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Effects of tire size

Old 07-27-04, 09:59 AM
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Effects of tire size

Hey Guys,

Need some skilled opinions.

I am going from 245/45 16 to 275/40-17.

The 245 set up weighed in at 36lbs. The 275 set up weighs in at 44lbs.

The 245 diam. was 24.5. The 275 is 25.3.

Typically I run at Thunderhill and run consistant 2:05's +- a few .10ths on the 245 set up.

The Question:

Will the increase in tread surface area, lower sidewall be enough to over come the larger diameter and increased weight, as it relates to lap times.

I expect that breaking distance will increase, but with more grip I won't need to break as much. Exit speed should therefore be faster, which is good as my gearing will be taller due to tire diameter.

You opinions please......

Thanks
Niles
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Old 07-27-04, 10:05 AM
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Honestly, I think the extra tire width should provide a HUGE advantage. Your car might not accelrate *quite* as quickly, but you'll be able to carry much more speed through the corners.
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Old 07-27-04, 10:52 AM
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Thanks Umr........

Also forgot to mention rin width....245 were on stock 8's and the 275 are on 10's.
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Old 07-27-04, 11:38 AM
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I think that the extra rubber will help you in every aspect except accelleration (minimal loss). More rubber on the road should mean better stopping, and cornering ability. You may have to do some fine tuning, but once that is done, your lap times should go down.

This all assumes that you are replacing your old tires with the same brand/model tire.
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Old 07-27-04, 12:03 PM
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All things are equal. Wider, taller, heavier tire and wheel are the only change.
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Old 07-27-04, 03:43 PM
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This reply is just for reference.

On a SpecMiata the tire to have is the 205 width. The 225 width is better in the curves but given a long enough straight the 205 shod car will pass the 225. The reason is the gearing advantage of the lower profile 205 tire.

Having said that your on your own as the only way to really know if the 17 is better than the 16 is back to back testing or just compare your track times from before.
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Old 07-27-04, 04:03 PM
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Originally Posted by MPM
This reply is just for reference.

On a SpecMiata the tire to have is the 205 width. The 225 width is better in the curves but given a long enough straight the 205 shod car will pass the 225. The reason is the gearing advantage of the lower profile 205 tire.

Having said that your on your own as the only way to really know if the 17 is better than the 16 is back to back testing or just compare your track times from before.
Please correct me if I am wrong, but I thought the miata spec cars run on the same size rims. In that case, a 225 tire is probably too wide for the same rim as the 205.

In the case of this thread, the wheels are 2" wider to accomodate the wider tires.
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Old 07-27-04, 04:16 PM
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Doesn't spec Miata run on the spec tire from Hankook?
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Old 07-27-04, 04:22 PM
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The wider track width will help you out cornering wise; less weight transfer in the corners.

The heavier weight will kill your acceleration in the lower gears, but not make much difference in 4th gear. You will have 4 new huge flywheels that you have to spin up when you want to accelerate.
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Old 07-27-04, 04:32 PM
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Originally Posted by speedturn
The wider track width will help you out cornering wise; less weight transfer in the corners.
Weight transfer has absolutely nothing to do with tire size. No matter what tire, spring or shock you bolt to the car the amount of weight transfer will always be the same.

weight transfer
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Old 07-27-04, 04:39 PM
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Originally Posted by speedturn
The wider track width will help you out cornering wise; less weight transfer in the corners.

The heavier weight will kill your acceleration in the lower gears, but not make much difference in 4th gear. You will have 4 new huge flywheels that you have to spin up when you want to accelerate.
Actually, if you have enough power, the extra weight will make no difference in the lower gears, and a bigger difference in the higher gears.
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Old 07-27-04, 05:15 PM
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Remember, only changing tire size, width, height and weight. Increasing all.
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Old 07-27-04, 05:15 PM
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Anybody else have first hand experience?
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Old 07-27-04, 05:17 PM
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MPM-- good info - thanks
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Old 07-27-04, 07:04 PM
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It will make a huge difference if you use the same type tires. I've driven on about everything and the fastest setup is the 285 18 30 size but the 275 40 17 is great for track use because the tires are very popular so there is lots of variety also its a lot of tire so they don't get as hot as some other setups. This of course means they stay sticky longer and last longer.

A spec miata already has plenty of tire it needs more power. The fd has plenty of power it needs more tire
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Old 07-27-04, 11:32 PM
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As FYI a 275/40/R17 isn't a lower sidewall than 245/45/R16. The height number is proportionate to the width, not a direct measure. The best tire calculator/comparsion tool I've found is: http://www.miata.net/garage/tirecalc.html

I'd think the improved breaking distances (provided you have enough braking power to lock up the new tires without melting your rotors) and cornering will make up for your loss of acceleration (since you can hopefully accelerate longer and carry more speed through the corner meaning you have to accelerate less).

Good luck out there.
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Old 07-28-04, 09:22 AM
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That's what I was thinking.

For Fritz... If the 285 has proven to be the best, would you prefer to have the 275 on 17's if the rear end was changed to say a 4.3? Or do you prefer the way the 18's feel on the Track.?

Thanks Guys,
Niles
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Old 07-28-04, 10:11 AM
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I don't think the braking is going to get better with a taller and heavier setup than before.
I think laterally is the only direction you might have improved things.
Whether or not it was enough can only be told by lap times.
Good luck
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Old 07-28-04, 10:21 AM
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Originally Posted by Travis R
I don't think the braking is going to get better with a taller and heavier setup than before.
If the foot print is bigger then braking will be better. The tires stop the car, not the brakes. Same reason cornering is better.
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Old 07-28-04, 12:42 PM
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The contact patch is wider... not longer. The tires ability to deflect (at the contact patch) is decreased front to back, but increased side to side.
Look at the kinds of racing that have tremendous acceleration and decceleration. Like drag racing and F1... they both have small diameter rims with large sidewalls so that the contact patch is both long and wide.
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Old 07-28-04, 12:51 PM
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Here we go again. Oh boy.

Bigger contact patch gives better braking, better acceleration and better cornering. The length/width ratio of the contact patch doesn't much matter. The more rubber you put in contact with the road the better your grip. Period. You bring more rubber, you get more grip in every direction.

https://www.rx7club.com/suspension-wheels-tires-brakes-archive-112/17-vs-18-racing-mind-narrow-vs-wide-tires-292194/
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Old 07-28-04, 12:52 PM
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Originally Posted by Travis R
they both have small diameter rims with large sidewalls so that the contact patch is both long and wide.

They both have small diameter rims because the rules say that's what they must use. The tire didn't dictate the wheel, the rules dictated the wheel which dictated the tire.
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Old 07-28-04, 01:10 PM
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Ah...Hahh! I thought we'd gone over this topic already!?

Last edited by SleepR1; 07-28-04 at 01:12 PM.
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Old 07-28-04, 01:11 PM
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Originally Posted by Travis R
The contact patch is wider... not longer. The tires ability to deflect (at the contact patch) is decreased front to back, but increased side to side.
Where's ArcWelder on this thread?? LOL
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Old 07-28-04, 01:13 PM
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Originally Posted by Travis R
The contact patch is wider... not longer. The tires ability to deflect (at the contact patch) is decreased front to back, but increased side to side.
Look at the kinds of racing that have tremendous acceleration and decceleration. Like drag racing and F1... they both have small diameter rims with large sidewalls so that the contact patch is both long and wide.
Actually, a tire with a larger diameter will have a larger contact patch front-to-back than a smaller diameter tire, given the same model tire and air pressure. A larger diameter tire will ALWAYS have a flatter curve, and thus a larger front to-back-patch. Combining this with a wider patch (which is much more significant) will provide a substantial increase in traction.
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