It's because Indy cars in an oval travel constantly at speed very close to peak, F1 cars do it for lot less time.slife wrote: A bit off-topic, but why is that Indy cars can use slipstreams to a greater extent and F1 cars cannot ?
Consider that, power of aero drag growing (roughly) with cube of speed, if a car has enough power to reach a speed in the order of 300km/h or more, at lower speed when drag is lot lower, it will have a conspicuous excess of power, usable to accelerate.
In that condition the reduction of drag given by the wake has relatively small impact because the "freed" power is relatively small amount compared with power that is anyway available at same speed also in clean air.
Only at very high speed when little power is left available (and so the acceleration rate is very small) the drag reduction from wake frees a comparatively big amount of power and gives big advantage to acceleration compared with clean air.
In an oval Indy cars travel only in that speed range, so slipstreaming is effective for the whole lap, 100% of time, F1 cars in road courses travel mostly outside of that speed range so slipstreaming can be used for little time.
To get a more quantitative idea, look at this graph:
That is derived analyzing engine sound from an onboard video of Alonso's FP in Malaysia. The two lines represent acceleration out of the last corner, in two consecutive laps (thus with basically same fuel load and basically same tyre conditions), one with DRS activated from corner exit, the other with DRS off. (in both cases KERS was used in basically same way).
As you can see up to about 230-240km/h the two lines are very close, only above that speed the "DRS on" case finally starts to really show a gain, albeit still rather slowly at first, as when red line hits 250 the blue is at 247, when DRS on car gets to 280, DRS off the car is at 274.
That's exactly for the reason explained above, till 240-250km/h, about 85% of the max speed car can reach (DRS off), the impact of drag on acceleration is relatively small, so even reducing it by 10-15%, as DRS does, changes only slightly the acceleration rate, and same obviously apply if the drag reduction, instead of coming from DRS, comes from wake.
Actually in case of wake the gain is probably even less and will need even higher speed to be really felt because, when cars travel at 250km/h, a gap of even just half second means already 30m distance (nose to tail), and at such distance probably the natural drag reduction felt in the wake is less than the DRS offers.
From that it follows that in all the parts of track with speed under 250km/h (or possibly even more), for a F1 car slipstreaming as help for acceleration is non-existent, in the wake or in clean air the longitudinal acceleration is virtually same. (incidentally, KERS makes things worse because increasing power available delays the moment drag becomes the limiting factor to acceleration)
Needless to say then that, in order to be really exploited, slipstreaming needs to be effective for various seconds so to allow the car behind to close the gap (that we have seen is already of 30+ meters even with car behind closely following at half second), which at that speed means several hundred meters.
Most of tracks, especially "new" ones, lack that kind of areas with high enough speed for several consecutive seconds, most have maybe 1, at best.
Ironically even when there is >1 km long straight it can be not enough because typically in Tilke's designs there is a very slow corner (or worse an hairpin) leading to longest straight, which means the first part of the straight itself is automatically made useless; for the first few hundred meters/seconds the cars travel at speed too low for slipstreaming so the effectively usable length is vastly reduced.
Last but not least... the above has nothing to do with downforce or its loss in the wake. The result would be identical if cars had half the downforce or even none, and/or had the perfect aero design that suffers no loss of downforce in the wake.
Without slipstreaming helping the acceleration of the car behind, the gap can't be closed so the overtake is physically impossible, and that even if the cars had in the wake exactly same cornering performance as in clean air.