I recently re-welded the mounts on my diff so the pinion angle lined up. So the yoke and rear pinion are on the same plane if that makes sense. Anyway notced today after cruising back from the Perth Jap day the uni's on the tail shaft were quite hot. Ive never taken any notice before, but do uni joints get hot?

Cheers

http://www.carcraft.com/howto/91758/index.html

"The goal is to create a straight line from the back of the crankshaft through the transmission, driveshaft, and the pinion of the differential under load."

Almost always there needs to be some pinion angle. If you are still running leaf springs, then 6 or 7 degrees of pointing down at rest.

Yeah, that pretty much explains what I have done

My query is that do uni's get hot under normal driving conditions (ie today for just over 1 hr at hwy speed)

I have heard of 4wd unis failing, but that due to excessive angles on the uni,s due to a lift kit.

Picking the right uni angle can be a bit hit and miss.
Depending on your application and use, will determine how far out of alignment you should made the uni's (Stationary) to compensate for the axle twist during acceleration and also work best in your desired driving condition.

Are there any audible noises coming from the diff under acceleration, coast (diff float) or deceleration?
This is usually a good indication of uni miss alignment. I remember getting it wrong once and chased a diff noise for weeks (new wheel bearing, diff centre rebuild) only to find the fix was to re-adjust the diff pinion angle.

Often internet articles that define a set amount of uni offset ("As the best") is based on one type of car and it's usage (i.e drag racing) Using that as an example you could require a large offset with leaf sprung cars due to the way the leafs/axles twist under heavy acceleration, but this would make it crap for highway or everyday use. It will remain miss aligned all other times except heavy acceleration. In this example, I would expect the uni's to "fight" each other and create heat.
I can't say I have ever touched a uni to know how hot they should be after straight after use, but I couldn't envisage it would be too hot too touch unless something is wrong??


Quick lesson why angle it's important: (Skip if not interested in this diatribe).

when the input and output of one uni don't run in the same plane (i.e the output is at an angle of incident to the input), the output runs at a cyclic angular velocity (i.e not a constant velocity), this is due to the effective radius of the output changes as the uni goes through 180 deg. The output velocity looks more like a sin wave.

This is often countered by having another uni on the other end of the output shaft which is used to cancel the cyclic velocity of the output shaft. There are two general methods used to do this (assuming the uni's on either end of the tail shaft have their mounting lugs aligned). 1. Re-establish the input shaft angle, 2. use a broken back design which has the rear uni mirror the front uni angle. With either method the rear uni angle is intending to cancel the output shaft cyclic velocity and recreates a constant velocity again.

This is what CVs joints (Constant velocity) were invented, to remove the requirement for two uni's and also enable the joint to run at higher incident angles.

pretty sure you've played with a well designed object and made it crap

sure 4x4's need to do this to compensate for lift kits, but it doesn't apply to our stock dattos, you think if Mr. Datsun Designer could reduce drive train loss by 1-2% by changing tailshaft angle he/she would not have known this and done this? i'm sure it would be at the optimum angle as per stock

no, i cant answer the question and google searching doesn't reveal anything, but you could check it by sticking your arm under any car that's just driven (rwd obviously)

I guess that is assuming that he engine or diff hasn't been replaced by something else?

well yes and no, your still going to retain the same angle essentially, unless the tail shaft is considerably longer or shorter. and i guess not many 1200s have original uni joints, most would be upgraded.

my point was more so... if it ain't broke don't fix it.... anyway it will be interesting to hear what the results are

Given that he is experiencing problems with uni's overheating, incorrect pinion angle is one of the probable causes. Personally, (unless the car has large lowering blocks or is purely drag racing) i would say 6-7 deg differential is too large and would be more inclined to make it closer to 2-3 deg max. Easy way to diagnose is to place wedges in, these can be bought and added permanently when the correct angle is determined.

Just to clarify a few things. It is a CA conversion with a BW78 (Not a stock 1200). I opted for a 2 degree difference when setting my pinion angle. Possibly the slight downward angle of the CA box could be an issue. Maybe 2 degrees is not enough

The diff also makes a slight noise when you back off the accelerator. It is quiet on acceleration and when the vehicle is de-accelerating. Going to ask the diff builder about this as it is fully re-built with new crown and pinion.

Car also mahes a hmmm noise at 100kph. It is not a constant noise. It is more Hmmm .... Hmmm .... Hmmm and the frequency increases as the speed goes up to 110 etc. The yuy from the tail shaft shop said this could be an indicator that the pinion is slightly out when the car is under load.

Spoke to the tailshaft guy who built the tailshaft which has new uni's and he said it is not un common for uni's to get too hot to touch, but he offered to come for a drive in the car next time I am in Perth and give his opinion.

jmac Quote:

old_school Quote:

If you have a noise then it might not be the diff as jmac said.