why do 18s wreck handling???

[Ricco_04]

The biggest reason for using 13" wheels in F1 is to minimise how big the brakes can be IIRC.

For years I had also believed that tyres where lighter than wheels.  (ie this inch difference)  If you buy a good wheel, say Gramm Lights, then the larger wheel with the smaller tyre will be the lighter.  But as Gary says, still has the smaller wall.

[RotorHeid]

R888 tyre
225/50/16=9.4 kg
255/35/18= 11.7 kg.
For track day, go for 16" cos it\'s cheapest and lots faster and handling better.

[FD3SNuttah]

i got 18\'s notice car now has more grip and looks SEXY

[gsb876]

Found an excellent article here : http://www.sportcompactcarweb.com/tech/0506scc_handling_cornering/light_wheels.html

Edited highlights follow...

Going up an inch in wheel diameter and running a lower profile tire is a good thing. A lower profile tire has shorter, stiffer sidewalls, which improve response to steering inputs and hold the tread flatter to the road surface during cornering load. However, it can be overdone.

Ultralow-profile tires are more sensitive to suspension tuning and camber changes. And stiff sidewalls don\'t conform to road surfaces easily. This makes ultralow-profile tires sensitive to shock, as the short, stiff sidewalls have very little compliance. Harsh surface inputs can make these tires skip and hop across the surface instead of digging in and finding grip. Large wheel and tire combinations also increase rotating and unsprung weight.

For example, most enthusiasts driving small-bore four-cylinder front-wheel-drive cars run 17x7-inch wheels with a 205/40-17 tire. The big wheel and low-profile tire look cool, but this combo is too large and too heavy for optimal performance. Hard-core track geeks driving these same cars almost always fit a lightweight 15x7-inch wheel with a 205- to 225/50-15 tire.

Huge wheels also increase your car\'s final drive. Their added weight increases the flywheel effect, slowing the car\'s acceleration and increasing load on the brakes. This means wheels larger than 18 inches are rarely used for performance, so even for large cars, 18 inches is the maximum practical wheel diameter.

Huge wheels can greatly increase unsprung weight-the weight of the components that aren\'t suspended. This includes the suspension arms, brakes, half of the shock absorber and the wheel and tire.

For the suspension to work well, the ratio of sprung to unsprung weight must be kept low. The best example of unsprung weight hurting handling is found on monster trucks. Even though they have wheel travel measured in feet instead of inches, it doesn\'t provide a significant measure of driver control.

This is why monster trucks have nearly as much unsprung as they have sprung weight, thanks to their huge tires and axles. This makes the shocks work much harder to damp wheel movement. Reduce unsprung weight and the suspension doesn\'t work as hard, the ride improves and the tires stay in contact with the ground.

The obvious way to make up for the disadvantages of increased wheel diameter and width is to use a light wheel. Light wheels are easier to accelerate and brake. They also reduce unsprung weight. Some examples of our favorite light wheels can be found in the table below. The mostly expensive wheels listed on this page are forged or semi-solid forged. These processes improve the grain structure and provide superior strength for the weight.

The value wheels are all cast. Casting is a less expensive way to make wheels and nearly all low-priced wheels are cast aluminum. Some of the Enkei offerings have MAT (Most Advanced Technology) formed rim sections. MAT helps improve the metal\'s mechanical properties using a spinning process that yields almost as much strength as forging for a fraction of the cost.

Jeeez , someone\'s been doing their research eh :rollin :rollin :rollin

[maximtaylor]

Correct english Era, but I ain\'t a Horse............just hung like one

There you got it from the horses mouth....:cool correct english ?

[banned advertiser]

Oh god, Max has been on the oats

[Adriansturgess]

Believe horses are rather partial to oats......

[clive]

Sorry Guys I meant to post on this thread yesterday but other things took over.

With 17inch wheels it is still passable to keep your rolling radius the same as 16inch wheels and keep all of your geometry settings as Mazda, in other words it has nothing to do with tyre sidewalls or anything else other that "the bigger you go the more you screw up the geometry of the car"

see picture and you will see that the main thing you will do amongst other things is move your roll center, which is not good as roll centers have dramatic effects on handling

[banned advertiser]

You only move you roll centre if you move you ride hieght, according to the diagram, so if you keep the car the same hieght the roll centre does not move.
When you change from 16" to 18" you lower your profile to almost an exact match in rolling radius, so some of it must come down to tyre wall etc.

I have to admitt I find it very confusing as to what to use especially if lower the car.

It might be better to change rims size and then use body kits etc to lower the look of the car than lowering springs, use aero kits to create the extra down force, leaving geometry the same, or take it to a geometry specialist.
Chris Wilson stopped posting on here and that was his speciality braking and susspension, he always said to keep to 16" or maybe 17" tops.

Interesting thread.

[McCARTHY]

the thing is guys i\'m not just talking about wheel sizes as i said in the first post.  i know that the stock 7 was set up to run upto a 17" rim ( std in the later models) so yes you go out side that to an 18" rim with all the stock gemoitery they it going to handle worse thats a given....

but 18s on a car with coilovers, roll bars, lower arm bars etc and an increased track width surely the 18s are a more than suitble choice for the car.... and with the right set up....

[oldone]

the thing is guys i\'m not just talking about wheel sizes as i said in the first post.  i know that the stock 7 was set up to run upto a 17" rim ( std in the later models) so yes you go out side that to an 18" rim with all the stock gemoitery they it going to handle worse thats a given....

but 18s on a car with coilovers, roll bars, lower arm bars etc and an increased track width surely the 18s are a more than suitble choice for the car.... and with the right set up....

Mac to be honest with you,the only way to gain experience on this is to do it on the track with a few combinations and a setup guru to adjust the car......the old saying is \'you either be a poser or a go\'er\'....racers have funny sayings but they are normally true and tested..:wiggle

[McCARTHY]

i can assure you this is not a "dress up" mod i\'m doing. I\'m hoping to get the whole car re spect for next year for the Time attack series. So a go\'er you could say :Thumbs-up