Question for the tire engineers

[Bandit]

As I recall there are some tire engineers that post here so here is a tech question;

 

If a tire is designed to be used on the rear of a rear drive car, how would it's construction differ and how would it react on the steering end?

[TiredBirds]

Just a hunch, I'd guess a tire designed for use on a FWD car might have a stiffer sidewall to distribute the weight???

[mender]

11 hours ago, Bandit said:

As I recall there are some tire engineers that post here so here is a tech question;

 

If a tire is designed to be used on the rear of a rear drive car, how would it's construction differ and how would it react on the steering end?

Do you have an example?

[Bandit]

24 minutes ago, mender said:

Do you have an example?

Nothing beyond a claim on another forum, which means it's dubious. Probably should have worded my question more along the lines of "If designing a tire to be used only on the rear of a rear drive car would it be designed in a way that would preclude them being used on the steering axle."

 

With Kumho 720 ACR's supposedly the rears wear much better than the fronts and there was a claim they were designed differently front to rear. Now, they are different sizes and possibly the compounds are different as they are designed for a specific car, but that is not how I took the comment that the rears were a different construction.

[mender]

To my knowledge, the tires available to us aren't tailored to a particular end of the car but you might find an advantageous bias with testing.   

[Bandit]

8 minutes ago, mender said:

To my knowledge, the tires available to us aren't tailored to a particular end of the car but you might find an advantageous bias with testing.   

The 720 ACR's are 200 tw and as has been said numerous times; "CC will not ban a specific tire".

 

355's will fit on the front with the unlimited flares we are allowed.  And parallel parking is not a concern.

 

Supposedly the rears wear much much better than the fronts so on all four corners they may be workable. Prices are not as bad as other tires anywhere near that size.

Edited July 18, 2021 by Bandit

[TiredBirds]

I would think tire wear would be specific to the individual car, driver, racing surface, and about 100 other things. Some cars will take more tire outback.. So I guess you might get a longer life out of them. 

[Ian]

10 hours ago, Bandit said:

The 720 ACR's are 200 tw and as has been said numerous times; "CC will not ban a specific tire".

 

355's will fit on the front with the unlimited flares we are allowed.  And parallel parking is not a concern.

 

Supposedly the rears wear much much better than the fronts so on all four corners they may be workable. Prices are not as bad as other tires anywhere near that size.

 

355s on the front?

 

You need to do this.  I need you to do this.  For science.

 

Please.

[mender]

10 hours ago, Bandit said:

The 720 ACR's are 200 tw and as has been said numerous times; "CC will not ban a specific tire".

 

355's will fit on the front with the unlimited flares we are allowed.  And parallel parking is not a concern.

 

Supposedly the rears wear much much better than the fronts so on all four corners they may be workable. Prices are not as bad as other tires anywhere near that size.

Better have good geometry with that width...

[Team Infiniti]

1 hour ago, mender said:

Better have good geometry with that width...

One thing about that guy, I’m sure he will!

[Bandit]

[collinskl1]

This is a gross oversimplification - but in general - nothing would prevent a tire from being run on either axle of a car.

 

The vast majority of cars are designed with square fitments in mind, and the tire's characteristics need to be suited for balanced performance and predictable handling as a package on both ends of the car. A few of those characteristics would be cornering stiffness, other force and moment metrics, various spring rates of the tire (lateral, longitudinal, torsional), and many many more.

 

I work in original equipment tires, so all my tires are specifically engineered for their unique application and performance targets. An aftermarket or replacement tire is engineered to give acceptable performance on a broader range of vehicles. The tires we use for our racing fall into that second category. Staggered fitments are sometimes more complicated, but allow separate tuning of the front and rear of the vehicle. There isn't a such thing as front tire design vs rear tire design, per se - again, nothing would prevent a tire designed for the rear of a staggered fitment car to work on the front of another car - it just might not be optimal for handling.

 

Sometimes, a certain tire construction can yield performances on a car that are non-linear... for example, if the tire on the front of the car is very responsive but the tire on the rear is more lazy or lags in response, you would feel the front of the car turn in quickly, then with a slight delay the rear of the car would "catch up" which wouldn't feel right, or potentially could unsettle the chassis.

 

My gut tells me that the rear ACR tire anecdotally wears better because it is so much bigger than the front tire.

[Bandit]

That's the sort of info I was looking for. Thanks.

 

I'm kind of curious if the compounds may be different on the rear. The rears are much larger and it is easier to get large amounts of downforce on the rear, so theoretically one could use a slightly harder tire.

 

Apparently the fronts on the ACR's are notorious for wearing very quickly, sometimes as little as one "track day", while the rears commonly go 3-4 times as long iirc.

[collinskl1]

I would doubt they used different compounds front to rear for the ACR tires - in my experience, I've never seen that before, including staggered fitments.

 

I wonder how much the front wear might be accelerated by hamfisted driving, understeering in corners, etc. If I recall correctly, the ACR is a 295/355 stagger. That seems like a small tire for a relatively heavy front engine car (at least compared to the 355 rear).

 

Camaro sizes are staggered by 20 mm at 285/305 on the ZL1 and 305/325 for the ZL1 1LE, and the C7 z06 was 285/335.

Edited July 19, 2021 by collinskl1

[Bandit]

7 minutes ago, collinskl1 said:

If I recall correctly, the ACR is a 295/355 stagger. That seems like a small tire for a relatively heavy front engine car.

The fronts have an very short sidewall as well. 295/25/19's as I recall. Might that play into it?

 

 

[mender]

6 hours ago, Bandit said:

I'm kind of curious if the compounds may be different on the rear. The rears are much larger and it is easier to get large amounts of downforce on the rear, so theoretically one could use a slightly harder tire.

As long as your low speed mechanical grip handling works with the different compounds, you can balance the high speed with aero. 

[collinskl1]

16 hours ago, Bandit said:

The fronts have an very short sidewall as well. 295/25/19's as I recall. Might that play into it?

 

 

 

I wouldn't say the low aspect ratio is a direct contributor to faster wear, but it could lead to a more responsive steering feel and/or saturating the available grip quicker than the rear tire loads up - which would support my understeer and sliding the front theory.

[Grant]

I think there are a few things going on with the ACR tires:

 

Firstly the fronts have tiny sidewalls and the load rating is only 1,323 lbs. I think this probably explains the catastrophic failures the fronts sometimes experience, e.g. on the Nurburgring (heavy car + bumps + downforce).

 

Just like the Gen 4 ACRs, the car has a massive downforce imbalance. It probably only understeers at speed, so ham-fisted drivers could easily be giving it too much front slip angle. Here's a picture of a twin turbo ACR Extreme at a standing mile event:

Lastly the fronts are said to be a different compound. This isn't unheard of in motorsport. Though I don't know anyone who's tried the rears on a different car, a 355 square setup should be an option for us in WRL.

[Bandit]

Curious if that car has the OE tires with that sidewall deflection.

 

Being a standing mile drag radials would be likely.

 

But yes, I'd say that's a pretty healthy aero imbalance at speed. The nose up reminds me of my 70 442 W30 at the big end and how light and unresponsive the steering was.

 

 

[Grant]

It's probably on R888Rs; those work better for most of the 1000+ whp roll racing guys. I'm pretty sure the car in the pic above was twin turbo.

[collinskl1]

Obviously can't tell what the tires are in that picture, but the section height looks taller than the OE ACR Kumho.

 

That phenomenon seen in the sidewall is called "standing wave" and that tire is not long for this world. The combination of inflation, load, and speed generates a resonance in the carcass that with enough time (not very much) will make it come apart. Yikes.

[Grant]

I got tired of trying to convince standing miles guys they need less wing, or something active

 

Excessive rear downforce seems to be unreasonably common in motorsport.