Smart Car of America Forum banner

1 - 20 of 35 Posts

·
Registered
Joined
·
24 Posts
Discussion Starter #1 (Edited)
If I replace the front tires with... UPDATED

185-60-15’s, will this reduce the wandering / increase the wandering at highway speed?

I’ve looked at a zillion posts, but seems no one clearly says if this was the fix. I’m ready for tires and would gladly change front wheels and upsize to reduce the wandering.

TIA

Marty in Kerrville

Installed Yoko 185-55-15’s. Haven’t been out on the highway yet, but the ride quality improved a ZILLION %.
More to come...
 

·
Registered
Joined
·
2,947 Posts
Almost all posts I've read have a positive connotation to them when going to a larger/wider tire up front.

You should be very careful in trying to keep front and rear around the same diameter for the stability/traction control, so probably also change the rears also...
 

·
Registered
Joined
·
922 Posts
Much of the wandering can be reduced by getting a quality alignment. The factory specs call for 1/4" toe in at each front corner. This is way too much and contributes to excessive understeer (which is what MB likely intended). What this also does is allow the tire with momentarily great road pressure (such as by hitting a bump) to slip less than the other tire, making the front end wander.

I've gone to 1/32" toe in at each front corner, which should result in zero dynamic toe at speed. This has mostly eliminated wandering and I am still on the OEM tires. It now only occurs when there are grooves in the road that catch the outside of the front tires and the inside of the rear tires. This problem might be mitigated by using a spacer up front to somewhat equalize the difference in track width.

Undoubtedly larger tires will help. They are heavier and more flexy, which absorbs some of the energy that would make the car wander. It's like how sometimes a bicycle with really skinny tires will develop a shimmy but put bigger and heavier tires on it and the shimmy goes away. Same general principle in that the larger bicycle tires absorb the resonant energy that contributes to generation of the shimmy.
 

·
Registered
Joined
·
922 Posts
I don't know where you got that nonsense. Factory toe-in specification is +0 deg. 36 min., +/- 10 min.
Exactly!

36 minutes is 0.6 degrees.

The formula for determining the toe, in inches, is:

Toe-in (in inches) = (tire diameter / 2) * (sin(toe-in angle)) * 2

tire diameter for a 155/60-15 is 22.3 inches
sin(0.6) = 0.5646424

Do the calculation and it results in a toe-in, per wheel, of 0.234 inches. Close enough to 0.25 inches to call it a quarter-inch of toe-in per wheel.

Aha!!! Found an online calculator for you:

https://robrobinette.com/ConvertToeDegreesToInches.htm

Set the toe-in to 1/32" per wheel and tracking/wandering problems mostly go away. This setting results in nearly zero dynamic toe at speed.
 

·
Registered
Joined
·
1,635 Posts
Exactly!

36 minutes is 0.6 degrees.

The formula for determining the toe, in inches, is:

Toe-in (in inches) = (tire diameter / 2) * (sin(toe-in angle)) * 2

tire diameter for a 155/60-15 is 22.3 inches
sin(0.6) = 0.5646424

Do the calculation and it results in a toe-in, per wheel, of 0.234 inches. Close enough to 0.25 inches to call it a quarter-inch of toe-in per wheel.

Aha!!! Found an online calculator for you:

https://robrobinette.com/ConvertToeDegreesToInches.htm

Set the toe-in to 1/32" per wheel and tracking/wandering problems mostly go away. This setting results in nearly zero dynamic toe at speed.
I think you need to revisit your calculations. Maybe the battery in your abacus is dead, I don't know. Saying the factory setting for toe-in is approximately 1/2 inch is just not plausible. If it were a half inch the front tires would be making skid marks driving straight down the road.

Toe-in is set relative to wheel diameter, not tire diameter, when a measurement is specified in inches. Think about it.
 

·
Registered
Joined
·
922 Posts
I think you need to revisit your calculations. Maybe the battery in your abacus is dead, I don't know. Saying the factory setting for toe-in is approximately 1/2 inch is just not plausible. If it were a half inch the front tires would be making skid marks driving straight down the road.

Toe-in is set relative to wheel diameter, not tire diameter, when a measurement is specified in inches. Think about it.
They are not "my" calculation, it is "THE" calculation. Math does not lie and I can find you other sites that will give you the same formula. They can't all be wrong and you be right.

Have you seen those stanced cars that have like five or six degrees of negative camber. Some of those run as much as an inch total toe in. They don't leave skid marks, because the tread is pliable enough.

Think of it this way. Let's say you take that same 155/60-15 tire, 22.3 inches in diameter, and drive a portion of it on a raised ridge that is one-half inch tall.

The portion sitting on the raised ridge is effectively rolling as if the tire RADIUS were one-half inch smaller. The rolling circumference of a tire is calculated by:

circumference = 2 * pi * r

This gives two circumferences of 70 inches and 68.5 inches. So in one revolution, the 70" length of tread can only travel 68.5 inches if it is riding on that ridge. Yet it doesn't leave skid marks there.
 

·
Registered
Joined
·
1,635 Posts
Okay, I give up. The internet is always right.

There is a reason you reset your toe-in to 1/32 in. per side. It's not because the factory does not know how to measure toe-in on their own cars.
 

·
Registered
Joined
·
922 Posts
The main takeaway from this is that it's known that Smart cars are designed with severe understeer built in from the factory, I guess due to problems during some "elk test". They use a significant tire size stagger, a significantly narrower front track, and then they specify an extreme amount of toe-in up front. This tends to increase straight line stability and blunt corner entry turn-in, while costing tire life. Fixing this problem goes a long way towards making a Fortwo a much more neutral handling vehicle and reduces wandering as an extra benefit.
 

·
Registered
Joined
·
922 Posts
Okay, I give up. The internet is always right.

There is a reason you reset your toe-in to 1/32 in. per side. It's not because the factory does not know how to measure toe-in on their own cars.
Don't give up. Show me how you calculate that 36 minutes is not one-quarter inch toe-in with a 22.3 inch diameter wheel. I'm always up for learning if I'm wrong.

And the reason I asked to have my toe set to 1/32" per side is because I understand how suspensions and alignments work, and have nearly 30 years of on-track experience using correct and incorrect setups. I know how I want a vehicle to handle, and after all this time and many mistakes, know how to get a vehicle there using what is available to me.

I've been proven wrong more than once before on this forum and I've only been here a couple of months. I'd gladly be proven wrong again in this case because that is how learning occurs. Only I'm pretty sure I'm not wrong but I'm **always** willing to listen. In this case, the argument is purely mathematical, and I think the math on how to do these calculations is well established.
 

·
Registered
Joined
·
24 Posts
Discussion Starter #12
OP here.

So......... if I go with 185-60-15’s all around (and do not change the alignment, whatever it may be), will this help or hinder the wandering issue?

I simply don’t want to change the setup if it makes it worse, or creates other issues.

Marty in Kerrville
 

·
Registered
Joined
·
1,635 Posts
I finally got time this morning to look at a couple of your links. I see what you are talking about but I will say that in the last 50 years in the automotive service industry I have never seen toe-in measured or specified at the outer diameter of the tire. That seems to be very imprecise. It is always specified as a linear measurement taken at the wheel rim or a angular machine measurement relative to the vehicle thrust center line. That is why the conversion chart you posted shows wheel diameter instead of tire diameter.

I am not going to re-do your calculations but if they are correct, which I have no reason to doubt, then the German engineer who came up with the smart toe-in specification was suffering from a rectal-cranial inversion. Or maybe it was Oktoberfest. Either way, it is implausible. Perhaps that is why a alignment tech at a local Mercedes dealer told me every smart he ever had on the alignment machine had the toe-in set incorrectly.
 

·
Registered
Joined
·
2,947 Posts
OP- Little doubt that you won't be happy with the results.

The alignment machines attach to the "metal" wheel, and measurements on the rack are based on that, but the calculations make sense dependent on the diameter of the tire, aka "wheel". (Shrugs in confusion...)
 

·
Registered
Joined
·
922 Posts
OP here.



So......... if I go with 185-60-15’s all around (and do not change the alignment, whatever it may be), will this help or hinder the wandering issue?



I simply don’t want to change the setup if it makes it worse, or creates other issues.



Marty in Kerrville


Marty, seems there are two things here. One is that changing the tire size without changing the alignment, which is what most people have done, is said to help the handing and wandering problem, especially in crosswinds. This part I haven't done (yet) but it makes some sense in that a heavier tire will dampen more of the forces that cause the car to wander. There's also been a lot of reports from those who have done so that say it works great. I'd believe that so I say go for it!

A quality alignment, at about $80-100 since you're only adjusting front toe, seems well worth it, but I seem to have been the only one so far to do that, so there's no one to back up my experiences.


Sent from my iPad using Tapatalk
 

·
Registered
Joined
·
922 Posts
I finally got time this morning to look at a couple of your links. I see what you are talking about but I will say that in the last 50 years in the automotive service industry I have never seen toe-in measured or specified at the outer diameter of the tire. That seems to be very imprecise. It is always specified as a linear measurement taken at the wheel rim or a angular machine measurement relative to the vehicle thrust center line. That is why the conversion chart you posted shows wheel diameter instead of tire diameter.



I am not going to re-do your calculations but if they are correct, which I have no reason to doubt, then the German engineer who came up with the smart toe-in specification was suffering from a rectal-cranial inversion. Or maybe it was Oktoberfest. Either way, it is implausible. Perhaps that is why a alignment tech at a local Mercedes dealer told me every smart he ever had on the alignment machine had the toe-in set incorrectly.


Rusted, I'm looking at the alignment printout and the initial toe settings were 0.19 and 0.21 inches in, which were set to 0.05 inches in at each front, so I guess 1/20th of an inch rather than 1/32nd of an inch. That matches what the initial settings for the rear were.

Have you seen another vehicles come through with a 36 minute toe-in specification? These must be known tire-eating cars, so I'd guess maybe something like an NSX or some of the earlier Porsches? Or possibly some vehicle with really soft suspension bushings so that the dynamic toe is quite different than static toe?

And totally agree with you on the imprecision of the measurement. I'd much rather use degrees and meters but that isn't the case yet.

EDIT: Rereading your post again, I guess based on your last sentence, you'd recommend every Smart buyer to get an alignment as well?


Sent from my iPad using Tapatalk
 

·
Registered
Joined
·
922 Posts
It's very hard to find a good alignment around here. Seems quite rampant....
When a car rolls down the manufacturing line, it gets anywhere from 1-3 minutes at the alignment station. That's not a lot of time, which is why most manufacturers have such a large tolerance in the alignment specification. Many alignment shops will just choose to hit that wide range and call it good, and in most cases it will be good. There's a lot of compliance in bushings and tires, and just the unevenness of most road surfaces make a sub-optimal alignment not that noticeable. However, you'll see things like uneven tire wear or poor tire life.

Where it does matter is when cars have minimal compliance in their suspension components, including tires, and when running on smooth surfaces. That would apply to sports cars on race tracks, which is why it's also commonly recommended to go to a place that specializes in aligning cars for racing or track use, or deals with higher end cars that typically have tighter suspensions. They'll hit the side-to-side numbers with much less difference, and will be willing to use specifications outside of the manufacturer's suggestions.

Where we are, I'm fortunate to have access to a number of different shops of this kind. They're all about twice as expensive as the cookie cutter stores but also take care not to ding up rims when attaching the equipment, and will let you actually sit in the car when the final settings are made (which in the case of a small vehicle like the Smart, makes a lot of difference).
 

·
Registered
Joined
·
861 Posts
I didn't follow a word of that soooo, what is the final word as to what the specs should be?

do the specs change when tire sizes change or is it a given regardless what tires you run?
I am going to 175/55 front and 195/55 rear if that matters


thanks guys
 

·
Super Moderator
Joined
·
11,309 Posts
alignment shouldn't change with tire size. If the car is aligned correctly to spec, it should be good unless there is physical damage or the tires wear unevenly.
 
1 - 20 of 35 Posts
Top