ET loss with bigger wheels

[Loochy88]

Im sure this topic has been discussed b4 on here somewhere but i couldnt find it. Anyway i have 2 questions.

How much of a loss should i expect in the 1/4 mile if i go from 16's to 17's?

and

Is it the diameter of the wheel or the weight of the wheel that accounts for the loss? For instance, if i found 17 inch wheels that weighed a significant amout less than my stock 16's would that actually improve my times?

[Arctc Wolf]

it depends more on weight.

[phoenix64]

Are you sure it depends more on weight, if you really change the tire wheel combo, it will make it like a lower(numerical) gear ratio. But at the same time, a 17 with a low pro tire is not much different than our cars stock tires.

[Dominic]

What your after is the change in rotational inertia between a 16" wheel with say P245/50R16 and a 17" wheel with a P275/40R17.

If you assume the tires weigh the same (they dont but lets assume so, the 17" tire weighs more because its bigger in width), and then assume that all of the mass is represented at the outer edge of the wheel diameter (even though it isnt since wheels have spokes)... or at least assume that the weight distribution across the diameter of the wheel is the same.

I = MR^2
16" wheel I = 19.5lbs * 16^2 = 4992
17" wheel I = 20.5lbs * 17^2 = 5925

The 20.5 comes from the weight of a SLP wheel (Firehawk/10spoke SS) since most others are heavier (i.e. ROH, Mille Miglia's are superheavy).

This is a +18.7% increase in rotational inertia. Just by going from stock 16's to lighter weight stock 17s.

Trivia Question: In order to keep the rotational inertia the same, how light would the 17" wheel need to be? [img]smile.gif[/img]

[17.27lbs]

[ February 08, 2003: Message edited by: Dominic ]</p>

[Loochy88]

thanks dominic, that answers a lot of my questions. And i dont think itd be easy to find a 17" wheel that weighs &lt;17.2 lbs.

Even if i did, a 17 inch wheel still has a slightly wider diameter unless youre riding on rubberband tires, which wouldnt look all that good on a 16 inch wheel.

Does anyone have time differences from going from a stock 16 inch wheel to a 20.5 lb 17 inch wheel?

[- I3lue - I3ird -]

Not sure if they make an application for our cars but the SSR competitions are pretty light...

[samfoh]

Also with most low-pro tires, you run higher pressure (more like 44psi instead of 33psi). It seems like running higher pressure would give you less grip. Would't that also contribute to lower ET's?

[Loochy88]

i dont really know about the lower/higher pressure thing. it seems like it might contribute a little. can anyone whos done the 16/17 switch post some times?

how bit a loss are we talkin here?

[David Blalock]

It may be insignificant with the times the V6 runs, but a low profile tire would also have less give in the side wall which can lower traction. Low profile tires are good for road racing because less roll in the tire but not as good for straight line runs.
David

[firebird1]

well, the most significant factor in this would be weight. The difference of rim+tire size will also effect your rpm's and shifting. My currernt times are around 15.15(15.141 and 15.166) with my 18"s that weigh 56lbs each, i am anticipating 14.9's maybe lower without rims which ill find out in a month or 2 , i will post withs/withouts when the time arises

[MustangEater8251]

With going to bigger rims...

there is the weight factro(already went over it)


Then there is the gearing factor... If you went with a lower profile tire so that technically the measurement from tread to tread on both sides of the tire it would be the same. Most likely the 17s will be slighlty larger, and will lower your gear ratio(more of a 3.08 effect over a 3.23)

Next issue with going to Wider tires is you have more friction on the surface of the ground while going down the drag strip, with more friction it will make the wheel feel "heavier".

With teh traction of going to 17 in. wheels you have a smaller sidewall, which is generally much stiffer, and will cause the tires to break loose more easily, not as much of a problem if oyu go form 235s to 275s, sinc you increase the tire contact surface.

Stiffer sidewalls will help you with cornering abilities.

[pgriffith]

you have to also think that the 17 inch sidewall is going to be smaller which probly means it will be more stiff. You will hook better with a softer sidewall, one that you would get from a smaller wheel.

[67-03 RIP]

Here's a formula for finding out how much effect a tire/rim switch would have on your gear ratio:

Effective ratio = (old dia / new dia) x rear gear

Here's how to figure the diameter from section width and aspect ratio:

Diameter= 2 x ((section width x aspect) / 2540))+ rim diameter

Section width is the bigger number 255, etc
Aspect ration is the smaller number 55, 45, etc

That will give a good indication of what your effective rear gear will change to with the new tire/rim.

Ron

[Arctc Wolf]

I wonder how much time and hp I'll lose w/ 17x11 w/ 315-35 tires. the next mod after that will be a converter, so i'll make it up w/ that and make best use the the new traction of those tires :D