I was researching tires and wheels for my next upgrade and found an ad for 16” Schmidt wheels (specifically for a smart ForTwo) on Craigslist. I was hoping to find a smoking hot deal on a set of slightly used tires and wheels, but instead found that the ad was placed by a dealer who recently got a big shipment in and were using Craigslist to maximize their advertising.
Since these wheels are what I decided on, I may just pull the trigger even though it is full retail. I have questions, though, that I haven’t been able to answer. I was hoping that this forum could come to my rescue….
1- The Schmidts come as a set, 6.5” wide for the front and 7” wide in the rear. The recommended tire for both the front and rear is 195 40/16. I want to square up the tires to reduce the car’s understeer, but why the staggered wheel sizes? Is the staggered wheel/square tire sizes OK?
2- I gather the tire pressure sensor can be reused when the new wheels are mounted. It seems that the sensor is part of the schrader valve. Is this correct? If so, it seems that it would be easy to install a simple schrader valve in the old tires (I plan on selling them to someone who wants a spare). Is this correct?
3- If I need to install a new pressure sensor, are there any lower cost aftermarket sensors that are known to work?
4- Lastly, anyone have a recommendation for a high quality 195 40/16 tire?
I was researching tires and wheels for my next upgrade and found an ad for 16” Schmidt wheels (specifically for a smart ForTwo) on Craigslist. I was hoping to find a smoking hot deal on a set of slightly used tires and wheels, but instead found that the ad was placed by a dealer who recently got a big shipment in and were using Craigslist to maximize their advertising.
Since these wheels are what I decided on, I may just pull the trigger even though it is full retail. I have questions, though, that I haven’t been able to answer. I was hoping that this forum could come to my rescue….
1- The Schmidts come as a set, 6.5” wide for the front and 7” wide in the rear. The recommended tire for both the front and rear is 195 40/16. I want to square up the tires to reduce the car’s understeer, but why the staggered wheel sizes? Is the staggered wheel/square tire sizes OK?
- I have the 16's, 6.5's and 7's, correcting the understeer?, not sure, but the ride handling and comfort is greatly improved and good match.
2- I gather the tire pressure sensor can be reused when the new wheels are mounted. It seems that the sensor is part of the schrader valve. Is this correct? If so, it seems that it would be easy to install a simple schrader valve in the old tires (I plan on selling them to someone who wants a spare). Is this correct?
- you could reuse them in new wheels, normal valves stems for spares, sure, I bought new ones from tirerack, I plan on swapping for winter here, if you buy new sensors from TR, make sure you ask for the nut/sleeves for additional $5.
3- If I need to install a new pressure sensor, are there any lower cost aftermarket sensors that are known to work?
- Tirerack, even though they say for package tires/wheels only, they sold them to me when I bought the tires, I called instead of a online order.
4- Lastly, anyone have a recommendation for a high quality 195 40/16 tire?
- Tirerack had yoko's and michelin's for the wheel size, I bought the mich's b/c the yoko's were backordered for a few weeks.
geosynch
- Have some pics in my gallery, will post more once I clean up the lil' guy.
I was researching tires and wheels for my next upgrade and found an ad for 16” Schmidt wheels (specifically for a smart ForTwo) on Craigslist. I was hoping to find a smoking hot deal on a set of slightly used tires and wheels, but instead found that the ad was placed by a dealer who recently got a big shipment in and were using Craigslist to maximize their advertising.
Since these wheels are what I decided on, I may just pull the trigger even though it is full retail. I have questions, though, that I haven’t been able to answer. I was hoping that this forum could come to my rescue….
1- The Schmidts come as a set, 6.5” wide for the front and 7” wide in the rear. The recommended tire for both the front and rear is 195 40/16. I want to square up the tires to reduce the car’s understeer, but why the staggered wheel sizes? Is the staggered wheel/square tire sizes OK?
2- I gather the tire pressure sensor can be reused when the new wheels are mounted. It seems that the sensor is part of the schrader valve. Is this correct? If so, it seems that it would be easy to install a simple schrader valve in the old tires (I plan on selling them to someone who wants a spare). Is this correct?
3- If I need to install a new pressure sensor, are there any lower cost aftermarket sensors that are known to work?
4- Lastly, anyone have a recommendation for a high quality 195 40/16 tire?
geosynch
Just in case you didn't know schmidt has 17'' space lines made specifically for the 451 check out my car in my gallory.
4- Lastly, anyone have a recommendation for a high quality 195 40/16 tire?
Yes: Don't install them.
The smart fortwo has different sized wheels and tires at each end for good handling with the significantly rearward weight bias (just as a rear-engined Porsche does). Consider that BRABUS fortwo Monoblock VI alloy wheels are still different in width, with 5.5 inch rims with 175/50 R 16 tires at the front and 7.5 inch rims with 225/35 R 17 tires at the rear.
When choosing your wheels, make sure that you get the same offset and that the tire/wheel that you choose weighs the same, or less, than the factory setup. Increasing unsprung mass (which is what wheels and tires are) really messes up ride and handling. Many a Miata's handling has been ruined by owners who replace the 12lb wheels with aftermarket wheels that weigh 18lbs or more.
I'm not trying to be a buzzkill -- just the opposite. I want people who spend a grand or two for wheels and tires to end up with a fortwo that looks good and handles at least as well, if not better, than stock. And certainly that is no less safe in emergency maneuvers.
P.S. I'm sure that there are people who will say that the car handles "fine" with identical wheels front and back. You'll also find people who say that "Miracle Fat Burning Cream" made them lose weight, that a magnet attached to their fuel line improved fuel economy, and that lottery tickets are a great retirement investment.
You forgot all the people who run around saying "The Sky is Falling".
If you're implying that I'm being an alarmist, how about a call to smart's Customer Service to get their take on it? I'm betting that they'll agree with me.
Ahhhh. You are opening Pandora’s Box. But your reply deserves a well thought out response.
Square tires (as in same size front & back). Why do I want them? Well, anyone who has pushed their smart into a corner at speed has felt the understeer whether they know what they were feeling or not. It feels like the car is “pushing” the trajectory in a wider arc than the driver is trying to achieve. The car was engineered this way so that the public does not get themselves into trouble, because understeer’s infamous cousin, oversteer, has a tendency to bring the back of the car around to the front when cornering at speed. I don’t like the excessive understeer. I suspect anyone who likes to drive “spirited” doesn’t either.
A few weeks ago there was a smart event at Infineon Raceway which I attended. At this event a vendor was showcasing his aftermarket products. smart had a representative there. At this event we had a chance to autocross our smarts, but more importantly, we had a chance to autocross a smart that had some of the aftermarket products. One of the modifications to this smart was square tires all the way ‘round. That smart did not have excessive understeer. In fact, it was fun to drive. Woo-Hoo! The owner said that the single most important transformation to improve the handling was the square tires sizes all the way ‘round.
Now the smart rep that was there with us at Infineon was quite clear that smart’s corporate position was don’t mess with changing the tires for safety reasons. He cited that the electronic stability control system could be defeated. Now I can honestly say that I am a bona fide expert in control systems – not automotive, but spacecraft – and as a control system expert I can say that I whole heartedly agree with smart. At least in theory, but not necessarily in practice. Changing the tires changes the dynamics (something that in control theory is called the “plant”). Changing the plant can make a system go unstable, but. There are things called gain and phase margins that any self respecting controls engineer is going to ensure are quite large. These margins ensure that the controlled system stays stable even if the plant changes. In this case the plant can change because someone changed the tires, or, more simply, someone changed the road surface and made it more slippery or gave it more traction.
That’s a long way of saying, I ain’t buying smart’s argument. It is pure CYA. Those who have gone before me and squared up their tires sizes help to shore up this line of reasoning. I want to improve the car’s handling by reducing the understeer. I think that squaring up the tires will be a big step towards that. If I’m wrong, I’m out about $1.5K.
Now I have strongly considered buying two rear tires and putting them on the front to achieve that end. I confess that the reason I’m not going down that road is pure vanity. I like the looks of the aftermarket wheels.
But the increased rotational inertia of the aftermarket wheels does concern me greatly and I have had a difficult time resolving the issue personally. This is in reference to the unsprung mass you were talking about. Rotational inertia is a function of the mass of the wheel & tire and the radius squared. The smart wheel is quite light – about 10 pounds. There simply isn’t an aftermarket wheel that comes close to it in weight. I went so far as to research carbon fiber wheels and found that the current crop of aftermarket carbon fiber wheels are about $20K a set and are marketed to the Lamborghini crowd. Even if I was determined to get a set for the smart, they aren’t made in a size that comes close.
By making a few educated guesses about something called radius of gyration tells me that the new wheels/tires that I am considering will increase the rotational inertia of the wheels by 60%. That’s a huge number which will require, surprise! 60% more torque just to get the wheels spinning. For a car that is so under-powered this simply doesn’t make any sense at all.
But I like the look of the aftermarket wheels, and I want to affect change in the handling by making the tire sizes “square.” It sorta puts the engineer in me at conflict with my vanity.
What will I do? Probably get the 16” Schmidts. There is no way I’d go to the 17” because as stated above, the inertia goes with the square of the radius. My vanity can push the engineer in me only so far.
In the end, if I can summarize your comment as “You shouldn’t change anything because the smart engineers know better than you do,” my reply would be simply, “Yes, they do. But they are optimizing a different variable than I am. They are designing a car for the masses. I see my smart as a blank canvas. The variable I am optimizing is my personal sense of what I think the car should be. There is no way a team of engineers 8 time zones away can do that.”
A fellow satellite engineer! I work in I&T at a satellite firm on the east coast, though all LEO S&T missions thus far. Previously, I had worked in flight software for them. My ACS experience is more limited, but I've got a good grounding in the principles. My background is embedded systems, instrumentation, satellites, etc. On the car side of things, I've done no small amount of chassis tuning and my current sports car is my Mazda Miata 10th Anniversary Edition. So, now that we've dispensed with the shop talk and resumes, I'd like to address a few of your points.
I'm not, for a moment, suggesting that nothing should ever be changed on the smart -- just that the changes be carefully reasoned and researched. It sounds like you've done that.
With regards to the understeer, is that solely a function of tire widths or is it also induced by the ESC firmware? And, given the fortwo's high cg relative to its track width, if you eliminate the understeer by increasing grip at the front end, does the higher lateral G-force when cornering significantly increase your chances of a rollover?
Disabling the ESC is not something that I'm willing to do. It's too valuable in compromised traction situations and in emergency situations -- especially in a high cg car with a narrow, short wheelbase (think Jeep CJ-5, for example).
I think that you're focusing too much on the rotational inertia and not nearly enough on the unsprung mass (we're not dealing with reaction wheels in the vacuum of space). It's the mass that could completely overwhelm the suspension's ability to keep the tires in contact with the road when the pavement is not glass-smooth. Everything from the shock damping rates to the spring rates come into play in controlling the mass as the wheel and tires accelerate up and down in response to dips, bumps, potholes, braking ripples, etc.
While inertial forces are directly proportional to the square of the radius, don't forget that the tires have mass also and I'm assuming that you'll be choosing your aspect ratio to get a specific diameter whether you go with 16" or 17" wheels -- so the radius at the tread will be, more or less, the same. Wider tires usually means greater mass, so choose them carefully.
Also, don't forget that if you do induce oversteer, the rearward weight bias/high polar moment of inertia, and the short wheelbase will cause the rear end to try to come around VERY quickly. It's not like oversteer in a front-engine, rear drive car where simply lifting off of the gas often brings the back end back in.
All food for thought. I hope that whatever choice you make is one that you're happy with and that, overall, is no less safe than stock.
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