to prolong the life of the parts. When we start putting extreme angles in the driveline, we follow certain rules. But ultimately, we accept that the parts are going to wear faster or break easier because of the stresses we have introduced.

 

[bobbywalter]

Define center. Depending on goal You do not want it center .

 

[scotts90ranger]

True, I didn't think of that. (assuming I got what you were going for...).. if you have the diff offset to the passenger side some it will counteract the torque of the driveshaft turning clockwise (looking at the input U joint) adding more down pressure to the drivers side tire, a longer lever on that side will lessen the effect, explains why the passenger side tire is always the one to spin out first since it is being lifted...

 

[RonD]

You want it in line with the Drive if possible
Front 4x4 differentials are in line with transfer case front Drive, not centered in vehicle
Rear differential is in line with transmission/transfer case output Drive
Rear axle differential is often off-set an inch or so to the right because engine and trans is off-set to the right, to allow clearance for steering shaft and in Rangers case the gas tank

Best practice is "parallels"
Drive and differential should be parallel from the side, like this: https://jniolon.classicpickup.com/drivelinephasing/phasing.jpg
This is so both u-joints have the same angle

If you have to have off-set Drive and differential looking down, then that should also be parallel, like B here: https://www.jalopyjournal.com/forum/attachments/upload_2019-8-20_11-38-16-png.4393758/
So u-joints have the same angle

Nothing wrong with left and right off-set as long as its done right, but there are limits, and people run into that when they LIFT vehicles, lol.

 

[don4331]

It doesn't. Offset is offset, whether it's horizontal, vertical or diagonal. The U-joint just knows that there is some angle between it's input and output. Any angle will cause wear, even angles that are below the max design spec of the joint. We just try to minimize it as best we can to prolong the life of the parts. When we start putting extreme angles in the driveline, we follow certain rules. But ultimately, we accept that the parts are going to wear faster or break easier because of the stresses we have introduced.

Careful @ericbphoto, no offset is actually worse than a little. Without any offset, the needles within the u-joint don't move and as a result the lubrication (grease) will breakdown, then the needles will fail and with them the u-joint. BTW, your video is very good at explaining the issue. If the front (transmission/transfer case) and back connection (axle) are not parallel both vertical and horizontal*, you will get vibration as the driveshaft attempts to speed up and slow down every revolution**.

@superj, On our drag car, we were looking for equal length axles, so when the last yellow light lit and our driver dropped the hammer, both axles twisted the same amount and the car launched straight as the green came on (similar to the torque steer issue in previous post). This actually results in the pinion being offset about 3/4" as the pinion doesn't line up perfectly with the center of the differential (Ford 9" axle).

@scotts90ranger, Explorer axles have the exact same offset as Ranger axles, both the drivers and passengers axles are longer by 3/8" - that's why we can swap them in so nicely. On other hand, Bronco II only has longer axles on passenger side, so axles doesn't swap in as nice. Most Mustang axles of same time period as our Rangers are equal length to ensure the car accelerate straight, so they wouldn't swap in either (but 2 driver's Ranger axles was simplest way to get 5 bolt rear axle in 'stang).

While you have the results correct, the passenger side tends to spin more because:

It often has several hundred pounds less weight (no driver or tank of fuel)​

The shorter/stiffer axle, means the tire is starting to turn while the longer shaft on the driver's side is still winding up.​

Lastly, the driveshaft is providing a rotational torque, so the the driveshaft applies 300 ft-lbs. to pinion, the drivers axle being 30" long, the wheel gets pushed down 120 lbs. force, but the passenger side at only 27" long gets lifted with 133 lbs. force.​

With lower weight, delay and more force, the passenger side spins 1st.

@Shran, for the original Jeep/early CJs, the front and rear differentials were lined up, so that if you missed the obstacle (rock) with the front differential, 99% of the time, you missed it with the rear. It also saves a gear in the Dana 18 transfer case - that matters when you are building hundreds of thousands. It is offset to passenger as driver's tend to preserve themselves at expense of someone in the other front seat - passenger always has option to vacate, driver not so much.

Unfortunately, it isn't as efficient to have 3 gears constantly under load to power the rear axle, so the later Dana 20/300 & np 231, run power out center to rear axle, leaving only the front axle offset, as it is difficult move the engine/transmission far enough (and to do so causes other weight issues). Power (drivetrain loses) only need be applied when 4x4 needed.

*It is possible to offset both the front and rear connections by exactly the same amount and get good results - technically that is what a dual cardan u-joint aka CV joint is doing. But it is very difficult to achieve in practice - Ford does it with 2 piece driveshafts in early SuperCab Rangers, and lots of members of this forum have had vibration issues after modifications (either lifting or lowering).

*Note: lighter driveshafts (Aluminum, Carbon fiber) have less mass to accelerate/decelerate, so you "feel" the vibration less.

 

[ericbphoto]

, no offset is actually worse than a little. Without any offset, the needles within the u-joint don't move and as a result the lubrication (grease) will breakdown,

I agree. But in these applications, there will be significant offset in at least one plane. The issue with no offset at all is that the needles don’t move in relation to the races and therefore vibrate little grooves in the races. So when they need to move, there is vibration because the races are no longer smooth.