Smart Car of America Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
13 Posts
Discussion Starter · #1 ·
I just got a 2008 Fortwo Passion and I've been looking at possible changes or upgrades. I know I'm going to want winter tires since I'm living in western New York state, so I've been looking at wheels and tires in general. I've been reading through the forums, but I'm not particularly familiar with how this all works. Before I started reading this I was pretty sure that the wheels were the round things at the bottom. So I may have some of this wrong, and still some questions.

I see that the 451 stock has front tires measuring 155/60R15 on rims measuring 4.5x15, with the rear tires measuring 175/55R15 on rims measuring 5.5x15. So that means they're the same diameter, but the front wheels are narrower than the rear wheels. I think I want to go with getting all four to match, but not entirely sure what to go with. I see a lot of people go with all four rear tires. I'm still trying to figure out how much a difference that makes, and how much of a difference going bigger or wider would make. I'm generally looking for something that gives a more comfortable ride, and preferably with a size that's common enough to have a lot of options.
 

·
Registered
Joined
·
927 Posts
It's a fairly complex subject with many factors, but to understand what you want to do, it helps to know why it is the way that it is. It seems that Mercedes wanted the smart car to have extremely safe handling behavior, and whether that was due to a rumored early failure at a European emergency swerve test, or due to some internal company decision, really doesn't change the outcome that the 451 has extreme understeer built into the handling behavior. This heavy understeer means that traction is lost at the front wheels first, that in steady state cornering, if you increase the speed you will require more steering input to maintain the same turning arc. That was accomplished in four ways.

First, as you've noted, the front wheels are smaller and narrower than the rear.

Second, the front wheels are closer to each other (this is called the track) than the rear wheels, by four inches. This creates a turn vector that tends to grind the front outside tire by increasing the slip angle at the front wheel.

Third, the alignment specifications that call for a lot of front toe-in with minimal rear-toe in, combined with minimal front negative camber combined with moderate negative rear camber. These specifications increase grip at the rear tires and minimize grip at the front.

Fourth, there is a front anti-roll bar, but no rear anti-roll bar. The spring rates don't compensate for the lack of a rear bar.

Changes you can easily make can increase the front traction but will only moderately affect the understeer attitude. Even if you were to use front tires twice the size of the rears, the car will still require increased steering input as you increase your speed in a steady state corner - this inherent handling trait can't be changed without doing something relatively expensive such as changing springs or adding a rear anti-roll bar.

You primarily want more comfort, and that usually means tires with a taller sidewall. That also helps because the speedometer tends to be about 6-7% optimistic, so that an indicated 60 MPH is only about 56 MPH. (The odometer, however, is spot on with the stock tires). Taller tires that can fit the stock front rims are hard to come by, and using the wider rear rim on the front helps to get the front and rear track a bit closer to each other. That's why one of the most common upgrades is to use four rear rims.

By doing that, you can run a taller tire since more sizes fit the rear rim width. Many people run the same sized tires at all four positions and that's a good improvement. You can then rotate tires as well, which will give you a bit more tire life overall. The traction and handling assist systems also won't mind that all four tires are the same size. The stock stagger has the front wheels being a bit more than 1% smaller in circumference than the rears, and the stability system seems to be okay with a range of 0% to around 2.5% smaller up front.

The other factor is that you have a slight rearward weight bias, so a slightly larger tire on the rear would usually be typical. On a low powered vehicle like a smart, with the inherent understeer built in, it's not a big deal and it's fine to go either way.

So with all that being said, for what you initially said about wanting comfort, you are probably best off by getting four similar sized rims of 5" or 5.5" width, and going with 175/55-15, 185/55-15, or 185/60-15, depending on how much cush you want in your ride opposed by how much acceleration you're willing to sacrifice (as the bigger tires will effectively increase your gearing and require more power to spin up to speed).

To reduce some of the understeering behavior, you can put some spacers behind just the front wheels. This will help equalize the track width of the front and rear. Also, getting an alignment that sets toe-in at the front wheels to around 1/16" or around 0.05 degrees, helps quite a bit to reduce sidewind sensitivity and minimizes tramlining on grooved pavement.
 

·
Registered
Joined
·
927 Posts
Sure. I don't have the experience of actually running four same sized rims but I have decades of experience in high performance driving situations and understand a lot about how suspension/tire modifications and adjustments can alter the driving characteristics. I'm still on the original tires and rims but have 20 mm spacers behind each front wheel, and have made the toe adjustments to the alignment, along with having my shop loosen all of the front suspension bolts so I could use up all of the manufacturing tolerances to get as much negative camber up front as possible.

I'm always happy to talk suspension so let me know if you gave any questions.
 

·
Registered
Joined
·
86 Posts
As you want more comfort, increasing the sidewall heights of the tires is one way to go. For example, you could change the standard 175/55R15 77T and 155/60R15 74T tires to 175/60R15 81T and 155/65R15 77T. These higher sidewall heights allow you to run lower tire pressures. So instead of the standard 36psi/29psi tire pressures, you could run 32psi/26psi.

Another option to increase comfort is to fit the standard tires to narrower rims. This works because the sidewalls flex more easily when fitted to narrower rims. So instead of fitting the rear 175/55R15 tire to a 5.5" rim, you could use a 5" rim or even a 4.5" rim. The Smart 452 Roadster front steel rim is 5x15 and has a 24mm offset (ET24) so would be a suitable choice. Or the Smart 451 ForTwo front steel rim is 4.5x15 and has a 23.5mm offset (ET23.5) so is also a suitable choice. For the front rim, a Smart 450 ForTwo front steel rim is either 3.5x15 ET20.5 or 4x15 ET27. The 4x15 ET27 would be a suitable choice for a narrow rim for the 155/60R15 front tire.

Or 195/50R15 rear and 175/55R15 front tires on standard 5.5x15 rear and 4.5x15 front rims, or 185/55R15 rear and 165/60R15 front tires on standard 5.5x15 rear and 4.5x15 front rims. Both these options you could run lower 32psi/26psi rear/front tire pressure due to both these tire options higher 82/77 load indexes, ie. they can support the same amount of weight using lower tire pressures.

If you can put up with the almost 5% increase in gearing, then 185/60R15 84T rear and 165/65R15 81T front tires is another option. Due to their even higher load indexes, you could reduce tire pressures to an even lower 29psi/23psi rear/front. Do bear in mind, that sometimes tire pressures are increased to more than what is needed to support the weight of a fully loaded car to give the best handling characterics. For example, you could add 3psi to get sharper handling at the expense of comfort. So instead of 29psi/23psi rear/front, you could try 32psi/26psi rear/front tire pressures with the 185/60R15 rear and 165/65R15 front tire option.

Take a look at the Peugeot iOn electric car. It came out in 2010 and uses Smart 450 ForTwo tire sizes, ie. 175/55R15 rear and 145/65R15 front. The 4x15 front rim that the Peugeot uses is the same size as what the 450 uses. However, the iOn uses a narrower 5x15 rear rim compared to the 450's 5.5x15 rear rim. This was no doubt chosen because a 175/55R15 tire gives a softer ride on a 5" wide rim than a 5.5" rim.

Whatever option you choose, make sure you don't select tires with rim protectors if you want maximum comfort. Rim protectors are thick rubber moulded into the sidewall which make the sidewall more inflexible and the ride harder. Online tire sellers often don't make it clear whether a tire has a rim protector, so make sure you always clarify this point before purchase if you want maximum comfort.
 

·
Super Moderator
Joined
·
29,220 Posts
Whatever you do, make sure the tires are within a couple of percent of the OEM radius or you'll have the ESP putting you into limp home mode....:)
 
1 - 6 of 6 Posts
Top