Gotta admit I'm having a little bit of trouble diagnosing what's going with my beloved 8. After and extended period of driving, about 30 minutes to an hour of freeway driving, my drivetrain begins to develop a shudder that's mostly noticeable around 3800-4200 rpm. Any idea as to what this may be? Has anybody else had this issue? It seems to go away once it has cooled down. Ideas?

 

How old / worn are the tires?

Might be related to tire heat. Some other heat related options i can think of probably wouldn't manifest as a 'shudder'

Edit:
My MSM got a very low frequency pulsing shudder on my nearly bald winter tires about 2 weeks ago, mostly apparent around 70-80mph, and I confirmed it wasn't 'drivetrain' by dropping to neutral on a long downhill in those speed ranges. Shudder still existed independant of RPM. Disappeared when I pulled them off and installed summer tires.

So you might try seeing if it's RPM or speed related, since I bet at those RPMs on the highway you are probably always in the same gear, so roughly the same speed

 

Tires are brand new so I cannot imagine thats what it is. Would a dying throwout bearing possibly be a cause for this? Its strange because the car drives smoothly when cold. No issues what so ever. They are winter tires so they are a softer compound but I can't imagine that it would cause this issue. I checked the rear half shafts and they seem to have zero play. Would driveshaft u joints cause this? If they were going bad would it be something that would occur right away or after a period of driving?

 

hmmm just saw the edit. The shudder is directly related to speed. I can kick it into neutral and it doesn't change anything so it doesn't seem to be engine related. They only thing it could be is the rotating assembly which would include the tires. That would be very interesting if this was the case. I mean if any joint was going bad I'm assuming that it would be apparent warm or not correct? Perhaps a PPF issue?

 

Since you have winter tires on, does that mean you have an alternate set of summers? If you do to throw those on for a quick test, you can rule the tires as in or out. It could be a balancing problem, it could be an actual tire defect problem. No indication where you are, but if it is starting to warm up, it's possible that the warmer winter tire is just very unhappy.

A wheel bearing is possible, but frankly I can't recall the last time i heard of an RX-8 wheel bearing failure. You can test for bearing failure by trying to rock each wheel against the direction of rolling (you will need to lock the steering wheel for the front wheel test). Grab two opposing sides of a wheel and push one side pull the other, then alternate, then change where you are grabbing by 90 degrees and repeat.

 

Did a check earlier in the week for wheel bearings and they all seemed okay on all 4's. I can't imagine my solid engine mounts and beatrush brace have anything to do with this. I see that you've been on the forum for a while, is this something that you have ever seen from a misaligned PPF? or should I overlook this before I redo everything. Unfortunately no summers to test on at the moment. Picking those up later in the week.

 

Is there anything you have done with the PPF?

 

Ive had to realign the PPF due to the MAZSPORT (old name drop) solid mounts I am running. Plus installing the beatrush trans brace resulted in having to realign the ppf however I did do so per FSM specs.

 

Ah, nod. Then yes, it could be a PPF sourced problem. The driveshaft will be spinning based on vehicle speed, and if the PPF isn't aligned correctly then it would result in some extra noise from driveshaft bearings, transmission output, and possibly the rear diff.

Are you sure you have the right spec and that it hasn't sagged?

 

Checked and it seems to be within factory spec. It did sag a bit so i brought it back up. Still no go. Perhaps its a driveshaft failure on one end. Perhaps slipping in and out of its center position and intermittently causing it to vibrate. Also noted that the smells now a bit of burning rubber after a drive but I have confirmed that tires are not rubbing anywhere. Very strange issue indeed.

 

Can you inspect the driveshaft surface itself? The driveshaft is carbon fiber wrapped in a sealant. If it's rubbing on something it could produce a smell and vibration.