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Wide Front Wheels

The standard narrow tyres fitted to many of the fortwos causes severe understeer, you turn the wheel in a corner but the car goes straight on. You need wider front wheels and tyres for increased grip.

Modification Details




The front wheels on the fortwo are narrow, so subsequently the tyres are also narrow. This was a design bodge to allow the car to pass the elk test (sometimes called the moose test). This is a driving field test that is a measure of stability when you throw the car into a quick left then right swerve as if you were avoiding something (like a moose, child or car reversing into the road). To pass, the car has to be doing 42mph or more and not flip over.

Elk Test Failure

This test had been undertaken since the 1970's and unfortunately for Mercedes (who ran smart at the time), the first car to fail the elk test was the 1997 Mercedes A Class which just fell over instead of swerve.

Mercedes immediate reaction was to panic and give the A Class narrow front wheels and tyres to increase understeer because the original tyres were gripping so much that the centre of gravity was being pushed outside
the envelope of the car. Being a tall and narrow car, it fell over.

As the smart city car (renamed to fortwo in 2004) was just a small version of the A Class, Mercedes had to do the same with the thin wheels and tyres. They also adjusted the front suspension setup which is why the 450 was rolled out with different front and rear wheel offsets. There was no time to fix it properly.

J Turn

This isn't a proper regulated test, it's just something you see in movies and car shows.
However, as with the elk test, having a tall car with wide front wheels is bad news.

You drive backwards as fast as the car will go, turn the wheel 180 degrees (356 fahrenheit) and the front
of the car spins around to face the other way. As is spins you put the car into 1st gear and straighten the
steering as the slide brings the front of the car around to the direction you want to face. Or in the case of
the fortwo, the front wheel grips and the car flips over on its side.

Conclusion

So, putting wide front wheels on your fortwo improves the way it handles by giving the front more grip.
Just remember that there is a limit to front wheel grip usefulness. If you swerve heavily at high speed in a
fortwo the car may fall over (instead of skidding uncontrollably in any way it wishes).


Don't swerve uncontrollably, slam your foot on the brake and let the ESP (or Trust) and
ABS do their thing before swerving around anything. As for the J turn, just don't do it.


Wider Tyres, The Easy Way

The obvious route to take for all of the smart range is to just fit wider tyres.
Depending on the width of the alloy, you have a range of tyre widths you can choose.
It is important not to exceed the minimum or maximum widths.

Check your existing wheel width and see how wide you can go.

Ideally, as the tyres become wider, the number depicting the profile (height) of the tyre should decrease.
The number on the tyre will be WWW/PP/DD where W is the tyre width, P is the tyre profile and D is the alloy
size in inches. For example 145/65/15. 145mm wide, 65% of the width is height, on a 15" alloys wheel.

In this instance, the alloy is 4" wide so using the table you can go up to 165mm wide.
The general rule is reduce the profile number by 5 every time the width increases by 10.
So, in this example you could consider 165/55/15 tyres.

 
Rim width Minimum tyre width Ideal tyre width Maximum tyre width
       
3.5 inches 125 mm 135 or 145 mm 155 mm
4.0 inches 135 mm 145 or 155 mm 165 mm
4.5 inches 145 mm 155 or 165 mm 175 mm
5.0 inches 155 mm 165 or 175 mm 185 mm
5.5 inches 165 mm 175 or 185 mm 195 mm
6.0 inches 175 mm 185 or 195 mm 205 mm
6.5 inches 185 mm 195 or 205 mm 215 mm
7.0 inches 195 mm 205 or 215 mm 225 mm
7.5 inches 205 mm 215 or 225 mm 235 mm
8.0 inches 215 mm 225 or 235 mm 245 mm
8.5 inches 225 mm 235 or 245 mm 255 mm
9.0 inches 235 mm 245 or 255 mm 265 mm
 
However, wider tyres are not a win/win situation. All the time you are going straight on you will have more grip.
Be aware that more grip also means more rolling resistance and therefore decreased fuel economy.

The wider the tyre, the more wallowing you will get in corners. This is where the tyre side wall
deflects allowing the movement of the wheel inside the envolpe of the tyre. Wallowing tyres will
decrease steering wheel feedback and increase the time it takes for the car to respond to steering.

 
Wider Front Wheels And Tyres

Wider wheels means wider tyres with less wallowing (side wall deflection), more grip and better handling.
The main drawback (apart from those mentioned before) is increased fuel consumption.

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