Yes they didn't include all those things because there are too many variables, and having a smaller diameter wheel (or shortened gear ratios) will help or hurt based on what track your on and if it creates/or saves shifts during a lap. As you said I suspect they used that speed range to keep the car in one gear and exclude as many variables as possible. Anyway I think it does answer my question. I plan to move to 19s next year and see how my laptimes are effected and then i'll have my real world answer as well

 

Hehe good luck with that. I said the same thing to myself and then I ran into the realities of testing...

couldn't get the same tire compound
couldn't get the same tire size
couldn't run the same wheel width with the smaller tire
etc etc

I did manage to compare a 305/30/19 and a 295/35/19 on the same wheel and though the gearing was taller, the 295 ran faster laptimes. Front tire was the same size and compound through both tests.

I'll never know if it was due to better conditions, or the taller gearing, or I'm just better or or or

 

Yes that is weird but interesting...

 

I agree that there are many variables at play. I’d suggest that if you are looking to improve lap times, the real comparison you are looking for is the VMax delta at the end of the longest straight on the track you most frequently drive. That is the first number to look at, and therefore you should also not change any aero if you want an accurate comparison. While acceleration is important, and can be effected be both tire diameter, and gearing, it’s the VMax to look at first. The second place to look is at the corner just preceding that longest straight. The tire that increases the VMin in that corner, and allows you to get to the accelerator sooner, will likely (but not 100%) generate the highest VMax, and be your winner. That VMin can also be influenced by overall tire/wheel weight combo. Speed carried out of that corner, will likely carry down the track, unless your tire diameters are causing sub-optimal (extra) gear changes.

 

I'm coming around to the conclusion that I don't have the budget, time, talent, or support to generate setup findings that would withstand peer review. The narrower taller tire being faster for example...'what the heck?' was all I could conclude. Thats after +10 years of HPDE and the last year of fairly intense tracking the GT4. Maybe I've just started up the slope of enlightenment...I'm not sure. My data logging setup isn't ideal to be fair, but its not **** either, and it didn't help me identify why the narrow tall tire was faster.

With those experiences behind me, this coming year I'm planning to just focus on making things consistent in my chassis setup and driving (sleep, hydration, mental focus, etc) and trying to use setups that are biased more for convenience and live-ability than laptime potential.

All that is background to say I would only go to the 19's if you felt like its going to solve a convenience issue you have.

 

Yes i really just started this thread out of curiosity. I am going to downside to 19" regardless due to decreased cost and increased tire options. But i do think it will make me faster, which i'm looking for because i've hit a plateau at my home track NJMP (specifically lighting)

 

Whats your laptime now? What was the 1:12.3 done on?

 

time now is 1:10.5 on dunlop sport maxx (oem sizes), and the 1:12.3 was on the oem cup 2, but i didn't know the car too well at that time (over a year ago). Don't really like the dunlops. My favorite tire i've had on the car thus far has been the goodyear sc3.

 

There have been more than a few disciplines where I descended from the mount of stupid , at some point in my life, after a few descents and then ascents on the slope of enlightenment, I really realized this effect. Germane to this topic are tracking cars and physics.

On the physics one, I 100% get why they did the 40 to 80 mph model, and I do think that model is very useful to predict what the SOTP effects will be in daily and spirited driving.

On the track, I expect the largest factor is going to be the time averaged power. However, for the speed at the end of a straight when running a hot lap, the 40-80 model ignores the entire reason for having a transmission, and most straights will have at least one shift. So you'll be in a much better place in the power band the entire time than the model concentrates on, and whether the 19s or 20s make a better top speed in the segment might even change depending on where you shift and how many times you shift after the apex... and where you end up in the power curve ahead of the braking zone. Is there one less shift with the 20s and you can just ding the rev limiter before braking? (advantage: 20s) Is there the same count of shifts and you end up a few hundo RPM less than the redline at the braking point (advangage: 19s). And so on.

For what I think will be smaller factors ... From the moment of inertia stadpoint, the lower moment of the 19" combination is helpful, but a lot of the effect is going to be offset by the fact that the wheels (and smaller factor, brakes and diff components) have to turn a larger ω for the same vehicle speed (this is somewhat straightforward to calculate), and the benefit turns into something along the lines of adding in the mass advantage again. There is a mass advantage, but then you have less overall rubber -> less heat capacity (if we're talking total lap times, and maybe adversely affect the exit speed of the prior corner a hair, though it could be helped if you're in a better place in the power curve at corner exit... this will be corner dependent). As far as contact patch is concerned, I'm unsure how much of that is driven by PSIG and how much is driven by the stiffness of sidewall and smaller radius; I suspect PSIG is a larger factor thus they'll be approximately equivalent from this standpint. As far as aero is concerned, if nothing else is changed, it'll prob still be affected by the 1/2" lower ride height -> more downforce & drag on a downforce car?

Looking at the above, I think 19s will more often have a small advantage over the 20s, but I don't think it'll always be the case.

So many factors, many of them not trivial.