ZCar Forum banner
Status
Not open for further replies.
1 - 9 of 9 Posts

·
Registered
Joined
·
1,770 Posts
Discussion Starter · #1 ·
Has anyone seen or done actual testing of the pros and cons of changing wheel diameter? (i.e., lateral g's, slalom speed, lap times, un-sprung weight, etc.} I'm mainly interested in 195x70x14 vs. 205x60x15, etc..and also in rim widths; 6 inch vs. 7 inch etc.
 

·
Registered
Joined
·
107,695 Posts
I hate to answer a question with a question, but I too would like some help in selecting new wheels and tires. I have a '77 that I bought recently. The PO had put 13 inch wheels on it and even though they are American Racing mags with BFG Radial TAs, I really want to go to at least 14 inch if not 15 inch wheels. Anybody have any suggestions?

> Has anyone seen or done actual testing of
> the pros and cons of changing wheel
> diameter? (i.e., lateral g's, slalom speed,
> lap times, un-sprung weight, etc.} I'm
> mainly interested in 195x70x14 vs.
> 205x60x15, etc..and also in rim widths; 6
> inch vs. 7 inch etc.
 

·
Registered
Joined
·
482 Posts
Some calculations...long

> Has anyone seen or done actual testing of
> the pros and cons of changing wheel
> diameter? (i.e., lateral g's, slalom speed,
> lap times, un-sprung weight, etc.} I'm
> mainly interested in 195x70x14 vs.
> 205x60x15, etc..and also in rim widths; 6
> inch vs. 7 inch etc.

If you already know this, then ignore this post, but if not reading on might help you somewhat.

Changing rim/tire diameter has effects on top speed, gear speed, acceleration etc....Therefore this is a choice that must be weighed in many areas.

In regards to tires:
Basically, a thinner tire will not 'flex' as much as a taller tire. This is why most race cars have such low profile tires and/or run extremely high pressures. This keeps the side loading of the tire in check, and allows a larger percentage of the 'contact patch' of the tire to stay in contact with the ground. The downside to this is although it definetely increases the load capacity of the tire, it diminsishes the cushioning effect the tire has, and provides a much stiffer ride. Of course on a nice smooth racetrack who cares, but on a regular road with potholes this might pose a concern. Rim size does not really have any affect on tire grip, outside of the fact that bigger rims allow wider tires.

The difference in Aspect Ratio of the two tires you mentioned is this:
70% of 195mm = 136.5mm
60% of 205mm = 123.0mm
So, although the 205mm tire is only 10mm wider than a 195, it's lower aspect ratio allows the tire to endure more side-loading. This would be a better choice for either rim.

In regards to rims:
Now we are talking about the total tire diameter in regards to acceleration, gear speed and top speed. We are also talking about the ability to accomodate different tires with a minimum of flex and side-loading.
Rim size affects how long it takes for the tire/rim to make one revolution. The larger the rim/tire, the longer this will take, hence hampering acceleration. Of course gearing changes and tire a/r can affect this also. This whole thing is really a balance of what you want vs. what you can get.
In regards to rim width, a larger rim width will safely allow a larger tire. Putting a wide tire on a narrow rim, although the rim is the same diameter will cause excessive side load and most likely will have a problem sealing the tire to the rim. This is why rims come in different widths.
As far as weight is concerned, the alloys are the lightest, (Magnesium is the lightest), with steel rims being the heaviest. Usually the difference in weight from a 14 alloy rim to 15 alloy rim is only a few pounds.
 

·
Registered
Joined
·
189 Posts
> Has anyone seen or done actual testing of
> the pros and cons of changing wheel
> diameter? (i.e., lateral g's, slalom speed,
> lap times, un-sprung weight, etc.} I'm
> mainly interested in 195x70x14 vs.
> 205x60x15, etc..and also in rim widths; 6
> inch vs. 7 inch etc.

Check out this link for some excellent wheel and tire information. It should prove very helpful to you. FYI, I run 205/60HR15 on my 260Z. Wheels are Enkie 15X7.
 

·
Registered
Joined
·
134 Posts
Re: Some calculations...long

I have to disagree on one thing. Rim width not only allows larger tires, but using the widest rim reccomended for a particular tire will support the sidewall better, allowing less rollover, very important in salom type events. To narrow, and even the tread can curve making all sorts of problems.

Alloy rims are not only lighter, but less flex also. Stiff is good for handling, and less chance of breakage.

Last point, tire size can be used to lower the car. A lower car means less roll and better handling. I run 205 60/14's on my autocross Z partly for that reason. Acceleration is better, and speedometer error is not a factor on the course.
 

·
Registered
Joined
·
1,610 Posts
14, 15, 16 or 17's

Rob -
From the way you worded your question I assume that you are planning on racing your Z. If you are not planning on flaring your fender wells the following are options if you want to keep your speedometer error to a minimum.
195/70-14, 205/50-15, 205/55 or 225/50-16, and 225/40-17. You can run 7 wide rims on all four corners with no problem.
Definition of Backspacing - the distance from the inside edge of the wheel to the inside surface where it mounts against the drums or rotor. Z suspensions were designed to have the wheel mounting point centered on the rim. On a 7 wide wheel that means 3 1/2 backspaced. to fit the 7 wide rims you can use the 3 1/2 backspacing and grind about 1/16 - 1/8 of material off the inside lip of your rear fender wells or go with 3 5/8 backspacing. If you are going for maximum tire/wheels under the stock fenders you can run 225/40-17's on the front with 17x7 wheels with 3 1/2 backspacing and 245/35-17's on the rear on 17x8 wheels with 4 5/8 backspacing. The critical area here is the rear. Check the clearances on your car for the wheel well and the suspension on the inside before springing for this set-up. These cars are old enough that they seem to develop dimensions of their own.
Phantom
 

·
Registered
Joined
·
19 Posts
Re: 14, 15, 16 or 17's

You accidentally said the wrong size for the 15s. They should be 205/60-15 to keep the speedo the same.

> Rob -
> From the way you worded your question I
> assume that you are planning on racing your
> Z. If you are not planning on flaring your
> fender wells the following are options if
> you want to keep your speedometer error to a
> minimum.
> 195/70-14, 205/50-15, 205/55 or 225/50-16,
> and 225/40-17. You can run 7 wide rims
> on all four corners with no problem.
> Definition of Backspacing - the distance
> from the inside edge of the wheel to the
> inside surface where it mounts against the
> drums or rotor. Z suspensions were designed
> to have the wheel mounting point centered on
> the rim. On a 7 wide wheel that means
> 3 1/2 backspaced. to fit the 7
> wide rims you can use the 3 1/2
> backspacing and grind about 1/16 - 1/8
> of material off the inside lip of your rear
> fender wells or go with 3 5/8
> backspacing. If you are going for maximum
> tire/wheels under the stock fenders you can
> run 225/40-17's on the front with 17x7
> wheels with 3 1/2 backspacing and
> 245/35-17's on the rear on 17x8 wheels
> with 4 5/8 backspacing. The critical
> area here is the rear. Check the clearances
> on your car for the wheel well and the
> suspension on the inside before springing
> for this set-up. These cars are old enough
> that they seem to develop dimensions of
> their own.
> Phantom
 

·
Registered
Joined
·
1,610 Posts
You're right Elmo

Elmo,
You're right. I did intend to spec 60 series for the 15 tires. I guess I got ahead of myself thinking about the 16 since that is what I run on mine.
I assume the photo of the red ZX is yours? Nice looking car!! My '77 is painted very close to the same color (Cadillac Eldorado Pearl Red) but also has a white racing stripe up the middle.
Z'ya
Phantom
 

·
Registered
Joined
·
19 Posts
Re: You're right Elmo

Thanks. Too bad you have no picture to post of yours. I'd like to see it. What kind of wheels do you have on it?

> Elmo,
> You're right. I did intend to spec 60 series
> for the 15 tires. I guess I got ahead
> of myself thinking about the 16 since
> that is what I run on mine.
> I assume the photo of the red ZX is yours?
> Nice looking car!! My '77 is painted very
> close to the same color (Cadillac Eldorado
> Pearl Red) but also has a white racing
> stripe up the middle.
> Z'ya
> Phantom
 
1 - 9 of 9 Posts
Status
Not open for further replies.
Top