Payload vs Upgrades -- a cautionary tale

[Jhawth]

You may want to reconsider your spare tire situation. Having a tire much smaller than your other tires will cause damage if you try to drive it with 3 37" tires and 1 33" tire on there.

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[Zandcwhite]

You may want to reconsider your spare tire situation. Having a tire much smaller than your other tires will cause damage if you try to drive it with 3 37" tires and 1 33" tire on there.

Like the millions of donut spares do? Your axle doesn't know the difference between turning a corner with same size tires or driving straight with different sized tires. That's the point of spider gears. If you have lockers, do not use them with different sizes tires. If you have a limited slip rear, make sure you put the smaller tire on the front. Follow those simple concepts and you could run hundreds or even thousands of miles on different sized tires without damage. The XJ's even came with a small temporary spare on some trims and they were 4wd too.

 

[bjm00se]

Like the millions of donut spares do? Your axle doesn't know the difference between turning a corner with same size tires or driving straight with different sized tires. That's the point of spider gears. If you have lockers, do not use them with different sizes tires. If you have a limited slip rear, make sure you put the smaller tire on the front. Follow those simple concepts and you could run hundreds or even thousands of miles on different sized tires without damage. The XJ's even came with a small temporary spare on some trims and they were 4wd too.

The truth is probably somewhere in the middle.

You'll probably get an ABS light almost immediately as the speed sensor on the wheel with the downsize tire picks up speed readings the computer can't reconcile.

And the spider gears do work to reconcile the mismatch the speeds, but they're not designed for long term high speed constant mismatch. This is why donut spares recommend max 50mph and max of 50 miles driving.

Now, do folks drive their donuts further than that? Sure. Especially in a day-to-day long term commute situation driving only five or ten miles a day before getting around to repairing the tire and changing out the spare. (And if you don't understand why that might happen, you've never worked in a shop that had prolonged stretches of heavy overtime. Lucky you.)

But a prolonged high-speed highway stretch (consistent with experiencing a flat on road trip out near East Overshoe, MonTexAvadaTah) might well cause those spider gears to overheat and fail under such extended use.

Don't misinterpret me. I'm not saying don't do it. I'm saying be aware of the stress that driving on a mismatched spare places on your equipment, and then plan and react and drive accordingly!

 

[DGT]

The biggest difference I've noticed from adding several hundred pounds of tires/wheels/accessories (JLURD, 6400# with two people and two large dogs) is the brakes could use, at a minimum, upgraded pads. Current braking vs OEM is noticeably worse.

 

[dragoneggs]

Since I knew I would be adding many little mods, including compressor, radio, aux lights, bigger tires, winch, recovery gear, camp stuff… the first thing I did was to remove the back seat and swap the OEM front bumper for an aluminum mid width. At least I figured I was starting with about 75lbs ‘in the bank’.

Not a lot of ways to significantly reduce weight after that. Now as far as loading up, one needs to be real particular on the weights of your mods and accessories. I chose very light wheels and tires, didn’t go overboard with my lights (so far). Stuff going inside can be loaded based on the mission.

 

[Zandcwhite]

The truth is probably somewhere in the middle.

You'll probably get an ABS light almost immediately as the speed sensor on the wheel with the downsize tire picks up speed readings the computer can't reconcile.

And the spider gears do work to reconcile the mismatch the speeds, but they're not designed for long term high speed constant mismatch. This is why donut spares recommend max 50mph and max of 50 miles driving.

Now, do folks drive their donuts further than that? Sure. Especially in a day-to-day long term commute situation driving only five or ten miles a day before getting around to repairing the tire and changing out the spare. (And if you don't understand why that might happen, you've never worked in a shop that had prolonged stretches of heavy overtime. Lucky you.)

But a prolonged high-speed highway stretch (consistent with experiencing a flat on road trip out near East Overshoe, MonTexAvadaTah) might well cause those spider gears to overheat and fail under such extended use.

Don't misinterpret me. I'm not saying don't do it. I'm saying be aware of the stress that driving on a mismatched spare places on your equipment, and then plan and react and drive accordingly!

The donut spare speed and distance limits are due to the garbage quality of tire not the spider gears. If a constant differential of speed overheated and cooked your spider gears, every sports car that goes out and enjoys a long windy road in the mountains or coast would suffer the same fate. They don't because it's not a thing. If you are overly concerned, put the smaller tire on the front passenger side and the FAD means it doesn't effect the spiders or gears or anything else even if you drive it across the entire country at freeway speed.

 

[bjm00se]

If a constant differential of speed overheated and cooked your spider gears, every sports car that goes out and enjoys a long windy road in the mountains or coast would suffer the same fate.

It's an interesting question and a chance to do a little arithmetic.

a 37" tire rotates 544 times per mile covered.
a 33" tire rotates 614 times per mile.

So, in one mile of perfectly straight freeway travel the spider gears have to reconcile 614 revolutions to 544 revolutions (we're only interested in the numeric difference here)

614 - 544 = 70 revolutions per mile.

Now, at 75 mph, the Jeep travels 1.25 miles per minute.

So at 75 mph freeway speed, the spiders are turning 70 * 1.25 = 87.5 RPM.

What about a sportscar in the twisties? I'm going to simplify down to a 1/8 mile perfectly round track. That's 660' long, has an inner diameter of 210' and a radius of 105'. (Keep in mind that a 105' radius curve would probably be marked for 20mph on most roads in the US.) But let's say we're blasting around that track at the same 75mph.

The inner diameter of that track is 210'. (660/3.14) The Jeep is about six feet wide at the axles. The outer diameter of that track is 222' (210 + 2*6)

So for every turn around the track, the inner wheel travels 660', while the outer wheel travels 697' (222*3.14)

A vehicle travelling 75mph is travelling 6600 feet-per-minute. (So in one minute, we get to make exactly 10 turns around the track (6600/660) -- assuming we're measuring speed off the inner tires)

So the inner tire goes 6600 feet, or 1.25 miles, in that minute. How far does the outer tire go?

We made 10 laps around the track. 10 * 697' = 6970 feet, or 1.32 miles, again distance travelled in one minute, measured in feet.

6600 feet = 1.25 miles travelled in one minute.
6970 feet = 1.32 miles travelled in one minute.

Again, that 37" tire is rotating 544 revolutions per mile.

So, the inner tire rotates 544 * 1.25 = 680 revolutions in one minute.
The outer tire rotates 544 * 1.32 = 718 revolutions in one minute.

718 - 680 = 38 RPM.

So, 75mph at freeway speed causes the spider gears to work more than twice as hard as blasting around a 1/8 mile circle continuously at 75mph - which I think you'll grant me is unlikely, if not completely impossible.

So what's my point? 37/33 mismatched tires makes the diff work *much* harder (realistically at least four times, probably more like ten times) than any amount of mountain road driving, where the speeds are going to be lower, the curve radiuses are going to be larger, and there will be inevitable periods of straightaway.

Guaranteed failure? Heck no. As they say, "your mileage may vary." But something to be aware of; which was my entire point all along.

That was fun.

 

[Jhawth]

Like the millions of donut spares do? Your axle doesn't know the difference between turning a corner with same size tires or driving straight with different sized tires. That's the point of spider gears. If you have lockers, do not use them with different sizes tires. If you have a limited slip rear, make sure you put the smaller tire on the front. Follow those simple concepts and you could run hundreds or even thousands of miles on different sized tires without damage. The XJ's even came with a small temporary spare on some trims and they were 4wd too.

I've read on here and other places that for 4wd having mismatch tire sizes would cause damage, which is why I was saying OP might want to reconsider. I see what you're saying about running it on the front in 2wd and being OK and that makes sense. Hopefully if you need to use the small spare whatever conditions you're in would allow for it.

I cut the sidewall on a tire at the back of a trail this last summer and there's no way I would of made it back out without 4wd. Only way out was the way I went in for that trail. If I had a small spare tire on the front I don't know if I would of caused any damage using 4wd to get out but it was nice to not have to worry about that.

 

[J0E]

I added 37” tires but kept a stock (mini) spare for the lighter weight.

That's what I do, with 4 full size spares at home that I picked up cheap on FB market.

You may want to reconsider your spare tire situation. Having a tire much smaller than your other tires will cause damage if you try to drive it with 3 37" tires and 1 33" tire on there.

Not true for the drive home. Even so, you can put it on the front where there's no differential involved on the ride home.

I've read on here and other places that for 4wd having mismatch tire sizes would cause damage, which is why I was saying OP might want to reconsider. I see what you're saying about running it on the front in 2wd and being OK and that makes sense. Hopefully if you need to use the small spare whatever conditions you're in would allow for it.

I cut the sidewall on a tire at the back of a trail this last summer and there's no way I would of made it back out without 4wd. Only way out was the way I went in for that trail. If I had a small spare tire on the front I don't know if I would of caused any damage using 4wd to get out but it was nice to not have to worry about that.

Absolutely nothing to worry about.

The donut spare speed and distance limits are due to the garbage quality of tire not the spider gears. If a constant differential of speed overheated and cooked your spider gears, every sports car that goes out and enjoys a long windy road in the mountains or coast would suffer the same fate. They don't because it's not a thing. If you are overly concerned, put the smaller tire on the front passenger side and the FAD means it doesn't effect the spiders or gears or anything else even if you drive it across the entire country at freeway speed.

I asked my hill billy cousin who's got 20K miles with a donut on the driving axle of his POS (he gets donuts for free) if he was worried about his spider gears. Spider what?

 

[zrickety]

Remember one mitigating factor is the increase in unsprung weight. Extra weight in tires and wheels shouldn't be factored in as a 1:1 increase because the truck itself doesn't have to support that weight.

Yes it still matters. But an extra 100# on each axle is not the same as 200# inside the Jeep.

In racing, 100 lbs unsprung becomes 1000 lbs on the vehicle, it's actually 10x increase (or decrease if you're going lighter). So it's much worse than 1:1.

 

[Jtclayton612]

I've read on here and other places that for 4wd having mismatch tire sizes would cause damage, which is why I was saying OP might want to reconsider. I see what you're saying about running it on the front in 2wd and being OK and that makes sense. Hopefully if you need to use the small spare whatever conditions you're in would allow for it.

I cut the sidewall on a tire at the back of a trail this last summer and there's no way I would of made it back out without 4wd. Only way out was the way I went in for that trail. If I had a small spare tire on the front I don't know if I would of caused any damage using 4wd to get out but it was nice to not have to worry about that.

typically that’s AWD systems that really caution against having mismatched tire sizes. Heck I think Subaru says you need to replace all 4 if getting a new tire due to a sidewall puncture and the difference between new and old tires is more than 3/32.

 

[bjm00se]

Here's my real world experience with off-size spares. In my Tacoma, the under-bed storage wasn't big enough to locate a 285/70R17 tire that matched the other four on the pavement, so I mounted a 265/75R16 down there - basically a 32" tire. (Later, I got a tire-gate bumper, and an exact match spare, but I digress)

Out on the trail one particular day, I chap in our group with an old 4-Runner tore a sidewall, and used his spare. He was running 35" tires. Later in the trip, he tore a *second* sidewall. (Turns out the tires on his rig were really old, and well past their "best-by" date... but again I digress.)

For the second flat, I lent him my 32" spare. We put it on the front. So he was running 35/35/35/32. He drove off the trail in 4-lo, then 2-hi for the roughly 200 mile pavement trip from the trailhead back to his place up north of Sacto, where I met him to reclaim my spare.

Didn't cause any problems that I'm aware of; though that was just a "trail buddy" and not someone I've spoken to since, so I wouldn't necessarily have heard if he had to tear down that diff a couple months later. I kinda doubt it though. Everything seemed copacetic for the trip.

 

[Zandcwhite]

It's an interesting question and a chance to do a little arithmetic.

a 37" tire rotates 544 times per mile covered.
a 33" tire rotates 614 times per mile.

So, in one mile of perfectly straight freeway travel the spider gears have to reconcile 614 revolutions to 544 revolutions (we're only interested in the numeric difference here)

614 - 544 = 70 revolutions per mile.

Now, at 75 mph, the Jeep travels 1.25 miles per minute.

So at 75 mph freeway speed, the spiders are turning 70 * 1.25 = 87.5 RPM.

What about a sportscar in the twisties? I'm going to simplify down to a 1/8 mile perfectly round track. That's 660' long, has an inner diameter of 210' and a radius of 105'. (Keep in mind that a 105' radius curve would probably be marked for 20mph on most roads in the US.) But let's say we're blasting around that track at the same 75mph.

The inner diameter of that track is 210'. (660/3.14) The Jeep is about six feet wide at the axles. The outer diameter of that track is 222' (210 + 2*6)

So for every turn around the track, the inner wheel travels 660', while the outer wheel travels 697' (222*3.14)

A vehicle travelling 75mph is travelling 6600 feet-per-minute. (So in one minute, we get to make exactly 10 turns around the track (6600/660) -- assuming we're measuring speed off the inner tires)

So the inner tire goes 6600 feet, or 1.25 miles, in that minute. How far does the outer tire go?

We made 10 laps around the track. 10 * 697' = 6970 feet, or 1.32 miles, again distance travelled in one minute, measured in feet.

6600 feet = 1.25 miles travelled in one minute.
6970 feet = 1.32 miles travelled in one minute.

Again, that 37" tire is rotating 544 revolutions per mile.

So, the inner tire rotates 544 * 1.25 = 680 revolutions in one minute.
The outer tire rotates 544 * 1.32 = 718 revolutions in one minute.

718 - 680 = 38 RPM.

So, 75mph at freeway speed causes the spider gears to work more than twice as hard as blasting around a 1/8 mile circle continuously at 75mph - which I think you'll grant me is unlikely, if not completely impossible.

So what's my point? 37/33 mismatched tires makes the diff work *much* harder (realistically at least four times, probably more like ten times) than any amount of mountain road driving, where the speeds are going to be lower, the curve radiuses are going to be larger, and there will be inevitable periods of straightaway.

Guaranteed failure? Heck no. As they say, "your mileage may vary." But something to be aware of; which was my entire point all along.

That was fun.

The math is all well and good when you reply to only part of my post. You aren't running 75mph in 4wd, so again put the smaller spare on the front and your spiders aren't turning at all. That car on the track or sports car in the twistys is probably turning 10x the rpm at the tires that you are getting off the trail thereby making their wear on those spider gears somewhere between equal and 6x greater (although being that the heat is what would cause issues in guessing that goes up exponentially with rpm meaning you could run trails for the life of your vehicle and never have a problem with your smaller spare).

 

[ChuckQue]

The biggest difference I've noticed from adding several hundred pounds of tires/wheels/accessories (JLURD, 6400# with two people and two large dogs) is the brakes could use, at a minimum, upgraded pads. Current braking vs OEM is noticeably worse.

What would be your pad of choice?

 

[DGT]

What would be your pad of choice?

Hawk Performance LTS is what has been recommended to me. I haven't tried them but plan to. Hawk claims 20%-40% greater stopping force vs OEM.

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