|
I asked this question to my good friend and team race engineer recently. My goal was to prove a 18x9.5 inch wheel with a 265/35 200tw tire is faster than a 17x9 with a 255/40 200tw tire despite the weight increase 5.75 lbs per wheel and .3" taller diameter. My theory is more tire = lower lap times. A 255 is the widest 200tw tire for a 17" wheel. Switching to 18's opens up a whole new world of tire widths. But is the weight and height worth it? This was his reply.
"As expected, the larger/heavier wheel/tire combo required more energy to cause an acceleration. However, since you're increasing both size AND weight, you kind of get double dinged so i was surprised by how much. angular momentum is increased by weight and size, and angular velocity is increased by size. Between the size and weight increase, the proposed tire/wheel combo will require roughly 7% more energy to accelerate, assuming an average speed of 91 mph at watkins glen. At tracks with a lower average speed, this effect will be less noticeable (the reciprocal goes for tracks with higher average speeds). Next I'll calculate changes to stopping capacity. I won't be able to calculate for the increase in thermal capacity (This won't affect outright speed anyway, but it will affect consistency) or the affect size will have on adhesion, so the assessment will be done strictly with regard to changes in overall friction forces. This will still be a good point of comparison though as the adhesion forces will increase marginally (yes, you're gaining contact patch, but you're loosing some temperature)."
Okay, now i've calculated the theoretical difference in lateral speed and have equated that to lap times. I found a study that was conducted to see how tires deflect under load and how that equates to contact patch (and more importantly, contact patch pressure). I made the assumption that your tires will react similarly, so I was able to simply scale their data based on your tire size increase. The interesting thing is that size and performance don't seem to track linearly. in this case, a 6% increase in size only equates to an effective 0.9% increase in cornering speed. Based on an assumed average corner speed at watkins glen and assuming you're using all available grip, this should equate to roughly a 0.7 mph increase in average cornering speed. This increase in speed will reduce lap times by roughly 0.4 seconds. The additional speed you carry down the straights will also be of benefit. I've assumed an average straight line speed increase of 0.5 mph (the taller tire will effectively reduce the car's mechanical advantage, which means it will be slightly more affected by drag and hills), which should yield 0.5 seconds. In total (and ignoring the effective torque loss from the larger/heavier wheels), you should be down 0.9 seconds over your old setup. It's really difficult to estimate how the torque loss will effect lap times, but i'm guessing at this point the larger wheel/tire package will still be an advantage (though it might not be 0.9 seconds in reality).
it's definitely a good, functioning theory. I didn't get a chance to look at braking, but I'm guessing that's going to be another few tenths. I'm not sure how much the larger/heavier wheels will affect lap times, but my guess is that's also going to be a few tenths. Let's say you've got -0.9 tenths for the turns, -0.4 tenths for braking, and maybe 0.4 tenths for the weight/size increase, you're still going to be down almost a second a lap. It's really hard to say exactly how much time the new setup will take off due to the heavily nuanced nature of the governing physics at play here, but it'll definitely be faster (my estimates as to how much faster are speculative and shouldn't be treated as gospel).
I realize this is more of a track based analysis but here we see that extra weight does not mean slower as long as it is met with more tire. I will do my best to answer questions about this but please don't ask too many. It is WAY over my head. I can attach his calculations if you really want. Hopefully this helps.
|