Pinion Operating Angles

[rvanstory]

Just got off the phone with a FFR engineer and had a VERY productive discussion about this. Here's what I learned.

The tolerances that I have been trying to achieve based on internet searches can be too restrictive. The MAIN concern is to get the the relative angle between the tranny shaft and pinion shaft to 2* or less (my internet sources said within 1/2*). He said at 2* or less, I will never have a vibration or wear problem on the roadster. He said they have cars with up to 5* difference in those two angles with no problems. So, my 2.5* would be fine, BUT if I can get it to 2*, it'll be great.

When doing this, the other angles I was concerned about was operating angles of u-joints. They'll be outside the 3* target I was looking for. He said 2 things about this... Up to 6* won't cause any problems. But even if you do exceed that, shorter life u-joints will be the result. I'm not worried about my u-joints lasting forever, I'm more worried about my rear end and tranny life.

Bottom line, he said the relative angle between the tranny and pinion shafts is the more important angle. So get it to 2* (or less) and don't worry about u-joint operating angles. Move on!

So, that's what I'm doing. At least now I feel like I've spoken to a knowledgeable engineer (at FFR) that not only understood my concerns, but had the knowledge to convince me my concerns were not as bad as imagined.

Thanks to all that have offered their insights and suggestions!

[rich grsc]

Just like I said. Been dealing with equipment and vehicles for 60 yrs.

[gbranham]

I've been monitoring this thread closely for future reference and was thinking all along that 'good enough was good enough', so I'm glad to hear FFR techs confirmed that. In my mind, if 'premature wear' on a u-joint means it last 50% less than normal, and a u-joint is probably good for, say, 100,000 miles, that'll be years and years of driving for me. Not part of the build I'm going to get too obsessed about being perfect. Flame on!

[Jeff Kleiner]

As I said…



Jeff

[neilkuy]

I posted this thinking it was following rvanstory’s last post on page one. I see now there are more and it has been concluded sufficiently. So take this as more evidence that we should all simply listen to Kleiner off the bat!

Speaking to the 2nd picture you reference. When an angle exists on a on a rotating joint the result is converting a smooth rotation before the joint (trans output) to a ‘lopey’ rotation after the joint (driveshaft). If you then insert an equal and opposite angle at the end of the driveshaft, it will turn that ‘lopey’ rotation back to a smooth rotation in the element following the second angle (diff input).
The lopey rotation of the driveshaft introduces some imbalance which can lead to vibration and added stress on the ujoint. If the angles are unequal that vibration is realized.
First image:
No angles so no lopeyness and minimal stress on the joints. The imbalance between the angles of the joints is zero.

Second image:
Equal and opposite angles. The driveshaft will have some lopeyness in its rotation- the magnitude of which is defined by the size of the angle of the first ujoint. But the diff input will have smooth rotation equal to that of the trans output since the second angle cancels all of the lopeyness. The imbalance here is 0 degrees which is the difference between the two ujoint angles since they are in opposite directions.

Third image:
No angle on the first ujoint and an angle on the other. This will result in a smooth rotation of the driveshaft since there is no angle between it and the trans followed by a lopey rotation of the diff input and consequently, the wheels. At really low speed you might feel a very slight surging or bucking, at any appreciable speed it would be a vibration, exaggerated at specific speeds due to resonance of the system. The imbalance is the angle of the second ujoint.

Fourth image:
Angles in the same direction on both joints. The lopey rotation of the driveshaft is exaggerated or amplified into the diff input by the second joint. The imbalance is the sum of the two ujoint angles since they are in the same direction.

Last case not in image
angle on both that is opposite but not equal, like what Jeff shows. The imbalance is the difference of the angles. 1 degree up and 1.5 down results in a 0.5 degree difference. This is the value the will correlate to a vibration and stress on the u-joint. This is the target case as it is easiest to minimize the imbalance when considering manufacturing tolerances.

In any of these cases the magnitude of the imbalance angle is what is concerning. A large enough imbalance leads to vibration in the car and stress on the ujoint which might lead to an early replacement of a ujoint. I can’t speak to what you would run into with adjusting this on these cars as I haven’t gotten that far on my build yet