Front suspension instability observation

[Flyfishing]

I have just installed Hummer H-2 wheels and Toyo open Country A/T 285/75 r17's ( at 75 psi) on my '07 3500 SRW 4x4. Other than a Thuren tracbar and DSS stabilizer the front end is stock. Prior to the wheel/tire conversion I was running the stock Michelins.

Today I noticed a vibration in the front end upon hitting some small, two inch deep depressions in the road from manhole covers at about 45 MPH. It is difficult to describe exactly but I'll try. It is a low amplitude oscillation at a frequency of approximately 4 or 5 cycles per second and it damps out in about a second or second and a half. I believe the engineering term would be a "Phugoid" oscillation. I'm sure the engineers have tuned this suspension for the stock wheel/tire combination and any deviation, especially with a heavier wider tire and different wheel width and back spacing changes the tuning.

I can begin to understand the dynamics of the "Death Wobble" a little better. I'm wondering if the so called death wobble is a form of "resonant frequency" instability. It may be,that by just adding aftermarket shocks and steering dampers, one is just masking the extent of the problem. and not changing the natural frequency of the front suspension?

It may be a worthwhile experiment by adding (clamp on) weights to the tie rod and/or drag link to change the frequency of the front end and retune it to different wheel/tire combinations.

I slept at a Holiday Inn once...



Regards, Richard

 

[Regcabguy]

Try dropping the pressure on your tires. According to your sig. ,you're not loaded with anything aside from the fine Caravan camper shell. I've got my 285 Toyo AT's set at 65psi all around with my 2000# camper on. Chuck the stock shocks for some Bilsteins. That should help. Hopefully,the tire shop did a dynamic balance with weights on inner and outer sides of the wheels. I'm sure others will suggest other ideas. Your E-rated tires are like rocks@75psi.

 

[MA2LA]

There are a couple components of the whole death wobble thing. I have spent years working on Jeep Cherokees and playing with different lifts and front end setups, the main things that make the wobble worse is any joint that may have any play, that is by far the worse thing. Next is your alignment, I do all my alignments with a tape measure and some string. I have found that lot of the shops that do them have a pretty wide rang of specs that they call good. I measured my dads grand Cherokee after he had it done and they were off of where i normal set my toe in at. Once I readjusted it, it drove sooo much better. another thing that can do it is a tire that is marginaly balanced. now everyones favorite bandage for the problem is steering stabilizers. They can do wonders but a well setup front end doesn't need one to stop death wobble, I rare have run them on my lifted jeeps and they have driven great except when I have had a loose track bar or tierod end.

 

[Flyfishing]

Bob, I agree with you regarding the high air pressure and shocks. When I ran the Michelins with 80 psi over the same road I did not experience the oscillations. These oscillations are very subtle and nowhere close to the often mentioned "Death Wobble". The tires were dynamically balanced last week. I agree that the stock shocks are pretty poor and when I installed Bilsteins on my '99 2500 CTD it made a world of difference. The reason the tires are at 75 psi is that I'm ready to go on a trip and the truck AND trailer will put a good bit of weight on her.

The purpose of the thread was twofold. One was an observation in the change of ride characteristics with a change of only one variable (Wheels/Tires at almost the same pressure). Second was a thought, not necessarily correct, that added mass on certain components would "cure" some of the problems people are having.

Regarding MA2LA response. True, all bets are off with worn/loose and mis-aligned componenets. My truck has 14,000 plus miles. I have not checked my alignment so it my not be within spec. or close to the optimum setting for my truck. I was under it yesterday to lube it and from what I could tell things were tight. The wear patern on the stock tires was fine.

For years I have been reading about the DW and a lot of the folks that have this problem of one degree or another claim that all of the joints are tight and the alignment was set at what the experts say are optimum and they still have DW.

The "add mass/weight" was just a thought and it may be something to consider when all else fails.

And Yes, the Caravan shell is very nice!



Thanks for "listening".



Regards, Richard