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Discussion Starter #21
Got it, thanks guys.
Regarding the rear control arm, it has a plastic cover over it and says "do not remove" so looks it's been epoxied. Too bad.
I'll have to replace all the control arm and linkage bushings in the near future. Looks like each on the left/right has 3 additional linkages aside from the main control arm. I'm not looking forward to replacing those. Hopefully they sell just the bushings for each linkage/arm.
 

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I don't know why people believe the crap about higher pressures wearing out tires. Have you had experience to back up your claims?
I have had tires last 90,000 miles at 50 psi on a sedan. Increased pressure does NOT reduce grip. It stabilizes the tire and keeps it from "rolling under" during cornering. It reduces hydroplaning too, because the cure cannot cave in as easily from the water pressure. Higher pressure helps the tire run cooler because there is less flex in the tread and sidewall.

It does transfer more energy to the suspension, but there has to be a tradeoff somewhere. Mfg pressure is for comfort, not handling, grip, wear, or hydroplaning. I used to work at a tire shop and most of the tires I replaced were worn on the edges and had better tread in the center, even when at mfg pressure.

I have had only good results from increased tire pressure
 

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All manufacturers recomend 32-35 psi for tires installed on cars and SUV's. That pressure is not for "comfort", is for safety. That's how their cars are approved by DOT to be driven on roads. The higher pressure written on side of the tires is also for safety. The air heats up when driven in the summer and that increases pressure.

High pressure reduces the tire contact patch and so it reduces grip. Plus, being so hard it actually bounces off road, loosing even more grip when going over bumps. That can be dangerous in turns.

Also, no radial tire would "roll under"! And even if they would slightly shift... the contact patch would be the same (if not bigger), so no reduced grip.
 

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Grip is a function of pressure and surface area. As the patch gets smaller the pressure goes up, maintaining grip.
I am not suggesting inflating to the point of having a bouncy ball. 40-45 psi is not going to do that. If that was my goal, then I would have said 80-100 psi, which is grossly stupid unless you are driving a heavy truck.

Remember the Ford/Firestone issue? Ford blamed Firestone for crappy tires, but Firestone made the tires to Ford's specs and Ford under inflated them, from the factory. That was a dot approved application and the customers paid the price when they were injured or died.

Radial tires can and do roll under or deform under cornering. I see it all the time. You can ask racers about it. Some tires are much worse than others. Touring tires roll under a lot. I can't stand driving a car with touring tires because I can't feel the road or what the tires are doing and they make the car feel like it's rolling off the turn.

There's a lot of mistruth out there about tires, and it seems to spread like wildfire and be taken for gospel even though most people have no experience to back it up.
 

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Pressure on the tire patch is given by car weight and that's all... Reaction and action must be equal. Otherwise car would either fly up or go into ground.

Firestone unions screwed up that batch from one plant, nothing to do with anything.

As for no experience, I guess the hundreds of engineers that wok in auto industry have no experience. Or they all conspire to say 32 PSI instead of 50PSI for some unknown reason.
I am done here, good night!
 

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Pressure on the tire patch is given by car weight and that's all...
That's correct. Pressure on the tire is from the car.

I used to teach physics, so I will try to explain this like I was teaching students.

Friction is dependent on surface area and pressure. If you decrease the surface area or contact patch, the pressure goes up. Friction stays the same.

Lets say there is 1,000 lbs of weight on a tire.
You would have a contact patch of 30 sq in at 32 psi.
You would have a contact patch of 27 sq in at 37 psi.
You would have a contact patch of 25 sq in at 40 lbs.

With 1,000 lbs on the tire...
A coefficient of .1 gives 100 lbs of friction.
A coefficient of .7 gives 700 lbs of friction.

The coefficient of friction determines friction when multiplied by the weight on the tire, regardless of surface patch area.


The recommended 32 psi is enough to not be "flat" and it limits how much force is transmitted to the cabin which increases comfort. That's all. I have been "over inflating" tires for ~15 years and have had no ill effects. In fact, it has saved our lives more than once. Better handling and hydroplaning resistance save lives. Any potential premature wear on components or tread is worth it. I have not seen any premature wear on anything in 500,000 miles, and I am an experienced mechanic with 20 years experience.

Just my very limited personal experience on the subject....
 

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I have a similar problem from the last month, but not only on 50mph speeds, i have rear end wandering from the lowest to high speeds.

Front & rear shocks are new, replaced the rear adjustable control arms, the wandering is still there.
I'm guessing it has to be the rear trailing arms. My car is 2006, has 239000 km and i think this is too much for the bushings.

Yesterday i visited a MOT facility and asked the guys to perform a stability/angle test & shock absorber efficiency (%). Shocks were fine, except an 20% difference on the response on rear ones (which replaced last week). Their conclusion was that the real trailing arms have play. Visually, their bushings do not appear worn, but they flex softer that they should. So i guess this has to be the problem. I am about to replace them soon, so i will let you know.
 

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When I decided to replace the bushings on one of my older vehicles (a Hyundai XG350), on the car, with the wheel in air, the bushings seemed fine. Only when I took them out and compared to the new ones I saw how bad they were.
I guess when the weight of the car really compresses those bushings, the situation changes. By hand is hard to achieve that level of compression, and you can "feel" only the absolute worst situations.
 

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I noticed my Rav4 wanders (almost to a scary point) when passing full power from about 50 mph as if one of the suspension bushings are loose or broken. Has anyone experienced this and know of a solution? I checked underneath and checked all the suspension bushings with a prybar and they are fine.
Take it into the mechanic and have it checked out.
 
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