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Safety of shims for rear alignment

Discussion in 'Gen 2 Prius Main Forum' started by sorka, Apr 19, 2011.

  1. sorka

    sorka Active Member

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    I shimmed my prius this weekend to remove about 0.44 degrees of positive out of 0.60 theoretically bringing total toe down to .16.

    The shims add about 3.8 mm between the hub and the suspension. The bolts are M10s I believe with 12 mm heads. There are 4 bolts per hub. I often take off ramps at a pretty good clip rather than braking when going from one freeway to another. The 4 bolts already seem like they wouldn't be enough to hold the hubs on.

    When adding the shims, it moves the hub further away resulting in a little more torque at the mating surface on the opposite side of steering. Also, the bolts just barely make it to the last thread.

    It's probably fine for someone who never corners hard, but I find myself nervous now when taking a corner wondering if first, the compressive force on the plastic shim won't eventually compress it permanently, and second, were the bolts weakened by the 52 ft lbs of torque needed with the shims over the stock 44 ft lbs? If just one of the two top bolts snap when corning, the wheel will come off instantly.

    Or is there enough engineering tolerance that the car would slide or flip long before there was ever a structural failure?

    I'll definitely be rechecking the torque in a week and in a few months.
     
  2. xs650

    xs650 Senior Member

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    Sorka, props for actually looking at the joint and thinking about it:rockon:

    If the bolts are grade 10.9 which is the metric equivalent of an SAE Grade 8 51 lb ft is on the high side of acceptable but acceptable for dry threads. There should be some numbers on the bolt head. If they are 8.8 then they have been over torqued but you don't normally get 8.8s in critical suspension applications

    The joint is stronger than it looks. I'm sure you have seen cars that slid into curbs and other obstacles and broke or bent wheels to the point that they weren't usable without the hub to axle joint failing. I wouldn't sweat that one.

    The tensile strength of one 10Mx1.5 grade 10.9 fastener is about 13,000 lb. You car has less that 1/2 it's weight on the rear wheels, so lets assume you are going around a 1 g corner with 1/2 the car's rear weight on the outer rear wheel. That is less than a 2000 lb side load. Bolt loads will be higher than 2,000 lb, but the spacer has no effect on bolt loads from cornering as long as things are torqued.

    You are right to be concerned about torque. Rechecking each week at Toyota shop manual torque until it stops taking any additional turn, then going to 1 month intervals, then 1 year intervals until no more movement would be a reasonable plan.

    If the spacer is designed right, there won't be any additional movement after you over-torque it at assembly. Please let us know what you learn.
     
  3. sorka

    sorka Active Member

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    Thanks for the quick reply.

    The bolt doesn't seem to have the usual grade stamp on it. This is what it looks like.

    Also I assume the overtorquing is meant to account for compression. i.e. when I check it next week, should I torque it to stock 44 or again make sure it's at 52.


    [​IMG]
     
  4. brick

    brick Active Member

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    I had similar concerns when I installed shims on my wife's car. I went back after a week and found that the bolts DID need to be torqued down again. (Bolts started turning at around 40 ft-lbs IIRC.) The second week they didn't need anything, but I plan to go back and check in another week or so.
     
  5. xs650

    xs650 Senior Member

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    Stick with the 44, the 52 was to mash things together at one time so they would stop compressing.

    Do the bolts go all the way through the plate they screw into and have at least one thread sticking out on the other side?
     
  6. sorka

    sorka Active Member

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    No. The end of the bolt only makes it through flush to the otherside. I think there's maybe half a thread that did not get used on the female side. I'd feel more comfortable with a longer bolt, but even the alignment shops which install this very shim use the stock bolt.

    What's even scarier is that my adjustment was very mild with the shim width varying from 3.5 mm to 4.1 mm. The shim covers a -1.5 to 1.5 (3 degree range). At this maximum setting, the thickness of the shim is about 6 mm at the thick point which means that a single bolt, if the shim lined up just right to correct an even amount of toe and camber would lose several threads. I can't imagine this is acceptable.
     
  7. jdenenberg

    jdenenberg EE Professor

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    You could buy longer bolts.

    JeffD
     
  8. sorka

    sorka Active Member

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    Yes, but without knowing exactly what the current ones are, this seems like it would be risky. Even the head design could matter. I'd only feel comfortable buying it from Toyota if they could match that bolt but just in a slightly longer version.

    BTW, even if this were a matter of matching grades, it would still be risky. Toyota has very high quality control and the test batches of bolts from their suppliers to make sure they really are at the proper grade.

    On my durango, a few months ago, I removed the middle row of seats to clean the carpets. There were M12 10.9 bolts holding the center row to the floor. Two of these bolts had their head broken off when I removed them.

    The first broke when I loosened it by hand with a 3/8 rachet. I used the torque wrench on the rest so that if one broke, I could look at the maximum torque used(digital snap-on torque wrench). The second bolt that broke did so at 18 ft lbs when I was loosening it.

    These bolts were obviously defective and came nowhere near their torque limit.
     
  9. FirstFlight

    FirstFlight Member

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    Sorka,

    I need to do what you did but I don't feel competent tearing apart my wheel. My toe was .42. Is there any chance you can provide a step-by-step instruction sheet or pictures?
     
  10. sorka

    sorka Active Member

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  11. FirstFlight

    FirstFlight Member

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    Thanks, that's a really good review. Looks too complicated for me. I want to do it but I feel like I may screw something up.
     
  12. xs650

    xs650 Senior Member

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    I think you are OK.

    If anyone needs longer bolts, Specialty Products sells 40mm (1.57 inches) long M10x1.5 bolts for that application. Since they are old timers in the business of supplying suspension parts, their quality should be OK.
     
  13. David Beale

    David Beale Senior Member

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    If you think about this for a minute you will realize you can relax. The -maximum- side load the Prius can develop as above is pretty low. While a -real- sports car such as a 3rd gen RX-7 -may- develop 1g in cornering, the Prius will never come close to that. -IF- the weight on the rear wheels is 1/2 the car's weight (it is less) then you -might- be able to load it to a max. of 1500 lbs. It will be closer to 1000 lbs. So you have more than 20X safety factor. The tire WILL slide long before you even get to a side load of 1500 lbs. Now, if you were to slide it into a curb, the wheel would break or bend long before you got over 10,000 lbs.

    As for the bolts that came apart when you removed them on another vehicle, they were cracked when they were installed (over-torqued). I've seen this before. Torque wrenches do fail, even the "high quality" ones on production lines.