Sorry, canzoomer, I must disagree with your calculation here. The plane of reference for offset calculation is from the hub mounting surface to the centreline of the wheel, so width doesn't matter. A 1" narrower wheel of the same 50 mm offset simply has 0.5" less width on each of the inside and outside half of the wheel. My reference source for calculating offset this way is the Tire Rack Tech Center's offset info page .

The 38mm offset with the 1" narrower wheel will maintain the vehicle track the same as the OEM 50mm wheel (ie width between the outside vertical planes of the wheels), whereas a 1" narrower 50mm wheels will decrease the track by 1".

Tire Rack's article does say This is very confusing - while they say that additional width for the same offset would be split evenly between the inside and outside, they don't follow on to explain why they say for most cars this won't work correctly! To my understanding, changing the offset will change the scrub radius (calculated through the centre of the contact patch), whereas just changing the width while keeping the same offset will keep the originally designed scrub radius for the suspension/wheel design.

In any case, I think anywhere from 38 to 50 mm would work satisfactorily on the RX-8.

Regards,
Gordon

 

:p Forgot to address this comment! Rice is in the eye of the beholder... or at least that's my hypocritical excuse.

Have you ever seen pics of the wheels and calipers I did on my BRG Miata? I'm a Lotus fan, so my car's colour scheme is sort of a homage to the 60s vintage Lotus racing colours. You know, the BRG cars with yellow wheels? Here's a pic...

 

And I disagree with YOU, Gordon!! {smile}

See:
http://toy4two.home.mindspring.com/offset.html

Enter, on the top row 8 and 50
Enter, on the next row 7 and 38

You will see that the outside of the wheel will move inwards 1mm (insignificant).

Now consider how rear suspension works on an RX-8.

The wheels move up and down, on an arc, with the centre of that arc on the pivot point of the suspension arms.

When the wheel travels downwards, the traction point moves towards the outside edge of the tire, away from the centre.

If you change the distance from the outside edge to the pivot point, then the total length of this assembly changes.
This means the leverage on the suspension changes too, as we have changed the length of a lever.
We ALWAYS want to keep the lever the same length, or the spring rates and damping rates will no longer be right.

What we want to do is to preserve the distance from the outside edge to the pivot point.

See:

 

Here is a good pictorial description of offset:
http://www.rx7turboturbo.com/robrobi...eel_offset.htm

As the caption about offset says:
Offset - Distance from the wheel center-line (CL) to the wheel mating surface - greater positive offset moves the tires inward (narrower track)

Therefore:
LESSER positive offset moves the tires OUTWARD (WIDER track)

If the wheels are 1" narrower, we need to compensate by moving the wheels outwards by 1/2" (12mm)
This is achived by REDUCING the positive offset by 12mm
So we need a wheel that has 12mm LESS offset than the stock wheels.

So, 50-12 = 38mm positive offset is required for a wheel that is 25mm (1") narrower than the originals.

 

Thanks for the pics canzoomer. I think I get it :o

 

knowing that my car will be here in the winter months. I asked dealer install the winter tires on the stock wheels before I pick it up. I do not know what brand winter tire dealer will use.

I am confused now ....

A) stock 18" wheels or 17" wheels for the winter?
should I order a set of the 17"wheels now and install the 17" winter. Leave the OEM wheels/tires for the summer.

B) Is it 18" winter/stock wheels just as good vs 17" for winter? Given 18" winter tire is already ordered. The 17" wheels/tire package is an alternatives (cost/choice). Then keep the 18" winter tire on stock wheel and buy a set new 18" wheels for summer OEM tires.

TY

 

17" wheels allow for tire with higher sidewall, makes choice of snow tires easier.
17" tires are less costly
17" wheels and tires allows for more wheel well clearance, so less issues with snow packing around tires and preventing steering
17" tires may have smaller contact patch allowing better snow traction.

17" wheels that clear the front brake calipers are a bit hard to find

17" x 7" is ( in my opinion) the best bet.

Narrower wheels offset is different than what you use for 18".
Use 38mm instead of 50mm offset for 17" x 7" wheels.

 

I agree with Canzoomer on most of his points (I still don't agree about the offset - the wheel movement diagram only applies to a straight old-fashioned swing-arm rear suspension, a la early Beetles, Corvairs, etc. On a multi-link or unequal-length a-arm suspension, the camber curves (and camber curves with chassis roll) for suspension compression and droop will look entirely different. Rear suspension aside, I think the scrub radius of the front suspension with it's steering implications is of greater significance, and changing the offset will change the scrub radius. But, again, I think anywhere from 38 to 50 offset is fine, my 17" wheels are 45mm, so the offset issue is more theoretical than practical).

In another thread, I posted some figures comparing swapping 18" winter tires vs. buying a separate set of 17" winter wheels and tires, it went like this:
Regards,
Gordon

 

I mainly agree with Gord.

One other factor to consider for winter tires is "how much snow vs how much ice vs how much clean roads?"

In general, we need to install some form of winter tire if we live in an area where we will drive on roads in conditions where driving temperatures will drop to below 38F (3C).

Summer tires, especially high performance tires like the Bridgestone Potenzas that come with the RX=8 are good for warm roads.

When the temperatures drop they then have characterisitics of a piece of rock. Traction rapidly becomes squirrelly, the tires have no flex, and on ANY amount of ice or snow they are like wearing roller blades on an ice rink. Totally dangerous.

When you are going to buy tires for winter, you have to look at the prevailing winter conditions in your area.

In some regions, it gets cold, but rarely snows, and you are usually driving on relatively dry roads.

For this condition the tire makers sell what are called "all season tires".
The problem with most of these is that the definition of "all season" varies quite a bit. In some cases it means wet, in others lower temperatures. In general, however, most of these are a really poor compromise.

There are some exceptions. For example the Nokian Hakka WR is an all-season that really means "Winter, with spring and summer, great on ice and light snow"

If you live in a region that gets fairly cold, with light and infrequent snowfalls, and lots of black ice and dry pavement, then this is good for you. Generally this means prairie regions in dry areas. Also good for areas further south that get infrequent snow or frost.

If you are in a region where you frequently get snow, especially wet snow, your requirement is qiute different. In this case you need a tire that can dig through the snow to get traction, with a fairly small contact patch to allow the weight to push the patch through the snow. You also need pretty large cuts between tire blocks to allow it tio "paddle" through snow without plugging up.
This is for regions with large amounts of precipitation. Mountain regions, coastal cold areas, and inland lake regions.
Once the snow season ends, get them off soon. Snow tires generally handle poorly on paecment, and wear quickly.

In the last case, where you are driving in mainly cold conditions, where the roads rarely thaw, but not much snow, you need an ice tire.
Generally these have similar treads to an "all season" but of a softer compound when cold, with lots of fine cuts, called "snipes" in the treads. Usually the tire compound has a high silicone content.
Good for US northern inland dry states, and the Canadian prairie provinces.

The last category is studded tires. Frankly, if the road conditions are so bad that you need these, you would be better off parking an RX-8 and taking out a 4 wheel drive, or riding public transport.

An other matter to consider, apart from tires is salt:

Lots of areas use salt to melt the ice.

It is a really good idea to frequently take the car to an indoor car wash, and clean off the ice and snow, so the salt is not in contact with metal parts of your car for long. Pay special attention to the wheel wells, the passages that exit the bottom of doors and fenders.
Do NOT USE SOAP, as it strips off wax.

Luke warm or cold water, and lots of it, is the best solution. Avoid wiping the body work to avoid scratching. Pressure washing first is the best way.

Wash the underside as well as you can, to avoid corrosion of exhaust and suspension components.

Rinse lots with clean water, and let it drain and chamois wipe it before going back out into freezing conditions.
If the car wash has compressed air, blow out any water from the door locks, window seals, and around the door weather seals. Make sure to get any water out from around the mirrors, especially if they are powered and heated. Heated outside mirror can break if the area around the mirror is packed with snow or ice.

If you park in a garage, avoid heating the garage above freezing temperatures. Taking a car into a heated garage with salty snow on it ensures that the parts will soak in salt water every time you park it. That will eat a steel body in little time.
Frankly , on salted roads, you are better off leaving it outside, in the shade, and best with a car cover.

Remember to wax (or better Zaino) your car with at least 2 coats before the onset of winter, and do not wash the car during winter with detergents.
Plain lukewarm or cool water is better, as it does not strip the wax.

Pick up a bottle of silicone spray, and wipe your door and window seals with it occasionally and they will not freeze stuck and will not tear weather seals.
This is good for windshiel washer baldes if used sparingly. Do not use much and do not get it on the glass. It smears.

Pick up a bottle of alcohol. Fill a spray bottle with it.
Keep it in the pocket of your winter coat, or somewhere handy for morning starts. Put the key in part way and dribble some on the key blade when opening the door and stuck locks melt clear.

If you are looking for an ice scraper, plastic ones are crappy. Try and find one with a blade made of brass. They cut the toughest ice, do not nick easily, and do not scratch glass.
Best is to use your credit cards. When you have broken them all you will find that you save a lot of money..

NEVER scrape a dirty windshield. The grit will scratch it.
If you have to thaw it in a hurry spray some alcohol on it.
Try to keep the alcohol off the paint. It strips wax.

Lastly, if you are new at winter driving, avoid using brakes. One should be able to drive with the least amount of braking, and this reduces chances of skidding. Allow lots of room in front of you for other cars, stop lights and signs.

If somebody takes the space in front of you, don't get angry. These idiots will pay the price soon enough, when they rear-end a bus or something..

For those who drive regularly in winter, and know all of this, please pardon my long response.

For those who are new to this, I live in Edmonton, where it freezes every night from early October to late April.

No accidents or rust in 30 years so far..

And wear sunscreen..

 

This might seem like a dumb question... but did any of you guys talk to your dealership before swapping tires? Is it possible to affect the warranty by putting on a "wrong" wheel?

 

Just chiming in for a sec -

I've been reading this thread since it started in whatever month that was, and I just wanted to say that the level of expertise that Gordon and Canzoomer have achieved is fairly impressive. I took their advice and purchased a set of 17 x 7 wheels with a harder compound winter tire for use here in Pennsylvania. Tested them on the car, and all was great. They drive nicely too!

I must admit that the whole offset thing was too much for me so I left it up to the guy at the Tirerack where I bought the tires, but at least I knew enough to ask the question, and to understand the answer.

I just wanted to say thanks because you guys really walked me through something that was pretty stressful for me, and you didn't even know that you were being helpful.

Just for my $.02 I did not contact my dealership about any warranty issues that might be involved, but my feeling is that it would not invalidate the warranty, even defeating the tire pressure sensor. The only thing is that if the wheel were to specifically damage the car, like the front calipers, that specific damage would not be under warrranty. That's my guess. Just a guess.

Thanks again - H

 

Can you elaborate on what you bought?

I still haven't decided what wheels/tires I'm going with... and anything I can add to the "it works" list would be helpful.

Thanks.

 

Hey there -

I got the 17 X 7 Sport Edition Fox 2 in Lt grey.

I wanted something that wasn't silver, and they looked slick to me.


For tires I got the Blizzak LM 50's in 225/50QR-17. They were cheaper than alot of the other tires for snows, but they are not speed rated for the Mazda. However I doubt if I will be racing around in the winter so this was OK for me.


I tried the package on my car and drove around a bit. The wheel cleared the caliper by at least an inch which was just fine, and they drove great around the block. I took them off then, and put them away waiting for winter, but the package seems to work well.

Anyway, it was the package that worked for me.

 

Thanks for posting.

 

Confused...No more!

I agreed on all points from Canzoome,Haze and Gord.

I will let dealer install the 18" winter unless they advise and supply me the 17".

I am not good in calculation. But I do not see great advantages in winter driving by changing to 17" if it is 225/45 or 205/50, atching the height etc... etc... comes to almost the same size of the stock. It comes down to the costs for 18" winter tire vs 17".

My first car is 72 beetle then Aspen, Cutlass Calais and now 94 Aerostar. All RWD without the latest technologies on tire compound and DSC/TCS. Winter tire is very important because the cold weather, especially freezing pavement and black ice.
Past 33 Quebec winter seasons driving technic is to keep a good distance, slow down, know your limit, leave home/work early to avoid rushing yourself through traffic jam created by those idiots driving 4X4 ended upside down.

Note that if I need to buy the whole set wheels/tires. I'll take the advise from Canzoomer and Gord. Thanks Guys.

PS. Mazda Canada should provide the search on web so that I can find out the status of my car!

 

TY_888, your in Montreal! Talk to Talon Tire Man!

 

Hey I thought I'd just mention that the CR model is "216".

They LOOK like the CR374s on Kal's site, but they are not. In fact, the CR216s aren't even listed!

I had to use the product code to get the right rim. Thanks!

 

In Canada I didn't realize that 18" snows get snapped up so fast. I found 2 Blizz LM-22s and that was it. Where are you getting yours?

 

I do not have the car yet (expecting last week of Nov). My dealer suppose to have the winter tire installed when I pick up the car. Sales manager promised me a set of good performance winter tire that fits the RX8. I do not know which brand name of winter tire. I leave it up to the dealer to locate the tires.

 

Chaeck the previous explanations in this thread and the Talon Tire Group Buy thread. In a nut shell, there are 2 points:

1. The 7-inch width of winter wheels & tires that everybody is buying from Talon Tire (or elsewhere) will give better traction by penetrating the snow deeper compared to the 8" wide ones.

2. The higher profile of 215/55R17 tires (which require that you switch to 17" diameter rims for the winter) will further improve traction due to the greater cushion of rubber between the hard metal rim and the snow-covered or ice-covered pavement. That additional height improves pliability of the tire on those surfaces. It would be even better if 16" rims could be used but I tried on many of them at Talon Tire and they were not fitting over the mongo brakes. That's why we ended up recommending the FRD 6 or FRD 30 rims with 215/55R17 Toyo Garit HT tires. In contrast, tires having a low profile (what you get on 18" rims) will be too inflexible or "hard" to get good traction in the snow. Think skates.

I understand that Mazda has not yet decided on their choice of 17x7 or 16x7 wheels to recommend or provide to their dealers' service departments. I don't mean to be alarmist, but if your dealer is going to put snow tires on the stock 18" rims, it means IMHO that he is making a buck from you at the expense of winter performance (at best) and your safety (at worst). What he did was choose to keep your business in Mazda service dept. (that's his best interest) whereas he should have advised you to find 17x7 or 16x7 wheels elsewhere (that's your best interest).