I've been searching for information on wider tires and I stumbled across this thread which discusses 15" vs 16" vs 17" rims a bit.

http://forums.maxima.org/showthread....threadid=85760

Many guys in there said that 15" rims will out-accelerate 17" rims. I do not think that is correct in all cases. As long as the overall diameter of the wheel+tire combo is the same and the weight is the same, the 15" and 17" combo will accelerate the same.

If I am full of it, please let me know, but I thought that was how it worked.

 

usually with 17" rims the diameter is slightly bigger so no big difference there. The difference comes in the weight. If you can get really light rims 15" or 17" you get good acceleration. But for the most part bigger rim equals heavier. That's why some people like having their 15" rims. Most don't because bigger rims look better.

 

There's more to it than that. A 17" rim has its weight spread further from the axis of rotation. This means it takes more energy to accelerate the wheel.

 

absolutly correct. Fuor example, if you have a 16 inch rim that weighs 15 lbs vs a 17 inch rim that weighs exactly the same, the 17's may still take more energy to rotate than the 15.

Rim weight has little to do with the slowing you down. It's the distribution of weight from the center that really determines how it accelarates.

 

I posted this in another thread, but hopefully some of you will find this useful too. Originally found this from the maxima.org site.

Tire size calculator:

http://www.miata.net/garage/tirecalc.html

 

That site does not help in this case because it does not take wheel/tire weight into consideration.

That site has been around for a long time and a lot of people (including myself) use it already.

 

17" wheels will be slower even if they weigh less than the 15"s which is doubtful unless you are comparing stock 15s to racing hart 17s. We have talked a lot about how big rims will kill acceleration.

 

This topic always starts trouble.

SuDZ

 

The drive axle applies a twisting force (torque) to the wheel/tire assembly to make it rotate.

The heavier the wheel/tire, and the further the weight is from the axis of rotation, the slower it accelerates.

If the wheel/tire combination is changed from a set of stock (heavy) 15 inch stockers to a set of liteweight 16" or 17" units (and the outside diameter of the setup is not changed) there shouldn't be a significant difference in acceleration.

If a heavy set of 19's or 20's is installed, the car's acceleration will be noticibly affected, especially if the overall diameter of the wheel/tire combination is increased.... as this kills the gearing.

Larger diameter wheels do put more wheel mass further from the axis of rotation, (the 'hoop' gets larger and moves further from the center) but as long as the wheel isn't that heavy, and the overall diameter of the wheel/tire is not changed, the car's overall acceleration shouldn't be affected that much.

Good Luck!

 

yeah! Here we go again!

 

Kinda OT, but how much do 17" Moda R6s weigh? I tried to look here but they didn't have MODA, so I was out of luck. Is there a better site where I can find weights of rims? Thanx.

 

Just do mr^2 (m=mass, r=radius which should be the same because you include tires). For your 15" wheels, and for your 17" wheels. The larger the number you get, the slower your car is going to be!
-Cyrus

 

What's the source and rationale behind that formula? Is it substantiated at all?

 

E=MC^2

 

It is the formula for rotational inertia, go look it up in an old Physics book.
-Cyrus

 

It's just not the extra weight of the larger rim or the fact that weight is spread out further from the axis, it's also about the tires. Lower profile tires are HEAVY because they typically use more steel belts and the sidewalls are very thick. The additional weight of tires further from the axis will only make things worse. You also need to factor in the extra rolling resistance of wider tires. 235 width tires are typically .7-.9" wider than stock. That's a lot of drag.

This week I went from the stock 15s with 215/60 tires to a new set of 17s with 235/45 tires. The 17s are very light (17lbs) and are lighter than the stock 15s. However, the tires are heavier, wider, and the additional weight to spread out further from the axis. Overall, I gained 3lbs in total weight. My car is slower. How do I know? It's not as responsive during normal launches and during "tip-in" acceleration. It's not a "wow, where did all my power go?" feeling, but I know I'm slower. I've owned 17s before and I ran with them at the drag strip. On average, I was .2 and 1-2mph slower than with 15s. Much of the difference was seen in the first 1/8 mile. I should note my old 17s were tanks though and weighed ~6lbs more than my current 17s (50lb wheel/rim combo).

It's all a trade off. On a roadcoarse track I'd run my 17s. At the drag strip I'll run my 15s. The 17s offer worlds more cornering traction, better feel, and better braking. The 15s offer better straighline traction off the line, better acceleration, and a softer ride.


Dave

 

Isn't the formula mv(squared) over r? for cetrifical force?

 

Hmmm according to the forumula, of you're able to keep the mass and the radius constant, you shouldn't loose that much acceleration going from a 15" to a 17" wheel... Even if you increase the radius, if you decrease the mass, you shouldn't loose that much from a 17" to a 15" wheel/tire combo... We need somebody to try 17" Rota Subzero's w. stockers to prove this...

LEMAR

 

Oops, sorry... I was a little drunk last night... saw that and it didn't click (in my brain)... E=mc^2

HAHA my bad!

 

Actually my bad, I wasn't quite in the right mind... you have to do it for the wheel and tire seperatley. The Radius factor does increase, since you have to calculate where the majority of the mass is. On a 17" rim, it's 8.5" from the rotational point, whereas on a 15" wheel is it 7.5". The tire weight also matters, people often forget that tire weights vary a lot as well. As far as I know, Pirelli P6000 Supersports are among the lightest.
-Cyrus