Mk48 Death Hop?

[Augdog1964]

Hey all,

Took the Mk48 out for a long drive today... awesome vehicle and LOTS of stares...

However between 35 and 40 there is a pronounced HOPPING that begins and gets fairly violent at times... anyone else have this?

It doesn't always do it, sometimes its not bad, other times very bad...

 

[patracy]

Is the MK14 attached?

 

[zout]

With a normal powered unit a rule of thumb is:

If you can drive out of a vibration - its in the tires
If you cannot drive out of a vibration - its in the driveline

 

[Stalwart]

I've been told by several current drivers of these that under certain conditions they can take a sudden and un-commanded hard turn without warning. One of these drivers was following one in his truck when the lead truck veered sharply off the road, crashed and rolled. Please be careful and enjoy the BEAST!

 

[Hoefler]

I have an MK48 with trailer. Mine is smooth all the way to about 45. After that, at times it feels a bit like my A3 death wobble before I put new tires on her. I drive at 40 MPH and she is smooth. Anything over that, and she feels like she could start shaking her head if the conditions are just right.
Pete

 

[dilvoy]

A guy that was in the USAF told me that when they would park them, they would keep them far away from other vehicles, because they could start wiggling side to side and beat themselves and whatever was near them up. It shure sounded weird to me, but how could someone make up a story like that? Anyone else heard of this?

 

[73m819]

A guy that was in the USAF told me that when they would park them, they would keep them far away from other vehicles, because they could start wiggling side to side and beat themselves and whatever was near them up. It shure sounded weird to me, but how could someone make up a story like that? Anyone else heard of this?

The Mk48s were MARINE, do not think the USAF had them

 

[Hoefler]

Given the steering linkage controls a valve that operates the two articulation cylinders, the only way to make this beating up stuff would be for the engine to be running to power the hydraulic pump and would need some operating the steering wheel. The articulation cylinders slave off the steering.
Your buddy in the AF was probably pulling chainski-tell him-8 hours bottle to throttle!
Pete

 

[tcody]

My truck bounced a lot after about 30 mph - just figured that was a combo of the suspension and tire balance. I won't take it on a trip until I get it licensed, but once I do I'll give you a better assessment. As Pete mentioned at low speeds there is a valve that operates the articulation cylinders. I found out the hard way that you have to watch it when firing it up standing by the cab or you will get a "love tap". Big trucks are like big dogs, they are easy to love but have their own personality and don't know their size. However, I have a narrow driveway and once you get the hang of it I can thread it down that narrow lane without trouble. I don't know if you can even balance those tires, but I am guessing that's your problem. Good luck!
Cheers

 

[bbf]

the tires are out of balance my mk dose it as well went to a truck tire shop and have no way to balance

 

[212sparky]

Can you add counteract beads to the tires? That should help with balance issues.

 

[Hoefler]

Looks like he likes chocolate cake! St Bernards are way cool-cooler still if you put a small barrel under hus neck.
Pete

 

[saddamsnightmare]

October 30th, 2012.

Unless you have a belt separation or other internal failure, your tire unbalance can be corrected with either the counterweight beads as noted above, or older truck tire shops have a lathe that will take off the high spots. The latter system works very well on NDCC's on deuces.

 

[Hoefler]

I had never heard of balance beads so went to a site to learn. Looks like they work so will try them.
Counteract Balancing Beads - Balancing the wheels of transportation, Improve fuel economy, Increase tire life, Reduce vibration, Reduce maintenance, Balance from truck to trailer, Trouble free and guaranteed, Environmentally friendly, North America,
Pete

 

[ZZ4x4]

Hey all,

Took the Mk48 out for a long drive today... awesome vehicle and LOTS of stares...

However between 35 and 40 there is a pronounced HOPPING that begins and gets fairly violent at times... anyone else have this?

It doesn't always do it, sometimes its not bad, other times very bad...

Dave,
Thanks for posting this. I was starting to think I was the only one having the issue! I've put over 200 miles on mine so far.
In my case, my MK can be smooth as silk for 2 miles and then starts banging up and down on one side or the other (or both at the same time) for another 2 miles before it smooths again. Can be a little scary! I beleive the tires being close together go in and out of sync on the flat spots. It is definitely not an articulation issue. I also don't beleive I have a balance issue ( relatively speaking) since it is not specific to any certain speed. The roughness comes and goes at any speed from 30 to 55mph. I have raised all tires and spun them.They are all out of round by 1/4 inch to 3/8 inch. I also rotated the RBU tires to the FPU with no improvement. None of the suspension links, ball joints , or bearings show any sign of trouble. I am looking for a company that can true the tires by shaving them round. I've found a few shops but they are too far away...


Pete , or anyone else, please post if you get the balance beads and they help....
Thanks,
Jeff

 

[Hoefler]

Will do.
I think you are on to the issue here-the wheels being so close together may demand the wheels/tires be near perfect.
Pete

 

[BiffJ]

One point to note on the unintended articulation issue is that the parking brake locks out the steering of the rear unit so on startup if you get a love tap you have problems other than that. If you're starting with the park brake off you shouldn't be. If your unit is moving(articulating) with the parking brake on than you have a problem elsewhere. Jump in the truck and make sure the parking brake is on, start it up and move the steering wheel. If the rear body moves something is wrong. If it doesn't than unlock the parking brake and it should steer and that is correct. As far as the bouncing its a combination of tires and harmonics. Some trucks sit for a long time and drive little. They end up with flat spots on the tires. When you turn the truck in the inside tires cover less ground due to the smaller turn radius. That means your tires are no longer in sync and all the flat spots don't line up anymore. For different combinations of where the flat spots are you will have different types of bounce. When they all line up again you have max bounce. Depending on how much weight you have on the truck and your speed the bounce can be more or less and occur at different speeds. The tire pressure can affect this as well because it changes the spring rate of the tires. Lots of little details can add up to a perfect storm of bouncing down the road. It may be an inherent issue with the trucks or it may be an oddity. From the small sample group we have on the civvie side its hard to tell.....

Frank

 

[ZZ4x4]

As far as the bouncing its a combination of tires and harmonics. Some trucks sit for a long time and drive little. They end up with flat spots on the tires. When you turn the truck in the inside tires cover less ground due to the smaller turn radius. That means your tires are no longer in sync and all the flat spots don't line up anymore. For different combinations of where the flat spots are you will have different types of bounce. When they all line up again you have max bounce. Depending on how much weight you have on the truck and your speed the bounce can be more or less and occur at different speeds. The tire pressure can affect this as well because it changes the spring rate of the tires. Lots of little details can add up to a perfect storm of bouncing down the road. It may be an inherent issue with the trucks or it may be an oddity. From the small sample group we have on the civvie side its hard to tell.....

Frank

I agree with all. One thing that also seems to support the flat spot diagnosis is that coming around large sweeping bends in the highway the bouncing sessions are very short or non-existant. Seems to support that the tires are turning at slightly different rates and get out of sync quickly which releives the bouncing. On a straight road the bouncing sessions can last a couple miles. I've had another driver drive next to me and all he could see was occasional bouncing of the tires. There was no wobbling or shimmying to speak off. If the tires shimmy then points more to steering king pins, tie rod, or ball joints.

 

[M2HB]

Maybe see if you can talk to someone at the manufacturer?

 

[Kayne]

Im new to Military trucks but very familiar with Large vehicles like the Cat articulated 740 dump trucks. They have about 6 foot tall tires. We had a huge rental fleet and one thing we had a small problem with (guess it depends on how you looked at be as its a 6 foot tall tire) with the tire getting flat spots in them. This didn't matter and didn't show up when they where in normal conditions, but on long haul roads where speeds of 30+ mph where reached it quickly became a huge issue with 160k lbs bounding around like a kid in a candy store. The only solution we found was to have them shaved down. which we had to ship the tires off to get done. Im trying to find where we did send them. Altho it may be cheaper in your case to simply buy new tires.

Kayne