Quote:
Originally Posted by WP0ZZZ
Bill thanks very much for those awesome plots! The comparison of tyre diameter with weight reduction was particularly illuminating.
If we look at the plot I'm quoting above 80 mph, assuming optimal gear selection for max thrust, it is clear that in some speed ranges the larger diameter tyre has more thrust and in some other ranges the smaller diameter tyre has more thrust. Do you have any comment on that?
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Yeah changing gearing isn't magic, it just shifts (pun!) stuff around.
Pulling hard through 3rd at 80 on the small tires the car is about to run out of gear and needs an upshift to 4th. On the larger tires there's still ~1,000rpm left to play with in 3rd. But note by 90mph the smaller tire crosses over to pulling harder in 4th as the larger tire car reaches redline in 3rd, then holds an advantage to about 105. They swap again at 115 and then a final time at 130.
An important thing to note is that gear changes take time (especially for us 915 guys) so while lowering the effective ratio looks appealing, in the real world it can end up slower, situation dependent. For example, if you geared a 911 to reach 60mph at the top of 5th the thrust chart would look insane. However, you'd need 4 shifts to get there, and gears 1-3 would be pretty useless.