Hi folks,
After a season of pot-holes and with the spring soon upon us, it is time to plan the spring tune-up and getting the tires ready for vacations. This note covers why wheel alignment is important, an approach and lessons learned.
Wheel Alignment
Wheel alignment toe, the angle in direction of motion, impacts vehicle stability (rolling straight hands off,) drag and tire wear. So high performance race car crews make temperature profiles of their tires after a run to diagnose alignment problems. With an infrared temperature tool, ~$40, anyone measure the tread temperatures just like the race car crews:
In these tests, I drove down the highway at different speeds and inflation pressures for about 5 miles and then pulled off to measure tire tread temperatures. Higher tire pressures reduced the temperature showing reduced wear from lower tire drag. Also, the right side tire had (and still has) an alignment problem, excessive camber. This caused uneven wear and premature replacement of my original tires. But notice that at higher tire pressures, not only were the right-side, tire treads cooler but the wheel camber problem was reduced.
Based upon these tests and having bought a new set of tires, I bought a 'life time alignment' service from Firestone to have it checked anytime I want. Other independent tire shops have similar programs with different terms and conditions. Every Spring and Fall, when I do my twice per year oil change and preventative maintenance, I also check the alignment.
Miss-alignment can occur from hard parking into a curb, a pot-hole or other road stresses. But early detection, before the tire wear is evident, allows us to avoid premature tire replacement and minimize handling issues like trying to wander off the road while changing radio stations.
Adjusting the front wheel toe alignment is easy by tweaking a threaded strut and locking nut and is included in the "life time alignment" service. In contrast, Prius front wheel camber requires loosening and tightening a bolt and in severe cases, the bolt needs to be changed. Yet curiously, neither the tire shops nor my local Toyota service center has shown any interest in equalizing the camber to my specifications since "it is in the green."
Tire alignment systems have a wide range of acceptable Prius toe and camber values. Of the two, toe is more important since this is the angle relative to the direction of travel, the same forces that steer the car. Camber is the vertical angle that helps the outside tire take the turning, centrifugal force but it has less impact on straight-line, tire drag. As my earlier temperature profile shows, keeping the tires well inflated to 50 psi, 345 KPa, reduces the effect of excessive camber and reduce rolling drag:
Image copied from:
http://www.procarcare.com/icarumba/r...dia_tires3.asp
Since most of the weight is on the front tires, we rotate them with the rear tires to equalize the overall wear. When tires are rotated each should have an inner and outer tire tread depth measurement. Tire tread depth lets us know how the side-to-side wear is working out, the combination of toe and camber. In the USA tread depth is traditionally given in "1/32" of an inch, ~.8 mm. If the inner and outer depths are off by even one unit, there is a problem that needs to be diagnosed and corrected!
Prius Rear Wheels
Prius rear wheels are bolted, 38 ft-lbs 52 N-m, to a solid axle that ordinarily is not adjustable:
Toyota has a special tool that can change rear wheel toe geometry but usually the dealer just replaces the axle after an accident. Independent shops lacking both the manuals and tool are even less able to deal with rear wheel alignment. Everyone will report a rear wheel alignment problem but typically only adjust the front wheel toe.
Shim Kits
There are after-market, wheel shim kits available that come with sets of metal tabs at "1/64", "1/32", "1/16" and even huge "1/8" shims (if you need a "1/8" shim, there is something dreadfully wrong!) Unfortunately, these kits don't come with Prius rear wheel instructions or even how to use the shims safely.
My rear wheels had both excessive toe and camber but nobody would correct them to my requirements. So I bought the Firestone "life time alignment" service, a shim kit and figured it out:
Since toe impacted rolling drag, I started with the left wheel (the upper pair of graphs) and installed a pair of 1/32 shims on the rear bolts. Shims must always be installed as same sized pairs because with four bolts, if there is any misalignment, the bolts that hold on the wheels can not tighten evenly. This is very, very important since
a mismatch can lead to a loose wheel, rocking, and over-stressing the bolts and a wheel falling off!
Unfortunately, I guessed wrong and the toe was worse than ever. But I was able to use the error to calculate that a 1/64 shim on the front bolts should minimize the rear wheel toe and they did a perfect job. Due to the bolt geometry, this also changed the camber angle (lower graphs.) To fix the camber, I used a pair of 1/16 inch shims
on the opposite side wheel only to find I over-corrected it. I replaced them with 1/32 inch shims and the camber was as near perfect as I could want and the toe remained perfect.
Now there is a small, shim-sized gap between the wheel and axle plates. This gap can allow salt-water from road ice and snow clearing or coastal areas to invade and attack the metal surfaces. The shims have a zinc coating, which protects them. However, if you live in areas with a lot of road or sea salt, it makes sense to fully remove the wheel from the axle and treat both surfaces with an anti-corrosion compound. Eventually, after two years, I plan to inspect my rear wheel and axle plates for corrosion and compare the rolling drag with and without the shims.
So far, the tire wear has been excellent. The tread depth has been even and the rate of wear per 10k miles indicates I will easily meet the expected tire life.
Questions? Comments?
Bob Wilson
Prius tire alignment
Re: Prius tire alignment
