RS4 Quick Launch

[sakimano]

Thanks Arthur. I'll give it a shot and report the results.

[ArthurPE]

a Veron launches at 3000 with 0 wheelspin (clutch must slip)

[sakimano]

a Veron launches at 3000 with 0 wheelspin (clutch must slip)

true

the best launching car I've ever seen was a Nissan GTR at a dragstrip. It was somewhat mindblowing how well it took off, but there was no drama whatsoever. I believe it was completely stock, and ran 11.1 all night long with modest MPH of around 120-123. The car just absolutely flew off the line in such a controlled fashion.

[ArthurPE]

if numbers aren't your thing or you think they are a sign of mental incontinence move along
looking at some RS4 data
wheel circum 6.8'
1st gear overall ratio 15.08, 5.1 mph/1000 rpm
a good baseline 0-60 ft time 1.9 or so at 32 mph
these are from several magazine tests

if no slip the wheel should make revs = D/6.8
based on engine speed allowing a slip factor the engine will drive the wheel revs = S x RPM/(60 15.08) x t revs...S = 1/slip and t = time
in order to equate the two, since we assume the engine is turning faster than required, the factor S < 1 is required and it's inverse is slip
equating the two and reducing:
D/6.8 = S x RPM/(60 15.08) x t or D = S x RPM x t / 133 or S = (D x 133) / (RPM x t) since D will be 60 so S = 7980/(RPM x t) or slip = (RPM x t)/7980

we also need to consider at 60' we are travelling 32 mph or engine speed 6250 rpm, so somewhere between launch and 60' we need to increase throttle, ie, we hook-up
now we can calc an avg slip or launch RPM based on 60' and a time
so avg RPM = launch RPM + (6250 - launch RPM)/(6250/launch RPM)
linear expressions are OK for such short times and fairly linear acceleration at these initial speeds

say t = 1.9 sec and launch RPM = 4000 so avg RPM = 5440
so slip = (5440 x 1.9)/7980 ~ 1.33 or 33% too much slip it would seem

say we launch at 3000 so avg RPM ~ 4560
slip = (4560 x 1.9)/7980 ~ 1.09 or 9% avg slip

what avg slip do we want? let's say a good bid to start (15% to 20% is considered optimum) and we want 5% (good traction) at 60', so average of 11% or so
if we solve for RPM = 1.11/(1.9) x 7980 ~ 4660
and launch = 3100 or so

my avg slip estimate would be 1/4 of the run at 15% and tailing off to <5% so avg of 7% or so 1/4 x 15% + 3/4 x (15% - 5%)/2 ~ 30/4 or 7%
RPM = 1.07/(1.9) x 7980 ~ 4500 or launch RPM 2900 or so

which would seem to be a good point to start experiment/testing
a similar method is used to calculate launch points for programs (obvioulsy with finer resolution and input from actual test runs)
time to generate some clutch stink

[adsgreen]

a Veron launches at 3000 with 0 wheelspin (clutch must slip)

Not a great example - with nearly 1000 lb/ft torque it can afford to launch low down!
It does have a degree of wheelspin as it leaves tyre marks when launching.
Iirc one of the people in our accountants building has one (and a sls) and commutes to work in it and parks it on the street!

A dragster launches from crazy high rpm and does have a huge amount of slip

[ArthurPE]

a Veron launches at 3000 with 0 wheelspin (clutch must slip)

Not a great example - with nearly 1000 lb/ft torque it can afford to launch low down!
It does have a degree of wheelspin as it leaves tyre marks when launching.
Iirc one of the people in our accountants building has one (and a sls) and commutes to work in it and parks it on the street!

A dragster launches from crazy high rpm and does have a huge amount of slip

actual thrust at 3000 rpm 1st gear launch
RS4 15.08 x 300/1.08 ~ 4200 lb thrust
Veyron 8.33 x 900/1.25 ~ 6000 lb thust ~ 43% more thrust but the veyron has 20% more mass
I was surpised to 60 ft they were close, a 1/10 or 2...after that the Veyron gets to 100 in the same time the RS4 gets to 60 mph
by the time the RS4 hits 100 the veron is doing ~170!!! on it's way to 250!