Should I Replace My Spare Tire?

jandrews · Jul 10, 2011

shane100700 said: ↑

You dont want two different size tires on any axle that has power to it. It will damage your vehicle, especially the differential.
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This is why so many manufacturers provide cars with donut spares, right?

Different radii tires will not hurt your vehicle for short term usage so long as it has an open differential.

shane100700 · Jul 10, 2011

horwitzs said: ↑

Then why aren't you harming it every time you turn? The different radii act no differently than having different tire sizes.
I realize turns are just now and then instead of full-time, but still would stress the diff, no? Added stress with the cute little spare. Also think about the popping sound when you make really sharp turns with 4wd engaged.

I'm not trying to be a dick, I believe you about different tires sizes being bad... just trying to understand the logic here since the diff was invented to allow the wheels to turn at different speeds. Actually they are designed to allow all multiple tires spin at the same time (remember the old 1 wheel wonders of the 70's and birth of posi-tracktion), sometimes at different rates of speed depending on differential type.
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It's because of our LSD-

The LSD viscose coupling, which has a special fluid that begins stiffen almost immediately when it is sheared between the VC plates, is isolated from the rest of the differential by rubber seals. The seals are between the differential (inner) case and the left side gear. When the two axles are turning at the same speed, the case, side gears and seals are all turning at the same rate.

However, when there is different axle rotation speeds (as with different circumference tires), the inner case turns at a rate corresponding to the average of the two axles. Consequently, the seals will now be subjected to slippage due to the different rotation speed of the left side gear and the differential case. The design, which has the seals lubricated by the regular gear oil, takes into account the expected short-term axle rotation differences in normal driving (e.g. turning a corner, following curves in the road etc), and the odd time one wheel loses traction (at least until the VC takes over). But extended driving with different circumference tires will mean the seals are subjected to continuous rubbing of the seal surfaces. While this might not cause immediate damage, cumulative wear of the seals can lead to leakage, mixing of regular differential gear oil with the special VC material, and loss of VC capability. In this case, you won't know until one day one wheel spins freely.

There's two reasons to avoid different tires. One is to prevent the VC fluid from being stressed due to the continuous shearing and eventual loss of viscosity (which means a permanent binding condition) and the other is to avoid undue wear on the seals, which can lead to mixing of the fluids.
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http://www.tacomaworld.com/forum/wheels-tires/45031-new-wheels-tires-what-do-u-guys-do.html

http://www.tacomaworld.com/forum/wheels-tires/119988-oversized-spare-tires.html

http://www.tacomaworld.com/forum/wheels-tires/108881-what-about-spare-when-going-larger-tires.html

Others may pipe in but we are also getting off topic. Do it if you want but I wouldnt recommend it.

BTW- jandrews, I was speaking mostly in regards to while in 4wd (because of the "thats what the differential is for" statement), otherwise remember the old 50 (miles)/50 (mph) rule!

Bolt79 · Jul 10, 2011

Good discussion...so would the small difference in tire size (one inch in diameter) make enough difference to require a different spare as long as the truck wasn't put in four-wheel drive?

jackrules · Jul 10, 2011

only replace it if you're offraoding...

shane100700 · Jul 10, 2011

Bolt79[OP] said: ↑

Good discussion...so would the small difference in tire size (one inch in diameter) make enough difference to require a different spare as long as the truck wasn't put in four-wheel drive?
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As long as you place it on the front and dont use 4wd, and keep it limited distances then there should be no issues.

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