How tall/short tire options

Discussion in 'The Hides' started by Nailhead Ronnie, Jan 12, 2018.

  1. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    When looking at this equation

    RPM = MPH x GEAR RATIO ×336 ÷ TIRE DIAMETER
    What the tallest and shortest tire that can be fit in a Buick 1964 and 1965 A Body like Skylarks and Specials.

    When does width start to play a factor in the tire and how wide can you go, what then starts to become disadvantages if any of trying to go too wide for traction sake. Will that also affect this formula?



    Thanks,
    Ronnie
     
  2. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    I pulled this formula off a Hot Rod magazine YouTube video. But why is the transmissions gear not in the formula above?
    I was thinking the transmission gear would be of importance since its all in the mix.
    Thanks in advance. I can post the video if need be
     
  3. Bigpig455

    Bigpig455 Fastest of the slow....

    One of the tallest tires I've used that fit easily is a 235-75-R15 at 29 inches - all those fancy 17 and 18 inch wheel /rim combos dont ever seem to get much taller than that due to wider aspect ratios.. Even a 275-60-15 is only like 28 inches.....as far as the short side, look at all those lowriders running vogues that arent much bigger than the brake drum, right? A short 215-75-R14 tire 60 foots unusually well in my 65 GS, by making a 3.42 gear feel more like a 3.73..that same 3.42 with a 29 inch tire would probably react more like a 3.08 or something.

    This thread talks about the absolute size of tires that people are able to fit on an A body and modifications required, but the conversations always seems to be more about width limitations..
    http://v8buick.com/index.php?threads/how-to-fit-a-275-315-on-the-front.328921/#post-2757284

    From a width and traction perspective, for street/strip cars people seem to find all the traction they need from a 9-10 inch drag radial or slick, I can run a 1.80 60 foot on a 275-60 15 street radial which is about 10 inches wide, Ive seen guys run 1.7x 60's on F-70-14's.... its more about how the car and suspension are set up. Go too wide on the tire, like a big 515-45-15 and that tire takes so much horsepower to turn I dont know if its worth it....and it'd never fit in the wheelwell. Others may have different experiences

    As far as transimission ratio, that does have an effect if you wanted to understand the absulute torque output of a drivetrain on a chassis dyno or something, but the formula you are using strictly measures RPM/MPH of a combo, not power....you could draw some conclusion from that, like you are keeping a car in the powerband, etc..
     
    Last edited: Jan 12, 2018
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  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Top gear in any non OD transmission is 1:1. Tire height and width are related by the aspect ratio. Take for instance the P245/60R-15. The aspect ratio is 60. What that means is the height of the sidewall is 60% of section width. The section width is 245, that is 245mm. The tire height is 2 times the sidewall height + the rim height. It is easier to convert everything to mm, then figure the result in inches. There are 25.4mm/inch. 15" is 381mm. The sidewalls for the above tire are .60 X 245 = 147mm. 2(147) +381 = 675mm. 675/25.4 = 26.57". So as the section width of a tire increases, so does the sidewall height if the aspect ratio remains the same.
     
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  5. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

  6. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    while crunching numbers in the rpm calculator Im getting running a taller tire will give me lower RPM at higher speed, and a shorter tire will result in the opposite logically I get it while picturing a mini bike having to work harder to keep up with a harly. So how does a tall tire hook up out the hole? What seems to be the performance route to go?
    Im gathering my choices of tire is 26-29 inches???
    Can we assume the BBB wants to stop pushing past 5,000r rpm stock so I can stop my calculation for top speed around 5,000 rpm?
     
  7. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    The specs on the stock 430 engine is 360 horsepower at 5000 RPM and 475 lb foot of torque at 3200 RPM.

    Does this mean all the fun from the torque is at 800 - 3200rpm and that at 5000 rpm I will reach my limit of power/speed/mph/output?
     
  8. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Geez Ronnie, you are so over thinking this. Gears are multipliers of torque. Higher numerical gears multiply torque more, but rev the engine higher. Lower numerical gears reduce RPM. A taller tire reduces the rear gear, shorter tires increases it. If you were setting up a 1/4 mile track car, you would gear it so that as you ran through the traps, you were at the RPM where your engine was making peak HP. In a street car, you gear it for your priorities.

    BTW, if the specs on an engine say it makes 475 TQ at 3200 RPM, that means it makes that TQ at WOT. If you ran the engine up to 3200 RPM and held it steady at whatever speed that was, the engine would not be making 475 ft lbs, it would be less, until you floored the gas.
     
    Last edited: Jan 21, 2018
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  10. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    Larry just learning trying to get it instead of remembering it. Thanks for being patient lol but I bet you want to shake me sometimes and say "are you listening boy!" Hahaha. But jokes aside I got what you said about the tire will give or take away from the rear gear. Nobody has really told me like that and NOW I get it. The 1st of your paragraphs is clear and I can understand it.
    But help me with this. Because the 2nd paragraph didnt regester with me.
    When I go into WOT at 3000rpm on the highway theoretically I got till 3200rpm untill I feal the snapback of the torque?
    And at a stop, like above will i fell the torque beginning to stop at 3200rpm?
     
  11. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    Anybody else that can chime in please do. I been asking Larry so many questions throughout my time on v8buick and he always is kind to give me the strait facts
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    No, the TQ changes with RPM. You would need to look at the actual Dyno sheet.
     
  13. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    Ok I looked at one just now. I seen how the torque stays significantlly on the higher side well past 3,4 and still present around 5000's rpm.
    Thank you. Also seen the hp and where they meet, thats would be around where you said you would want to gear a car for the 1\4mile passing the traps? Or a little before that?

    So getting a shift kit is essential to try to benifit from each individual engine right?
     
  14. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    One thing I would like to know is whats important to get a good lanch and even lift the tires.

    I think lifting the tires might hurt your overall time(i aint no expert just speculating) but Ive always like when a car lifts it front tires when racing.

    Whats the combo that makes this happen? TQ + Tires + rear gear + weight of car?
    How much or what is needed?
     
  15. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    :cool:Now i get why the Buick engine is so cool. It makes alot of power quick and for a while too:cool:
     
  16. Nailhead Ronnie

    Nailhead Ronnie Well-Known Member

    Where is the rpm range that a BBB likes to cruise in?
     
  17. Bigpig455

    Bigpig455 Fastest of the slow....

    so Ronnie, to answer a couple of your questions..my BBB can cruise easily at 3500, but most people will be more comfortable with about 2800...and on another thread you asked about trap RPM. That will depend on your motor and build, but 5300 is a good number to shoot for. A short formula to get you there would be 3.73 gears and a 28 inch tire, like maybe a 275-60-15 or a 245-70-15. If you want to bring your cruise rpm down but keep your trap RPM, go with 3.42's, and use a shorter tire at the track, like a 26" 235-60-15.

    Here's a link to a good calculator to model the relationship between tire, gear, rpm and speed: http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

    Here's a link to a good tire height reference chart:https://tiresize.com/height-chart/


    To pull a tire at launch, you'd need about 375hp at the wheels and a well set up suspension, and that means probably 475 hp and 525 tq at the flywheel. Pulling the wheels isnt the worst thing in the world, but there are plenty of cars that go just as fast with similar power that dont. I cant give you a specific formula, but the only car to pull a wheel at this year's Pure Stock drags was a 3750 lb Buick with 365 RWHP and 3.73 gears that ran high 12's with a 1.89 60 ft ( a measure of how hard it launches). Cool car, and an impressive launch on bias ply G-60-15 tires, but it was by no means the fastest car there.
     

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    Last edited: Jan 22, 2018
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