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post #1 of 45 (permalink) Old 04-13-2011, 01:52 PM Thread Starter
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Bigger Tires

I'll be in the market for new tires some time this season, and I would like to get a little more rubber under this car. What is the largest tire size I can fit on the stock wheel size (18 x 8, if I remember correctly) withouth rubbing? I seem to recall someone recommending 265/35R18s, but I'm a little concerned about losing rolling radius.

Has anyone run that tire size in an autocross? Do you have trouble with topping out 2nd gear on larger courses (maybe not so large courses with the shorter tire)? Does 3rd become at all useful with those tires?
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post #2 of 45 (permalink) Old 04-13-2011, 04:25 PM
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5%

that is a 5% loss in diameter - according to the 1010tires.com calculator - the 265/35/18s fit wheels from 9 to 10.5" - whereas stock 245/45/18s fit from 7.5" to 9" wheels.

aslo lets you compare multiple sizes a the same time - 265/40/18s are only 1.28% smaller though still indicate 9 - 10" wheel.

255/40/18 is 2.44% smaller - fits the same wheel.

that site also has a warning that more than 3% difference can result in brake failure - doesn't explain the potential failure modes

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post #3 of 45 (permalink) Old 04-14-2011, 03:45 PM Thread Starter
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Your brakes won't fail in any catastrophic fashion, but you may run into problems if you lose vacuum at your booster due to reduced braking force at the road with a larger diameter tire... I'm a brake engineer, so I've already thought all that through.
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post #4 of 45 (permalink) Old 04-14-2011, 06:47 PM
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While we're on the subject, as I will also be in the market for new tires before NASSM, are there any wider tires we can put under these cars without rubbing or altering the wheel well? I already have my replacements picked out so more than likely I'll be getting those and sticking with the OEM size anyway, but it'd be nice to know if there's an option for a wider footprint, especially in the rear.

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post #5 of 45 (permalink) Old 04-14-2011, 10:23 PM
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I know there are some with bigger tires than I, but I have 275/35/18 on 9" Rims in the back.

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post #6 of 45 (permalink) Old 04-17-2011, 12:16 PM
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I have a set of Hooziers that I run autocross on. 295/30-18 A6s all 4 corners. Stock wheels. They do fit. They also lower the car about 1". Inside of the right front tire does hit the front sway bar at full left steering lock (rubs a little but not enough to really hurt if you rotate them). But they DO hold in the turns.

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post #7 of 45 (permalink) Old 04-18-2011, 02:02 PM
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wow

Quote:
Originally Posted by SkySoldier View Post
I have a set of Hooziers that I run autocross on. 295/30-18 A6s all 4 corners. Stock wheels. They do fit. They also lower the car about 1". Inside of the right front tire does hit the front sway bar at full left steering lock (rubs a little but not enough to really hurt if you rotate them). But they DO hold in the turns.
interetingly those tires are larger diametr than stock - compared to the smaller than stock diameter for the tires the original post indicated.

I say wow becuase those tires are speced for 10.5 - 11.5" wheels - compared to stock tires fitting 7.5 - 9.5" wheels. While I am not saying that you cannot get a tire to mount on a wheel outside the defined range - you can end up with problems with the sidewall and load and rolling friction etc.


As for earlier posts about brakes - I suspect they are refering to changes in the centifugal forces based on changes in the mass of the wheel/tire combination - having an impact on the load under braking which could adversely affect the life of the brakes - and outside of a track environment where you might be doing a brake inspection every day (or more often) you coudl end up with a problem that could result in a failure in a time frame that you would not normally expect for a stock setup.
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post #8 of 45 (permalink) Old 04-18-2011, 05:32 PM
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Quote:
Originally Posted by MCW Sky View Post
interetingly those tires are larger diametr than stock - compared to the smaller than stock diameter for the tires the original post indicated..
Your pic has the size listed as 295/30/19....the size quoted above is 295/30/18

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post #9 of 45 (permalink) Old 04-19-2011, 11:30 AM
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Oops

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Originally Posted by GeeTeeOh! View Post
Your pic has the size listed as 295/30/19....the size quoted above is 295/30/18
Oops - here is an update with 4 sizes compared to stock.

295/30/18 is almost 6.5% smaller diameter.

And a second chart with other sizes - not saying that I recommend any of these sizes or that they will fit without issue - just trying various data points to see what they look like.

It really depends on whether you are wanting to stay with the stock wheel and or stock wheel diameter.

sorry - the attachments don't seem to be working so well - they are clear and readable on my system - but after uploading they are too small and zooming in is not so good - in any case I am using this website Tire Size Calculator - tire & wheel plus sizing - there are others - the Miata Garage has one as does TireRack.

hmm - trying upload again with jpg to see if that is more readable.

that works better - changed my default screen capture to jpg from png

defaults write com.apple.screencapture type image_format
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'09 RL Ruby Red Manual
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Retracting motorized front license
HID low beam & fogs
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Last edited by MCW Sky; 04-19-2011 at 12:40 PM.
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post #10 of 45 (permalink) Old 04-20-2011, 08:25 AM
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Have any of you here ever had a flat tire on the road? What did you do?
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post #11 of 45 (permalink) Old 04-21-2011, 04:07 PM Thread Starter
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Haven't gotten a flat with this car, but if it was just a nail or screw throught the tread, then the fix-a-flat and air compressor that came with the car should do the trick... for a little while. Other than that, you're going to have to call a tow truck.

Going back to the brake stuff, your brakes don't really care how much centrifigul force your wheels and tires are seeing. They do care about the inertia of the wheel/tire combo, but the change in inertia due to a large tire will be relatively small when compared to the vehicle inertia acting on that corner. As I mentioned before, the change in rolling radius (for a larger tire) will decrease the braking force at the road, meaning that you need to put more force into the pad to stop the car in the same distance you would with a stock tire. This will cause premature wear out, but we're talking a drop of probably 10 - 15%... so it's not like they will wear out in three months. Changing tire size (in either direction) can screw with your ABS/TC/ESC system, but as long as you tell the car what the new rolling radius is it shouldn't be a big problem. This, of course, assumes you are purchasing quality brake parts. Cheap pads and rotors will have a whole host of issues that will be made worse by changing tire size.

Finally, going back to the A6s, at what speed do you top out 2nd gear now? Do you find yourself in 3rd more often, or do you just let it bounce off the rev limited for longer?
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post #12 of 45 (permalink) Old 04-22-2011, 04:01 AM
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Quote:
Originally Posted by kudelt07
As I mentioned before, the change in rolling radius (for a larger tire) will decrease the braking force at the road.
Do you happen to have a quick explanation or a link to something that explains this? My brain is refusing to grasp this concept (granted it is late) but I don't want to derail this thread with a physics discussion

The things that seem to be keeping me from grasping this:

- I have seen way too many people say that the unsprung weight is a big deal to readily accept how you dismiss it as not mattering in regards to braking. I am not an engineer or an expert mechanic though, so I ask questions.

- Also... bigger tire (to an extent) = bigger contact patch (more on this in a second) = more friction/traction area = better braking? Unless... Are you are just saying that keeping width equal, a smaller radius tire has better braking action? (which isn't quite the same as the unsprung mass concept, even though the unsprung mass would be less in a smaller radius tire)

Ugg! Not tracking this AT ALL.

---

I did have a few other things to mention...

Looking at the generic sizes and what those are supposed to yield is one thing. Make sure you are also checking out tread width as well. I was also looking to see if there were contact patch gains to be found in changing up tire sizes. I noticed that an increase in size does not always yield a bigger tread width. (sadly, tire rack does not always list tread width for every tire)

Here's the first example I could find (tread width is in bold):
Continental ExtremeContact DW
275/40ZR17 25lbs 9-11" 10.2" 25.7"
285/40ZR17 27lbs 9.5-11" 10.2" 26"

So, in this case you add 2lbs per tire, raise the car .15" and have esentially the SAME amount of rubber meeting the road (maybe .1" more do to the increased circumference).

I have seen this on several lines of tires. Knowing how the tire molding process works (thank you so much "How It's Made"), I am gonna guess that some companies reuse the same tread mold for different sizes.

It's just one more thing to check when looking for tires

---

And then...

Quote:
Originally Posted by MCW Sky
I say wow becuase those tires are speced for 10.5 - 11.5" wheels - compared to stock tires fitting 7.5 - 9.5" wheels. While I am not saying that you cannot get a tire to mount on a wheel outside the defined range - you can end up with problems with the sidewall and load and rolling friction etc.
Those Hoosiers are just racing slicks in disguise

Hoosier A6

They're stupid soft and insanely sticky with some groves cut into them so they can pretend to be "street legal"

If you look around the threads in this section you'll see people talking about the funky tire pressures and alignments to make these beasts work. Even then the life expectancy is pretty darn low with in comparison to a normal tire.

---

Ok... off to bed...
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post #13 of 45 (permalink) Old 04-23-2011, 09:12 AM
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Centrifugal force

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Originally Posted by Nova-Exarch View Post
Do you happen to have a quick explanation or a link to something that explains this? My brain is refusing to grasp this concept (granted it is late) but I don't want to derail this thread with a physics discussion
short answer - centrifugal force - as you increase the diameter of the wheel and or width of the wheel/tire - you are generally not only increasing mass overall but also moving that mass farther out from the center of the hub - meaning there is more momentum in the spinning wheel that needs to be overcome by the braking force applied.

Or in other words - two identical vehicles traveling at the same speed - where one had 245/45/18s and the other has 285/35/19s - unless the second has magnesium wheels or some other factor that compensates - will have more momentum that spinning mass will have - meaning more braking effort required.

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post #14 of 45 (permalink) Old 04-23-2011, 12:33 PM
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Quote:
Originally Posted by kudelt07 View Post
I'll be in the market for new tires some time this season, and I would like to get a little more rubber under this car.
Always curious why people want wider tires. A theoretical performance advantage, or a style statement? Just curious.

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post #15 of 45 (permalink) Old 04-23-2011, 12:37 PM
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I really did not read all of this but I did notice the post right above me here... (Oh, not you Bill, you posted while I was typing!) And Bill, ABSOLUTELY there is a need for bigger tires other than style or looks. My car on stock tires would be dangerous. The GTO's are a perfect example. Those cars are super limited on rear tire sizes, they'd be happy to have tires as big as our stock sizes. What happens is they end up getting huge HP out of the motors, but all they do is spin the tires. The only way to make a fast GTO is to roll the fenders or tub it.

The theory of centrifugal force I think is a minor factor when you're talking about braking force. I could be wrong, and I'm not going to research it for an hour to find out for sure, but this is how I see it...

The analogy of the cars traveling at the same speed is flawed when you're talking about centrifugal force on braking. I'm not saying the results are wrong, I'm saying it's the wrong way to look at it. A valid comparison would be to take the vehicle weight out of the equation. Spin up a heavy wheel/tire combination on a fixture, or with the car on a rack, and measure how fast the brakes can stop the wheel from a certain speed. Then spin up a light wheel to the same speed and measure. Obviously you're going to stop the lighter wheel faster, I'm not arguing that. But without trying to slow down 4000lbs along with the wheel and tire weight, the difference between the two combinations is going to be minor. I've stopped plenty cars while running them on the rack, and believe me, you can stop all that centrifugal force pretty damn fast. WAY faster than stopping the actual moving car. I'm gonna really guess here but I'd say 90% of the braking force is used to slow down the actual vehicle weight and 10% is used to slow down the actual wheel/tire/hub. I'm judging this by how much faster you can stop the wheels in the air vs. on the ground. Running the car up to 100mph on the rack and you can stop it in a second or so, run it that fast on the road and it will take maybe 10 times that long.

As far as all this tire size crap goes... Search the forums for my posts on what tires work on the stock rims. TIRE RACK IS NOT THE END ALL AUTHORITY ON WHAT TIRES FIT WHAT RIMS. For several reasons, most of all, liability.
275/40/18's WILL fit on the stock rims, WILL NOT rub and WILL wear perfectly evenly. I'm running 275/45/18's on the back of my Sky right now (mo powwa, mo rubba). They did have a few rubbing issues, but nothing that's not fixable. Absolutely the best traction I've had so far, they are insanely better than the stock crap rubber and size. Last time I ran the car hard, the front wheels were so hot you couldn't touch them for at least an hour after it was parked. I know what works and what doesn't.

Last edited by gmtech16450yz; 04-23-2011 at 12:43 PM.
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