2015 IRS vibration

[wallace18]

I just finished up a 427 MK4 with the new IRS for a customer and he has a vibration under acceleration and at a steady 60mph. Not coming from engine or trans. Seems like it is coming from half shafts. All parts were brand new Ford stuff. Anyone else run into this problem? If so what fixed it? Any help is appreciated. TW:confused:

[edwardb]

Have you checked tires? Inflation, out of balance, out of round?

[Duke]

Does it start around 50/60 or is it all the way through the speed range? If starting at speed then it's most likely wheel balance. If it's throughout the speed range it's most likely driveshaft balance. The chance of it being the half shafts are very slim. You can check the half shafts by getting it up in the air (on all 4 jacks) and have someone put it in gear (1st) and see from under it if they appear out of balanced.

[NAZ]

Vibration under acceleration and at speed is a classic symptom of a driveshaft imbalance due to phasing. That does not necessarily eliminate half-shafts but the driveshaft would be my prime suspect. Pinion angle must still match trans output angle under load even with IRS. If your IRS is rubber mounted you must take the rotational flex into account when setting static pinion angle. An old trick they taught us at Ford Factory Training: if you suspect the driveshaft is actually out of balance you can use a hose clamp as weight when troubleshooting this problem. When it comes to balance -- if you can make the problem worse by adding weight you can make it better by moving the weight around. This is best performed on a lift. It helps to eliminate balance as an issue if all other parts of the driveshaft check out.

[wallace18]

Vibration under acceleration and at speed is a classic symptom of a driveshaft imbalance due to phasing. That does not necessarily eliminate half-shafts but the driveshaft would be my prime suspect. Pinion angle must still match trans output angle under load even with IRS. If your IRS is rubber mounted you must take the rotational flex into account when setting static pinion angle. An old trick they taught us at Ford Factory Training: if you suspect the driveshaft is actually out of balance you can use a hose clamp as weight when troubleshooting this problem. When it comes to balance -- if you can make the problem worse by adding weight you can make it better by moving the weight around. This is best performed on a lift. It helps to eliminate balance as an issue if all other parts of the driveshaft check out.
we have the drive shaft inline static with trans and diff. Should i have it angle slightly down at diff? Wouldn't the drive shaft vibrate at all speeds if out of balance?

[Mark Reynolds]

I've read that this is a huge problem with 2015 Mustangs in general. Google it.

[NAZ]

Wallace, under load (acceleration) the pinion angle should match the trans output shaft angle. If there is too much misalignment you will get a vibration and it will be worse under load as the pinion angle will change (the front where the u-joint mounts will rise as the housing rotates) due to deflection in the elastomer mounts attaching the differential to the chassis. So yes, you would expect to have the static pinion angle adjusted so that it is lower at the front by one to several degrees. How much depends on how much the pinion angle changes under acceleration. The key observation for me was that you experience the vibration under load. That indicates to me that there is misalignment with the pinion angle due to flex in the mounts. If you can determine how much the housing rotates under load it's a simple matter to adjust the pinion angle lower in front by the amount it rotates (compared to the angle of the trans output shaft). Or you can experiment with the pinion angle through trial and error until the vibration is mitigated. Also, if you have the driveshaft perfectly straight (no angle) from the trans to the rear end you may experience premature u-joint failure. We usually set the driveshaft up so that we have at least 1-deg of angle so the needle bearings will pick up grease and spread it across the races keeping the u-joint bearings properly lubed. If the u-joint bearings do not rotate they tend to wear prematurely. Let us know how you fix the problem.

[edwardb]

I've read that this is a huge problem with 2015 Mustangs in general. Google it.

True enough. But seems to be mainly related to the 2-piece driveshaft. Found this pic of one. A lot going on there. Not applicable to our builds. Hopefully...

http://www.lethalperformance.com/info/assests/IRS_7.jpg

[wallace18]

It can't be from the mustang drive shaft since the MK4's is about 4" long and one piece. Thanks for the thought. Will try to look at half shafts and pinion angle.

[rickscobra]

Have 700 miles on my 2015 IRS and have experienced no vibration. I am running Forte 306/T-5. Just bolted everything together with no shims of any kind.

[wallace18]

My customer Bill took the car on a long ride yesterday and now says he feels it in the shifter especially in 3rd gear. I will get to drive it this week and make my own evaluation. Thanks for all the input. We will figure this out I am sure.

[kcorn]

I have modified and built a number of drive shafts in my home shop for various projects. I have run into vibration issues where I ended up with the front and rear universal joints slightly out of phase because they weren't quite square to the driveshaft tube. So even though your drive shaft is in effect two universal joints welded together don't automatically dismiss it as the source of your customers vibration problem.

[wallace18]

I have modified and built a number of drive shafts in my home shop for various projects. I have run into vibration issues where I ended up with the front and rear universal joints slightly out of phase because they weren't quite square to the driveshaft tube. So even though your drive shaft is in effect two universal joints welded together don't automatically dismiss it as the source of your customers vibration problem.

10/4 will definitely look into it. Thanks.

[wallace18]

Today, Bill brought his 427 MK4 by. After driving the car it did have a vibration under acceleration. We drove my car back to back with his. After putting the car on the lift and checking a few things we decided the angle of the drive shaft was incorrect. I welded up an 1.38" spacer for the rear of the transmission and bolted it in place with longer bolts. We also raised the rear ride height by 3/4". Problem solved no more vibration. Thanks to everyone who lended their opinion. It was a great help. I prayed to God this morning we would fix the car and he blessed us with find the fix. I hope this may help someone else down the road. TW.


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[wareaglescott]

What was the angle of the driveshaft and what does it sit at now?

[wallace18]

What was the angle of the driveshaft and what does it sit at now?

It was lower at the trans than the diff. Now it is slightly higher than the diff at the trans.

[Mark Reynolds]

Good for you guys! Must be satisfying to actually solve a vibration issue like that.