Bounce - Cant stand it any more!

[macattack]

I agree with Fifthwheel. Check powerdivider and driveshaft for wear

how would you go about checking the power divider?

[leversole]

Jacked each wheel off the ground individually and ran at speed, could not generate any shaking...hoping the front balance will reveal the issue...

[leversole]

Had the front wheels/tires balanced on the truck at the Mack dealer...no change in bounce but the steering wheel is smooth as a cheerleaders a$$...

Thought maybe by jacking up one wheel at a time, I was preventing whatever is causing the issue by compressing the suspension on the jack...so today...

I jacked up each side at a time under the trunnion...ran it at the "culprit" speed at length...NO sign of shaking, wobble, nuthin!

Dont know what I'll do next...

http://youtu.be/VFzNrUmWdwQ

[jaysm35a2]

I thought about this thread the other day. I put new rear tires on my Mack and I get a hop at higher speeds. like 50 and up. They dont wobble.

Would 10 lbs of air diff in a tire vs. another one cause mine?

[Swishy]

Get the air out

Ifn U run n bobtail try 50psi steers n 30psi Drive

Tyres R jist air suspension

cya

§wishy

[DCR]

Be very careful running tires under inflated. I have heard both Goodyear and Michelin tire reps state that a 20 percent under inflation will damage the side walls in just a few miles. If running 100 psi anything under 80 psi is going to cause future problems.

[Swishy]

DCR

I never beleive any thing a salesman tells me

they only wanna tell U wot u'd like to hear

LOL

Yes they B right if runnin with a load

But run n bobtail there aint enuff weight on drives to damge sidewalls

a M8 tried run 20psi in a tandem drive bobtail to improve ride

he said twaz a tuch to lo in pressure

Give it a try n U'll notice the tyre looks no different with no wall damage

cya

§wishy

[DCR]

Swishy,

The reps were tire engineers not sales people that addressed a group of Prevost bus owners at at Prevost Owners Group rally. I'll stay with what they advised. Thanks for your input.

[barbe]

_{have you checked the brake drumd i have had some trucks with that prob and found the drums eather mounted out of round or out of ballanced if you have budd type hubs it is a prob more offen than you may think}

[jaysm35a2]

They are Goodyears and spoke hubs. Its a 66' east coast Mack. Im going to take the PSI to around 70- in each tire and see if that works. They are all around 70-80 psi right now. Just didnt know if 10 psi in one tire and 80 in another tire would do that. Also , I dont pull trailers ... yet. I just dont have one or a place to keep it. The drums are good, I looked them over today , Jay

[dirtywhiteboy]

most trucks bounce pretty good bobtailing....ant no way around it....tie sum weight to it or hook it to a trailer an see if it smooths out

[jaysm35a2]

Finding a fat girl my wife approves of to tie to the back of my trucks another issue...

[hatcity]

[Rob]

Slurpy!!!

Rob

[leversole]

As the bounce occurs at the same speed EVERY time,no matter the phasing of the wheels side to side or front to rear, I must assume that if the problem is the wheels, it is only one of them, further, the tires and wheels have been replaced, that only leaves the drums, or the wheel hub. If the wheel hub was bent, I would see that in tire wobble.

My current thought is that one of the drums is way out of balance and setting the bounce in motion. The mack dealer balanced the front at 65 mph, and his equipment spun the tire up to speed pretty fast, so it might not have revelaed itself, as I can accelrate the truck quickly and blow right through the bounce before it has a chance to get going.

I guess I will have to live with it until I can get hold of some replacement drums...

[rhasler]

We have pretty much the same balancing equipment and I have found that if you spin up too fast you can miss the spot where a wheel is out of balance. I usually try to spin the wheel up a little slower so I can get a "feel" for a vibration and not have to spin the tire up several times.

I like fixit's suggestion. I've run into trucks that had out of round drums that caused really bad vibrations at certain speeds and was only able to track the vibration down by temporarily disabling the brakes on the suspect wheel.

[Rob]

I've seen a lot of drums new, and out of the box be off in balance. Some as much as six ounces depending on size. As they age and wear, it is a definate possibilty of being off. However, the circumference in which the unbalanced condition occurs is much less of a problem near the hub, as opposed to the outer surface of a tire where the actual speed or surface feet per minute is much greater.

I've had drums sandblasted, then turned true, and then balaced on my tire machine and acheived satisfactory results. I can only go to about 1/8th ounce accuracy using this method but it's close enough for who it's for.......

Have you looked into the power divider yet?

Rob

[jaysm35a2]

_{I aired all my tires to the exact same PSI , I wasnt able to drive it after I did that but do you think that could be an issue for mine hopping is having a diff PSI up to 10 pounds diff in each tire?}

_{Sorry to ask a question in your thread Leversole. Im with rhasler, could it be you had the wheel spun up to fast to miss the spot?}

[leversole]

Have you looked into the power divider yet?

Rob

What/How can I check the power divider? Will also try and back the brakes off two at a time...I will also indicate the front and rear drive shafts.

[Rob]

What/How can I check the power divider? Will also try and back the brakes off two at a time...I will also indicate the front and rear drive shafts.

I like to butt the truck up against something solid, or chain it down to check for torque bias. Basically make sure the axles want to pull together and spin tires together. If that passes alright, pull it apart and look at the peanuts and drum for scoring, or wear in the "gullets" meaning the high and low transitions of drum and the cam housing for deformation. These are wear items but if it is not smooth in operation, (meaning binding) you can get a shake or bounce originating in this unit that is difficult to track down.

Rob

[leversole]

I like to butt the truck up against something solid, or chain it down to check for torque bias. Basically make sure the axles want to pull together and spin tires together. If that passes alright, pull it apart and look at the peanuts and drum for scoring, or wear in the "gullets" meaning the high and low transitions of drum and the cam housing for deformation. These are wear items but if it is not smooth in operation, (meaning binding) you can get a shake or bounce originating in this unit that is difficult to track down.

Rob

Thanks Rob...can I check it on gravel? Or does it need the friction that asphalt provides? Does having either the front or rear off the ground and it not trying to bias any torgue to the rear on the ground mean anything? I would think running either rear up through the rpm range it would be trying to bias at least some of the torque to the stationary set...maybe not...I have read the manual and have kind of an understandstanding on how the unit operates...

[Rob]

Thanks Rob...can I check it on gravel? Or does it need the friction that asphalt provides? Does having either the front or rear off the ground and it not trying to bias any torgue to the rear on the ground mean anything? I would think running either rear up through the rpm range it would be trying to bias at least some of the torque to the stationary set...maybe not...I have read the manual and have kind of an understandstanding on how the unit operates...

I forgot to mention gravel or dirt!! Not asphalt or concrete as you don't want to break anything. If you lift one axle only that axle will spin. The power divider actually bias the driveline torque trying to keep both axles together yet allows differential action between the two for turning the truck. On gravel or dirt, the truck should try to keep both rear axles trying to pull the truck and being unable to move, should spin both sets of drive tires, (on set on each axle). If it won't hold the axles together during a pull, it is shot.

Another trick I like to use requires a sharp incline. I attempt to drive up an incline to see how far I can progress till the front axle starts to spin. By design, (I'm told) the Mack power divider should be able to make a single axle hold approximately 70% of what both axles can take. Typically in a good setup, I can almost get the front axle to start lifting from the ground before it starts to spin free. This is true in either forward, or reverse. If the torque bias isn't correct and working, nothing else about it works right either. Wear can cause loss of traction and certainly a driveline vibration as this thing is always working in series with the driveline torque, both on, and off power.

Rob

[leversole]

This is very interesting! In the last few days, I have been throwing the trans in nuetral during the bounce and sometimes it goes away! Thought at first that it was descending thru the "bounce zone" too fast...maybe you are on to something! It is not in time with the tire rotation (bounce cycle is much slower). Maybe I am just being hopeful!

Now, to find a strong building with a dirt parking lot!

[Rob]

This is very interesting! In the last few days, I have been throwing the trans in nuetral during the bounce and sometimes it goes away! Thought at first that it was descending thru the "bounce zone" too fast...maybe you are on to something! It is not in time with the tire rotation (bounce cycle is much slower). Maybe I am just being hopeful!

Now, to find a strong building with a dirt parking lot!

Find a dirt pile out of a dump truck. Mash it a few times easily with the front tires, then ease into it.

Rob