Say Goodbye to the Flat Spotted Tires / Shuttering Theory

[ntmehan]

This is so obvious now that we're talking about it. BMW has issues (maybe that's the wrong word, my BMW tech buddy is constantly replacing them on older cars, so they're just a wear item) with their flex disc (guibo, in BMW terminology) but the difference is that they sell it as a seperate part. That part can range from ~$45-200, but even at the high end that's nowhere near the cost to buy an entire driveshaft assembly! I would accept replacing a flex disc at my mileage even (58,000) for a car of the weight and power of my 5.0, so that's where the problem is. It's not necessarily bad design, it's just the inability to replace an inexpensive wear item (which is in no way compromised separate from the assembly) and instead having to shell out to replace the entire assembly 90% of which is just fine. Does anyone know if there are any aftermarket options? Looks like this would be a very simple DIY for anyone with a torque wrench, right, as long as you can get the flex disc for a reasonable price by itself? I would pony up $50 bucks to buy something and give it a shot... And hell, even if it only lasts 3 years/45000ish miles, I'd replace it again at $100k for the price of a damn oil change...

 

[arcman]

I posted this previously, the SGF part number for the front flex disc is GAH04-004 ($88). The guy I spoke to last at PowerTrain Industries said they have a new part number GAK01-006 ($111). Need to verify that number and I would like to know why the upgraded part is so much more expensive (the reason for contacting SGF). Though the disc isn't listed on their website, Eric there said they could get the part.

Just to let you know... these guys think the issue is in the U-joint.

If you are going to do this, know that the driveshaft side bolts are left handed and the trans bolts are right. What for Hyundai?

 

[Kraig Wilkinson]

I posted this previously, the SGF part number for the front flex disc is GAH04-004 ($88). The guy I spoke to last at PowerTrain Industries said they have a new part number GAK01-006 ($111). Need to verify that number and I would like to know why the upgraded part is so much more expensive (the reason for contacting SGF). Though the disc isn't listed on their website, Eric there said they could get the part.

Just to let you know... these guys think the issue is in the U-joint.

If you are going to do this, know that the driveshaft side bolts are left handed and the trans bolts are right. What for Hyundai?

Arcman you seem very knowledgeable about this subject . I appreciate you and others like you on this forum. Just wanted to say Thank You for input and the time you spend educating others.

 

[phdegraff]

That is correct. That's why some owners report the problem coming back after Hyundai replaces the driveshaft.

Maybe I'm talking out my butt here but that is why, after (and before) speaking to that engineer, I feel it's a problem that won't get fixed and Hyundai is just appeasing owners by replacing parts. The small crowd getting parts tossed at their cars is way cheaper than Hyundai admitting there's a problem, engineering a fix, tooling up and possibly having to recall tens of thousands of cars. Why else would Hyundai replace all these expensive parts when we know for sure it's coming from the rear driveshaft? I proved that by removing three bolts, and I am not the first to figure this out.

Arcman, you have done us all an invaluable service! I stand in awe of your perseverance, your calm, logical approach, and your reasoned conclusions - all of them incl. Hyundai's treatment of this. Remarkable work, sir! You put into practice what some of us have been hoping would be done, and you did a superb job of it.

Gennadiy questioned how much to replace this flex-joint but I fear he missed your main take-away: That replacing the coupling will not solve the vibration problem. It is a design flaw and it's there to stay - unless ... unless nymehan's parking suggestion helps. Maybe, just maybe, unloading that coupling when parking will help. I am eager to try it.

(2016 G2, AWD, all options, 27,400 miles as of yesterday.)

I had, some time ago, accepted that I will have to live with it. Knowing that the cause has finally been found somehow makes it easier.

Thank you again, arcman! We all owe you a debt of gratitude.

 

[TinsleyC]

I am curious how many with this flutter issue park their car on a slope with sufficient grade to put stress on the flex joint.

For me, both work and home (garage) are about as flat as you can get (Dallas TX). I don't have the problem myself.

 

[pmckechnie]

Arcman , This is about a totally different car but maybe it could shed some light on the problem.
I have a 1984 s class MB that has a drive shaft that looks very much like the Genesis. Rubber disk at each end, U joint in the center and a bearing mounted in a rubber mount in the center support bracket.
I had a similar problem with it but it was at about 40 mph. After completely rebuilding the shaft with the parts I could get (front and read disk, center support with new bearing, rubber boots at the u-joint) there was very little change. You have to buy a complete shaft to get the u-joint.
After going over the directions of how to set up the shaft assembly and installing it I saw something I had missed. I assume the Genesis is the same but not sure. There is a slip joint the allows the shaft length to be adjusted. It has a threaded collar that you can loosen to shorten the shaft so you can get it out and reinstall it. The part I missed was to snug the collar up just enough to take any slop out of the shaft and then put the car back on the ground and role it by hand, not drive it, at least 10 feet. This allowed the shaft to adjust itself to the car. Then you can raise the car back up, tighten the collar back up to some torque figure (don't remember how much) and you are finished. It worked. Vibration was gone. It only took be around a year and a ton of money to fix a problem that was my fault all along.
Do you remember any adjustment sleeves on the Genesis drive shaft? A picture of the Genesis shaft I found looked exactly like the MB shaft.

 

[dboz1031]

This is good stuff. Wondering if it's possible to just go with a different coupler? Something that resists distortion. So my thinking, it would be worse in the heat. The part is more pliable and then cools down and "hardens" into a more deformed position. In the 100 degree heat from the garage, it would still be deformed and "softer" but the vibration probably goes away much faster. That would explain for mine why some days its absolutely horrible and others its not there at all. It could also depend on the spot that coupler was when I parked. If the max strain was at the position of the bolt, it may not distort as much? Thanks for taking the time to do your own testing. Impressive. I also wonder if I could get a drive shaft company to cut off and come up with a better "end" there to try to eliminate this? Of course that probably opens up warranty issues also? I also can feel the vibration at times around 40 and also 70 but its at a different frequency.

 

[dboz1031]

My dealer kept having a problem with the shaft coming apart during shipping and they felt when they put it back together the balance may be off if not on the perfect spline. So yes, I believe its a complete two piece. And they took mine to get balanced to be certain it was for the 3rd install.

 

[dboz1031]

As to parking on a slope, YES, when I am pointed down hill on a steep slope in drive with my foot on the brake, my car has significantly more vibration. That is what led me to believe it might be the torque converter.

 

[dboz1031]

Anyway, ARCMAN, many thanks for your info and efforts here!!!!!!!

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[dboz1031]

Also, the longer my car sits, the worse the vibration can be. So if it sits for 4-5 days, it tends to be really bad and it stays really bad for several days. This makes sense to me with the coupler because the longer it stays deformed in one spot, probably takes it longer to reshape? And if I get unlucky and it just happens to stop at complete 180 from the previous point, that is what probably causes my car to feel like its going over rumble strips.

Sorry for all the posts, brainstormed and did not edit.

 

[arcman]

Thanks to all for the kind words. I don't give up easily. Like my previous CAN bus issue, I will get to the bottom of this. I'm 99% convinced it's the coupler but there's still some out there in the driveline biz who tell me it's the U-joint. Weather we end up living with this or we find a fix... I need to know.

This week I will try contacting SGF Germany by phone since they won't respond via email. Hopefully Dana/Spicer's work force is returning and I can get some info from them. I tried every extension and left messages to the engineering group but the only line to pick up was the security staff. No one's been home. If SGF will give me the skinny on the upgraded flex disc, I will buy one. Again, I'm not holding my breath.

TinsleyC... Preloading in the axial plane isn't what I'm thinking. It's the flex in the forward/backward due to the angle difference of the trans flange and the driveshaft. That's why I said to try moving the car to rotate the coupler 180 degrees after sitting all night. This would put the part of the coupler that was bent towards the rear of the car now bending to the front. If you look back at the photo I posted, you would bend that part of the coupler the opposite direction. The coupler doesn't flex a whole lot over one rotation. I'm guessing an 1/8"-1/4"?

Couple days ago after leaving the house, I did three full throttle launches from stop in forward and reverse to see if it had any 'loosening' effect. After maybe a mile and a half on a smooth road, I didn't hear the usual fluttering sound from the console. Normally, I continue to hear it at least five miles down the road. I will try the coupler rotation this weekend.

pmckechnie... There is no lock ring or any device to keep the splines held together. Just the slip fit and a thin rubber bellows type boot.

 

[Suburbazine]

I've got a driveline shop near my house that does custom work, I'll see if they want to take a crack at building a custom non-wobbly driveshaft with non-wobbly U-joints. I can move the shaft around by hand just by pushing and pulling it.

 

[TinsleyC]

I'm pretty clueless on the driveshaft and flex-mount technology, but why use rubber to handle the flexing? Why not use some sort of metal configuration using ball bearings, or ball joints, or similar?

 

[ntmehan]

I'm pretty clueless on the driveshaft and flex-mount technology, but why use rubber to handle the flexing? Why not use some sort of metal configuration using ball bearings, or ball joints, or similar?

This one is actually pretty simple. It's all to improve NVH. The rubber will absorb driveline vibration and flex a bit to absorb shift shock as well, generally aiding in the refinement of the car. It also provides a bit of protection for all the other hard parts of the drivetrain. A CV or U-joint is a wear item, just like a flex disc, so you might as well serve a dual purpose to help sell smooth and quiet luxury off the new car lot. (Ironically a flex disc is much cheaper and easier to replace than a CV or U-joint in the same application, if only they'd sell it as a separate part from the Hyundai parts catalog.) Problem is, if our cars weren't so quiet and smooth, it could make some noise and we'd never notice, but alas.

 

[arcman]

Not the result I was hoping...

Received the new flex disc Friday and installed it today. Noticeably more noisy then the original (so the vibration must be worse too, I just can't feel it). Odd thing is now instead of it making the noise at 29mph, it's down at 25mph. This still makes me think it's the disc.

For some reason, the importer ground off three sets of mold numbers around the perimeter of the disc. The proper part number is still there but I would think taking any weight off the outer diameter (probably just a few grams) isn't going to help the balance. I spoke a while back with someone there who wants me to balance the shaft but they can't balance it with the discs. I don't think that will do much good, seeing as the disc is three times the diameter of the shaft.

I guess the next approach is to make a rig to check the balance on just the disc alone. Another project...

 

[TinsleyC]

Not the result I was hoping...

For some reason, the importer ground off three sets of mold numbers around the perimeter of the disc.
View attachment 30213

That's strange. Import / customs issue?

 

[Suburbazine]

The rotating mass of the guibo shouldn't be enough to cause significant wobble at less than 70mph. If the propshafts themselves are not true they would explain the low speed vibrations. Putting the new, less forgiving guibo on probably more firmly connects the wobble to the car. It's also possible the new guibo isn't running true either and makes the propshaft wobble.

 

[arcman]

I'd like to try leaving just the disc on the trans flange and running it up to speed to see if it vibrates. Problem is, I don't think I can pull the driveshaft flange back far enough to give the disc clearance to rotate without hitting it. No way am I pulling the exhaust, heat shield, center bearing and driveshaft to test this theory.

It works in the opposite way below but there's a large bung that holds the bushing that protrudes into the disc from the driveshaft flange.

 

[Kraig Wilkinson]

I'd like to try leaving just the disc on the trans flange and running it up to speed to see if it vibrates. Problem is, I don't think I can pull the driveshaft flange back far enough to give the disc clearance to rotate without hitting it. No way am I pulling the exhaust, heat shield, center bearing and driveshaft to test this theory.

It works in the opposite way below but there's a large bung that holds the bushing that protrudes into the disc from the driveshaft flange.

View attachment 30255

Also having trans running with no load may act differently?