Weird front end shimmy

2010 FJ Cruiser has an intermittent shimmy that you can feel in the steering wheel at highway speeds. Noticed it not long after new tires. Go around a long curve and it starts, then stays until you go around another curve and then stops. It is as if the wheels have to align a certain way for some harmonic effect. I had the tires rebalanced and rotated. My mechanic said they balanced up very well. Shimmy is better but not gone. Wierdest thing is that when it happens, if I keep my foot on the gas and gently press the brake the shimmy stops and the returns as soon as I let up on the brake. When I asked my mechanic if it could be 2 slightly warped rotors he said the shimmy would be worse braking not better. Any ideas anyone? Thanks.

Are the new tires the stock size or did you put on oversized off road tires?

This one could be tough to solve. The FJ Cruisers have the large, heavy tires, the short wheelbase, the high Cg, and the hard suspension that tends to want to shimmy going around corners. And no matter how well you balance the wheels, if there’s any erratic wear it’ll never go away without new tires.

So, a couple of questions:
First, have the alignment done and everything related checked out. It might be as simple as an out of alignment condition. Or a worn bushing, although your truck is young for worn out suspension components. But you never know, you could have a split bushing or a leaky shock.

How did your mechanic balance the wheels? If he used a regular spin balancer, you could still have an eccentricity or anomaly in the tires that he cannot pick up. If he doesn’t have a machine that does “road force balancing”, try to find a shop that does RF balancing will detect anomalies that a regular spin balancer won’t.

Do you have any irregular wear? If so, the only solution might be “rounding” the tires. In the old days tire shops used to have machines that would turn the wheel while a small spinning cutting blade went back & forth cutting surface rubber off to “true” the tires. I don’t know if anybody even has those machines anymore.

You may also need a “steering damper” added. It’s basically a shock absorber mounted sideway to the steering system that dampens sideways wobbles.

Is this a 4WD or 2WD FJ? I’d guess 4WD.

This SUV has a full-time 4WD with all 4 wheels driven all the time. It has Torsen gear-type differentials front, rear and middle that compensate for turning. I think maybe the front tires are not quite the same circumference so driving straight sets up rotation in the front differential that it tries to resist.

To fix it, jack up front corner at a times and take a flexible tape measure and measure the circumference (how big around - right at the center) of each front tire. If they are more than 1/2 inch or so different, that may be your problem. Check the rears to see if you can create a matched pair with very similar circumference. Put those 2 on the front. I think the problem will go away.

The FJ is know for this issue. The steering rack is incredibly sensitive to road and tire conditions. There are a few things that can help, first make sure the balance mark on the tire is aligned with the valve stem. Second have the tires force balanced and the alignment set, you want everything in the alignment to be dead on. If you have aftermarket wheels ensure that they center on the hub or use centering rings.