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  • Putting bigger shoes on your ride

    So just about everything short of Snow traction and LSR means bigger meats on your ride. Short of buying and trying (obviously the best idea), can you get a general feel for what will fit under there by using the published specs on a tire?

    Example:
    My Riviera takes a 225/75R15 on a 15x6in wheel. I want to squeeze the biggest 18in tire under the front as I can. Looking on Tirerack for the spec of the average 225/75R15 and I'm looking at a section width of ~8.8in on a 6in wheel. This tells me I have ~1.4in a side outside the bead. By going to an 18in tire of close to the same overall diameter I get a 235/50R18 with a 9.6in section width on a 8in wheel. This tells me I have ~0.8in outside the bead. Doesn't this mean I can increase the backspacing by around 0.5in since the tire is the closest to hitting something on the stock setup due to the decrease in "bulge"? Could I also widen the tire selection out into the space by keeping the same backspacing and overall diameter if I keep the "bulge" on the inside to less than the stocker?

    Thanks,
    Central TEXAS Sleeper
    USAF Physicist

    ROA# 9790

  • #2
    Re: Putting bigger shoes on your ride

    The tread is not as wide as the bulge on the 75 series tire, so you have to consider that also....could be that under cornering/bump conditions, the lower profile tires will scrape somewhere that the old ones clear.

    You can get close by doing the math, but you should be a bit conservative, because things are not always what they should be. And you'll only find out after you spent the $$$.

    My fabulous web page

    "If it don't go, chrome it!" --Stroker McGurk

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    • #3
      Re: Putting bigger shoes on your ride

      riviera.

      one has to account for FWD and the torque angles. under throttle, it could go farther than measurements.

      18s are rough ride.

      they have been doing this to subarus alot lately. still tiny cars, same wheel wells potential for the last 25 years...paying more for a bearing snapping axle cracker on a footprint way too big for the weight.

      the trend sucks.

      I understood the wheel change to be some environmental statement. less rubber, more rim.

      I stay 15 inch, and get it over the 25 inch diameter, as a goal for minimum (however it works out on anything) , stays nice, and in the science that created it years ago. :D
      Previously boxer3main
      the death rate and fairy tales cannot kill the nature left behind.

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      • #4
        Re: Putting bigger shoes on your ride

        old rivieras are rwd
        My fabulous web page

        "If it don't go, chrome it!" --Stroker McGurk

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        • #5
          Re: Putting bigger shoes on your ride

          air shocks

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          • #6
            Re: Putting bigger shoes on your ride

            Riviera's were RWD till 1979, mine is only the second year of production, 1964.

            I'm planning on going no wider than the 235 in the front and using the fact that it's impossible to get wheels with proper backspacing so I have to run an adapter to adjust it in the fine tune department by cheating to the outside and turning them down if I need/can.

            Trying to keep it general enough so that lots of other people that have either cars that no one has established the "limits" of tire size or maybe it's just an excercise in mental math that I enjoy and I know others do too. Do tire stores let you "check out" a tire in a particular size and test it in the wheel well unmounted?
            Central TEXAS Sleeper
            USAF Physicist

            ROA# 9790

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            • #7
              Re: Putting bigger shoes on your ride

              with an 18" wheel the sidewall shouldn't move much side to side in a corner.. the issue is where the widest part of the tire falls with the bigger inch rim, you'll hit tierods before you get a wide enough tire under it..
              as the 18" rim is 1.5 or 2" taller than the stocker from the center line of the rim..

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              • #8
                Re: Putting bigger shoes on your ride

                I suggest you use the tire size calculator.



                It's very helpful and it gives you the idea what size of tires you would need to be able to maintain your original ride height. My Mercury Monterey has stock 225/75/15, I was able to stuff in 275/55/17 at the rear. Using the calculator, more or less gave me the idea how big I could go. I ended up using 1" spacers though

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                • #9
                  Re: Putting bigger shoes on your ride

                  most, if not all, tire stores will mount up tires either on their rim or your rim to allow you to check and see what fits.... it's how I got the tires I have under the rear of my Buick... we tried several sizes, most of which they had to have specially brought in for my fitting session..... hardly seemed worth it (to me), but they were glad to do it. Of course, I've also blessed them with quite a bit of business afterwards as well... but I know where I'm at Tire Factory and Les Schwab both will do that.... otherwise, there are tire gauges that give you the size and profile of a tire and you can see what hits or fits by the gauge..... http://www.summitracing.com/parts/PHP-01201/
                  Doing it all wrong since 1966

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