Rear shimmy at 50+mph

dalmationlovers wrote:
Why would 80lbs on the rear be too much. I have always run my rear duals at or near max and my front tires below max. Both checked and filled cold. Which is easy in the PNW since we don't seem to like it warm.

I will call the tire shop tomorrow.

Because you are only an 11000 lb RV like me. Winnebago says 61 PSI on mine. My Contis say 80 PSI max and that would be at max loading (per tire) which you and I are nowhere near. All you are doing by running at that pressure is making your ride harsher. There is not going to be any significant improvement in fuel economy. You should have a sticker on your door stating what tire pressures the MFR recommends. Start there.

If this came on suddenly, after you added Extenders, i'd say look at how the wheels were reinstalled, for low pressure due to leakage from a new source, and there's always possibility a tire has failed internally. Last thing you want is a blowout.

Gently stated, you should weigh your rig and adjust tire pressure according to a Load/Pressure chart, available on sites like Michelin. Doesn't matter who made your specific tires, so long as the excat specs (Size, Profile, Load Rating) all match. Note the Chart will have pressures for Single and Dual, so read the right lines. You ARE over pressure.

Strongly stated, IF you must have Extenders, the Valves in the Rims must be METAL. Extenders on Rubber stems WILL lead to failure.

If the shimmy has been developing, then I'd include Driveshaft among my list of suspects. A loose or really stuck U-Joint'll cause commotion.

For those reading this I have been driving old farm trucks, trucks, pickups ect for over 62 years. Many with used tires and very old tires. When you start to get a shake on your rig.. trailer, motorhome, car, pickup ect that you did not have before it may be a wheel weight but odds are its not. Most common is a tire coming apart. The tubless tires are made with a thin layer of rubber inside to hold the air in. When that thin layer of rubber gets a hole or crack (from setting) inside the air will leak into the area between it and the tread pushing the tread off. A bounce or bump you did not have before should send up red flags. Odds are you have a problem..

Dalmationlovers, First off, I am no expert but I do have 64 years under my belt. From experience Over inflation is a very real concern. Dakzuki and Snowdance have given you good information and I agree. One of the better investments I have made was an Infrared Thermometer. If something starts bumping or shimming stop and use it on the tires. It saved me twice, well worth the $20.00 I spent on E-bay. Exact temperature is not important but if five tires are around 118 deg F and one is at 145 deg F, you have a problem.

The primary thing is that all tires are around the same temperature. The same logic can be applied to wheel bearings on trailers as well.

I would wager you have a separation, possibly caused by over inflation.

I am no expert and advice is only from experience.

80 psi is for the tire's maximum load rating. With the weight of your rig being relatively low, the tires will be overinflated for the actual load they are carrying. Just find a closed WSDOT weigh station and you can check your axle weights, then look the up the correct pressure on the tire manufacturers' website.

Why would 80lbs on the rear be too much. I have always run my rear duals at or near max and my front tires below max. Both checked and filled cold. Which is easy in the PNW since we don't seem to like it warm.

I will call the tire shop tomorrow.

I have been thru that before several times. And it was a tire coming apart. I would have a good tire shop check. In my case I could not find it but the fellow at the tire shop I go to saw it 15 feet from the rig and was right both times. One tire was near new.. I to question your tire pressures.. Sounds like way to much..

.......and did you remove the back duals yourself to install the extenders, and then make sure the wheels were seated correctly back on the hub, and then tighten the lug nuts to the proper torque?

First make sure the valve extenders aren't leaking - which can always be an unseen problem on the inner wheels. If the wheels didn't cause shimmy before playing around with the extensions they shouldn't shimmy now. What kind of extenders are you talking about?....the solid metal ones which are actually valves and installed at a tire dealer; or the rubber ones or the braided ones that screw onto the original valves??.....these are the ones that tend to leak if not installed very carefully. Lots of postings about air pressure problems with the braided screw on extensions. Not everyone has those problems, but lots do.

dalmationlovers wrote:
Have a question for the experts out there. This weekend was the first extended trip after I installed my value extenders and was finally able to adjust all the air pressure. 03 Chateau Sport 26Q, 8.1 Vortec with 480L tranny. Set the duals at 80lbs (max) and the fronts at 75 lbs with 80 as the max. Weigh about 11,500 fully loaded.

The rears before were at 90 lbs and the short trips I took I didn't notice any shacking or shimmy. Friday and Today, each way about 2.5 hours time, while traveling to and from the campground the shimmy was bad. No issue with the front tires and would only begin above 50 mph. I was thinking that my duals are out of balance. Just need some ideas.

Thanks
Arnold

At first blush it looks like you have too much air in your tires for that weight. I'm thinking more like 65-70 PSI max....probably closer to the 65 end. Pressures should be set when tires are cold (not driven on). Not sure what your tires are rated for but 90 sounds like way too much air. You will need to get the wheels/tires looked at. Your could have a tire fixing to let go in the worst case.