Tire question

[Guest]

It's about time for some new rubbers and I'm thinking of down sizing.

I recently replace the tires on my Nissan truck. I went from a 17-205-60

to a 17-195-55. Not a lot of difference in the side wall height, but I do notice that the sidewall is more perpendicular to the road. It doesn't bulge out like the 205's. Now I know that new tires can make it seem like your car runs better. So I did expect some change but not as much as I think I have. It's seem like there has benn a bigger improvment then when I put heavier sway bars in the front and back + 40lbs of lead bolted to the frame over each rear wheel to get the weight distribution a little more even.

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Does anybody think that going with a narrower tire on the Reatta is a problem?

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I'm thinking it will reduce the body roll I fell is induced by the bulging sidewall on the stock size. Front rear struts were replaced a few years back.

It would be nice to go with wider rims but most Reatta's with aftermarket rims I've seen just look out of place or to ghettofabulous.

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Thanks! RiNgO

I like soft sticky tires. You don't get a lot of miles out of them, but with the litte miles I put on the car I wind up getting news ones well before they're worn.

I do most of my driving on back country roads (all are paved), the Blueridge Parkway and Skyline Drive. (Now I see why my ST taxes are so high, these roads are kept nearly perfect!) I'm not a speed demon, Just don't like to slow down!

[Guest EDBS0]

Down size NO! NO! NO! Oversize! +1 or +2

You are really going 57.2mph in your Nissan when the speedo is reading 60mph. You are reading 4.6% too fast.

Tiresize calculator.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> 40lbs of lead bolted to the frame over each rear wheel to get the weight distribution a little more even. </div></div> Lighter is better. Try sliding your seat all the way back, move the battery to the trunk.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">heavier sway bars in the front and back </div></div> Probably a good idea but first try heavy duty bushings and endlinks to make a firmer more responsive car. Energy Suspension Makes great parts. buy them here.

do some reading at the TireRack

Read this too.

Had a fun fall ride down the Blueridge Parkway to Ashville a few years ago and yes it is a beautiful road and also has the thickest pea soup fog that I have seen on a road.

[padgett]

My rule is that for high performance you want tread width the same as rim width. For racing the rim should be wider than the tread. Have a large box full of autocross trophies (wife got tired of looking at them) to say am right.

Reason is simple if you plot tire movement on hard cornering. With a tire wider than the rim, as the tire deforms sideways, on the outside of the trun (the side that is getting loaded) the outside sidewall straightens up while the inside sidewall deforms more. This lifts the inside edge of the tire and rolls the ouside sidewall under so about half the tread is effectively off the ground. This is not a good.

With a rim wider than the tire, the opposite occurs and lateral deformation plants the whole tread on the ground.

For street use the 205x60x15 on 15x7s on my Fiero is a pretty good compromise. The 225x60x16s on 16x7s on my Reatta are for street/touring only.

BTW a rule I used for autocrossing was to take the weight of the front of the car (all the weight is going to be on the outside tire in a hard corner), divide by the load rating of the tire, and multiply by 32. This gave an inflation pressure for max cornering on street tires. Did same for rears. Was using 50-55 psi on front of my Sunbird and enbarassing a lot of Porsches. (car was Florida regionional champion, Fla State Champion, & SoWest Div Champion SCCA Solo II)

[Barney Eaton]

I too believe you errored on the tire selection on your Nissan. Not only are you going to put about 5% more miles on the odometer. (500 per 10,000) but you lowered the load rating of the tire with the change. A 205/60 in any size has a higher load rating per tire than a 195/55 of the same rim size.

[padgett]

Interesting, Tire Rack has no match for a 195/55/17,though Nissan pickups come with 195/75/14s.

That said about 30 yar ago I discovered two things about tires - the first was the rim vs tread factor described above. The second was that a street tire requres a certain amount of loading to work properly. In the bad old daze of two ton pony cars it was difficult to experience but Porches, Fieros, and Corvairs experience it all of the time.

Basically if you plot load vs cornering force on a tire (assuming proper geometry and wheel size) it looks something like a skewed bell curve. Overload the tire enough and cornering force falls off a cliff as the tire slides. What few realize is that cornering force drops off on the front side of the curve also. If you do not load a tire properly, it can never develop proper cornering. This is why late braking and weigh transfer going into a corner is so important.

Put a superwide tire on a car and you are liable to get a double whammy: if the rim is not wide enough, you get the lifting mentioned above, and if planted properly the tire does not have enough load to develop maximum force.

True, sometimes one can cancel the other but that generally results in hot spots and in extreme cases can result in chunking.

So for a light pick em up truck weighting around 2800 lbs (2wd 1994 Nissan) 195/55/17s with a basic load rating per tire of around 1200 lbs are not that small. Do think a 215/55/17 on an 8" rim would be a better choice.

[Barney Eaton]

Padgett, I too went to tirerack to see if I could find load ratings and they do not list the 205/60r17.

Depending on the age/model of the pick up it probably has enough load rating, unless you put that 1/2ton load in the back. I ran out of time searching.

[2seater]

I can't keep up with Padgett's math and reasoning, but I am dead sure it is sound. The only thing I could add is, the OEM's usually puts the minimum tire on that they can get away with. Partly from an economic standpoint, but the minimum size with higher inflation pressure can help fuel mileage as well (CAFE ratings). A small amount of tire bulge is preferable for shock absorbtion and ride quality, but not for optimum handling. It's really a balancing act for the wide variety of normal use a tire goes through in everyday driving. If one would follow the conjecture further and further along the original path suggested, an even skinnier tire would handle better yet <img src="http://www.aaca.org/ubbthreads/images/graemlins/crazy.gif" alt="" />

[padgett]

Tires defy physics. According to the math you cannot exceed one gee something the dragsters passed long ago. The fact is that a wide soft (remember Sly picking up quarters ?) tire molds into irregularities in the road (why traction is best on a Florida shell road).

Also reality has an evil way of rearing up and biting you - why race tires are illegal in stock autocross classes.

That said I have 225x60x16s on 16x7 rims on my Reatta for three reasons:

1) were cheaper/more available than 215x65x16s

2) are same size tires and wheels as our Bonneville

3) not planning to race it

As I mentioned, for good street handling you want rims as wide as the tread. For all out performance you want rims the same as the section width of the tire. That means you want 9 inch rims for a 225 section tire (and for all out, the lower the aspect ratio the better).

Now many people seem to like really wide tires (guess they don't have rain). I see a lot worn out in the center also. That should say something about contact patch by itself. Personally prefer a balanced car.

My V-8 Sunbird won a Lot of autocrosses & was 3200 lbs on 195x60x13s. All I was permitted were 13x6 wheels and I had to check the fronts for cracks after every run - once broke a center right out. Also was running 50-55 psi in the front tires. All I can say is that it worked (all wheel steering helped).