Wheel sizing vs. power question
 

I recall that when I went from 18" to 19" wheels with my 350Z, I could definitely feel a loss of power, likely due to the increased rotational enertia of a bigger wheel.
Has anyone who has gone from 19's to 20's on their 370Z felt the same thing?

Aside from the raw effects of wheel weight and wheel diameter in terms of rotational mass / inertia, the effective gearing is also changed. Making the wheels larger has the same gearing effect as changing your LSD's ratio in the bad direction (closer to 1:1), whereas making them smaller moves the ratio back in the good direction (closer to larger_number:1).

I went from the stock 19's to a lighter (but wider) set of 18's and the difference was very noticeable.

circumference stays the same. gross weight is wat matters...even that you're using the right size wheels and tires.

The circumference has to be larger by definition when you upsize the wheels, right? (well, unless you offset larger wheels with thinner tires, or vice-versa)

The diameter would, of course, stay the same.....tires with 1" lower profile would be necessary. However, I've found that a larger wheel, even though the diameter of the tire stays the same, will sap some power. Am I wrong about this?

the tire's most outer circumference stays the same. you offset the larger diameter wheel by using shorter sidewall tires.

remember that the purpose of using larger diameter wheels it to allow larger brakes. quicker turn-in response is just a nice sideeffect.

i switched up from 19in te's to 20in ssr's hoping it wud slow me down...it didn't.:ugh2:

and yes, as everyone above has stated, circumference stayed the same

kenchan is right. ive heard this reffered to as "rolling diameter". usually they will offer you the thinner tire to compensate the growth in wheel size, unless you specify otherwise. and vegas, right again about the power sap on the bigger rim, regardless of the "rolling diameter" remaining the same. why? aside from obvious weight gain? i dont know. but i have experienced this on two vehicles. must, maybe the rims you chose were close enough to the weight of your old rims, so you didnt get power loss? just guessing

I am thinking of replacing my rears with 285/35,i wonder what effect wiil have.??

i never changed width and i have no idea how that would be. maybe affect handling? my biggest concern (for myself) about going up on rim size is not any marginal power loss, but ride comfort. the less rubber under you, the bumpier it will feel. and on louisiana roads, its a good way to beat the crap out of your car and have everything rattling in no time.

Okay increasing diameter of the wheels will allow for bigger brakes, increase turn-in response, reduce acceleration but increase top speed.

Also well a 17inch rims and 18 inch rim may have the same rolling diameter (difference in sidewall height) and mass. Having more of the mass near the outside of the wheel will hurt performance compared to having more the mass near the center of the wheel. So going with a lighter weight tire helps a lot.

it wont increase top speed....

some of you guys are making generalizations without understanding how it works.

the overall diameter of the wheel + tire is the item your looking for + weight.

If the wheel weights more, even if its the same size as your OEM, it would slow you down. Whether or not you will feel it, would be determined by how much weight is added.

If you upsize your wheels, you can downsize the side wall ratio to keep the same overall OEM diameter and therefor your TOP speed / acceleration would not be altered DUE to the size. it would, most likely be altered due to the weight and the shifting of the weight load to the outer radius where it would sap the most power.

I went to a 20x12 wheel, with a 325/25/20.

Run the calculations on the overall diameter compared toa 275/35/19. You will find my overall diameter is shorter by about .17" which technically means i should accelerate faster. but since its a 20x12 wheel with massive rubber, i know it weighs more than the OEM and therefore realistically would slow me down. Can i feel it? not really. I dont go full throttle everytime i drive my car all day long and so i wasnt attuned to the oem vs these. the car is still quick as hell.

In my case, when I dropped from 19 to 18, combined with my tire changes I did reduce the overall circumference. And yes, mass matters more the further it is from the center of the wheel.

The "top speed" factor isn't worth mentioning since the gearing on our cars goes way past the point of drag-limit anyways. You'll never reach the top speed of the gears+wheels regardless.

"Increased turn-in response" isn't a function of wheel size AFAIK. It's a function of running shorter (as in thinner) tires, which you can do on any reasonable wheel size.

The only good "performance" argument for increasing your wheel size is to fit larger brake rotors. Otherwise the ideal is a smaller wheel with a low profile tire.

found this yesterday . motordyne comparing different wheel size and there effects . Motordyne Test of Wheel Diameter Vs HP

I meant increasing overall diameter will increase top speed.

I remember my stock 17 inch 350Z wheels with tires weighed 54 pounds
Swapped to my 19 inch Volk TE37s with tires they weighed around 49 pounds

By going +2 in rim sizes but reducing the unsprung weight i actually noticed a positive change...

Its always funny to hear someone go +2 and add like 10 pounds per wheel and wonder why their car feels slower! haha

Tire Size Calculator - tire & wheel plus sizing

The above link should help some still debating.

simple useful Tire size calculator

A positive change how? By moving the some of the rotational mass outward 2" you should be losing power, not gaining it. From what I've read and experienced with my 350Z, a gaining power doesn't seem possible. I went from 18's to 19's and the setup was lighter than stock.....yet the car still felt like I had lost some power.

Folks - you are confusing torque, total tractive effort availble and power.

The amount of torque produced by an engine at a given engine-speed will be constant (near enough).

Power is the amount of "actual work" that can be done and is actually torque multiplied by time (usally expressed in engine rpm).

Total tractive effort is the torque available at the road and is influenced by overall final drive ratio (including OD of the wheel/tyre assembly)

The weblink to dyno charts graphs the same "power" but it is achieved over slight differing time periods and IMHO misrepresents the true facts .. I beleive the graph is showing "tractive effort" on the vertical axis. It is the slower time to acheive the same output (tractive effort) that you can perceive as "less power".

What is not clear is the relative diameters of the wheel/tyre combination as this will cause the time for a given torque (available tractive effort at the tyre/dyno roller interface) to differ as it influences the gearing. Bear in mind that a gearbox and fnal drive ratio are effectively a torque multiplication device and will influence the time it takes for a given amount of torque to achieve a given rotational speed (and hence tractive effort measure).

The seat of your pants will tell you that a 20" wheel with 35 profile tyre will accelerate more slowly that a 19" wheel wth the same profile tyre and this wil accelerate more slowly than a 18" wheel fitted with a 35 profile tyre - this is due to the realtive overall gearing ratio diferences.

In fact, the same torque and engine rpm mean the same "power" figure at the flywheel, it is just that by changing the effctive gear ratio, you are altering the relative "total tractive effort" and this is what manifests as acceleration which you feel thru the seat.

Differing weights of wheel/tyre assemlbies will not affect the torque (and therefore power) generated by the engine at the flywheel, but it will affect the ability of the engine to generate "tractive effort" as a consequence of gearing changes.

To determine the differences in tractive effort due to rotational inertia alone (differing wheel assembly weights), you would need to repeat the tests with wheel/tyre combo having exactly the same rolling diameter.

That said, lightweight wheels will reduce unspung weight which will improve mechanical grip as the damper has less weight to control and this makes the suspension more predictable in its behaviour - they will contribute to lower overall vehicle weight which improves the ability of a given amount of torque to accelerate the vehicle, as well as a reduction in torque losses attributed to accelerating the wheel and tyre itself.

The discussion thread is about "power" but I really think the peception is about "time to accelerate the vehcile" and these are not the same thing.

RB

I said a positive change not gaining power per say...
You are sure the 19s were lighter then your stock 18s?

Its common knowledge lighter wheel/tire better performance for cornering, accelerating etc.

The circumference of the entire wheel/tire was the same as stock so its not going 2inches larger.. The car dynamics stay the same as far as the height of the tire... But the sidewall gets reduced aiding in cornering and the overall weight of the rim being reduced allows for the power being put to the ground to move the wheel with less overall effort.

Its like saying lift a 50 pound weight and then lift a 30 pound weight... It doesnt mean you are stronger by lifting the 30 pound weight easier it just takes less overall effort to lift. (my best analogy for this situation)

All of the above is true, but all other things (wheel/tire mfg, weight distribution/total, etc) being equal, if you kept the same sidewall height and dropped back to an 18" wheel, the car would perform even better.

The only performance reason to intentionally upsize wheels is to fit bigger brake rotors. You can run lighter wheels and shorter sidewalls in any diameter, and the smaller you get the outside diameter the better it is for acceleration (the gearing effect mentioned at length two posts up).

Volt Z saw the positive change becaue he lowered the unsprung weight of the vehicle.
Unsprung weight is anything that isn't supported by the shocks and springs. Lighter brake calipers and suspension pieces can help improve the handling a car. The spring and shock are trying to press them down. With a lower inertia because of the reduced mass makes it easier to control. think if as catching a ping pong ball and a golf ball both traveling at the same speed. WHen it hits your hand the golf ball is going to make your hand move more and be harder to stop even though they are of similar size and speed.

Reducing rotating mass pays big dividends. The spinning wheels work like gyoscopes so turning them because more difficult as they gain speed. The extra inertia from a heavier wheel makes if harder to start moving, change direction, and to slow. Interia isthe tendency of a body to preserve its state of rest or uniform motion unless acted upon by an external force. the more mass it has the more external force is required.

It would make sense that if VolkZ bought a set of 18in rims of the same style as his 19's the 18's would be lighter and he would gained even better results. You also have to take in to acount the extra weight of the tires. To maintain the same tire diameter or rotational circumfrence(sp) an 18in tire is going to have 1 more inch of rubber on both sides off setting the weight reduction of the smaller rims. light brake disks also have the same effect. This is the reason for racing cross drilled rotors.

Lighter unstrung weight has a huge effect on motorcycles. This is why people pay $4000 set for CF rims that save them 4lbs. Since bikes don't have a built in camber setting , it is easier to tilt the wheel makes for a much more responsive ride.

Using the same sidewall ratio on 18's as you would a 19 would reduce the overall gearing ration. It would help it accelerate. If depends on the grip level of the tires if you over gear your car you can overload your tires and just melt them. I seriously down that it would be the issue on case the gearing change wouldn't be that great. It would throw off your speedometer, and raise the RPMs to go any given speed.

Good write up! Rep+

Whenever you change wheel size, you change profile of the tire to keep gearing the same, and also it would look ridiculous by either losing or gaining room in the wheel well. Total weight is the only thing that could effect acceleration. It's as simple as that.

Moving from 18s to 19s or the other way around is not going to make a huge difference. Ive come to accept, after all these years, the butt dyno is not a good reference. The butt dyno reference is used way too much on this forum and almost all non track drivers are not going to be able to tell the difference. I know you want to believe you can feel it but let it go folks. A big difference in weight will induce an affect but it has to be a HUGE difference.

Not true. If the majority of the weight is closer to the outside diameter, this WILL affect the effort needed to turn the wheels/tires. It has to do with the rotational mass being pushed further away from the center of the wheel.

True VegasZ. Weight distribution is also a factor. If you have wheels that weight the same. If the design has a large mass centrally located by the hub with thin spokes going out the the rim, it will be require less energy to accelerate. If you have a rim that is has a thinner hub but has a more solid spoke design the weight is moved further out from the axis giving the outside mass more leverage to resist movement.

For those that need a visual. Take peice of rope and tie a weight close to your hand now spin it so the end of the rope is making an arc. Now grab the other end of the rope and spin it so all things being the same. The end of the rope is going to the travel the same distance but with weight further from the axis. Tell me which os more difficult. To start and stop.