Larger wheel diameter - slower acceleration

[rognvald261]

I found going from stock discs to big brake setup a huge drag, apparently unsprung rotational mass carries a magical performance penalty that's hard to pin down a clear explanation as to why. Search web, some say is 2x up to 10x factor, but whatever it is it does make a big difference to momentum / torque.

Some example rim weights:

VE Redline Forged rims 12.6 kg all

VF Redline Forged rims 10.7 kg front
VF Redline Forged rims 11.8 kg rear

VFII Redline cast rims 13.83 kg front
VFII Redline cast rims 13.97 kg rear

Vertini Dynasty cast 20“ 14.0 kg front
Vertini Dynasty cast 20“ 14.3 kg rear

DBA T3 5000 Series lightweight two piece rotors have same design and ring mass as the venerable T3 4000 Series but are actually 1.9 kg lighter each corner. Brembo make similar thing too and are fitted to the VF Motorsport edition.

So what seems an innocuous choice of upgrade aftermarket rims and brakes could easily cost you 5.5kg per corner, and if you agree with multiplying due to complicated physics, that's horrendous as even the slightest change in unsprung weight makes a huge difference to acceleration, braking, suspension and handling.

BTW if you have access to the transmission tune, measuring and entering the correct wheel diameter makes for far more predictable hence accurate performance and economy. Ultimately the powertrain tune is based on rear wheel revolutions per km, so 1.5% discrepancy at the wheels can mean accuracy (predictability) is out. +/- 100 RPM may not seem much on the dampened nacho needle inside the dash display but can make a noticeable difference to expected / actual performance, because it is perhaps the most important coefficient.

2010 SS A6 tune uses 478 wheel revolutions per km
2014 SS A6 tune uses 476 wheel revolutions per km
275/35R19 tune should use 471.5
275/30R20 tune should use 473

 

[Skylarking]

I have a set of 19 inch MY15 redline wheels in a box made in USA compare to the ones on my car which are made in Thailand.

So which if any of the Thailand and USA rims have drop forged markings on them?

 

[Poor old Dad]

I found going from stock discs to big brake setup a huge drag...etc...

Thanks for all of that. I must admit that I had no idea that a relatively small weight difference at the wheels could be so noticeable (or measurable).

I'll let my fat brother know that he has to sit in the middle of the back seat...

 

[Anthony121]

So which if any of the Thailand and USA rims have drop forged markings on them?

I'll check the USA ones. They are defiantly lighter and finished differently on the inside.

 

[Forg]

I'll check the USA ones. They are defiantly lighter and finished differently on the inside.

Did you buy those from Holden - is it at all possible they could have fitted them to Chevrolets but not Holdens for MY15?

 

[Anthony121]

Did you buy those from Holden - is it at all possible they could have fitted them to Chevrolets but not Holdens for MY15?

Yes got them from Holden and parts were listed against the Australian VF Redline.

 

[3rspecB]

Following on from this. I now have all 3 types of the VF Redline 19" staggered rims

Their are subtle differences between the forged MY15 and cast MY16/MY17 Redline rims like, weight, spokes are thinner and slightly narrower and the paint finish on the back of the spokes (which you can't see when on the car).

The MY14 VF1 forged are slightly lighter around 500g then the MY15 forged. The MY16/MY17 are around 2.5kg each more heavier. According to my dodgy home scales.


MY14 VF1 19" Redline, Alcoa USA, forged:

MY15 VF1 19" Redline, Alcoa USA, Forged:

MY16/MY17 VF2 19" Redline, Cast, made in Thailand:

 

[426Cuda]

The X factor in unsprung weight may be explained due to our use of aluminium and the Yanks preference for aluminum.

 

[Forg]

Ta for the interesting info, 3rspecB and Anthony121.

 

[Immortality]

Thanks for all of that. I must admit that I had no idea that a relatively small weight difference at the wheels could be so noticeable (or measurable).

I'll let my fat brother know that he has to sit in the middle of the back seat...

Rotational mass, go back to the old spin on a rotating bar stool trick with your legs pushed out as far as you can and then bring your legs on close and then push your legs out again...

If you watch any youtube videos about dyno tuning you will note that the ones that know there **** will mention the effect of rotating mass of the rear wheel effect on power output.