adriver

iTrader: (1)

Could somebody please EXPLAIN to me, WHY this is a bad thing? I know its not the best option, but in a "preparing for the worst" mindset and trying to fully understand its limitations.

This question came up on another site about using a spare. First we had two different opinions on whether or not it should go on the front or rear..

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. 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.

(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.


JUST TO BE CLEAR RIGHT AWAY; I am not a mechanic, engineer, or someone who knows how well the diff should work. There could very well be a really simple answer that I don't know.

I'm curious if there is a better answer to this but for the sake of argument/discussion lets just say, WHAT IF you were driving down the road and get two flats because you slip off road or hit something. Lets say there is a combination of you have larger wheels, larger brakes, and maybe your only option for a second spare is a friend, or a stranger who pulls over and sells you there spare on the spot for $100. LETS JUST SAY, this other spare is smaller and wouldn't even fit on your front because of the caliper. Maybe a full size spare doesn't fit for you, or you just don't want to buy one. ETC....


Eaton MLocker (G80) mechanical locking differential, automatic locking differential
MLocker (G80) Operation
I read that as there is no gradual engagement.

In the factory owners manual, it doesn't reference the G80 at all. It only says if you have a flat, to just change that wheel. Then drive straight to somewhere you can fill with air, then only drive as long as needed to get it fixed.

The G80 gets 100% lockup at 100rpMinutedifference (I also read 120, but 100rpm is from eaton.com), so I'll stick with conservative. I was curious and 265/70/17 VS 265/70/18 has a difference of 19.37rpMile.

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. (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 diameter size difference). The difference for circumference 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, with a 4.5" difference). The last time I used a factory spare, IIRC it said do not exceed 50mph, and go less than 50 miles.

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?

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 jack up the rear, 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. 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.


Now this is all theoretical numbers, but there may very well be something really simple I am missing. It has been brought up that this is bad for the spider gears, but WHY? How is running different size tires different than turning? Is there damage being caused to your spider gears every time you turn, or is there no problem until it engages.

madmann26

iTrader: (7)

adriver

iTrader: (1)

Yes, but until that g80 gets 100rpM difference between the two wheels it is open. On the two specific tires I mentioned there is a 4.5" diameter difference and you would not get 100rpMinute difference until you got to 57mph. I'm trying to find out if there is any/gradual damage being done prior to that 57mph, or if the mechanics create an all or nothing type of wear/problem...

My understanding is it wouldn't. I am getting several people telling me it would cause damage when it is still open, but nobody can tell me why. Those who disagree with me, are sounding as if they are pulling numbers out of their rear ("a few miles is okay", "don't go over 5-10mph,"). I keep being told I am wrong, but nobody can give me a clear, detailed answer, as to why.

2002_Z28_Six_Speed

iTrader: (20)

The only thing I can think of is the difference in tire size makes the spider gears turn 100% of the time. Are they designed to do that? Is cooling sufficient? Will turning 100% of the time wear them out prematurely (like in 50K more miles)? Factory has to tell you to operate in a way that makes the vehicle last 100K miles and minimize warranty claims. CYA.

Brian68

"It has been brought up that this is bad for the spider gears, but WHY? How is running different size tires different than turning? Is there damage being caused to your spider gears every time you turn, or is there no problem until it engages."

I think the best answer to what you have here is to concentrate on the last 3 sentences. The spider gears are straight cut, poorly and cheaply machined parts that are engineered for only turning corners. Most people do not spend a ton of time or distance on corners or even a steep degree curve so spider tooth quality does not matter like a ring and pinion tooth. Gradual curves would cause a differential action, but at such a low load and RPM difference so the gear oil would be able to mask any wear as long as the oil properties are in order. Spider gears are actually always in play as a vehicle changes or maintains speed, but there is no spinning action in straight lines so the gear teeth just transfer power in a stationary manner. Increased differential action between poorly matched radii/diameters/circumferences or turning the vehicle increases friction and therefore heat and wear at the point of the cheap little spider gears, G80 or not. Tires never wear at the exact same rate anyway, so spiders will always be needed to some extent, even a fraction turn per mile with out a turn or lane correction. Also, tire sizes are more of a target than an exact measure, much like shoe sizes.

Now for the G80 part. This limited slip has a spring loaded flyweight that engages around 100-ish rpm difference between axles. It isn't going to be exactly 100rpm, but within a percentage that makes someone in management happy. The system uses clutch plates behind the side spider gears, so a small amount of slip will be happening (ramped) until friction stops completely or parts break. I think the system limits maximum slip control or complete lock up, but can't remember for sure. Slip does not stop instantly, so loss of control from an instant apply won't happen. Continued diff locking application to the point of parts breakage is instant, but has a whole bunch of variables so yes, loss of control could happen here. I had a few G80 failures to repair (did not unlock) that led to spider gear wear failure due to high friction past what the oil could protect against. These teeth were worn down so much there was nothing left. There was no loss of control, just stopped working or the driver finally noticed the noise/crow hop on turns.

You have answered your own question with math abilities and understanding about the RPM differences on tire sizes and speed/distance. I filled in the balance of what else you needed with limiting differential friction and use good oil. I also have different size tires on my truck front to back and would rather not think of having a flat tire, but your math helped me. BTW 1010tires.com has a tire size calculator that you can compare tires with quickly, just remember it wont be perfectly exact for each tire brand/model.

arthursc2

iTrader: (5)

MikeGyver

iTrader: (5)

As simply put as possible, different size tires will put noticeable wear on the spider gears in short order.