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What matters more: wheel & tire weight or overall diameter?

Sportwagen SEL

Ready to race!
Location
Burlingame, CA
After doing quite a bit of research, it seems like there are two schools of thought regarding this debate. I'm curious to know — which makes a bigger difference in acceleration, running lighter wheels or reducing your overall tire circumference? (i.e. lowering your gearing)

I've tried both at the same time, which made a huge difference together, but unfortunately I have not tried each concept separately (hence why I don't know which one made the bigger difference).
 

nuSchule

Ready to race!
Location
NC
I think there are a LOT of variables to consider with your question... are you talking about acceleration in terms of speed: 0-60? 0-100? 50-100? Or in terms of distance: 400m? 800m? 1000m? When you say "lighter" wheels, do you mean lighter by 5lbs? 10lbs?

Sorry for all the questions but I believe, generally, a shorter wheel/tire will have a greater impact to the change in acceleration when compared to a lighter wheel. Disclaimer: that's a blanket statement without considering any other factors.
 
Location
St. Olaf
WHAT actual weight difference we're talking about, WHAT actual change in diameter?

The difference between 225/45 R17, 235/45 R17, 225/40 R18, 235/40 R18, 225/35 R19
and 235/35 R19 is noticeable, but still minor. Sadly you didn't mention which ones you
tried though.
 

Sportwagen SEL

Ready to race!
Location
Burlingame, CA
I think there are a LOT of variables to consider with your question... are you talking about acceleration in terms of speed: 0-60? 0-100? 50-100? Or in terms of distance: 400m? 800m? 1000m? When you say "lighter" wheels, do you mean lighter by 5lbs? 10lbs?

Sorry for all the questions but I believe, generally, a shorter wheel/tire will have a greater impact to the change in acceleration when compared to a lighter wheel. Disclaimer: that's a blanket statement without considering any other factors.

Good point, I think I'm most likely talking in terms of the butt dyno as a DD lol. That's what I thought too until I read this:

In theory, yes. Beware, science follows.

The kinetic energy in the wheel is a measure of how much energy you've spent just getting the wheel to spin rather than investing that energy into the road. Here's the equation for the kinetic energy of a rotating object:

r is radius
m is mass
I is rotational inertia
E is energy
w is angular velocity
P is the constant Pi

E = 1/2 I w2

rotational inertia is obtained by this equation (for hollow cylinders):

I = 1/2 m r2

but w is radians per second, which is rotations per second (rps) times radians per rotation, which is 2 P

w = 2 P rps

rotations per second (rps) is velocity/circumference
circumference is 2 P r
so rps = v/(2 P r) therefore:

w = 2 P (v/(2 P r)) = v/r

This gives us an energy equation of:
E = 1/2 I (v2/r2)

Now we work on the I term, which as stated above is based on mass and radius.
But of course mass depends on radius. The mass of a cylinder is:
m = density 2 P r
Let's let the constant C be "density 2 P":
m = C r
So:

I = C r r2 = C r3

and then

E = 1/2 C r3 (v2/r2)

collapse the 1/2 into the constant, and combine terms, and we get:

E = C r v2

This means that for any given forward speed of the vehicle (v), the energy required just to get the wheel up to speed is greater for larger radius wheels. And this relationship is linear, e.g. a wheel that is 50% larger (in radius) will require 50% more energy to reach the same speed.

So this issue is finally settled, right? Unfortunately not. It takes this much energy to get the wheel up to speed, but once it's there, you only need to add more energy to make up for what has been lost to friction, and here the larger wheel wins.

Larger wheels have less rolling resistance, for several reasons. First, they won't drop (as much) into a smaller hole as a smaller wheel would. Second, they have greater leverage for lifting the wheel over bumps. Third, there is less deformation of the tire at the contact patch on the ground.

The main effect of changing the wheel diameter on a car is the need to change the gears which change the ratio of engine speed to wheel rotation speed; larger wheels clearly rotate more slowly for a given car speed. However, the acceleration and top speed of a car does not depend on the wheel diameter if the gear ratio is optimized for that diameter.

More important is the weight of the wheel. The lighter the wheel, the less is the kinetic energy of the wheel at a given speed and so the more of the work of the engine is used to drive the car forward.

Large diameter wheels and tires effectively put the car in a slightly higher gear, which in principle could be compensated for by having a different set of gears in the power train, but in the real world that change isn't usually cost-free. If your engine is overpowered or "torquey" with its given gear set, a larger tire diameter helps you use that for speed. Conversely many small engines will need small tires to do their (very modest) best acceleration.

The rotary inertia of huge tires can reduce a vehicle's acceleration, but that's only a factor when the wheels and tires are a substantial fraction of the vehicle's mass. Other than monster trucks or beginner-designed RC or robot vehicles, it usually does not matter much.

TL;DR Weight is more of a factor than size due to rotational inertia.

Do you think this is true?
 

Sportwagen SEL

Ready to race!
Location
Burlingame, CA
WHAT actual weight difference we're talking about, WHAT actual change in diameter?

The difference between 225/45 R17, 235/45 R17, 225/40 R18, 235/40 R18, 225/35 R19
and 235/35 R19 is noticeable, but still minor. Sadly you didn't mention which ones you
tried though.

I was trying to decide between 2 different setups at the time:

#1) 17x7.5 wheels (17lbs) & 225/45 tires (23lbs)

#2) 17x8 wheels (20lbs) & 245/40 tires (24lbs)

The goal was to find the lightest combo at a reasonable cost that also lowered the gearing as much as possible while at the same time leaving enough sidewall (nothing lower than a 40 series) to protect against potholes, as well as have a little tire overhanging the rim to protect against curb rash. Based on the tires I was deciding between (EC Sport and PSS), the only size that fit the equation that actually lowered the gearing was a 245/40/17 size tire (225/40/17 and 235/40/17 weren't available). However, I wasn't sure if the slight decrease over stock (24.7" vs 25.1") would outweigh the 4lbs of additional weight from that setup.

I ended up going with setup #1 after discovering that EC Sports weigh only 20lbs in a 225/45 size (vs 23lbs for the PSS). However I'm still curious to know which makes a bigger difference. I used to run a 245/35R17 setup (which was a night and day improvement on my Sportwagen) however, because I lost 7lbs and went down over 5% in overall diameter, I was never able to tell which factor was the one that really made the biggest difference.

I've also tried 225/40R18 (stock), 255/35R18, 215/35R18, and 245/40R18.
 

snobrdrdan

former GTI owner
Well there really isn't a negative to going with lighter wheels/tires (except the cost)

Going smaller diameter though....that will change the gearing, obviously, and affect your MPG's when up to speed for example (turning 3k instead of 2,500RPM on the highway, for example). And mess with the speedo too, throwing it off.
Plus going smaller will look a little goofier, if you aren't lowered to compensate
(this is assuming a "drastic" change)
 
Location
St. Olaf
Tire weight does matter even more than wheel weight since the wheel's mass is
more close to the center. Unfortunately, as most UHP tires actually weigh more
compared to modern higher-quality touring tires. UHP tires need to be stiffer for
improved cornering and that's why they come with heavier reinforcements inside.
As often we have to accept compromises > acceleration vs cornering performance.
I doesn't harm when the wheel is as light as possible as long as you can afford it
and it's strong enough. It's isn't that simple when it comes to tires though. I still
think the PSS is a great compromise between (low) weight and (cornering) perfor-
mance. All lighter tires (I'm aware of) are too soft (except perhaps the PS Cup),
while very most (if not all) EP and R-compounds are heavier, again except the PS
Cup/PS Cup 2, which I wouldn't run on the street all the time.
 

nuSchule

Ready to race!
Location
NC
Tire weight does matter even more than wheel weight since the wheel's mass is
more close to the center...

^This.

The math OP provides is very compelling and makes a valid point, but I don't believe it considers where the weight is located on the cylinder (wheel).

I think at the end of the day, the method you've already confirmed to give measurable benefit is the best way to go: do both! ;)
 

Sportwagen SEL

Ready to race!
Location
Burlingame, CA
Well there really isn't a negative to going with lighter wheels/tires (except the cost)

Going smaller diameter though....that will change the gearing, obviously, and affect your MPG's when up to speed for example (turning 3k instead of 2,500RPM on the highway, for example). And mess with the speedo too, throwing it off.
Plus going smaller will look a little goofier, if you aren't lowered to compensate
(this is assuming a "drastic" change)

Yes, I was aware of all that when I made the plunge (on both my cars — Sportwagen & GTI), and I have to say that it was 100% worth the slightly worse MPG, speedo inaccuracy (I actually like it reading fast — helps me avoid tickets!), and not so sexy appearance.

While it did make 1st gear a little bit too low, it really put all the other gears in a much more usable range (especially 6th, which was always too tall for my tastes). Made both cars more responsive & less laggy both off the line and at speed due to the torque increase.
 

Sportwagen SEL

Ready to race!
Location
Burlingame, CA
Tire weight does matter even more than wheel weight since the wheel's mass is
more close to the center.

Yes, that is why I ended up going with the EC Sport (set of 4 is 12lbs lighter than PSS in size 225/45R17!) since they are supposedly on par with the super sports. I do realize that I will lose some cornering sharpness over the PSS, however for me personally it will be more than made up for by the superior ride quality.
 

Kinogod

Passed Driver's Ed
Location
Long Beach



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Kinogod

Passed Driver's Ed
Location
Long Beach
Clearly wheel weight before rim weight but both are SUPER IMPORTANT for performance. A lot of owners miss this fact and over tire their cars and make them look cool but perform worse.

I notice a diff between flickability of a 225 and a 235. The way I like to drive the lighter wheel tire combo is king before spacers and offsets and fat rubber and heavy batcave rim stock.

(2015 GTI PP manual, neuspeed 18x8 RSE12's, Michelin pilot sport cups, VWR sport springs, spulen engine mount, drop in K&N air filter)


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Location
St. Olaf
Again, I'm convinced you'll find some more threads where you could place your pictures.
I'd suggest you start a built thread. ;)
 

Sportwagen SEL

Ready to race!
Location
Burlingame, CA
Just got my new 37lb, 25.1" diameter wheelset, and while I do really like it a lot, I have to say it's not quite as satisfying as the thrill of flying through your gears that I got with my 45lb, 23.9" diameter wheelset. Even thought the lightness DOES really make a difference in how easy it feels to accelerate from a stop, not having all that extra torque and responsiveness from the lower gearing makes it feel not that much different from stock while on the road IMHO. If only I could go small diameter AND lighter weight while still having enough clearance for my PP calipers & and being strong enough to resist damage from potholes...
 
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