RePost: Highway Wind Stability Problem Summary

Summary of my experiences and opinions:
The stock Goodyear Integrity tires with 40k miles (10 months) on my 2004 Prius were replaced with Bridgestone RE950 tires of the same size-diameter. The Bridgestones are a huge improvement in handling-stability at freeway speed, have fantastic dry and wet grip and are better on irregular surfaces. After the first 4,500 miles they have an average amount of noise (the Integrities are quiet when new), provide a firmer ride and get about 3-4 mpg less (now averaging about 49mpg combined driving). They are an excellent value ($70 each sale price w/mount-balance). I would definitely buy these tires again.

RE950 Performance Summary (see attached file)
Tested over 4,500 miles combined driving. MPG done on 160 mile commute (30 mi 2 lane country-55 mph, 30 mi freeway with 1,000 foot climb-decent 75mph, 20 mi stop-go, city driving 35 mph).

Prius Background
Many 2004-2005Toyota Prius owners have been struggling with stability issues at highway speeds on the 2004-2005 models. These issues include poor cross wind stability, poor tracking and twitchy handling on irregular surfaces. The magnitude of this problem can be seen on the popular PriusChat.com website with over 10,000 views of the topic “Poor Steering and Handling on the Highwayâ€. Considering that about 24,000 Prius models were sold in 2004 and many magazine reviews have made note of this issue, highway handling is an area that needs improvement on the Prius.

Most people buy a Prius because they want outstanding gas mileage. This has caused many Prius owners to inflate their tires from the stock 34front/32rear psi factory suggestion to as high as 42/40 psi. Increasing tire pressure does increase mileage by about 1 to 2 mpg. Increased pressure also helps reduce the rolling and yawing tendency of the Integrity tires (at the price of a very firm to stiff ride quality at 42/40 psi interior parts start to vibrate over irregular surfaces). However, even at 42/40 I think the Goodyears have terrible side wall flex which leads to very twitching handling at highway speeds with the relatively narrow and tall Prius. The stock Prius shocks and springs are rather stiff (possibly to compensate for the weak tires ?). The front sway bar-bushings and rear torsion bar perform their jobs adequately. Adding further turmoil to the Prius’s handling woes is an electrically boosted power steering system with excessive boost at all speeds and a moderate amount of dead center play. This tends to cause the driver to over correct at highway speeds. The Prius also has a very sealed up under body (presumably to help minimize aerodynamic drag). Testing with different air dam locations on the front of the car has shown some improvements in highway speed handling.

All this raises the question “Is it the Goodyear Integrity tires or something else in the Prius (or a combination)?†To answer this I rented a Ford Focus and Toyota Corolla S for two days each of highway driving. The Focus has nearly identical dimensions to the Prius and drives much more stable even with Goodyear Integrity tires (60 series vs. the 65 series on the Prius). The Focus has an excellent ergonomic steering wheel with just enough boost in the power steering to feel “rightâ€. The Corolla has the same size wheels and tires as the Prius, but handles like an average small 4 door car. The steering setup in the Corolla is much better then in the Prius. The Corolla “S†package has front chin spoilers, side skirts and a small rear deck spoiler to help aerodynamics.

Based on the contrasting test drives I would conclude that the problem is very poor tires combined with poor front end aerodynamics and steering. On my test car I ran the stock and modified front air dam positions with the old and new tires. Moving the air dams forward helped with both manufacturer’s tires, but the Goodyears still felt very unstable at highway speeds. During crosswinds and lane changes the Goodyears felt like the back end was ½ a second behind the reaction of the front. Some Prius drivers describe this as the back end wanting to swap with the front. I feel like the front tires roll first, then turn while the rear tires are still flexing until they roll over enough to start to follow the front. Combined with the poor power steering setup this can cause a quite unnerving ride when passing large trucks or in a cross wind.

With the Goodyear tires you can push on the side of the rear bumper of a parked Prius and see the car yaw left to right. The wheel to fender distance stays about the same, while the stock tires “twistâ€. Pushing on the top of the “B†pillar produces a similar effect but in a “roll†mode. With the new Bridgestone RE950 tires neither of these two effects can be seen. This a crude test, but does indicate a much more stable side wall.

Data
Alignment- done twice a Tracy Toyota, Tracy CA. Test car drives straight and has no accidents.
FrontLeft FrontRight FRONT RearLeft RearRight
Camber 0.8 0.8 0.0 -1.0 -1.0
Caster 3.4 3.1 0.2 0.18 0.23
Toe -0.02 -0.01 -0.02 0.18 0.23
Incl angle 9.7 8.3 ….. ….. ….



Tread Depth (mils) Integrity after 40,080 rotated every 10k
FrontLeft FrontRight RearLeft RearRight
Inside 14 15 17 13
Middle 12 15 16 13
Outside 10 11 12 12

Tread Depth RE950 after 4,500 (no rotations)
FrontLeft FrontRight RearLeft RearRight
Inside 26 27 28 29
MiddleIn 32 33 32 32
Outside 26 27 27 27

The RE950 depths indicate under inflation (tread depth in middle of tire is thicker).
The front driver tire is subject to extra wear. This has been reported on Prius models since 2003.