changing Sport Car Tire Aspect Ratio Questions...is it safe?

[johnnynomani]

I am changing tires on a 350Z for the winter and wanted to know what you guys think. I have talked to Nissan and TireRack.com and I get the feeling that they sometimes do not know what they are talking about.

This is currently the OEM tire setup on the car:
Front: 225/45/18
Rear: 245/45/18

What I would like to place on the car is the following:
Front: 225/45/18
Rear: 245/40/18 (RFT)

I would like to change the aspect ratio of my rear wheels because I have found a very nice Run Flat winter tire that I would like to purchase (which only comes in 245/40/18).

My Questions are:
1. What is the possible dangers of having a 45 in the front and 40 in the back?

2. What is the cons of going to a lower Aspect Ratio (ex: 45 to 40) besides having it through off your speedometer?

 

[cibachrome]

RSVP with O.E. brands, model/construction and pressures and the winter rear ditto. I would expect more brand & construction intermix variation than aspect effects. What is your spare plan: eliminate the spare as an issue, leave it in the garage, use an oem summer as the spare (where will you stash it?), inflator kit + Slime, AAA, drive on it until I get home, ...

If you stay within brands AND the Summer construction is a sibling to the Winter RF, there may be a slight understeer increase if the winter meats have higher cornering stiffness. If they have less (it can go either way) then a slight decrease. BFD. THERE IS NO GOOD RULE OF THUMB because Brand and tire parts (breakers, ribs, tread weave, belt angle, shoulder recipe, compound, etc. all dominate the handling performance. especially in an OEM vs. aftermarket fitment. One thing is certain, though, your gas mileage will go down because of the jump in FX with the run flat. Some structural concerns may crop up because the body might get angry about the higher transmisibility of the run flats, but that again will depend on the type of run flat: light or heavy.

 

[johnnynomani]

Thanks for the info. What I want for winter tires are as follows:

Front: 225/45/18 (Bridgestone Blizzak LM-25)
Rear: 245/40/18 (Bridgestone Blizzak MZ-03)

There is multiple reasons for this setup but what I need to know is if there is going to be harm done to the car. Is this safe?

 

[cibachrome]

Not sure I have all up to date material for an answer, but here goes:

The 350Z with its OEM Bridgestones is an exceptional performance vehicle with low compliance, high bandwidth and a below avg amount of linear range understeer (about 2.0 deg/g). Roll compliance is also low, but typical for this class of car. Since the steering system is relatively stiff, the developers had to use tire size and wheel rim width (8.5"F & 9"R) to get the LR understeer: The absolute best way to do this IMHO.

That being said, switching to RFTs will produce a stiffer ride and higher steering loads. Not sure from your rear tire model name (Blizzak vs. what I have RE050A-RFT data on what is correct next, but if the RE050A is your new tire, it is equal or superior to the OE tire for cornering and a bit heavier for steering. If you believe the OE setup is a bit "quick" at high speed (85 -100 mph), you may like this switch.

On the fronts, I see there is info on an RE040-RFT but not in the 225/45R18. Do you mean the front RFT will be a 225/40R18 (instead of 225/45R18) ??.

Other than the gas mileage & tire wear and the past sins of tire noise and feathering due to some production alignment problems, you might find you like the Winter car better than the Summer car.

If you are driving in snow, the damage to the car will now be in the rear, as you will outstop the car behind you, so watch your backside.
Let us know what the outcome is for correlation to my SWAG.

 

[NickE]

From someone who has a bunch of experience on winter tires and in extreme winter conditions.

1) Dont use the run-Flats, you dont really want a stiffer sidewall on snow, slush, and ice. The tire needs to be more compliant over all on most of the surfaces you will see in the winter.

2) Wider is only better on two types of winter surfaces, Glare Ice and Panked snow.

Normally going a little narrower while upping the sidewall a bit will provide better performance in snow and ice. Alternatively it is common to get a second set of wheels for the winter months.

Particular snow tires are always reccommended in sets of four. This balences the response corner to corner.

I would recommend:
the LM-25 all the way around in the stock size.

Or purchasing a 17" wheel and tire set for winter. You then have more options.

Fronts and rears at: 225/50x17

If you expect deep snow and slush then you could go to a narrower tire also. That puts you into a factory specific Porche size though. 205/55x17.... No blizzaks available. Though you might be able to find a Nokian tire in that size. (The Nolkian winter tires are generally considered to be the next step up from Blizzaks.)

Anyway I dont think you will gain any benefit with the staggered widths, and may see significant losses in rear end traction once the wide flat tires start to float on the snow and slush.

Or just buy a front wheel drive beater with poopy tires and keep the beautiful 350 in the garage till the sun comes back out.

 

[gt6racer2]

Of course there are many trade-offs and how you weight the factors will give you your answer. Some other points to consider though : 1) Snow driving - if so you'll want thinner tires to cut through to good traction (Just look at the WRC set-ups for winter). For me this was the biggest factor as the wide tires just polished the surface to ice. 2) Temperature - some extra compliance from a taller sidewall can be nice. I dropped from 18's summer to 17's for winter. This allowed me to keep same rolling diameter. BTW you may create issues with different front rear diameter as regards ABS, tire pressure monitor or others as your rear wheels will always spin faster than the fronts..