315/35/20’s ??

[CrystalRedTintcoat]

This has been discussed on this forum many times in the past. Check around for a post from me linking to Al Oppenheiser’s video on YouTube about Chevy’s desire to stretch the rear tire over those 11” rims to reduce understeer. Now if you’re going to re-engineer your car in other ways to solve the understeer problem then you could increase your tire size. But do also check at the biggest tire you can put on the your rims. There are limits.


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[1 Long onE]

I put 315/35/20 and the car is definitely not as sharp as before but it is also a taller tire and the ride improved as well.

[acammer]

Originally Posted by CrystalRedTintcoat

This has been discussed on this forum many times in the past. Check around for a post from me linking to Al Oppenheiser’s video on YouTube about Chevy’s desire to stretch the rear tire over those 11” rims to reduce understeer. Now if you’re going to re-engineer your car in other ways to solve the understeer problem then you could increase your tire size. But do also check at the biggest tire you can put on the your rims. There are limits.


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I've been searching for the video you referenced without success. I sure would be interested in watching that, for my own edification. If you could link it, that would be awesome.

[gajagfan]

Originally Posted by acammer

I've been searching for the video you referenced without success. I sure would be interested in watching that, for my own edification. If you could link it, that would be awesome.

This should be what yo are looking for....

[Scotty D]

Very cool
Thanks

[acammer]

Thanks. Al doesn't do anything to explain the stretch, just that they went with the smaller tire to mitigate under-steer by not shifting too much traction to the rear of the car. If there is a clip where the speak specifically as to why the purposely went for the stretch, that would be interesting.

[gajagfan]

Originally Posted by acammer

Thanks. Al doesn't do anything to explain the stretch, just that they went with the smaller tire to mitigate under-steer by not shifting too much traction to the rear of the car. If there is a clip where the speak specifically as to why the purposely went for the stretch, that would be interesting.

Speaks directly to it at about the 35 second mark when explaining that the smaller tire was stretched over the wider wheel to reduce the contact patch and help with under steer.

[acammer]

Originally Posted by gajagfan

Speaks directly to it at about the 35 second mark when explaining that the smaller tire was stretched over the wider wheel to reduce the contact patch and help with under steer.

Yea, he states, from what I understand, that they didn't want as big of a contact patch in the back (as compared to a ZL1), so that the car would be less likely to under-steer. That makes perfect sense, I understand balancing the chassis with tire sizes.

But, it doesn't explain at all why they would stretch it over an 11" wheel, instead of using 20x10's all around. If he had said something like "we choose to stretch the rear tires to change the responsiveness of the tire" then I would buy that, as stretching certainly has an impact on tire feel and response. But I don't hear him say anything like that, just that they wanted to have the right sized contact patch to help achieve the chassis balance they were looking for.

I think they had already ordered/produced x amount of 20x10 and 20x11's when they made the decision to go to a square stance on the 1LE, so they just stretch the rears on to get the car "square" and have balance they want. Nothing wrong with that. If there is an actual reason they've given for the stretch (not related to getting the contact patch they want while still using the 20x11's) I've yet to hear it.

[gajagfan]

Originally Posted by acammer

Yea, he states, from what I understand, that they didn't want as big of a contact patch in the back (as compared to a ZL1), so that the car would be less likely to under-steer. That makes perfect sense, I understand balancing the chassis with tire sizes.

But, it doesn't explain at all why they would stretch it over an 11" wheel, instead of using 20x10's all around. If he had said something like "we choose to stretch the rear tires to change the responsiveness of the tire" then I would buy that, as stretching certainly has an impact on tire feel and response. But I don't hear him say anything like that, just that they wanted to have the right sized contact patch to help achieve the chassis balance they were looking for.

I think they had already ordered/produced x amount of 20x10 and 20x11's when they made the decision to go to a square stance on the 1LE, so they just stretch the rears on to get the car "square" and have balance they want. Nothing wrong with that. If there is an actual reason they've given for the stretch (not related to getting the contact patch they want while still using the 20x11's) I've yet to hear it.

The 285 tire on a 10 inch rim has a greater contact patch then it does on an 11 inch rim, thus reducing the traction of the rear of the car. He states that. That reduced traction or push reduces the under steer. I believe that to be the reason they went with the set up they did. You do get he is saying that the car handles better with the 11 inch wheels on the back then it would have with the 10 inch wheels on the back using the 285 tire right? To follow your logic of they already had extra 11 inch wide wheels, then I guess you assume they just threw away the wider ZL1 tires they had for those wider rims?

[acammer]

Originally Posted by gajagfan

The 285 tire on a 10 inch rim has a greater contact patch then it does on an 11 inch rim, thus reducing the traction of the rear of the car.

I don't believe that is correct. It's my understanding that stretching a tire on a rim increases the contact patch. A 285 on and 20x10 would have a smaller contact patch than if it was mounted on a 20x11.

He states that. That reduced traction or push reduces the under steer. I believe that to be the reason they went with the set up they did. You do get he is saying that the car handles better with the 11 inch wheels on the back then it would have with the 10 inch wheels on the back using the 285 tire right? To follow your logic of they already had extra 11 inch wide wheels, then I guess you assume they just threw away the wider ZL1 tires they had for those wider rims?

I would think tooling and production of wheels is far more specific in terms of quantity and thus much more finite, than for something like a tire, that is used on several different vehicles, and is likely to be replaced many times over the life of the vehicle. I would agree that my supposition that they had a surplus of 20x11 wheels (or a shortage of 20x10s) is just that - a speculation. It has no basis beyond just thinking out loud on reasons the 20x11 may have been elected over a 20x10 all around.

I hear him saying that they wanted a smaller contact patch in the rear than a ZL1, so they went with the 285 out back. That makes perfect sense. I guess you could suppose they went the extra inch on the rim to get just a little more contact patch than up front for the same given size of tire. To me, and I've listened to that interview over and over, as well as read everything I can find on the subject, it sounds like they wanted to reduce the contact patch to balance the chassis, so they went down in tire size. The stretch was not the goal of the size change, but rather a (potentially desirable?) by-product. If the goal was to reduce understeer, then stretching the 285 on a 20x11 would produce more understeer (bigger contact patch) than if a 20x10 was used. Maybe they thought that would swing the bias too far...

Of course unless you can actually ask the engineers, or find it in print some where, it's speculation. You could also argue that there are other 5th gen examples of stretching the rears on a square set-up - the Z/28 does this. Certainly I'm not an expert on why GM would make the choices they would.

I can agree that there are reasons you would want to stretch a tire to change it's performance characteristics. I'm not convinced that there weren't other corporate pressures that may have influenced the decision to use the 20x11 instead of squaring the wheels, or using a 20x10.5.

It's just interesting to talk about and discuss.

[gajagfan]

Originally Posted by acammer

I don't believe that is correct. It's my understanding that stretching a tire on a rim increases the contact patch. A 285 on and 20x10 would have a smaller contact patch than if it was mounted on a 20x11.



I would think tooling and production of wheels is far more specific in terms of quantity and thus much more finite, than for something like a tire, that is used on several different vehicles, and is likely to be replaced many times over the life of the vehicle. I would agree that my supposition that they had a surplus of 20x11 wheels (or a shortage of 20x10s) is just that - a speculation. It has no basis beyond just thinking out loud on reasons the 20x11 may have been elected over a 20x10 all around.

I hear him saying that they wanted a smaller contact patch in the rear than a ZL1, so they went with the 285 out back. That makes perfect sense. I guess you could suppose they went the extra inch on the rim to get just a little more contact patch than up front for the same given size of tire. To me, and I've listened to that interview over and over, as well as read everything I can find on the subject, it sounds like they wanted to reduce the contact patch to balance the chassis, so they went down in tire size. The stretch was not the goal of the size change, but rather a (potentially desirable?) by-product. If the goal was to reduce understeer, then stretching the 285 on a 20x11 would produce more understeer (bigger contact patch) than if a 20x10 was used. Maybe they thought that would swing the bias too far...

Of course unless you can actually ask the engineers, or find it in print some where, it's speculation. You could also argue that there are other 5th gen examples of stretching the rears on a square set-up - the Z/28 does this. Certainly I'm not an expert on why GM would make the choices they would.

I can agree that there are reasons you would want to stretch a tire to change it's performance characteristics. I'm not convinced that there weren't other corporate pressures that may have influenced the decision to use the 20x11 instead of squaring the wheels, or using a 20x10.5.

It's just interesting to talk about and discuss.

No doubt interesting to discuss, at least for you and I. I disagree with your thoughts on contact patch, as I have stated before, and am trying to determine a way to define that. In my opinion the wider rim prevents the tire from having any bulge to it, and the tire on the edge of the contact patch actually is raised of the ground in an effort to get back to the rim (think not quite enough tire width). That being said, I think that contact patch is only half the battle of traction.

The ability of the sidewall to flex also would seem to play a key role in the tires ability to maintain traction. I would think that stiffness at some point could lead to a reduction in traction.

In 2013 GM knew they planned on making ZL1's for 3 more years in the current configuration. I feel that would have been plenty of time to work through the inventory and adjust purchasing in the future of the 11 inch wide rim. I do believe that the GM engineers had a box of current parts to work with to come up with the best handling package they could, and the car was found to work better withe the wider rim and smaller tire in the rear. This does not explain the development of the front splitter and the unique rear spoiler.

We do agree that without asking actual guys involved with the project we may never know.

[acammer]

Originally Posted by gajagfan

No doubt interesting to discuss, at least for you and I. I disagree with your thoughts on contact patch, as I have stated before, and am trying to determine a way to define that. In my opinion the wider rim prevents the tire from having any bulge to it, and the tire on the edge of the contact patch actually is raised of the ground in an effort to get back to the rim (think not quite enough tire width). That being said, I think that contact patch is only half the battle of traction.

The ability of the sidewall to flex also would seem to play a key role in the tires ability to maintain traction. I would think that stiffness at some point could lead to a reduction in traction.

In 2013 GM knew they planned on making ZL1's for 3 more years in the current configuration. I feel that would have been plenty of time to work through the inventory and adjust purchasing in the future of the 11 inch wide rim. I do believe that the GM engineers had a box of current parts to work with to come up with the best handling package they could, and the car was found to work better withe the wider rim and smaller tire in the rear. This does not explain the development of the front splitter and the unique rear spoiler.

We do agree that without asking actual guys involved with the project we may never know.

I agree on production of the rims, they could have likely changed it if they wanted to.

From what little reading I have done, when you stretch a tire, you increase the sidewall stiffness, and improve the responsiveness and break-away characteristics of that tire. This site has some interesting reading. Up to a point it supposedly adds contact patch and grip, and then would begin to take away grip when taken too far.

You can go the other way too - like mounting a 315 on a 9" rim. You'll get a "ballooning" of the tire, and it won't effectively plant all that rubber on the ground. And it'll feel terrible, because the sidewall will be flexing all over.

[Matty Ice]

I

[mma]

I have 315/35/20 DSW's, but I don't track it or anything. Definitely feels a bit different from a squared setup using 285s