Running a smaller spare with larger tires, and a G80..

Discussion in 'Engine & Drivetrain' started by adriver, Sep 14, 2018.

  1. adriver

    adriver Full Access Member

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    This was asked in another thread on another forum, and frankly, this site seems to give better answers. So Here is the question:

    Hello
    Those running larger than oem tires; does anyone switch out the spare to match? Would there be any reason for concern of putting on the spare even tho its temporarily smaller diameter?




    If you do run a spare, put it on the front. Do not put it on the rear. This is especially important of you have the G80 or any other locker or limited slip


    (This was my quote, not trying to be bias, just get a fair honest answer) I'm going to contradict this... If you have a spare you need to put on, and are willing to swap around two tires (if needed), it should go on the rear. The simple reason is its more important to steer and stop as safe as possible then it is to try to accelerate. If that small spare were to blow, or if you have bad tires you are making last as long as possible, the good ones go in front. If you ever have a blowout, its easier to control if the blowout is in the rear. THE BIGGEST REASON, everyone is taught to use two hands on the wheel, is because if you have a blowout: and are just chilling, you won't be able to control the vehicle if it happens in the front.

    Even if you have a good smaller spare on the front, its still going to pull to that side when you brake.




    Even an open diff is hell on the spider gears. So I wouldn't go far or fast. The spiders aren't meant to be turning fast all the time. Granted they aren't turning as fast as a one tire fire burnout, but still not great for them.



    Ok, I could see that. I read up, and the G80 gets 100% lockup at 100rpMinute difference (I also read 120, but 100rpm is from eaton.com). I was curious and 265/70/17 VS 265/70/18 has a difference of 19.37rpMile. I feel like this should be an easy answer, (and I'm blaming this on being tired).

    For the sake of argument, lets say you are going straight, and gentle enough to never cause your tires to spin. Put the smaller spare on the left side, and decided to only make right turns, so that your spare is the only one that would be turning more than straight (or figure out what your turning radius/angle is, and calculate the difference in)... . You would have to go in excess of 300mph before your G80 differential would lock up on those two 265/70 size tire in a 17" vs an 18", (19.37 rpMILE difference. (60 minutes per mile) means that you have to go more than 5 miles per minute (with no spin) to lock up the diff at 100 rpMINUTE...…


    One of the factory size tires (I just picked a 2006 Silverado for the hell of it), is 275/55/20. I just picked what I think could be a donut size 225/70/17, ( a 4.5" tire size difference). The difference is 103.85 RPMile.

    100 (max rpMinute) / 103.85 (the RPMile = .96292 etc..% of revolutions per minute. then multiply that percentage times 60minutes = 57.775 MPH... would be your max speed before your diff would lock up with what I'm thinking are real size tires and a donut, (275/55/20 VS 225/70/17, 4.5" difference).


    To me this seems like a simple formula that if you are running a smaller size spare tire, and a g80, would be easy to figure out to just keep that information in the back of your mind for whatever size tires you got. Or am I missing something simple?





    I know not everyone can fit their full size spare underneath, and some people don't want to spend several hundred or more on a new wheel to match, and a couple hundred or more on a new tire, when they may never use it, and it may do nothing more than to add weight. So in the purpose to best understand what options we have...…………………. What you got?
     
  2. Scottydoggs

    Scottydoggs Full Access Member

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    i failed to read all that, just your question. aint got no time for math crap. its wrong, end of story.

    if the flats on the rear you need the full size spare on there or you eat the rear end up. if your going 5 miles to a shop to get the tire fixed, run it. just no long term or miles should be ran.

    just like a fwd car, you run the full size tire on the front always. has nothing to do with steering or braking, the rear is in the tranny now, it will kill it having a smaller tire on one side.

    one better, my regals front brakes are up graded. the spare wont fit over them now. so i carry two jacks. end of story there right.....
     
  3. adriver

    adriver Full Access Member

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    Not exactly the end of the story. Not sure if that was supposed to be funny or lazy, but you really didn't give any reason at all other than IF the spare wont fit over the caliper.. ok, that's one reason if you needed to do it that way. Now can you tell me why a diff that's running unlocked will be damaged if it stays unlocked? Also, what does the tranny have anything to do with it?

    Why would two jacks help you? Did you mean spares? Were you trying to say you carry two spares in a regal, when one full size would work?
     
  4. Sasquatch

    Sasquatch Full Access Member

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    Do you know the difference in the diameter of the tires? If there is a minor difference about a half inch i wouldn`t worry but any bigger than that i wouldn`t do it. When i switched to 20 inch wheels i went to a junkyard and got a spare from a tahoe with 20 inch wheels. The spare is an 18 inch wheel with a tire that is very close to the diameter of the 20 inch tire. If GM thought it was a good idea to run different size tires with the weak G-80 locker they could have just left a smaller spare under the truck and saved a few bucks. The money i spent on the new spare is far less than the cost of replacing an exploded differential.
     
  5. Scottydoggs

    Scottydoggs Full Access Member

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    your diff will take a crap if left on for weeks on end, spare tires are a temp deal, not meant to be left on.

    and i said i carry two jacks, not two spare tires. a front wheel drive car should really never have one smaller tire on it. the differential is in the tranny of a fwd car. if i get a front flat i jack up that side of the car, (why two jacks), put the back on the front, spare on the back. ill do this if i can get it fixed asap or not. im not killing my tranny over a jack and 5 lug nuts. now add the fact that my spare wont fit over the camaro calipers on the regal. im really forced to live by this rule.

    when i had my 03 gmc 2500 hd i ran 33's. i also had a 33 spare tucked under the back. same reason, not killing my rear end. that truck did lots of long distance driving, to the middle of no where sometimes.
     
  6. adriver

    adriver Full Access Member

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    Just to include it for ealier: RED was other poster BLUE was what I wrote.








    Oh ok, about the two jacks. I didn't get that it was because if the flat was on the rear, you knew you were going to be changing two wheels. Definitely the right thing to do on a FWD.


    As for the diff, I would guess its not ideal, but WHY? Now just to be clear, I do not have in depth knowledge of why this would hurt a diff. This question was originally specifically towards the G80. The G80 is unlocked until there is a 100rpm difference between the two wheels. Then it locks. Several comments were to just get a full size spare, which is not a way of understanding WHY. The way I understand it, is as long as you don't do that 100rpm difference then


    This is what eaton says about the g80

    http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differentials/mlocker/index.htm
    MLocker (G80) Operation
    "During normal driving conditions, the MLocker (G80) functions as a light-bias limited slip differential. When a low-traction situation occurs that causes a wheel speed difference greater than 100 RPM, a flyweight mechanism opens to engage a latching bracket. The stopped flyweight triggers a self-energizing clutch system, forcing a cam plate to ramp against a side gear. Cam plate ramping will continue to increase until both axles turn at the same speed (full lock), which prevents further wheel slip. When the need for improved traction is gone - unlocking occurs automatically and the differential resumes normal operation."

    I read that as there is no gradual/partial engagement. That it is completely unlocked until it hits that "100rpm" (its been brought up, not sure how accurate that 100rpm is). So how is there any problem happening to the G80?


    AGAIN, I am not a mechanic, engineer, or experienced with the G80. There may be something very simple that I am missing. I don't need to be right, I just need to know why.


    I checked an owners manual I found online for a 2005 silverado
    http://www.stpeteblue.com/chevy/manuals/2005SilveradoManual.pdf

    Changing a flat starts on 5-96. It doesn't specifically mention the G80 (that I see). It doesn't differentiate whether a different size spare should go in the front or rear, just to put the spare on where the bad wheel/tire is, and drive it immediately to somewhere you can check and fill the air, then to get it fixed. The closest was the 2nd to last paragraph in the section:
    "Your vehicle may have a different size spare tire than the road tires-those originally installed on your vehicle. This spare tire was developed for use on your vehicle, so it is all right to drive on it. If your vehicle has four-wheel drive and the smaller spare is installed, keep the vehicle in two-wheel drive as much as possible."




    So I'm going to keep the options going of what should THEORETICALLY keep you within that 100rpm change.


    To use a tire size that comes standard, 265/65/18.. 265/65/18 has a circumference of 99.15" (Start with your tire size and circumference)
    99.15" x 100 rpMinute (supposed rpm before lockup on a G80) = 9,915 inches before lockup on a g80
    9,915" / 12 = 826.25 Ft per minute before lockup

    1mph = 5280 ft (in a mile) / 60 (minutes in an hour) = 88ft per minute = 1mph
    826.25ft (per minute before lockup) / 88 ft (per minute) = 9.389 mph before your wheels should lock up (if you were to put your vehicle on a lift with wheels loose, or put on stands and have only one wheel spinning)...


    On that same note...


    If you want to know how fast you could go with two different size wheels before you lock them up in your own vehicle:
    You could either put your vehicle on a lift, or jack stands with the wheels in the air. Put it in gear with pressure on the other wheel and slowly increase speed until your wheel locks up.. If you test your wheels and they lock up at 8mph
    8mph times 88ft per minute = 704 ft per minute before lockup..
    704 ft x 12" = 8,448" per minute before they locked up.
    8,448" / your circumference of the wheel = (just going to throw in) 81.24" (the circumference of a random 225/50/17) = 103.998 (actual rpms before locked).

    Not sure how accurate 100rpm is, (not sure how "magical" that number is), but you could use this method to find out what rpm your diff is locking. Its accuracy would be based on how well you could accurately read your (better be) calibrated speedometer. You could always drop a mph or two to give yourself a safety net or or turning allowance.

    Then compare the tires you are using to your spare again to figure out what speed you THEORETICALLY could safely go before lockup if you ever needed to.

    AGAIN; DISCLAIMER: JUST TO BE CLEAR:
    I am not a mechanic, engineer, or someone knows how well the diff should work. The OP's thread got me interested in this, and up to this point I don't believe there has been a legitimate counter-response to me using math, and just trying to answer this question in an actual in-depth understanding way.
     
  7. Sasquatch

    Sasquatch Full Access Member

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    I am no expert on the matter but i would think that if the difference in tire size is great enough to activate the locker while traveling in a straight line it would put stress on not only the locker but on the axles as well.

    As we know when the locker is activated both axles are locked together and if one tire is smaller than the other it will be forced to match the rotation of the larger one causing it to put stress on the axles. I`m not sure if it will cause a problem but i am not willing to find out.

    I looked around for a good answer and most agree that its not a great idea unless you have an open differential. Here is one example,

    Using a smaller spare...

    W/locking diff. Dont do it unless it is your last resort...as it would be hard on the components and driving as it would act like you were constantly turning, drive slow...

    W/LSD Don't do it unless it is almost your last resort(I wouldnt do it) unless it was the last resort. On a LSD with clutches it would cause wear and is hard on it...still drive slow but would be easier than a locked diff

    W/open I would do it, but don't do it for long periods as it makes the diff work harder and causes wear as it would be constantly working the spider gears.

    I might end up running a smaller spare, but I wouldnt run it long. Just for me to get somewhere to get the org. tire fixed. This is much better than no spare....
     
  8. adriver

    adriver Full Access Member

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    Well, the G80 is unlocked until (found 120rpm, and 100rpm), lets say 100rpm to be conservative. With the 4.5" tire diameter difference I used, I figured you could go near 60mph before the diff would lock THEORETICALLY.... I would think if that G80 locks up while you are doing 60mph on the highway with two different size tires, you are talking CATASTROPHIC FAILURE... If your diff doesn't explode when that engages, your rear end would try to turn and that would definitely snap a mount or something.


    I know you don't want to drive around on any different size for long no matter what diff you have. The last time I used a spare, I swear it said don't go faster than 50mph and drive less than 50 miles on it. I drive 1,000 mile road trips across I-10 in the southwest, and through west texas, between el paso and san Antonio, there are two stretches where you go over 200 miles and see next to nothing. I often go through in December/January, and have been caught in a snow storm.
     
  9. corvette744

    corvette744 2004 Z-71

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    I see no problem running it for a short period of time till you get your tire fixed or a new tire.Your right about the 100 rpm scale-the reason i got a 33 inch spare tire under mine is i live on the wisconsin border and i tow even in winter-so if that happened to me with a smaller tire and it spun it would blow the spider gears out of the rear end.To much of a risk just get the same size diameter tire for a spare.I got a headache reading all of that but you put in alot of work in your searching thats why im replying.But if your traveling in non winter roads i see no problem with it-but would feel better with the right size especially over 200 miles with no repair station around that would be a serious tow.
     
  10. corvette744

    corvette744 2004 Z-71

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    I have to add this after taking an aspirin and looking at your math again-i think your off on tire circumference-which is tire diameter 265/65/18 is 31.56 not 99.15 like you said big diff so would be 263 ft per minute before lockup. To use a tire size that comes standard, 265/65/18.. 265/65/18 has a circumference of 99.15" (Start with your tire size and circumference)
     

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