1. #1
    Mary Ellen
    Firehouse.com Guest

    Post International Chassis Problem - ideas????

    Our dept recently took delivery of a new engine. It's on an International 4900 series chassis. The GAWR front axle is 12,000 and the GAWR rear axle is 23,000.

    The chassis weight distribution - fully loaded - is 8,000 front axle and 19,500 rear axle.

    Driving is fine below 35 mph and above 45 mph. In the range from 35 to 45 there is a serious bouncing problem. Almost like out of round or out of balance tires on the front.

    Every tire has been measured, balanced, spin balanced and swapped out. Same for the rims. (Dealer swapped tires and rims from new truck on lot that did not bounce, did not help). Every drive line angle has been measured and is within factory spec.

    Today, International said they were going to remove the 12,000 pound springs from the front and replace them with 10,000 pound springs. They believe the distribution of the weight is too far back. (Center of gravity). Maker says, not possible - they followed all International guidelines for weight distribution.

    Since the truck does the same bouncing when empty, this makes no sense to me but does anyone have any other ideas? Ever seen anything like this?

    Not really interested in slamming maker. At this point, they are working with us. They have delivered many similar models on same chassis with same weight and had no problems.

    Your thoughts gentlemen?????

  2. #2
    Firehouse.com Guest


    Were the wheels spin balanced on the truck? If not, you might have an out of balance brake drum.

  3. #3
    Truckie from Missouri
    Firehouse.com Guest


    We have International 4700's under our ambulance boxes that had similar problems. We changed the tires out to a different brand that reduced the speed range vibrations to near nothing.


  4. #4
    Firehouse.com Guest


    What is the wheelbase? Fairly short?

  5. #5
    Mary Ellen
    Firehouse.com Guest


    Yes, can't find the exact figure right off hand but its a short wheelbase two door chassis.

  6. #6
    Firehouse.com Guest


    Mary Ellen;

    Had some problems as you discribed on various chassis makes. Tried every thing changing front tire brands, balancing drive shafts the works.

    You,re describing a bounce as opposed to vibration. One thing that worked a couple times was lowering the air pressure in the tires. Right now you are at around 110 psi. This makes the tire very hard and bouncy.

    I assume you are running 11R22.5 in front and 12R22.5 in the rear. Based on the weights you provided you can bring the air pressure down to 90lbs each and still be able to transport the wieghts provided safely.

    I suggest that you start lowering the air pressure in all your tires by 5 lbs and road testing the truck each time you remove 5 lbs. This will take some of the bounce out of the tire. Do not go below 90 psi, at 85 psi you are getting marginal on the wieght ratings. This has worked a couple times.

    Best luck

  7. #7
    Firehouse.com Guest


    Mary Ellen,

    Have the front springs been replaced on the new engine yet? With a 12,000 GVW on the front and no more weight than you have on the front, I believe that International is on the right track. The description of the bounce and the speed that you experience it matches what I have seen on other trucks. Is the rear suspension a leaf spring or air suspension? I would keep the tire inflation at the tire manufacturer's recommended pressures. Running the tires with lower air pressure can make the rig squirrely on the road. Please,let us know the outcome of the repairs.


  8. #8
    Mary Ellen
    Firehouse.com Guest


    Thanks for all the tips. Unfortunately, none of them worked. Tire pressures were increased and decreased. Front springs were replaced with 10,000 pound springs, made no difference. Rear suspension is leaf springs.

    Chief had settlement meeting with maker on Friday. They are taking back vehicle. Since they have dozens of this model in service, they are sure it's chassis related but we are at the point where even if problem was solved today, 5 years from now when something wears out we'd suspect it was realted to this original problem.

    Thanks for the ideas.

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