Nice write up. Flyboy, have you had the chance to take a look at the spreadsheet I linked to in
this other thread? I was wondering if the gentleman who created it is using the same formulas as those you are relying on. It would appear from his figures that the RKE/TKE ratio can be up around 4:1. Perhaps you can shed some light on where your two models diverge from each other. (I have a hard time following either of the methods, so any light you could shed would be appreciated.)
Here's some factors that
might come into play:
The spreadsheet models tire weight seperately from wheel weight. This might be important because it's easy to drop 7 lbs. off the weight of the tire alone. (Runflats are heavy!) Garfield's tires aren't particularly light either, but he probably saved 3-4 lbs. That weight could probably be "lumped" out around the 21" diameter. (Your method of lumping the total weight out aound 17" seems plausible too.)
Also, exactly
how quickly the tire/wheel is accelerating is important, isn't it? Much of the "perceived" benefits of lighter wheels are reported during high acceleration at relatively low speeds. (That's mostly because high accelerations are really only possible at low speeds. In first gear, in other words.) This makes me suspect that heavy wheels "consume" a greater chunk of power when the car (and wheel) is accelerating quickly.
Yes, I plugged a couple different 0-60 times into the spreadsheet, and got very different RKE/TKE ratios. A 10 second run gave a ratio of around 3.5:1, whereas a 5 second 0-60 run spit out something closer to 6:1. I can replicate your 1.6:1 ratio by assuming a very slow 0-60 time. (40 seconds) What rate of acceleration did you use? Would your ratio come out different if you assumed a different rate of acceleration? Say, the very fast 5-15mph rate, as opposed to the much slower 50-60mph rate?
I will soon be able to give you some more real-world data. I'll be measuring/graphing my car's acceleration at various speeds with both the stock 17" runflats, and with a tire/wheel package weighing about 33 lbs. I will seek to minimise traction and driver-technique factors, so it should give us more to look at than Garfield's 0-60 times, which were also determined by traction and launch technique. I should have something to post in another couple weeks. (Getting my car tomorrow!)
-Dave
