Tow truck with poor handling

GregH · January 27, 2011

We earned a customer that owns a tow company recently. His '08 F-450 rollback had some sort of drivability issue, another dealer worked on it, came back with more problems, finally got fed up. Brought it to us and we fixed the drivability problem correctly on the first visit.

So we're golden.

However, this customer had a second line on his RO. Stated that at 45mph and above, if you have to turn the steering wheel sharply (avoid a box in the road, or drift onto the shoulder and jerk the wheel back) then the truck swerves drastically and makes you feel as if the truck is going to swap ends.

Vehicle has about 110,000 miles, on his third or so set of tires. He's been living with it for the last 6 months to a year, and at the suggestion of the previous dealer he bought new tires for it - no improvement. Tires are correct for the vehicle. Front end pieces are not worn. No issues noted with the rear suspension. No suspension modifications related to the rollback. No obvious wreck damage. The owner cannot identify a specific event corresponding with the onset of symptoms, although he is sure it hadn't always handled this poorly.

I test drove the vehicle extensively on their first visit. I live 60 miles from work, and I drove it home one night. I didn't believe the customer when he said how poorly it drove. I was wrong. On a straight stretch at 60 miles per hour, I moved the vehicle so it straddled the center dotted line. I then turned sharply about 5 degrees, as if to make an aggressive move to pull out and pass someone. It felt as if the rear end was on ice! The truck heeled around, and I automatically steered in to correct what felt like a slide. Of course, the truck was not sliding around, and my abrupt correction was a fraction after the truck caught itself and stopped yawing. The result is that by reversing the direction of the steer, I induced a second rotation event in the other direction. Ack! Quick turn the wheel back! Three or for oscillations later, I got the truck under control by using little nudges on the steering wheel.

Below 45mph, the symptoms are gone.

On the return trip to work the next day, I experimented again. At 60, I jerked the wheel again. But this time, I did not try to correct what felt like a slide. I got the impression that the rear end moved over in the opposite direction of the steering wheel direction. Almost as if the rear axle were able to move side to side, or the rear sidewalls were exceptionally weak. It felt so much like it that I tried it several times while keeping the left rear wheel in the rear view mirror. Obviously the rear axle did not move side to side that I could tell.

I told the owner that after a thorough inspection and test drive, I did not find a cause for the condition. I explained gently that it appeared to be a vehicle characteristic, and I did not have a repair to recommend at that time. The rear axle is located at the center point of the rollback platform, and there is a wheel lift under the rear of the rollback. So the vehicle's center of gravity is much further back than on a stock truck.

The owner was cool with it, and was happy that we were able to fix the drivability problem. However, I heard today that he is bringing it back for a small oil leak, and could we please look into the handling problem again?

Anyone felt something similar?

Bruce Amacker · January 27, 2011

Wow, tough one. I'd check the suspension bushings real closely on both the rear axle and front axle, especially spring eye bushings and wheel bearing adjustment on all 4. I'd probably also have someone follow me on the highway and watch the truck as it was wallowing around to identify what's loose.

I'm assuming radials all 6, and tire pressures are correct? If no problems were found I would borrow 4 rear tires from another truck and try them.

Good Luck!

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Keith Browning · January 27, 2011

Any really weird issues like this I have seen ended up being steering gears. Some gears would bind and free up usually causing the driver to over correct. Of course I would want to eliminate any other possibilities first. I would think that any suspension issues would have to be really obvious to cause this. Posted Image

Brad Clayton · January 27, 2011

Worn track bar bushings give all sorts of nightmarish handling problems on these chassis', they have to be torqued wicked tight also.

Jim Warman · January 27, 2011

Ditto on track bar bushings and steering gear... Have someone move the wheel side to side with the weight on the tires... Feel both ends of the track bar.

The steering gear on late 450/550 trucks isn't adjustable for mesh preload in the field. First, check the mesh preload over centre. If that feels acceptable (it may not be 'in spec' but it shouldn't have any "wall slap" either), then have an assistant rock the steering wheel again - weight on the wheels, and feel carefully at the lower sector shaft bushing. If the sector shaft moves side to side at all - replace the steering gear.

Toe in.... if the front/rear weight bias is too high, giving it a skootch more toe in may help.

Is this a 4X4? And does it have a steering damper?

hitechmech · January 27, 2011

Remove the shocks and check them on the floor, i had an experience once and found a seized shock. What a wicked ride.

GregH · January 28, 2011

Thanks for the suggestions. I will check these when the truck arrives.

From the top - the tires were considered suspect by the other dealer. They were not proper for the truck, and seemed like a likely candidate. Swapping to the correct tires did not affect the condition. Based on that, the tires are low on the list of suspects.

That's a good idea about having someone follow me. I just had it that one night by myself, and I wasn't able to see anything with just the rear view mirrors.

Front bearings are good, rears have not been checked yet.

I'm not sure about the steering gear being suspect. I will check it closely as you all have described, but the issue really seems to be in the rear.

Did this chassis have a rear track bar? I don't recall seeing one - although I don't recall not seeing one either...

Again, thanks for the help!

ktmlew · January 28, 2011

I would inspect the frame closely for cracks at the crossmembers...

cbriggs · January 31, 2011

I worked for a tow company a few years ago. We had 4 rollback decks, international chassis. We put a set of tires on one of them and it did exactly as you describe. As near as we could figure it was due to the tread design; deeper, smaller lug blocks with more syping seemed to be too soft, especially with no load on. We tried a set with larger more solid blocks and the concern was gone.

dieseldoc · February 5, 2011

My service truck is an 05' 550 with a service body with crane and miller generator, very top heavy. We just put new tires on and had the alignment done. We run a very aggressive tire on our trucks because of where we have to drive to work on our customers equipment. The old tires it felt fairly stable. However with the new ones(deep tread depth)you barely touch the wheel and she is heading for the ditch. May have to do with the tires even though they are new. . . . . I asked the tech that used to run this truck and he said the first 5000-10,000 miles it is like that but gets better as the tires break in a bit.

Keith Browning · February 5, 2011

You mean as the tires wear and the shallower tread depth makes the tire tread stiffer.

dieseldoc · February 8, 2011

Actually I have only put 3,500 miles on the new tires now and the handling is back pretty much to normal the way it was with the old tires. The wandering wobbliness is gone. I do need to order a new steering stabilizer and put on it though

jimmy57 · February 19, 2011

I know what Dieseldoc is talking about all too well. My personal vehicle is an F450 cc hauler bed 2006 MY. I hate getting new tires. Not as scary as truck OP described but the handling with new tires is awful. General/Continentals in the OEM version are the least offensive and a set of Michelin ZXE (I think that was the model) were damn near impossible to deal with and were traded back for Conti's.

Full alignment has been verified a few times and toe-in has been set above and below spec with no noticeable difference while tires are at nearly new full tread depth. Road surface does make huge difference. I would describe the Michelins as road bike on metal grate bridge surface bad and the conti's as road bike on grooved pavement worn almost back to smooth if those terms are something you can relate to.

Anyway, I can easily see the worst end of tread design and some looseness in steering up front or spring eye bushings in rear turning the tire wiggles into a monster.