bacalhau16

iTrader: (32)

Hey that was a nice stab at me. You must be a pretty funny guy! I believe you got most of the answers you needed though so Im all done here.

First350, thanks again for that fuel tap, and good luck this season with the new tires.

Triple8Sol

iTrader: (218)

Very nice Peter...I'll have to check these out some time. I'm running the same size in the Avon R-comps, and once I kill the tires this year, will be in the market for something new...

Kirkster

iTrader: (2)

I am running 285 / 30 /18s all the way around on my track rims...

z-u-later

iTrader: (43)

Awesome! Do you have some pics of your setup? I just wanted to know what the 285s look like on the track rims. Also, what tires are you running with since I did not see the RT-615 in 285s?

jun14scr

iTrader: (1)

no stats needed. we are telling you what we all have learned through experience and through rational judgement. like many have said, the effective gearing on your car will cause it to be a bit slower than the 285 18 como, a 275 18 combo, and even a 245 18 combo in relation to how fast it accelerates. i would consider ullose's opinion to be one of the best out there, he is not a national champion for no reason. also, having multiple people who have been racing for quite a while also believe this should be more than enough reason to take this to heart.

twenzel

iTrader: (27)

Yea, I ran 265/35/18 on the track wheels front and rear before I had the Advans. It really didn't have as much overhang as you would think.

drivesolo

iTrader: (6)

IMO... 275/35-18 FTW.

275/40-17 FTWL (For The Win - Locally).

jun14scr

iTrader: (1)

285/30/18 FTW.

275/40/17 FWTL.

ofcourse, this also depends on what trim level your car is.

first350

iTrader: (24)

automatically defaulting to the lowest raidus Wheel/tire combination is not the reaon why a 30/35 is better than a 40 series tire...while going to a smaller radius wil help with acceleration, there's a reason why everyone is not on go-kart wheels for auto-X - we'd be in 6th gear!

IMO, tire profile is more of a function for suspension/handleing. Rim width/radius is performance/gearing (even though the profile is included in the total radius)

-peter


*Hoosier's and Kuhmo's autoX tires are both 275/40/17...does anyone know why they don't go with a 30/35 series?

ULLLOSE

Most tires sizes are a combination of OEM sizes and rules of the various racing organizations.

For instance in SCCA club racing the GCR list a spec tire size for most SS and Touring class cars. The GCR also allows most cars to vary by 20mm in width and 10% aspect ratio.

So if the GCR specs your car with a 275-40-17 you could run a 295-255mm width and 30-50 aspect ratio. Road racers use more tires in a weekend than autocrossers do all year so they tend to drive the sizes.

In autocross stock class the only rule we have is whatever size DOT tire we can cram on a stock size wheel. So most of the time you will see people going for the widest shortest tire possible.

btw Hoosier and Kumho both make a 295-17 as well.

DavesZ#3

iTrader: (26)

I agree with you that handling is an important consideration when picking profile, that's one of the reasons I went as low as I could. Obviously you don't want to go so small as to need to shift beyond 2nd gear. In the 2½ years I've been autocrossing the Z, I have hit the rev limiter just but a few times in 2nd. On stock tires, that's around 60mph. Since we rarely have courses that allow speeds that high, I figured a shorter tire would allow me to get closer to the limiter in the 50mph range that we typically max out at.

My autocross tires (265/35/18) are 1.3" in diameter smaller than my stock ones were. The acceleration difference is very noticeable. It's somewhat harder to measure the handling improvement as it's an apples-to-oranges comparison. I picked the 265 over the 275 (Falkens) because they had the same tread width but the 265's have reinforced sidewalls and the 275's don't. That helps with handling but again, I have no direct way to measure it or feel it.

first350

iTrader: (24)

in my mind, it's similar to 1/4 drag racing...you want to finish the 1/4 mi just at redline (minimize # of shifts, while maximize your power). It's a bit harder for AutoX b/c we don't have a set course, so our max speed changes...I've actually had an AutoX course where I was near 85 MPH (in Cali w/ a group that had mostly American muscle cars); but since then, I'll hit the rev limiter in 2nd gear only 10-15% of the time...that's also why I increased my redline.

-Peter


***I just got the tires mounted tonight...is there a set way to break in AutoX tires? I know you can get tires that have gone through heat cycles...should I try something like that before taking them AutoXing?

Fluid1

iTrader: (19)

There is a give and take for every tire size. If only one tire performed better than the rest, it would be the onlyone selling. As ULLLOSE said, you can argue all you want about those tires, but they likely won't win you Nationals. If you're not after the Nat'l title, you will have a great time autoxing on them locally.

jun14scr

iTrader: (1)

+1.

so.. when can i see those wheels fluid!

Kirkster

iTrader: (2)

I see the rev limiter on almost every autocross that we attend. Sometimes not on the first run but usually by the fourth I have hit it at least once or twice on part of the course.

I am running Hoosier A6 tires to answer the tire brand question.

DavesZ#3

iTrader: (26)

In what gear, 1st?

I know from talking to different people that some clubs run huge courses that allow for higher speeds. In those cases I can see where hitting it in 2nd would be more common.

z-u-later

iTrader: (43)

Below are the Falken Azenis RT-615 specs. I looked at the 18" tires closely and noticed that the 265/35/18 clearly stands out as the best choice for performance. The RPM alone @ 852 blows the other sizes away. It also has a nice thread width (10.0) and is reinforced. It only weighs 28.7 lbs since the OD is only 25.4.

http://www.falkentire.com/rt615_sizes.html

betamotorsports

iTrader: (1)

One thing to keep in mind when discussing handling and tire sidewall height is compliance. A tire is a spring just like a coil spring. Being a spring, a tire has a spring rate that comes from the air pressure and sidewall stiffness - with most coming from the air pressure. Typical tire spring "rates" are around 1,200 to 1,500 lb. in.

A siffer sidewall reduces how responsive the tire is to small bumps that are not big enough to move the suspension due to friction and inertia in the suspension itself. Taken to an extreme with very short sidewalls and high air pressures a tire stops gripping as it should and becomes skatey. It feels responsive but ultimate grip is reduced. Once way race engineers reduce this effect is to reduce friction and intertia in the suspension itself with the most effective methods being unsprung weight reduction and high dollar, multi-adjustable shocks.

DavesZ#3

iTrader: (26)

Yeap, that's pretty much why I picked them. I couldn't see an advantage to the 275s since they have the same tread width and the sidewall is only 0.1" higher.

z-u-later

iTrader: (43)

My head is hurting from reading what you just posted . . .