Yet another rear tire wear thread

This is a long story, but does have a happy ending if you have the patience to read it (or just skip to the bottom).

I recently purchased a used '08 Prius with 18k miles on it and noticed some pretty severe inside rear tire wear on both of the rear tires (inside inch of the tire is going bald whereas the rest of the tire is still very new looking). After digging around on this website I realized that getting a rear alignment might not be so simple, so I tried to call the local Toyota dealer about the problem - "try" being the operative word.

I called the service department at Putnam Toyota 3 times to speak to somebody about a rear alignment and was routed to their service manager and left voicemails (2 different voicemail boxes). Basically I did not want to make an appointment without first talking to somebody there to make sure they knew what they were doing regarding the rear beam axle and the use of shims to correct the alignment. So after getting the brush off I realized I should just go somewhere else. Afterall, if they weren't helpful before taking my money, I somehow doubt they would be any easier to work with after the fact if something happened down the road.

I remember stopping by a local shop called Mach III in San Francisco a while back. They sell a lot of aftermarket rim / tire packages and subsequently do quite a few alignments. I talked to the owner (Wally) on the phone and he sounded like he knew what he was doing and assured me they could do the rear alignment as they've worked on Prius before.

When I arrived for my appointment, they put the car on the rack and it turns out the the driver's side rear was slightly out of spec. -1deg camber and .31deg toe. It was close enough to where the owner said that it was my choice, but it would probably be fine to leave alone. He offered to order the shim and install it if I wanted and when I decided to have them order the shim and do the install myself, he only charged $20 for an alignment check (rather than $80 for the alignment) and $19 for the shim. What a great shop! If anybody near San Francisco has some odd rear tire wear or just wants to check their alignment, I highly recoomend them.

Anyhow back to the rear tires. The alignment was close enough to being within spec that it doesn't really explain why the rear tires were so bald on the inside inch. It looked even worse than my other car that has -3.5deg camber which is quite aggressive. Also if the tire pressures had been low, the wear would look a little different.

Then I realized something the original owner had told me when I bought the car - the car was used solely for a 100 mile trip for work. Couple that with the fact that there was a lot of dirt and junk in the trunk (indicating he was hauling stuff), is it possible that the rear tires wore so unevenly because the prius suspension is too soft and/or there was too much weight in the rear?

In anycase I'm going to pop the shims in and rotate the tires and see what happens. I'm hoping the problem goes away

 

Thanks for the replies. I hadn't thought about the tires being rotated already, but it doesn't look to be the case as all of the other wheels were within spec - actually pretty much middle of the road and the tires were inflated to 38F/35R when I checked them.

LF
toe -.09deg
camber -.08deg

RF
toe -.05deg
camber -1.0deg

LR
toe .31deg
camber -1.0deg

RR
toe .05deg
camber -1.7deg

Total rear toe was within spec, but the LR itself was out of spec, but not horribly so. Especially with only -1deg camber it didn't really explain what I saw on the tire.

Anyhow the shim came in same day and I put it in this morning. It was perhaps an hour's work and anybody with typical handtools can do it pretty easily

Off the top of my head the tools list is:
- jack
- jackstands
- something to block the front tires when the car is up in the air
- 24mm socket for lug nuts
- 12mm socket for bolts behind the drums
- sharpie and scissors to mark and cut the shim
- dremel with sanding belt or a round file

The shim is made by SPC and at first when I was reading the instructions, I thought I was given the wrong part as the included instructions didn't have the Prius listed. I looked online for an updated manual and it was there with the 01-09 Prius listed. I won't go into the details of how to install it as the SPC website explains it better than I could (there is also a short install video and an online shim size calculator that is great). Two things I will mention though. The shim is actually 2 discs together. It took me far too long to figure out that it wasn't 1 piece and the two pieces should be rotated to align the numbers. Also the inside hole (think of a doughnut) is not quite large enough to clear the drum spindle so I used a dremel to enlargen it. The shim is fairly rigid plastic, so a rounded file would work too or even scissors if you get desperate.

What I liked about this shim is that it allowed me to accurately choose how much camber and toe I wanted to add or subtract.

I have some crude measuring tools at home that can measure total toe and before the shim install, total toe was ~5/32. After putting the shim in, total toe is now ~1/32 which is right on the money. I can't really measure camber accurately down to 1degree as I don't have a decent bubble gauge, but the one I have shows it to be slightly less than the RR tire which is at -1.7deg. Only time will tell if the "new" rear tires wear evenly.

The instructions with the shim say to check/retorque the bolts on the back of the drum after 28k miles. I'll probably just check them as well as the alignment at the next oil change.

 

Update:

I put about 2k miles on since the alignment and retorqued the 4 rear hub bolts to 44lb/ft this weekend when I had the car up for an oil change. The bolts turned quite a bit - perhaps 1/2 turn which was surprising. I am using the same torque wrench as before.

I checked my other prius and same thing - the bolts turned a surprising amount before hitting 44lb/ft (again about 1/2 turn). I'm going to continue keeping an eye on them - hopefully that was it. If they keep needing more attention, I'm going to stop recommending these plastic shims