Vribration in 5th gear, please HELP!

[peaksja]

I have a 90 base S13 FB. Basically during driving in 5th gear the car often shakes [mainly felt in the steering wheel]. The issue also started bleeding into 4th and 3rd. I currently have balanced 18'' tires and rims. I was told that it may be the u-joints so I bought an intirely new[used] drivesahft and installed it last night. After driving the car for a few hours it slowly came back. Is the problem in the shaft itself or could it be the suspension? Is it possible for a brake pad to be appliying costant pressure noticable at higher RPM?

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The wheels were balanced less than 3 months ago. I do see that one does not hold more than 20lbs. I am going to buy new one. The tire is a 35 series Kohmo. I wonder since 20lbs is less than half the max 44lbs; considered a flat?

I also had this problem with the original factory rims and hubs.Any help would be greatly appreciated.

[JeromeS13]

Is it at a certain speed or rpm range? 20 psi in a tire isn't considered "a flat". And, just because the max pressure for a tire is 44psi, that doesn't mean that's what it needs to be. Tire pressure varies greatly with the tire construction AND weigh balance of the vehicle.

[peaksja]

Yes, today I noticed that the vibration would begin at 55 mph. So after coming to a complete stop, I shifted up and road in 4th until 55mph; same result. Then I thought back as to when it occured in 3rd, was when I was speeding at shifting gears right at around 4,000 rpm and noticed that I was slightly over 55 mph in 3rd gear.

Sorry, as for the tire pressure I should have stated that all other three tires at kept at 40psi.

[JeromeS13]

40psi seems a bit high. What size tires were you running? IIRC, when I was running 215/35/18 or 225/40/18, Yokohama had recommended 35psi all around. As far as the vibration, it sounds very typical of tire balancing. They may be "balanced" or whatnot. But, I'm willing to bet money that its in the wheels/tires. And, if you're noticing it more in the steering wheel, its in the fronts.

[rotorimp]

Since you said that you had the problem with your old wheel/tire combo you can mostly rule out your new wheels. While your at speed--55-gently apply pressure to the brakes. Rotors can become unbalanced. you can also have movement in the ball joints, control arms and in particularly the tension rods.

[peaksja]

Looking closer at the issue... it begain exactly at 55 mph and is becoming consistant. At this speed if I lightly press the brakes my steering wheel does vibrate in the same manor but at a smaller scale. I also noticed that once I hit a bump in the road the issue stopped, then hit another and it slowly increased. It has to suspenion related. I am going under.....