Control, we are detecting a shimmy.

[Shiflett]

Seeking advise on where to look for an issue. I drove and parked my truck, stock M1083, on a Saturday without issue, the following day the truck had a shimmy/vibration at 25mph which goes away, and then again at 35mph which also goes away the closer you get to 40mph.

1. engine rev's in neutral VERY smooth
2. rear drive shafts are tight.
3. front drive shaft has the slightest of play
4. transmission shifts normal
5. tires are evenly inflated and don't appear to have any damage
6. rear lower control arms are solid
7. I don't feel the shimmy through the steering wheel, leads me to think its in the rear


Any thoughts?

 

[Ronmar]

What is the frequency of the shimmy?

Big tires turn relatively slowly, about 6 revolutions per second(HZ) at highway speed. I would expect a tire to oscillate about 3HZ at 30MPH... A DS would be 7.8X that... My limit for counting reliably is about 12HZ, so even at 30MPH a DS would be too fast for me to count.

On level ground, Jack up one wheel at a time, chock the other wheels. While it is in the air check for side to side play, top to bottom and front to rear(bearings and steering). Then put someone in the cab, start and engage a gear to spin that one wheel(and guard the brakes). look for out of round at the tread and out of true at the edge of the tread, sidewall and rim. Repeat for all wheels...

Where is the play on the front DS? yoke, u-joint or slip-joint? If you can feel play, it is too much... A DS would produce a very high frequency vibration, and not likely a shimmy. But at our steep DS angles and the fact we spin them way into the red zone for their angle, THEY MUST BE PERFECT otherwise bad things ensue...

Probably the biggest benefit of eco hubs, they bring our DS RPM back onto the acceptable range on the angle vs RPM chart...

If you steering is tight, its probably a tire, but could be a rim, or even a bearing...

Good luck

 

[Ronmar]

Oh, does the shimmy change if you apply the brakes?

 

[GeneralDisorder]

Rotate your tires and rotate in the spare and see if it changes.

And definitely fix that driveshaft before it lets go and shreds the transmission - front DS failures usually destroy the transmission or at least the control valve body ($5,000 part).

 

[Shiflett]

What is the frequency of the shimmy?

Big tires turn relatively slowly, about 6 revolutions per second(HZ) at highway speed. I would expect a tire to oscillate about 3HZ at 30MPH... A DS would be 7.8X that... My limit for counting reliably is about 12HZ, so even at 30MPH a DS would be too fast for me to count.

On level ground, Jack up one wheel at a time, chock the other wheels. While it is in the air check for side to side play, top to bottom and front to rear(bearings and steering). Then put someone in the cab, start and engage a gear to spin that one wheel(and guard the brakes). look for out of round at the tread and out of true at the edge of the tread, sidewall and rim. Repeat for all wheels...

Where is the play on the front DS? yoke, u-joint or slip-joint? If you can feel play, it is too much... A DS would produce a very high frequency vibration, and not likely a shimmy. But at our steep DS angles and the fact we spin them way into the red zone for their angle, THEY MUST BE PERFECT otherwise bad things ensue...

Probably the biggest benefit of eco hubs, they bring our DS RPM back onto the acceptable range on the angle vs RPM chart...

If you steering is tight, its probably a tire, but could be a rim, or even a bearing...

Good luck

Thank you, it’s definitely in the drive hrz range. The flex is in the slip joint. I’m going to pull it and have it professionally refurbished.

 

[Shiflett]

Oh, does the shimmy change if you apply the brakes?

I don’t know. I’ll check.

 

[Keith Knight]

What tire type do you have and what year are they?

 

[Shiflett]

What tire type do you have and what year are they?

395/85r20 Goodyear MV/T from 2012

 

[coachgeo]

395/85r20 Goodyear MV/T from 2012

13yr old tires..... eek.. that may well be your issue. at least their MVT's

 

[Keith Knight]

After balancing my drive shafts and new unjoints. I still had vibrations changed tires and up the psi it went away. When mine was still an M1078 I ran 85 psi up front which stopped the heavy wearing of the tires. 65 psi in the back yes I had to disconnect the CTIS and do it manually. I feel 60-65 is way to low for on road.

 

[Ronmar]

After balancing my drive shafts and new unjoints. I still had vibrations changed tires and up the psi it went away. When mine was still an M1078 I ran 85 psi up front which stopped the heavy wearing of the tires. 65 psi in the back yes I had to disconnect the CTIS and do it manually. I feel 60-65 is way to low for on road.

Yep, these things are pretty nose heavy when empty. you need to run different tire pressures with different axle weights to balance out the tire wear.

55PSI is considered an Offroad/Cross-country tire pressure in the Goodyear MVT documentation…

 

[GeneralDisorder]

Yep, these things are pretty nose heavy when empty. you need to run different tire pressures with different axle weights to balance out the tire wear.

55PSI is considered an Offroad/Cross-country tire pressure in the Goodyear MVT documentation…

All the heavy trucks - 1088, 1089, etc run 81 psi for the CTIS.

 

[Ronmar]

Saw a M1088 that a local agency had acquired that had 89PSI stenciled above every wheel-well...

 

[GeneralDisorder]

The tire's load rating is at 100 psi according to the sidewalls on my 2022 MV/T's

But I prefer not to subsidize my dentist's kid's college fund. I find that I tend to run the 55 psi spec daily around town and switch to the 81 psi spec for high speed, longer distance travel.

CTIS is great. Push button, walk away.

 

[littlesfmtv]

My gigantic shimmy was a tire with no beadlock in it. I bought my truck from a third party that claims "the shop goes through the whole truck", which is complete bs.

 

[littlesfmtv]

All the heavy trucks - 1088, 1089, etc run 81 psi for the CTIS.

Weirdly the 1087 with a 10k pound box on it runs 55 hwy. After reprogramming mine to 81, its highway characteristics improved dramatically.

 

[MatthewWBailey]

My gigantic shimmy was a tire with no beadlock in it. I bought my truck from a third party that claims "the shop goes through the whole truck", which is complete bs.

Yeah I've heard that line before. Pretty sure everyone knows which shop it is too.

 

[littlesfmtv]

Yeah I've heard that line before. Pretty sure everyone knows which shop it is too.

Even better, they installed AC and wired the fan in backwards so the system overheated. Now it won't hold a charge and they don't even warranty their own work. Tip of the iceberg of fun things I've found.

 

[MatthewWBailey]

Even better, they installed AC and wired the fan in backwards so the system overheated. Now it won't hold a charge and they don't even warranty their own work. Tip of the iceberg of fun things I've found.

Ugh. That sucks. Mine is missing CTIS hub seals so the gear oil flows into the CTIS. Luckily lol they never replaced the frayed CTIS lines so I saw the oil flowing out onto the ground. "Gone through by our shop" means they changed the oil and put tire shine on the sidewalls.

 

[GeneralDisorder]

Yeah I've heard that line before. Pretty sure everyone knows which shop it is too.

Not just those guys either. Acela has done that to a bunch of trucks because the owner didn't want to source the bead-locks. Friend of mine with an Acela built A1R had one of his tires go flat from a leak and it popped the bead off the rim. He was not aware till that moment that his truck didn't have any bead-locks in the rims.

IMHO you are always better off getting a surplus truck and taking no one's word for anything having been done to it. The evil you don't know about vs. the evil masquerading as a warranty.