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Larger wheel diameter - slower acceleration

VFBlack

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Swapped the 18inch stockies for 20inch x 9 rims with 245/35 tyres, noticed a significant drop in acceleration however not too sure whether top speed has changed. Overall wheel diameter increased by 51mm which pretty much explains the issue, had i known this would have made such a difference probs would have gone 19".

Anyone else experience this?
 

Poor old Dad

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What size were your 18's ?

According to https://tiresize.com/calculator/ the difference between 245/45/18 and 245/35/20 is 5mm which equates to 0.5kph speed increase at 110kph.

(The rolling diameter is basically the same)
 

VFBlack

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My mistake read the calculator wrong, but I always thought the stock rims were 245/45/18 based off this website https://www.wheel-size.com/size/holden/commodore/2016/

Could there be another reason why car feels way more sluggish than before after fitting new rims? Might need to hop down to local mechanic
 

abuch47

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Better grip, less wheelspin.
 

Forg

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Depending on the wheel design itself, it may be significantly heavier. And the mass is concentrated further from the centre of the wheel than before - don't forget that rotational acceleration & momentum relate to mass with the square of the radius, so the extra effort involved in spinning-up larger wheels with mass further from the centre of a wheel is squared with the moving of that mass further from the centre.

I gut-feel used to struggle with being able to see any difference at all, how can an extra kg of mass that's distributed an extra 2" from the centre require that much extra juice to spin-up? But people have tested it, it does make an appreciable difference. Probably more so if you've also changed the effective gearing, which you have.

If it's a problem, fit a numerically-larger diff … simples! :)
 

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Wheels magazine did a test on a VF some time ago where they commented that the previous model with 18” rims was quicker as compared to much the same vehicle with 19 or 20” rims of the same width. They suggested this was due to the higher sidewall on the 18” tyres squatting on acceleration which had the effect of increases the footprint as compared to the same width 19” or 20” rims with much lower stiffer sidewalls.

Maybe a combination of the above wheels mag view and the rotary moment of inertia fighting your desire to accelerate as suggested by Forg.

Oh, as for changing wheel diameter and tyre aspect ratios, the overall rolling circumference does not change by any meaningful amount as tyre aspect ration drops by the appropriate amount as rim diameter goes up. Just check the revolutions per km as used to be quoted by tyre manufacturers to see the trivial difference. Put on 20” rims with tyres of an aspect ration of 65 rather than 35, if they’d even fit that is, and we are the singing to a different tune.
 
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VS 5.0

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Wheels magazine did a test on a VF some time ago where they commented that the previous model with 18” rims was quicker as compared to much the same vehicle with 19 or 20” rims of the same width. They suggested this was due to the higher sidewall on the 19” tyres squatting on acceleration which had the effect of increases the footprint as compared to the same width 19” or 20” rims with much lower stiffer sidewalls.

Maybe a combination of the above wheels mag view and the rotary moment of inertia fighting your desire to accelerate as suggested by Forg.
Top Gear Oz also did a comparo between 19" & 20" rims on a VE around their test track. The 19s went around quicker, with (IIRC) the conclusion being that the greater sidewall, allowing greater flex = greater grip, was the main benefit. I don't recall them doing anything on basic acceleration though.
 

VS 5.0

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Depending on the wheel design itself, it may be significantly heavier. And the mass is concentrated further from the centre of the wheel than before - don't forget that rotational acceleration & momentum relate to mass with the square of the radius, so the extra effort involved in spinning-up larger wheels with mass further from the centre of a wheel is squared with the moving of that mass further from the centre.

I gut-feel used to struggle with being able to see any difference at all, how can an extra kg of mass that's distributed an extra 2" from the centre require that much extra juice to spin-up? But people have tested it, it does make an appreciable difference. Probably more so if you've also changed the effective gearing, which you have.

If it's a problem, fit a numerically-larger diff … simples! :)
Yes, bigger rim/tyre combo will add more weight, but I wouldn't have thought that much. Would it be as great as the difference between a full tank and 1/4 tank of fuel, or carrying a 70kg passenger ?

Or is it something to do with unsprung weight ?
 

426Cuda

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Wheels magazine did a test on a VF some time ago where they commented that the previous model with 18” rims was quicker as compared to much the same vehicle with 19 or 20” rims of the same width. They suggested this was due to the higher sidewall on the 19” tyres squatting on acceleration which had the effect of increases the footprint as compared to the same width 19” or 20” rims with much lower stiffer sidewalls.

Maybe a combination of the above wheels mag view and the rotary moment of inertia fighting your desire to accelerate as suggested by Forg.
Interesting. I dont recall there being a difference in say 0-100 in the VF Redlines with light weight forged 19" wheels, compared to the SS with the heavier cast rims. Although from memory the SS rims were 18". So, maybe the difference in weight was negated by the 18" rims and slighty heavier car, with all the add on bits on the Redline?
 

Forg

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Yes, bigger rim/tyre combo will add more weight, but I wouldn't have thought that much. Would it be as great as the difference between a full tank and 1/4 tank of fuel, or carrying a 70kg passenger ?

Or is it something to do with unsprung weight ?
I can't recall any quantifications, sorry … just that it's noticeable. What I'm talking about is not to do with unsprung weight, that affects handling but will hardly affect acceleration (unless it contributes to something like axle-tramp or other loss of traction); I'm talking about the change in ability to accelerate the wheels due to the mass being further from the centre.

OK try this if you're sitting in a chair that spins (at work or in a home office). Consider how little your hands/arms weigh compared to the rest of you. Spin the chair around with your arms held out, at a (safe) decent rate … then pull your arms in. You rotate faster, and noticeably faster. You've not changed the total mass involved, just pulled it in closer to the centre, and without any energy input the rate of rotation increased.
To accelerate your car, the diff has to increase the rate at which those wheels are turning; the further from the centre the mass is, the more energy it takes to increase the wheel's rate of rotation (and without spinning the wheels faster the car can't go faster).

Interesting. I dont recall there being a difference in say 0-100 in the VF Redlines with light weight forged 19" wheels, compared to the SS with the heavier cast rims.
There's enough variation from day to day & track to track that you'd need to set out to test back-to-back to tell, I suspect.
Unless you got someone who'd put in a few years' worth of experience driving one with the Forg'd wheels and then fitted the cast ones.

I could be wrong, but I do believe handling is affected more by wheel weight than acceleration is.
 
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