First Time Buying After Market Wheels

[brandonc]

Hello All,
I have a 16 ecoboost. No brembo brakes. I plan on getting a staggered set of KMC district wheels in silver Part numbers KM68520512435 and KM68528512435. the Fronts being 8.5s and the rears 10.5. They are both offset 35 mm. i wanted to go 10.5 all the way around for the deep concave look all around but the supplier im ordering them from was pretty much no help saying "we cant advise that". Ive seen Gt owners do it that is why i asked. I will live with 8.5's up front if needed but i plan on running Mickey Thompson Street Comps all around, 305/35r20 in the back and 255/35r20 in the front. i am not lowered. Will this setup work? the supplier said yes, but he couldn't even tell me if i could run 10.5s all around so i dont find much confidence in trusting him. I know this was lengthy and probably a no brainer, but i just wanna be sure, as i cannot return the wheels when ordered.

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[MELLOWYELLOW06]

It should work however you may be stretching the hell out of your front tire.

At worst, depending on the design of the wheel, you may need a few mm spacer.

 

[Paul@PKAUTODESIGN]

Hello All,
I have a 16 ecoboost. No brembo brakes. I plan on getting a staggered set of KMC district wheels in silver Part numbers KM68520512435 and KM68528512435. the Fronts being 8.5s and the rears 10.5. They are both offset 35 mm. i wanted to go 10.5 all the way around for the deep concave look all around but the supplier im ordering them from was pretty much no help saying "we cant advise that". Ive seen Gt owners do it that is why i asked. I will live with 8.5's up front if needed but i plan on running Mickey Thompson Street Comps all around, 305/35r20 in the back and 255/35r20 in the front. i am not lowered. Will this setup work? the supplier said yes, but he couldn't even tell me if i could run 10.5s all around so i dont find much confidence in trusting him. I know this was lengthy and probably a no brainer, but i just wanna be sure, as i cannot return the wheels when ordered.

Hey brotha so I carry the KMC wheels and I can supply them for you if you desire. So if you do the 20x8.5 & 20x10.5 then you can easily do the 255/305 M/T tires without a issue.

Now if you want to rock the 20x10.5 all around you can do it, its unique just needs a bit of a spacer upfront but we can make it work.

Let me know if I can help.

 

[PistolPete]

Hey brotha so I carry the KMC wheels and I can supply them for you if you desire. So if you do the 20x8.5 & 20x10.5 then you can easily do the 255/305 M/T tires without a issue.

Now if you want to rock the 20x10.5 all around you can do it, its unique just needs a bit of a spacer upfront but we can make it work.

Let me know if I can help.

Hey Paul, I thought you said I couldn't run 305s in the rear? I'm confused...

 

[Ewheels]

Track guys run 305 square all day long. I do not know for certain if ecoboost cars have the same fender space that GT cars do so don't quote me on that. IF ecoboost cars and GT cars do in fact have the same wheel well space, then you can fit 305 tires on an 11 wheel at ever corner. 19x11 ET 50 wheel with 305/30/19 tire and a 25mm spacer on the front will fit.

It seems that you are going for looks over functionality, which is fine, but just know that a 305 tire all around is extremely unnecessary unless you're doing some track time.
Also, any time someone tells me they want to put 20s on their car, I cry a little bit on the inside...but that's just me.

 

[Paul@PKAUTODESIGN]

Hey Paul, I thought you said I couldn't run 305s in the rear? I'm confused...

Correct sir back here is the thing, It is based on the offset

20x10.5 ET 35 with a 305 is aggressive and needs camber to work properly.

Typically 20x10.5 ET 45 with a 305/35/20 is what the aim is for to work best.

 

[Cory S]

You will also lose traction going to a tire that’s wider than 255 in rear. You will gain better handling though.

 

[GuardEcoBeast]

You will also lose traction going to a tire that’s wider than 255 in rear. You will gain better handling though.

How do you lose traction going to a wider tire?????

 

[Cory S]

How do you lose traction going to a wider tire?????

Weight distribution. Mustangs are fairly light in the rear. Going to a super wide tire spreads the weight out, causing less down force for the tread to dig into the asphalt. Theres a fine line there. I have found it many times. This doesn’t apply to R compound drag radial/slick tires on a drag strip surface however.

 

[NightmareMoon]

Weight distribution. Mustangs are fairly light in the rear. Going to a super wide tire spreads the weight out, causing less down force for the tread to dig into the asphalt. Theres a fine line there. I have found it many times. This doesn’t apply to R compound drag radial/slick tires on a drag strip surface however.

Thats not correct. More contact area (less weight per unit area) would improve grip a small amount. Tire grip is not quite linear with weight, with the advantage to the lighter car, otherwise we’d all be on 8” skinny tires.

As for handling, a front 255 8.5” tire with a 305 tire on a 10.5” rear is going to understeer noticeably. Sever staggers don’t improve handling in our cars, quite the opposite. Square would handle better. It doesn’t sound like its a major concern for the OP or he wouldn’t be looking at MT tires in the first place.

 

[Ewheels]

You will also lose traction going to a tire that’s wider than 255 in rear. You will gain better handling though.

F**king WHAT?!?! Yeah, this is why ever drag racer uses skinnies on the drive wheels and EVERY sports car manufacturer tries to get the skinniest tires they can comfortably fit. Look at the top S550 times (drag or track) and NONE of them will be using 255 in the rear.

.....no one here knows everything. Ask questions all you want, that's how people learn. But it's so irritating to see people spew nonsense as if it's a hard truth.

 

[Cory S]

F**king WHAT?!?! Yeah, this is why ever drag racer uses skinnies on the drive wheels and EVERY sports car manufacturer tries to get the skinniest tires they can comfortably fit. Look at the top S550 times (drag or track) and NONE of them will be using 255 in the rear.

.....no one here knows everything. Ask questions all you want, that's how people learn. But it's so irritating to see people spew nonsense as if it's a hard truth.

There are limits on the street with weight and friction. I’ve played around with many tire widths over the past 25 years. Many times I have tested 275-285mm tires grip better in a straight line than 295-325’s. We are talking street tires with street tread.

A wider tire does not always give more traction because of the width of it. Surface area does not impact friction. Friction is calculated by force of the contact (in this case weight) and the friction coefficient. They use a different compound for the wider tires that require the size, so the sidewalls can support the vehicle. Having a wider tire will increase chance of hydroplaning. You have less pressure with wider tires and it is more difficult to break the surface tension of the water. Here's a link detailing the tire and it's friction.http://www.physlink.com/Education/AskExperts/ae200.cfm

 

[abbeynormal]

"... the coefficient of friction decreases as the vertical load, Fz, increases": https://en.wikipedia.org/wiki/Tire_load_sensitivity

So, (all things being equal) a wider tire will have more overall traction.

 

[Cory S]

The ultimate grip of a tire (maximum lateral force) is not generated solely by simple friction. The tire material interacts with the surface microscopically and macroscopically, physically interlocking with features of the track/road surface.

This interaction produces a non-linear relationship between vertical/normal force and ultimate grip, whereby the ultimate grip increases less and less for a given increase in normal force.

This means that a tire of a given contact patch size with 1000N of normal force might produce 1000N of ultimate lateral force, but the same tire with 1200N of normal force might only produce 1100N of ultimate lateral force. Increasing the size of the contact patch reduces unit loading and brings the force back to a more favorable range in the normal-to-lateral-force relationship.

If you don't understand how/why tires interact with the surface this way, imagine the ultimate extreme: an off-road tire driving on a lumpy solid surface. The tire's tread blocks could interlock with the lumps in the surface such that the tread blocks would have to be sheared off or completely deflected in order for the tire to slip sideways. The same thing happens on tarmac; the features of the tire and road surface are just smaller. Throw your basic friction out the window. This is a much more complex interaction and hard to model.
Since the Mustang has been notoriously light in the back end, spreading the downward weight out can cause the tread pattern not to grip the texture of the asphalt as well as a narrower tire given the down force per psi and the road surface is the same. So with the S550, I’m willing to bet a 255-285 rear would be ideal over a 305-325. Strictly for asphalt straight line grip. Different tire brands and models can also change the results obviously.

 

[mavisky]

The ultimate grip of a tire (maximum lateral force) is not generated solely by simple friction. The tire material interacts with the surface microscopically and macroscopically, physically interlocking with features of the track/road surface.

This interaction produces a non-linear relationship between vertical/normal force and ultimate grip, whereby the ultimate grip increases less and less for a given increase in normal force.

This means that a tire of a given contact patch size with 1000N of normal force might produce 1000N of ultimate lateral force, but the same tire with 1200N of normal force might only produce 1100N of ultimate lateral force. Increasing the size of the contact patch reduces unit loading and brings the force back to a more favorable range in the normal-to-lateral-force relationship.

If you don't understand how/why tires interact with the surface this way, imagine the ultimate extreme: an off-road tire driving on a lumpy solid surface. The tire's tread blocks could interlock with the lumps in the surface such that the tread blocks would have to be sheared off or completely deflected in order for the tire to slip sideways. The same thing happens on tarmac; the features of the tire and road surface are just smaller. Throw your basic friction out the window. This is a much more complex interaction and hard to model.
Since the Mustang has been notoriously light in the back end, spreading the downward weight out can cause the tread pattern not to grip the texture of the asphalt as well as a narrower tire given the down force per psi and the road surface is the same. So with the S550, I’m willing to bet a 255-285 rear would be ideal over a 305-325. Strictly for asphalt straight line grip. Different tire brands and models can also change the results obviously.

While not technically incorrect, your assumptions here are broken by the actual math.You're assuming that the difference between a 255 and a 325 is going to throw the weight off so drastically that despite it's extra tread pattern the tire will actually grip less. In reality there is plenty of weight to go around and the wider tire is able to utilize the larger surface are just fine.

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