20 inch rims

Here's another.
 

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I kinda like the 20" rims although I will stick with my stock style for my GN. Glad others are doing what makes them happy. FWIW the math behind the extra brake load is complex but here is a basic pass at it. As mentioned earlier the moment of Inertia (I) is what is important here (rotational not reciprocating). Basic formula for a cylindrical disc is

I=1/2mr^2

and is a function of the wheel mass (m) and the radius (r). Those with keen eyes might say that a wheel isn't a a true cylindrical disc, and you would be right, as there are mass variations, but for the purposes of showing the effect of wheel diameter and because the actual math is stupid crazy this is the simple version.

Let's assume you can find a 20" rim that weighs the same as a 15" steel wheel that the stock brakes were designed for so the mass (m) is the same. This means that the moment of inertia for

Factory approximation:
m= 15 lb
r=15"/2" or 0.625'
I= 2.93 lb-ft^2

Sweet looking 20's
m=15 lb
r= 20"/2" or 0.833''
I= 5.21 lb-ft^2

This means you increased the moment of inertia with the same weight wheel by 77% by simply changing diameter.

Now this doesn't include the tires and it is unlikely you will find a 15 lb 20" rim, and when the weight of the tires works in it gets worse, but basically because the radius (r) works on the square, moment of inertia goes up rapidly.

Now the last formula to relate moment of inertia to torque can be basically represented by

T=I*alpha

where:

T= Torque
I= Moment of Inertia
alpha= angular acceleration (deceleration in our case)

without going into meaningless sample calcs, your brakes create a fixed torque and since the brakes are the same with only a wheel upgrade you can see an increase in moment of inertia translates into a decrease in the amount of angular acceleration which basically means more time to stop.

Time for a beer.
 
Last edited:
I rolled like this before my gnx style wheels.
 

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20" ride quality suffers a little. A little stiff for everyday. Just do the gnx fender mod's. Adjustable shocks would be a good up grade. I only went with 20's because the wheel style I really like that was the smallest diameter.
ImageUploadedByTapatalk1407945437.493601.jpg
 

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Raeban said:
I kinda like the 20" rims although I will stick with my stock style for my GN. Glad others are doing what makes them happy. FWIW the math behind the extra brake load is complex but here is a basic pass at it. As mentioned earlier the moment of Inertia (I) is what is important here (rotational not reciprocating). Basic formula for a cylindrical disc is

I=mr^2

and is a function of the wheel mass (m) and the radius (r). Those with keen eyes might say that a wheel isn't a a true cylindrical disc, and you would be right, as there are mass variations, but for the purposes of showing the effect of wheel diameter and because the actual math is stupid crazy this is the simple version.

Let's assume you can find a 20" rim that weighs the same as a 15" steel wheel that the stock brakes were designed for so the mass (m) is the same. This means that the moment of inertia for

Factory approximation:
m= 15 lb
r=15" or 1.25'
I= 23.4 lb-ft^2

Sweet looking 20's
m=15 lb
r= 20" or 1.66'
I= 41.7 lb-ft^2

This means you increased the moment of inertia with the same weight wheel by 77% by simply changing diameter.

Now this doesn't include the tires and it is unlikely you will find a 15 lb 20" rim, and when the weight of the tires works in it gets worse, but basically because the radius (r) works on the square, moment of inertia goes up rapidly.

Now the last formula to relate moment of inertia to torque can be basically represented by

T=I*alpha

where:

T= Torque
I= Moment of Inertia
alpha= angular acceleration (deceleration in our case)

without going into meaningless sample calcs, your brakes create a fixed torque and since the brakes are the same with only a wheel upgrade you can see an increase in moment of inertia translates into a decrease in the amount of angular acceleration which basically means more time to stop.

Time for a beer.
Click to expand...
Yes. But
215/65/15 = 26" overall dia
305/30/20 = 26.5" overall dia
So not much difference?
 
MDavis said:
Yes. But
215/65/15 = 26" overall dia
305/30/20 = 26.5" overall dia
So not much difference?
Click to expand...

Yay more maths! Anyways, I had to edit my above post for a miscalculation, but I will take a stab at the tires. Keep in mind this is an approximation as the tire sidewalls and tire tread have different mass distributions. This time the formula is

I= 1/2*m*(Major Radius^2 - Minor Radius^2)

Here is the math done in excel so I don't have to type it all out. Keep in mind in this example to show how the radius effects inertia I have kept the mass of the tires the same. Generally tires for 20" wheels tend to weigh more than tires for 15" wheels of similar diameter so just for kicks I tossed that into the calculations at the bottom. I don't have good information on how much a 20" wheel weighs, but this example was never meant to be anything other than an approximation to help with understanding the effect of radius.

Stuff from other post=
upload_2014-8-13_17-44-0.png


Just the tires, both at 21 lbs each:

upload_2014-8-13_17-44-30.png


Now with more probable tire weights courtesy of tirerack.com and my own tire weight for the 15"

upload_2014-8-13_17-46-20.png


Looks like roughly a 50% total increase when you look at realistic tires.

Hope this helps.
 
Not a big fan of anything bigger than a 18 on a GN. I run 17's The black 20's that were posted look pretty good the other two that were chrome. :vomit: but to each there own.
 
Raeban said:
Yay more maths! Anyways, I had to edit my above post for a miscalculation, but I will take a stab at the tires. Keep in mind this is an approximation as the tire sidewalls and tire tread have different mass distributions. This time the formula is

I= 1/2*m*(Major Radius^2 - Minor Radius^2)

Here is the math done in excel so I don't have to type it all out. Keep in mind in this example to show how the radius effects inertia I have kept the mass of the tires the same. Generally tires for 20" wheels tend to weigh more than tires for 15" wheels of similar diameter so just for kicks I tossed that into the calculations at the bottom. I don't have good information on how much a 20" wheel weighs, but this example was never meant to be anything other than an approximation to help with understanding the effect of radius.

Stuff from other post=
View attachment 231527

Just the tires, both at 21 lbs each:

View attachment 231528

Now with more probable tire weights courtesy of tirerack.com and my own tire weight for the 15"

View attachment 231529

Looks like roughly a 50% total increase when you look at realistic tires.

Hope this helps.
Click to expand...

Yep, makes sense.
Didn't mean to hit the report button.
 
I have 20s installed on my Chevelle and compared to the 15 rallys I can honestly say I feel no difference in braking and accelerating. With the hydroboost, it will car send you through the windshield if hammer on the pedal and i haven't installed my Baer 14" brake kit. The nitro tires 305 30 20 actually hook great compared to the bf goodrich 15 tires believe it or not with 650hp Big block. A 20" rim is no way consider a "donk" styled rim in 2014, those guys are running 24" rims at a minimum. The older generation may feel different but I had some guys my age at car meet say my rims are kinda small. At the end of the day buy whatever make you think looks great,
 
I think you would be good with 20's. I'll stay smaller than that on my GN cuz I like the GNX style rim look. I do have 22's on my Skylark tho - w/no "mods" to suspension or brakes & it works/drives nice! I didn't upgrade it, but I replaced it all with newer parts. But I don't drive it everyday, just a weekend cruiser. I still keep the rally's tho cuz I like that look as well.

Do what makes you happy!

image.jpg

image.jpg
 
turbodean87 said:
20" ride quality suffers a little. A little stiff for everyday. Just do the gnx fender mod's. Adjustable shocks would be a good up grade. I only went with 20's because the wheel style I really like that was the smallest diameter. View attachment 231489
Click to expand...

R u from Washington State? I'm from Olympia. Not living there anymore. But I visit cuz my mom & Stepdad is still there. Jus wanted to ask since I seen your Lic. Plates.
 
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