175x55x15 stock rear, diameter = 22.57" rev per mile = 921.5... 1.1% error from the factory
195x45x16 = plus 1, diameter = 22.90" rev per mile = 908.2...1.45% error
215x35x17 = plus 2, diameter = 22.92" rev per mile = 907.5...1.53% error
225x35x17 = Plus 2, diameter = 23.20" rev per mile = 896.5... 2.72% error
This particular system of computer controlled traction control, Stability program, and anti lock brake system uses 4 discrete sensors on each wheel/axle/hub assy. This system really does not care about tire diameter... it is a differential comparator that knows the rpm of each wheel, vehicle speed, throttle position, Yaw sensor input and acceleration/deceleration signals...
By designing it this way, the different versions of a smart car can be supplied with different size tires and diameters and not adversely impact the safety systems
07 and 08 451 series new design smart fortwo can be configured as Pure, Passion, Pulse, Cabrio, and Brabus ...within this family are three distinctly different tire dimensions from the factory
Because the speed signal for the speedo and ECU is from a fixed gear (analog) to digital pulse generator on the transmission the tire diameters are not relevant to the vehicle speed computation.
That all said, there IS a practical error front axle to rear axle to be concerned with...
I do NOT know what the max variance can be before the comparison computer thinks there is a condition that needs ESP or ABS activity.
But I do know that we should not put large diameter tires on the back with out having the front axle diameters fairly close or weird things can happen This was my main point of concern. I hear people changing to same size wheels and tires for front and rear. This is a bad idea!
On the 450 series it was possible to default the ESP to OFF.
The new 451 uses different logic and the simple workaround was deliberately eliminated by MB for product liability reasons. Just a matter of time before we find a way to hack in and default OFF the ESP and then all these issues go away for those of use who want to race this car without computer intervention
Sorry but I do not need the safety gurus to chime in about how dangerous that is...
Some owners will need to do this for closed course sanctioned racing events... most of us are adult enough to know these modifications (defeating the ESP) are not for daily street use.
I found your email quite thorough - full of design level knowledge. As members of Smartcarofamerica, we are lucky to have your knowledge.
Calculators are fine for averages, but if you take a tape measure around the outside of the front and rear tires on my stock 451 you will find that they are within 1/8", which doesn't agree with this calculator. Tire companies are like that, I have sold and use thousands of tires on all sorts of vehicles from competition to street and they vary. True the width of the front vs. rear will impact the under/oversteer conditions, but straight or braking they are turning basically the same rpm, so changing to four alike will not impact that. Yes it may impact any yaw or slip sensing, but for ABS there will be no difference.
I think the issue you are concerned with is putting wider tires on the front to match wider tires on the rear and changing the narrow front vs wide rear ratio...
smart MB used 155mm in the front and 175mm in the rear to DELIBERATELY induce Understeer...
They do NOT want a neophyte driver (typical inexperienced American?) having to correct a very short wheel based car from Over-steer
Under steer is where your front end skids out in the turn or fails to turn (wants to go straight)
Over steer is where the rear end breaks loose or skids to the out side and causes you to correct by counter-steering into the skid...
On these very short wheel based cars, over steer can get the typical driver in a lot more trouble because they snap loose too fast to correct effectivly....( if you are not ready for the skid)
Those of us who are Technical drivers and are fully in tune with the car, environment, and behaviors prefer the over-steer to the under-steer....
By changing the very narrow front tires to wider and adding some width to the rear OR just making them all the same.... understeer is reduced...
Some will change just so they can have 4 of the same for rotation and spare tire reasons
Some will go wide all the way around for looks
Some will ratio the front and rear wider for better slalom control
All I am saying is as long as the DIAMETER is kept reasonably close the computers will stay happy.....
You keep DIAMETER close by carefully selecting width and side wall height ratios that result in similar tire dimensions
The tire dimensions are;
Width.... in this case we are concerned with 155mm front or 175mm rear...
notice that the stock tires are all on 15" rims but the front and rear have different width and different ASPECT ratios
Aspect ratio = side wall height
Aspect ratio ... as in 155 x 60 x 15... or 175 x 55 x 15
the 60 or 55 are ratios compared to the tire width.... so they can put a narrow tire in front and a slightly wider tire in the rear but both fit 15" rims and are basically the same diameter...
Calculators are fine for averages, but if you take a tape measure around the outside of the front and rear tires on my stock 451 you will find that they are within 1/8", which doesn't agree with this calculator. Tire companies are like that, I have sold and use thousands of tires on all sorts of vehicles from competition to street and they vary. True the width of the front vs. rear will impact the under/oversteer conditions, but straight or braking they are turning basically the same rpm, so changing to four alike will not impact that. Yes it may impact any yaw or slip sensing, but for ABS there will be no difference.
EXACTLY!
The calculator shows that "theoretically" the diameter is nearly identical. After measuring stacks of tires looking for the right stagger and matching up tires for Surarus, I know that different manufactures have different ideas for what all that stuff on the side really measures and then add manufacturing tolerances and you have a wide range of measured sizes, all with the same numbers.
So the reason I gave the calculator was to show how close the two are already and with the wiggle room they need to allow... no problemo.
I changed to 16" rims (and tires of course) on my 2009 Smart. They are exactly the same rotational diameter as the 15" tires and I'm still having ABS issues as I go around corners (mostly left corners). The front brakes grab - even with my foot still on the gas - its caused me to almost go off the road a couple of times as the front end of the car skids out of control. I went to the local (San Diego) Smart dealer and they told me it was my tires/rims even though they're stock diameter. Obviously they couldn't find the problem and were looking for an excuse to charge me for their labor. So, I still have the problem... My current tire pressure is 32#'s front, and 34#'s rear. I noticed that the dealer added air - one tire was 39#'s, another 2 (one front and one rear) were 37#'s and one was 35#'s.... Great Job! I sure got my money's worth...
When the ESP steps in, which brake is it applying (inside rear or outside front)? Outside front would kick it into turning less, inside rear may kick the rear end out a tad...
If the inside rear is activating, I'd lower front tire pressures and raise rear tire pressures by a few pounds. If it's the outside front, raise front pressure and lower rear pressure.
...The front brakes grab - even with my foot still on the gas - its caused me to almost go off the road a couple of times as the front end of the car skids out of control. ...
Joe - I recently switched my stock passion wheels/tires for 17 inchers and have been experimenting daily with a range of tire pressures. What you describe can easily be induced by incorrect pressure. I'm finding that the wider, ultra low-profile tires require much less pressure.
I would test the pressure changes described by deepblueQ.
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