the how wide can I go debate

63XL500

Junior Member
Joined
Jun 14, 2010
Posts
28
Reaction score
0
Location
Fort Worth
Well, the car ran pretty good. I wasn't trying to launch too hard for the first time out at the track so best 60-foot was only 2.02. Best et was 12.43 at 117. Happy with the mph but now I need to get it off the line. The first run it roasted down as soon as I let out the clutch because I couldn't go around the water box and didn't clean the tires off good enough and I missed 3rd and still hit 115 (13.4). After that I was sneaking up on my 60-foot, basically launching at idle and rolling into it. The 1-2 shift resulted in no spin at all so for a sticky non-drag radial tire that will last a little longer I am happy with them. But next time I'll be pushing them harder now that I am more comfortable with the car. It has an Exedy Mach 500 clutch and didn't seem to have any issues with slippage.
 

Quorra

Junior Member
Joined
Oct 4, 2014
Posts
6
Reaction score
0
There is a ton of bad info on tires and rims on this forum and the internet in general. I’ve tried to resist posting but just can’t anymore. I’m an engr. I’ve worked for 2 OEM’s and 1 major tire company. I normally don’t post much because I don’t like posting my resume to prove that I know what I’m talking about. But here goes. Sorry it got a bit long.

First off, tire manufactures do not establish tires sizes or wheel sizes. The Tire and Rim Association does. It was invented over 100 yrs ago to insure compatibility across manufactures. They also establish speed rating tests plus load and inflation requirements. All the info you see on Tire Rack comes directly from T&RA. 90% of the info tire manufactures publish is straight from T&RA. Some tire manufactures do publish data measured from their tires in production. But they will never publish data outside of the T&RA guidelines. In order to be D.O.T. approved, you must meet the T&RA standards.

l_b7wuQ2bGOMJQdkhMSpFUyU70TjV4-XFditel4rIrCDnYUYloIFU5stCWP0w6q7ndOlPxpUxeRj1wPDIEoWn4ifcOM4c_7mKqTuhTxjnH20GKEFS1LnS7dw_SETZGCm9e2Pv3tr5FK7e-lF8VpO70Y_Es3MzMmna_IRW2cAcfZFqktk1s1por6nahsNwD0yKj0ErkAL-trS6kS-AcZM_rKaw0RZhkBzvHHMSoq5xLP5VBV-QRdtVdNIDKdFfO1dvI0BwGk3aPKzsEHafWn5oNh2VJfgnESyuJQpL6fZ7q1pCr4bAKTtX9T_PTswys2YlH0wEdJDGJ05wJVGSVBhVqfPt4Y9zgOcW_dzq8lKZdBqt3K89WC5pbw2hCRJueJ7rr7rHhRIxQmtbCl_pzSNPN-5M71k25-qLzXybkKbK-M3CE4XTow3tZpDjq4iBniU3FAw5G8627t_4qZFcX_jj9-8X7WCdteavRcDx52L_8mUyIIMn-0LB0dHpVBFLLaCiXVu5iLF4ISq7sbPMAWW5j72MJC4DMB99a-x9d5O_FXedrvNiKnVNiU0ANN7epiHgkhdxt67cOOWN0dn-ZJZQnoDxXf3SMHEWQwC0U4yhP1UFiDJw4mO=w1079-h972-no


Included in the T&RA specs are minimum and maximum tire sizes. And yes, there is enough tolerance such that a tire that is actually a 300 width could be sold as either a 295 size or a 305 size. Car and truck tire sizes are every 10 mm width ending in 5 (275, 285, 295, etc). Motorcycle tires end in 0 (140, 150, 160).

JvWu71XhNKWIKxNGJVdQhk0J07DFLtopKfhLo6YTGhgB6hJUGiJISZfmRbp5cZOmarkJjkP8wjnYWZVnyEr6Mcfmkfv11w_JmWhnpHJHsfkKwF__XsmaCjlnHVdqBqFP7iXc3aUmOoGeMIlcs2nxri57Fmq6vdTRTC85MZOu4ZCDCFez4gxn7qFztBoGyBtNbqGN3j197cBZXTUGSJufLxUrCjJIA-9f4ijJf2cbIbKNCYWejnffws0HI4mUcnr_1lajDAbRewj4N87f4zNvyjuAAdCTJ530vXjdLlbrDWYbIzP_k-nyldfRRsGVZdMlYDXQhtq8y7qzKvWHOCeYmD8RUQj0ZkeRtC9egLozyDJFlI0oB0KCvuvfjVxrHY3_e1XIlR5l2pC0XiU4XvNkg3tBdnnjJF8AgHWM6MtXoXyJJW0w563ooTFgj1SXmYCDPKTstBfi00Gqs3NlBCSZcZPFvuU48i1KgqmG9jmbPxI9OLHJ2MZMH95PVmDR7VPx9eH2qjvz-jK-fd39rZbBeWJ6SkrWJ9m5EGc9TeU93qWZdr8WY97PPPqhysWHV_3JuRUcOMlgez3YwtTh8yYiiUqANbqRjzVenVR96wz2VHEOyYfmKO9N=w698-h883-no



“I've come to the conclusion that tire sizing is basically this....rim width + 1, then multiply by 25.4 and round up to the next closest tire width in the appropriate aspect ratio for the tire height you want.”

“General rule is to stay within 1/2 inch +/- of the tread width (not section width) of the tire. Considering most 275's float around 10 inches of tread width, wheels ranging from 9.5 to 10.5 inches would work best.”

Both of these are false. Aspect ratio is an important factor is determining rim width, especially when going outside of the approved rim width range.


“Aspect ratio doesn't really matter in this rule of thumb. You just need to select an aspect ratio that provides the desired overall diameter that is needed.
Overall section width is independent of aspect ratio.
A 275 tire, whether a 40, 50, 60, or what have you will always be about 10.8"-10.9" in width”


This is completely false. The lower the aspect ratio, the wider the recommended rim width.
245/60 7.0” 7.0-8.5” range
245/50 7.5” 7.0-8.5” range
245/45 8.0” 7.5-9.0” range
245/40 8.5” 8.0-9.5” range
That’s a 1.5” difference in recommend width and 1” difference in the range for the same 245 wide tire varying only the aspect ratio.


“M&H recommends 12" wide rims for that size, and minimum is probably 11"

This is not 100% accurate. M&H simply chose to only publish the T&RA recommended size. A 325/40 tire has a lot of sidewall and can easily go 1 inch outside of the T&RA approved list without failing due to rim width.

Here are a couple pages from the T&RA book.


Ny6nl09VgTXw3_rYXvI6RarJVumE74qbGoLRgWTHtYirfK-jlcKPfAkp4yWhXu8X6BtiD3lRUo_R21CB_vHjSK7AciiMH4_CZ2PgQZQx2oOrl-qLIMh7W_04j46U_FDTNfmb22CvqhXZDsIMlAfkFFwLH6rv5T_RHPUHGyadDJz9kXUfZHEOY_p2CdKMuVJyMpXwPGr0zYs4Zl3wI0PtaOp0TJQP6U4UiJfN-88oTJSxKRH7D_Ac8xo5xHFvB64dINtN7jCmlJI1M06SvaX7FrRVJNSy95wO0XQUKF__fabZhxi43wYkKmszB8VlxlfIvQT3EB5iFkupJkJSD2wR22XoSI5t09yukvm9PGSySLvfks5mh76b75OPDoyr8DA_sIPiTC-Is2VQLY4MhwHnkwlL1PmK5NCeJ29ZvIKKIdrOgfRk4ihyANTurh6fC_5CwaLhkjKOtHmavDXc7GZ1YF3ZoVTpH8v49QRgQp4U-UQpXqRfhSdYZcaIvn_XbF67addF5THgt37zXkRtaUbiu9F2Eoa_8VNbGoaw_qEZFPgjva3fxKzySoWBtn08A7D1qNwFSFLvrxy1OGooOC0DQE6KpA71mNS33lEwWlzWjS6K5ZofT7-z=w912-h740-no



qd_Hx6wzfxcCADWcnEUqSsOl_L_eM_itTs5EMQU_lvNXcweG33wgdfhfOSjCM5icQr9Pk-AVJ2Y2nYXHJ6W8ciLqp_PmGj3zy8FdNK08gjmhNX24q2aDT_NJuMuahoimOixsOzz82laCTqo_bJAiCgvX2pux7lgNGHaDaHjrqS3nza8j4A1bfmtY5MwAju0rHtP6L42VdzfRL0rthPJl_NpjNBVhxtnZBrxiO8aeuqdrGv-f0sVDOepXWsIi8erdm5WaTEtCX6urMY5zAnhx3b9UzwX8cpYzRvzpL2JNAolRqHl9xAn7NmxqB2u7vLl4jeOXaYxzmPgVNPgCbOE68C7rRaImeIs_SR2wAbpgZSlwXHoVpwU0IxwEl6kEfLEOoVoX1GayLys17YCUsRN-YtY-3AeQI2xHzW6RD6dKpCs7cIa2VivPAS1ApfzXnPydeOttpD5x_RQEhzmLlzWsPtHbB9YEIBs3pZnr6IrAFjavZRBcVVRgRc1dIUKLZC3Gl2lWk_WxG1DTJdt87S5dtuIIhFeNi2BmqfgCjqkh6xrhhkz7728p2IVNU_g1lbqVDYr5QMnmMWSH8UinHCuDWDrFEUtFM0fGkp82aZQo4h3OJmGJvRyY=w893-h940-no



“but like Frank said, the Mustang world is filled with people who don't think much past getting wide tires and you're at least partly taking their word for it. An 11" vs 10" shootout, same day, same car could prove enlightening whichever way it might favor.”

Sure that tire probably would be slightly better on an 11” or 12” wide wheel. But that’s not the point. You have a 10” wide wheel. A 295/40R18 is the largest size approved by T&RA for a 10” wide wheel. But that 325/40R18 is going to provide more traction than a 295/40R18 on that same 10” wheel (all other variables being equal).


“post 13, 275's on 8.5 rims
And it's still wrong. Just because Boltzman posts a picture of an non-recommended tire and wheel combination showing that it holds air still doesn't make it OK for general street use.”

That’s incorrect. That rim is only ½” below the T&RA approved minimum rim width. It is perfectly safe for road use. There is a huge and I mean huge safety factor built into tire specs.

As an example, most people don’t know that passenger car tires and wheels are designed to withstand 120 psi. That is the max that most home air compressors will put out. This requirement was enacted so that you could not kill yourself by over-inflating your tire at home. However, when you go to auto-x forums and talk about running high tires pressures, say 40 psi, there is always some Dudley Do-Right preaching that the sidewall says 35 psi max therefore you will die if you inflate to 36 psi.

My point is that one half inch is fine. In most cases a full inch too wide or too narrow is fine. In some cases, even more than that is safe. The tire will not fail. I’ve done it. I’ve tested it with lab equipment.


“It gets really hot here, though, so I don't want to have a tire heating up the sidewall too much from bulging out that it causes a blowout.”

This is not true. The bulge (or stretch) will not fail the tire from heat. Low inflation fails tires from heat. That bulge is relatively constant, it is not flexing as it goes around the tire. However low inflation causes excessive flexing as it rolls into and out of the footprint. That builds heat and fails tires.


Lastly, if you keep the rim size constant, the bigger the tire, the greater the traction. This holds true until way past what most Dudley Do-Rights think. Let’s use the 8.5” wide wheel. T&RA says that 255/40R18 is the widest tire they approve. But in practice a 265/40R18 will provide more available traction. So will a 275 and 285 but with decreasing improvements. At some point no further traction will be gained. Yes, the tire bulges (it can easily take it). Yes, there will be a slight reduction in steering feel or more precisely, it’s a delay in steering response because it takes a split second longer to develop lateral forces due to the non-ideal sidewall. But this factor is way less than internet wisdom would lead you to believe. On a racetrack (road course, auto-x, drag) a 275 wide tire will outperform a 255 wide tire on the same 8.5” wide wheel, all other variables being equal (which is never completely achievable).

As an example, look at the top header pic of this SCCA Super Vee forum. http://apexspeed.com/forums/ Do you think that rear tire is anywhere close to the T&RA tire width recommended for that narrow rim width? Wider and wider tires kept being developed to reduce lap times while the wheel size remained the same due to the rules.

However we should also consider the opposite approach. The opposite is to keep the same tire size and vary the rim width. If you do this you will find that there is a sweet spot in performance. And that sweet spot will line up very closely to the T&RA approved rim widths. And this is more like what the OEM’s do. They pick a tire size first and then the rim width or do it at the same time. Most of us already have a set of wheels and then try to put on the best tires that will fit, even if it is outside of the T&RA approved limit.

Ok, I’m done, flame away if you must.



Racer -

I realize this post is years old but wondering if you would cover one final piece to this debate for me..
I'm currently ready to switch to 275/40r19's on my 8.5" OEM rims conservatively covered with the factory wrapped 245/45r19's from conception..

What was left out of the applause worthy treatise above, or at least, the final piece I need before retail purchase is whether or not a staggered tire setup would enhance bite on cornering or if there's more merit in going with a wide-stance square setup all-around.

In this particular case: 275/40 rears with 245/45 fronts versus 275/40's on all 4

Would you be so kind to address this when you have a moment as this inquiring mind would surely love to put this question to rest, adding it to the vault of gems you've previously provided.

~Cheers~
 

Latest posts

Support us!

Support Us - Become A Supporting Member Today!

Click Here For Details

Sponsor Links

Banner image
Back
Top