I wonder why a car has 4 wheels but has only one spare wheel. Is it that only one tyre can puncture at a time? If not, what if more than one tyre punctures on the go?
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3I'd argue in many cases ( compact city car ) you maybe don't need one at all. Given the space it takes and the good reliability of modern tires and the fact that half the population wouldn't change it on their own anyway.. (I mean that even without introducing run-flats) – agentp Sep 05 '17 at 15:00
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9Anyone else think that changing a flat should be part of the driving test? – Solar Mike Sep 05 '17 at 18:19
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5@SolarMike That's funny but also absurd. Just try pulling the lug nuts after some garage torqued them on at 50 ft-lbs. Further, that would rule out physically handicapped. To say nothing of the cars which do not even have a spare. – Carl Witthoft Sep 05 '17 at 19:32
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@CarlWitthoft 50 ft-lbs easy : the challenge we had as apprentices was to "click" a torque wrench mounted in a vice set to 100 ft-lbs WITHOUT bracing yourself in any way against the bench... only 4 in a group of 30 could do it... And my worry is I can and have snapped too many large bolts etc But your point is true however it is sad that so many able-bodied will call the breakdown service when they could do it themselves and release the service to those with more need... – Solar Mike Sep 05 '17 at 20:01
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3when was the last time someone you know has needed a second spare wheel? – njzk2 Sep 06 '17 at 04:48
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@njzk2 I had two flat tyres on an 18 mile journey in 2009. I'd had a flat tyre, fitted the spare then ran across a pothole which caused another tyre to go flat. Luckily the second flat happened just a few hundred yards from my destination so it could be argued I didn't "need" the second spare. – Steve Matthews Sep 06 '17 at 08:54
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"...what if more than one tyre punctures on the go?" Call the local wrecking service to tow you a repair shop nearby. It's a simple risk-benefit trade-off. – NoDataDumpNoContribution Sep 06 '17 at 08:55
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7Having travelled the equivalent of 10-15 earth equators, I needed a spare tire exactly once. Spare wiper blades or spare batteries would have made more sense. – phresnel Sep 06 '17 at 09:31
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Bad potholes in my area frequently result in losing 2 tires (and wheels) at the same time... I've had to use spare tires a number of times in my life, mostly in a prior job. Working in construction was likely the culprit as there were nails and screws everywhere... – Brian Knoblauch Sep 06 '17 at 13:14
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1there was this guy I knew who decided to ignore the 50mph speed rating on his doughnut spare, blew that out too. Long walk (So I heard..) – agentp Sep 06 '17 at 14:20
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Many modern cars have no spare wheel at all. BMW have recently started implementing run-flat tyres across the range. Tyres which can, even with a puncture and no air pressure, be used at a reduced road speed.
Some other manufacturers have begun to include a compressed can of "tyre weld" or similar which attaches to the valve of a flat tyre and injects a sealant foam, again so the vehicle can be driven at a reduced road speed to a tyre bay for proper repair.
Carrying a spare wheel is quite an expensive business. It's expensive from a perspective of space, it's expensive from a perspective of weight and its expensive from the added production costs. However, generally a single spare is seen as a good compromise. There is nothing to stop a multiwheeled vehicle picking up more than one puncture at a time but this is an unlikely scenario.
Continent crossing vehicles (such as Dakar rally cars) will generally carry multiple spare wheels strapped to bonnets, roof bars or on the tailgate. This is because they may have to travel hundreds of miles before finding a garage.
For most consumer vehicles, they're usually never more than a few miles from a town or village with a tyre bay. Also, they're usually used on roads where busses operate and mobile phone coverage is good. If you were unlucky enough to have two punctures, the likelyhood is you can call a local garage, continue your journey on the bus or at worst case, have to flag down another passing car.
I should add that the most common reason why a four wheeled vehicle requires two tyre repairs is that a puncture has happened and the owner has found that the spare is also flat.
Steve Matthews
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Cars on long distance rallies in remote locations are likely to carry an even bigger collection of inner tubes for quick puncture "repairs". In an emergency, you only really need a new tire if the casing is damaged badly - e.g. cut by a sharp rock, or damaged in an accident. An inner tube is enough to stop an air leak. – alephzero Sep 05 '17 at 15:12
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2@alephzero, if you're in a race, changing the whole tire is a lot faster than repairing a flat, especially if you have to unmount the tire and put in a tube. – JPhi1618 Sep 05 '17 at 16:15
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4Also you typically drive much more carefully and slowly when you're running a spare tire... So you would need to puncture 2 tires. Chances are, if 2 of your tires need replacing, so does the part of your car between those two tires. – corsiKa Sep 05 '17 at 20:40
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Yeah I found my spare nearly flat when I first went to use it. I had a sharp word with my mechanic about that one. Free tire pressure check with oil change didn't include the spare. – Joshua Sep 06 '17 at 18:08
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1@corsiKa - That would be true in most civilized nations, but around these parts you see people running up the highway at speed on the things ... STU-PID! – Pᴀᴜʟsᴛᴇʀ2 Sep 07 '17 at 00:01
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Take a look at some classic antiques. Certain models have two spares, one mounted in each of the front quarter panels behind the axle.
I found approximately a zillion examples here
As tires, and roadbeds, got better designed, the probability of getting a flat (or more) rapidly moved asymptotically close to zero. Savings in cost and weight are critical to sales success, hence the disappearance of spares.
Carl Witthoft
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Many of those "zillion examples" show a vehicle with just one spare, either on the side or in the back, or else it's hard to tell from the image if there are two spares or just one. (I'm not saying you're wrong; I'm merely saying that the link by itself isn't all that convincing.) – Sep 05 '17 at 21:32
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"Asymptotically close to zero"?! I've bought two new tyres in the last year. Roads in the UK are not good. – Andrew Leach Sep 05 '17 at 23:20
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@phresnel On that car, well under 3000 miles. Some major, government-maintained (rather than County-maintained) roads are literally falling apart. – Andrew Leach Sep 06 '17 at 09:33
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@AndrewLeach: Okay (we can talk metric, if you want; I am from Germany). And how many miles and tires did you go/consume in total? Personally, I travelled something beyond 400000 kilometers, and needed a single spare tire in that time (on German roads, that is). – phresnel Sep 06 '17 at 09:36
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That's it. In under 3000 miles (5000km), I've had to replace two tyres which have been damaged because of poor roads. It doesn't take many potholes! The amount of actual wear of those tyres and the ones I haven't replaced is minimal. [But we shouldn't be discussing this here.] – Andrew Leach Sep 06 '17 at 09:41
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1@AndrewLeach you're a statistical outlier. I've had three punctures in ~200,000 miles on British roads. One of those I know was from a nail when parking on waste ground. In all cases the puncture was slow enough that I could drive to a tyre repair garage. – slim Sep 06 '17 at 13:05
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@slim I also had two flat tyres this summer. I live in a very rural area of the UK with chalk geology, so the minor roads are full of sharp flints that wash out of the fields. Two in one summer is unusual, but I average more than one per 10,000 miles. – Mike Scott Sep 06 '17 at 15:29
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With modern tires on decent-quality roads, you can expect a flat tire about once every 150,000 km. So the chance of having a flat within one given trip is very small (let's say 1/1000 if your trips are 150 km long on average). The chance of having two flat tires at the same time independently of each other becomes 1/1,000,000.
That chance is so small, the cost of carrying a second spare with its drawbacks (weight ~25 kg, increased fuel consumption, and the space it takes up) far outweighs the benefits.
If you have two flats simultaneously, chances are you'll have 4 flats because you've driven over agressive one-way traffic enforcement measures or a police blockade. 2 spares aren't going to help you in those situations.
Hobbes
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