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Why don't all cars use BMW i3 style tires?


#1

The BMW i3 uses extremely thin 155/70R19 tires, which is way different than any other car on the market. The idea is that even though they are so narrow, they are also extremely tall, which means the contact patch area is similar to a more conventional tire. But after I thought about it more, tall and narrow tires should have quite a lot of other advantages as well:

  • Better aerodynamics
  • More space for steering/suspension
  • Taller wheels look better on the side profile
  • Possibly better performance on snow/dirt?

I honestly couldn’t think of any disadvantages to such a design, except maybe if you are just trying to maximize grip for a sports car. But it seems to me that every family hatchback, sedan, or SUV should be using something similar. Any thoughts?


#2

One downside I can think is with the size. It can cause the fender to possibly cut into the doors reducing the opening of the door making it harder to get in and out of. Also, for sedans it would make the front of the car need to be taller which would mess up aerodynamics, and design. Plus with the larger diameter it would need still need a wide fender in the front since at the same steering angle the tips of the tires extend into the body more so you don’t save space in the engine bay.


#3

Load capacity might also be impacted with such a unique tire, but I havn’t done enough research on it.


#4

Well, when thinking about it, many cars today are using tire sizes that is more extreme than those on a Ferrari Testarossa. You really don’t need it on a diesel family station wagon… mostly looks and prestige.


#5

I would think load capacity would be the biggest issue. On a small light car it wouldn’t be a problem. Like the tiny wheels and tires on a Geo Metro for instance. But go a bit larger with a pickup truck or large SUV and that’s a lot of weight you’re concentrating on a small surface area. Hence the existence of dual wheels for larger trucks.


#6

Well, as an example the Volvo 145 wagon used 165R15 (82-profile) with a gross weight of 1820 kg, you have to go for quite heavy vehicles to make load capacity an issue actually…


#7

For some reason I’d think thin tires like the i3’s would increase the risk of rollover.


#8

I dont think they have better performance on snow amd dirt. The reason being is that you want to spread the weight of the car out when travelling on loose terrain so you dont end up stuck. Thin tyres do not help in this.


#9

Narrow tyres do perform much better than wide ones in snow and dirt…for exactly the reason you mention, the weight is spread over a lesser area, and because of that the pressure is higher per m2… which gives better traction, the car won’t “float” on the loose surface. The alternative is to make the pressure low enough to drive on the loose surface without sinking down… but then you need a really huge area or a very light vehicle… Like a snowmobile with tracks and a weight of only some hundreds of kilograms…

Also, wide tyres hydroplanes easier.


#10

I think it’s important to emphasize that the contact area is supposed to be the same. It would be longer and narrower, but the actual area should be the same, which means the same amount of grip, the same pressure, and the same load.

Nowadays car hoods and beltlines are getting higher and higher due to pedestrian impact regulations, so using taller thinner wheels would allow them to maintain good proportions while also getting the other advantages.


#11


That’s not how it works.

As I said, the entire reason why you want less pressure on your tyres is because you don’t want your car to sink. By having greater force exerted in a smaller area, you cause the car to sink.
With wider tyres, you spread the weight more evenly across a wider area. Its like lying on a bed of needles. If you were lying on a single needle, it would hurt like hell, but by spreading your weight across many needles, it hurts less.

This is the exact same reason why tanks use tracks, why you wear snow shoes, and why when driving on loose surfaces, you deflate your tyres slightly. You don’t want you vehicle to get sink and get stuck.

On road surfaces thinner tyres may be better suited, but offroad in loose snow and dirt as it was implied, wider tyres are better.


#12

That’s exactly how it works… with normal tires on a car you can’t get an area big enough to get such little pressure for the car to be able to drive on top of the snow, mud or whatever it is. It will sink anyway. With tracks, yes, but that is a completely different story, you never get that area with regular tyres.

But you’re welcome to prove me and everyone else that lives here wrong, I still got 1 meter of snow outside my window, as every year…


#13

But he did just prove you wrong


#14

No. :wink:

But I am not interested in winning the internet tonight, so if you want to believe that wide tyres are superior in the snow, go ahead… I know that they aren’t.


Seems like there is more people out there that lacks knowledge in the same way I do after living in this climate for over 30 years though. :wink:


#15

I find it really odd that more people don’t realize this. Narrow tires give you far better traction in snow because they act like pizza cutters and slice through the snow rather than trying to float or bulldoze it out of the way… Seriously, if you all think @Knugcab is wrong, spend a winter in Sweden or hell Michigan where I’m from. You don’t see people putting F1-esque snow tires on their cars in these parts.

But more on topic. A big reason the BMW i3 uses such a weird tire size is for maximum efficiency; a narrow, large diameter tire reduces rolling resistance versus a wide small diameter tire. There are some downsides to using it though:

  • Although its a 70 profile tire, that’s only 70% of a teeny 155 mm width, so as with all your high performance cars you see with the 40 - 45 profile tires, potholes are the devil. They will fuck your wheels up bad if you’re not careful. This is why I can’t fathom why GM started offering factory 22 in wheels on their trucks and why people buy them. Your making and buying a truck for the wrong reasons with wheels like that i.e. having poorly defined “shit to haul”… apparently on only the finest of paved roads.

  • Again the profile thing - it isn’t great for comfort. Although this is sort of moot given how prolific low profile tires are nowadays.

  • More space for suspension? Well kind of. Lateral space definitely but I would think you actually have less vertical space since to achieve the same ride height as a smaller diameter tire, you would need to move the hub up. And then if you wanted to maintain the same suspension travel, the fender and upper suspension mounts need to move up too. And then that means making the entire car vertically larger. The BMW i3 doesn’t really care about this though because its mean to be a roomy, comfy compact MPV like a Ford Cmax.

  • More space for steering? Again, that’s probably a kind of. A large diameter tire would mean that to achieve the same turning angle / radius, the wheel well would possibly need to be deeper. And that’s not good for interior / engine bay space. The BMW i3 again doesn’t really care about this though because electric motors are virtually always smaller to equivalent piston engine.

Point is, its complicated. The BMW i3 did what it did for reasons centered around the fact that its a high miler or electric depending on what you think of its gasoline-powered range extender.


#16

Well, here is what WhatCar? magazine had to say about the i3’ handling and ride comfort: https://www.whatcar.com/bmw/i3/hatchback/review/on-the-road/

Ride Comfort:

''Even on standard 19in wheels, you’ll notice plenty of thunks and bumps along pockmarked urban roads – that’s a shame, given that the car is primarily designed to be used in cities. The ride on the optional 20in wheels is, unsurprisingly, even harsher and crosses the line into uncomfortable.

The i3s offers a marginally better ride than the standard car. Its suspension may be firmer but wider tyres help take the edge off expansion joints and potholes, whereas the lighter 20in wheels reduce unsprung mass, smoothing out the ride at higher speeds.’’

Handling:

''At faster speeds, the standard i3 can feel a bit twitchy and nervous, though, so it isn’t as fun to drive along a twisting country road as you might imagine. Overly quick yet uncommunicative steering can also force you to take multiple bites at corners. And despite the fact that the car’s relatively skinny tyres produce more grip than you might expect, sudden bumps can easily knock the i3 off line.

Thankfully, the i3s goes some way to correcting this imbalance. A 40mm wider rear track (the distance between the wheels), 10mm lower ride height and specially tuned springs, dampers and anti-roll bars ensure the i3s stays flat through quick changes in direction. And, thanks to wider tyres at both the front and rear, there’s less of the skipping and pogoing that you get in the standard car.’’

That’s kind of what I thought - the incredibly narrow tyres make for a fairly uncomfortable ride. Handling would be similar to an F1 bolid from 1950s.

The main reason why this was done was to lower rolling resistances and drag which would kill it’s range. It makes sense. However, while there certainly is a huge benefit to having narrow snow tyres in rally conditions, those aren’t exactly JUST narrow budget radials. Rally cars will use studded tyres with very coarse tread which dig into snow for grip rather than skimming over it like a regular tyre would. A narrow road tyre would hold well but still won’t be able to dig into snow anywhere near as well as a rally tyre would.

I did once catch a glimpse of an i3 parked up with wheels turned towards the kerb and the tyre width caught me a little off guard. Then again, it’s not really a racing car. Their primary focus is to keep consumption as low as possible.

While lower tyre pressure will help spread the load and increase traction on dirt and roads and prevent your car from sinking into muddy terrain (at least partially), snow is a different case where digging in is actually desired. I guess it would be kind of similar to those crazy swamp buggies over in the US of A, with gignatic studded wheels cutting through mud and water.
http://barrybland.typepad.com/photos/uncategorized/2008/08/22/swamp_buggy_10.jpg


#17

No, we use to change tyres in the winter time, rally tyres aren’t legal on a street car, but at least we change to tyres with a better compound suited for cold climate and most often studs too…

Summer tyres are pretty much worthless in the winter here anyway, regardless of width, size or anything else.


#18

I would change tyres in the winter too but there is no ‘‘winter’’ as such in the UK. We got a total of 3 days of snow this winter. Yes, my 235s weren’t much good. I’ll need a set of winters at some point though, even if to just deal with lower temps and icy conditions a little better.

I’m simply saying that i3 tyres are not winter tyres. They won’t have studs, coarse tread or the correct compound for true winter conditions.


#19

Of course, there is drawbacks with a size that by todays standards is as odd as 155/70R19. I just think that the tyres of todays cars are unnecessarily wide and low profile if it’s only seen from a practical standpoint. The Ferrari Testarossa, old but still a competent sports car, used 225/50R16 up front (though to be fair 255/50 in the rear), and that’s the same dimension that the 4-cyl N/A Mercedes C180 station wagon I once had was using. You can’t really say that it is justified then, by safety, practicality or anything else, to run the same dimension on a 125 hp family wagon that was enough for a 390 hp supercar 10-15 years earlier, it’s really not needed.

Looks is one thing, the belt line is so much higher on cars of today, for safety reasons as mentioned earlier. The 16 inch wheels that would have been impressive in the 80s almost looked small on that car, with all of the sheetmetal between the top of the wheel and the rear side glass. You have to make big wheels, and most people think that rims are more beautiful than tyres, so they make an even bigger rim (to be fair, to be able to fit bigger brakes too), lower profile tyres, and to get the right proportions, wider tyres too. The BMW i3 is bit of an oddball, people will accept that things are looking a bit weird, buyers of more regular cars don’t want to drive around on “some silly bicycle tyres”.

With that said, narrow tyres (to a limit) have many advantages for everyday use, like lower rolling resistance, better aerodynamics, lower purchase prize, lighter steering (though hardly an issue today with all cars having power steering), lower risk of hydroplaning and often lower noise levels too. Not the same grip on dry asphalt, however the question is how often that’s needed for the average driver in an average car…

But I guess that all of us here is gearheads and because of that we want cars to be sexy too. And that’s of course easier to succeed with when using some fat lo-profile rubber instead of skinny ones on small steel wheels.


#20

I’m not going to pretend I’m an expert in any sense, but I get the impression that it’s the road beneath the snow that gives narrow tires grip in those conditions. If there were no road beneath, or the snow was extremely thick, wider tires would prevent sinking and therefore be less likely to get stuck.

In terms of pure traction, for example on ice, the size of the contact patch shouldn’t affect the total amount of friction. The weight per square inch will increase, but the total area will decrease. All that matters it the weight of the vehicle and the coefficient of friction. Though of course things are more complicated in real situations.

I was also able to find these images, where the tires appear to be much wider:

image

Again, I’m not trying to argue anything, just putting my two cents in. Feel free to correct anything.