When are tires too wide......

[Mad Hatter]

With the current trend of bigger is better.... is their a point when its too much? A buddies gen 5 ZL1 is running on R7 335/30-18's all the way around, and though he has amazing grip he seems to one second slower then he used to be. I have 295's now and would like to go to 315's which seems to be working well from what you see on TMO.... Several of my friends are looking for the widest rubber they can fit in.... I imagine that once you get to a certain width you are starting to affect all kinds of suspension variables.

 

[Grant 302]

Still takes HP to push all that extra 'meat' around...Can't say exactly where the returns from more rubber are diminished, but lower power to weight ratio cars will suffer more/first.

 

[Mad Hatter]

I think that would be the logical conclusion, but the Zl1 is putting down 520rwhp.... So I was a little surprised that he lost time. Was wondering if such a big tire was upsetting the suspension setup in some way. I think I remember a thread were someone tried 305's up front and did not like the results, was looking for it.

 

[Grant 302]

Yeah, might also be more grip than its spring rates can keep 'control' over the geometry. IOW, too much roll.

 

[302BOB]

Tires are surely a plus for your expected performance...but too much may exceed your car's handling capacity..be careful..............

 

[Fair]

We have NEVER seen a situation where you can make the "the tires are too wide" on a road course car. This is something we have been preaching - and testing - on track/autox situations for over 30 years. I've tested this theory on many different cars, but I will share the S197 an S550 tests we've done since they are more relevant.



You can ignore our 3 decades of testing and proving that "wider is better" at your own peril. We have had online "arguments" with folks about since the 1990s, and we have heard the tired old wive's tales of "too much rolling resistance" and "can't get enough heat into the tires" before. This line of thinking is broken, and just not factual.



If giant tires can make even a 4 cylinder Miata faster in a 45 second autocross, they damn sure can get enough heat in them in a 15-20 minute track session. People that think otherwise are either A) very, very slow or B) very misinformed and C) likely don't pay attention to lap times. The results sheets and the timers don't lie...

2011 Mustang GT S197 Test

On cars as big/heavy/powerful as S197 or S550 Mustangs, there is literally not a "too wide" size available! We have tested over 5 years of autocross and time trial competition on our S197 Mustang GT, from 2010 - 2015 seasons. We started with 255mm stock tires and moved up to 18x9" wheels with 265mm 140 treadwear tires - which was the widest limit for the class we ran for 1.5 years (SCCA STX).



It was painful to drive with tires this narrow - we were constantly fighting rear tire spin even with stock power levels (380 whp), but it was laughable with 430 whp on the 265s. Autocross and track testing with multiple National level drivers and it was a hot mess. We tested with FOUR brands of 265/40/18 tires, all of the "hot" tire models, raced it dozens of times. Futile!



Then we switched classes briefly (STU) when we went up to 275mm street tires, the car got faster and easier to drive. By a lot. Could we still overpower the rears? Sure, but in the same SCCA region our "PAX" finishes went from "top 75" (out of 150 cars) to "top 10" on 275s. Same 140-200 treadwear, just a bump up to 18x10" wheels and 275/40/18 Re-11s.



Then we tried 295s, and the car kept dropping time and won more races than before. We started winning at Goodguys and even some portions of Optima series qualifier events on this size 295/35/18 street tire.



We switched to 315mm 200 treadwear tires and the car WOKE UP. This was the max width we could fit under stock fenders and the car won an entire event at an Optima series qualifier, with only 430 whp. Some of the cars we raced against had 1000 hp...



Meanwhile the car was going from "sucking" in NASA TTB class on 265 tires to KILLING IT on 315mm Hoosier A6 rubber in TT3 class. The car was unstoppable on the 315 Hoosier A6 DOT tires, and lap times dropped significantly from the 295mm street tires at the same track (from 2:03 to 1:50). TIRES MATTER. A LOT.



The next season we cut fenders, added flares, and switched to 335F/R sizes for 200 treadwear competition, and 335F/345R Hoosier A7s for NASA TT3 class. Our previous lap records fell by 2-3 seconds from the 315mm Hoosiers.



At this point the 345/35/18 Hoosier was and still is the BIGGEST TIRE THIS COMPANY MAKES, and few other tires are even made that wide. We maxed out the width available and ran the fastest times in the last 2 years we owned that car. If Hoosier had made a 355mm A7 we would have bought it.

2018 Mustang GT S550 Test



On our 2018 GT we went from the skinny 235mm tires on 18x8" wheels to 305/30R19 RE-71Rs on 19x11" wheels and dropped TEN SECONDS at the same test track (MSR-C) with the same driver, in just one week's time. We proved the "wider is better" game on the S197, so why screw around on this car? We made one big jump right off the bat.



We will test 315mm A7 tires on this car next and watch the lap times drop drop drop. When the wife allows me to cut fenders I will go straight to 335F/345R sizes - because that's the biggest we can get.



edit: Of course with increasing levels of grip, the spring rates and dampers needed to control them need to go up. I've got examples of where we ignored this and it bit us in the ass, too. Real R-compounds in real widths need REAL shocks - not the rubbish that Ford installs, but real monotube adjustable dampers. The tires aren't EVERYTHING, but they are MOST of the game. We do all of our suspension tricks just to keep the tires happy and working efficiently. So yes, as width/grip/compound increase, so must suspension spring/damper/bar/bushing upgrades.

Cheers,

 

[Mad Hatter]

Terry, Been a big fan of your BIg Red S197 for a while. Even thought about buying it once! I guess the main thrust of this thread is right here......

Of course with increasing levels of grip, the spring rates and dampers needed to control them need to go up. I've got examples of where we ignored this and it bit us in the ass, too. Real R-compounds in real widths need REAL shocks - not the rubbish that Ford installs, but real monotube adjustable dampers. The tires aren't EVERYTHING, but they are MOST of the game. We do all of our suspension tricks just to keep the tires happy and working efficiently. So yes, as width/grip/compound increase, so must suspension spring/damper/bar/bushing upgrades.

Would love to see those examples of being bit in the ass, so we do no repeat them! Ass..uming they are not literal.... if so... really do not want to see them!!

 

[Grant 302]

^Exactly. I don't think anyone here needs to be convinced that wider is better.

The title of this thread could have just as well been "what's wrong with this ZL1 on 335 R7s"

 

[Mad Hatter]

I would very much like to know.....But even though the ZL1 is a friends car.... He is also my competition on Saturday.. So maybe I will fill him in later.. Like in December!

 

[Grant 302]

Like in December!

2019 or 2020?

 

[Busdriver]

We switched to 315mm 200 treadwear tires and the car WOKE UP. .

Terry, How much spring rate did you end up needing with 315s?

I ordered up some MCS coil-overs (from you) and went with the lowest rate package since it's also a daily driver. It's gotta be better than the Steeda springs/Konis I have now.

 

[Mad Hatter]

Good coilovers with serious shocks are in a different league then the regular spring and Koni Setup. I found P springs and Konis to be harsher then even the Cortex koni setup with #600/#350.

 

[JeffV8]

Good read. Thanks Terry

 

[DocWalt]

NA BRZ was tested by various folks to be faster on 225s vs 245s, so wider isn't always faster. Add power then that balance swings the other way.

Anecdotal evidence from my own testing... I went to Shenandoah circuit with 100% stock suspension and tires (215s) but NA power adders on my BRZ. I went back the next spring with wide sticky tires (245s), good suspension, etc etc. I saw 10 mph faster exit speed onto the longest straight and only a couple of mph faster at the braking point. That's a lot of drag...

Sure, pretty much every modern-ish Mustang has more power, but there's obviously a limit to "wider is better"

 

[Mad Hatter]

Thats the one thing I do not like about the BRZ or GT86....... severely power limited.... Would be a great car with 250hp.

 

[blacksheep-1]

I think it's more of a function of weight and not so much power. Each tire MFG has a load rating for a particular tire, those are static, but once you start moving stuff around via suspension movements, you can exceed those limits pretty easily, in IMSA we were forced to use itty bitty 275s and they were way to small, in Pirelli we ran 305/680s and we've stuffed 315s on a mustang on occasion..some of them like the T1 stuff could use 335s on the rear, IMO. The corvettes in GT2 are running 345s.
I think in a general sense, a light car can use a stickier tire in a specific size, where an identical, but heavier car, say about 500 pounds heavier, needs a wider tire in a harder compound, especially if going the distance is an issue. I think it makes sense to run the smallest tire possible..that works, it's less crap to haul around the track. Karts are a prime example of this, sometimes going to a narrower tire can cause the the kart to run a lot faster.

 

[JeffV8]

Karts with Briggs & Stratton engines ran great with small tires. My shifter Kart liked wide

 

[Mad Hatter]

That makes lots of sense from a physics point of view. F=MA etc. In the case of the ZL1 he weighs 300 kilos more then me! I suspect its probably the fact that he still has his magnaride OEM shocks combined with lower stiffer springs, sway bars, bushings?

 

[Fair]

Would love to see those examples of being bit in the ass, so we do no repeat them! Ass..uming they are not literal.... if so... really do not want to see them!!

Yes I have examples... this 1992 Corvette we built a few years ago is a good one. We built it for NASA's former TTC class, which was a "points" based class. It barely fit into TTC - they never had anyone actually try to race one there - so we had severe limitations on what we were allowed to do in "C".



We built the entire car around a 245/40/17 Hoosier R7. Only mods we could do were springs, that's it. No points left for better dampers, so we had to have the 1992 Bilsteins rebuilt and work around that limitation. We used a custom made VBP 1200# transverse front spring and 550# 84 Z51 rear spring.

It was balanced surprisingly well on the 245mm tires and we shattered some track records in this car, then National Office threw down a lot of weight and rules rer-writes the in 2016, making it not competitive in that class. But during the '15 season I got a wild hair and decided to test fit a 335mm Hoosier A7 to the car... we made a bunch of measurements, spec'd out a set of 18x12" wheels and had them shipped...



Damn - it fit! Well... it rubbed a bit up front so we cut the fenders. So then I stuck a 345 A7 on the rear!



Yea it was pretty nasty looking but I knew how well this 335F/345R Hoosier package worked on our S197. So we ran it for two events on the Corvette in TT2 class. And you know what?



You cannot jump up four classes and +100mm of tire and from R7 to A7 compound and not have consequences. THIS NEEDED REAL DAMPERS. We tried the other $300 set of "new" Bilsteins and it was just as bad. The car was bouncing around like it had no shocks whatsoever - and literally all we changed was tires. I have never been scared in a race car before... this.



Moral of the story is what I wrote above: you have to match the spring rates AND dampers to the tires you are building around. We have setup this car for 275mm Falken street tires and it will soon be sold to a ChampCar, WRL or AER racer - where the springs and shocks it has are more appropriate.

Terry, How much spring rate did you end up needing with 315s?

I ordered up some MCS coil-overs (from you) and went with the lowest rate package since it's also a daily driver. It's gotta be better than the Steeda springs/Konis I have now.

Yea, anything that is a real monotube adjustable not made in China is better than lowering springs with twin tubes.



We ran a variety of spring rates on 4 different shock packages on this car over the years. When we first went to the 315s we were on the 450/175 package and the car had too much roll and dive at the grip level. When we moved up the our 550/250 setup (streetable) it was a lot better, but with 315 Hoosiers it still had a little bit of roll (above).



Later on with high aero loads on the BIG Hoosiers (335/345) we went all the way to 800/275 (not streetable). That is a huge jump, admittedly.

• Vorshlag's GT Spring Package is 450 #/in Front / 175 #/in Rear
• Vorshlag's GTS Spring Package is 550 #/in Front / 250 #/in Rear
• Vorshlag's GTR Spring Package is 800 #/in Front / 275 #/in Rear

NA BRZ was tested by various folks to be faster on 225s vs 245s, so wider isn't always faster. Add power then that balance swings the other way.

Wait... what? Tested HOW? I have tested the 86 chassis many times with anything from 215 to 255mm tires and it was ALWAYS faster on the wider stuff. Dramatically faster.



I think "various folks" need to check their test methods. Seriously, these cars WAKE UP with added tire width, even at 2600 pounds and 200 hp. I would bet large sums of money on that before/after track test. Because it works.

Anecdotal evidence from my own testing... I went to Shenandoah circuit with 100% stock suspension and tires (215s) but NA power adders on my BRZ. I went back the next spring with wide sticky tires (245s), good suspension, etc etc. I saw 10 mph faster exit speed onto the longest straight and only a couple of mph faster at the braking point. That's a lot of drag...

But what about lap time drops - that's what matters, not mid-corner or end of straight speeds.

Sure, pretty much every modern-ish Mustang has more power, but there's obviously a limit to "wider is better"

Again, I don't see an obvious limit even on "under powered cars". In SCCA autocross the tiniest cars are going faster with ever wider tires (275-295mm Hoosiers on 1.6L Miata Street Prepared cars, 315-335mm Hoosiers on 3.0L DSP BMWs, etc). The track guys are more reluctant to test these wider tires because "conventional wisdom" talks them out of it so often. Shame.

 

[Mad Hatter]

Thanks for all the info! but the photos got dropped for some reason.