[funked1]

Originally Posted by mecorn

I have just taken delivery of a set of 305/30 fronts and 315/30 rears. I will be mounting them on 19s with stock widths. Any more input on how the 305s respond to being pinched???

I am very interested to hear how this works. Aggressive as ****. Send it!

[TrackClub]

Originally Posted by funked1

I am very interested to hear how this works. Aggressive as ****. Send it!

Aggressive in a racing world is going the OTHER way. There is nothing to recommend oversizing a tire on a rim performance wise.

[mecorn]

Originally Posted by TrackClub

Aggressive in a racing world is going the OTHER way. There is nothing to recommend oversizing a tire on a rim performance wise.

Appreciate the perspective, but there have been exceptions... the Hoosier A6 comes to mind. I know I’m not using the recommended tire for wheel width, which is why I’m asking if anyone else has experience with this setup.

Any yes, this IS aggressive!

[Mountain]

Originally Posted by joelster

I noticed a big difference in tire pressures on the 660's. The sweet spot for me is 30lbs. I ran them for 2 events at 31-32lbs. On the 3rd event I had a decent lead so I tried lowering them. OMG what a difference. Took them to 30 and the front end was noticeably better. Lowered my time by 1.1 seconds. Went to 4th event and ran them at 30 for the whole event and set top pax. I run 19x11, 19x11.5, and 315/30/19 square. Car is roughly 3650lbs.

That's interesting because I just did an autocross test and tune with my RT660's with the sole-purpose of playing with tire pressures and found the opposite. I am running 275/30 front and 315/30 rear on 19x10/19x11 wheels.

Day was about 55-60F, sunny, dry day. Surface is some weird blacktop. I used a tire pyrometer and digital tire pressure gauge (Longacre stuff). I started at 29.6 psi cold and ended up adding pressure after two or three test runs at that setting. Hot pressure coming in from the runs would only get to about 32psi and the tire temps showed the same temp middle and inside with the outsides being about 2-3F cooler (front and rear). My notes indicate that running at those pressures the tires were not coming in quickly [to grip] and the rear was squirrely. I actually ended up adding pressure the rest of my trials. I can't comment what the true "cold" psi is (I can go check them in the garage as I haven't touched them), but the pre-run psi I ended at was around 34psi front, with the rear having just a touch less psi and rendering a 2-3F spread across the outside, center and inside of the tread (i.e. 92-94-97). My notes indicate the tires came in a lot quicker and the car had a good balance of grip. Times dropped at least 1-second.

But, again, I'm on 275/315 tires on 10"/11" wheels, maybe your ambient temp was a lot hotter and/or your track surface was a lot more grippy.

[Mountain]

Also... I ran the OEM SC3 (20" setup) that same day, same course. Playing with the tire pressures as well. I will say this: the SC3 can run a similar time as the RT660, but they need heat in them: the RT660 for sure felt to come in way quicker than the SC3 (I noted that the RT660 comes in, for the first run, after just a few maneuvers putting some slip into them, while the SC3 seemed to need almost 1/3 of the course to feel as confident). If autocross is the goal, the RT660 is the way to go over the SC3.

I am going to bring the RT660's to TT Nats with me and run them for a practice session to see how they do. I've read a few others notes on them on track, including Strano's notes, but I want to try them myself. Maybe I'll end up running them for the event if they seem to show some kind of advantage over the SC3 (which I love as a 200tw track tire). We'll see.

[TrackClub]

Originally Posted by mecorn

Appreciate the perspective, but there have been exceptions... the Hoosier A6 comes to mind. I know I’m not using the recommended tire for wheel width, which is why I’m asking if anyone else has experience with this setup.

Any yes, this IS aggressive!

Yes, i am aware of some folks ignoring min rim width requirement. Whether this is "aggressive" depends on how we understand the word. I'd call it "sub optimal" regarding tire performance and "not recommended" due to possible safety aspects. While one can get away with it with very stiff side wall tire, it will be overly pinched by a rim and hence "balloon" beyond its design architecture.
While this may not lead to tears in autox, going WOT thru the esses at the Glen may lead to different outcomes, given extreme loads.

Note what Mountain has done rim vs tire sizes and imo this is the way to get the most out of a tire: "not the other way around".

All pro race cars maximize rim size, to ensure stiffest side wall possible and hence the most feedback and responsiveness possible from the tires. Going the other way will reduce both and hence it makes zero sense for performance. Sure, an A7 will deliver better pace over streets even in a sub optimal set up, buy why leave pace on the table and push tires outside their safety design specs?

Lastly, this is of particular importance for fronts, as they do a lot more work under braking and cornering, usually dealing with much higher loads.

Anyhow, you asked the question, which suggests you either didn't know, or simply sought validation from others to "justify" your decision. If the latter: be careful, as we don't all track our cars at the same pace, the same venues, or at the same limits. Just my 2 cents, that's all.

Cheers!

[Mountain]

Originally Posted by TrackClub

Yes, i am aware of some folks ignoring min rim width requirement. Whether this is "aggressive" depends on how we understand the word. I'd call it "sub optimal" regarding tire performance and "not recommended" due to possible safety aspects. While one can get away with it with very stiff side wall tire, it will be overly pinched by a rim and hence "balloon" beyond its design architecture.
While this may not lead to tears in autox, going WOT thru the esses at the Glen may lead to different outcomes, given extreme loads.

Note what Mountain has done rim vs tire sizes and imo this is the way to get the most out of a tire: "not the other way around".

All pro race cars maximize rim size, to ensure stiffest side wall possible and hence the most feedback and responsiveness possible from the tires. Going the other way will reduce both and hence it makes zero sense for performance. Sure, an A7 will deliver better pace over streets even in a sub optimal set up, buy why leave pace on the table and push tires outside their safety design specs?

Lastly, this is of particular importance for fronts, as they do a lot more work under braking and cornering, usually dealing with much higher loads.

Anyhow, you asked the question, which suggests you either didn't know, or simply sought validation from others to "justify" your decision. If the latter: be careful, as we don't all track our cars at the same pace, the same venues, or at the same limits. Just my 2 cents, that's all.

Cheers!

Well, slicks also 1. have much stiffer/stronger sidewalls than street tires and 2. many times they are designed with the mindset they may be squeezed onto wheels of sub-optimal width due to various racing series/class requirements. The Falken RT660 has a fairly soft sidewall and that's one reason why I went with 275/30 and 315/30 (I wanted 305/30) over 275/35 and 295/35. Interesting enough, I've heard equal amounts of people say the taller sidewall Falkens have great steering response/feel and don't have great steering response/feel, lol...

[TrackClub]

Originally Posted by Mountain

Well, slicks also 1. have much stiffer/stronger sidewalls than street tires and 2. many times they are designed with the mindset they may be squeezed onto wheels of sub-optimal width due to various racing series/class requirements. The Falken RT660 has a fairly soft sidewall and that's one reason why I went with 275/30 and 315/30 (I wanted 305/30) over 275/35 and 295/35. Interesting enough, I've heard equal amounts of people say the taller sidewall Falkens have great steering response/feel and don't have great steering response/feel, lol...

Yep for sure race tires have stiffer sidewalls. As far as pinching them, usually class requirements dictate tire sizes vs rims, but perhaps not all. Agree, that with softer sidewall tire, running wider rim is beneficial, if not an outright requirement. I think what folks comment about tires alone much depend on the rest of their set up and shouldn't be taken in isolation. At least I wouldn't. Cheers!

[DaveC113]

If we're looking at 305s on 10" wide rims that's close. For Michelin PS4S recommended width is 10.5 - 11.5".

[TrackClub]

Originally Posted by DaveC113

If we're looking at 305s on 10" wide rims that's close. For Michelin PS4S recommended width is 10.5 - 11.5".

Exactly.

[Msquared]

Originally Posted by TrackClub

Lastly, this is of particular importance for fronts, as they do a lot more work under braking and cornering, usually dealing with much higher loads.

Strictly speaking, the larger tire will handle higher loads than the smaller one, even on the same wheel size. Load capability is mostly a function of the volume and pressure of the air contained inside the tire, and obviously for a given pressure the 305 will hold a lot more air than the 275. The 275/30/19 is rated for a max load of 1565lb, whereas the 305/30/19 is rated for 1874lb. That is irrespective of rim width.

I am sure there is no safety concern. People have been mounting 305/30s on 19x10s with our cars since the first 6th-gen 1LEs came out. I am sure there will be a difference in feel. One question will be how the bigger front tires wear: will they do anything strange. Again, that has not been a problem on the RE71 or Rival S 305 fitment; so probably not, but every tire model is different. They may end up wearing better simply because of their higher load rating. In terms of the stopwatch, it may very well end up being a wash: more contact patch offset by bigger diameters. In that case, it would come down to feel.

Two indisputable advantages to the 275s: they are cheaper and one of the fits into the Camaro trunk a lot more easily than the 305!

[TrackClub]

Originally Posted by Msquared

Strictly speaking, the larger tire will handle higher loads than the smaller one, even on the same wheel size. Load capability is mostly a function of the volume and pressure of the air contained inside the tire, and obviously for a given pressure the 305 will hold a lot more air than the 275. The 275/30/19 is rated for a max load of 1565lb, whereas the 305/30/19 is rated for 1874lb. That is irrespective of rim width.

I am sure there is no safety concern. People have been mounting 305/30s on 19x10s with our cars since the first 6th-gen 1LEs came out. I am sure there will be a difference in feel. One question will be how the bigger front tires wear: will they do anything strange. Again, that has not been a problem on the RE71 or Rival S 305 fitment; so probably not, but every tire model is different. They may end up wearing better simply because of their higher load rating. In terms of the stopwatch, it may very well end up being a wash: more contact patch offset by bigger diameters. In that case, it would come down to feel.

Two indisputable advantages to the 275s: they are cheaper and one of the fits into the Camaro trunk a lot more easily than the 305!

Agree on the load rating. Disagree that this is irrespective of the rim size, as the tire will behave differently on an undersized rim, as the sidewall will assume a different shape and likely flex differently/more vs projected design - and that's why we have manufacturer limits clearly specified. If 305 was ok for a 10 rim, manufacturers would include it as approved and increase their sales as a result - zero doubt - but NONE of them do. Why?

What makes you SURE there is no safety concern? Would you also suggest mounting a 305 on 9.5? 9.0? Where exactly does a safety concern kick in, based on your musings?

The fact that some ppl mount tires beyond manufacturer specified range, doesn't mean it is a safe practice, or should be blindly copied, or condoned.The fact they get away with it doesn't prove anything, as who knows how they use their cars, or how far they push, etc.

No matter how good a driver, or how well prepped a car, what keeps us safely connected to the pavement is just 4 small contact patches. There really is no good reason to push the envelope here just to save a cost of 2 rims.

[TrackClub]

Just to add some of my own musings regarding load ratings being directly related to tire size and amount of air:
While this is the case with these 2 specific Falken sizes, this wisdom does not apply to all of them and in some cases smaller tires have higher ratings. So something else must play a role here, perhaps carcass construction, side wall stiffness, aspect ratio, etc. But to be clear: i am not sure. Also note, for example, A7 has much lower load ratings for comparable sizes vs street tires. Hoosier also recommends slightly different rim sizes too - in some cases - to make it even more complex.
Another consideration is cold and hot tire pressure. It is no secret, that running pressures too low to improve performance can lead to tire failure. Happens in Nascar almost every race. Id imagine this to be even more influenced by incorrect rim size.
Bottom line, i would never put an oversized tire on a front, given they work much harder than rears. Simply because this hobby already carries plenty of risk and i wouldn't want to wonder if i am gonna lose a tire while cornering at 100+ mph under extreme *lateral* loads. Another consideration, would be a potential risk for others following me. But that's me and my personal principles. Cheers!

[Msquared]

Originally Posted by TrackClub

Agree on the load rating. Disagree that this is irrespective of the rim size, as the tire will behave differently on an undersized rim, as the sidewall will assume a different shape and likely flex differently/more vs projected design

It's literally the specs the manufacturers publish. There is no array of load ratings for a given size tire plotted across rim width. There's just one load rating, no matter the rim width.

While this is the case with these 2 specific Falken sizes, this wisdom does not apply to all of them and in some cases smaller tires have higher ratings. So something else must play a role here, perhaps carcass construction, side wall stiffness, aspect ratio, etc. But to be clear: i am not sure.

Carcass construction does have something to do with it. That's why there are different "ply" or load range ratings. And again, air pressures change it too. The max load rating assumes the tire is inflated to its max rated pressure (the pressure molded into the sidewall). Note that those things are related: the heavier-duty construction allows higher air pressures, and therefore higher load ratings. So no, it's not just about air volume and pressure, but for a given tire model in which the sizes have the same construction and are inflated to the same pressures, the load is always higher for the larger tire. And again, that's irrespective of rim width. So the 305 RT660 has a higher load rating than the 275 (they are both in the XL load range category), and the difference is pretty big.

- and that's why we have manufacturer limits clearly specified. If 305 was ok for a 10 rim, manufacturers would include it as approved and increase their sales as a result - zero doubt - but NONE of them do. Why?

There's no such thing as a "manufacturer limit" on rim widths. The rim width range is the range for which the manufacturer designed the tires, but that doesn't mean it's unsafe to mount tires to wheels that are outside that range. It just means the tires may not perform optimally. There are various examples of manufacturers equipping cars from the factory with tire fitments outside of the tire company's rim width range.

What makes you SURE there is no safety concern? Would you also suggest mounting a 305 on 9.5? 9.0? Where exactly does a safety concern kick in, based on your musings?

Based on many, many data points of people running 305/30/19s on 10" wheels and then running those in autocross and road course timed laps, that data supports the conclusion that there is not a safety issue. I would not recommend smaller rim widths than that because: 1) the tire would probably begin wearing strangely and not perform as well, and 2) there is probably a more appropriate size from the tire manufacturer.

The fact that some ppl mount tires beyond manufacturer specified range, doesn't mean it is a safe practice, or should be blindly copied, or condoned. The fact they get away with it doesn't prove anything, as who knows how they use their cars, or how far they push, etc.

Literally the data tells us it's not a safety concern. That's how research works: you gather data, and more samples that show a particular outcome mean the confidence factor (i.e. lower probability of error in conclusion) gets higher. In this case, we have numerous people who have competed in 1LEs with 10" wheels shod with 305 tires. This includes national championship drivers. We haven't had failures. Nobody said anyone should "blindly copy" anything. That's a straw man argument. This would be a very well researched trial.

There really is no good reason to push the envelope here just to save a cost of 2 rims.

Ahh, I think I see the disconnect here. You are assuming that we can all just choose whatever wheel sizes we want, but that is not the case. In SCCA B Street competition, we cannot stray from the stock rim widths at all. We are stuck with 10" front wheels. It has nothing to do with saving money. So when considering front tires from the Falken RT660 lineup, we have to choose between the 275/30/19 or the 305/30/19. There's no option in between, and no option to use wider wheels. Ergo the choice to try 10mm smaller than Chevy's OE front, or 20mm bigger.

Another good reason to consider the 305/315 combo is the ABS system. Using the 275/305 combo increases the diameter split from front to rear. That may - may - induce ice mode more easily. If so, then the 305/315 combo would not. That alone is enough of a reason to consider the safety and consistency of the larger combo. I can tell you that I've gotten ice mode in my 2020 on the 275/305 combo whereas I did not on the stock Goodyears. That may have to do with the bigger size split, or maybe it's just down to the increased grip, slip characteristics, or other construction differences. I don't know. But that would be a reason to try the larger combo.

I wish I had the funds to do a controlled test of the two combos on the same 1LE with a couple really consistent and fast drivers, a la the GRM tests that Whitener and Hollis do. It would be very interesting to directly compare times across temps and conditions. Unfortunately, I don't.

I do agree that in general it's not ideal to mount tires on wheels that are narrower than the manufacturer rim width. However, that's based on performance and handling traits rather than safety concerns. I'm very confident that safety is not a concern. The problem here is that in certain competition classes we can't change rim widths and the current best tire option doesn't offer a "just-right" size option for the front of our cars. Covnersely, you could view the problem as being that Chevy should have made our cars with 11" square rim widths. That would have solved everything. But we have what we have, and we are just mulling over two imperfect-but-very-good choices. I had the option to try the larger size and chickened out because of the unknowns in performance. I will be interested to see mecorn's results!