New Thomas with heavy shaking at 25 mph

[Rob_Rocker]

Driving a brand new Thomas with a Cummings B6.7 engine

. I currently have 2.5k miles on it and from the beginning when traveling between 24-30 mph the bus has a noticeable front end vibration that dissipates at 30-35 mph. Its bad enough that panels are now creaking, fasteners, bolts and screws are loose and coming off. Our shop has spent many hours trying to fix the issue. They have regreased fittings, changed front wheels and tires and checked all steering and braking components. The best the mechanics can say it there might be a harmonic imbalane between the engine and transmission that a future software update might resolve. Our zone gets three new buses a year and one other new Thomas is having the same issue. Anyone have any ideas or advice regarding this?

 

[Mr P]

Google a Sirometer. I have a Treysit brand. It tells the frequency of the vibration. From there, you figure out rotational speeds of bus components. When you get a match, you've found the problem.

 

[Dbacks2k4]

Only idea I'd have is to re-balance all 6 tires. I've seen one tire being out of balance cause some of the vibrations you're experiencing.

 

[PorchDog]

Also, you can balance an egg, but it will not roll very well. Tires or wheels with significant runout can cause this. After re-reading your post-- if they swapped the front wheels off another unit-- I guess that should have eliminated runout as an issue.

 

[nikitis]

I would check your drive shaft, it could be imbalanced and needs to have weights added to it. Especially if it adjusts during certain speeds which would mean at certain frequencies it's more of an issue.

Drive shaft would be my first go to before tires etc, but yeah anything rotational would be the cause.

Take the bus to a drive shaft shop and have them balance it out, and if they do that and it's still there may go ahead and have them replace your carrier bearings as well while they have the shaft out.

 

[Mr P]

Sirometer....frequency....facts.....fix.

 

[Ronnie-SKO]

Never heard of a sirometer, but sure sounds like a good idea. I like good data.

 

[Ronnie-SKO]

Just looked it up and Amazon sells a cheap one for only $28

 

[#1FordFan]

Since it has 2500 miles shouldn’t it be under the factory warranty???


I did note this sirometer idea, I know the Jeep Off-roaders will want to hear about it.

 

[nords]

A friend of mine's bus would have very bad shaking that would come and go. He finally asked the perfect question "do the front wheels travel different amounts when you turn?" Rebalancing the front tires fixed it. When they were in sync, it would go away. He would turn and they might end up unsynchronized and shake until the next turn.

 

[hcarter951@gmail.com]

The drive shafts should turn about 5 times faster than the tires. Tires are x circumfrence, at 60 mph turning n rpm. Low frequency, high amplitude (tires) vs high frequency, low amplitude (drive shaft(S))