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Unread 06-01-2010, 12:20 AM   #1
JLC08JK
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Has anyone used a 32" spare with larger tires?

I am wondering how the computer will react if a small spare tire is used in combination with a 35" tire on the same axle.
This would only be done in emergency situations and not in 4wd.
Has anyone tried this and what happens with the ESP, BLD and Antilock?


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Unread 06-01-2010, 03:23 AM   #2
fernjack
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Just not a good idea .......... you will damage drive train .
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Unread 06-01-2010, 08:17 AM   #3
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The way I see it, 32" tire circumference is 100". 35" circumference is 110". So there's about a 10% difference that the differential would have to compensate for. It does that all the time during turns. But I would keep my speed down to minimize heat.

I've never tried running a smaller spare with a JK, so I can't answer your question from experience. But I think most of us have run donut spares on passenger cars, which is essentially the same thing.

BLD attempts to give some torque to a non-spinning wheel by braking the other wheel on the axle. I think (but I don't know, but I think) BLD requires a radical difference in torque between the wheels to kick in. I wouldn't be too concerned about BLD.

I'm less familiar with ESP. But ESP can be disabled. So in such an emergency situation, you could disable ESP.

Antilock brakes? I have no idea how the system would work in an emergency stopping situation. I think, but I don't know, that the brakes work independently, so any one wheel will be acted upon independent of what the others are doing. That seems like a tire difference wouldn't matter. But, I have no practical knowledge.

Great topic. This sounds like a great question for a Jeep engineer...
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Unread 06-01-2010, 08:30 AM   #4
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Yes, that is a small difference in speed, but the EPS system knows if you are turning or not and also which direction the vehicle is actually going - so the EPS will know you are going straight, but the wheel speed differential will make it think you are turning or "sliding" and probably activate the BLD system or anti-skid feature...definitely NOT a good idea...if you are trying to save some bucks and only buy 4 large tires...my strong advice is skip it and buy five matching tires...
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Unread 06-01-2010, 08:46 AM   #5
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There are advantages to running a smaller spare. Saves a lot of weight, which helps performance. Saves money, not only the tire and wheel itself, but also the necessary tire carrier. Preserves visibility out the rear window above the tire. And maintains the convenience of having the tire mounted directly to the tailgate (only one thing to swing out to access the cargo area, rather than two). For normal driving on 4 good tires, which is 99.99999% of the time, the smaller spare is awesome. So the question is, is the Jeep drivable on mismatched tires?

We need to understand more about these electronic systems so we can answer this question. The OP has identified the three: ESP, BLD, and ABS. ESP can be disabled. So it's only a question of BLD and ABS.

I would be more concerned about ABS than BLD, as we can see how BLD works in videos and such. Try to climb over something, spin a tire, and soon thereafter BLD kicks in. Doesn't seem like a very precise system that cares much about 10% differences in anything. It just applies brakes to give your non-moving wheel some torque. It's designed as a pseudo differential locker, not as a traction control system.
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Unread 06-01-2010, 12:09 PM   #6
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grinner has all the reasons right!!!
I'm not to concerned with getting a flat other than on a long road-trip.
I logged over 200k miles on my F350 and never carried a spare. When I did get a flat I would just get towed to a tire shop and get it fixed. It would end up taking the rest of the day but it only happened 3 times in all those miles!
On my Jeep I would like to have the convenience of having a spare. Using the stock spare would allow me to save a little money on 35's, there are great deals on 4 tires no deals on the fifth
If I use a 35 as the spare then a tire carrier needs to move to the top of the list!
Tire carrier+spare = $800+
Every time I cross one thing off my list something else gets moved to the top and three things get added to the middle
Here is an example of the list; $800, $800, $800, $800, $800............
I would just like some reassurance that if I need to use the spare to get 30 miles out of the desert it's not going to lock everything up and be un-driveable. I would only use the spare on the rear!
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Unread 06-01-2010, 12:57 PM   #7
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Im gonna tell you a story about the dude with a real nice Porsche, and his flat tire, COSTLY, experience. Any of you know Porsches, they all come with an air compressor, nice little logo on the plastic carrier deal, pretty cool. They come with becuase in the event of a flat, the spare is stored under the hood flat, space is limited in little cars like those. So, this friggin rich dude pulls into the shop, on fire. I put the fire out(almost), put it on the lift and put out the rest of the fire. Then I asked him to come into the shop and explain to me what exactly he did. "I got a flat, put on the spare and drove here"

In about 4 miles, he destroyed the following:
1) Paint Job - fire melted it to bare metal, whole car painted
2) spare wheel and tire
3) brake line, rotor, pads, caliper, couple other things....and in a car like that you do both sides, not one.
4) oil cooler lines, oil cooler(line burnt up to the cooler, melted it)
5) trunk carpet
6) passenger side tail light housing, wiring, bulbs
7) soft top(bottom right corner melted)

He never inflated his spare, I showed him the compressor and got a "Oh thats what that thing is for".

It has traction control, just like our Jeeps. So, all 4 wheels have to spin at the same time. Not for nothing, driving anywhere on stock spare when the other 3 are 35's, not good to do.

Edit - forgot to tell you why he burned, the traction control, or for us its ESP which eats rear brakes......Uses brake pressure to make sure the spinning wheel(which will be spinning faster) slows to gain traction. Smaller spare = spinning faster, and after a while red hot rotors ignite pads, then lines, then your screwed......
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Unread 06-01-2010, 01:17 PM   #8
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I understand the reasons, but personally don't agree with the non-matched spare idea. That's just me.

Another solution would be to get a tire repair kit and portable compressor so you can fix the minor issues on the trail. I've also seen more than a few situations where the guy carries the spare inside the back of the Jeep on some sort of mount.
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Unread 06-01-2010, 04:17 PM   #9
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Just don't do it... you are running a high, high risk of damaging something - or many things - a lot more expensive than a 5th spare 35" tire that costs $225-300.

And the tire carrier excuse isn't that great of one. Sure, the stock carrier or something like the besttop/highrock carrier that mounts on the stock gate ($99) isn't good to run a 35" tire for 50,000 miles or anything, but many people have run a year+ with a 35" tire on the stock carrier.

I have the beststop carrier for $99 and it holds my 35" close to the gate (closer to gate the better bc its not like being on a springboard with leverage bouncing the gate) and very snug. I don't leave the gate hang open with it on there. I don't have any odd squeeks, rattles, or noticable wear on the hinges or the spot welds on the inside of the gate. I've been running this for 5k miles on crappy PA roads. I plan on a $400-600 carrier sometime, but you can run a 35" without a solid steel aftermarket carrier for quite some time. THAT's a much better idea than running a spare that's 3" difference from the other 3 wheels. I'd personally rather run no spare running around highways and just get it AAA'd if you get a flat that fix-a-flat can can't fix than to run a 32" spare with 35s.

Oh and a 35" spare will also fit in the cargo area just fine if you want to go that route (spareless on gate = best vision!)...
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Unread 06-01-2010, 05:59 PM   #10
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I asked about this at one time, on here and other boards, and the general strong consensus was not to do it on the JK...there were many reasons, but the consensus was that with all the JK electronics, etc...damage would occur...
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Unread 06-01-2010, 06:33 PM   #11
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If you are looking for a lower cost alternative $50 solution. Get a used 35" tire with 20% of the tread left on it used wheel if it looks fugly Paint the wheel flat black or put a cover on it.

Back in my college days driving my 61 International scout I never paid more than $20 per tire some times a lot less LOL.

If you have a brand new tire and you are sticking with the same wheels most used tire shops would be happy to trade you a Used 35" tire for that new 32" spare.

Either way - Buy a compressor, bottle of slime + some plugs so you hopefully never have to use the spare. They have kits at Wally World for $25 - $50.
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Unread 06-01-2010, 06:47 PM   #12
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There is a reason I suspect that all 4WD vehicles come with full size spares. Why would even want to test if you can run a different size than the other 3?
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Unread 06-01-2010, 09:08 PM   #13
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Quote:
Originally Posted by CGScotty View Post
There is a reason I suspect that all 4WD vehicles come with full size spares. Why would even want to test if you can run a different size than the other 3?
Not true, My old WJ did not have a full size spare.
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Unread 06-01-2010, 09:46 PM   #14
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If we were to actually try to find the answer, rather than guess, the benefit is we'd learn a lot more about our 4WD and traction control systems. I'm enjoying the research on this one.

Check this out: JP Magazine Article
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Unread 06-01-2010, 11:21 PM   #15
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Im very glad someone was ballsey enough to ask this question. I have 35's and came across a awesome deal for 4 38" tires in decent shape. So far I havent read anything mechanically wrong with running a smaller tire for a short amount of time... Now I have bypassed my ESP, BLD and ABS using a little toggle switch. Would I be OK with running a 35" spare just to get off the trail with so I can call for a tow?
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