Bias plies vs. radials on a foxbody mustang, or Casey's Ghetto Tire Comparison (long) - Ford Mustang Forums : Corral.net Mustang Forum
 
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Bias plies vs. radials on a foxbody mustang, or Casey's Ghetto Tire Comparison (long)

On July 17th, 2016, I attended auto-x #5 of the New England Region of the Sports Car Club of America with the intention of conducting a same-day slick tire comparison. This is the club I have been auto-xing with since the beginning of the 2015 season. Their site, Devens Airfield in Ayer, MA, is sort of like a baby brother to the Riverside Annex in College Station, TX; itís a small airport complex. However, unlike the Annex, it doesnít have the massive patch of concrete from a 4-10 split.

Since re-joining the SCCA last year (after a 19? year hiatus), I have focused on transitioning my dedicated auto-x car, a barely-street legal (registered and inspected in MA) 1992 Ford Mustang, to C-Prepared with the ultimate goal in the distant future of turning it into a CMC or AI car. It is by no means a full prep car; it currently weighs somewhere close to 2900 lbs. without driver (200 pounds over minimum weight) and has a 306 (0.30 over 302) small block that maybe makes 275-290 rwhp and has 3.55 gears with a T-5. The suspension is a fairly basic Poor Manís 3 Link with Koni yellows, coil-overs up front, and torque arm rear springs. 1998 Cobra brakes are another major upgrade that this car has received during the 20+ years that I have owned it. Spring rates are 375# coil overs front, 415-515 conventional rear. Front bar is í92 stock with SN95 front control arms and 94-95 spindles; rear bar is a hollow í98 Cobra bar with SN95 axles. Car has a bunch of caster and a pinch of toe out.

So why a same day tire comparison? Being unable to afford new A7s or Hoosier radial slicks, for the past several years I have kept my eye out for good deals on used racing tires. In the spring of 2014, I purchased a barely-used set of Continental GT-R slicks (285/645/18 front and 305/650/18 rear) from GT Track Guys shipped for $591.55 and had them mounted on some cheap 18*10s I had purchased from American Muscle. I was turned on to these tires for auto-x by Chris Masy (CMasy). For a low-budget slick option, I really canít recommend this used slick tire enough. As a side note, in 2015, the GT Track Guys told me that they think the Pirelli slicks would be even faster for auto-x, as they are lighter and heat up faster, but I have not yet had a chance to test any yet.

Then, in 2015, I purchased a set of 16*12 Real Racing Wheels from Chris Masy. After all, if I am going to run in CP, I am going to need some REALLY big tires. Bigger even than 285s and 305s. With the wheels came an absolute plethora of Hoosier bias ply racing slicks that Chris had collected for his CP foxbody car. With eyes that were perhaps bigger than my stomach, I had the brand new 27x14-16 and used 25x12-16 R35As mounted up. With a bunch of clearancing, the 27x14 rears and 25x12 fronts looked like they might actually fit.



Yeah, no. During the 2015 Devens Tour, my stock driver-side tailpipe rubbed against the 14-inch-wide tireís inner sidewall. Near the end of my 3rd run, the tire blew. Oops.



Luckily I had had a backup set of used 275/35-18 Hoosiers R6s, found locally via Craigslist, mounted up on my 18*10s prior to the 2-day event in place of the Continentals in case I was in the hunt for free tires due to contingency. (I wasnít, I foolishly entered XP because CP wasnít going to have enough entries to qualify for tire contingency from Hoosier and because my car was illegal for CP due to a ricer wing and NASCAR Car of Tomorrow front splitter. As good olí Jim Ross would say, I got whipped like a government mule in XP by the Panda and company. Those guys arenít playing around.)

Since then, I have auto-xed either on the Continentals or on a different Hoosier bias ply setup: I moved the 25x12s to the rear (where they fit perfectly) and ordered some 23.5x11.5-16s on clearance from Hoosier for the front. After cording the Continentals (after getting a ton of runs out of them) during the summer of 2015, I have been exclusively auto-xing on the bias ply Hoosiers described above. Until the most recent auto-x, that is. More on that in a bit.

Several months ago, I finally decided to give Track Night in America a try. Despite having worked as a test driver for Slick 50 and as a corner worker for organizations such as CVAR, NASA, and PCA at Texas World Speedway off and on since the 90ís, I donít actually have a ton of HPDE experienceómaybe just a session or two, really (although I am positive that I have set some track records for a corner worker going from Turn 7 to the pits; my secret is that I skipped the road course and took the NASCAR back straight and turns 3 and 4 flat out. That oval is stupid bumpy). Worse, during the last such session, at Texas World Speedway sometime in 2009 or 2010, I was experiencing overheating issues due to a pinhole leak in my radiator so I really wasnít able to get after it.

So when I saw TNiA advertising a $35 work/run (run 1 session out of 3) deal for the newly opened Palmer Motorsports Park in June, I signed up. However, while I had a great time, I was on my Riken Raptor street tires (275/40-17s on 17*9s; are you getting that I am cheap? Parents, donít let your kids grow up to be cowboys and also donít let them get multiple liberal arts degrees) as I had flat-spotted my front right Hoosier during the NER auto-x earlier that month and I hadnít swapped out the corded Continentals yet on my 18*10s. Womp womp. Still, I had a great time at Palmer and decided to sign up for TNiAís next event at Thompson Motor Speedway in CT. However, this time, I was hoping to be on slicks.

https://www.dropbox.com/s/zwsscobo60...GoPro.mp4?dl=0

Luckily, I had two free sets of tires fall into my lap: Arnold Beebe, who races his foxbody in CP, tossed in his 100+ run used Avon bias ply slicks (25x12.5 rears and 23.5x11 fronts) as part of a deal when I purchased a pair of foxbody doors from him (I want to compare the weight of my power window/mirror doors vs. his manual window/mirror doors). I just had to drive to Schenectady, NY, to get them. The trip, which I made in my foxbody with a trailer behind it (I have a hitch setup thanks to Kent Kroll, aka Thinkkker), was no big dealóexcept my freakiní left tailpipe decided to divorce itself from the muffler after 20+ years of happy marriage while I was coming home on the Masspike.

Despite the high number of runs, the Avonís tread indicators still showed that they had plenty of rubber left on them. I decided to just have the rears mounted. I now had a pair of 25x12.5-16 Avons for the rear of the car and a pair of 25x12-16 Hoosiers for the front. Not ideal but I wanted to try out the bigger front tire setup as I had only been able to make 2.9s with the 12Ē slicks up front last year.

I also visited a local Porsche racecar builder, who sometimes mounts slicks for me, and asked if he had any decent 18Ē slicks in his discard pile (which is kept under a tarp, protected from sunlight). Porsche Bill told me he did have some recent discards from some customers of his and that I was free to take what I liked (thereby saving him his $2/tire disposal fee). As we all know, Porsche guys are rich; they throw away perfectly good tires (Jordon, once you start tracking that beast, you can ship me your old tires. Thanks)! I was able to patch together a set of 27/65-18 Michelin Touring/GT slicks; each had at least 2mm of rubber left according to the wear indicators. Two were of the medium compound (A8H) and two were of the hard compound (A9H). I had these mounted up on my 18*10s; to my eye, they fit perfectly even though Michelin would prefer an 11Ē rim be used for them. I then used the Michelins at a TNiA session at Thompson. Much better.

https://www.dropbox.com/s/6r6zqvatpb...GoPro.mp4?dl=0

With functional tires on both sets of auto-x rims, it occurred to me that I would be able to do a same-day tire comparison at the next auto-x. In the New England Region, we almost always get 3 morning runs and 3 afternoon runs. As it would be very easy for me to swap tires in between my run groups, on July 17th, that is just what I set out to do. It was a beautiful, low-90* day and the course was free of marbles by the time I ran in the third heat. While there was a threat of rain, it never materialized.



In addition, I had a co-driver; a former roommate of Grant Reeveís, John is a former MR2 road racer who has done exactly two auto-xesóone 13 years ago and one 10 years ago. John has heard me talk about doing TNiAs and wants to drive my car at one. I told him he would have to auto-x first, so he decided to co-drive with me at this event. Despite his lack of experience, both in auto-xíing and in my car, I also thought it might help to have data coming from someone who was completely unfamiliar with the car and the tires. Plus, he could give me a hand checking pressures, taking tire temps, and taking durometer readings.

For the tire comparison test, I borrowed a pyrometer and durometer from my friend and former regular co-driver, Jeff Seeger (Batchman). My methodology was planned to be as follows:
1. The morning runs would be done on the bias ply tires as those were the ones the carís front camber (-2*) was set for; plus, I figured it would be easier to go from the mushy bias plies to the radials rather than the other way around.
2. Measure the starting cold durometers of all 8 tires. If I had been really smart, I would have done this before mounting the wheels to the car and then put the softer tires on the right wheels (the course ran counter-clockwise).
a. Bias ply (all on 16*12s)
i. Fronts (25x12-16 R35A Hoosiers)
1. Left: 64 Right: 73
ii. Rears (25x12.5-16 Avons)
1. Left: 53 Right: 60
b. Radials (all on 18*10s)
i. Fronts (27/65-18 Michelin GT/Touring slicks, A8H medium compound)
1. Left: 57 Right: 52
ii. Rears (27/65-18 Michelin GT/Touring slicks, A9H hard compound)
1. Left: 68 Right: 68
3. Set the initial starting cold pressures to the following psi:
a. Bias Ply (25 front/23 rear hot goals)
i. Fronts: 23
ii. Rear: 21
b. Radials (based upon advice from Porsche Bill that they worked best with hot pressures of 32-36 psi)
i. Fronts: 34
ii. Rear: 30
4. Measure each tireís average durometer after each run.
5. Measure each tireís Inner-Center-Outer tire temperatures after each run.

Now, the really astute reader will have already figured out one major flaw in my plan: by being a two-driver car, we would have very little turnaround time between drivers. Plus, my co-driver, being inexperienced with auto-xing, just wasnít familiar with the speed of the two-driver routine; so there were some growing pains (we forgot to re-number the car after my first run, etc.).

The first speedbump occurred before I had even taken my first run (I was driving first so that he could ride with me to see the course at speed): we had overinflated the tires the night before to make sure we didnít have to add air in the morning. While letting air out of the bias ply tires, John got the fronts set but then left to use the facilities. Due to a miscommunication, I thought he had also got the rears set. So, when I hopped in to take my first run, the fronts were at 23 while the rears were somewhere between 35 to 40. Yeah, not good. That run sucked.

Once we got the pressures sorted out, the bias ply tires quickly stabilized and we left the fronts at 23.5 front and rears at 21.5. We did not make adjustments based on tire temps as we just did not have the time in between each run to take temps; instead, I based these pressures off of data Jeff and I had taken during previous events.

After my second run, I was able to take durometer readings of the bias ply tires:
1. Fronts (Hoosiers)
a. Left: 64 (no change) Right: 62 (down from) 73
2. Rears (Avons)
a. Left: 53 (no change) Right: 51 (down from 60)

After my third run, I once again took durometer readings:
1. Fronts (Hoosiers)
a. Left: 65 (up from 64, probably an error) Right: 65 (up from 62, ditto?)
2. Rears (Avons)
a. Left: 53 (no change) Right: 51 (no change)

After Johnís third run, we finally were able to take tire temps (seen from plan view, car front towards top of page):
1. Fronts (Hoosiers)
a. Left: 140-145-143 Right: 140-147-152
2. Rears (Avons)
a. Left: 137-137-136 (nailed it!) Right: 137-144-146

Looking at the tire temps, it is now clear that the front bias ply tires could, in fact, use more negative camber. Also, the inside tire was a touch over-inflated. Normally, I do keep the inner tire a psi or two lower than the outer; I just failed to do that today as I wanted to simplify checking tire pressures and not confusing myself during hectic driver changes. Not a lot I can do right now to add negative camber to the rear. Mathisís rear axle camber trick may be in this carís futureÖ

Ok, that was a lot of information, but how did the car feel and how were we doing? The car, to put it mildly, felt mushy as hell. Iíve already been over my carís setup with Jack Hidley of MM and he has helped me to understand that my carís rear is under-sprung and needs about 135# more spring rate to get the front roll stiffness distribution down (the car understeers, which is why I planned to mount the harder compound Michelins on the rear for the afternoon runs). The course was, imo, VERY busy and the bias ply tires were terribly slow to react to steering inputs; as a result, both John and I felt like we were continually behind the car (which youíll see from the video linked below) and, despite having auto-xed on bias ply tires for almost a year now, I still canít get used to the slip angles. I was looking forward to throwing the radials on for the afternoon and dropping lots of time as I felt they would respond better to the busy course.

https://www.dropbox.com/s/9rocallisv...n%203.avi?dl=0

'C Prepared' - Heat 1 of 2
Scott Jessurun 1983 Chevrolet Camaro 60.731+1 59.361 58.730
Phil Mackaronis 1994 Chevrolet Camaro 61.145 58.788 58.748
Casey Brown 1992 Ford Mustang 61.097+1 60.684 59.614+1
John Brennan 1992 Ford Mustang 65.384+1 63.334+1 62.058+dnf

A quick note about my competitors in order to make myself feel better (i.e., excuses).
1. Scott is in a ďrealĒ CP car and has been to Nationals before. However, back then, the car had a v6. Nowadays, his car weighs 2720 lbs, has a 4.8 aluminum LS block, and makes good power (400 hp? 450? I always forget). He completed the swap last year but has been fighting oil, sound, and belt throwing issues ever since. Sunday was his first auto-x of the year and, luckily for him, he had no issues. However, he is hampered by his tires: he is running 15*10s with used circle track Hoosiers.
2. Philís catfish is similar to my foxbody in that it is a work in progress and is not yet a ďrealĒ CP car. However, he is on A7s (3rd or 4th event on them). Bastard.
a. Wow, look at happens to the A7s once they get warmed up and Phil gets one run under his belt.

After our first set of runs, John and I parked the car to let the wheels and brakes cool and then grabbed lunch from a food truck that is on site for each event. They started showing up last year and this has been huge for me; obviously I canít drive the car on R-compounds into town to grab lunch (and I always forget to pack a sammich because my wife thinks I should have to do that sort of thing on my own).

After lunch, we just barely had enough time to swap on the 18*10s with the Michelins before having to report for our afternoon work assignments. After that heat was completed, we had another free heat during which I did one final once over of the car before pulling it into grid. Once there, we double checked the pressures (TWICE! I really wanted to be sure we started off correctly this time), took durometer readings, and then it was time to race.

During my first run, I was convinced that the car was at least 2 seconds faster. The radials were much easier to drive as they were more predictable with crisper response and turn in. John felt the same after he took his first run of the afternoon. The pressures stabilized within a run or two at 32 rear, 36 front, and we left them there. We did spray the tires down before my final run, which was slower but I may have pooched 2 sections (but Iíll blame the colder tires, thank you very much). Unfortunately, we just didnít have the time to take durometer or temp readings until after my final run (I had gotten a re-run and took it out of order, so I drove first and last in the heat).
1. Fronts
a. Left: 49 (down from 57) Right: 50 (down from 52)
2. Rears
a. Left: 47 (down from 68!) Right: 44 (down from 68!)

Holy crap, thatís a huge durometer change for the hard compound tires that I was not expecting.
After my last run, temps were:
1. Fronts
a. Left: 132-134-125 Right: 133-150-153
2. Rears
a. Left: 120-129-122 Right: 120-141-135

As expected, the radials need more negative camber. Last time I ran radials, the Continental GT-Rs, they seemed to like -3.75*. Looks like 32 psi was too much air for the rear tires. Not sure yet what to make of the front left readings. Over-inflated by a psi or 2 and needs more camber, I think.

https://www.dropbox.com/s/d1fq2ke7xt...rerun.avi?dl=0

ďBut what about the times, man? Were they any faster?Ē I hear you, I hear you.
'C Prepared' - Heat 2 of 2
Phil Mackaronis 1994 Chevrolet Camaro 59.149+1 58.410 57.633+1
Scott Jessurun 1983 Chevrolet Camaro 59.007 58.672 58.490
Casey Brown 1992 Ford Mustang 60.214 59.279 60.545
John Brennan 1992 Ford Mustang 61.925+dnf 62.138+2 63.475+2

I was dead certain the radials had proven to be significantly faster. I was wrong. Extrapolating that I would have had a small improvement in times based on the average of the improvement both Phil (0.338 seconds) and Scott (0.24 seconds) both showed in the afternoon (clean runs only, average = 0.289 seconds) with no changes to their cars, I am comfortable in guessing that I would have run a 59.325 clean, probably on my 2nd run of the afternoon heat (I tend to over push it on my final run if I am not in the lead, a flaw I am aware of. See the results above, and from events dating back to 1992, for confirmation).

ďBut, Casey, you based your estimate on your fastest RAW time from Heat 1, not your fastest clean time.Ē
Shut up, itís my simulation. The important thing to note here is that the estimated 59.325 bias ply time is less than 5/100ths of a second from the actual 59.279 I ran on the Michelins.

But how did John do? Yeaaaaah, weíre not going to talk about that today. Freakiní MR2 guys.

https://www.dropbox.com/s/7an16eln8a...n%203.avi?dl=0

Some quick tire size info and math:
ē The bias ply tires provided a total of 49 inches of tread width. Assuming the car was ~2,900 lbs with 210 lbs. for driver with helmet (Iíve been doing the 21-day Fix, thank you very much), thatís ~63.5 lbs/tire inch. The rear tires had a diameter of 24.75 inches per the Avon spec page. My CVS Pharmacy bathroom scale says the Avons weigh 41.8 lbs each. I didnít measure the Hoosiers but they should be similar.
ē The Michelins provided a total of 1,080 mm of tread width. Converting that to inches, because the French have not yet put a man on the moon, thatís 42.5 inches of tread width (~13% less tread width than the bias plies). The tires have a diameter of 25.4 inches per the Michelin spec sheet. My scale says they weigh 47.6 lbs each. Thus, this setup is ~23 lbs heavier. Thatís 73.7 lbs/tire inch.

There was no appreciable performance difference between the two different tire setups I used yesterday. While the Michelin radials felt stupendously easier to drive, more predictable, crisper, sharper, insert another adjective here, the over-size oddball bias ply tires used their ~13% contact patch advantage and slightly smaller rear tire diameter (which equates to slightly more aggressive rear gearing) to be just as quick.

If I get really froggy, I will attempt to do a similar comparison at Thompson in CT or Palmer in MA. With fewer transitions, my money would be on the bias plies being faster, if I have the stones to push them.

Conclusion: The Michelins are, inch for inch, a faster tire than the bias ply combo tires.

Bench racing conclusion: Had the camber been adjusted to better utilize the Michelins, and if lightweight 18*12 wheels were available to run 31/71-18 Michelin slicks (48.8Ē inches of total tread width), Iíd have gone 13% faster and won CP and would have lost TToD only to the B-Mod cars. Thatís how tires and math work, right?

Begging conclusion: If anyone out there has any connections with SportsCar, Grassroots Magazine, etc., please tell them that I would like to test, for real, a set of Hoosier radial slicks on the 16*12s against a set of Hoosier A7s on some 18*12s. I canít afford any of the above, so theyíll just have to have their sponsors help out. Happens all the time, right? Lookiní at you, Andy Hollis.

Thank you.

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post #2 of 4 Old 07-19-2016, 10:06 AM
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I just want to know how did you even get on the track with the illegal wheel/tyre sticking fast the fender, I had to have fender flares up front in order to pass tech. And not going to lie those old slicks and old wheels look makes me cringe, i dont know why just not my style, Yes i know its racing who cares what it looks like but that screams 1980s.


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post #3 of 4 Old 07-19-2016, 10:30 AM Thread Starter
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In the 80s, people were running 16*10s. We'd consider those small tires in CP these days.

At TNiA at Thompson, I ran the 18*10s, which don't stick out (see the second auto-x video). Plus, there's no tech at TNiA.
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post #4 of 4 Old 07-19-2016, 04:22 PM
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Quote:
Originally Posted by aurdraco View Post
In the 80s, people were running 16*10s. We'd consider those small tires in CP these days.

At TNiA at Thompson, I ran the 18*10s, which don't stick out (see the second auto-x video). Plus, there's no tech at TNiA.
My track wheels are 18x10 Nitto 275s (baby tyre) i know

I cant imagine or actually i can, running 18x12s with 315s all four

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