[agl]

Hi all,

I currently have a stock 2021 V6 1LT 1LE, which comes with 8.5" et 25 wheels with 245/40R20 tires in the front and 9.5" et 39 wheels with 275/35R20 tires in the rear.

I'm looking to get a second set of wheels and tires for track days and time trial events in my area, and due to SCCA classing rules, the max tire width I could go with is a 285 to stay in my Sport 3 class. I'm looking for a square setup, and have my eyes on a set of 20"x10" et 25 wheels, and 285/35R20 tires on all four corners. I was thinking the 35 sidewall profile would be ideal, due to having the closest overall wheel diameter to the stock wheel/tire setup.

However, I have a few concerns with fitment/clearances/rubbing. After trying to do some research on the forums, I see a lot of people running 285/30R20s vs the 35s, but have seen multiple users with the 35s as well. And while I've seen multiple posts showing their 20"x10" et 25 wheels, specifically in the front, it seems like et 23 is the accepted value.

I want to ensure this setup will work and fit without any rubbing of any sort, especially during sustained track time. Do you think this setup will work? Or do you have any other recommendations? All help is appreciated!!

Thanks guys!!

[foshjowler]

I think that should be fine. They will be slightly taller than the stock 275s but shouldn't be enough to cause an issue.

As far as width/offset, you'll be good. I run a 19x10.5 et 22 all around with 305/30-19 and don't have any issues with rubbing. It's possible you'll run into issues with getting the max camber, but that's all I would foresee, if at all.

[DaveC113]

I can't make any recs based on experience but FWIW, I recently ordered smaller diameter wheels to save weight vs the 20s. I think you could run 285/30/19 or 285/35/19.

I run 265/40/19 square for winter w/ no issues. Stability controls work fine.

[Msquared]

A 285/35/20 is 27.9" in diameter. That's taller than the 1LE tires, front or rear. There is no advantage to running tall tires on a road course or autocross. It will raise your car's CG and instant centers, which means your car will ride a good bit higher and transfer more load during any form of acceleration. That's usually good for a drag launch but it's bad for cornering and braking. Plus they will be heavy. I would strong recommend going down to 285/30/19 front and rear, which is 25.7" in diameter. Your car will literally sit more than an inch lower on those tires which is meaningful in reducing weight transfer, plus they will be a lot of pounds lighter.

[lt4camaro]

Originally Posted by Msquared

A 285/35/20 is 27.9" in diameter. That's taller than the 1LE tires, front or rear. There is no advantage to running tall tires on a road course or autocross. It will raise your car's CG and instant centers, which means your car will ride a good bit higher and transfer more load during any form of acceleration. That's usually good for a drag launch but it's bad for cornering and braking. Plus they will be heavy. I would strong recommend going down to 285/30/19 front and rear, which is 25.7" in diameter. Your car will literally sit more than an inch lower on those tires which is meaningful in reducing weight transfer, plus they will be a lot of pounds lighter.

Can I pick your brain please, I am into 1/4 passes and of course anything that minutely helps launch. So your basic statement is a taller tire versus a shorter tire in the rear and everything else equal is that the car should weight transfer better to the rear on a launch? please elaborate if you would. Also would removeing the rear sway bar help the car dig or weight transfer better, I already have the front swaybar off. I always go after the crumbs to do anything for a little more weight transfer from a dig just get a little more bite.

[Msquared]

Originally Posted by lt4camaro

Can I pick your brain please, I am into 1/4 passes and of course anything that minutely helps launch. So your basic statement is a taller tire versus a shorter tire in the rear and everything else equal is that the car should weight transfer better to the rear on a launch? please elaborate if you would. Also would removeing the rear sway bar help the car dig or weight transfer better, I already have the front swaybar off. I always go after the crumbs to do anything for a little more weight transfer from a dig just get a little more bite.

So yeah, a taller rear tire should allow a little more CG height, which means more weight transfer. When cornering or braking, you want the least weight transfer possible. But for a drag launch you want more, at least up to the point that the front tires leave the ground (100% weight transfer).

The tradeoff is that a taller tire of course increases your overall gear ratio (lowers it numerically), which may not be a good thing. You ideally want to cross the line near the top of 4th or maybe 5th, so you have to work that part out.

I don't know for sure about removing the sway bar on a car like this. Unlike the front, I don't think the rear bar would be an issue for limiting travel. And with the IRS on these cars, there's no issue of solid-axle twist to address. So I'm think you can just leave connected, but maybe someone else has real experience drag racing these cars would have a different opinion.

[lt4camaro]

Originally Posted by Msquared

So yeah, a taller rear tire should allow a little more CG height, which means more weight transfer. When cornering or braking, you want the least weight transfer possible. But for a drag launch you want more, at least up to the point that the front tires leave the ground (100% weight transfer).

The tradeoff is that a taller tire of course increases your overall gear ratio (lowers it numerically), which may not be a good thing. You ideally want to cross the line near the top of 4th or maybe 5th, so you have to work that part out.

I don't know for sure about removing the sway bar on a car like this. Unlike the front, I don't think the rear bar would be an issue for limiting travel. And with the IRS on these cars, there's no issue of solid-axle twist to address. So I'm think you can just leave connected, but maybe someone else has real experience drag racing these cars would have a different opinion.

Thank You Matt for sharing your knowledge. Regarding the rear sway bar, I assume by removing it , rear squat tendencies might entice weight transfer? yes/no? but may also entice less downward force on the rear tires because the rear suspension is compressing? I dont know, just thinking and my thoughts maybe wrong. Would the rear sway bar promote a more even downward force by keeping the body more level in the rear during a launch and by pushing up on the unibody and creating rear wheel down force ?

[Msquared]

Originally Posted by lt4camaro

Thank You Matt for sharing your knowledge. Regarding the rear sway bar, I assume by removing it , rear squat tendencies might entice weight transfer? yes/no? but may also entice less downward force on the rear tires because the rear suspension is compressing? I dont know, just thinking and my thoughts maybe wrong. Would the rear sway bar promote a more even downward force by keeping the body more level in the rear during a launch and by pushing up on the unibody and creating rear wheel down force ?

A few things here. A sway bar should never oppose wheel movement where both rear wheels are moving in the same direction. It only opposes roll, where they are in opposite directions or one is moving further than the other in the same direction. IOW, it is just trying to make both wheels move the same amount in the same direction. So it should never promote or prevent squat.

If uneven (left to right) traction due to uneven loading of rear tires were an issue, then a rear bar could help. On a solid-axle car, it would help prevent the unwanted tendency to pick up the right tire and plant the left, and you could even preload it to counter that. But with an IRS car, that shouldn't be an issue.

Squat is not a cause of weight transfer - it's the result of it. But it does reduce traction during a launch because the full amount of weight transfer won't happen until the suspension has reached its static point - i.e., until it's done getting into its "squat position." So while the suspension is moving toward its final squatting position, the rearward weight transfer is not fully loading the rear tires. So for a standing-start launch, the less squat the better. This is why cars set up for drag racing have stiff springs and lots of compression damping rate on the rear. Some, like Top Fuel cars, have no rear suspension at all! They get all the extra traction from weight transfer instantly. The problem is you don't really want to drive around the street like this. And it is possible to create suspension geometry with more than 100% squat (a lot more possible with a solid axle than IRS), that actually causes the rear end to rise up with power application rather than squat. This helps plant the rear tires by pushing up on the mass of the car. But you can't do that with a sway bar, and again this isn't good for cornering.

[lt4camaro]

Originally Posted by Msquared

A few things here. A sway bar should never oppose wheel movement where both rear wheels are moving in the same direction. It only opposes roll, where they are in opposite directions or one is moving further than the other in the same direction. IOW, it is just trying to make both wheels move the same amount in the same direction. So it should never promote or prevent squat.

If uneven (left to right) traction due to uneven loading of rear tires were an issue, then a rear bar could help. On a solid-axle car, it would help prevent the unwanted tendency to pick up the right tire and plant the left, and you could even preload it to counter that. But with an IRS car, that shouldn't be an issue.

Squat is not a cause of weight transfer - it's the result of it. But it does reduce traction during a launch because the full amount of weight transfer won't happen until the suspension has reached its static point - i.e., until it's done getting into its "squat position." So while the suspension is moving toward its final squatting position, the rearward weight transfer is not fully loading the rear tires. So for a standing-start launch, the less squat the better. This is why cars set up for drag racing have stiff springs and lots of compression damping rate on the rear. Some, like Top Fuel cars, have no rear suspension at all! They get all the extra traction from weight transfer instantly. The problem is you don't really want to drive around the street like this. And it is possible to create suspension geometry with more than 100% squat (a lot more possible with a solid axle than IRS), that actually causes the rear end to rise up with power application rather than squat. This helps plant the rear tires by pushing up on the mass of the car. But you can't do that with a sway bar, and again this isn't good for cornering.

Again, thank you for sharing your knowledge. Any thoughts on lifting the front like a inch or lowering the back a inch by a spring spreader and or by compressing the rear springs to add small amounts of weight to the rear tires and promote weight transfer.

[Msquared]

Originally Posted by lt4camaro

Again, thank you for sharing your knowledge. Any thoughts on lifting the front like a inch or lowering the back a inch by a spring spreader and or by compressing the rear springs to add small amounts of weight to the rear tires and promote weight transfer.

Sure. So changing front or rear ride height doesn't significantly change the loading on either set of tires. Weight doesn't "slide" on the chassis rake, if you know what I mean. Lifting the front end might help launch the car just because it would raise the CG, though. That would cause more weight transfer to the rear tires on launch. Again, this would be terrible for braking or cornering. Lowering the rear shouldn't help at all.