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

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.

Thanks Umr........

Also forgot to mention rin width....245 were on stock 8's and the 275 are on 10's.

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.

All things are equal. Wider, taller, heavier tire and wheel are the only change.

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.

Doesn't spec Miata run on the spec tire from Hankook? :confused:

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.

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

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.

Remember, only changing tire size, width, height and weight. Increasing all.

Anybody else have first hand experience?

MPM-- good info - thanks

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 :)

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.

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

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

If the foot print is bigger then braking will be better. The tires stop the car, not the brakes. Same reason cornering is better.

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.

Here we go again. Oh boy. :D

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/

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.

Ah...Hahh! I thought we'd gone over this topic already!?

Where's ArcWelder on this thread?? LOL :)

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.

I'm through with it. I just provided the link ;)

If the link doesn't work I will suggest some books. If that doesn't work it's not worth me worrying about :p:

Exactly, but let's not forget the heavier rotating mass of the bigger tire and wheel.

The braking load will be increased............right?

Will the increase in contact patch make up for the breaking loss due to increased weight?

Well, I don't want to rehash old arguements... but Damon, if F1 has the rim dictated, why don't they run a shorter tire for lower rotational inertia?

When you are increasing rim size from 8" wide to 10" wide, what happens to the backspacing or offset? Are you putting the extra 2" inside the wheels, or is some going outside?
If some wheel is going outside, then your track width will be wider.
If your track width is wider, then your weight transfer in the turns will be reduced, and you can corner faster.

That is why there are rules limiting the maximum track width of a race car.

Adding rotating mass affects acceleration in lower gears, and makes minimal difference in acceleration as you approach top end.

If you want to quickly test the effects of rotating mass, then get on a Dynojet dyno. Those dynos measure how quickly you accelerate a rotating mass. Make back to back runs with your old light tires and then your new heavy tires, and then ponder why the dyno says you lost so much horsepower even though you made no changes to your engine.

Because the tire size is also dictated by the rules. If you want to run in F1 you must run that size tire. Thems the rules.

Be very careful of basing any argument on "such and such racecar does it this way so that's best". Racecars are built to satisfy the rules under which they compete and the rules are not written to make the cars "fast", they are written to satisfy other conditions. In fact if you look over the last 20 years most new rules in every series (other than safety issues) have been written to make the cars slow. Cheaper follows a close second.

In autox for instance tire size is unrestricted but wheel size is dictated by the rules of your class. Many autox cars run as wide a tire as they can with the smallest circumference available. This has some advantages for autox: It lowers your overall gearing. It leaves little sidewall deflection. The tire will heat quicker since it must turn at higher RPM for a road speed. The tire will retain its heat longer after the run because there is not as much surface area to radiate the heat out of the carcass when parked.

As for your F1 tire example if you run a smaller diameter tire your tire temps automatically go up because even if you keep the contact patch size the same size you're asking a lesser volume of rubber in the tire's contruction to do as much work as the larger diameter tire was before. This is going to make more heat and the compound would automatically have to be harder in order to give life to the tire. The tire would run hotter because each rubber molecule sees the road more often and at the same time there is less surface area available to radiate heat from the tire.

F1 currently is using the tires as air brakes. They wanted to give the cars less grip so they grooved the tire; this leaves less rubber in contact with the road. They could have accomplished the same thing by mandating narrower tires but those give less aero drag than the wider ones. The current grooved tire example is a perfect illustration of the folly in deciding since it's in F1 it must be the best way. Bolgona. You could easily get the same grip AND have less drag on the car, so why doesn't anyone do that? Because the rules don't let you...

I love how you skirt around the heart of the issue. ;)

Yes, unless you have really shitty brakes, in which case your car won't stop anyway :)

In theory, a larger contact patch will mean you can actually apply MORE braking force than before, assuming your braking system can generate the forces required without overheating.

In my opinion, one huge plus about running wider tires in bigger diameters is the ability to install, and actually USE, bigger brakes.

PaulC

Sorry, I disagree with most of that.

Apparently you think that adding wider wheels to the inside, to accomodate wider tires, won't increase cornering ability.......that somehow track width is the only factor. Thats wrong.

Weight transfer??????? What weight transfer?

There are some rules in some organizations regarding track widths. Not all of them have such rules. For example, SCCA street prepared classes specifically allow fenders to be modified to allow much wider wheels (very wide track widths). I have seen some very fast (and ugly :) ) Dodge neons with tires sticking out at least 4" past the fenders.

Acceleration and dyno HP are different things. Acceleration is limited by traction and HP. You can have 500 hp, and not be able to accelerate very quickly because of bad traction. With a powerful car at low speeds, especially when cornering, heavier wheels wont make any difference because you are already near the limit of traction. Low speed on a dyno is meaningless. It is at higher speeds where the extra weight will come into effect.

I don't think that's what he is saying. He recognizes the difference.

If you put on a wider wheel and add all your extra wheel width to the outside of the wheel this makes your track width wider. If the track is wider the weight transfer across the car during cornering will in fact be less.

Rotating mass inertia affects how quickly we can change the rpm of that heavy wheel that we are trying to accelerate.

In low gears, a car accelerates very quickly.

In 5th gear, a car accelerates very slowly. This is because aero drag is very high, and the torque coming out of the transmission is very low (compared to 2nd gear), therefore car acceleration is very low, therefore the wheel is changing rpm very slowly,
therefore a heavy wheel makes very little difference on 5th gear acceleration.

^^^Yeah, but then there's those guys who say all you need is gearing in order to get to 200mph ;) :o:

OK Gents.......

Struck a nerve :-)

Damon, thanks for the link....Good stuff and most of it way over my head.

I'll post hard data after my next event at T-hill on the 7th.

Thanks for the drama............

Niles

drama?? That was tame. You want drama, go ask why you can't remove the glass in the door over on the IT.Com forum.

lol

PaulC

False, oversimplified about brakes and wheel-tires. As roadway contact diameter increases, leverage to the brake [rotor] decreases. Into exaggerated diameters, and without scaling the brake force, the brake cannot grip the assembly with enough leverage to effectively slow the vehicle. The opposite is seen with reduced wheel size, eventually skidding/lockup for the same brake force.

*Brake force being the pressure applied at brake contact surface, and not the rotational wheel force at the roadway.

BOLOGNA!!

The brake system has much more force available than you can possibly use because once the tire starts sliding that extra brake force is in reserve.

Hydraulic systems are extremely powerful. Everyone keeps thinking leverage plays a big role in braking systems and it really doesn't. All less leverage means is that the system must clamp the brakes harder and that's easily done: you push the pedal harder.

http://=http://scirocco.dyndns.org/f...n/pfpage1.html

https://www.rx7club.com/suspension-wheels-tires-brakes-archive-112/do-rotors-lines-pads-really-make-big-difference-231994/

https://www.rx7club.com/suspension-wheels-tires-brakes-archive-112/cross-drillled-slot-brake-rotor-207758/