Anyone good at trouble shooting driveline vibrations?

[John Milmont]

For a long time my race car, and old 3rd gen Firebird, has had a terrible driveline vibration at around 65-70mph which is where they always seem to happen. I have always just kinda lived with vibration but now that I need to get my new engine to the dyno to get tuned, I want everything to be perfect.

Today I went and checked the runout of the driveshaft and found that its .005" at the rear and about .006 at trans output. I think this is right about the upper range of sepc. One thing I did notice was that the rear bushing on the trans was getting a little sloppy. Its hard to tell how much, but I was reading that may not be too important. Anyone have experience with that?

The u-joints seem to be tight and I had the driveshaft balanced not too long ago, but yet the vibration is still there.

Anybody have any advice for hunting down this vibration? I'm absolutely sure its not wheels and tires.

 

[Jac Mac]

Quick check is to get a suitable size hose clamp, place the clamp screw at the 'low' point and try the car, probably best to do front & then rear [ie one thing at a time].
another somewhat risky test is put the car up on secure axle stands & get a friend with some mechanical understanding to run it up to that speed range while you hold a piece of white marker chalk real close to the tube to check for tube runout at speed, then place the hose clips with weight 180° from 'high' spot. Do the test with/without tires to eliminate that possibility [ fit wheelnuts when without to retain drums/discs if reqd].. take care!!
There are some types of ujoint that are a mismatch with the pinion yoke location cups, make sure you are using the correct type here..eg no circlips or inner circlips only..whatever the yoke requires...


MAKE SURE YOU HAVE THE AXLE STANDS UNDER THE CAR AXLE_NOT THE BODY_ TO REPLICATE THE AS RUN DRIVESHAFT ANGLES

 

[John Milmont]

Sounds a little scary to have the car up on stands and running it out to 70mph. I guess I can try it without anyone under the car to see how it goes then take it from there.

 

[Ian Anderson]

Are the U joints in phase? or out of phase?

Could simply be need to unbolt, move to different spline and then re bolt

Ian

 

[Jim Cowden]

One or 2 piece shaft.

Jim

 

[Randy V]

I used to race a 3rd gen Camaro in the SCCA's American Sedan class. In my car I had this problem due to the lower ride height I had the car at. This changed the pinion angle. Ultimately I changed the torque arm for one that I could adjust the pinion angle with, but before I was able to do that, I adjusted the ride height up a little in the back and put a couple of washers between the transmission crossmember and the sub-frame of the car to effectively lower the tailshaft.
This might work for you..
Oh also, I put in an Energy Suspension poly transmission mount to replace the Moroso solid mount.

 

[John]

Randy i think you may have hit the nail on the head, i was just going to explain to John that the diff flange must be at the same angle as the rear out put flange [if there was one ] of the trans, other wise all sorts of harmonics can occure.

cheers John

 

[Larry L.]

I'm going to throw this out there because it happened to me:

Have you checked the balance on your wheels/tires?! 'Dead serious.

I spent several hundred dollars chasing just such a vibration. 'Turned out that the new, state-of-the-art balancing machine at my local tire shop WAS NOT WORKING PROPERLY which resulted in improper weights being placed in the wrong location on the wheels! THAT was the source of the problem.

'Rebalanced the wheels/tires on a properly functioning machine and presto - problem gone.

 

[John Milmont]

you guys have provided a lot of information here, now i just got to check all this stuff out.

It is a one piece shaft. It is not possible to change the phase of the u-joints since the sliding part is in the back of the trans, not on the shaft. the car is lowered, but its not that low. i have had the pinion angle checked in the past and it was found ok, but i should confirm this.

I also have a poly mount for the trans that's on a aftermarket transmission cross member. Now that I think about it, I have noticed that the rear of the engine is lower than the front. I'm thinking that the shaft angle off the rear of the trans might be wrong due to this, it could be made worse with the car being lowered like Randy said.

 

[John Milmont]

After checking all the angles and shimming the back of the trans just to see if that makes a difference, and still finding nothing wrong and no differences; I'm just going to take it to a shop to see if they can figure it out. I'm thinking that the rear bushing in the trans is too sloppy and causing the issue.

 

[John Milmont]

I think i finally figured this thing out. After taking the car to a drive line shop which found nothing wrong, I was wondering if the vibration was coming from the front.

My car has very large spacers on the front wheels - 4". They are the kind that bolt to the hub, and then you bolt the wheel to the spacer.

I put a indicator on the front wheel to find that it had a radial runout of .040" which is quite large. I then took the wheel off and put the indicator on the spacer which also had a very large runout. The runout could even been seen with the naked eye.

It turns out that the center hole of the spacer is a about 1mm bigger than the hub center and I believe this is causing the problem. Hard hits or cornering surely moves the spacer off center.

I was able to make a ring out of some thin sheet metal to fill this 1mm gap between the spacer and the hub. I haven't tested the car yet but I'm pretty sure this is going to fix it.